JIST - Society for Imaging Science and Technology
JIST - Society for Imaging Science and Technology
JIST - Society for Imaging Science and Technology
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
<strong>JIST</strong><br />
Vol. 51, No. 5<br />
September/October<br />
2007<br />
Journal of<br />
<strong>Imaging</strong> <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong><br />
imaging.org<br />
<strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>
Editorial Staff<br />
Melville Sahyun, editor<br />
sahyun@infionline.net<br />
Donna Smith, production manager<br />
dsmith@imaging.org<br />
Editorial Board<br />
Philip Laplante, associate editor<br />
Michael Lee, associate editor<br />
Nathan Moroney, associate editor<br />
Mitchell Rosen, color science editor<br />
David S. Weiss, associate editor<br />
David R. Whitcomb, associate editor<br />
<strong>JIST</strong> papers are available <strong>for</strong> purchase<br />
at www.imaging.org <strong>and</strong> through<br />
ProQuest. They are indexed in<br />
INSPEC, Chemical Abstracts, <strong>Imaging</strong><br />
Abstracts, COMPENDEX, <strong>and</strong> ISI:<br />
<strong>Science</strong> Citation Index.<br />
Orders <strong>for</strong> subscriptions or single<br />
copies, claims <strong>for</strong> missing numbers,<br />
<strong>and</strong> notices of change of address<br />
should be sent to IS&T via one of the<br />
means listed below.<br />
IS&T is not responsible <strong>for</strong> the accuracy<br />
of statements made by authors <strong>and</strong><br />
does not necessarily subscribe to their<br />
views.<br />
Copyright ©2007, <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong><br />
<strong>Science</strong> <strong>and</strong> <strong>Technology</strong>. Copying<br />
of materials in this journal <strong>for</strong> internal<br />
or personal use, or the internal or personal<br />
use of specific clients, beyond<br />
the fair use provisions granted by the<br />
US Copyright Law is authorized by<br />
IS&T subject to payment of copying<br />
fees. The Transactional Reporting Service<br />
base fee <strong>for</strong> this journal should be<br />
paid directly to the Copyright Clearance<br />
Center (CCC), Customer Service,<br />
508/750-8400, 222 Rosewood Dr.,<br />
Danvers, MA 01923 or online at<br />
www.copyright.com. Other copying<br />
<strong>for</strong> republication, resale, advertising or<br />
promotion, or any <strong>for</strong>m of systematic<br />
or multiple reproduction of any material<br />
in this journal is prohibited except with<br />
permission of the publisher.<br />
Library of Congress Catalog Card<br />
No. 59-52172<br />
Printed in the USA.<br />
<strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong><br />
<strong>Technology</strong><br />
7003 Kilworth Lane<br />
Springfield, VA 22151<br />
www.imaging.org<br />
info@imaging.org<br />
703/642-9090<br />
703/642-9094 fax<br />
Manuscripts should be sent to the<br />
postal address above as describe at<br />
right. E-mail PDF <strong>and</strong> other files as requested<br />
to dsmith@imaging.org.<br />
Guide <strong>for</strong> Authors<br />
Scope: The Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> (<strong>JIST</strong>) is dedicated to the advancement of imaging science knowledge, the<br />
practical applications of such knowledge, <strong>and</strong> how imaging science relates to other fields of study. The pages of this journal are<br />
open to reports of new theoretical or experimental results, <strong>and</strong> to comprehensive reviews. Only original manuscripts that have not<br />
been previously published nor currently submitted <strong>for</strong> publication elsewhere should be submitted. Prior publication does not refer<br />
to conference abstracts, paper summaries, or non-reviewed proceedings, but it is expected that Journal articles will exp<strong>and</strong> in scope<br />
the presentation of such preliminary communication. Please include keywords on your title <strong>and</strong> abstract page.<br />
Editorial Process/Submission of Papers <strong>for</strong> Review: All submitted manuscripts are subject to peer review. (If a manuscript appears better<br />
suited to publication in the Journal of Electronic <strong>Imaging</strong>, published jointly by IS&T <strong>and</strong> SPIE, the editor will make this recommendation.)<br />
To expedite the peer review process, please recommend two or three competent, independent reviewers. The editorial staff, will<br />
take these under consideration, but is not obligated to use them.<br />
Manuscript Guidelines: Please follow these guidelines when preparing accepted manuscripts <strong>for</strong> submission.<br />
• Manuscripts should be double-spaced, single-column, <strong>and</strong> numbered. It is the responsibility of the author to prepare a succinct,<br />
well-written, paper composed in proper English. <strong>JIST</strong> generally follows the guidelines found in the AIP Style Manual, available<br />
from the American Institute of Physics.<br />
• Documents may be created in Microsoft Word, WordPerfect, or LaTeX/REVTeX.<br />
• Manuscripts must contain a title page that lists the paper title, full name(s) of the author(s), <strong>and</strong> complete affiliation/address <strong>for</strong><br />
each author. Include an abstract that summarizes objectives, methodology, results, <strong>and</strong> their significance; 150 words maximum.<br />
Provide at least four key words.<br />
• Figures should con<strong>for</strong>m to the st<strong>and</strong>ards set <strong>for</strong>th at www.aip.org/epub/submitgraph.html.<br />
• Equations should be numbered sequentially with Arabic numerals in parentheses at the right margin. Be sure to define symbols<br />
that might be confused (such as ell/one, nu/vee, omega/w).<br />
• For symbols, units, <strong>and</strong> abbreviations, use SI units (<strong>and</strong> their st<strong>and</strong>ard abbreviations) <strong>and</strong> metric numbers. Symbols, acronyms,<br />
etc., should be defined on their first occurrence.<br />
• Illustrations: Number all figures, graphs, etc. consecutively <strong>and</strong> provide captions. Figures should be created in such a way that<br />
they remain legible when reduced, usually to single column width (3.3 inches/8.4 cm); see also<br />
www.aip.org/epub/submitgraph.html <strong>for</strong> guidance. Illustrations must be submitted as .tif or .eps files at full size <strong>and</strong> 600 dpi;<br />
grayscale <strong>and</strong> color images should be at 300 dpi. <strong>JIST</strong> does not accept .gif or .jpeg files. Original hardcopy graphics may be sent<br />
<strong>for</strong> processing by AIP, the production house <strong>for</strong> <strong>JIST</strong>. (See note below on color <strong>and</strong> supplemental illustrations.)<br />
• References should be numbered sequentially as citations appear in the text, <strong>for</strong>mat as superscripts, <strong>and</strong> list at the end of the document<br />
using the following <strong>for</strong>mats:<br />
• Journal articles: Author(s) [first/middle name/initial(s), last name], “title of article (optional),” journal name (in italics), ISSN<br />
number (e.g. <strong>for</strong> <strong>JIST</strong> citation, ISSN: 1062-3701), volume (bold): first page number, year (in parentheses).<br />
• Books: Author(s) [first/ middle name/initial(s), last name], title (in italics), (publisher, city, <strong>and</strong> year in parentheses) page reference.<br />
Conference proceedings are normally cited in the Book <strong>for</strong>mat, including publisher <strong>and</strong> city of publication (Springfield, VA, <strong>for</strong> all<br />
IS&T conferences), which is often different from the conference venue.<br />
• Examples<br />
1. H. P. Le, Progress <strong>and</strong> trends in ink-jet printing technology, J. <strong>Imaging</strong> Sci. Technol. 42, 46 (1998).<br />
2. E. M. Williams, The Physics <strong>and</strong> <strong>Technology</strong> of Xerographic Processes (John Wiley <strong>and</strong> Sons, New York, 1984) p. 30.<br />
3. Gary K. Starkweather, “Printing technologies <strong>for</strong> images, gray scale, <strong>and</strong> color,” Proc. SPIE 1458: 120 (1991).<br />
4. Linda T. Creagh, “Applications in commercial printing <strong>for</strong> hot melt ink-jets,” Proc. IS&T’s 10th Int’l. Congress on Adv. In<br />
Non-Impact Printing Technologies (IS&T, Springfield, VA 1994) pp. 446-448.<br />
5. ISO 13655-1996 Graphic technology: Spectral measurement <strong>and</strong> colorimetric computation <strong>for</strong> graphic arts images (ISO,<br />
Geneva), www.iso.org.<br />
6. <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> website, www.imaging.org, accessed October 2003.<br />
Reproduction of Color: Authors who wish to have color figures published in the print journal will incur color printing charges.<br />
The cost <strong>for</strong> reproducing color illustrations is $490 per page; color is not available to those given page waivers, nor can color page<br />
charges be negotiated or waived. Authors may also choose to have their figures appear in color online <strong>and</strong> in grayscale in the printed<br />
journal. There is no additional charge <strong>for</strong> this, however those who choose this option are responsible <strong>for</strong> ensuring that the captions<br />
<strong>and</strong> descriptions in the text are readable in both color <strong>and</strong> black-<strong>and</strong>-white as the same file will be used in the online <strong>and</strong><br />
print versions of the journal. Only figures saved as TIFF/TIF or EPS files will be accepted <strong>for</strong> posting. Color illustrations may be<br />
also submitted as supplemental material <strong>for</strong> posting on the IS&T website <strong>for</strong> a flat fee of $100 <strong>for</strong> up to five files.<br />
Website Posting of Supplemental Materials: Authors may also submit additional (supplemental) materials related to their articles<br />
<strong>for</strong> posting on the IS&T Website. Examples of such materials are charts, graphs, illustrations, or movies that further explain the<br />
science or technology discussed in the paper. Supplemental materials will be posted <strong>for</strong> a flat fee of $100 <strong>for</strong> up to five files. For<br />
each additional file, a $25 fee will be charged. Fees must be received be<strong>for</strong>e supplemental materials will be posted. As a matter of<br />
editorial policy, appendices are normally treated as supplemental material.<br />
Submission of Accepted Manuscripts: Author(s) will receive notification of acceptance (or rejection) <strong>and</strong> reviewers’<br />
reports. Those whose manuscripts have been accepted <strong>for</strong> publication will receive correspondence in<strong>for</strong>ming them of the issue <strong>for</strong><br />
which the paper is tentatively scheduled, links to copyright <strong>and</strong> page charge <strong>for</strong>ms, <strong>and</strong> detailed instructions <strong>for</strong> submitting accepted<br />
manuscripts. A duly signed transfer of copyright agreement <strong>for</strong>m is required <strong>for</strong><br />
publication in this journal. No claim is made to original US Government works.<br />
Page charges: Page charges <strong>for</strong> the Journal is $80/printed page. Such payment is<br />
not a condition <strong>for</strong> publication, <strong>and</strong> in some circumstances page charges are<br />
waived. Requests <strong>for</strong> waivers must be made in writing to the managing editor prior<br />
to acceptance of the paper <strong>and</strong> at the time of submission.<br />
Manuscripts submissions: Manuscripts should be submitted both electronically<br />
<strong>and</strong> as hardcopy. To submit electronically, send a single PDF file attached to an e-<br />
mail message/cover letter to jist@imaging.org. To submit hardcopy, mail 2 singlespaced,<br />
single-sided copies of the manuscript to: IS&T. With both types of submission,<br />
include a cover letter that states the paper title; lists all authors, with complete<br />
contact in<strong>for</strong>mation <strong>for</strong> each (affiliation, full address, phone, fax, <strong>and</strong> e-mail); identifies<br />
the corresponding author; <strong>and</strong> notes any special requests. Unless otherwise<br />
stated, submission of a manuscript will be understood to mean that the paper has<br />
been neither copyrighted, classified, or published, nor is being considered <strong>for</strong><br />
publication elsewhere. Authors of papers published in the Journal of <strong>Imaging</strong><br />
<strong>Science</strong> <strong>and</strong> <strong>Technology</strong> are jointly responsible <strong>for</strong> their content. Credit <strong>for</strong> the<br />
content <strong>and</strong> responsibility <strong>for</strong> errors or fraud are borne equally by all authors.<br />
JOURNAL OF IMAGING SCIENCE AND TECH-<br />
NOLOGY ( ISSN:1062-3701) is published bimonthly<br />
by The <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>,<br />
7003 Kilworth Lane, Springfield, VA 22151. Periodicals<br />
postage paid at Springfield, VA <strong>and</strong> at<br />
additional mailing offices. Printed in Virginia,<br />
USA.<br />
<strong>Society</strong> members may receive this journal as part of<br />
their membership. Forty-five dollars ($45.00) of<br />
membership dues are allocated to this subscription.<br />
IS&T members may refuse this subscription by written<br />
request. Domestic institution <strong>and</strong> individual nonmember<br />
subscriptions are $195/year or $50/single<br />
copy. The <strong>for</strong>eign subscription rate is $205/year.<br />
For online version in<strong>for</strong>mation, contact IS&T.<br />
POSTMASTER: Send address changes to JOURNAL<br />
OF IMAGING SCIENCE AND TECHNOLOGY,<br />
7003 Kilworth Lane, Springfield, VA 22151.
<strong>JIST</strong><br />
Vol. 51, No. 5<br />
September/October<br />
2007<br />
Journal of<br />
<strong>Imaging</strong> <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong> ®<br />
Feature Article<br />
391 Digital Color Image Halftone: Hybrid Error Diffusion Using the Mask<br />
Perturbation <strong>and</strong> Quality Verification<br />
JunHak Lee, Takahiko Horiuchi, Ryoichi Saito, <strong>and</strong> Hiroaki Kotera<br />
Special Section: Digital Printing Technologies, including papers presented at<br />
IS&T NIP<br />
402 Human Perception of Contour in Halftoned Density Step Image<br />
Phichit Kajondecha, Hongmei Cheng, <strong>and</strong> Yasushi Hoshino<br />
407 Advanced Color Toner <strong>for</strong> Fine Image Quality<br />
Akihiro Eida, Shinichiro Omatsu, <strong>and</strong> Jun Shimizu<br />
413 Preparation of Microemulsion Based Disperse Dye Inks <strong>for</strong> Thermal<br />
Bubble Ink Jet Printing<br />
S. Y. Peggy Chang <strong>and</strong> Y. C. Chao<br />
419 Effects of Molecular Substituents of Copper Phthalocyanine Dyes on<br />
Ozone Fading<br />
Fariza B. Hasan, Michael P. Filosa, <strong>and</strong> Zbigniew J. Hinz<br />
424 Relationship between Paper Properties <strong>and</strong> Fuser Oil Uptake in a<br />
High-speed Digital Xerographic Printing Fuser<br />
Patricia Lai, Ning Yan, Gordon Sisler, <strong>and</strong> Jay Song<br />
431 Simulation of Traveling Wave Toner Transport Considering Air Drag<br />
Masataka Maeda, Katsuhiro Maekawa, <strong>and</strong> Manabu Takeuchi<br />
438 High Speed <strong>Imaging</strong> <strong>and</strong> Analysis of Jet <strong>and</strong> Drop Formation<br />
Ian M. Hutchings, Graham D. Martin, <strong>and</strong> Stephen D. Hoath<br />
445 Effects of Thin Film Layers on Actuating Per<strong>for</strong>mance of Microheaters<br />
Min Soo Kim, Bang Weon Lee, Yong Soo Lee, Dong Sik Shim, <strong>and</strong> Keon Kuk<br />
continued on next page<br />
imaging.org<br />
<strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>
IS&T BOARD OF DIRECTORS<br />
President<br />
Eric G. Hanson<br />
Department Manager<br />
Hewlett Packard Company<br />
Immediate Past President<br />
James R. Milch Jim<br />
Director Research & Innovation Labs.<br />
Carestream Health, Inc.<br />
Executive Vice President<br />
Rita Hofmann<br />
Chemist, R&D Manager<br />
Il<strong>for</strong>d <strong>Imaging</strong> Switzerl<strong>and</strong> GmbH<br />
Conference Vice President<br />
Robert R. Buckley Rob<br />
Research Fellow<br />
Xerox Corporation<br />
Publication Vice President<br />
Franziska Frey<br />
Assist. Prof., School of Print Media<br />
Rochester Institute of <strong>Technology</strong><br />
Secretary<br />
Ramon Borrell<br />
R & D Director<br />
Xaar Plc<br />
Treasurer<br />
Peter D. Burns<br />
Principal Scientist<br />
Carestream Health, Inc.<br />
Vice Presidents<br />
Choon-Woo Kim<br />
Inha University<br />
Laura Kitzmann<br />
Marketing Dev. & Comm. Manager<br />
Sensient <strong>Imaging</strong> Technologies, Inc.<br />
Michael A. Kriss<br />
MAK Consultants<br />
continued from previous page<br />
Special Section: Digital Fabrication, including papers presented at IS&T DigiFab2006<br />
452 Hybrid Stacked RFID Antenna Coil Fabricated by Ink Jet Printing of Catalyst with<br />
Self-assembled Polyelectrolytes <strong>and</strong> Electroless Plating<br />
Chung-Wei Wang, Ming-Huan Yang, Yuh-Zheng Lee, <strong>and</strong> Kevin Cheng<br />
456 Top Contact Organic Thin Film Transistors with Ink Jet Printed Metal Electrodes<br />
Kuo-Tong Lin, Chia-Hsun Chen, Ming-Huan Yang, Yuh-Zheng Lee, <strong>and</strong> Kevin Cheng<br />
461 Ring Edge in Film Morphology: Benefit or Obstacle <strong>for</strong> Ink Jet Fabrication of<br />
Organic Thin Film Transistors<br />
Jhih-Ping Lu, Ying-pin Chen, Yuh-Zheng Lee, Kevin Cheng, <strong>and</strong> Fang-Chung Chen<br />
465 Digital Fabrication Using High-resolution Liquid Toner Electrophotography<br />
Atsuko Iida, Koichi Ishii, Yasushi Shinjo, Hitoshi Yagi, <strong>and</strong> Masahiro Hosoya<br />
IS&T Conference Calendar<br />
For details <strong>and</strong> a complete listing of conferences, visit www.imaging.org<br />
IS&T/SID’s Fifteenth Color <strong>Imaging</strong><br />
Conference cosponsored by SID<br />
November 5–November 9, 2007<br />
Albuquerque, New Mexico<br />
General chairs: Jan Morovic<br />
<strong>and</strong> Charles Poynton<br />
Electronic <strong>Imaging</strong><br />
IS&T/SPIE 20th Annual Symposium<br />
January 27–January 31, 2008<br />
San Jose, Cali<strong>for</strong>nia<br />
General chair: Nitin Sampat<br />
CGIV 2008: The Fourth European Conference<br />
on Color in Graphics, Image <strong>and</strong> Vision<br />
June 9–June 13, 2008<br />
Terrassa, Spain<br />
General chair: Jaume Pujol<br />
Archiving 2008<br />
June 24–June 27, 2008<br />
Bern, Switzerl<strong>and</strong><br />
General chair: Rudolf Gschwind<br />
Ross N. Mills<br />
CTO & Chairman<br />
<strong>Imaging</strong> <strong>Technology</strong> International<br />
Jin Mizuguchi<br />
Professor, Yokohama National Univ.<br />
David Weiss<br />
Scientist Fellow,<br />
Eastman Kodak Company<br />
Chapter Director<br />
Franziska Frey – Rochester<br />
Patrick Herzog – Europe<br />
Takashi Kitamura – Japan<br />
Executive Director<br />
Suzanne E. Grinnan<br />
IS&T Executive Director<br />
ii J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 391–401, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Digital Color Image Halftone: Hybrid Error Diffusion<br />
Using the Mask Perturbation <strong>and</strong> Quality Verification<br />
JunHak Lee, Takahiko Horiuchi, Ryoichi Saito <strong>and</strong> Hiroaki Kotera<br />
Graduate School of <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba-Shi, Chiba,<br />
263-8522, Japan<br />
E-mail: leejunhak@graduate.chiba-u.jp<br />
Abstract. Error diffusion is widely used in digital image halftones.<br />
The algorithm is very simple to implement <strong>and</strong> very fast to calculate.<br />
However, it is known that st<strong>and</strong>ard error diffusion algorithms, such<br />
as the Floyd Steinberg error diffusion, produce undesirable artifacts<br />
in the <strong>for</strong>m of structure artifacts, such as worms, checkerboard patterns,<br />
diagonal stripes, <strong>and</strong> other repetitive structures. The boundaries<br />
between structural artifacts break the visual continuity in regions<br />
of low intensity gradients <strong>and</strong> there<strong>for</strong>e may be responsible <strong>for</strong><br />
false contours. In this paper, we propose a new halftone method to<br />
reduce the structural artifacts <strong>and</strong> to improve the gray expression,<br />
called hybrid error diffusion, by using the concept of “error diffusion<br />
by perturbing the error coefficient with a mask.” The proposed algorithm<br />
consists of two steps in each pixel position. In the first step, a<br />
perturbation is calculated using the internal pseudor<strong>and</strong>om number<br />
<strong>and</strong> a selected 44 mask, similar to a dither mask. In the second<br />
step, error diffusion weights are calculated with the criterion <strong>for</strong> each<br />
pixel value. The proposed hybrid method can reduce the structural<br />
artifacts while keeping the advantage of the error diffusion. This<br />
paper discusses the per<strong>for</strong>mance of the proposed algorithm with<br />
experimental results <strong>for</strong> natural test images. Then, objective assessment<br />
results are given using statistical tools <strong>and</strong> the structural similarity<br />
measure <strong>for</strong> color images. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5391<br />
INTRODUCTION<br />
Halftoning is a method of producing the pseudocontinuous<br />
tone images using only a finite number of gray levels. Because<br />
of the inherent characteristic of the human visual system<br />
with regard to observing average gray level over an area,<br />
the human observer perceives intermediate tones. Generally,<br />
halftoning is considered as a simple “on” <strong>and</strong> “off” modulation<br />
technique, where the sensation of intermediate tones<br />
is created by the presence <strong>and</strong> absence of a pixel. The digital<br />
halftone technology plays an important role in trans<strong>for</strong>ming<br />
a continuous tone (gray or color) image to an image with a<br />
reduced number of gray levels <strong>for</strong> display devices.<br />
For halftone technologies, each pixel value is determined<br />
to be white or black when compared to the threshold<br />
value, <strong>and</strong> the quantization error is then fed back <strong>and</strong> added<br />
to adjacent pixels. 1,2 The conventional error diffusion algorithm<br />
has the advantages of simple implementation <strong>and</strong> fast<br />
calculation speed. It uses the concept of overflow <strong>and</strong> diffusion<br />
of the quantized error, <strong>and</strong> then resets the diffusion.<br />
Received Oct. 28, 2006; accepted <strong>for</strong> publication Jun. 5, 2007.<br />
1062-3701/2007/515/391/11/$20.00.<br />
However, the conventional error diffusion algorithm introduces<br />
distortion, reducing the visibility, worms, <strong>and</strong> false<br />
textures or additive noise. In order to solve these problems,<br />
many digital halftone algorithms have been proposed. Examples<br />
include using variable thresholds, 3 <strong>and</strong> variable filter<br />
weights 4–6 with input data. There were also approaches that<br />
considered the color channel correlation 6–8 <strong>for</strong> improving<br />
the color halftone visibility in color images. However, these<br />
methods require a complex process <strong>and</strong> long calculation<br />
time or many lookup tables.<br />
In this paper, we propose a well-organized halftone algorithm,<br />
hybrid error diffusion (HED) to improve the conventional<br />
halftone artifacts <strong>and</strong> enhance the visibility of<br />
color. The proposed algorithm is very simple, easy to implement,<br />
<strong>and</strong> can reduce the structural artifacts, keeping with<br />
the advantage of the error diffusion algorithms. We use the<br />
concept of a perturbing error filter weight by using the mask<br />
value, which is perturbed with a pseudor<strong>and</strong>om number.<br />
The proposed algorithm is basically the same as using the<br />
four-tap style error filter similar to that of the Floyd–<br />
Steinberg error diffusion. The basic procedure of the proposed<br />
algorithm is as follows:<br />
1. Determine the mask value <strong>for</strong> each color plane <strong>and</strong><br />
added to pseudor<strong>and</strong>om number.<br />
2. Calculate the error filter weights <strong>for</strong> each color<br />
plane.<br />
The mask that is used is similar to the ordered dither mask.<br />
The mask value is selected by pixel, line, <strong>and</strong> each color<br />
plane. We also use the different error filter weights <strong>for</strong> each<br />
color plane to enhance the color visibility. The error filter<br />
weights were calculated by using a different mask <strong>for</strong> each<br />
color plane. The results of the proposed algorithm show<br />
good per<strong>for</strong>mance <strong>for</strong> reducing artifacts, worms, <strong>and</strong> false<br />
textures.<br />
The paper is organized as follows. In the next section,<br />
we review the conventional error diffusion algorithm <strong>and</strong><br />
investigate its general problem. In the Hybrid Error Diffusion<br />
Algorithm section, we explain the proposed HED algorithm<br />
in detail. In Experimental Results, we introduce the<br />
conventional halftone evaluation tools, pair correlation, <strong>and</strong><br />
radially averaged power spectrum density. As another<br />
method of halftone evaluation, a structural similarity mea-<br />
391
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Figure 1. General block diagram of conventional error diffusion.<br />
sure between color images is also derived. Then, the simulation<br />
results of the algorithms are given with natural images.<br />
We also compare our algorithm with conventional methods<br />
using the objective assessment, halftone statistical analysis,<br />
<strong>and</strong> color image structural similarity measure. Finally, there<br />
is the Conclusion.<br />
CONVENTIONAL ERROR DIFFUSION ALGORITHM<br />
There are many error diffusion algorithms <strong>for</strong> improving the<br />
halftone quality. 1–12 Almost of the conventional algorithms<br />
are designed based on the Floyd–Steinberg error diffusion<br />
algorithm. In this section, we investigate the Floyd–Steinberg<br />
algorithm 1 <strong>and</strong> the Jarvice et al. 2 error diffusion algorithm,<br />
as a representative conventional error diffusion algorithm, to<br />
simulation results. In Figure 1, each signal can be defined as<br />
follows:<br />
=1, if jx,y T,<br />
bx,y 1<br />
0, otherwise,<br />
jx,y = ix,y − a jk ex − j,y − k,<br />
ex,y = bx,y − jx,y,<br />
2<br />
3<br />
Figure 2. Simulation results of conventional error diffusion algorithm.<br />
Figure 3. Block diagram of hybrid error diffusion.<br />
where ix,y is the input image <strong>and</strong> the bx,y is the output<br />
image of the halftone process. The signal jx,y represents<br />
the modified input, a jk are the error filter weights, a jk =1,<br />
<strong>and</strong> T is the threshold value. The signal ex,y is the accumulated<br />
error value that will be diffused to adjacent pixels.<br />
The conventional error diffusion algorithm has the advantage<br />
of simple implementation <strong>and</strong> fast calculation. However,<br />
the conventional error diffusion introduces the distortion<br />
that reduces image quality <strong>and</strong> produces worms, false<br />
textures, <strong>and</strong> additive noise. The simulation results of the<br />
Floyd–Steinberg error diffusion algorithm <strong>and</strong> the Jarvice et<br />
al. 2 error diffusion algorithm are given in Figure 2. The input<br />
image is the “gradation ramp” of increasing gray levels<br />
from 0 to 128 gray levels with the slope of 4 pixels per gray<br />
level. From the simulation results, we can see the prominent<br />
discontinuity <strong>and</strong> a white dot in the middle of the gradation.<br />
In the low gray area, we can also see diagonal stripes. The<br />
worm patterns are also in the middle of the gradation ramp.<br />
The reduction of these kinds of conventional artifacts is the<br />
objective of the proposed algorithm. At the end of this paper,<br />
we compare the results of conventional error diffusion<br />
to the results of the Floyd–Steinberg error diffusion, vector<br />
color error diffusion, 7 <strong>and</strong> Shiau–Fan error diffusion 10 with<br />
natural images.<br />
HYBRID ERROR DIFFUSION ALGORITHM<br />
We use the concept of perturbing error filter weight by using<br />
the mask value, which is perturbed with a pseudor<strong>and</strong>om<br />
number. The concept of the proposed hybrid error diffusion<br />
is shown in Figure 3, in which each signal can be defined as<br />
follows:<br />
=1, if jx,y T<br />
bx,y 4<br />
0, otherwise,<br />
jx,y = ix,y − ha jk ex − j,y − k,<br />
ex,y = bx,y − jx,y,<br />
Ma jk = MaskValue <strong>for</strong> each color/line/pixel ,<br />
ha jk = RndNum + Ma jk ,<br />
5<br />
6<br />
7<br />
8<br />
392 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Figure 5. Determination of the error weight ha jk .<br />
Figure 4. Results of gray level 28: a R, G, B same mask <strong>and</strong> b R, G,<br />
B different mask.<br />
ha jk =1, ha jk 0, 9<br />
where ix,y is the input image <strong>and</strong> bx,y is the output<br />
image of the halftone process. jx,y represents the modified<br />
input, <strong>and</strong> ex,y is the accumulated error value that will be<br />
diffused to adjacent pixels. Ma jk is the selected mask value,<br />
which is dependent on the pixel position <strong>and</strong> color plane.<br />
ha jk are the error filter weights, <strong>and</strong> T is the threshold<br />
value. The four-tap style error filter weight of the proposed<br />
algorithm is similar to that of the Floyd–Steinberg error diffusion<br />
algorithm. However, the internal pseudor<strong>and</strong>om<br />
number generator <strong>and</strong> mask selector is newly added to that.<br />
The error filter weights ha jk are varied with the internally<br />
calculated pseudor<strong>and</strong>om number <strong>and</strong> the mask value. The<br />
mask that is used is similar to the ordered dither mask. This<br />
mask value is selected by pixel, line, <strong>and</strong> color plane. As a<br />
result, the error filter weight varies with the pixel position,<br />
line, <strong>and</strong> color plane. Finally, the error filter weight ha jk <br />
values are determined pixel by pixel by the criterion of<br />
Eq. (9).<br />
The procedure of calculating the error carry is as follows.<br />
First, the mask value is determined based on the color<br />
plane, the pixel position, <strong>and</strong> the line position. For example,<br />
if the color plane is red R, the (pixel, line)(3, 3), the<br />
mask value will be 9 in the R of Fig. 3. Next, a pseudor<strong>and</strong>om<br />
number is added to the predetermined mask value.<br />
Finally, the error filter weights are determined by a normalizing<br />
process <strong>for</strong> each pixel <strong>and</strong> color plane. Then, the diffusion<br />
process is carried out, which is similar to conventional<br />
error diffusion. In view of hardware implementation,<br />
<strong>for</strong> example, the mask values <strong>and</strong> pseudor<strong>and</strong>om number<br />
Figure 6. System block diagram <strong>for</strong> CISM.<br />
seeds can already be stored in the internal RAM (r<strong>and</strong>om<br />
access memory) area. The generation <strong>and</strong> reading process<br />
can be carried out in pixel calculation duration.<br />
We use a different mask <strong>for</strong> each color plane to improve<br />
the color visibility. The results are compared in Figure 4. The<br />
result of a comes from using the same R, G, <strong>and</strong> B masks,<br />
while the result of b is from using different R, G, <strong>and</strong> B<br />
masks as shown. We can see that the white dots in the result<br />
of a are replaced by the mixing of R, G, <strong>and</strong> B dots in the<br />
case of b. We can confirm the increasing effect of halftone<br />
carry density in these results. The green (G) <strong>and</strong> blue (B)<br />
masks are generated from the red mask. The red mask is<br />
generated with the relation of check board weight. That is,<br />
the green mask is generated by moving 1 pixel to left of the<br />
red mask. The blue mask is generated by moving 2 pixels to<br />
left of the red mask. In Figure 5, the example is given in the<br />
case of the RndNum=7 <strong>for</strong> each color plane.<br />
It is possible to control the error diffusion pattern by<br />
controlling the mask value. Because the error diffusion pattern<br />
mainly depends on the error filter shape, the scan direction,<br />
<strong>and</strong> error filter weights, we can control the error<br />
diffusion pattern by controlling the R, G, <strong>and</strong> B masks.<br />
EXPERIMENTAL RESULTS<br />
Introduction to Halftone Evaluation<br />
In this section, we introduce assessment tools used to evaluate<br />
the HED algorithm. In order to evaluate the halftone, we<br />
use conventional halftone statistics. However, it is difficult to<br />
verify the similarity of the structural pattern using conventional<br />
verification measures. In this paper, we use the color<br />
image similarity measure <strong>for</strong> evaluating the structural<br />
pattern.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 393
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Figure 7. System block diagram <strong>for</strong> color HVS part in CISM.<br />
Figure 8. System block diagram <strong>for</strong> the structural similarity measure part in CISM.<br />
Halftone Statistics: Point Process<br />
We introduce the conventional point process statistics to<br />
evaluate the halftone image. The c<strong>and</strong>idates are pair correlation<br />
<strong>and</strong> radially averaged power spectrum density<br />
(RAPSD). 11 Pair correlation, the first c<strong>and</strong>idate, is the influence<br />
that the point at y has at any x in the spatial annular<br />
ring. The pair correlation is a strong indicator of the<br />
interpoint relationships <strong>for</strong> a given pattern. The pair correlation<br />
Rr is known as<br />
Rr = ER yry <br />
, 10<br />
ER y r<br />
where y is the point that is influenced by x. is the sample<br />
of point process. Spectral analysis was first applied to stochastic<br />
patterns by Ulichney 12 to characterize patterns created<br />
via error diffusion. To do so, Ulichney developed the<br />
radially averaged power spectra along with a measure of anisotropy.<br />
The radially averaged power spectrum is as follows:<br />
Pˆf = 1 K DFT 2D i 2<br />
, 11<br />
K i=1 N i <br />
where DFT 2D represents the two-dimensional, discrete<br />
Fourier trans<strong>for</strong>m of the sample , N is the total number<br />
of pixels in the sample , <strong>and</strong> K is the total number of<br />
periodic area being averaged to <strong>for</strong>m to estimate. Finally, the<br />
RAPSD is defined as follows:<br />
Pf =<br />
1<br />
Pˆf,<br />
NRf fRf <br />
12<br />
where Rf is the series of annular rings <strong>and</strong> NRf is the<br />
number of frequency samples in Rf .<br />
Color Image Similarity Measure<br />
In this section, we introduce the evaluation method, color<br />
image similarity measure (CISM), used to evaluate the HED<br />
algorithm. The color image similarity measure is largely<br />
composed of two blocks. The first one is the color consid-<br />
394 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Figure 9. Results of gray level 28 with the Floyd–Steinberg algorithm:<br />
Pair correlation/RAPSD.<br />
ering block with the human visual system (HVS) <strong>and</strong> color<br />
image structural similarity calculation block as shown in<br />
Figure 6. As introducing the color HVS model, we could<br />
consider the interrelation <strong>for</strong> each color channel in structural<br />
similarity measure. A color HVS model takes into account<br />
the correlation among color planes. The HVS model<br />
is based on a trans<strong>for</strong>mation to CIELab color space <strong>and</strong><br />
exploits the spatial frequency sensitivity variation of the luminance<br />
<strong>and</strong> chrominance channels. Having a model of the<br />
HVS allows us to measure the distortion seen by a human<br />
viewer.<br />
Color HVS block<br />
The color HVS block is composed of several subblock, color<br />
space conversion, discrete Fourier trans<strong>for</strong>m, <strong>and</strong> human visual<br />
filters as shown in Figure 7. We carry out the color space<br />
conversion to use the human visual frequency response<br />
model. The RGB image was trans<strong>for</strong>med to CIEXYZ, <strong>and</strong><br />
then to CIELab color space. We denote the L, a * , b * as the<br />
Y y , C x , C z <strong>for</strong> the convenience of equation, respectively.<br />
As a luminance HVS filter, we use the model that is<br />
proposed by Sullivan et al. 13 <strong>and</strong> Nasanen. 14 The contrast<br />
sensitivity of the human viewer to spatial variations in<br />
chrominance falls off faster as a function of increasing spatial<br />
frequency than does the response to spatial variations in<br />
luminance. This HVS chrominance filter is based on the<br />
experimental results obtained by Mullen. 15 The details of the<br />
HVS model are in Appendix I (available as Supplemental<br />
Material on IS&T website, www.imaging.org).<br />
The flow of the color HVS block is as follows. Let<br />
x R,G,B m,n <strong>and</strong> y R,G,B m,n denote the continuous tone<br />
image <strong>and</strong> distorted image, respectively. x Yy ,C x ,C z<br />
m,n <strong>and</strong><br />
y Yy ,C x ,C z<br />
m,n are obtained by trans<strong>for</strong>ming x R,G,B m,n<br />
<strong>and</strong> y R,G,B m,n to the Y y C x C z color space,<br />
X Yy ,C x ,C z<br />
k,l = DFTx Yy ,C x ,C z<br />
m,n,<br />
Y Yy ,C x ,C z<br />
k,l = DFTy Yy ,C x ,C z<br />
m,n,<br />
13<br />
14<br />
Figure 10. Results of gray level 28 with the hybrid error diffusion algorithm:<br />
Pair correlation/RAPSD <strong>for</strong> each RGB channel.<br />
H HVS k,l = H Yy k,l,H Cx k,l,H Cz k,l,<br />
P XYy<br />
,C x ,C z k,l = X Y y ,C x ,C z<br />
k,lH HVS k,l,<br />
P YYy<br />
,C x ,C z k,l = Y Y y ,C x ,C z<br />
k,lH HVS k,l,<br />
15<br />
16<br />
17<br />
x Yy ,C x ,C z m,n = DFT −1 P XYy k,l, 18<br />
,C x ,C z <br />
y Yy ,C x ,C z m,n = DFT −1 P YYy k,l, 19<br />
,C x ,C z <br />
where DFT is the discrete Fourier trans<strong>for</strong>m <strong>and</strong> DFT −1 is<br />
the inverse discrete Fourier trans<strong>for</strong>m. The HVS filters are<br />
applied to the luminance <strong>and</strong> chrominance components in<br />
the spatial frequency domain. Finally, the output of the color<br />
HVS part is x R,G,B<br />
m,n <strong>and</strong> y R,G,B<br />
m,n, which is trans<strong>for</strong>med<br />
to RGB color space, taking HVS into account. This is<br />
the input to the structural similarity measure block.<br />
Structural similarity measure block<br />
The system block diagram <strong>for</strong> the structural similarity<br />
(SSIM) measure block in CISM is shown in Figure 8. The<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 395
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Figure 11. Inputs of the structural similarity measure <strong>and</strong> the color image similarity measure.<br />
SSIM algorithm is exp<strong>and</strong>ed to RGB color. The structural<br />
similarity method was proposed by Wang et al. 16,17 The<br />
SSIM compares local patterns <strong>for</strong> pixel intensities that have<br />
been normalized <strong>for</strong> luminance <strong>and</strong> contrast between a reference<br />
image <strong>and</strong> a distorted image. The MSSIM is the mean<br />
structural similarity measure <strong>for</strong> entire image. The details of<br />
the SSIM <strong>and</strong> MSSIM are in Appendix II (available as<br />
Supplemental Material on IS&T website, www.imaging.org).<br />
In this paper, we exp<strong>and</strong> the concept of MSSIM to color<br />
images. The input of the structural similarity measure part is<br />
x R,G,B<br />
m,n <strong>and</strong> y R,G,B<br />
m,n, which was already processed<br />
with consideration of the HVS. The final output of CISM is<br />
the weighted sum of the MSSIM value <strong>for</strong> each channel in<br />
RGB color as shown in<br />
CISMx,y = w i MSSIMx,y,<br />
i<br />
20<br />
where w i is the weight <strong>for</strong> each channel in RGB color. In this<br />
paper, we used the value of w i =1/3.<br />
Figure 12. Comparison of the structural similarity measure <strong>and</strong> the color<br />
image similarity measure: Floyd–Steinberg error diffusion raster scan.<br />
Evaluation <strong>and</strong> Experimental Results<br />
In order to verify the proposed algorithm, we compared the<br />
conventional error diffusion algorithm, Floyd–Steinberg error<br />
diffusion algorithm, 1 vector color error diffusion, 7 <strong>and</strong><br />
Shiau–Fan error diffusion 10 with the proposed algorithm.<br />
Results of Halftone Statistics<br />
The results of pair correlation <strong>and</strong> RAPSD are shown in<br />
Figures 9 <strong>and</strong> 10. The input image resolution is a 128<br />
128 pixel image with a gray level of 28 as shown in Fig. 4.<br />
Figure 9 is the result of the Floyd–Steinberg algorithm, <strong>and</strong><br />
Fig. 10 is the result of hybrid error diffusion. For the result<br />
of pair correlation, Rr=0 <strong>for</strong> r3.5 is a consequence of<br />
the inhibition of points within a distance of 3.5 of each<br />
other. The more frequent occurrence of halftone result is in<br />
the distance of 4.5r7.5 with the condition of Rr1.<br />
There is no area <strong>for</strong> the case of Rr=0 in Fig. 10. This<br />
means that the hybrid error diffusion can offer the chance of<br />
occurrence in the RGB mixed model.<br />
For the result of RAPSD, it has a power spectrum that is<br />
composed entirely of high frequencies, in the case of the<br />
Floyd–Steinberg algorithm in Fig. 9. However, the power<br />
spectrum of hybrid error diffusion is extended to the lower<br />
Figure 13. Objective evaluation results: Color image similarity measure.<br />
frequency area <strong>and</strong> the high frequency components are suppressed.<br />
This means that the density of a dot is increased<br />
<strong>and</strong> spread with a good pattern profile.<br />
Results <strong>for</strong> the Gradation Characteristic<br />
First, we try to show the capability of MSSIM <strong>and</strong> CISM to<br />
assess the halftone image quality. We used the Floyd–<br />
Steinberg error diffusion (raster scan) as a test halftone<br />
396 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Table I. Numerical data: Color image similarity measure CISM.<br />
Table I.<br />
Continued.<br />
Input<br />
Gray<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Proposed<br />
HED<br />
Vector Color<br />
E.D.<br />
Input<br />
Gray<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Proposed<br />
HED<br />
Vector Color<br />
E.D.<br />
0 1.0000 1.0000 1.0000 1.0000<br />
1 0.9556 0.9556 0.9556 0.9556<br />
2 0.7842 0.7842 0.7842 0.7867<br />
3 0.5526 0.5526 0.5526 0.5779<br />
4 0.3760 0.3760 0.3760 0.4222<br />
5 0.2615 0.2615 0.2718 0.3143<br />
6 0.1886 0.1886 0.2793 0.2661<br />
7 0.1410 0.1410 0.2448 0.2307<br />
8 0.1089 0.1089 0.2751 0.2290<br />
9 0.0863 0.0863 0.3149 0.2324<br />
10 0.0705 0.0701 0.3280 0.2643<br />
11 0.1362 0.0883 0.2964 0.2610<br />
12 0.2079 0.2076 0.2693 0.2654<br />
13 0.1956 0.1956 0.2798 0.2658<br />
14 0.1793 0.1793 0.3135 0.2778<br />
15 0.1654 0.1654 0.3217 0.2902<br />
16 0.1534 0.1534 0.3268 0.2956<br />
17 0.1429 0.1429 0.3303 0.2963<br />
18 0.1338 0.1338 0.3150 0.2969<br />
19 0.1301 0.1269 0.3305 0.3084<br />
20 0.1677 0.1621 0.3421 0.3242<br />
21 0.1908 0.1768 0.3544 0.3302<br />
22 0.1972 0.1969 0.3651 0.3252<br />
23 0.1883 0.1883 0.3619 0.3353<br />
24 0.1800 0.1800 0.3753 0.3509<br />
25 0.1724 0.1724 0.3813 0.3644<br />
26 0.1654 0.1654 0.3933 0.3639<br />
27 0.1599 0.1591 0.3886 0.3733<br />
28 0.1782 0.1734 0.4098 0.3836<br />
29 0.1947 0.1925 0.4162 0.3998<br />
30 0.2076 0.2063 0.4266 0.3994<br />
31 0.2015 0.2015 0.4403 0.4037<br />
32 0.1951 0.1951 0.4449 0.4155<br />
33 0.1890 0.1890 0.4494 0.4232<br />
34 0.1881 0.1849 0.4626 0.4283<br />
35 0.2044 0.2015 0.4685 0.4377<br />
36 0.2166 0.2148 0.4741 0.4529<br />
37 0.2206 0.2211 0.4898 0.4608<br />
38 0.2155 0.2155 0.4954 0.4701<br />
39 0.2103 0.2099 0.4990 0.4704<br />
40 0.2150 0.2112 0.5138 0.4835<br />
41 0.2269 0.2275 0.5202 0.4876<br />
42 0.2361 0.2371 0.5257 0.4983<br />
43 0.2350 0.2357 0.5348 0.5041<br />
44 0.2304 0.2304 0.5397 0.5160<br />
45 0.2348 0.2319 0.5523 0.5264<br />
46 0.2459 0.2455 0.5540 0.5297<br />
47 0.2516 0.2535 0.5645 0.5396<br />
48 0.2520 0.2516 0.5714 0.5501<br />
49 0.2527 0.2487 0.5815 0.5567<br />
50 0.2593 0.2594 0.5873 0.5658<br />
51 0.2668 0.2682 0.5938 0.5725<br />
52 0.2689 0.2694 0.5994 0.5774<br />
53 0.2700 0.2667 0.6072 0.5850<br />
54 0.2759 0.2765 0.6147 0.5956<br />
55 0.2829 0.2846 0.6214 0.5936<br />
56 0.2848 0.2847 0.6223 0.6044<br />
57 0.2869 0.2870 0.6304 0.6112<br />
58 0.2936 0.2952 0.6393 0.6215<br />
59 0.2992 0.3012 0.6459 0.6268<br />
60 0.3005 0.3005 0.6531 0.6405<br />
61 0.3062 0.3084 0.6607 0.6345<br />
62 0.3121 0.3155 0.6627 0.6458<br />
63 0.3146 0.3163 0.6686 0.6475<br />
64 0.3220 0.3265 0.6780 0.6646<br />
65 0.3231 0.3240 0.6805 0.6640<br />
66 0.3286 0.3274 0.6794 0.6772<br />
67 0.3327 0.3319 0.6911 0.6753<br />
68 0.3371 0.3393 0.6970 0.6796<br />
69 0.3423 0.3418 0.7011 0.6922<br />
70 0.3465 0.3468 0.7049 0.6892<br />
71 0.3518 0.3527 0.7124 0.6953<br />
72 0.3559 0.3554 0.7172 0.7046<br />
73 0.3613 0.3618 0.7217 0.7085<br />
74 0.3661 0.3663 0.7300 0.7145<br />
75 0.3703 0.3699 0.7317 0.7230<br />
76 0.3762 0.3764 0.7370 0.7297<br />
77 0.3800 0.3805 0.7352 0.7341<br />
78 0.3853 0.3866 0.7420 0.7338<br />
79 0.3903 0.3914 0.7495 0.7333<br />
80 0.3948 0.3961 0.7538 0.7430<br />
81 0.4001 0.4018 0.7533 0.7423<br />
82 0.4043 0.4074 0.7612 0.7474<br />
83 0.4097 0.4143 0.7646 0.7281<br />
84 0.4146 0.4191 0.7675 0.7570<br />
85 0.4199 0.4230 0.7739 0.7248<br />
86 0.4219 0.4213 0.7736 0.7264<br />
87 0.4290 0.4275 0.7817 0.7065<br />
88 0.4333 0.4331 0.7842 0.7808<br />
89 0.4390 0.4379 0.7883 0.7991<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 397
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Table I.<br />
Continued.<br />
Table I.<br />
Continued.<br />
Input<br />
Gray<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Proposed<br />
HED<br />
Vector Color<br />
E.D.<br />
Input<br />
Gray<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Proposed<br />
HED<br />
Vector Color<br />
E.D.<br />
90 0.4437 0.4430 0.7902 0.7993<br />
91 0.4490 0.4486 0.7945 0.8058<br />
92 0.4538 0.4531 0.7995 0.8049<br />
93 0.4591 0.4586 0.7993 0.8039<br />
94 0.4638 0.4631 0.8047 0.8022<br />
95 0.4694 0.4690 0.8071 0.8051<br />
96 0.4743 0.4734 0.8132 0.8034<br />
97 0.4795 0.4789 0.8179 0.8061<br />
98 0.4847 0.4840 0.8142 0.8030<br />
99 0.4898 0.4890 0.8236 0.8107<br />
100 0.4952 0.4948 0.8257 0.8131<br />
101 0.4999 0.4996 0.8257 0.8099<br />
102 0.5055 0.5052 0.8305 0.8227<br />
103 0.5106 0.5088 0.8342 0.8284<br />
104 0.5157 0.5152 0.8346 0.8295<br />
105 0.5213 0.5207 0.8379 0.8275<br />
106 0.5262 0.5256 0.8417 0.8309<br />
107 0.5315 0.5312 0.8460 0.8361<br />
108 0.5369 0.5363 0.8489 0.8406<br />
109 0.5417 0.5412 0.8477 0.8444<br />
110 0.5472 0.5467 0.8512 0.8397<br />
111 0.5527 0.5521 0.8534 0.8407<br />
112 0.5577 0.5569 0.8557 0.8400<br />
113 0.5628 0.5622 0.8601 0.8449<br />
114 0.5686 0.5678 0.8627 0.8441<br />
115 0.5734 0.5726 0.8633 0.8369<br />
116 0.5783 0.5778 0.8665 0.8336<br />
117 0.5843 0.5836 0.8701 0.8411<br />
118 0.5891 0.5884 0.8713 0.8350<br />
119 0.5939 0.5934 0.8731 0.8203<br />
120 0.5989 0.5982 0.8762 0.8230<br />
121 0.6043 0.6035 0.8790 0.8226<br />
122 0.6092 0.6085 0.8804 0.8277<br />
123 0.6143 0.6137 0.8826 0.8299<br />
124 0.6194 0.6187 0.8873 0.8472<br />
125 0.6251 0.6236 0.8869 0.8352<br />
126 0.6306 0.6434 0.8894 0.8440<br />
127 0.6368 0.6395 0.8894 0.9160<br />
128 0.6401 0.6357 0.8909 0.8827<br />
129 0.6447 0.6318 0.8917 0.8286<br />
130 0.6494 0.6486 0.8925 0.8077<br />
131 0.6548 0.6539 0.8982 0.8820<br />
132 0.6603 0.6590 0.8989 0.8629<br />
133 0.6656 0.6645 0.8986 0.8762<br />
134 0.6705 0.6696 0.9034 0.8641<br />
135 0.6759 0.6746 0.9044 0.8604<br />
136 0.6810 0.6795 0.9045 0.8611<br />
137 0.6859 0.6848 0.9066 0.8692<br />
138 0.6911 0.6893 0.9109 0.8713<br />
139 0.6959 0.6945 0.9112 0.8768<br />
140 0.7009 0.6996 0.9126 0.8751<br />
141 0.7059 0.7047 0.9131 0.8752<br />
142 0.7105 0.7093 0.9147 0.8802<br />
143 0.7151 0.7138 0.9158 0.8828<br />
144 0.7200 0.7186 0.9188 0.8798<br />
145 0.7245 0.7231 0.9209 0.8933<br />
146 0.7293 0.7279 0.9228 0.8984<br />
147 0.7338 0.7326 0.9216 0.8969<br />
148 0.7382 0.7370 0.9247 0.9007<br />
149 0.7429 0.7418 0.9246 0.9018<br />
150 0.7473 0.7465 0.9288 0.8984<br />
151 0.7521 0.7510 0.9283 0.9070<br />
152 0.7568 0.7571 0.9299 0.9031<br />
153 0.7612 0.7607 0.9307 0.9020<br />
154 0.7654 0.7649 0.9312 0.8913<br />
155 0.7700 0.7695 0.9365 0.9016<br />
156 0.7747 0.7739 0.9354 0.9045<br />
157 0.7786 0.7781 0.9349 0.9021<br />
158 0.7832 0.7822 0.9357 0.9049<br />
159 0.7873 0.7868 0.9404 0.9027<br />
160 0.7913 0.7911 0.9415 0.9157<br />
161 0.7956 0.7953 0.9391 0.9194<br />
162 0.8000 0.7992 0.9431 0.9201<br />
163 0.8038 0.8040 0.9431 0.9225<br />
164 0.8077 0.8077 0.9446 0.9274<br />
165 0.8119 0.8114 0.9447 0.9292<br />
166 0.8159 0.8157 0.9460 0.9268<br />
167 0.8200 0.8192 0.9454 0.9168<br />
168 0.8248 0.8242 0.9486 0.8530<br />
169 0.8332 0.8321 0.9489 0.8756<br />
170 0.8336 0.8312 0.9499 0.8611<br />
171 0.8360 0.8328 0.9502 0.8943<br />
172 0.8393 0.8348 0.9540 0.8480<br />
173 0.8429 0.8399 0.9526 0.8902<br />
174 0.8461 0.8440 0.9542 0.8993<br />
175 0.8498 0.8482 0.9557 0.9003<br />
176 0.8531 0.8514 0.9541 0.9073<br />
177 0.8564 0.8551 0.9568 0.9158<br />
178 0.8596 0.8588 0.9570 0.9254<br />
179 0.8622 0.8625 0.9595 0.9313<br />
398 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Table I.<br />
Continued.<br />
Table I.<br />
Continued.<br />
Input<br />
Gray<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Proposed<br />
HED<br />
Vector Color<br />
E.D.<br />
Input<br />
Gray<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Proposed<br />
HED<br />
Vector Color<br />
E.D.<br />
180 0.8649 0.8662 0.9587 0.9290<br />
181 0.8675 0.8699 0.9594 0.9261<br />
182 0.8702 0.8731 0.9623 0.9246<br />
183 0.8726 0.8764 0.9609 0.9273<br />
184 0.8754 0.8791 0.9630 0.9370<br />
185 0.8782 0.8827 0.9634 0.9316<br />
186 0.8800 0.8860 0.9643 0.9432<br />
187 0.8838 0.8888 0.9655 0.9429<br />
188 0.8847 0.8920 0.9658 0.9475<br />
189 0.8865 0.8974 0.9679 0.9477<br />
190 0.9059 0.9015 0.9689 0.9424<br />
191 0.9025 0.9034 0.9709 0.9466<br />
192 0.9006 0.9030 0.9708 0.9201<br />
193 0.9035 0.9051 0.9719 0.9375<br />
194 0.9043 0.9075 0.9718 0.9325<br />
195 0.9073 0.9099 0.9737 0.9457<br />
196 0.9105 0.9117 0.9727 0.9461<br />
197 0.9145 0.9159 0.9750 0.9470<br />
198 0.9132 0.9168 0.9738 0.9409<br />
199 0.9132 0.9193 0.9760 0.9496<br />
200 0.9166 0.9220 0.9765 0.9498<br />
201 0.9182 0.9245 0.9772 0.9595<br />
202 0.9202 0.9275 0.9769 0.9547<br />
203 0.9237 0.9303 0.9788 0.9594<br />
204 0.9263 0.9333 0.9785 0.9589<br />
205 0.9279 0.9352 0.9798 0.9637<br />
206 0.9309 0.9381 0.9799 0.9637<br />
207 0.9327 0.9404 0.9799 0.9620<br />
208 0.9359 0.9427 0.9809 0.9630<br />
209 0.9386 0.9458 0.9812 0.9664<br />
210 0.9425 0.9483 0.9822 0.9655<br />
211 0.9574 0.9569 0.9817 0.9628<br />
212 0.9460 0.9528 0.9824 0.9644<br />
213 0.9478 0.9536 0.9823 0.9612<br />
214 0.9486 0.9556 0.9836 0.9662<br />
215 0.9520 0.9583 0.9829 0.9665<br />
216 0.9533 0.9605 0.9841 0.9633<br />
217 0.9547 0.9616 0.9845 0.9728<br />
218 0.9561 0.9618 0.9853 0.9676<br />
219 0.9595 0.9632 0.9861 0.9703<br />
220 0.9627 0.9651 0.9851 0.9718<br />
221 0.9641 0.9664 0.9856 0.9729<br />
222 0.9658 0.9674 0.9855 0.9710<br />
223 0.9686 0.9671 0.9860 0.9737<br />
224 0.9698 0.9679 0.9865 0.9743<br />
225 0.9785 0.9738 0.9861 0.9764<br />
226 0.9732 0.9700 0.9863 0.9751<br />
227 0.9726 0.9705 0.9861 0.9775<br />
228 0.9754 0.9704 0.9863 0.9754<br />
229 0.9737 0.9732 0.9867 0.9783<br />
230 0.9739 0.9744 0.9866 0.9758<br />
231 0.9744 0.9765 0.9860 0.9760<br />
232 0.9777 0.9789 0.9863 0.9744<br />
233 0.9746 0.9784 0.9860 0.9713<br />
234 0.9749 0.9775 0.9862 0.9731<br />
235 0.9752 0.9742 0.9859 0.9682<br />
236 0.9756 0.9745 0.9850 0.9669<br />
237 0.9757 0.9757 0.9855 0.9696<br />
238 0.9772 0.9748 0.9841 0.9672<br />
239 0.9727 0.9718 0.9855 0.9622<br />
240 0.9714 0.9736 0.9836 0.9487<br />
241 0.9720 0.9710 0.9843 0.9528<br />
242 0.9689 0.9692 0.9838 0.9414<br />
243 0.9711 0.9733 0.9838 0.9394<br />
244 0.9630 0.9640 0.9834 0.9379<br />
245 0.9596 0.9622 0.9831 0.9370<br />
246 0.9553 0.9599 0.9836 0.9379<br />
247 0.9529 0.9560 0.9825 0.9432<br />
248 0.9512 0.9520 0.9838 0.9505<br />
249 0.9508 0.9517 0.9856 0.9578<br />
250 0.9555 0.9561 0.9870 0.9644<br />
251 0.9694 0.9957 0.9896 0.9794<br />
252 0.9964 0.9957 0.9923 0.9813<br />
253 0.9964 0.9957 0.9957 0.9918<br />
254 0.9964 0.9957 0.9957 0.9976<br />
255 0.9964 0.9956 0.9956 1.0000<br />
algorithm. The input image (128128 pixels) is the constant<br />
valued continuous tone image having 0 to 255 gray<br />
level, <strong>and</strong> its corresponding halftone image as shown in Figure<br />
11. The MSSIM <strong>and</strong> CISM take the value between 0 <strong>and</strong><br />
1. When there is no difference between reference image <strong>and</strong><br />
halftone image, the value is 1. The MSSIM was applied to<br />
assess the image quality, such as a jpeg image, noise added<br />
image, video source etc. 16,17 From Fig. 12, we can see that the<br />
MSSIM must be modified <strong>for</strong> assessing the halftone visibility.<br />
The MSSIM value is close to 0 throughout the gray level<br />
besides the high <strong>and</strong> low gray area. But in the result of<br />
CISM, the luminance distortion is mainly shown in the low<br />
gray area. The contrast distortion of the halftone image is<br />
mainly shown in the high gray area. Although the Gaussian<br />
weight (1111, =1.5) is applied to the image in the spatial<br />
domain, the MSSIM values do not fully comprise the<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 399
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
Figure 14. Results of the “elliptical ramp.”<br />
concept of the HVS. The color HVS filters contribute to<br />
detecting the disturbance of gradation <strong>and</strong> color correlation<br />
of the RGB channel.<br />
In Figure 13, we try to show the visibility characteristics<br />
throughout the gray level of various kinds of halftone methods:<br />
Floyd–Steinberg error diffusion (raster scan), Shiau–Fan<br />
error diffusion, vector color error diffusion, <strong>and</strong> proposed<br />
algorithm. In the cases of conventional error diffusion, the<br />
CISM values are lower than that of the proposed algorithm<br />
throughout the gray level of 0–255. Especially, the discontinuity<br />
of the gradation characteristic is shown in the case of<br />
the vector color error diffusion. From the results, the visibility<br />
characteristic of HED is outst<strong>and</strong>ing compared to that of<br />
the conventional halftone method. The numerical data of<br />
CISM is given in Table I.<br />
Results <strong>for</strong> Natural Images<br />
We compare the results of Floyd–Steinberg error diffusion<br />
(raster scan), Shiau–Fan error diffusion, vector color error<br />
diffusion, <strong>and</strong> the proposed algorithm with the source of the<br />
“elliptical ramp” image in Figure 14 <strong>and</strong> the “closed rose”<br />
image in Figure 15. The size of source images is 256256<br />
pixels. In Figs. 14(b)–14(d), false textures are prominent in<br />
the middle of elliptical ramp gradation. But as shown in Fig.<br />
Figure 15. Results of the “closed rose.”<br />
Table II. Numerical data <strong>for</strong> natural images: Color image similarity measure CISM.<br />
F/S E.D.<br />
Raster Scan<br />
Shiau–Fan<br />
E.D.<br />
Vector Color<br />
E.D.<br />
Proposed<br />
HED<br />
Elliptical ramp 0.69 0.69 0.87 0.90<br />
Closed rose 0.74 0.61 0.69 0.75<br />
14(e), there is no structural pattern caused by the error diffusion<br />
in the case of the proposed algorithm. In addition,<br />
the proposed algorithm does not suffer from the directional<br />
artifacts, such as diagonal worms, which appear in the elliptical<br />
ramp edge <strong>and</strong> highlight area. The gradation of color<br />
rendition is also better <strong>for</strong> the proposed algorithm. The<br />
white dots in Figs. 14(b) <strong>and</strong> 14(c) are replaced by the mixture<br />
of red, green, <strong>and</strong> blue, which is less visible. The numerical<br />
data of CISM are given in Table II.<br />
CONCLUSION<br />
In this paper, we proposed a new error diffusion algorithm.<br />
The proposed algorithm is very simple, easy to implement,<br />
<strong>and</strong> can reduce the structure artifacts while keeping the advantages<br />
of the error diffusion. We use the concept of perturbing<br />
error filter weight using the mask, which is selected<br />
400 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lee et al.: Digital color image halftone: Hybrid error diffusion using the mask perturbation <strong>and</strong> quality verification<br />
with a pseudor<strong>and</strong>om number. The results of the proposed<br />
method <strong>and</strong> conventional error diffusion were compared to<br />
the natural image. In addition, the proposed algorithm was<br />
evaluated with the objective assessment tools, halftone statistics,<br />
<strong>and</strong> CISM. We improved the gray expression using<br />
the mask <strong>and</strong> pseudor<strong>and</strong>om number. The color visibility<br />
was also improved by using the mixed error diffusion weight<br />
method. The proposed algorithm has good per<strong>for</strong>mance <strong>for</strong><br />
improving the gradation characteristics <strong>and</strong> reducing the<br />
structural pattern induced by conventional error diffusion.<br />
For future work, we will try to investigate the possibility of<br />
adaptation to the flat panel display.<br />
REFERENCES<br />
1 R. W. Floyd <strong>and</strong> L. Steinberg, “An adaptive algorithm <strong>for</strong> spatial gray<br />
scale”, Proc. SID 17, 75 (1976).<br />
2 J. F. Jarvis, C. N. Judice, <strong>and</strong> W. H. Ninke, “A survey of techniques <strong>for</strong><br />
the display of continuous tone pictures on bilevel displays”, Comput.<br />
Graph. Image Process. 5, 13 (1976).<br />
3 N. Damera-Venkata <strong>and</strong> B. L. Evans, “Adaptive threshold modulation<br />
<strong>for</strong> error diffusion halftoning”, IEEE Trans. Image Process. 10, 104<br />
(2001).<br />
4 P. Li <strong>and</strong> J. P. Allebach, “Tone dependent error diffusion”, IEEE Trans.<br />
Image Process. 13, 201 (2004).<br />
5 R. Eschbach, “Reduction of artifacts in error diffusion by means of input<br />
dependent weights”, J. Electron. <strong>Imaging</strong> 2, 352 (1993).<br />
6 V. Monga <strong>and</strong> B. L. Evans, “Tone dependent color error diffusion”, Proc.<br />
IEEE Int. Conf. on Acoustics, Speech, <strong>and</strong> Signal Proc. (IEEE, Piscataway,<br />
NJ, 2004) Vol. 3, p. 101.<br />
7 N. Damera-Venkata <strong>and</strong> B. L. Evans, “Design <strong>and</strong> analysis of vector<br />
color error diffusion halftoning systems”, IEEE Trans. Image Process. 10,<br />
1552 (2001).<br />
8 J. H. Lee, “Plasma display apparatus <strong>and</strong> image processing method<br />
thereof”, US Patent Application, 0,253,782A1 (2005).<br />
9 D. J. Lieberman <strong>and</strong> J. P. Allebach, “Model based direct binary search<br />
halftone optimization with a dual interpretation”, Proceedings of the<br />
IEEE Int. Conf. on Image Proc. (IEEE, Piscataway, NJ, 1998) Vol. 2, p. 44.<br />
10 J. N. Shiau <strong>and</strong> Z. Fan, “Set of easily implementable coefficients in error<br />
diffusion with reduced worm artifacts”, Proc. SPIE 2658, 222 (1996).<br />
11 D. L. Lau <strong>and</strong> G. R. Arce, Modern Digital Halftoning (Marcel Dekker,<br />
New York, 2001), pp. 27–45.<br />
12 R. Ulichney, Digital Halftoning (MIT Press, Cambridge, MA, 1987).<br />
13 J. Sullivan, L. Ray, <strong>and</strong> R. Miller, “Design of minimum visual<br />
modulation halftone patterns”, IEEE Trans. Syst. Man Cybern. 21, 33<br />
(1991).<br />
14 R. Nasanen, “Visibility of halftone dot textures”, IEEE Trans. Syst. Man<br />
Cybern. 14, 920 (1984).<br />
15 K. T. Mullen, “The contrast sensitivity of human color vision to<br />
red-green <strong>and</strong> blue-yellow chromatic gratings”, J. Physiol. (London) 359,<br />
381 (1985).<br />
16 Z. Wang, A. C. Bovik, <strong>and</strong> E. P. Simoncelli, “Structural Approaches to<br />
Image Quality Assessment”, in H<strong>and</strong>book of Image <strong>and</strong> Video Processing,<br />
2nd ed. (Academic Press, New York, 2005), Chapter 8.3.<br />
17 Z. Wang, A. C. Bovik, H. R. Sheikh, <strong>and</strong> E. P. Simoncelli, “Image quality<br />
assessment: From error visibility to structural similarity”, IEEE Trans.<br />
Image Process. 13, 600 (2004).<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 401
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 402–406, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Human Perception of Contour in Halftoned Density Step<br />
Image<br />
Phichit Kajondecha, Hongmei Cheng <strong>and</strong> Yasushi Hoshino<br />
Department of Systems Engineering, Nippon Institute of <strong>Technology</strong>, Miyashiro, Saitama, 354-8501, Japan<br />
E-mail: s3054602@sstu.nit.ac.jp<br />
Abstract. Contour is an important element of an image, usually<br />
expressed by the difference in density between two areas of an<br />
image. Objects are usually recognized from observation of their contours.<br />
In some cases, emergence of a false contour is a problem in<br />
image coding <strong>and</strong> decoding. Halftoning is an essential image process<br />
<strong>for</strong> digital printing of continuous tone. Contour perception is<br />
studied experimentally under various halftoning conditions, observation<br />
distances, <strong>and</strong> illumination conditions. The results show that in<br />
the human visual system, contour perception becomes easier with a<br />
decreasing stimulus of dots, which serve as the component of a<br />
halftone image. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong><br />
<strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5402<br />
INTRODUCTION<br />
In image processing, contours or boundaries in an image are<br />
important factors because our perception <strong>and</strong> recognition of<br />
an object depend on contour. In a sense, there<strong>for</strong>e, our eyes<br />
can be said to search <strong>for</strong> contour in an image. 1 Contour is<br />
the boundary between two areas of different density, color,<br />
or texture. If the dots are sufficiently small, the eye cannot<br />
detect the dot pattern 2 <strong>and</strong> we can recognize contour. In<br />
order to achieve high-quality image printing, the contour<br />
expression characteristics of an image are considered important.<br />
One way to underst<strong>and</strong> <strong>and</strong> evaluate image quality is<br />
use of the human visual system (HVS). 3–5<br />
Digital halftoning is essential <strong>for</strong> processing in digital<br />
printing <strong>and</strong> can be achieved by various methods. A conventional<br />
halftone is a fundamental method that has a wide<br />
application in printing. Digital halftoning has been evaluated<br />
from various viewpoints, but we have not yet gained a sufficient<br />
underst<strong>and</strong>ing of the relationship between the ease of<br />
perceiving contour, which is one of most fundamental elements<br />
of an image, <strong>and</strong> half-toning methods.<br />
In this study, images with contours of two different density<br />
values are processed to halftoned images of various halftone<br />
cycles <strong>and</strong> halftone angles of 0° <strong>and</strong> 45°. These<br />
halftoned images are observed under several conditions of<br />
illumination <strong>and</strong> observation distance. With respect to halftone<br />
frequency, images of relatively large cycles are prepared<br />
<strong>for</strong> accurately expressing the density of the image. If the<br />
observation is conducted at long distances, the condition of<br />
view angle from the eye is the same as that of a small cycle.<br />
The perception of contour is examined under various conditions,<br />
<strong>and</strong> the dependence of perception ratio on the difference<br />
in density is obtained. As the stimuli of the halftone<br />
cycle decrease relative to the density difference between regions,<br />
the contour is perceived more easily.<br />
EXPERIMENTAL<br />
As shown in Figure 1, two types of images were produced,<br />
each of which differed with respect to the direction of the<br />
contour line: (a) Images with perpendicular contour lines<br />
<strong>and</strong> (b) images with diagonal contour lines. The images were<br />
printed at a size of 10 cm10 cm with a resolution of<br />
300 dpi. The image printing conditions are shown in Table<br />
I, <strong>and</strong> as can be seen in the table, in some cases the density<br />
level of area A was fixed at 192 while the density level of area<br />
B was varied between 191 <strong>and</strong> 180. In other cases, the density<br />
level of area A was fixed at 64 while that of area B was<br />
varied between 63 <strong>and</strong> 52.<br />
Figure 2 illustrates the halftone dot arrangement, screen<br />
dot shape, <strong>and</strong> screen angle. The screen dot shape of the<br />
halftone is circular <strong>and</strong> produced at two screen angles (0°<br />
<strong>and</strong> 45°). As shown in Figure 3, the cycle of halftone dots<br />
was prepared in four levels: 1mm, 2mm, 3mm, <strong>and</strong><br />
4mm.<br />
The observation experiments are carried out as illustrated<br />
in Figure 4. The halftoned images were presented under<br />
three illumination conditions at four different observation<br />
distances. The perception ratios are obtained as the<br />
ratio of testees who perceived contour (responded “yes”)<br />
verses total testees. Testees who could not perceive contour<br />
responded “no.”<br />
RESULTS AND DISCUSSIONS<br />
The perception ratios obtained in the experimented conditions<br />
are plotted against the density difference between areas<br />
Received Feb. 2, 2007; accepted <strong>for</strong> publication May 7, 2007.<br />
1062-3701/2007/515/402/5/$20.00.<br />
Figure 1. Example density difference patterns: a perpendicular contour<br />
pattern <strong>and</strong> b diagonal contour pattern.<br />
402
Kajondecha, Cheng, <strong>and</strong> Hoshino: Human perception of contour in halftoned density step image<br />
Table I. Preparation of observation samples.<br />
Density<br />
Difference<br />
Density Values of Areas A <strong>and</strong> B<br />
A-B<br />
A-B<br />
1 64-63 192-191<br />
2 64-62 192-190<br />
3 64-61 192-189<br />
4 64-60 192-188<br />
5 64-59 192-187<br />
6 64-58 192-186<br />
7 64-57 192-185<br />
8 64-56 192-184<br />
9 64-55 192-183<br />
10 64-54 192-182<br />
11 64-53 192-181<br />
12 64-52 192-180<br />
Figure 3. Patterns of dot cycle at screen angle 45° at density level 189-<br />
192: a 1 mm cycle, b 2 mm cycle, c 3 mm cycle, <strong>and</strong> d 4mm<br />
cycle.<br />
Figure 2. Explanation of halftone dot arrangement: a angle of halftone<br />
dots <strong>and</strong> b cycle of halftone dots.<br />
A <strong>and</strong> B. The results show that the ratio increases with the<br />
density difference. The ratios are obtained while four factors<br />
are varied: cycle of halftone dot, screen angle, observation<br />
distance, <strong>and</strong> illumination. We discuss three points: effect of<br />
halftone dot cycle, effect of halftone screen angle, <strong>and</strong> effect<br />
of illumination.<br />
Effect of halftone Dot Cycle<br />
Figure 5 shows the perception ratio dependence on density<br />
difference of four distances <strong>and</strong> four cycle conditions when<br />
the screen angle is 45°. From Fig. 5, <strong>for</strong> all observation distances,<br />
perception ratios generally increase with a decreasing<br />
dot cycle. Concerning observation distance, when the dot<br />
cycle is 4mm, the contour is very difficult to perceive at<br />
0.5 m, but the contour becomes easier to perceive at 3m.At<br />
every observation distance, the difference in perception ratio<br />
among the dot cycles of 1–4 mm decreases with an increase<br />
in observation distance.<br />
Similarly, at every observation distance, the increase in<br />
the perception ratio is considered from the viewpoint of the<br />
human visual system (HVS). As shown in Figure 6, within<br />
the employed experimental range, the sensitivity of the HVS<br />
decreases as the dot cycle increases. As the stimulus of halftone<br />
dot decreases, the perception ratio increases. In the<br />
observers’ opinions, there are two cases: perception of only<br />
Figure 4. Schematics of experiment.<br />
dots in the halftone image <strong>and</strong> perception of contour. As a<br />
whole, when the stimulus of the dots is strong, the observer<br />
has difficulty in perceiving the contour. In addition, the decrease<br />
in the differences between the dot cycle at certain<br />
observation distances derives from the decrease in dot sensitivity<br />
in the HVS.<br />
The results above are consistent with our subjective estimation.<br />
Figure 5 shows that the perception ratio increases<br />
as observation distance increases. This is considered to be<br />
due to the decrease in the stimulus of dots as observation<br />
distance increases. 1<br />
Effect of halftone Screen Angle<br />
Figure 7 shows the perception ratio dependence on density<br />
difference at observation distances of 0.5 m <strong>and</strong> 1.0 m with<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 403
Kajondecha, Cheng, <strong>and</strong> Hoshino: Human perception of contour in halftoned density step image<br />
Figure 5. Perception ratio of average patterns diagonal <strong>and</strong> perpendicular pattern vs density differences<br />
192-180 at four distances <strong>and</strong> a dot cycle of 1–4 mm when the halftone angle is 45°. The plates show the<br />
conditions where the observation distance is a 0.5 m, b 1m,c 2m,<strong>and</strong>d 3m.<br />
halftone frequencies of 1mm, 2mm, 3mm, <strong>and</strong> 4mm.In<br />
a digital system, the halftone screen angle is simulated by<br />
placement of the dots within the halftone cells. Figure 7(a)<br />
shows a comparison between a screen angle of 0° <strong>and</strong> 45° at<br />
an observation distance of 0.5 m relativetoaveragevalueof<br />
diagonal <strong>and</strong> perpendicular pattern under an illumination of<br />
1.010 3 Lux. The result shows that at a halftone cycle<br />
1mm, the halftone image with the screen angle 45° can be<br />
detected more easily than the halftone image with the screen<br />
angle 0°.<br />
According to the results shown in Fig. 7, the perception<br />
ratio of the halftone image with a screen angle of 45° is<br />
higher than that with a screen angle of 0° at a halftone cycle<br />
of 1mm. The sensitivity of our human visual system is reported<br />
to decrease at angles of 45° <strong>and</strong> 135°. 6 There<strong>for</strong>e, the<br />
stimuli of the halftone dot when the halftone angle is 45°<br />
decreases, <strong>and</strong> we perceive the contour more easily when the<br />
angle is 0°. However,atcyclesof2mm, 3mm, <strong>and</strong> 4mm,<br />
the ratios do not show as clear a difference as when the dot<br />
cycle is 1mm, possibly because the dot stimulus is too<br />
strong. There<strong>for</strong>e, in some cases, the opposite relationship<br />
can arise <strong>for</strong> all of the other cycles (2 mm, 3mm, <strong>and</strong><br />
4mm).<br />
Effect of Illumination<br />
One factor that affects contour perception of a halftoned<br />
image is illumination. Figure 8 shows the perception ratio<br />
dependence on density difference at a halftone angle of 45°.<br />
The results show a comparison of when the illumination<br />
conditions are 1.010, 1.010 2 , <strong>and</strong> 1.010 3 Lux <strong>and</strong> observation<br />
distance is 1m<strong>and</strong> 3m, under halftone cycles of<br />
1mm<strong>and</strong> 4mm, respectively. Figure 8(c) shows that the<br />
perception ratio increases with illumination when the dot<br />
cycle is 1mmat an observation distance of 3m. Figures<br />
8(b) <strong>and</strong> 8(d) show that the perception ratio decreases when<br />
the dot cycle is 4mmat observation distances 1m<strong>and</strong> 3m<br />
with increasing illumination. In summary, the perception<br />
404 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Kajondecha, Cheng, <strong>and</strong> Hoshino: Human perception of contour in halftoned density step image<br />
Figure 6. Relationship between discrete Fourier trans<strong>for</strong>m of halftone frequency <strong>and</strong> modulation transfer function<br />
of HVS: a halftone frequency 1 dot/mm <strong>and</strong> b halftone frequency 0.33 dot/mm.<br />
Figure 7. Perception ratio of average pattern diagonal <strong>and</strong> perpendicular dependence on density differences.<br />
Density values ranged between 192 <strong>and</strong> 180, the observation distance was a 0.5 m <strong>and</strong> b 1.0 m,<br />
<strong>and</strong> illumination was 1.010 3 Lux.<br />
ratio increases with decreasing illumination at a dot cycle<br />
4mm, whereas the perception ratio increases when illumination<br />
increases at a dot cycle of 1mmat an observation<br />
distance of 3m.<br />
In the perception of contour, competition arises between<br />
Stevens effect <strong>and</strong> the decrease in sensitivity in a highfrequency<br />
area under low illumination. When the dot cycle is<br />
1mm, the Stevens effect becomes dominant, <strong>and</strong> the perception<br />
ratio increases with illumination. However, when the<br />
dot cycle 4mm, the perception ratio increases with a decrease<br />
in illumination. The reason <strong>for</strong> the increasing in the<br />
perceptionratioatadotcycleof4mm under decreased<br />
illumination is considered to be that the MTF of the HVS<br />
shifts to the low frequency range, resulting in a decrease in<br />
the sensitivity of the halftone dot. 7<br />
CONCLUSIONS<br />
Contour perception was investigated under various halftone<br />
<strong>and</strong> observation conditions. Perception ratio was observed<br />
to increase with density difference. At large halftone dot<br />
cycles of 3mm, the ease of contour perception increases as<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 405
Kajondecha, Cheng, <strong>and</strong> Hoshino: Human perception of contour in halftoned density step image<br />
Figure 8. Perception ratio dependence on density difference between 64-52 <strong>and</strong> an angle of 45°, with a<br />
cycle condition <strong>and</strong> observation distance of a 1mm<strong>and</strong>1m,b 4mm<strong>and</strong>1m,c 1mm<strong>and</strong>3m,<strong>and</strong><br />
d 4mm<strong>and</strong>3m.<br />
observation distance increases. This is due to the decrease in<br />
dot stimulus relative to the contour signal in HVS. The perception<br />
ratio of halftone images with screen angles of 45°<br />
was found to be higher than that observed when the screen<br />
angle is 0° with a cycle of 1mmat observation distances of<br />
0.5 m <strong>and</strong> 1m. For illumination, contour perception was<br />
found to improve when illumination was decreased at the<br />
large halftone dot cycle of 3mm.<br />
ACKNOWLEDGMENTS<br />
The authors would like to thank Shigeru Kitakubo of the<br />
Nippon Institute of <strong>Technology</strong>, Japan, Kazuhisa Yanaka of<br />
Kanagawa Institute of <strong>Technology</strong>, Japan, <strong>and</strong> the members<br />
of the Hoshino Laboratory <strong>for</strong> their kind contributions <strong>and</strong><br />
ef<strong>for</strong>ts, support, <strong>and</strong> valuable comments.<br />
REFERENCES<br />
1 P. Kajondecha, H. Cheng, S. Huang, <strong>and</strong> Y. Hoshino, “Contour<br />
perception of halftoned density step image”, Proc. IS&T’s NIP 22 (IS&T,<br />
Springfield, VA, 2006) pp. 351–355.<br />
2 H. R. Kang, Digital Color Half-toning (SPIE, Bellingham, WA, 1999),<br />
p. 61.<br />
3 J. Dusek <strong>and</strong> K. Roubik, “Testing of new models of the human visual<br />
system <strong>for</strong> image quality evaluation”, Proc. IEEE International<br />
Symposium on Signal Processing <strong>and</strong> its Applications, Vol. 2 (IEEE, Los<br />
Alamitos, CA, 2003) pp. 621–622.<br />
4 S. Kitakubo <strong>and</strong> Y. Hoshino, “Some characteristics on human visual<br />
sensitivity or spatial frequency of digital halftone images”, Proc. IS&T’s<br />
NIP 21 (IS&T, Springfield, VA, 2005) pp. 118–121.<br />
5 M. Ikeda, Image Processing in Human Vision (Heibonsha, Japan, 1999),<br />
p. 137.<br />
6 D. W. Heeley <strong>and</strong> B. Timmey, “Meridional anisotropies of orientation<br />
discrimination <strong>for</strong> sine wave gratings”, Vision Res. 28, 337–344 (1988).<br />
7 F. L. Van Nes <strong>and</strong> M. A. Bouman, “Spatial modulation transfer in the<br />
human eye”, J. Opt. Soc. Am. 57, 401–406 (1967).<br />
406 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 407–412, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Advanced Color Toner <strong>for</strong> Fine Image Quality 1<br />
Akihiro Eida, Shinichiro Omatsu <strong>and</strong> Jun Shimizu<br />
Per<strong>for</strong>mance Chemicals Research Laboratories, Kao Corporation, Wakayama, 640-8580 Japan<br />
E-mail: eida.akihiro@kao.co.jp<br />
Abstract. Application of full color printers has spread rapidly in the<br />
office environment, <strong>and</strong> dem<strong>and</strong>s on the full color printer have been<br />
also increasing; especially, high-speed printing, oilless fusing <strong>and</strong><br />
fine image quality are strongly requested. Recently, several types of<br />
chemical toner have been launched. Because they have small <strong>and</strong><br />
narrow particle size distributions, they have an advantage in fine<br />
image quality. However, the resin of chemical toners are usually<br />
styrene-acrylic, which is not suitable <strong>for</strong> high-speed printing <strong>and</strong><br />
glossy images <strong>for</strong> pictorial applications because the styrene-acrylic<br />
has to be high molecular weight to make toner have sufficient durability.<br />
On the other h<strong>and</strong>, polyester has better properties <strong>for</strong> highspeed<br />
printing than styrene-acrylic because of the existence of a<br />
polar group that enhances a compatibility with the cellulose of paper.<br />
Polyester has good durability even if it has a low molecular<br />
weight <strong>and</strong> thus can provide a glossy image. Thus polyester has<br />
several advantages over styrene-acrylic. In general, it is difficult to<br />
use polyester as the binder resin of chemical toner. On the other<br />
h<strong>and</strong>, it is easy to make polyester color toner by the pulverization<br />
method, but it is difficult to make pulverized toner having small <strong>and</strong><br />
narrow particle size distribution <strong>for</strong> fine image quality, <strong>and</strong> it is also<br />
difficult to make pulverized toner having both oilless fusing ability<br />
<strong>and</strong> sufficient durability. In this article, design of oilless fusable, pulverized<br />
color toner with small <strong>and</strong> narrow particle size distribution<br />
was investigated. This advanced color toner named “MC toner” can<br />
provide high-speed printing, oilless fusing, <strong>and</strong> also fine image<br />
quality. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5407<br />
INTRODUCTION<br />
In recent years, as color printers have become widely used,<br />
dem<strong>and</strong>s on the color printer have also been increasing: <strong>for</strong><br />
instance, high image quality, high speed, <strong>and</strong> low cost per<strong>for</strong>mances<br />
are required <strong>for</strong> the toner. These properties imply,<br />
<strong>for</strong> example, small particle size, narrow particle size distribution,<br />
good transfer property, low temperature fusing, etc.<br />
To achieve these requirements, several types of chemical<br />
toner were developed <strong>and</strong> launched. It is said that per<strong>for</strong>mance<br />
of chemical toner is better than pulverized toner with<br />
respect to various points of per<strong>for</strong>mance. 1,2<br />
Dem<strong>and</strong>s on color printing <strong>and</strong> requirements <strong>for</strong> color<br />
toner in order to achieve these dem<strong>and</strong>s are listed below.<br />
Comparison of typical chemical toner <strong>and</strong> pulverized toner<br />
<strong>and</strong> a comparison of styrene-acrylic <strong>and</strong> polyester resins<br />
with respect to these requirements are also discussed.<br />
Fine Image Quality<br />
For fine image quality, faithful dot reproduction of latent<br />
image, faithful transfer property, glossy image <strong>for</strong> pictorial<br />
image, wide color gamut, are required.<br />
Faithful Dot Reproduction<br />
Figure 1 shows a dependency of raggedness of line defined as<br />
TEP on toner particle size. This indicates that the raggedness<br />
increases in proportion to toner particle size. Thus, <strong>for</strong> faithful<br />
dot reproduction, toner is required to be a small size.<br />
From this point of view, chemical toner has an advantage<br />
because chemical toners are made by a buildup method.<br />
Figure 2 shows toner size distribution of conventional pulverized<br />
toner <strong>and</strong> typical chemical toner. The toner size distribution<br />
of the chemical toner is smaller <strong>and</strong> narrower.<br />
Faithful Transfer<br />
In general, it is said that a spherical toner shows a good<br />
transfer efficiency. 3,4 Indeed, Figure 3 shows transfer efficiency<br />
of chemical toner having a spherical shape<br />
circularity=0.98 is much better than irregular-shaped<br />
toner circularity=0.92, where, transfer efficiency is represented<br />
by E, which is the color difference between blank<br />
tape on paper <strong>and</strong> tape removing residual toner from the<br />
organic photoconductor (OPC). (For more detail, see Experimental<br />
section.) On the other h<strong>and</strong>, scatter of spherical<br />
toner is much worse than <strong>for</strong> irregularly shaped toner, which<br />
indicates that spherical toner tends to roll in the transfer<br />
process because of high flowability. Moreover, cleaning properties<br />
of spherical toner are also bad. Thus, it is not clear<br />
whether spherical shape is beneficial <strong>for</strong> fine image quality.<br />
Recently, some chemical toners have adopted moderately<br />
spherical shape (i.e., circularity is 0.96; <strong>for</strong> example, potato<br />
shape, spindle shape, or dimple shape). Transfer efficiency,<br />
1 Presented, in part at IS&T NIP21 Baltimore.<br />
Received Jan. 9, 2005; accepted <strong>for</strong> publication Jun. 10, 2007.<br />
1062-3701/2007/515/407/6/$20.00.<br />
Figure 1. Dependency of image quality TEP on toner particle size.<br />
407
Eida, Omatsu, <strong>and</strong> Shimizu: Advanced color toner <strong>for</strong> fine image quality<br />
Figure 4. Comparison of pigment dispersion of pulverized toner <strong>and</strong><br />
typical chemical toner.<br />
Figure 2. Comparison of particle size distribution <strong>and</strong> image quality of<br />
conventional pulverized toner <strong>and</strong> typical chemical toner.<br />
Figure 5. Offset b<strong>and</strong> of St/Ac toner, polyester toner, <strong>and</strong> crystalline<br />
polyester containing toner.<br />
Figure 3. Comparison of per<strong>for</strong>mance of spherical shape toner <strong>and</strong> irregular<br />
shape toner.<br />
scattering property, <strong>and</strong> cleaning property of toner with such<br />
a moderately spherical shape are well balanced.<br />
Glossy Image<br />
For pictorial image printing, a glossy image is preferable. To<br />
print an image with high gloss, it is necessary that a toner<br />
resin have a low molecular weight, because in the fusing<br />
process, viscosity of the resin having low molecular weight is<br />
low <strong>and</strong> the surface of fused image becomes smooth, <strong>and</strong> the<br />
image having such smooth surface specularly reflects light<br />
well <strong>and</strong> shows high gloss.<br />
From this point of view, polyester resin has an advantage<br />
over styrene-acrylic resin. Polyester resin has sufficient<br />
durability owing to hydrogen bonded polar groups, while<br />
styrene-acrylic resin has poor durability <strong>for</strong> the same low<br />
molecular weight distribution as polyester.<br />
Wide Color Gamut<br />
For wide color gamut, fine dispersion of the pigment is necessary.<br />
Figure 4 shows dispersion of the pigment of pulverized<br />
toner <strong>and</strong> chemical toner. In general, dispersion of a<br />
pulverized toner pigment is better than <strong>for</strong> a chemical toner,<br />
because the kneading process enhances dispersion of the<br />
pigment. In general polyester resin has better pigment<br />
dispersability than styrene-acrylic resin, because the polar<br />
groups of polyester resin enhance dispersion of the pigment.<br />
There<strong>for</strong>e, <strong>for</strong> a wide color gamut, pulverized toner <strong>and</strong><br />
polyester resin have advantages over chemical toner <strong>and</strong><br />
styrene-acrylic resin. 5<br />
High-Speed Printing<br />
For high-speed printing, low-temperature fusing ability is<br />
required <strong>for</strong> color toner. As described above, polyester resin<br />
has sufficient durability with low molecular weight. Moreover,<br />
polyester has polar groups that enhance compatibility<br />
with cellulose of paper. Thus, polyester resin has an advantage<br />
<strong>for</strong> high-speed printing over styrene-acrylic resin. Recently<br />
a crystalline polyester resin that has ideal viscoelasticity<br />
<strong>for</strong> low-temperature fusing has been proposed (Figure 5)<br />
<strong>for</strong> higher speed printing. 6–8<br />
Oilless Fusability <strong>for</strong> Low Cost Machine<br />
For low machine cost, oilless fusing ability is required <strong>for</strong><br />
color toner, so that the oil applying unit can be removed<br />
408 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Eida, Omatsu, <strong>and</strong> Shimizu: Advanced color toner <strong>for</strong> fine image quality<br />
Resin<br />
Table I. Properties of the experimental polyester resin.<br />
Acid Value a<br />
mg KOH/g<br />
T1/2 b<br />
°C<br />
T g<br />
c<br />
°C<br />
Polyester 20 115 63<br />
a The acid value was measured according to ASTM D1980-67.<br />
b The softening point T1/2 was measured according to ASTM E-28-67.<br />
c The glass transition temperature T g was measured by a differentital scanning calorimeter “DSC<br />
Model 200” manufactured by Seiko Instruments Inc., at a heating rate of 10°C/min.<br />
Toner<br />
Wax<br />
%<br />
Table II. Toner samples.<br />
Kneading<br />
Machine<br />
Preconditioning<br />
Target Size<br />
µm<br />
A 7.0 Twin-screw None 8<br />
B 7.0 Open-roll None 8<br />
C 6.0 Open-roll None 5<br />
D 6.0 Open-roll Silica=1.0% 5<br />
E 6.0 Twin-screw Silica=1.0% 5<br />
• to contain plenty of wax with high durability, <strong>for</strong> oil-less<br />
fusing<br />
In this article, the design of an oilless fusable polyester pulverized<br />
color toner with small <strong>and</strong> narrow particle size distribution<br />
is presented. Factors that enable toner to be pulverized<br />
with a small <strong>and</strong> narrow particle size distribution<br />
were investigated in detail.<br />
Figure 6. Open-roll-type kneader.<br />
from the printer system. Generally, a resin <strong>for</strong> color toner<br />
has a low <strong>and</strong> narrow molecular weight distribution <strong>for</strong><br />
glossy image <strong>and</strong> high transparency, but such a resin cannot<br />
give a wide nonoffset range in fusing because elasticity of<br />
such resin fusing conditions is low <strong>and</strong> offset problems tend<br />
to occur. There<strong>for</strong>e, to prevent offset problems, under a conventional<br />
oil applying unit may be attached to the fuser unit,<br />
which is costly. Thus, oilless fusable toner is described <strong>for</strong><br />
design of a low cost machine. To achieve oilless fusing, it is<br />
necessary to add wax to the toner. In the fusing process, the<br />
wax melts <strong>and</strong> diffuses out from inside the toner <strong>and</strong> works<br />
as a releasing agent instead of oil applied to the fusing roller.<br />
However, it is difficult to disperse sufficient wax by a twinscrew<br />
extruder, such as is conventionally used in the kneading<br />
process. Also, thereby the dispersed size of wax domains<br />
in the toner becomes very large, <strong>and</strong> such poor dispersion of<br />
wax causes poor durability of the toner. 9,10 On the other<br />
h<strong>and</strong>, chemical toner can include plenty of wax, which does<br />
not exist on the toner surface; thus, chemical toner is oilless<br />
fusable without a durability problem.<br />
As described above, both chemical toner <strong>and</strong> pulverized<br />
toner have good points <strong>and</strong> bad points. But polyester resin<br />
has a certain advantage over styrene-acrylic resin. In general,<br />
it is difficult to use polyester resin in chemical toner. On the<br />
other h<strong>and</strong>, it is easy to make polyester color toner by the<br />
pulverization method, but it is difficult to make pulverized<br />
toner with small <strong>and</strong> narrow particle size distribution <strong>for</strong><br />
fine image quality. To summarize, the ideal toner is expected<br />
as follows:<br />
• to use polyester <strong>for</strong> glossy image <strong>and</strong> high-speed<br />
printing<br />
• to be small <strong>and</strong> narrow size distribution, <strong>for</strong> fine image<br />
quality<br />
EXPERIMENTAL<br />
Preparation of Polyester Resin<br />
Bisphenol-A propylene oxide adducts, ethylene oxide adducts,<br />
terephthalic acid, C12-succinic anhydride, <strong>and</strong> trimellitic<br />
anhydride were allowed to react <strong>for</strong> condensation polymerization<br />
at 230°C with a small amount of the catalyst in<br />
a glass flask, which was equipped with a thermometer, a<br />
stainless steel stirring rod, a reflux condenser, <strong>and</strong> nitrogen<br />
inlet tube. Physical properties of this polyester resin are<br />
listed in Table I.<br />
Preparation of Toner Samples: For Investigation<br />
of a Factor of Oilless Fusable Toner<br />
Toner samples A <strong>and</strong> B comprised the resin described above,<br />
wax, charge control agent, <strong>and</strong> colorant. The colorant was<br />
Quinacridone (Pigment Red 122); the melting point of the<br />
wax was 80°C. Content of the colorant was 6.5 wt. %; content<br />
of the wax was 7.0%, which is the amount that enables<br />
oilless fusability. The materials were premixed in a batch<br />
mixer, <strong>and</strong> then they were kneaded. We used two types of<br />
kneader, a conventional twin-screw extruder <strong>for</strong> toner A <strong>and</strong><br />
an open-roll-type kneader (Figure 6) <strong>for</strong> toner B. The toners<br />
were pulverized <strong>and</strong> classified to 8 m. Toner samples are<br />
listed Table II.<br />
Preparation of Toner Samples: For Investigation<br />
of a Factor of Efficient Pulverizing<br />
Toner samples C, D, <strong>and</strong> E also comprised the resin described<br />
above, wax, charge control agent, <strong>and</strong> colorant. The<br />
colorant was Quinacridone (Pigment Red 122); the melting<br />
point of the wax was 80°C. Content of the colorant was<br />
6.5 wt. %, <strong>and</strong> content of the wax was 6.0%, which is the<br />
amount that enables oilless fusability.<br />
The materials were premixed in a batch mixer, <strong>and</strong> then<br />
they were kneaded. Toners C <strong>and</strong> D were kneaded by the<br />
open-roll-type kneader, <strong>and</strong> toner E was kneaded by the<br />
twin-screw extruder. Be<strong>for</strong>e pulverizing, toner chips <strong>for</strong> tonners<br />
D <strong>and</strong> E were mixed with 1.0 wt. % of hydrophobic<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 409
Eida, Omatsu, <strong>and</strong> Shimizu: Advanced color toner <strong>for</strong> fine image quality<br />
Figure 7. Comparison of wax dispersion <strong>and</strong> durability.<br />
silica; we call this process “preconditioning.” Then, they<br />
were pulverized <strong>and</strong> classified. The target size was 5 m.<br />
Toner samples are listed Table II.<br />
MEASUREMENTS<br />
Durability was tested by using a toner cartridge of a color<br />
laser printer <strong>for</strong> which the developing system was a single<br />
component. Toner was put into the cartridge, <strong>and</strong> the developer<br />
roll was rotated at 60 rpm without developing the<br />
toner to the organic photoconductor. Durability was defined<br />
as time when filming of the toner to the doctor blade occurred<br />
<strong>and</strong> a streak appeared in the toner layer on the developer<br />
roller.<br />
The particle size distribution was measured by a Coulter<br />
Multisizer II with a 100 m size aperture. The size of dispersed<br />
wax domains was observed by TEM. For convenience<br />
of evaluation, a cross section of the toner chip be<strong>for</strong>e pulverizing<br />
was observed.<br />
Toner was developed in a nonmagnetic singlecomponent<br />
printer whose resolution was 600 dpi. A dot image<br />
on the paper be<strong>for</strong>e fusing was observed under the microscope.<br />
Dispersion state of silica <strong>and</strong> free silica rate were<br />
measured by Particle Analyzer FT-1000 (Horiba). Circularity<br />
was measured by FPIA 2100 (Sysmex).<br />
Transfer efficiency was defined as the amount of residual<br />
toner on the photoconductor after the transfer process.<br />
Areal density on the photoconductor was controlled at<br />
0.40–0.45 mg/cm 2 . The residual toner was removed by<br />
clear tape <strong>and</strong> placed on white paper. The amount of the<br />
residual toner is defined as the difference of hue E between<br />
the residual toner sample <strong>and</strong> the blank tape on the<br />
same paper.<br />
RESULTS AND DISCUSSION<br />
Wax Dispersion <strong>and</strong> Durability<br />
Figure 7 shows wax dispersion in toners A <strong>and</strong> B. The size of<br />
the dispersed wax domains in toner A that was kneaded by<br />
conventional twin-screw extruder is very large. This indicates<br />
the twin-screw extruder cannot disperse wax finely<br />
enough. On the other h<strong>and</strong>, the dispersed size of wax of<br />
Figure 8. Result of pulverizing of toners C, D, <strong>and</strong> E.<br />
toner B was very small. Toner B was kneaded by an openroll-type<br />
kneader, <strong>and</strong> this type of kneader can knead toner<br />
at a lower kneading temperature, which means toner is<br />
kneaded more strongly at a higher resin viscosity compared<br />
to the twin-screw extruder.<br />
Durability of toner B is over six times that of toner A,<br />
<strong>and</strong> is enough <strong>for</strong> practical use. This result indicates that<br />
large domains of wax decrease durability of toner because, it<br />
is thought, that such a large wax domain tends to (i) exist on<br />
the toner surface after pulverizing <strong>and</strong> (ii) stick to the doctor<br />
blade <strong>and</strong> developer roller. To avoid this problem, it is necessary<br />
to disperse wax finely in the toner; thus, toner<br />
kneaded by the open-roll-type kneader can contain plenty of<br />
wax with finely dispersed size <strong>and</strong> can yield both efficient<br />
fusing ability <strong>and</strong> durability <strong>for</strong> oilless fusing.<br />
Result of Pulverizing of Toners C, D, <strong>and</strong> E<br />
Figure 8 shows the result of pulverizing. Toner D was pulverized<br />
to 5 m size with narrow distribution successfully.<br />
Toner C, which was not preconditioned, was not pulverized<br />
to a small size, <strong>and</strong> it contains large-size particles. Toner E,<br />
which was preconditioned but kneaded by the twin-screw<br />
extruder, also was not pulverized to 5 m size. Dot image<br />
quality with toner D is as good as with typical chemical<br />
toner shown in Fig. 2. On the other h<strong>and</strong>, the dot images<br />
with toners C <strong>and</strong> E are worse because of toner scatter.<br />
These results indicate that small <strong>and</strong> narrow size distribution<br />
is necessary <strong>for</strong> fine image quality. The reason why toners C<br />
<strong>and</strong> E were not pulverized successfully is discussed below.<br />
Difference between Toners C <strong>and</strong> D<br />
In general, as toner particle size in the pulverizer becomes<br />
smaller, the adhesion <strong>for</strong>ce between particles increases; small<br />
toner particles tend to agglomerate. Such agglomerated particles<br />
cannot be given pulverizing energy efficiently, <strong>and</strong> it is<br />
difficult to pulverize toner to small size, as was observed <strong>for</strong><br />
toner C. But in the case of toner D, silica is present in the<br />
410 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Eida, Omatsu, <strong>and</strong> Shimizu: Advanced color toner <strong>for</strong> fine image quality<br />
Figure 9. Effect of the preconditioning silica.<br />
Figure 11. Difficulty of pulverizing in case of large domain of wax.<br />
Figure 10. Wax dispersion in toners D <strong>and</strong> E.<br />
pulverizer, <strong>and</strong> the silica decreases the cohesive <strong>for</strong>ce of the<br />
toner particles (Figure 9). Such dispersed particles can be<br />
pulverized efficiently, <strong>and</strong> the efficiency of classifying is<br />
also improved. Thus, the preconditioning method is suitable<br />
<strong>for</strong> pulverizing toner to a small <strong>and</strong> narrow size<br />
distribution.<br />
Difference between Toners D <strong>and</strong> E<br />
Figure 10 shows wax dispersion in toners D <strong>and</strong> E. Wax<br />
domain size in toner E is much bigger than in toner D<br />
because of the difference of kneader, as described above. It is<br />
thought that this difference of wax dispersion also affects<br />
pulverizing efficiency. Figure 11 illustrates the influence of<br />
wax domain size on pulverizing. When the wax domain is<br />
small, the effect of preconditioned silica is sufficient. On the<br />
other h<strong>and</strong>, in the case of large wax domains, such as toner<br />
E, the toner chip tends to be pulverized at the surface of a<br />
wax domain. Consequently, the large domains of wax exist at<br />
toner surface <strong>and</strong> cause decreasing flowability <strong>and</strong> sticking<br />
to the pulverizer, even if silica preconditioning is employed.<br />
Thus, such large domains of wax disrupt efficient pulverizing.<br />
These results indicate that both preconditioning <strong>and</strong><br />
fine wax dispersion are necessary to pulverize toner to small<br />
size with narrow distribution.<br />
Effect of Silica Preconditioning<br />
Toner pulverized by the preconditioning method already has<br />
silica added during the pulverizing process. Thus, the preconditioning<br />
method is a technique of both pulverizing <strong>and</strong><br />
surface treatment. The properties of silica added by the preconditioning<br />
method are analyzed by a particle analyzer<br />
Figure 12. Comparison of silica dispersion of toner surface.<br />
(Horiba). Figure 12 shows a comparison of the state of silica<br />
on the toner surface. Silica dispersion of toner added by the<br />
preconditioning method is better than conventional silica<br />
dispersion by addition in the mixer, <strong>and</strong> free silica rate is<br />
less. It is supposed that silica added in the pulverizing process<br />
is strongly bound to the particles, <strong>and</strong> extra free silica<br />
agglomeration is avoided or eliminated on classification. In<br />
general, free silica agglomeration is a cause of filming; thus,<br />
toner made by the preconditioning method has good<br />
durability.<br />
In summary, toner pulverized by the preconditioning<br />
method has a small <strong>and</strong> narrow size distribution that is suitable<br />
<strong>for</strong> fine image quality <strong>and</strong> also has good silica dispersion<br />
on the surface <strong>for</strong> uni<strong>for</strong>m charging ability <strong>and</strong> good<br />
durability. Accordingly, we named the preconditioning<br />
method “MechanoChemical” technology, <strong>and</strong> we call toner<br />
made by mechanochemical technology, “MC toner.”<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 411
Eida, Omatsu, <strong>and</strong> Shimizu: Advanced color toner <strong>for</strong> fine image quality<br />
(Figure 14). Transfer efficiency E of this potatolike shape<br />
toner 4.5 m was 5.2.<br />
CONCLUSIONS<br />
The present investigation leads to the following conclusions:<br />
Figure 13. Dependency of circularity on toner particle size.<br />
Figure 14. SEM photograph of small-size toner made by preconditioning<br />
method.<br />
Tendency of Toner Shape on Particle Size<br />
Figure 13 shows circularity of toner pulverized by the preconditioning<br />
method. Circularity increases as toner particles<br />
become small. This indicates that small particle size toner<br />
tends to be rounded by frequent collision in the pulverizer<br />
1. To achieve both oilless fusability <strong>and</strong> high durability,<br />
fine wax dispersion is necessary. An open-roll-type<br />
kneader is suitable <strong>for</strong> kneading the requisite<br />
amount of wax because low kneading temperature is<br />
possible.<br />
2. To pulverize toner to small size with narrow distribution,<br />
both preconditioning <strong>and</strong> fine wax dispersion<br />
are necessary.<br />
3. Distribution of silica added to toner surface by preconditioning<br />
is uni<strong>for</strong>m, <strong>and</strong> the resulting toner has<br />
less free-silica agglomerations.<br />
4. Shape of toner pulverized to small size become<br />
rounded because of frequent collisions in the<br />
pulverizer.<br />
REFERENCES<br />
1 Y. Matsumura, P. Gurns, <strong>and</strong> T. Fuchiwaki, Proc. IS&T’s NIP17 (IS&T,<br />
Springfield, VA, 2001) p. 341.<br />
2 M. Yamazaki, M. Uchiyama, <strong>and</strong> K. Tanigawa, Proc. of Japan Hardcopy<br />
2000 Fall Meeting (<strong>Imaging</strong> Soc. Japan, Tokyo, 2000) p. 1.<br />
3 C. Suzuki, M. Takagi, S. Inoue, T. Ishiyama, H. Ishida, <strong>and</strong> T. Aoki, Proc.<br />
IS&T’s NIP19 (IS&T, Springfield, VA, 2003) p. 134.<br />
4 T. Aoki, Proc. IS&T’s NIP19 (IS&T, Springfield, VA, 2003) p. 2.<br />
5 A. Eida, S. Omatsu, <strong>and</strong> J. Shimizu, Proc. IS&T’s NIP20 (IS&T,<br />
Springfield, VA, 2004) p. 102.<br />
6 E. Shirai, K. Aoki, <strong>and</strong> M. Maruta, Proc. IS&T’s NIP18 (IS&T,<br />
Springfield, VA, 2002) p. 258.<br />
7 E. Shirai, K. Aoki, <strong>and</strong> M. Maruta, Proc. IS&T’s NIP19 (IS&T,<br />
Springfield, VA, 2003) p. 119.<br />
8 T. Kubo, E. Shirai, <strong>and</strong> K. Aoki, Proc. IS&T’s NIP20 (IS&T, Springfield,<br />
VA, 2004) p. 73.<br />
9 A. Eida <strong>and</strong> J. Shimizu, Proc. IS&T’s NIP16 (IS&T, Springfield, VA, 2000)<br />
p. 618.<br />
10 J. Shimizu, S. Omatsu, <strong>and</strong> Y. Hidaka, Proc. IS&T’s NIP19 (IS&T,<br />
Springfield, VA, 2003) p. 130.<br />
412 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 413–418, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Preparation of Microemulsion Based Disperse Dye Inks<br />
<strong>for</strong> Thermal Bubble Ink Jet Printing<br />
S. Y. Peggy Chang <strong>and</strong> Y. C. Chao<br />
Institute of Organic <strong>and</strong> Polymeric Materials, National Taipei University of <strong>Technology</strong>, No. 1, Sec. 3,<br />
Chung-Hsiao E Rd., Taipei, 106, Taiwan<br />
E-mail: ycchao@ntut.edu.tw<br />
Abstract. This study describes the preparation of microemulsion<br />
inks based on commercially available disperse dyes <strong>for</strong> thermal<br />
bubble ink jet printing. The approach to make the inks is by <strong>for</strong>mulating<br />
to <strong>for</strong>m water-soluble emulsions (o/w), which were then optimized<br />
to reach a dynamically stable isotropic condition. The dyes in<br />
three primary colors—cyan, magenta, <strong>and</strong> yellow—were systematically<br />
investigated. Different species <strong>and</strong> amounts of dispersants,<br />
emulsifiers, <strong>and</strong> dyes were selected to prepare microemulsions after<br />
intense stirring. The system, D50/963H/ Pannox140/glycerin/water,<br />
per<strong>for</strong>med excellently in particle size reduction <strong>and</strong> size stabilization.<br />
The surface tension, pH value, viscosity, <strong>and</strong> stability of the<br />
microemulsion inks meet the requirement of the applied printhead to<br />
provide both good printing quality <strong>and</strong> excellent printing consistency<br />
without clogging in the nozzle. The interaction between dyes <strong>and</strong> the<br />
microemulsion system can be comprehensively understood based<br />
on the results of particle size analysis, lightfastness, water fastness,<br />
<strong>and</strong> the characterization of printing quality. The low particle sizereducing<br />
efficiency can be attributed to the compatibility between<br />
dye <strong>and</strong> dispersant structures as well as particle aggregation. The<br />
lightfastness could be enhanced with smaller particle size but decreased<br />
with dye content. However, the water fastness of the pre<strong>for</strong>med<br />
sample was identically high without being affected by the<br />
dye particle size in the range between 80 nm <strong>and</strong> 96 nm. The printing<br />
per<strong>for</strong>mance was found to be closely correlated with the dye<br />
structure <strong>and</strong> ink concentration. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5413<br />
INTRODUCTION<br />
With superiority in cost <strong>and</strong> manufacturing technique, the<br />
thermal bubble ink jet technology shares a valuable ink jet<br />
printing market with the piezo ink jet. To <strong>for</strong>mulate water<br />
miscible inks with water insoluble colorants, the dyes or pigments<br />
must be processed via microparticle reduction <strong>and</strong><br />
intensive dispersing to <strong>for</strong>m stable jet inks. The present article<br />
focuses on applying microemulsion technology to render<br />
the ink of disperse dye base a homogeneous <strong>and</strong> thermally<br />
stable system <strong>for</strong> the thermal bubble printhead. As<br />
previously defined, 1 the microemulsion is <strong>for</strong>med by mixing<br />
oil phase, surfactant, cosurfactant, <strong>and</strong> water to provide a<br />
large oil-<strong>and</strong>-water interfacial area <strong>and</strong> low viscosity. Different<br />
surfactants <strong>and</strong> composition determine the microscopic<br />
structure of oil-in-water droplets, bicontinuous systems, <strong>and</strong><br />
<br />
Present address: 10F, No. 3, Linsen North Road, Taipei City, Taipei,<br />
Taiwan. E-mail: sychang168@gmail.com<br />
Received Jan. 16, 2007; accepted <strong>for</strong> publication Mar. 29, 2007.<br />
1062-3701/2007/515/413/6/$20.00.<br />
water-in-oil droplets. These small structures enable not only<br />
the reaction of oil-soluble phase <strong>and</strong> water-soluble phase but<br />
also the synthesis of ceramic pigments 2 as well as phase<br />
transfer catalysis. 3 Both categories of normal microemulsion<br />
<strong>and</strong> reverse microemulsion have been reported to have very<br />
low water solubility, <strong>and</strong> consequently, low solid content in<br />
the preparation of pigment containing inks, 4 although the<br />
nanoparticles were produced with narrow distribution <strong>and</strong><br />
no agglomeration. That is because the microemulsion phase<br />
trans<strong>for</strong>ms to another state when the water amount exceeds<br />
a certain level. An effective ink, comprising high solid content<br />
<strong>and</strong> merits of microemulsions, should be <strong>for</strong>mulated<br />
with a choice of a microemulsion or reverse microemulsion<br />
system, optimization of preparation conditions, <strong>and</strong> application<br />
of bicontinuous structure. However, most prior research<br />
work 5–10 focused only on the preparation <strong>and</strong> characteristics<br />
of well-dispersed nanoparticles by the reverse micoremulsion<br />
method, without experiments on practical application<br />
in the ink jet printer. Moreover, comparatively little in<strong>for</strong>mation<br />
has been published on how ink jet printability of dyestuffs<br />
correlate with dyestuff structure <strong>and</strong> molecular weight,<br />
ink composition, concentration, <strong>and</strong> solvent used. Only by<br />
systematically exploring inherent possibilities <strong>and</strong> limitations<br />
of the technique can strategies be envisioned <strong>for</strong> printing<br />
with a large number of different compounds in future. 11<br />
In order to prepare an isotropic <strong>and</strong> highly disperse dye<br />
containing ink jet ink, this work first optimized the dye dispersion<br />
<strong>for</strong>mulation to obtain well-dispersed <strong>and</strong> storage<br />
stable preink, followed by studying effects of dye structure<br />
on the microemulsion region in the quasi-ternary phase diagram.<br />
The particle site in extremely purified disperse dye<br />
presscake should be reduced, <strong>and</strong> the dye well suspended<br />
<strong>and</strong> free of impurities to be capably incorporated into a<br />
microemulsion. Then, conditions <strong>for</strong> the maximization of<br />
water-solubility were identified as the basis <strong>for</strong> the ink<br />
preparation. Precisely speaking, the microemulsion of disperse<br />
dyes in this work was prepared by <strong>for</strong>ming droplets of<br />
water, containing dye, suspended <strong>and</strong> stabilized in an appropriate<br />
mixture of cosurfactant <strong>and</strong> cosolvent. Finally, the<br />
preferred ink physicochemical properties were determined<br />
according to printability tests. In addition to the effect of dye<br />
structure, the particle size of dyes <strong>and</strong> the concentration of<br />
inks influenced printing consistency <strong>and</strong> the light stability.<br />
413
Chang <strong>and</strong> Chao: Preparation of microemulsion based disperse dye inks <strong>for</strong> thermal bubble ink jet printing<br />
Table I. Preparations <strong>and</strong> particle sizes of stabilized disperse dye dispersions dye content: 20%.<br />
EXPERIMENTAL<br />
Preparation of Dispersion of Disperse Dye<br />
Disperse dyes in the <strong>for</strong>m of presscake with high purity is a<br />
prerequisite in the ink <strong>for</strong>mulation to ensure an accurate<br />
reaction system unaffected by side products of the synthesized<br />
dye. The dyes applicable <strong>for</strong> ink jet ink should be exhibited<br />
with neutral color shade <strong>and</strong> demonstrated dyeing<br />
fastness. In the interest of our investigation, we selected the<br />
dyes resembling the process colors in commercial ink jet<br />
printing <strong>and</strong> analyzed the effects of different dyes on the ink<br />
<strong>for</strong>mulation. We selected C.I. Disperse Blue 60 (bright<br />
greenish blue, C.I. Constitution No. 61104, Widetex, Taiwan)<br />
<strong>for</strong> cyan by comparing to C.I. Disperse Blue 79 (dark reddish<br />
blue, C.I. Constitution No. 11345, Widetex, Taiwan),<br />
C.I. Disperse Red 60 (bright bluish red, C.I. Constitution<br />
No. 60756, Widetex, Taiwan) <strong>for</strong> magenta in contrast with<br />
C.I. Disperse Red 109 (bright yellowish red, C.I. Constitution<br />
No. 11192, Widetex, Taiwan), <strong>and</strong> C.I. Disperse Yellow<br />
77 (bright greenish yellow, C.I. Constitution No. 70150,<br />
Widetex, Taiwan) <strong>for</strong> yellow by analogy with C.I. Disperse<br />
Yellow 99 (bright greenish yellow, C.I. Constitution No.<br />
48420, Widetex, Taiwan). A combination of dispersants <strong>and</strong><br />
wetting agents applicable <strong>for</strong> disperse dyes in ink <strong>for</strong>mulation<br />
were divided into two systems of alkyl polynaphthalene<br />
<strong>for</strong>maldehyde sulfonate (D50, Pleasant <strong>and</strong> Best Chemical<br />
Co.) with the sodium salt of di-butyl naphthalene sulfonic<br />
acid (MT 830L, Pleasant <strong>and</strong> Best Chemical Co.), <strong>and</strong> maleic<br />
anhydride diisobutylene copolymer (PT 245L, Pleasant <strong>and</strong><br />
Best Chemical Co.) with Triton X-100 (PerkinElmer). Those<br />
disperse dye dispersions with particles of small size, i.e., precursors<br />
of microemulsion, were prepared by grinding with a<br />
planetary micromill (Pulverisette 7, Fritsch) where zirconium<br />
oxide grinding bowls <strong>and</strong> 0.5 mm beadswereadopted<br />
to effectively degrade particle size. Each sample was ground<br />
at 400 rpm <strong>for</strong> different times to obtain the derived particle<br />
size. Then, the dispersion liquid was filtered to remove contamination,<br />
such as particles derived from unwanted wear of<br />
the grinding elements, <strong>and</strong> finally stabilized with an<br />
antiprecipitating agent, NL428 (Jintex Co., Taiwan). Preferred<br />
dye dispersing methods are listed in Table I, where<br />
systems have been optimized to achieve extraordinary stability<br />
when particle size dropped below 200 nm. Although<br />
such preink obtained was well dispersed <strong>and</strong> stable, its physicochemical<br />
properties did not meet the dem<strong>and</strong>s of ink jet<br />
printing <strong>and</strong> immediately clogged printing nozzles because<br />
of high dye content (20%), high surface tension<br />
50–60 dyne/cm, <strong>and</strong> high viscosity 7–15 cps, whereas<br />
the relative values <strong>for</strong> thermal bubble ink jet inks were<br />
3–10% of dye content, 20–40 dyne/cm surface tension,<br />
414 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Chang <strong>and</strong> Chao: Preparation of microemulsion based disperse dye inks <strong>for</strong> thermal bubble ink jet printing<br />
Table II. Composition of disperse dye microemulsion microemulsion system: Sinopol<br />
80:Sinopol 963H:Pannox 140:Glycerin:Deionized water=2.4%:4.8%:7.2%:<br />
26.4%:49.2%–54.2%.<br />
Ink<br />
Number<br />
Maximum<br />
Amount of<br />
Dye in<br />
Microemulsion<br />
%<br />
Balanced<br />
Amount<br />
of Deionized Water<br />
%<br />
Particle<br />
Size<br />
nm<br />
Particle Size after<br />
Stability test<br />
three cycles of 5°C <strong>for</strong><br />
3 h <strong>and</strong> 70°C <strong>for</strong> 3 h<br />
nm<br />
B60-am 8.0 50.2 a 80.9 80.1<br />
B60-bm 5.0 53.2 a 129.7 131.4<br />
B79-am 6.0 52.2 a 88.6 88.1<br />
B79-bm 6.0 52.2 a 128.3 130.2<br />
R60-am 10.0 49.2 49.5 48.5<br />
R60-bm 8.0 51.2 89.5 91.6<br />
R109-am 7.0 52.2 86.2 86.8<br />
R109-bm 6.0 53.2 96.5 98.4<br />
Y77-am 5.0 54.2 97.5 99.3<br />
Y77-bm 10.0 49.2 99.6 98.7<br />
Y99-am 8.0 51.2 83.8 84.2<br />
Y99-bm 5.0 54.2 102.2 104.5<br />
a The addition of 1% diethylene glycol monoethyl ether.<br />
<strong>and</strong> 2–5 cps viscosity. The results of surface tension <strong>and</strong><br />
viscosity were shown in Table I.<br />
Determination of Microemulsion System<br />
The microemulsions of disperse dyes were obtained by<br />
intensively mixing the dye dispersion with an emulsifier<br />
(Sinopol 963H, polyoxyethylene nonyl phenyl ether,<br />
HLB=12.0, Sino-Japan Chemical), a coemulsifier (Pannox<br />
140, nonyl phenol polyethylene glycol ether, HLB=17.7, Pan<br />
Asia Chemical), a cosolvent (glycerin, Ferak), sorbitan oleate<br />
(Sinopol 80, HLB=4.3, Sino-Japan Chemical), <strong>and</strong> deionized<br />
water. The mixture was homogenized (1500 rpm <strong>for</strong><br />
10 min) to disperse the dye into very small emulsion droplets,<br />
whereby the microemulsion spontaneously <strong>for</strong>med. The<br />
system achieved its stability due to unique size compatibility<br />
<strong>and</strong> adhesion of the dye particles inside the oil phase of the<br />
microemulsion. Microemulsions of magenta <strong>and</strong> yellow dyes<br />
were thermodynamically stable <strong>for</strong> an indefinite period of<br />
time <strong>and</strong> an survive freezing <strong>and</strong> thawing cycles. However,<br />
the cyan dye samples were observed precipitating in the storage<br />
stability test (three cycles of 5°C <strong>for</strong> 3h<strong>and</strong> 70°C <strong>for</strong><br />
3h) whereas others did not. Despite the stabilization of<br />
microemulsion structure by coemulsifier, hydrotropic<br />
amphiphiles (Diethylene glycol monoethyl ether, Riedel–de<br />
Haen AG Seelze–Hannover) were needed <strong>for</strong> cyan dyes to<br />
surround each microemulsion droplet in order to prevent<br />
the microscopic droplets from coalescing. In order to determine<br />
the maximum amount of water content <strong>for</strong> each system,<br />
the quasiternary phase diagram was used to establish<br />
the largest microemulsion regime, where the ratios of emulsifier,<br />
coemulsifier, <strong>and</strong> cosolvent <strong>for</strong>med the upper region of<br />
the boundary between the total mass of dye phase, water<br />
Figure 1. Good dispersion of B79-a <strong>and</strong> R109-b in comparison to<br />
B79-b under an electronic microscope Scope: 100.25.<br />
phase, <strong>and</strong> emulsifiers. The optimized compositions <strong>for</strong> each<br />
dye microemulsion are summarized in Table II.<br />
Preparation of Disperse Dye Inks<br />
The dye microemulsions met the viscosity requirements <strong>for</strong><br />
thermal ink jet printing, i.e., 2–6 cps., measured by Brookfield,<br />
LVDV-III. However, other additives, e.g., fungicide <strong>and</strong><br />
buffer solution (ammonium solution), employed in ink to<br />
optimize ink per<strong>for</strong>mance, decreased the viscosity without<br />
affecting other properties of the microemulsion. In general,<br />
the lower the viscosity the greater the velocity <strong>and</strong> amount of<br />
fluid propelled <strong>for</strong>ward, which usually leads to the <strong>for</strong>mation<br />
of long tails behind the head of the drop. Glycerin was there<strong>for</strong>e<br />
added additionally about 0.5–2 wt. % to restore the<br />
viscosity <strong>and</strong> optimize printing consistency. The characteristics<br />
of the inks were determined using a Horbita pH meter<br />
<strong>for</strong> pH measurement, Face CBVP-A3 <strong>for</strong> surface tension<br />
measurement, <strong>and</strong> ZetaPlus particle analyzer <strong>for</strong> particle<br />
size evaluation. Phase separation was determined by visual<br />
observation. The printing quality of line sharpness, edge<br />
acuity, successiveness, <strong>and</strong> nozzle clogging was evaluated by<br />
printing on a Lexmark Z43 printer on paper. The lightfastness<br />
of inks printed on fabric (100% polyester) was evaluated<br />
according to AATCC16-1998A.<br />
RESULTS AND DISCUSSION<br />
Effects of Disperse Dye on Dispersion Preparation<br />
The effect of different disperse dyes on particle size were<br />
displayed in Table I. The dispersion <strong>for</strong>mulations with D50/<br />
MT830L have revealed smaller particle size with shorter<br />
grinding time except <strong>for</strong> Y77-a. The samples thereof, B79-a<br />
<strong>and</strong> R109-a, achieved average particle size of 105 nm, <strong>and</strong><br />
moreover, R60-a achieved the nanoparticle size of 53.2 nm,<br />
which might be due to D50, polynaphthalene <strong>for</strong>maldehyde<br />
sulfonate, having better wetting <strong>and</strong> deagglomeration than<br />
PT245L, maleic anhydride copolymer, under the mechanical<br />
<strong>for</strong>ce of micromilling. B79-b presented the largest particle<br />
size with agglomerates, indicating an unsuitable combination<br />
with PT245L/Triton X-100 (Figure 1), <strong>and</strong> then further<br />
failed to <strong>for</strong>m microemulsion. By comparing B79-b to<br />
R109-b, more alkyl groups in the diazo structure decreased<br />
the efficiency of particle size reduction <strong>and</strong> consequently<br />
required more time or more active surfactants to accomplish<br />
the desired result. Among the systems of PT245L/Triton<br />
X-100, only the structure of Y77-b had more satisfactory<br />
compatibility with the copolymer allowing reduction in particle<br />
size to 106.5 nm in 120 h. The slight difference in<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 415
Chang <strong>and</strong> Chao: Preparation of microemulsion based disperse dye inks <strong>for</strong> thermal bubble ink jet printing<br />
Table III. The physicochemical properties of microemulsion based disperse dye inks.<br />
Ink-<br />
Dye Content<br />
%<br />
Surface<br />
Tension<br />
dyne/cm<br />
Viscosity<br />
cps<br />
pH<br />
Value<br />
Particle<br />
size<br />
nm<br />
Lightfastness<br />
AATCC16-<br />
1998A<br />
Printabiliy<br />
h<br />
B60-am-4 34.9 4.28 7.7 82.6 4 8<br />
B60-am-8 33.7 4.58 7.6 80.8 5 8<br />
R109-am-3.5 36.0 3.51 7.7 90.2 4 8<br />
R109-am-7 41.2 3.57 7.6 84.4 5 6<br />
Y77-bm-5 41.4 5.80 7.3 90.7 5 8<br />
Y77-bm-10 41.1 3.96 7.3 95.5 4 6<br />
Y99-am-5 41.5 4.50 7.5 82.9 4 8<br />
Y99-am-8 40.6 4.43 7.6 83.2 4 8<br />
Figure 2. Quasi-ternary phase diagrams of the microemulsion systems,<br />
where E+C+S represents the total mass of emulsifier, coemulsifier <strong>and</strong><br />
cosolvent, O is the mass of oil phase Sinopol 80, <strong>and</strong>Wisthemassof<br />
water. Lines 1–5 represent the ratio of emulsifier to coemulsifier <strong>and</strong><br />
cosolvent in mass 2:3:11, 2:1:11, 3:2:11, 2:3:7, <strong>and</strong> 2:3:13. I is<br />
the system of Sinopol 963H HLB=12.0/Pannox 140<br />
HLB=17.7/Glycerin. II is the system of Sinopol 960H<br />
HLB=10.0/Pannox 119 HLB=15.8/Glycerin. III is the system of<br />
Sinopol 966H HLB=14.1/Pannox 150 HLB=18.0/Glycerin.<br />
particle-reducing results between the two dispersion systems<br />
at both 120 h <strong>and</strong> 168 h occurred in samples of C.I. Disperse<br />
Blue 60 <strong>and</strong> C.I. Disperse Red 109 probably due to<br />
lower molecular weight <strong>and</strong> reduced steric hindrance of the<br />
dye structures. The antiprecipitating additive, NL428 was<br />
found to successfully prevent reaggregation in both dispersing<br />
systems.<br />
Determination of Disperse Dye Microemulsion System<br />
The microemulsion system of Sorbitan trioleate/<br />
polyoxyethylene nonyl phenyl ether/nonylphenol polyethylene<br />
glycol ether/glycerin/water had been chosen by comparing<br />
to other compositions in the quasi-ternary phase<br />
diagrams at 25°C, shown in Figure 2. The experimental results<br />
shown in Table II indicated that the system had good<br />
compatibility with most dispersing systems <strong>and</strong> could be<br />
optimized to prepare the disperse dye inks of different colors.<br />
Figure 2 demonstrated that the microemulsion could be<br />
<strong>for</strong>med if the HLB values of emulsifiers lay between 10.0 <strong>and</strong><br />
18.0. The microemulsion area, <strong>for</strong>med in the upper region<br />
of the boundary line, depends on the ratios of emulsifier to<br />
coemulsifier <strong>and</strong> cosolvent. A maximum ratio was found as<br />
2:3:11 <strong>and</strong> decreased in the following sequences: 2:1:11,<br />
3:2:11, 2:3:7, <strong>and</strong> 2:3:13. When the cosolvent was the ratio of<br />
11, additional emulsifier (emulsifier:coemulsifier2:3) enlarged<br />
the microemulsion area more than those<br />
(emulsifier:coemulsifier2:1) with the three different HLB<br />
value combination systems. However, more emulsifier of<br />
HLB value 14.1 than coemulsifier of HLB value 18.0 was<br />
required in system (III) to <strong>for</strong>m the largest microemulsion<br />
area. It could be inferred that the emulsifiers of HLB values<br />
of 10.0 <strong>and</strong> 12.0 had more affinity to the oil phase <strong>and</strong><br />
emulsified effectively at the lower ratio of 2 than the emulsifiers<br />
of HLB value 14.1, but required more coemulsifier of<br />
HLB value 15.8 to <strong>for</strong>m the largest microemulsion area, i.e.,<br />
maximum water dissolving capacity. When the ratio of<br />
emulsifier to coemulsifier was accordingly set at 2:3, the<br />
cosolvent at ratio of 13 increased water solubility more than<br />
at the ratio of 7, especially in the system (I), where the HLB<br />
value was 12.0 <strong>for</strong> emulsifier <strong>and</strong> 17.7 <strong>for</strong> coemulsifier. Nevertheless,<br />
the cosolvent at the ratio of 11 was most preferred<br />
to <strong>for</strong>m the maximum water solubility in three systems.<br />
Relatively, the ratio of 2:3:11 (emulsifier:coemulsifier:<br />
cosolvent) in system (I) presented the largest microemulsion<br />
area among other two systems. The Sinopol 963H/Pannox<br />
140/Sinopol 80/Glycerin microemulsion system had a maximum<br />
microemulsion area, implying maximum solubility.<br />
This ratio dependence indicated that the interfacial area generated<br />
by the emulsion drops can be exp<strong>and</strong>ed with the increased<br />
amount of coemulsifier <strong>and</strong> cosolvent. The maximum<br />
dye amount of each dispersion <strong>for</strong>mulation dissolved<br />
in the given microemulsion system was determined by particle<br />
size variation 2.5% after the storage stability test, as<br />
shown in Table II. The resulting microemulsions were referred<br />
to as disperse dye inks, listed in Table III. In short, the<br />
composition corresponding to the maximum solubility was<br />
Dye:Sinopol 80:Sinopol 963H:Pannox 140:Glycerin:Deionized<br />
water5%:2.4%:4.8%:7.2%:26.4%:54.2%.<br />
Effects of Disperse Dye on Microemulsion System<br />
In Table II, the maximum amount of disperse dyes in a given<br />
microemulsion system was determined as the amount at<br />
which 2.5% or larger variation of the particle size was detected<br />
after the storage stability test. The samples of B60-am,<br />
R60-am, R60-bm, Y77-bm, <strong>and</strong> Y99-am could be incorporated<br />
more effectively inside the microemulsion droplets to<br />
allow the highest dye content, up to 8–10%. Other samples<br />
imply the maintenance of at least 5% of the dye, <strong>and</strong> all the<br />
required ink properties can also be met. Notably, to avoid<br />
the dye agglomeration mentioned above, 1% amount of<br />
amphiphile (diethylene glycol monoethyl ether) was added<br />
into samples of B60-a, B60-b, B79-a, <strong>and</strong> B79-b. The particle<br />
416 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Chang <strong>and</strong> Chao: Preparation of microemulsion based disperse dye inks <strong>for</strong> thermal bubble ink jet printing<br />
Table IV. The water fastness of microemulsion based disperse dye inks.<br />
Water-fastness Test AATCC-61-2003-2A<br />
Figure 3. The printing quality of inks, B60-am-8%, R109-am-7%, <strong>and</strong><br />
Y77-bm-5%, which were ink jet printed with Lexmark Z43 printer.<br />
size of dispersed dye was further reduced in the microemulsion<br />
after ball milling. In the storage stability test, the particle<br />
size of B79-am <strong>and</strong> R60-am decreased further while the<br />
R109-am, B60-am, Y77-bm, <strong>and</strong> Y99-am did not change<br />
apparently (nearly 0%). It could be concluded from the<br />
above analyses that when the ratio of Sinopol 963H/Pannox<br />
140/Sinopol 80/Glycerin was 2:3:1:11, the system of disperse<br />
dye/D50/MT830L/water maintained good stability when the<br />
temperature changed from 5°C to 70°C.<br />
Effects of Ink Concentration on Physicochemical<br />
Properties<br />
With the superiority in particle stability, the samples of<br />
R109-am, B60-am, Y77-bm, <strong>and</strong> Y99-am were selected <strong>for</strong><br />
further study on the effect of ink concentration. Since the<br />
process color of light cyan <strong>and</strong> light magenta are usually<br />
<strong>for</strong>mulated in half concentration of cyan <strong>and</strong> magenta, the<br />
concentration effect on lightfastness, particle size, <strong>and</strong> printability<br />
there<strong>for</strong>e becomes important. In this work, sample<br />
B60-am-8% was investigated with the concentration of 4%,<br />
while the sample R109-am-7% is investigated with the concentration<br />
of 3.5%. Y77-bm-10% <strong>and</strong> Y99-am-8% are per<strong>for</strong>med<br />
with the lowest dye content of 5% in microemulsion<br />
according to the above analyses. To be applicable as inks <strong>for</strong><br />
the thermal bubble ink jet printing, each dye microemulsion<br />
was <strong>for</strong>mulated with the added fungicide, the buffer solution<br />
to stabilize pH value of the system, <strong>and</strong> a little glycerin (the<br />
cosolvent applied in the above microemulsion system) to<br />
optimize viscosity. The above agents could all be incorporated<br />
into <strong>for</strong>mulations without disrupting microemulsion<br />
stability. The inks <strong>for</strong> thermal ink jet printing were produced<br />
with acceptable physicochemical properties, summarized in<br />
Table III. The surface tension <strong>and</strong> viscosity of each sample<br />
met the dem<strong>and</strong>s of ink drop per<strong>for</strong>mance, to enable efflux<br />
through the capillary tube of the printer nozzle on to be able<br />
to print continuously <strong>and</strong> present good printing quality,<br />
sharp line, <strong>and</strong> accurate edge, either on paper or polyester<br />
fabric, without nozzle clogging <strong>and</strong> edge feathering (Figure<br />
3). However, the inks of R109-am-7% <strong>and</strong> Y77-bm-10% had<br />
printing continuity of 8 h, but it can be extended much<br />
longer when the dye content was reduced, e.g., R109-am-<br />
3.5% <strong>and</strong> Y77-bm-5%. By comparing the particle size to<br />
other samples, the structure or <strong>for</strong>mulation of R109-am-7%<br />
would not tolerate the temperature or vibration variation<br />
inside the thermal bubble printhead. The failure of Y77-bm-<br />
10%, however, could be attributed to the largest particle size<br />
causing inferior printability. The samples of B60-am-4% <strong>and</strong><br />
R109-am-3.5% exhibited an increase in particle size <strong>and</strong> a<br />
slight decrease in lightfastness when the concentration was<br />
Ink-<br />
Dye Content<br />
%<br />
Color<br />
Change<br />
Staining on<br />
Cotton Fiber<br />
Staining<br />
on Acrylic<br />
Fiber<br />
B60-am-4 5 5 5<br />
B60-am-8 5 5 5<br />
R109-am-3.5 5 5 5<br />
R109-am-7 5 5 5<br />
Y77-bm-5 5 5 5<br />
Y77-bm-10 5 5 5<br />
Y99-am-5 5 5 5<br />
Y99-am-8 5 5 5<br />
diluted in half. This implies that excessive amount of surfactant<br />
<strong>and</strong> solvent breaks up the microemulsion system <strong>and</strong><br />
induces the primary particles to generate larger particles, i.e.,<br />
agglomerates. Moreover, the greater amount of small dye<br />
particles on the medium surface would reflect more light<br />
<strong>and</strong> protect prevent the printed fabric from the effects of<br />
light exposure. Such particle effects are also seen in the yellow<br />
samples, although the differences are insignificant. The<br />
anthraquinone dyes per<strong>for</strong>med better in lightfastness than<br />
the diazo ones, <strong>and</strong> their lightfastness grade can reach 5 by<br />
decreasing the particle size. No <strong>for</strong>mulation in Table III per<strong>for</strong>ms<br />
less than grade 4, which demonstrates that all the<br />
<strong>for</strong>mulated inks meet the requirement of light stability <strong>for</strong><br />
thermal bubble ink jet printing.<br />
The water fastness of each sample in Table IV was identically<br />
at grade 5, according to AATCC-61-2003-2A. There<br />
was no color change in shade <strong>and</strong> no staining on cotton <strong>and</strong><br />
acrylic fiber. The cotton <strong>and</strong> acrylic fiber are usually easily<br />
stained by all kinds of dyes in the water-fastness test. It could<br />
be inferred that the microemulsion ink drops containing<br />
dyes (from 3.5% to 10%) were jetted on dem<strong>and</strong> by the<br />
thermal bubble ink jet nozzle <strong>and</strong> fixed well on the polyester<br />
filament. Excessive dye that could be carried away by water<br />
did not occur on the fiber surface. Moreover, those dye particle<br />
sizes in the range between 80 nm <strong>and</strong> 96 nm did not<br />
affect the fastness to the fiber in water but slightly in the<br />
light. The high water fastness is thus another advantage of<br />
the studied microemulsion ink <strong>for</strong> the application of textile<br />
ink jet printing.<br />
In the future, more systematic study of ink rheology in<br />
the dyestuff ink jet printability will be carried out to investigate<br />
the relationship between dye structure, ink composition<br />
<strong>and</strong> fluid viscoelasticity during ink jet printing.<br />
CONCLUSIONS<br />
The microemulsion system of Sinopol 80/Sinopol 963H/<br />
Pannox 140/Glycerin/Deionized water was determined to be<br />
compatible with the best dispersing composition of disperse<br />
dyes <strong>and</strong> exhibited excellent behavior in the quasi-ternary<br />
phase diagrams. It can be concluded that only C.I. Disperse<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 417
Chang <strong>and</strong> Chao: Preparation of microemulsion based disperse dye inks <strong>for</strong> thermal bubble ink jet printing<br />
Yellow 77 had better compatibility with other ingredients to<br />
achieve printability of more than 8h <strong>and</strong> lightfastness of<br />
grade 5. Moreover, the samples of B60-am, R109-am, Y77-<br />
bm, <strong>and</strong> Y99-am in the given microemulsion presented best<br />
stability without particle size variation in the storage stability<br />
test. Nevertheless, the samples of B79-am <strong>and</strong> R60-am<br />
demonstrated slightly decreased particle size in the<br />
microemulsion.<br />
After the microemulsions were <strong>for</strong>mulated into inks, the<br />
samples of R109-am-7% <strong>and</strong> Y77-bm-10% could not be<br />
printed with thermal bubble ink jet <strong>for</strong> more than 8h,<br />
which is probably due to poor heat resistance of diazo structure<br />
or larger particle clogging in the nozzle after the ink has<br />
been frequently heated in the thermal bubble print head.<br />
When the ink concentration was diluted in half, B60-am-4%<br />
<strong>and</strong> R109-am-3.5% exhibited a particle size increase <strong>and</strong><br />
slight decrease in lightfastness, which is due to the generation<br />
of agglomerates in the unbalanced system with higher<br />
amounts of surfactants <strong>and</strong> solvent. Notably, it was found<br />
that anthraquinone dyes yield higher lightfastness than diazo<br />
dyes to attain the lightfastness grade of 5.<br />
The microemulsion inks of this study had many advantages<br />
over other inks containing just the dye dispersions.<br />
One advantage was that microemulsion ink was a<br />
microheterogeneous system that provided a large interfacial<br />
area to accommodate dye content up to 8–10% <strong>and</strong> low<br />
viscosity to effectively meet the dem<strong>and</strong> of ink drop per<strong>for</strong>mance<br />
without adding other viscosity modifiers. Another<br />
advantage was that microemulsion inks produced printed<br />
dot sizes that were nearly independent of the surface properties<br />
of the medium; they could present good printing quality<br />
either on paper or 100% polyester fabric without nozzle<br />
clogging <strong>and</strong> edge feathering. The most important advantage<br />
was that dyes in the given microemulsion exhibited smaller<br />
particle size than in dispersion <strong>for</strong>mulations <strong>and</strong>, thereby,<br />
good storage stability with very little particle size variation.<br />
ACKNOWLEDGMENTS<br />
The authors thank S. P. Rwei in our department <strong>for</strong> helpful<br />
discussions. The authors also thank Widetex, Pleasant <strong>and</strong><br />
Best Chemical, Jintex Co., Pan Asia Chemical, <strong>and</strong> Sino-<br />
Japan Chemical <strong>for</strong> offering materials. Nevertheless, we<br />
gratefully acknowledge Taiwan Textile Research Institute <strong>for</strong><br />
support.<br />
REFERENCES<br />
1 K. J. Lissant, Emulsions <strong>and</strong> Emulsion Tech., Part III (Marcel Dekker,<br />
New York, 1984) pp. 140–162.<br />
2 A. García, M. Llusar, S. Sorli, J. Calbo, M. A. Monros, <strong>and</strong> G. Page, J.<br />
Eur. Ceram. Soc. 23, 1829–1838 (2003).<br />
3 M. Häger et al., Colloids Surf., A 183-185, 247–257 (2001).<br />
4 R. Guo, H. Qi, Y. Chen, <strong>and</strong> Z. Yang, Mater. Res. Bull. 38, 1501–1507<br />
(2003).<br />
5 C. Mo, M. Zhong, <strong>and</strong> Q. Zhong, J. Electroanal. Chem. 493, 100–107<br />
(2000).<br />
6 K. Aikawa, K. Kaneko, T. Tamura, M. Fujitsu, <strong>and</strong> K. Ohbu, Colloids<br />
Surf., A 150, 95–104 (1999).<br />
7 R. Guo, H. Qi, Y. Chen, <strong>and</strong> Z. Yang, Int. J. Inorg. Mater. 18, 645–652<br />
(2003).<br />
8 C. A. Miller, R. N. Huan, W. J. Benton, <strong>and</strong> T. Fort, Jr., J. Colloid<br />
Interface Sci. 61, 554–568 (1977).<br />
9 V. K. Bansal, D. O. Shah, <strong>and</strong> J. P. O’Connell, J. Colloid Interface Sci. 75,<br />
462–475 (1980).<br />
10 R. Guo, H. Qi, Z. Yang, <strong>and</strong> Y. Chen, Ceram. Int. 30, 2259–2267 (2004).<br />
11 B. D. Gans, P. C. Duineveld, <strong>and</strong> U. S. Schubert, Adv. Mater. (Weinheim,<br />
Ger.) 16, 203–213 (2004).<br />
418 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 419–423, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Effects of Molecular Substituents of Copper<br />
Phthalocyanine Dyes on Ozone Fading<br />
Fariza B. Hasan, Michael P. Filosa <strong>and</strong> Zbigniew J. Hinz<br />
ZINK <strong>Imaging</strong> Inc., Waltham, Massachusetts 02451<br />
E-mail: fariza.hasan@zink.com<br />
Abstract. Copper phthalocyanine dyes, widely used in various imaging<br />
systems, are susceptible to fading under ambient conditions.<br />
One of the main factors responsible <strong>for</strong> such fading is the presence<br />
of ozone. Addition of suitable antiozonants has been shown to be<br />
effective in improving ozone stability. Although the exact mechanism<br />
of such stabilization is not fully understood, the electronic structures<br />
of the additives have shown to have significant impact on their effectiveness.<br />
In order to increase the ozone stability of copper phthalocyanine<br />
dyes without any additives, several copper phthalocyanine<br />
dyes containing substituents of varying electronic structures<br />
were synthesized <strong>and</strong> tested <strong>for</strong> ozone stability. The structure of a<br />
copper phthalocyanine dye with significantly improved stability to<br />
ozone is described in this paper. Possible mechanisms leading to<br />
such stability are also discussed. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5419<br />
INTRODUCTION<br />
Copper phthalocyanine CuPC dyes are extensively used in<br />
various imaging systems because of several advantages, including<br />
high spectral absorptivity, solubility in various common<br />
solvents, <strong>and</strong> relatively greater lightfastness compared<br />
to several other classes of dyes. However, CuPC dyes are also<br />
susceptible to ozone fading under various ambient conditions,<br />
which can cause deterioration of images. A large number<br />
of papers related to the effects of ozone exposure on<br />
image stability have been published. Some of the recent publications<br />
are referred to in this article. 1–11<br />
Several types of materials commonly used as antiozonants<br />
in other industries, such as p-phenylenediamines <strong>for</strong><br />
reducing degradation of rubber due to exposure to ozone,<br />
are not suitable <strong>for</strong> imaging systems because of the highly<br />
colored products <strong>for</strong>med due to their reactions with ozone.<br />
These reactions have been shown to proceed through electron<br />
transfer mechanisms. 12 Other methods used <strong>for</strong> minimizing<br />
degradation of rubber due to exposure to ozone,<br />
such as coating with impermeable materials, are not easily<br />
applicable to imaging products, except in a limited number<br />
of imaging systems where polymeric materials can be transferred<br />
over the printed images. However, in such systems the<br />
efficiency of these materials depends on the continuity of the<br />
transferred polymeric layer, <strong>and</strong> any defect present in this<br />
Received Jan. 11, 2007; accepted <strong>for</strong> publication Mar. 1, 2007.<br />
1062-3701/2007/515/419/5/$20.00.<br />
layer would allow ozone to diffuse through <strong>and</strong> cause degradation<br />
of the dyes in images.<br />
Several aminoanthraquinone dyes having similar functional<br />
groups as p-phenylenediamines are efficient antiozonants,<br />
but they do not <strong>for</strong>m highly colored reaction products,<br />
due to the presence of electron withdrawing carbonyl<br />
groups in the fused ring systems. 13,14 Aminoanthraquinone<br />
dyes, as well as p-phenylenediamines, are essentially ozone<br />
scavengers <strong>and</strong> thus reduce the availability of ozone to react<br />
with CuPC dyes. It is expected that the presence of molecular<br />
substituents, which can act as ozone scavengers would<br />
also render the CuPC dye molecules more resistant to fading<br />
<strong>and</strong> discoloration due to exposure to ozone. In order to test<br />
this hypothesis, <strong>and</strong> since the reaction of olefins with ozone<br />
is well known, a CuPC dye with allyl substituents was synthesized.<br />
To verify the mechanism, two other CuPC dyes<br />
with different substituents were also synthesized <strong>and</strong> tested.<br />
EXPERIMENTAL PROCEDURES<br />
Substituted Copper Phthalocyanine Dyes<br />
The structures of the synthesized CuPC dyes with varying<br />
substituents, labeled as CuPC-B, CuPC-C, <strong>and</strong> CuPC-D, are<br />
shown in Figure 1. A commercially available CuPC dye, Solvent<br />
Blue 70 (CAS No. 12237-24-0) was used as the control<br />
<strong>for</strong> the tests. Although the exact structure of this dye is not<br />
known, it is possibly a mixture of several isomers containing<br />
various ring substituents, <strong>and</strong> can be represented as<br />
CuPC-A.<br />
Synthesis of Copper Phthalocyanine Dyes<br />
The synthetic method used <strong>for</strong> the substituted copper phthalocyanine<br />
dyes is described in Scheme 1. 15 Copper<br />
phthalocyaninetetrasulfonic acid tetrasodium salt (Aldrich),<br />
containing four −SO 3 Na groups as ring substituents was<br />
suspended in a 4:1 mixture of sulfolane <strong>and</strong><br />
dimethylacetamide. A 32-fold excess of phosphorus oxychloride<br />
was added, <strong>and</strong> the reaction mixture was allowed to<br />
reflux <strong>for</strong> 4h, after which the reaction mixture was cooled to<br />
room temperature <strong>and</strong> poured into rapidly stirring ice water.<br />
The blue precipitated solid was collected, washed with water,<br />
<strong>and</strong> dried in a vacuum desiccator at room temperature <strong>for</strong><br />
24 h. The crude wet cake, with some water present, was<br />
dissolved/suspended in methylene chloride <strong>and</strong> cooled to<br />
4°C. A large excess of diallyl amine (<strong>for</strong> CuPC-B),<br />
dibutylamine (<strong>for</strong> CuPC-C) or diethoxyethylamine (<strong>for</strong><br />
419
Hasan, Filosa, <strong>and</strong> Hinz: Effects of molecular substituents of copper phthalocyanine dyes on ozone fading<br />
without any polymeric binder from a donor sheet to a porous<br />
receiver. 16 The material used as donor sheets <strong>for</strong> these<br />
experiments was 4.5 m thick PET with backcoat containing<br />
lubricant suitable <strong>for</strong> thermal printing. The receiver consisted<br />
of a coating of fumed silica with a hydrophilic polymeric<br />
binder system on a polyester base. The coverage of the<br />
dyes coated on donor sheets was maintained constant<br />
285 mg/m 2 . No protective coating was transferred over<br />
the images. St<strong>and</strong>ard-type thermal printheads <strong>for</strong> monochrome<br />
prints were used <strong>for</strong> transferring the images, at<br />
maximum energy of 2.5 J/cm 2 .<br />
Figure 1. Structures of copper phthalocyanine dyes tested.<br />
Scheme 1.<br />
CuPC-D) was slowly added to the cooled solution <strong>and</strong> then<br />
allowed to warm to room temperature <strong>and</strong> stirred <strong>for</strong> 16 h.<br />
The crude reaction mixture was placed onto a plug of dry of<br />
silica gel <strong>and</strong> eluted with methylene chloride. A greenish<br />
colored material came off <strong>and</strong> was discarded. The silica with<br />
the product adhering to it was sucked down until nearly dry.<br />
The elution was continued with a mixture of ethyl acetate/<br />
hexanes, 1:1 by volume. This process removed a dark brown<br />
impurity from the product. The eluent mixture was changed<br />
to one containing 4:1 ethyl acetate/hexanes. This fraction<br />
contained the desired copper phthalocyanine dye.<br />
<strong>Imaging</strong> Method<br />
Printed images of the dyes were generated by thermally<br />
transferring the coated dyes <strong>and</strong> suitable thermal solvents,<br />
Spectra of Copper Phthalocyanine Dyes<br />
Absorption spectra of the dyes in solution were obtained,<br />
after dissolving the dyes in methylene chloride, using a<br />
HP8452A diode array spectrophotometer. The reflection<br />
spectra of the dyes were measured at the D max regions of<br />
printed images of the dyes, using a Gretag SPM500 densitometer.<br />
The conditions <strong>for</strong> the measurements were:<br />
illuminationD50, observer angle2°, density<br />
st<strong>and</strong>ardANSI A, reflection st<strong>and</strong>ardwhite base, <strong>and</strong> no<br />
filter.<br />
Ozone Stability Test Method<br />
To evaluate the ozone stability of the synthesized CuPC dyes<br />
containing various substituents, as well as the commercially<br />
available CuPC dye, the D max regions of the transferred images<br />
were exposed in an ozone chamber constructed from a<br />
Pyrex jar 1.2 ft 3 <strong>and</strong> a mercury-argon lamp. 17 Ozone was<br />
produced in situ by the direct photolysis of oxygen in the<br />
ambient air within the chamber. A fan within the chamber<br />
ensured that all samples were uni<strong>for</strong>mly exposed. The temperature<br />
in the ozone chamber was between 21°C <strong>and</strong> 23°C<br />
<strong>and</strong> relative humidity was 47–50%. The images were exposed<br />
to ozone <strong>for</strong> definite periods of time. For each set of<br />
experiments, a “control” image, which was an image of the<br />
control commercially available dye, CuPC-A with cyan reflection<br />
density of very close to unity, was exposed <strong>for</strong> 1h<br />
with the tests samples. A comparison of the extent of change<br />
of each of these “control” images was used as a method to<br />
calibrate the effective ozone concentration in the chamber<br />
<strong>for</strong> each experiment. However, during the short period of<br />
time in which these experiments were conducted, the<br />
changes of the control images were practically identical,<br />
which indicated that the concentration of ozone in the<br />
chamber during these experiments remained unchanged.<br />
Since the aim of these experiments was to compare the<br />
ozone resistance of the CuPC dyes under identical conditions,<br />
it was not necessary to measure the exact concentration<br />
of ozone in the chamber. The ozone stability of each of<br />
these dyes was compared by overall spectral changes <strong>and</strong><br />
quantified by the changes of the reflection densities (cyan,<br />
magenta, <strong>and</strong> yellow) <strong>and</strong> colorimetric parameters (L * , a * ,<br />
<strong>and</strong> b * ) of the images after ozone exposure, using a Gretag<br />
SPM500 densitometer. The conditions <strong>for</strong> the measurements<br />
are as described in the previous paragraph.<br />
420 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Hasan, Filosa, <strong>and</strong> Hinz: Effects of molecular substituents of copper phthalocyanine dyes on ozone fading<br />
Figure 3. Comparison of spectra of thermally transferred images of<br />
CuPC dyes.<br />
Figure 4. Ozone induced spectral changes on thermally transferred images<br />
of CuPC-A <strong>and</strong> CuPC-B.<br />
Figure 2. Comparison of spectra of CuPC dyes in methylene chloride<br />
solution.<br />
RESULTS AND DISCUSSION<br />
Figure 2 shows the absorption spectra of the commercially<br />
available <strong>and</strong> the three synthesized copper phthalocyanine<br />
dyes in methylene chloride. Although the solution spectra of<br />
these dyes are not identical, they are similar to each other.<br />
These results indicate that the electronic structures of the<br />
substituents have minor or insignificant effects on the chromophore.<br />
However, a comparison of the reflection spectra of<br />
the transferred images of the three dyes, shown in Figure 3,<br />
indicates significant differences between the spectra of the<br />
dyes. Although all three dyes show blueshifts of the transferred<br />
images compared to their spectra in solution, which is<br />
an indication of H-aggregation, CuPC-B shows greater shift<br />
than the other two synthesized dyes. It is likely that the<br />
planar allyl groups facilitate the stacking of the dye molecules<br />
more than the nonplanar butyl <strong>and</strong> diethoxyethyl<br />
groups, causing such aggregation.<br />
Figure 4 shows the reflection spectra of images of<br />
CuPC-A <strong>and</strong> CuPC-B, be<strong>for</strong>e <strong>and</strong> after exposure to ozone<br />
<strong>for</strong> 1h. The control CuPC-A image showed large changes,<br />
including a significant decrease of cyan density <strong>and</strong> increase<br />
of yellow density, which is due to the chemical degradation<br />
of the chromophore by ozone. The CuPC-B image after exposure<br />
to ozone under identical conditions remained almost<br />
unchanged. In order to determine if the observed ozone stability<br />
of CuPC-B is due to the steric hindrance caused by the<br />
allyl groups on the reaction of ozone with the chromophore,<br />
two other dyes, CuPC-C <strong>and</strong> CuPC-D, in which the allyl<br />
groups were replaced by butyl <strong>and</strong> diethoxyethyl groups, respectively,<br />
were also tested. Figures 5 <strong>and</strong> 6 show larger spectral<br />
changes <strong>for</strong> both CuPC-C <strong>and</strong> CuPC-D, compared to<br />
the changes <strong>for</strong> CuPC-B. These results indicate that the<br />
higher stability of CuPC-B to ozone is unlikely to be due to<br />
simple steric hindrance.<br />
The changes in cyan (C), magenta (M), <strong>and</strong> yellow (Y)<br />
densities, <strong>and</strong> L * , a * , <strong>and</strong> b * values of images from each of<br />
the four dyes after exposure to ozone are listed in Tables I<br />
<strong>and</strong> II, respectively.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 421
Hasan, Filosa, <strong>and</strong> Hinz: Effects of molecular substituents of copper phthalocyanine dyes on ozone fading<br />
Table I. Effects of ozone exposure on C, M, Y densities of printed images of CuPC dyes<br />
Dyes Density DC DM DY<br />
CuPC-A Initial 1.22 0.23 0.10<br />
After exposure 0.46 0.15 0.26<br />
Ratio, final/initial % 38 65 260<br />
CuPC-B Initial 1.20 0.31 0.25<br />
After exposure 1.19 0.31 0.26<br />
Ratio, final/initial % 99 100 104<br />
CuPC-C Initial 1.20 0.30 0.20<br />
After exposure 0.82 0.24 0.28<br />
Ratio, final/initia% 68 80 140<br />
Figure 5. Ozone induced spectral changes in thermally transferred images<br />
of CuPC-A <strong>and</strong> CuPC-C.<br />
CuPC-D Initial 1.15 0.30 0.20<br />
After exposure 0.80 0.23 0.26<br />
Ratio, final/initial % 70 77 130<br />
Table II. Effects of ozone exposure on L * , a * , <strong>and</strong> b * values of printed images of CuPC<br />
dyes.<br />
Dyes L * a * b *<br />
CuPC-A Initial 68.82 −45.86 −38.88<br />
Final 81.12 −27.49 −1.29<br />
Change 12.30 18.37 37.59<br />
CuPC-B Initial 62.78 −46.42 −30.19<br />
Final 62.75 −46.43 −29.81<br />
Change −0.03 −0.01 0.38<br />
Figure 6. Ozone induced spectral changes in thermally transferred images<br />
of CuPC-A <strong>and</strong> CuPC-D.<br />
CuPC-C Initial 63.73 −45.35 −36.36<br />
Final 70.45 −41.70 −16.59<br />
Change 6.72 3.65 19.77<br />
CuPC-D Initial 64.56 −44.71 −34.32<br />
Final 70.88 −40.31 −17.96<br />
Change 6.32 4.40 16.36<br />
A comparison of gradual cyan density loss with time of<br />
printed images of CuPC dyes due to exposure to ozone over<br />
an extended period of time is shown in Figure 7. The<br />
CuPC-A image shows 15% retention after 8.5 h of exposure,<br />
whereas the most stable CuPC-B image retains 73% of the<br />
initial cyan density under identical conditions. The other<br />
two dyes, CuPC-C <strong>and</strong> CuPC-D show much less density<br />
retention than CuPC-B, but are slightly more stable than<br />
CuPC-A. These results show the same trend in ozone stability<br />
of the CuPC dyes as observed after 1h of ozone<br />
exposure.<br />
The data indicate that the ozone stability of CuPC-B,<br />
containing allyl substituents, are considerably greater than<br />
that of the butyl or diethoxyethyl substituted CuPC-C <strong>and</strong><br />
CuPC-D dyes. As discussed be<strong>for</strong>e, a comparison of the<br />
spectra of the four dyes shows that greater extent of<br />
H-aggregation as indicated by the blue shift in the spectrum<br />
of CuPC-B, may be associated with the observed ozone stability.<br />
The spectra of the control CuPC-A dye <strong>and</strong> the synthesized<br />
dyes, dissolved in methylene chloride, do not show<br />
any such difference. It is possible that the planar allyl groups<br />
facilitate the required rearrangement of the molecules to<br />
cause such aggregation more than the other substituents <strong>and</strong><br />
result in a more stable <strong>for</strong>m of dye in solid state, as in a<br />
transferred image. In addition, the allyl groups may also<br />
react with ozone <strong>and</strong> act as ozone scavengers, thus protecting<br />
the chromophore.<br />
CONCLUSIONS<br />
Difference in molecular substituents of copper phthalocyanine<br />
dyes produces large effects on ozone fading of the<br />
printed images. Images <strong>for</strong>med with the allyl substituted dye<br />
show greater ozone stability than those with butyl or<br />
diethoxyethyl substituted dyes. The allyl substituted dye also<br />
422 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Hasan, Filosa, <strong>and</strong> Hinz: Effects of molecular substituents of copper phthalocyanine dyes on ozone fading<br />
Figure 7. Rates of cyan density loss due to ozone exposure of thermally<br />
transferred images of CuPC dyes with varying substituents.<br />
exhibits a larger extent of H-aggregation <strong>and</strong> blueshift in<br />
transferred images than the dyes containing other molecular<br />
substituents, when compared to their absorption spectra in<br />
solution. An increase in stability to exposure to ozone is<br />
likely to be due to the reaction of the allyl substituents with<br />
ozone, which accordingly act as ozone scavengers. Another<br />
possible factor may be the ease of <strong>for</strong>mation of H-aggregates<br />
that are more stable than the monomeric dyes.<br />
REFERENCES<br />
1 M. Berger <strong>and</strong> H. Wilhelm, Proc. IS&T’s NIP19 (IS&T, Springfield, VA,<br />
2003) pp. 438–443.<br />
2 M. Thornberry <strong>and</strong> S. Looman, Proc. IS&T’s NIP19 (IS&T, Springfield,<br />
VA 2003) pp. 426–430.<br />
3 K. Kitamura, Y. Oki, H. Kanada, <strong>and</strong> H. Hayashi, Proc. IS&T’s NIP19<br />
(IS&T, Springfield, VA, 2003) pp. 415–419.<br />
4 D. Bugner, R. Van Hanehem, P. Artz, <strong>and</strong> D. Zaccour, Proc. IS&T’s<br />
NIP19 (IS&T, Springfield, VA, 2003) pp. 397–401.<br />
5 J. Geisenberger, K. Saitmacher, H. Macholdt, <strong>and</strong> H. Menzel, Proc.<br />
IS&T’s NIP19 (IS&T, Springfield, VA, 2003) pp. 394–395.<br />
6 S. Wakabayashi, T. Tsutsumi, M. Sakakibara, Y. Nakano, <strong>and</strong> Y. Hidaka,<br />
Proc. IS&T’s NIP19 (IS&T, Springfield, VA, 2003) pp. 203–206.<br />
7 R. A. Barcock <strong>and</strong> A. J. Lavery, J. <strong>Imaging</strong> Sci. Technol. 48, 153–159<br />
(2004).<br />
8 D. Brignone, D. W. Fontani, <strong>and</strong> A. Sismondi, Proc. IS&T 13th<br />
International Symposium on Photofinishing <strong>Technology</strong> (IS&T,<br />
Springfield, VA, 2004) pp. 47–50.<br />
9 C. Halik <strong>and</strong> S. Biry, Asia Pac. Coatings J. 16, 20–21 (2003).<br />
10 S. Kiatkamjornwong, K. Rattanakasamsuk, <strong>and</strong> H. Noguchi, J. <strong>Imaging</strong><br />
Sci. Technol. 47, 149–154 (2003).<br />
11 A. Kase, H. Temmei, T. Noshita, M. Slagt, <strong>and</strong> Y. Toda, Proc. IS&T’s<br />
NIP20 (IS&T, Springfield, VA, 2004) pp. 670–672.<br />
12 X. Zhang <strong>and</strong> Q. Zhu, J. Org. Chem. 62, 5934–5938 (1997), <strong>and</strong><br />
references cited therein.<br />
13 F. B. Hasan <strong>and</strong> S. E. Rodman, Proc. IS&T’s NIP20 (IS&T, Springfield,<br />
VA, 2004) pp. 729–733.<br />
14 F. B. Hasan, J. <strong>Imaging</strong> Sci. Technol. 49, 667–671 (2005).<br />
15 US Patent, pending.<br />
16 US Patent 6,537,410.<br />
17 Designed by M. A. Young, ZINK <strong>Imaging</strong>, LLC.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 423
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 424–430, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Relationship between Paper Properties <strong>and</strong> Fuser Oil<br />
Uptake in a High-speed Digital Xerographic Printing<br />
Fuser<br />
Patricia Lai <strong>and</strong> Ning Yan <br />
University of Toronto, 33 Willcocks Street, Toronto, Ontario M5S 3B3, Canada<br />
E-mail: ning.yan@utoronto.ca<br />
Gordon Sisler<br />
Xerox Research Center of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada<br />
Jay Song<br />
Cincinnati <strong>Technology</strong> Center, International Paper Company, 6283 Tri-Ridge Blvd,<br />
Lovel<strong>and</strong>, Ohio 45140-8318<br />
Abstract. The fuser roll in high-speed xerographic printers is typically<br />
coated with a silicon-based oil to facilitate clean splitting of the<br />
fused image <strong>and</strong> paper as it exits the fusing nip. If oil uptake by<br />
paper is excessive, the fuser roll will become insufficiently coated<br />
with oil. Consequently, both image quality <strong>and</strong> fuser roll life will be<br />
negatively impacted. A variety of commercial papers were characterized<br />
<strong>and</strong> evaluated <strong>for</strong> their oil uptake per<strong>for</strong>mance. Using partial<br />
least-squares regression, key paper properties correlated with oil<br />
uptake were identified. Surface energy of paper was found to have<br />
the strongest correlation with oil uptake. Furthermore, based on<br />
contact mechanic theories, a contact area index (CI) was used to<br />
describe the degree of contact between paper <strong>and</strong> the fuser roll at<br />
the nip. A positive correlation between oil uptake <strong>and</strong> CI suggests<br />
that roughness <strong>and</strong> stiffness also have significant influences on oil<br />
uptake. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5424<br />
INTRODUCTION<br />
High-speed digital xerographic printing presses are versatile<br />
with the capability to produce high resolution prints on a<br />
variety of media at rapid speeds. The focus of this study is<br />
on the fusing section, where the image is fixed onto paper.<br />
Hot roll fusing is the most common fixing method <strong>for</strong> these<br />
printers, where toner is fused onto the substrate by heat <strong>and</strong><br />
pressure within a nip <strong>for</strong>med by two rotating rolls, which are<br />
typically a fuser roll <strong>and</strong> a pressure roll 1 (Figure 1).<br />
Clean splitting between the fuser roll <strong>and</strong> paper with a<br />
toner image must be achieved to produce a sharp image<br />
without offset at high reproduction speeds. To facilitate this,<br />
the fuser roll is typically lubricated with a release agent, in<br />
most cases a silicon-based oil, that is applied continuously<br />
using a donor roll. The printing media takes away oil at the<br />
fusing nip <strong>and</strong> over time, with high volume production, excessive<br />
oil uptake may occur, resulting in oil depletion within<br />
the system. Consequently, the fuser roll will become insufficiently<br />
coated, resulting in toner offset, poor print quality,<br />
<strong>and</strong> a reduction in fuser roll life.<br />
Over the years, extensive research in the area of fusing in<br />
xerographic printing has focused on the interaction between<br />
toner <strong>and</strong> paper in terms of the effect of paper properties,<br />
toner properties, <strong>and</strong> operational conditions on fusing fix. 2–4<br />
However, an important aspect in the fusing process that has<br />
not been well studied is the effect of fuser roll lubrication on<br />
print quality. It has been observed that the oil depletion rate<br />
varies with different types of papers in high-speed xerographic<br />
printing, but thus far the specific paper properties<br />
that control this oil depletion rate are not clear.<br />
This study aims to bridge this gap in the literature by<br />
identifying the key paper properties that control fuser oil<br />
uptake <strong>and</strong> by obtaining a mechanistic underst<strong>and</strong>ing of the<br />
interactions between oil, paper, <strong>and</strong> the fuser roll material.<br />
An underst<strong>and</strong>ing of the interactions between fuser oil <strong>and</strong><br />
paper is of fundamental importance in improving fuser roll<br />
life <strong>and</strong> print quality. Meanwhile, underst<strong>and</strong>ing these interactions<br />
will help papermakers optimize paper properties to<br />
minimize fuser oil consumption <strong>and</strong> eliminate downstream<br />
problems, such as oil streaks on the printed copy.<br />
<br />
IS&T Member.<br />
Received Jan. 22, 2007; accepted <strong>for</strong> publication May 10, 2007.<br />
1062-3701/2007/515/424/7/$20.00.<br />
Figure 1. Schematic of the fusing section in a high-speed xerographic<br />
printing press.<br />
424
Lai et al.: Relationship between paper properties <strong>and</strong> fuser oil uptake in a high-speed digital xerographic printing fuser<br />
Table I. Properties measured <strong>for</strong> all paper samples.<br />
Property Range Test Method<br />
Basis weight 76–269 g / m 2 TAPPI T410<br />
Caliper 0.10–0.24 mm TAPPI T411<br />
Porosity 12–3500 Gurley sec TAPPI T460<br />
Bending stiffness MD 0.84–32.6 mN TAPPI T543<br />
rms roughness 1.20–5.83 µm WYKO NT-2000<br />
Surface free energy 35.1–51.2 mJ/ m 2 Wu’s geometric mean<br />
method<br />
Contact angle of oil<br />
on paper<br />
57.9°–67.2° See Materials <strong>and</strong> Method<br />
Section<br />
MATERIALS AND METHODS<br />
Characterizing the Test Paper Set<br />
The test paper set consisted of 16 commercial papers<br />
(samples 1–16), five of which were uncoated papers (samples<br />
1, 4, 5, 7, 8), <strong>and</strong> the remaining were coated papers with<br />
varying coating compositions. A typical commercial siliconbased<br />
fuser oil with a viscosity of 5.75 cm 2 /s 575 cSt at<br />
25°C <strong>and</strong> surface tension of 20.6 mJ/m 2 at 25°C was used in<br />
this investigation.<br />
All samples were characterized <strong>for</strong> their physical, topographical<br />
<strong>and</strong> surface chemistry properties. Physical properties<br />
(basis weight, porosity, caliper, bending stiffness) were<br />
determined <strong>for</strong> each sample according to the st<strong>and</strong>ard test<br />
methods listed in Table I.<br />
Surface topography of each sample was evaluated in<br />
terms of the root-mean-square (rms) roughness. Measurements<br />
were obtained using WYKO NT-2000, an optical<br />
noncontact surface profiler system. Surface profile measurement<br />
data were collected <strong>for</strong> each sample. The data were<br />
median smoothened, <strong>and</strong> the tilt in the data was removed to<br />
ensure that a completely horizontal surface was analyzed.<br />
The surface chemistry of paper was characterized in<br />
terms of contact angle measurements of fuser oil on paper,<br />
surface free energy of paper, <strong>and</strong> work of adhesion between<br />
paper <strong>and</strong> oil. Static contact angles of fuser oil on all the<br />
papers were measured using the Fibro DAT contact angle<br />
machine. All samples were conditioned <strong>for</strong> at least 24 h in a<br />
controlled room at 23°C <strong>and</strong> 50% relative humidity (T402<br />
TAPPI St<strong>and</strong>ard). The contact angle measurements were also<br />
carried out in the conditioned room. For each drop of oil on<br />
the paper substrate, the static contact angle was measured as<br />
a function of time due to spreading on the substrate. For<br />
each paper sample, 16 contact angle-time profiles were obtained<br />
<strong>and</strong>, from these profiles, the contact angles at 1 sec<br />
were averaged. This resulted in an average contact angle of<br />
oil on paper at 1 sec <strong>for</strong> each sample.<br />
The surface free energy of paper <strong>and</strong> the work of adhesion<br />
between paper <strong>and</strong> oil were also determined <strong>for</strong> all<br />
samples. Wu’s geometric mean method, with diiodomethane<br />
<strong>and</strong> water as test liquids, was used to determine the surface<br />
free energy of the papers. 5 Literature values <strong>for</strong> the surface<br />
tension, dispersive <strong>and</strong> polar components <strong>for</strong> diiodomethane<br />
( D =50.8 mJ/m 2 ; D D =49.5 mJ/m 2 ; D P =1.3 mJ/m 2 )<br />
<strong>and</strong> water ( W =72.2 mJ/m 2 ; W D =22 mJ/m 2 ;<br />
W P =50.2 mJ/m 2 ) were used. 5 The work of adhesion between<br />
paper <strong>and</strong> oil at ambient conditions W PO,AMB was<br />
calculated using the Young–Dupré equation. 6 A W PO,AMB<br />
value was calculated <strong>for</strong> each paper using ambient contact<br />
angle measurements of fuser oil on paper at 1 sec. The surface<br />
tension of fuser oil at ambient temperature, OIL,AMB<br />
=20.6 mJ/m 2 , was used. 7<br />
Evaluating the Oil Uptake Per<strong>for</strong>mance of the Test Paper<br />
Set<br />
All paper samples were evaluated <strong>for</strong> their oil uptake per<strong>for</strong>mance<br />
by quantifying the amount of oil that each paper<br />
sorbed in the fusing nip. The amount of oil uptake <strong>for</strong> all<br />
blank paper samples was obtained using a prototype highspeed<br />
xerographic fusing system <strong>and</strong> inductively coupled<br />
plasma mass spectroscopy (ICP-MS). The detailed experimental<br />
procedure is described in a previous paper. 8<br />
Partial Least-Squares Regression Modeling<br />
After characterizing the papers <strong>and</strong> obtaining oil uptake<br />
amounts <strong>for</strong> each sample, these two data sets were analyzed<br />
using partial least-squares (PLS) regression to determine the<br />
correlation structure between paper properties (input or x<br />
variables) <strong>and</strong> oil uptake (output or y variable). Partial leastsquares<br />
(PLS) regression is an established multivariate statistical<br />
method that can extract the correlation structure between<br />
input <strong>and</strong> output variables from experimental data. 9<br />
The dimensions of large data sets are reduced by projecting<br />
the relevant in<strong>for</strong>mation onto low-dimension subspaces defined<br />
by principal components. 10 Using graphical methods,<br />
the data can then be analyzed easily to underst<strong>and</strong> processes<br />
<strong>and</strong> to identify key factors that predict response variables.<br />
In this study, partial least-squares (PLS) regression is<br />
used to model the relationship between paper properties x<br />
<strong>and</strong> oil uptake y. Traditionally, multiple linear regression<br />
(MLR) is used in relating a response variable y toasetof<br />
x variables, but PLS is more robust because unlike MLR, PLS<br />
can h<strong>and</strong>le large numbers of x variables, highly dependant<br />
variables, <strong>and</strong> noisy data. 10 Simca-P software by Umetrics<br />
was used to analyze the data.<br />
The PLS model per<strong>for</strong>mance was assessed using the<br />
loading plot <strong>and</strong> the parameters, R 2 Y <strong>and</strong> Q 2 . R 2 Y is a multiple<br />
correlation coefficient between the measured response<br />
y <strong>and</strong> the predicted response ŷ PRED values. The Q 2 parameter<br />
denotes the model’s ability to predict the response.<br />
An ideal model 10 has R 2 Y <strong>and</strong> Q 2 values of 0.5 while minimizing<br />
, the difference between R 2 Y <strong>and</strong> Q 2 . Further details<br />
of the PLS regression method used in this study are<br />
given in a previous paper. 8<br />
RESULTS AND DISCUSSION<br />
Test Paper Set<br />
All papers were characterized <strong>and</strong> the samples cover a broad<br />
range of the assessed properties as summarized in Table I.<br />
Paper samples range from text weight to cover stock <strong>for</strong> both<br />
coated <strong>and</strong> uncoated papers.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 425
Lai et al.: Relationship between paper properties <strong>and</strong> fuser oil uptake in a high-speed digital xerographic printing fuser<br />
Figure 2. Oil uptake amounts <strong>for</strong> all paper samples. Sheet size is 8.5<br />
11 in.<br />
Oil Uptake Data<br />
Oil uptake data <strong>for</strong> all paper samples are shown in Figure 2.<br />
It is important to note that the experimental technique used<br />
to quantify oil uptake results in higher oil uptake values than<br />
observed when printing on a commercial press. There<strong>for</strong>e,<br />
the results should be interpreted in terms of relative oil uptake<br />
amounts among the paper samples.<br />
In general, coated papers appear to have higher oil uptake<br />
amounts than uncoated papers. However, some coated<br />
papers, such as sample 6, have small oil uptake values that<br />
are similar to that of uncoated papers. To gain insight into<br />
the relationships between paper properties <strong>and</strong> oil uptake,<br />
the oil uptake data are correlated with the properties of all<br />
the samples using PLS regression.<br />
Figure 3. Loading plot <strong>for</strong> the PLS model. Key paper properties x correlated<br />
with oil uptake y are identified. R 2 Y=0.72 <strong>and</strong> Q 2 =0.61 with<br />
=R 2 Y−Q 2 =0.11.<br />
Figure 4. Observed oil uptake data y measured vs predicted oil uptake<br />
ŷ PLS predicted values from the PLS model. R 2 Y=0.72 <strong>and</strong> Q 2 =0.61. Each<br />
data point represents a different paper sample.<br />
Key Paper Properties Identified by the PLS Regression<br />
Model<br />
PLS regression was per<strong>for</strong>med on the oil uptake y <strong>and</strong><br />
paper properties x data sets. All the paper properties listed<br />
inTableIwereusedasx-variable inputs into the initial PLS<br />
regression model. The data were mean centered <strong>and</strong> scaled<br />
to unit variance. One principal component, calculated<br />
through cross-validation, was found to sufficiently explain<br />
the variance in the data, resulting in R 2 Y <strong>and</strong> Q 2 values of<br />
0.72 <strong>and</strong> 0.61 respectively with =R 2 Y−Q 2 =0.11.<br />
The correlation structure between paper properties <strong>and</strong><br />
oil uptake are identified in the loading plot (Figure 3). Variables<br />
with a PLS weighting factor, w * c, of the same magnitude<br />
<strong>and</strong> sign as that of oil uptake have strong positive correlations,<br />
while variables with negative w * c values have<br />
negative correlations with oil uptake. rms roughness <strong>and</strong><br />
contact angle measurements, <strong>for</strong> example, are furthest from<br />
zero in the negative direction w * c−0.5, <strong>and</strong> thus have a<br />
strong negative correlation with oil uptake. Surface free energy<br />
has the highest PLS weight w * c0.5, <strong>and</strong> there<strong>for</strong>e<br />
has the strongest positive correlation with oil uptake. Basis<br />
weight, bending stiffness, <strong>and</strong> caliper have moderate positive<br />
correlations with oil uptake, with PLS weight values between<br />
0.2 <strong>and</strong> 0.4. With the lowest PLS weight w * c0.2, porosity<br />
based on Gurley seconds has the weakest positive correlation<br />
with oil uptake. There<strong>for</strong>e, the loading plot reveals that papers<br />
with high roughness <strong>and</strong> high surface energy correspond<br />
with large oil uptake amounts.<br />
For this PLS regression model, Figure 4 compares the<br />
observed y measured <strong>and</strong> the predicted values ŷ PLS predicted of<br />
oil uptake. The model’s ability to predict oil uptake well is<br />
shown in Fig. 4, where a good scatter of the data points<br />
around the 45° line is present.<br />
The equation <strong>for</strong> the final PLS regression model is<br />
ŷ PLS predicted = 0.22x SFE + 0.16x BWT + 0.13x STF + 0.11x CAL<br />
+ 0.08x POR − 0.22x RMS − 0.20x CA + 6.4, 1<br />
where SFEsurface free energy of paper, BWTbasis<br />
weight, STFbending stiffness, CALcaliper,<br />
PORporosity in Gurley seconds, RMSrms roughness,<br />
<strong>and</strong> CAambient contact angle measurements of oil on paper.<br />
The coefficients are based on mean centered <strong>and</strong> unit<br />
variance scaled data. To accurately predict absolute values<br />
<strong>for</strong> oil uptake, this model must be validated with an external<br />
data set, consisting of a large number of new <strong>and</strong> well characterized<br />
paper samples.<br />
426 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lai et al.: Relationship between paper properties <strong>and</strong> fuser oil uptake in a high-speed digital xerographic printing fuser<br />
Table II. Ambient work of adhesion between fuser oil <strong>and</strong> paper <strong>for</strong> all paper samples.<br />
Paper Sample No.<br />
W PO,AMB = OIL 1 + cos <br />
mJ/m 2 <br />
Figure 5. Oil uptake vs surface free energy of paper, R 2 =0.65.<br />
Mechanistic Underst<strong>and</strong>ing of Fuser Oil-Paper<br />
Interactions in the Fusing Nip<br />
Although PLS regression reveals positive <strong>and</strong> negative correlations<br />
between x <strong>and</strong> y variables, PLS regression does not<br />
ascertain cause <strong>and</strong> effect relationships between the two data<br />
sets. There<strong>for</strong>e, it is necessary to gain a mechanistic underst<strong>and</strong>ing<br />
of how these interacting paper properties affect the<br />
oil uptake process. The key properties identified by PLS suggest<br />
that oil uptake is highly related to two effects: the wetting<br />
behavior of oil on paper <strong>and</strong> the nip contact area between<br />
paper <strong>and</strong> the fuser roll. These two mechanisms are<br />
further examined to explain how oil is physically transferred<br />
to paper within the fusing nip.<br />
Wetting Behavior of Fuser Oil on Paper<br />
From the PLS regression model, a negative correlation between<br />
oil uptake <strong>and</strong> ambient contact angle measurements<br />
of oil on paper was observed. As the contact angle of oil on<br />
paper decreases, wetting increases <strong>and</strong> the result is more oil<br />
uptake. There<strong>for</strong>e, contact angle measurements of oil on paper<br />
at ambient conditions can be used as a tool to predict<br />
the wetting behavior of oil on paper during uptake in a<br />
fusing nip.<br />
The wetting behavior of oil on paper was further investigated<br />
by looking at the effect of surface free energy (SFE)<br />
of paper on oil uptake. From the PLS model, a positive<br />
correlation between oil uptake <strong>and</strong> SFE is present. The surface<br />
tension of the fuser oil at room temperature is low<br />
20.6 mJ/m 2 , <strong>and</strong> the oil will preferentially wet papers that<br />
exhibit higher surface energies. As the surface energy of paper<br />
increases, further wetting of oil occurs <strong>and</strong> the result is<br />
an increase in oil uptake. This result follows the same approach<br />
by S<strong>and</strong>ers et al., 11 where it was found that higher<br />
energy surfaces promoted wetting <strong>and</strong> spreading of molten<br />
toner on paper. The positive correlation between oil uptake<br />
<strong>and</strong> SFE can also be seen in the independent linear regression<br />
plot of these two variables (Figure 5), where an increase<br />
in SFE of paper results in an increase in oil uptake, as the<br />
surface tension of oil remains constant. With a good positive<br />
correlation R 2 =0.65 between these two variables, SFE of<br />
paper is shown to be an important paper property in explaining<br />
the effect of wetting on oil uptake; there<strong>for</strong>e, it is a<br />
Uncoated papers 1 28.80<br />
4 29.58<br />
5 28.70<br />
7 29.02<br />
Coated papers 2 31.45<br />
3 31.40<br />
6 31.51<br />
9 30.80<br />
10 31.15<br />
11 31.49<br />
12 31.20<br />
13 31.56<br />
14 31.51<br />
15 31.14<br />
16 30.01<br />
mechanism by which the oil uptake trend can be partially<br />
explained. It is important to note that while the correlation<br />
between oil uptake <strong>and</strong> SFE of paper is determined at ambient<br />
conditions, it is expected that the surface energy of the<br />
paper will not change significantly with fusing temperature<br />
(185°C) due to a short residence time of the paper within<br />
the fusing nip 30 ms.<br />
The surface chemistry effect of paper on oil uptake is<br />
also investigated by looking at the ambient work of adhesion<br />
between paper <strong>and</strong> oil W PO,AMB . The calculated values <strong>for</strong><br />
all paper samples are listed in Table II. In general, the work<br />
of adhesion between oil <strong>and</strong> uncoated papers is slightly<br />
lower than the work of adhesion between oil <strong>and</strong> coated<br />
papers. Figure 6 shows a positive correlation between oil<br />
uptake <strong>and</strong> W PO,AMB . This indicates that to minimize oil<br />
uptake the work necessary to separate oil from paper must<br />
be minimized, which is achieved by lowering the surface free<br />
energy of the paper surface.<br />
Figure 6. Oil uptake vs ambient work of adhesion between paper <strong>and</strong><br />
oil, W PO,AMB .<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 427
Lai et al.: Relationship between paper properties <strong>and</strong> fuser oil uptake in a high-speed digital xerographic printing fuser<br />
Relative Contact Area Between Paper <strong>and</strong> the Fuser Roll<br />
in the Nip<br />
Oil uptake is also examined with respect to the contact area<br />
between paper <strong>and</strong> oil within the fusing nip. This contact<br />
area is examined on the microscopic scale, where surfaces<br />
exhibit asperity peaks <strong>and</strong> valleys. As a result, when two<br />
surfaces touch, the real contact area is less than the nominal<br />
contact area. The relative contact area between paper <strong>and</strong> the<br />
fuser roll at the nip is modeled using theory from contact<br />
mechanics <strong>and</strong> in relation to the identified key nonsurface<br />
chemistry related properties (basis weight, caliper, stiffness,<br />
<strong>and</strong> rms roughness). The transfer of oil from the fuser roll to<br />
paper is then examined as a function of this relative contact<br />
area at the nip.<br />
Greenwood <strong>and</strong> Williamson developed a theory <strong>for</strong> the<br />
elastic contact of rough surfaces that takes into account surface<br />
topography <strong>and</strong> elastic de<strong>for</strong>mation based on Hertz<br />
theory. 12 The model assumed that contact occurs at the asperity<br />
peaks of the surface, <strong>and</strong> that the total contact area is<br />
the sum of the contacting areas of the peaks. According to<br />
Greenwood <strong>and</strong> Williamson, 12 <strong>for</strong> a negative exponentially<br />
distributed rough surface, the relative contact area parameter,<br />
A re , is defined as<br />
A re =k E,<br />
P 2<br />
where is the root-mean-square (rms) roughness value, E<br />
is a complex modulus defined in Eq. (3), <strong>and</strong> k is the rms<br />
value of the peak curvature <strong>and</strong> is defined in Eq. (4), 12<br />
1 1−<br />
E = paper <br />
+<br />
E paper<br />
2<br />
1− 2 roll <br />
. 3<br />
E roll<br />
A re is a dimensionless parameter describing the relative<br />
contact area between two rough surfaces or the ratio of real<br />
contact area to nominal contact area. This parameter describes<br />
the contact area between surfaces as a function of<br />
surface texture <strong>and</strong> compressibility of the substrate. The surface<br />
texture parameter was also defined as a dimensionless<br />
parameter, R f =k, by Yan <strong>and</strong> Aspler 13 ; <strong>and</strong> k are obtained<br />
from surface profile measurement data using Eq. (4),<br />
where z i−1 , z i , <strong>and</strong> z i+1 are three consecutive height measurements;<br />
h is the horizontal step interval between the height<br />
measurements; <strong>and</strong> N A is the total number of peaks measured.<br />
The rms roughness <strong>and</strong> k values <strong>for</strong> the test paper set<br />
are shown in Table III, <strong>and</strong> these values are in agreement<br />
4<br />
Table III. rms roughness <strong>and</strong> curvature parameter k values <strong>for</strong> all paper samples.<br />
Paper<br />
Sample<br />
No.<br />
rms<br />
Roughness<br />
µm<br />
St<strong>and</strong>ard<br />
Deviation<br />
<strong>for</strong> rms Roughness<br />
with those published by Yan <strong>and</strong> Aspler. 13 As expected, rms<br />
roughness values <strong>for</strong> uncoated papers are greater than those<br />
of coated papers. Curvature parameters appear to be greater<br />
<strong>for</strong> uncoated papers than <strong>for</strong> coated papers.<br />
Based on the work by Greenwood <strong>and</strong> Williamson 12 as<br />
well as Yan <strong>and</strong> Aspler, 13 a contact area index (CI) is developed<br />
to describe the degree of contact between the paper<br />
<strong>and</strong> the fuser roll at the nip [Eq. (5)]. This index represents<br />
the relative contact area as a function of rms roughness ,<br />
asperity peak curvature k, <strong>and</strong> complex modulus E. E<br />
combines the modulus of paper <strong>and</strong> that of the fuser roll. In<br />
this study, the fuser roll was silicon rubber <strong>and</strong> <strong>for</strong> Eq. (3),<br />
E roll =2 MPa, <strong>and</strong> Poisson’s ratio, roll =0.5, at 175°C were<br />
used 7 as estimated values <strong>for</strong> the fuser roll at fusing temperature.<br />
These values remained constant throughout the study.<br />
E paper was calculated using the bending stiffness of paper S<br />
<strong>and</strong> Eq. (6). 14 Changes in Poisson’s ratio among the paper<br />
samples were assumed to be negligible, <strong>and</strong> a constant Poisson’s<br />
ratio of paper =0.4 was used. 15 Pressure P within the<br />
fusing nip was estimated as a function of caliper (C), where<br />
P=C <strong>and</strong> is a constant, which depends on the spring<br />
constant of the nip,<br />
CI =<br />
A re<br />
= 1 C<br />
k E ,<br />
E paper = 12S<br />
C 3 .<br />
Average k<br />
1/µm<br />
St<strong>and</strong>ard<br />
Deviation<br />
<strong>for</strong> k<br />
Uncoated papers 1 5.83 0.38 1.35 0.05<br />
4 3.24 0.11 0.74 0.04<br />
5 4.84 0.17 1.21 0.03<br />
7 4.10 0.38 1.02 0.14<br />
8 4.73 0.05 1.16 0.12<br />
Coated papers 2 1.25 0.01 0.17 0.02<br />
3 1.20 0.11 0.20 0.02<br />
6 1.40 0.04 0.20 0.01<br />
9 1.61 0.11 0.30 0.04<br />
10 1.44 0.03 0.24 0.01<br />
11 1.61 0.06 0.25 0.01<br />
12 1.81 0.13 0.29 0.02<br />
13 1.80 0.16 0.26 0.01<br />
14 1.58 0.09 0.24 0.01<br />
15 1.50 0.02 0.28 0.004<br />
16 1.26 0.05 0.17 0.01<br />
Although CI is not a measurement <strong>for</strong> the actual contact<br />
area between paper <strong>and</strong> the fuser roll at the nip, it is an<br />
5<br />
6<br />
428 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lai et al.: Relationship between paper properties <strong>and</strong> fuser oil uptake in a high-speed digital xerographic printing fuser<br />
Table IV. Contact area index, CI, values <strong>for</strong> all paper samples.<br />
Paper Sample No. CI 10 −7 StDev 10 −7<br />
Uncoated papers 1 0.21 0.007<br />
4 0.36 0.12<br />
5 0.19 0.08<br />
7 0.35 0.02<br />
8 0.17 0.002<br />
Coated papers 2 2.23 0.03<br />
3 0.96 0.08<br />
6 1.99 0.01<br />
9 0.63 0.06<br />
10 0.86 0.02<br />
11 0.79 0.03<br />
12 0.70 0.04<br />
13 0.73 0.03<br />
14 0.82 0.04<br />
15 0.78 0.002<br />
16 0.90 0.05<br />
Figure 7. Positive correlation between oil uptake <strong>and</strong> contact area index<br />
CI is illustrated.<br />
indication of the degree of contact that would result when<br />
paper contacts the fuser roll. A paper sample with a lower CI<br />
value indicates a lower relative contact area with the roll than<br />
that of a paper with a higher CI value. A CI value was<br />
calculated <strong>for</strong> each sample, which allowed <strong>for</strong> a relative comparison<br />
among papers <strong>for</strong> their degree of contact with the<br />
fuser roll. The results are summarized in Table IV.<br />
When two surfaces touch, the actual contact is assumed<br />
to occur at the tips of the paper surface asperity peaks. 12 In<br />
the case of coated or smooth papers, the asperity peaks are<br />
broader than the asperity peaks on uncoated or rougher papers.<br />
As a result, the peak surface area in contact with the<br />
fuser roll is larger <strong>for</strong> coated papers, <strong>and</strong> CI will be greater<br />
<strong>for</strong> coated papers. The trend in Table IV, that coated papers<br />
have greater CI values than uncoated papers, is in good<br />
agreement with this explanation. Furthermore, as peak contact<br />
area increases with the fuser roll or as CI increases,<br />
further oil uptake by paper is expected. Figure 7 shows a<br />
positive linear trend between oil uptake <strong>and</strong> CI. While R 2 is<br />
not too high, the positive trend indicates that surface texture<br />
(roughness) <strong>and</strong> stiffness of paper (which encompasses effects<br />
from basis weight <strong>and</strong> caliper) have an affect on oil<br />
uptake.<br />
The approach in applying contact mechanics to examine<br />
oil uptake by paper has been simplified, where nip residence<br />
time <strong>and</strong> oil thickness were considered constant due<br />
to steady-state conditions in the fusing apparatus. A good<br />
basis <strong>for</strong> underst<strong>and</strong>ing oil uptake by paper during fusing<br />
has been established, <strong>and</strong> further study is warranted to investigate<br />
the affect of oil rheology on the oil uptake process.<br />
Simplified Model <strong>for</strong> Oil Uptake<br />
Overall, the surface chemistry of paper <strong>and</strong> the degree of<br />
contact that occurs between paper <strong>and</strong> the fuser roll at the<br />
Figure 8. Oil uptake measured y measured vs oil uptake predicted by the<br />
MLR model ŷ MLR predicted , R 2 =0.8.<br />
nip are shown to be significant parameters in predicting <strong>and</strong><br />
underst<strong>and</strong>ing oil uptake. These parameters characterize two<br />
separate mechanisms by which paper takes up oil from the<br />
fuser roll. Surface free energy characterizes the dominant<br />
surface chemistry effect of paper within the fusing nip,<br />
where wetting of oil on paper is promoted as paper surface<br />
energy increases. CI, on the other h<strong>and</strong>, accounts <strong>for</strong> the<br />
effects of physical paper properties on oil uptake, where an<br />
increase in nip contact area between paper asperity peaks<br />
<strong>and</strong> the fuser roll results in an increase in oil uptake. There<strong>for</strong>e,<br />
with CI <strong>and</strong> SFE as dominant factors in describing oil<br />
uptake by paper, a multiple linear regression (MLR) model is<br />
developed using CI <strong>and</strong> SFE as independent parameters. After<br />
scaling CI <strong>and</strong> SFE to unit variance, the MLR model is<br />
ŷ MLR predicted = 2.25x SFE + 1.27x CI − 1.65,<br />
with R 2 =0.8. Figure 8 compares y measured with ŷ MLR predicted ,<br />
which shows that SFE <strong>and</strong> CI describe oil uptake well.<br />
Figure 9 is a three-dimensional (3D) contour plot summarizing<br />
the relationship between oil uptake, surface free<br />
energy (SFE) of paper, <strong>and</strong> contact area index (CI). The<br />
linear relationship between oil uptake <strong>and</strong> CI, <strong>and</strong> between<br />
oil uptake <strong>and</strong> SFE are evident from this plot.<br />
Of the paper properties included in the PLS model, further<br />
study is required to ascertain the relationship between<br />
7<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 429
Lai et al.: Relationship between paper properties <strong>and</strong> fuser oil uptake in a high-speed digital xerographic printing fuser<br />
Figure 9. The 3D contour plot describes the relationship between oil<br />
uptake, surface free energy of paper, <strong>and</strong> contact area index CI.<br />
porosity <strong>and</strong> oil uptake. It is expected that porosity may play<br />
a minor role in the initial transfer of oil from the fuser roll to<br />
paper. The effects of pore size, volume, <strong>and</strong> distribution on<br />
oil uptake all require thorough investigation. In addition,<br />
work of adhesion between oil <strong>and</strong> paper at fusing conditions<br />
has a good potential <strong>for</strong> describing the oil uptake process<br />
within the fusing nip. At ambient conditions, W PO,AMB was<br />
found to have a positive correlation with oil uptake; however,<br />
with work of adhesion values calculated at conditions<br />
similar to fusing conditions W PO,fusing , the correlation between<br />
W PO,fusing <strong>and</strong> oil uptake may improve. There<strong>for</strong>e,<br />
W PO,fusing may also help to account <strong>for</strong> the unexplained variance<br />
in the oil uptake data. Moreover, surface tension of oil<br />
<strong>and</strong> accurate contact angle measurements of oil on paper at<br />
fusing conditions require further investigation in order to<br />
accurately calculate W PO,fusing .<br />
CONCLUSIONS<br />
A systematic study was carried out to investigate the relationship<br />
between fuser oil uptake <strong>and</strong> paper properties in<br />
high-speed xerographic printing. A partial least-squares<br />
(PLS) regression model was developed to identify the key<br />
paper properties that are significantly correlated with the<br />
uptake of a typical silicon-based oil; rms roughness <strong>and</strong> ambient<br />
contact angle measurements of the oil on paper were<br />
found to have strong negative correlations with oil uptake.<br />
Caliper, basis weight, bending stiffness, <strong>and</strong> surface free energy<br />
had positive correlations with oil uptake.<br />
Contact angle measurements <strong>and</strong> SFE were both included<br />
in the PLS model. Although both parameters describe<br />
the surface chemistry of paper, the contact angle measurements<br />
were more convenient to obtain as it involved<br />
only one fluid, fuser oil. The PLS results showed that both<br />
parameters indicate the same trend with oil uptake, providing<br />
the experimenter with confidence to use the more convenient<br />
method to determine the correlation between oil<br />
uptake <strong>and</strong> surface chemistry of paper. Furthermore, the<br />
predictive ability of this PLS model will improve with validation<br />
using a new paper set.<br />
Surface free energy (SFE) of paper was found to have<br />
the strongest positive correlation with oil uptake in the PLS<br />
regression model, <strong>and</strong> this positive correlation was also confirmed<br />
in a linear regression of the two parameters with an<br />
R 2 =0.65. There<strong>for</strong>e, SFE was determined to be a dominant<br />
property that describes the wetting behavior of oil on paper.<br />
A contact area index (CI) was used to compare the degree<br />
of contact between paper <strong>and</strong> the fuser roll at the nip.<br />
CI was developed as a function of surface texture (rms<br />
roughness <strong>and</strong> surface curvature), nip pressure as estimated<br />
as a function of caliper, <strong>and</strong> elastic moduli of paper <strong>and</strong> the<br />
fuser roll. CI <strong>and</strong> oil uptake were found to have a positive<br />
correlation, suggesting that surface texture <strong>and</strong> stiffness of<br />
the paper have significant influences on oil uptake.<br />
A multiple linear regression model was developed by<br />
regressing oil uptake against CI <strong>and</strong> SFE. Both CI <strong>and</strong> SFE<br />
were treated as independent factors affecting oil uptake.<br />
These two factors were found to be dominant parameters in<br />
predicting oil uptake, as the MLR model accounted <strong>for</strong> 80%<br />
of the variance in the oil uptake data. There<strong>for</strong>e, oil uptake<br />
can be explained mechanistically in terms of the wetting<br />
behavior of oil on paper <strong>and</strong> by a contact area index to<br />
describe the degree of contact between paper <strong>and</strong> the fuser<br />
roll at the nip. Other factors that potentially may have significant<br />
affects on oil uptake are surface porosity of paper<br />
<strong>and</strong> the work of adhesion between oil <strong>and</strong> paper at fusing<br />
conditions. Both parameters require further investigation.<br />
ACKNOWLEDGMENTS<br />
Financial support from NSERC Canada <strong>and</strong> the member<br />
companies of the Surface <strong>Science</strong> Consortium at the University<br />
of Toronto Pulp <strong>and</strong> Paper Centre are gratefully acknowledged.<br />
As well, the support from Xerox <strong>and</strong> International<br />
Paper in carrying out experimental work is gratefully<br />
appreciated.<br />
REFERENCES<br />
1 L. Leroy, V. Morin, <strong>and</strong> A. G<strong>and</strong>ini, “Electrophotography: Effects of<br />
printer parameters on fusing quality”, Proc. 11th International Printing<br />
<strong>and</strong> Graphic Arts Conference (TAPPI, Bordeaux, France, 2002).<br />
2 M. Alava <strong>and</strong> K. Niskanen, “The physics of paper”, Institute of Physics<br />
Publishing: Rep. Prog. Phys. 69, 669 (2006).<br />
3 J. S. Aspler, “Interactions of ink <strong>and</strong> water with the paper surface in<br />
printing”, Nord. Pulp Pap. Res. J. 8, 68 (1993).<br />
4 M. B. Lyne <strong>and</strong> J. S. Aspler, “Ink-Paper Interactions in Printing: A<br />
Review”, in Colloids <strong>and</strong> Surfaces in Reprographic <strong>Technology</strong>, edited by<br />
M. Hair <strong>and</strong> M. D. Croucher (ACS, Washington DC, 1982), p. 385.<br />
5 S. Wu, Polymer Interface <strong>and</strong> Adhesion (Marcel Dekker, New York, 1982).<br />
6 A. V. Pocius, Adhesion <strong>and</strong> Adhesives <strong>Technology</strong> (Hanser/Gardner<br />
Publications, New York, 1997).<br />
7 Xerox Research Centre of Canada, private communication.<br />
8 P. Lai <strong>and</strong> N. Yan, “The relationship between paper properties <strong>and</strong> fuser<br />
oil uptake in high-speed xerographic printing”, Proc. 92nd Paptac<br />
Annual Meeting (PAPTAC, Montreal, Quebec, 2006).<br />
9 R. Shi <strong>and</strong> J. F. Macgregor, “Modeling of dynamic systems using latent<br />
variable <strong>and</strong> subspace methods”, J. Chemom. 14, 423 (2000).<br />
10 L. Eriksson, E. Johnansson, N. Kettaneh-Wold, <strong>and</strong> S. Wold, Multi- <strong>and</strong><br />
Megavariate Data Analysis: Principles <strong>and</strong> Applications (Umetrics<br />
Academy, Sweden, 2001).<br />
11 D. J. S<strong>and</strong>ers, D. F. Rutl<strong>and</strong>, <strong>and</strong> W. K. Istone, “Effect of paper properties<br />
on fusing fix”, J. <strong>Imaging</strong> Sci. Technol. 40, 175 (1996).<br />
12 J. A. Greeenwood <strong>and</strong> J. B. P. Williamson, “Contact of nominally flat<br />
surfaces”, Proc. R. Soc. London, Ser. A, 295, 300 (1966).<br />
13 N. Yan <strong>and</strong> J. Aspler, “Surface texture controlling speckle-type print<br />
defects in a hard printing nip”, J. Pulp Pap. Sci. 29, 357 (2003).<br />
14 J. Levlin <strong>and</strong> L. Söderbjelm, Pulp <strong>and</strong> Paper Testing: Papermaking <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong>, Book 17 (Fapet Oy, Finl<strong>and</strong>, 1999).<br />
15 N. Stenberg <strong>and</strong> C. Fellers, “Out-of-plane Poisson’s ratios of paper <strong>and</strong><br />
paperboard”, Nord. Pulp Pap. Res. J. 17, 4 (2002).<br />
430 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 431–437, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Simulation of Traveling Wave Toner Transport<br />
Considering Air Drag<br />
Masataka Maeda<br />
Graduate School of <strong>Science</strong> <strong>and</strong> Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki<br />
316-8511, Japan<br />
Katsuhiro Maekawa<br />
The Research Center <strong>for</strong> Superplasticity, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki<br />
316-8511, Japan<br />
Manabu Takeuchi<br />
Department of Electrical <strong>and</strong> Electronic Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi,<br />
Ibaraki 316-8511, Japan<br />
Abstract. A simulation method to analyze the behavior of a toner<br />
cloud driven by a traveling electrostatic wave with air drag is proposed.<br />
The two-fluid flow model, which regards air <strong>and</strong> toner cloud<br />
as incompressible <strong>and</strong> viscous fluids, is employed. A method of the<br />
calculation of the electric potential using the moving particle semiimplicit<br />
(MPS) method, which is often used in the study of fluid dynamics,<br />
is developed in this article. In the present method, all of the<br />
motion of the toner, the airflow, <strong>and</strong> the electric potential are calculated<br />
by using the moving particle semi-implicit method. Common<br />
calculation points are used in the electrostatic calculation, the toner<br />
motion calculation, <strong>and</strong> the airflow calculation in order to avoid the<br />
complexity of data exchange among those calculations. The validity<br />
of the calculation of the electric potential using the MPS method is<br />
confirmed by comparing the results to those of a model that has a<br />
known strict solution. Modeling of the behavior of toner cloud in<br />
toner transport with a traveling electrostatic wave is per<strong>for</strong>med. An<br />
increase in the maximum synchronous frequency due to air drag is<br />
demonstrated. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong><br />
<strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5431<br />
INTRODUCTION<br />
Since Masuda et al. 1 demonstrated that powders could be<br />
transported by means of a traveling wave of an electric field,<br />
the method has been studied in various applications. In electrophotography,<br />
theoretical <strong>and</strong> experimental studies of<br />
toner transport have been made using this method. Melcher<br />
et al. 2–4 <strong>and</strong> Schmidlin 5,6 provided much insight into the<br />
underst<strong>and</strong>ing of the different modes of toner transport with<br />
a traveling electrostatic wave. Numerical analysis of toner<br />
transport has also been improved. In early studies, the numerical<br />
analyses focused on the motion of a single particle<br />
to avoid the complexity of the interaction between particles.<br />
In recent years, the motion of many particles has been analyzed.<br />
By using the particle-in-cell method <strong>for</strong> many particle<br />
systems, Gartstein <strong>and</strong> Shaw 7 showed that the velocity of<br />
Received Nov. 8, 2006; accepted <strong>for</strong> publication May 1, 2007.<br />
1062-3701/2007/515/431/7/$20.00.<br />
particle flow could vary all the way from zero to the phase<br />
velocity of the traveling wave. Kober 8 numerically solved the<br />
equations of toner motion by superimposing the electric<br />
field due to toner charge <strong>and</strong> image charge on the electric<br />
field applied by the electrodes.<br />
These analyses provided insight into the behavior of<br />
large ensembles of toner particles. However, the calculation<br />
of the motion of the toner <strong>and</strong> the electric field was complicated<br />
by the dual approach of using both mesh <strong>and</strong> particle.<br />
The motion of the toner is calculated using particles,<br />
but the electric field is calculated using a mesh. The variables<br />
are calculated independently <strong>and</strong> must be interpolated between<br />
the mesh <strong>and</strong> the particles at each time step. Moreover,<br />
in a many-particle system, the motion of air dragged by<br />
the toner particles is not negligible. In many previous numerical<br />
analyses of the traveling wave toner transport, the air<br />
was supposed to be quiescent <strong>and</strong> the motion of the air<br />
caused by particle motion was not taken into account. A<br />
calculation method that not only simplifies the calculation of<br />
electric field but also includes the air effect is required.<br />
In order to avoid the complexity of the calculation, we<br />
use calculation points that move with the toner or air rather<br />
than fixed meshes or grids. This technique makes the calculation<br />
of electric field associated with the moving charge <strong>and</strong><br />
moving air more manageable. The smoothed particle hydrodynamics<br />
(SPH) method <strong>and</strong> the moving particle semiimplicit<br />
(MPS) method, which are often used in the study of<br />
fluid dynamics, are well-known modeling methods that use<br />
moving particles.<br />
In this paper, we propose a simulation method that can<br />
solve the problem of toner <strong>and</strong> air motions <strong>and</strong> electric field,<br />
simultaneously, using a particle method. Using the present<br />
method, we calculate the toner motion in traveling wave<br />
toner transport <strong>and</strong> show an overview of the behavior of<br />
toner cloud that is affected by airflow as the speed of traveling<br />
wave increases.<br />
431
Maeda, Maekawa, <strong>and</strong> Takeuchi: Simulation of traveling wave toner transport considering air drag<br />
MODEL<br />
Physical Model<br />
Let us consider the motion of charged toner particles driven<br />
by a traveling electrostatic wave taking into account the effect<br />
of air motion. We examine the effect of electrostatic<br />
<strong>for</strong>ce <strong>and</strong> airflow as a major factor of basic driving mechanism<br />
of toner transport with traveling wave.<br />
The electric potential <strong>for</strong>med by toner charge is given<br />
· = Q,<br />
where is the permittivity of media <strong>and</strong> Q is the space<br />
charge density due to toner charge. The electric field can be<br />
expressed as<br />
E =−.<br />
In the analysis of a solid-gas flow, such as motion of<br />
toner particles in air, the flow structure is different according<br />
to whether the particulate phase is dispersed in a fluid phase<br />
or separated from a fluid phase. It was reported that toner<br />
particles are transported in b<strong>and</strong>s that are spaced by the<br />
wavelength of traveling wave. 4 There<strong>for</strong>e, the objective of our<br />
method to be developed is to obtain an overview of the<br />
behavior of a cluster of toner particles rather than of a discrete<br />
toner particle. We regard a cluster of toner particles in<br />
a b<strong>and</strong> as a separated phase <strong>and</strong> as an incompressible fluid.<br />
In other words, we employ the two-fluid flow model of air<br />
<strong>and</strong> toner cloud in this study.<br />
From mass <strong>and</strong> momentum conservation <strong>and</strong> incompressibility,<br />
the equations <strong>for</strong> air are<br />
D a<br />
Dt =0,<br />
1<br />
2<br />
3<br />
Du a<br />
=− 1 P a + a 2 u a + g − 1 F at , 4<br />
Dt a a<br />
air by pressure <strong>and</strong> viscosity of toner cloud. F ta is the interaction<br />
<strong>for</strong>ce that is exerted on toner cloud by pressure <strong>and</strong><br />
viscosity of air. D/Dt is denotes the Lagrangian time derivative.<br />
Since we discretize these equations using a calculation<br />
point, as discussed later, these are described as calculation<br />
points <strong>for</strong> air <strong>and</strong> toner, not <strong>for</strong> a volume. Thus, the concentration<br />
of toner cloud is not expressed in the equations.<br />
Numerical Model<br />
In order to avoid the complicated procedure of interpolating<br />
between grid <strong>and</strong> particle, only calculation points are employed<br />
in the calculation of the electric field, motion of<br />
toner, <strong>and</strong> airflow. Mesh <strong>and</strong> grid are not employed in this<br />
study. The calculation points are placed evenly in an analysis<br />
area. A toner particle corresponds to one calculation point.<br />
The calculation points assigned <strong>for</strong> toner particles move<br />
with the motion of the toner cloud. In order to calculate the<br />
electric field, the airflow, <strong>and</strong> the flow of toner cloud, we<br />
examined the particle methods, such as the SPH method<br />
<strong>and</strong> the MPS method, which are usually used <strong>for</strong> fluid analysis.<br />
The SPH method was used to calculate electric potential<br />
in our previous study, 9 but this method requires special<br />
schemes to treat incompressible fluid. 10 In the present study,<br />
the MPS method, 11 which is often used in studies of incompressible<br />
flow, is employed the calculation of the electric<br />
field. The calculation points assigned <strong>for</strong> air also move with<br />
the airflow calculated using the MPS method.<br />
In the MPS method, calculation points (also called particles<br />
in the MPS method) with the physical quantity f are<br />
placed evenly in an analysis area. The gradient of the physical<br />
quantity f is modeled as the average slope weighted with<br />
a weighting function between the object calculation point i<br />
<strong>and</strong> its neighboring calculation points j, <strong>and</strong> is expressed at a<br />
position of calculation point i as<br />
f i = d <br />
n ji f j − f i r ij<br />
wr 0 ij ,<br />
r ij<br />
r ij<br />
F at =−− 1 a<br />
P a + a 2 u at. 5<br />
r ij = r j − r i , r ij = r ij , 9<br />
The equations <strong>for</strong> toner cloud are<br />
D t<br />
Dt =0,<br />
Du t<br />
Dt =− 1 P t + t 2 u t + 1 QE + g + 1 F ta ,<br />
t t t<br />
6<br />
7<br />
where r i is the position vector of calculation point i, <strong>and</strong> d is<br />
the spatial dimension. The weight function w uses the following<br />
function:<br />
e<br />
wr =r r −1 0 r r e<br />
, 10<br />
0 r e r<br />
F ta =− 1 t<br />
P t + t 2 u ta, 8<br />
where u is velocity, is density <strong>and</strong> is viscosity of air <strong>and</strong><br />
toner cloud. Subscript a, t denote air <strong>and</strong> toner cloud, respectively.<br />
Q is volume charge density of toner cloud, <strong>and</strong> E<br />
is electric field. F at is the interaction <strong>for</strong>ce that is exerted on<br />
where r e is the size of weight function that defines interacting<br />
calculation points <strong>and</strong> n 0 is the number density of calculation<br />
points in the initial configuration, <strong>and</strong> is defined by<br />
n 0 = wr ij .<br />
ji<br />
The Laplacian is modeled as<br />
11<br />
432 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Maeda, Maekawa, <strong>and</strong> Takeuchi: Simulation of traveling wave toner transport considering air drag<br />
where<br />
2 f i = 2d<br />
f<br />
n 0 j − f i wr ij ,<br />
ji<br />
12<br />
wrr 2 d<br />
=V<br />
. 13<br />
V<br />
wrd<br />
In order to discretize Eq. (1) <strong>for</strong> the electric potential<br />
using the MPS Laplacian model, Eq. (12), we use the identity<br />
· = 1 2 2 − 2 + 2 ,<br />
<strong>and</strong> we can obtain the discretization of Eq. (1) as<br />
· i =<br />
d<br />
<br />
n 0 j + i j − i wr ij =−Q i .<br />
ji<br />
14<br />
15<br />
Solving Eq. (15) simultaneously <strong>for</strong> unknown with<br />
the charge density <strong>and</strong> the boundary potential we obtain the<br />
potential at each calculation point. Here, the charge densities<br />
Q i are given as a property of the calculation points <strong>for</strong> toner,<br />
<strong>and</strong> the boundary potentials are given as a property of the<br />
calculation points at the boundary. From Eq. (9), the electric<br />
field is obtained by<br />
E i =− i =− d <br />
n ji j − i r ij<br />
0 wr ij . 16<br />
r ij<br />
Two-fluid flow of toner cloud flow <strong>and</strong> airflow is<br />
discretized by calculation points with toner mass <strong>and</strong> calculation<br />
points with air mass, respectively, in MPS. The continuity<br />
equations [Eqs. (3) <strong>and</strong> (6)] are satisfied by keeping<br />
the number of calculation points constant during the motion<br />
in the analysis area. Keeping the number density of the<br />
calculation points constant, n 0 satisfies the incompressibility.<br />
The Navier–Stokes equations [Eqs. (4) <strong>and</strong> (7)] are calculated<br />
as follows. 12<br />
In a time step k, the terms with the exception of the<br />
pressure term of Eqs. (4) <strong>and</strong> (7) are explicitly calculated,<br />
<strong>and</strong> temporal velocities u * are obtained as<br />
u * a = u k a + g + t, 17<br />
a<br />
a<br />
2 u aa<br />
r ij<br />
+ a<br />
a<br />
2 u at<br />
u * t = u k t + t<br />
2 u<br />
t<br />
tt<br />
+ t<br />
2 u<br />
t<br />
ta<br />
+ g + QE<br />
<br />
t. t<br />
18<br />
In Eqs. (17) <strong>and</strong> (18), a <strong>and</strong> t denote interactions from<br />
air <strong>and</strong> toner, respectively. Temporal positions r * are obtained<br />
as<br />
r a * = r a k + u a * t<br />
r t * = r t k + u t * t<br />
where t is time step. The viscous term is calculated as<br />
2 u i = 2d<br />
<br />
n 0 u j − u i wr ij .<br />
ji<br />
19<br />
20<br />
21<br />
Since temporal velocities u * do not take the pressure<br />
term into account, the velocities of the next step k+1, u k+1<br />
should be corrected as follows:<br />
u a k+1 = u a * + u a ,<br />
u t k+1 = u t * + u t ,<br />
22<br />
23<br />
where u are the correction velocities. The correction velocities<br />
u due to the pressure term can be written as<br />
1<br />
P a<br />
a<br />
k+1tt, 24<br />
u a =− 1 a<br />
P a<br />
k+1a<br />
+<br />
u t =− 1 P t<br />
t<br />
+ 1 P t<br />
<br />
k+1t t<br />
k+1at. 25<br />
The first <strong>and</strong> second terms on the right-h<strong>and</strong> side of Eq.<br />
(24) denote pressure gradients due to interactions neighboring<br />
calculation points <strong>for</strong> air <strong>and</strong> toner, respectively. Adding<br />
both sides of Eqs. (24) <strong>and</strong> (25) gives<br />
u a <br />
1−c<br />
t + c t u t <br />
a t =− 1 P k+1 ,<br />
a<br />
P k+1 = 1−cP a a + P a t + cP t t + P t a ,<br />
26<br />
where c is concentration of toner.<br />
Since the temporal position r * does not satisfy the incompressibility<br />
constraint, that is to say, the temporal number<br />
density of calculation point at the temporal position n * is<br />
not n 0 . The temporal number density of calculation point n *<br />
should be corrected to n 0 as<br />
n 0 = n * + n,<br />
27<br />
where n is the corrected value of the calculation point number<br />
density. The correction value of calculation point number<br />
density n is related to the velocity correction u through<br />
the mass conservation equation.<br />
n<br />
n 0 t + ·1−cu a + c t<br />
u t<br />
a<br />
=0. 28<br />
With Eqs. (26)–(28), Poisson equations of pressure are<br />
obtained as<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 433
Maeda, Maekawa, <strong>and</strong> Takeuchi: Simulation of traveling wave toner transport considering air drag<br />
From Eq. (12), we obtain<br />
2 P a k+1 = a<br />
t 2 n * − n 0<br />
n 0 , 29<br />
2 P t k+1 = t<br />
t 2 n * − n 0<br />
n 0 . 30<br />
2d<br />
n 0 ji<br />
P j − P i wr ij =− m n * i − n 0<br />
t 2 n 0<br />
m = a,t.<br />
31<br />
Solving the simultaneous equations [Eq. (31)] <strong>for</strong> unknowns<br />
P gives the pressure at each calculation point. With<br />
Eq. (9), the gradient of the pressure is obtained as<br />
P i =− d <br />
n ji P j − P i r ij<br />
0 wr ij . 32<br />
r ij<br />
Finally, the velocities of air <strong>and</strong> toner at next step k1<br />
are obtained using Eqs. (22)–(25) <strong>and</strong> (32). Here, the equations<br />
<strong>for</strong> air <strong>and</strong> toner cloud are described separately in order<br />
to point out the interaction between toner cloud <strong>and</strong> air<br />
explicitly. However, the program code can be written in the<br />
same way with one flow model using two kinds of densities<br />
<strong>and</strong> two kinds of coefficients of viscosity. As described<br />
above, we can obtain the motion of the toner, airflow, <strong>and</strong><br />
the electric field using only calculation points.<br />
TEST CALCULATIONS<br />
Electric Potential<br />
In order to confirm the validity of the application of the<br />
Laplacian model in the MPS method <strong>for</strong> the calculation of<br />
electric potential, we per<strong>for</strong>med a calculation of two cases of<br />
potential distribution with <strong>and</strong> without charge. The first case<br />
is an analysis of the gap between parallel plates with sinusoidal<br />
potential <strong>and</strong> ground potential as shown in Figure 1.<br />
The analysis area is of width L=1 mm, <strong>and</strong> height<br />
h=0.2 mm. The potentials applied to the upper <strong>and</strong> lower<br />
plate are<br />
r ij<br />
upper x =0,<br />
lower x = V 0 sin 2 x .<br />
33<br />
34<br />
The peak voltage is V 0 =200 V <strong>and</strong> the wavelength is<br />
=L. The calculation points are placed in a 10020 matrix.<br />
The top <strong>and</strong> bottom rows of the calculation points are set to<br />
Figure 1. Calculation geometry of parallel plates.<br />
Figure 2. Comparison between numerical solutions by the MPS Laplacian<br />
model <strong>and</strong> analytical solution of the electric potential <strong>for</strong>med between<br />
the parallel plates with applied sinusoidal voltage.<br />
be the potentials upper <strong>and</strong> lower , respectively, as the<br />
Dirichlet boundary condition. The periodic boundary condition<br />
is applied in the x direction. The size of the weight<br />
function is r e =2.1l 0 ,wherel 0 is the distance between neighboring<br />
calculation points. Two rows of dummy calculation<br />
points are added at the outside of the plates to avoid the<br />
error of number density of calculation points near the<br />
boundary.<br />
Figure 2 shows a comparison between the numerical<br />
results by the present method <strong>and</strong> the analytical solution. As<br />
a whole, the calculated potential is in good agreement with<br />
the analytical solution. The maximum error, which is generated<br />
near the lower plate, is 2.5%. The error is caused by<br />
the fact that in order to simplify the h<strong>and</strong>ling of the boundary<br />
condition, the boundary condition is located not between<br />
calculation points but at the calculation points themselves.<br />
The second case is an electric potential analysis that<br />
involves toner charge. A toner layer, with depth of<br />
0.035 mm, dielectric permittivity of 2.0, <strong>and</strong> a charge density<br />
of 1.5 C/m 3 , is placed on the lower plate in the geometry<br />
as shown in Fig. 1. The potential distribution <strong>for</strong>med<br />
between the parallel plates with potentials of −200 V <strong>and</strong><br />
0V was calculated using the Laplacian model in MPS as<br />
represented in Eq. (15). Figure 3 shows a comparison between<br />
the numerical results by the present method <strong>and</strong> the<br />
analytical solution. The left vertical dotted line denotes the<br />
surface of the toner layer. These results show that the Laplacian<br />
model in MPS correctly calculates the electric potential<br />
with or without space charge.<br />
Airflow<br />
A test calculation of the airflow caused by the motion of a<br />
toner cloud <strong>and</strong> the <strong>for</strong>ce exerted on the surface of toner<br />
cloud by air in motion were per<strong>for</strong>med using the present<br />
method. To verify the interaction between the airflow <strong>and</strong><br />
the surface of the toner cloud, insofar as the motion of the<br />
toner generates airflow, we used a toner cloud moving with a<br />
uni<strong>for</strong>m velocity <strong>and</strong> a uni<strong>for</strong>m shape. The calculation<br />
points representing the toner cloud are placed in the shape<br />
of a trapezoid <strong>and</strong> move at a constant velocity of 0.1 m/s in<br />
434 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Maeda, Maekawa, <strong>and</strong> Takeuchi: Simulation of traveling wave toner transport considering air drag<br />
Figure 3. Comparison between numerical solutions by the MPS<br />
Laplacian model <strong>and</strong> the analytical solution of electric potential with toner<br />
charge.<br />
Figure 5. Comparison of results calculated between by the present<br />
method <strong>and</strong> by the FEM method, a x-component of viscous <strong>for</strong>ce, b<br />
y-component of viscous <strong>for</strong>ce, <strong>and</strong> c pressure that exerted on the surface<br />
of a toner cloud in the shape of trapezoid by airflow.<br />
Figure 4. Distribution of velocity of air calculated a by the present<br />
method <strong>and</strong> b by the FEM method.<br />
the x direction. The analysis area is of width L=1 mm, <strong>and</strong><br />
height h=0.2 mm. The periodic boundary condition,<br />
whereby the calculation points leaving from one boundary<br />
are returned through the other, is applied in the x direction.<br />
The air density <strong>and</strong> viscosity are =1.205 kg/m 3 <strong>and</strong><br />
=1.8210 −5 Pa s, respectively. The system is initially at<br />
rest. The time step used is t=110 −6 s. The steady state<br />
of the same problem is calculated using a commercial finite<br />
element method (FEM) code <strong>for</strong> comparison. In this FEM<br />
calculation, the toner cloud is modeled by a boundary of a<br />
trapezoidal shape. The boundary condition is the inflow/<br />
outflow condition with the velocity of the toner cloud instead<br />
of moving the trapezoidal geometry to simulate the<br />
motion of toner cloud. The no-slip condition is applied at<br />
the upper <strong>and</strong> lower plates.<br />
Figure 4(a) shows the distribution of the velocity of air<br />
at t=50 s calculated by the present method. Figure 4(b)<br />
shows the result of the steady-state condition calculated by<br />
the FEM code. The viscous <strong>for</strong>ce <strong>and</strong> pressure acting on the<br />
surface of toner cloud by air in motion was calculated using<br />
the present method <strong>and</strong> are shown in Figure 5. The results<br />
calculated by FEM are also plotted <strong>for</strong> comparison.<br />
The velocity field calculated using the present method<br />
including the vortex over the trapezoidal shape is in good<br />
agreement with the result using the FEM code <strong>for</strong> the most<br />
part. There is a small discrepancy between the present<br />
Figure 6. Initial configuration of calculation points <strong>for</strong> analysis of traveling<br />
wave toner transport.<br />
method <strong>and</strong> the FEM code in calculating the viscous <strong>for</strong>ce<br />
<strong>and</strong> the pressure. However, the results are adequate <strong>for</strong> our<br />
qualitative estimate.<br />
TRAVELING WAVE TONER TRANSPORT<br />
The results of the verification tests described above show<br />
that the present method is adequate <strong>for</strong> the calculation of<br />
the electric field, the airflow caused by the motion of toner<br />
<strong>and</strong> the viscous <strong>for</strong>ce, <strong>and</strong> the pressure acting on the surface<br />
of toner cloud by air in motion. We employed the present<br />
method <strong>for</strong> the analysis of the behavior of a toner cloud<br />
transporting with a traveling electrostatic wave.<br />
The initial configuration of the calculation points is<br />
shown in Figure 6, <strong>and</strong> the parameters of the calculation are<br />
listed in Table I. The periodic boundary condition is applied<br />
in the x direction. The traveling potential applied to the<br />
lower plate is as follows:<br />
0 x,t = V 0 sin 2 x − vt , v = f, 35<br />
where v is the phase velocity.<br />
In our first simulation by the present method, we per<strong>for</strong>med<br />
calculations to obtain the frequency characteristics<br />
of the transport velocity of the toner. The frequency of the<br />
traveling wave is swept from 0Hzto 3.0 kHz in the period<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 435
Maeda, Maekawa, <strong>and</strong> Takeuchi: Simulation of traveling wave toner transport considering air drag<br />
Table I. Parameters of toner transport model.<br />
Analysis Area<br />
Width L m<br />
Height h m<br />
Pitch of calculation-point l 0 m<br />
1.0 10 −3<br />
0.2 10 −3<br />
0.01 10 −3<br />
Traveling Electrostatic Wave<br />
Wavelength m<br />
1.0 10 −3<br />
Pressure 4.0l 0<br />
Applied Voltage V 0 V 200<br />
Figure 7. Distribution of toner particle velocity vs phase velocity of traveling<br />
wave.<br />
Frequency kHz 0–3.0<br />
Toner<br />
Charge Density Q C/m 3 0.3<br />
Relative Permittivity 1.2<br />
Bulk Density t kg/ m 3 300<br />
Viscosity µ t Pa s 0<br />
Air<br />
Relative Permittivity 1.0<br />
Density a kg/ m 3 1.2<br />
Viscosity µ a Pa s<br />
1.82 10 −5<br />
Time Step<br />
Step s<br />
1.0 10 −6<br />
Span s<br />
75 10 −3<br />
Size of Weight Function<br />
Electric Potential 2.1l 0<br />
Number Density 2.1l 0<br />
from t=0 ms to 75 ms in the calculation. In other words,<br />
the phase velocity is varied from 0msto 3.0 ms at the wavelength<br />
of traveling wave =1 mm. We focus on the effect of<br />
electrostatic <strong>for</strong>ce <strong>and</strong> air drag. The viscosity of toner cloud<br />
flow is taken to be zero to exclude the effect of friction<br />
between toner <strong>and</strong> the plate, to which the traveling wave<br />
potential is applied, <strong>and</strong> the effect of friction among toner<br />
particles.<br />
Figure 7 shows the velocity distribution of the toner<br />
particles with airflow caused by the motion of toner as a<br />
function of the phase velocity of traveling wave. The solid<br />
line in Fig. 7 denotes the phase velocity of the traveling wave,<br />
<strong>and</strong> the dotted lines denote the critical velocity at which the<br />
Stokes’ <strong>for</strong>ce is equal to the electrostatic <strong>for</strong>ce of one particle<br />
in quiescent air. In Fig. 7, as the wave phase velocity of the<br />
traveling wave increases from zero, the toner particles move<br />
synchronously with the wave phase velocity until a certain<br />
wave phase velocity. Beyond this phase velocity, the number<br />
of toner particles that move synchronously with the wave<br />
phase velocity decreases gradually with an increase in phase<br />
velocity. Finally, the velocities of all toner particles do not<br />
synchronize with the wave phase velocity.<br />
Figure 8. A snapshot of position of toner particles <strong>and</strong> velocity field of<br />
airflow: a Initial starting step, b the step when the toner particles<br />
rearrange in response to the electric field, c the step when the toner<br />
cloud de<strong>for</strong>ms due to air <strong>for</strong>ce, d the step when part of toner particles<br />
separate from the toner cloud due to air <strong>for</strong>ce, <strong>and</strong> e the step when all<br />
the toner particles are out of phase with the traveling wave <strong>and</strong> the initial<br />
shape of toner cloud disappears.<br />
The motions of toner particles <strong>and</strong> air were observed as<br />
an animation created using the numerical results. Figure 8<br />
shows snapshots of the toner particle position <strong>and</strong> airflow<br />
velocity from the viewpoint of a coordinate moving with the<br />
traveling wave. The contour line of the electric potential,<br />
which is <strong>for</strong>med by the traveling wave potential applied on<br />
the plate <strong>and</strong> toner charge, is superimposed. Figure 8(a)<br />
shows the positioning of calculation points <strong>for</strong> toner, which<br />
436 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Maeda, Maekawa, <strong>and</strong> Takeuchi: Simulation of traveling wave toner transport considering air drag<br />
Figure 9. a Maximum synchronous frequency as a function of number<br />
of toner particles to be transport <strong>and</strong> b average velocity of airflow as a<br />
function of number of toner particles to be transported.<br />
are placed in the shape of a chord, geometrically, at the<br />
initial step. Figure 8(b) shows the snapshot of the step when<br />
the toner particles rearrange in response to the electric field.<br />
Figure 8(c) shows the snapshot of the step when the toner<br />
cloud de<strong>for</strong>ms due to air <strong>for</strong>ce. In Fig. 8(c), the cluster of<br />
toner, which is around the trough of potential wave at low<br />
velocity in Fig. 8(b), moves to negative direction of x axis<br />
due to air resistance. As shown in Fig. 8(d), at higher phase<br />
velocity, the airflow around the surface of the cluster of toner<br />
causes a separation of the toner particles at the surface of the<br />
cluster of toner <strong>and</strong> the separated toner particles do not<br />
move in synchronization with the phase velocity of the traveling<br />
wave any longer. Finally, all the toner particles are out<br />
of phase of the traveling wave <strong>and</strong> the initial shape of cluster<br />
of toner disappears, as shown in Fig. 8(e).<br />
In our second simulation, we per<strong>for</strong>med calculations of<br />
various numbers of toner particles <strong>and</strong> considered the influence<br />
of air drag. Figure 9(a) shows the relationship between<br />
the maximum frequency at which the particles move synchronously<br />
with the wave phase velocity <strong>and</strong> the number of<br />
toner particles to be transported. Figure 9(b) shows the average<br />
airflow velocity in x direction as a function of the<br />
number of toner particles. As the number of toner particles<br />
increases, the velocity of airflow increases <strong>and</strong> the maximum<br />
synchronous frequency increases.<br />
As mentioned above, we confirmed that air drag caused<br />
by toner motion increased the maximum synchronous frequency,<br />
at which the toner particles moved synchronously<br />
with a phase velocity of a traveling wave.<br />
CONCLUSION<br />
A simulation method to analyze the behavior of a toner<br />
cloud driven by a traveling electrostatic wave, taking into<br />
account the effect of air motion, is proposed. Several calculations<br />
of traveling wave toner transport using this method<br />
were per<strong>for</strong>med. The results demonstrate that airflow<br />
dragged by toner particles plays an important role in the<br />
behavior of toner particles transporting with a traveling electrostatic<br />
wave.<br />
In the traveling wave toner transport, the friction between<br />
toner <strong>and</strong> plate, to which traveling potential is applied,<br />
or the friction among toner particles in addition to<br />
electrostatic <strong>for</strong>ce <strong>and</strong> air resistance are important. These<br />
frictions can be simulated using the viscosity of toner in our<br />
two-flow model. Actually, adhesion of charged toner to electrode<br />
<strong>and</strong> cohesion between toner particles occur. We will<br />
approach these subjects in the future.<br />
However, the present method calculates the electric potential<br />
<strong>and</strong> the motion of the toner with common calculation<br />
points to avoid the complexity of data exchange between<br />
the electric potential analysis using meshes or grids<br />
<strong>and</strong> toner motion analysis using particles. This present<br />
method allows a direct calculation of the electric potential<br />
distribution that varies with the motion of charged toner<br />
particle. Moreover, using the same calculation points, airflow<br />
can also be calculated. This makes it possible to calculate the<br />
behavior of the charged particles in a fluid. The present<br />
method can be used to simulate not only toner transport but<br />
also in eletrophotographic liquid development in which the<br />
charged toner moves in a carrier fluid or an electrophoretic<br />
paperlike display.<br />
ACKNOWLEDGMENT<br />
The authors are grateful to Chin-Che Tin of Auburn<br />
University <strong>for</strong> his critical reading, kind suggestion, <strong>and</strong> improvement<br />
of the English of this article.<br />
REFERENCES<br />
1 S. Masuda, K. Fujibayashi, K. Ishida, <strong>and</strong> H. Inaba, “Confinement <strong>and</strong><br />
transport of charged aerosol clouds by means of electric curtains”, Trans.<br />
Inst. Electr. Eng. Jpn., Part B 92-B, 9–18 (1972) [in Japanese].<br />
2 J. R. Melcher, E. P. Warren, <strong>and</strong> R. H. Kotwal, “Theory <strong>for</strong> puretraveling-wave<br />
boundary-guided transport of tribo-electrified particles”,<br />
Part. Sci. Technol. 7, 1–21 (1989).<br />
3 J. R. Melcher, E. P. Warren, <strong>and</strong> R. H. Kotwal, “Theory <strong>for</strong> finite-phase<br />
traveling-wave boundary-guided transport of triboelectrified particles”,<br />
IEEE Trans. Ind. Appl. 25, 949–955 (1989).<br />
4 J. R. Melcher, E. P. Warren, <strong>and</strong> R. H. Kotwal, “Traveling-wave delivery<br />
of single-component developer”, IEEE Trans. Ind. Appl. 25, 956–961<br />
(1989).<br />
5 F. W. Schmidlin, “A new nonlevitated mode of traveling wave toner<br />
transport”, IEEE Trans. Ind. Appl. 27, 480–487 (1991).<br />
6 F. W. Schmidlin, “Modes of traveling wave particle transport <strong>and</strong> their<br />
applications”, J. Electrost. 34, 225–244 (1995).<br />
7 Y. N. Gartstein <strong>and</strong> J. G. Shaw, “Many-particle effects in traveling<br />
electrostatic wave transport”, J. Phys. D 32, 2176–2180 (1999).<br />
8 R. Kober, “Simulation of Traveling Wave Toner Transport”, Proc. IS&T’s<br />
NIP18 (IS&T, Springfield, VA, 2002) pp. 453–457.<br />
9 M. Maeda, S. Ozawa, <strong>and</strong> M. Takeuchi, “Calculation of potential<br />
distribution in the vicinity of photoconductive drum <strong>and</strong> rotating high<br />
resistive roller with particle method”, J. Imag. Soc. Jpn. 45, 90–96<br />
(2006).<br />
10 J. P. Morris, P. J. Fox, <strong>and</strong> Y. Zhu, “Modeling low Reynolds number<br />
incompressible flows using SPH”, J. Comput. Phys. 136, 214–226 (1997).<br />
11 S. Koshizuka <strong>and</strong> Y. Oka, “Moving-particle semi-implicit method <strong>for</strong><br />
fragmentation of incompressible fluid”, Nucl. Sci. Eng. 123, 421–434<br />
(1996).<br />
12 H. Gotoh, Computational Mechanics of Sediment Transport (Morikita<br />
Publishing, Tokyo, 2004), p. 110 [in Japanese].<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 437
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 438–444, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
High Speed <strong>Imaging</strong> <strong>and</strong> Analysis of Jet<br />
<strong>and</strong> Drop Formation<br />
Ian M. Hutchings, Graham D. Martin <strong>and</strong> Stephen D. Hoath<br />
Inkjet Research Centre, Institute <strong>for</strong> Manufacturing, Department of Engineering,<br />
University of Cambridge, Mill Lane, Cambridge CB2 1RX, United Kingdom<br />
E-mail: gdm31@cam.ac.uk<br />
Abstract. New techniques have been developed <strong>for</strong> analyzing, in<br />
detail, the shape <strong>and</strong> development of ink jets <strong>and</strong> drops. By using<br />
flash illumination of very short duration (ca. 20 ns), high quality,<br />
single-event digital images of jets <strong>and</strong> drops can be captured. A<br />
computer program, PEJET, has been written to automate the processing<br />
of such images <strong>and</strong> to generate quantitative data about the<br />
whole ink stream. From this data, it is then possible to compute the<br />
variation in fluid volume, volume flow, <strong>and</strong> velocity as a function of<br />
both position <strong>and</strong> time. The method has been shown to have high<br />
accuracy. The results can be used to study the influences of nozzle<br />
design, drive wave<strong>for</strong>m, <strong>and</strong> fluid properties on jet <strong>and</strong> drop <strong>for</strong>mation,<br />
as well as to provide accurate data <strong>for</strong> comparison to the results<br />
of computational modeling. Examples of results from a dropon-dem<strong>and</strong><br />
system are presented that illustrate the potential of the<br />
method to compare quantitatively the per<strong>for</strong>mance of print systems<br />
<strong>and</strong> inks. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5438<br />
INTRODUCTION<br />
As the range of applications of ink jet systems exp<strong>and</strong>s, so<br />
the need grows to underst<strong>and</strong> their per<strong>for</strong>mance with an<br />
ever-widening range of fluids, which are sometimes difficult<br />
to print. These may be fluids with complex rheologies or<br />
fluids containing high loadings of solid particles.<br />
The visualization of jet <strong>and</strong> drop <strong>for</strong>mation close to the<br />
print head can lead to insights into how the system is<br />
behaving. 1–3 From detailed measurement of the shape <strong>and</strong><br />
size of jets as they develop over time, the volumes <strong>and</strong> velocities<br />
of the jets <strong>and</strong> the drops which <strong>for</strong>m from them can<br />
be computed. These results can then be used to compare the<br />
per<strong>for</strong>mance of print head designs <strong>and</strong> printing fluids, <strong>and</strong><br />
check the validity of numerical models.<br />
This paper describes techniques that have been developed<br />
to make such quantitative observations <strong>and</strong> gives examples<br />
of different ways in which this in<strong>for</strong>mation can be<br />
used to explore the per<strong>for</strong>mance of inks <strong>and</strong> print heads.<br />
We have examined model inks jetted from a Xaar dropon-dem<strong>and</strong><br />
print head as part of development work at the<br />
Cambridge Inkjet Research Centre, in some cases replicating<br />
previous studies in order to establish techniques <strong>and</strong> to underpin<br />
more novel analyses. In particular, we present some<br />
work used to provide input to current theoretical models<br />
developed by our research partners. 4<br />
APPARATUS AND EXPERIMENTAL TECHNIQUES<br />
Figure 1 illustrates the equipment used in this work. The key<br />
elements are as follows:<br />
• a high resolution digital camera with a lens system capable<br />
of imaging a field of view of a few millimeters,<br />
connected to a PC <strong>for</strong> control <strong>and</strong> image storage<br />
• an ink jet print head with drive electronics <strong>and</strong> data<br />
source<br />
• a very short duration 20 ns flash light source with<br />
delivery optics<br />
• means to delay the flash relative to the initiation of the<br />
printing event <strong>and</strong> to measure that delay time accurately<br />
The experiments <strong>for</strong> the measurements described below<br />
used a Xaar 126-200 print head with a linear array of<br />
50 m diam nozzles. The fluid was a simple semitransparent<br />
UV-curable ink with a Newtonian viscosity of<br />
20 mPa s at 25°C.<br />
Jets <strong>and</strong> drops emerging from ink jet printers are commonly<br />
visualized stroboscopically by creating composite images,<br />
superimposing tens or hundreds of individual events in<br />
a single frame. 5 This procedure relies on the repeatability of<br />
the drop <strong>for</strong>mation process to give the impression of observing<br />
individual jets <strong>and</strong> drops. In some cases, as shown in<br />
Figure 2(a), the events are not completely reproducible,<br />
which results in some blurring of the image, particularly at<br />
long times after drop ejection. For stroboscopic images of<br />
this kind the flash duration is typically 1 s, which, given<br />
the high velocity <strong>and</strong> small size of the objects being ob-<br />
Received Mar. 1, 2007; accepted <strong>for</strong> publication Jun. 20, 2007.<br />
1062-3701/2007/515/438/7/$20.00.<br />
Figure 1. Schematic of apparatus.<br />
438
Hutchings, Martin, <strong>and</strong> Hoath: High speed imaging <strong>and</strong> analysis of jet <strong>and</strong> drop <strong>for</strong>mation<br />
Figure 2. Comparison of images from different techniques: a composite<br />
image strobed illumination <strong>and</strong> b single event 20 ns flash<br />
illumination.<br />
served, can result in significant movement blur. However,<br />
some in<strong>for</strong>mation about the development of the jet <strong>and</strong><br />
drops can be obtained from strobed images by changing the<br />
timing or phase of the illuminating flashes relative to the<br />
printing event.<br />
Alternatively, a high-speed camera can be used to observe<br />
single events as they occur. 6 This requires the use of a<br />
camera with a very high framing rate (in megahertz), but<br />
typically the pixel resolution of such equipment is poor <strong>and</strong><br />
only a small number of frames can be captured.<br />
All the images used in this work were obtained by using<br />
a short duration 20 ns flash <strong>and</strong> a high resolution still<br />
camera. Hence, each captured image was of a unique event.<br />
In cases where the events are reproducible, a sequence can be<br />
built up by taking pictures of successive events at increasingly<br />
greater time intervals from an appropriate event trigger<br />
signal. The time delay be<strong>for</strong>e firing the flash was set by using<br />
signal delay electronics <strong>and</strong> was measured with an oscilloscope<br />
detecting the trigger signal <strong>and</strong> an output from the<br />
flash.<br />
The apparatus shown in Fig. 1 was used to capture individual<br />
printing events. A typical image is shown in<br />
Fig. 2(b). Because of the very short duration of the flash<br />
illumination, there is no significant motion blur. In cases<br />
where the events are reproducible, the time development of<br />
the event can be investigated by capturing a series of images<br />
with different delays. Less reproducible events can also be<br />
captured repeatedly <strong>and</strong> in<strong>for</strong>mation on their variability determined.<br />
Because of the high quality of the imaging, it is<br />
possible to determine drop <strong>and</strong> ligament sizes <strong>and</strong> shapes<br />
<strong>and</strong> then to use this in<strong>for</strong>mation to compute drop <strong>and</strong> ligament<br />
volumes. By comparing images within timed sequences,<br />
the velocities of the various components of the ink<br />
stream can also be determined.<br />
The Xaar XJ126-200 drop-on-dem<strong>and</strong> print head uses a<br />
shared-wall piezoelectric design to reduce the head drive<br />
voltage needed to <strong>for</strong>m jets of a given velocity. As a result,<br />
neighboring nozzles are influenced by the need to actuate<br />
the shared walls. The print head can be driven in a singleshot<br />
mode that fires a particular group of nozzles at one<br />
time, cycling around the three groups A-B-C-A-B-C- <strong>and</strong> so<br />
on, but in our application, a single trigger event caused the<br />
print head to print in the sequence A-B-C with fixed short<br />
time delays between the three groups. The actual nozzles<br />
fired depend on the image presented to the print head. In<br />
this work, the images were solid blocks arranged either to<br />
print all the nozzles, a designated block of 16 nozzles, a<br />
single A-B-C set, or a single nozzle from group A, B, or C,<br />
according to the phenomenon under study.<br />
A logic circuit was devised to h<strong>and</strong>le the asynchronous<br />
nature of the print comm<strong>and</strong> <strong>and</strong> the print head cycle time<br />
clock, in order to ensure reproducible firing of individual<br />
nozzles on a timescale of 1 s. The actual ink jet printing<br />
starts at a fixed time in relation to the clock edge, so that<br />
without the use of this logic circuit, an additional undesirable<br />
timing jitter, equal to the clock cycle time of 1.75 s,<br />
would be introduced, thereby disrupting pseudosequences<br />
obtained with shorter time steps. Residual timing uncertainties<br />
were 0.70 s, due to r<strong>and</strong>omness in the triggering of<br />
some flashes, which were also associated with weaker light<br />
output <strong>and</strong>, thus, darker images. An oscilloscope was used to<br />
determine the precise relative timing of the nozzle firing <strong>and</strong><br />
image capture to a precision of 20 ns. The timings of all<br />
images presented here are accurate to 0.1 s.<br />
IMAGE PROCESSING AND DATA EXTRACTION<br />
To extract quantitative in<strong>for</strong>mation from the images, it is<br />
necessary to analyze them <strong>and</strong> decide which parts of the<br />
image belong to the background <strong>and</strong> which to the ink drops<br />
<strong>and</strong> ligaments. St<strong>and</strong>ard techniques <strong>and</strong> various proprietary<br />
image processing tools are available to find edges <strong>and</strong> objects<br />
within images. 7 However, there are particular features of<br />
these backlit images, which make the use of such tools laborious<br />
<strong>and</strong> sometimes inaccurate. In particular, the background<br />
intensity often varies both within each image <strong>and</strong><br />
from image to image. Within a single image, there is often a<br />
thin extended ligament that starts at or near the nozzle.<br />
Shading by the nozzle means that the background intensity<br />
varies significantly along the length of the ligament. Although<br />
the light source provides a very short duration pulse,<br />
the nature of the source means that the intensity can vary by<br />
10% or more from image to image. Drops <strong>and</strong> ligaments<br />
from a transparent fluid often show light central regions<br />
because light from the bright-field source passes straight<br />
through these areas, rather than being refracted away from<br />
the optical path as it is at the edges of these features.<br />
The example in Figure 2(b) illustrates some of these<br />
issues. An image processing method was developed, which<br />
would cope with these variations <strong>and</strong> artifacts <strong>and</strong> allow<br />
hundreds of images at a time to be analyzed automatically.<br />
Physically reasonable assumptions can be made about the<br />
objects being imaged to simplify this task. For example, liquid<br />
drops <strong>and</strong> jets will not have holes within them <strong>and</strong> the<br />
edges should be represented by smooth curves at a pixel<br />
level. The area within the image in which the drops <strong>and</strong><br />
ligaments are expected to appear is often known. Fixed features<br />
within all the images in a timed sequence can be used<br />
to compensate <strong>for</strong> any small, inadvertent positional movement<br />
between the object <strong>and</strong> the imaging system during the<br />
process of image capture.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 439
Hutchings, Martin, <strong>and</strong> Hoath: High speed imaging <strong>and</strong> analysis of jet <strong>and</strong> drop <strong>for</strong>mation<br />
Figure 3. Image processing sequence.<br />
Figure 4. Raw image above <strong>and</strong> processed version below, showing<br />
the results of automated feature selection <strong>and</strong> edge detection. The largest<br />
drop in this image has volume of 80 pl <strong>and</strong> the smallest a volume of<br />
0.2 pl.<br />
The image processing is carried out in several steps, as<br />
shown in Figure 3. First, from a selected area of interest (a)<br />
a relatively fast but inaccurate technique is used to find the<br />
approximate edges of the features by looking <strong>for</strong> regions of<br />
rapid brightness change (b). In the next step (c), the edges<br />
are examined in detail to determine which parts of the edges<br />
are inside <strong>and</strong> which parts are outside the feature, based on<br />
a threshold level determined by the local ranges of brightness<br />
levels. Finally, any “holes” within the features are filled<br />
(d).<br />
A computer program, PEJET, was written, which incorporates<br />
these processing techniques, together with a way to<br />
select a region of interest within a set of images. The program<br />
allows a fixed datum to be defined within the image,<br />
which is then used to correct <strong>for</strong> any slight shift of the camera<br />
relative to the object over the course of the experiment.<br />
The program includes a way to detect <strong>and</strong> label each feature<br />
<strong>and</strong> to output images indicating the features selected. It also<br />
outputs the size data associated with each feature into a text<br />
file or spreadsheet. The program can be set up to process a<br />
complete set of images (<strong>for</strong> example, a timed sequence)<br />
without operator intervention. Figure 4 is an example of the<br />
image output from this program in which the various drops<br />
<strong>and</strong> ligament have been recognized <strong>and</strong> their edges indicated.<br />
The program has not been optimized <strong>for</strong> speed because<br />
it is not used to process images as they are captured but<br />
rather to batch process them subsequently. The sensitivity of<br />
the edge detection algorithm can be adjusted to suit the<br />
contrast <strong>and</strong> noise level within the image. It can also be set<br />
to reject unwanted or spurious images by defining the region<br />
of interest <strong>and</strong> rejecting spots below a preset level. However,<br />
our experience with the images we have captured has been<br />
that the program will reliably detect features four or more<br />
pixels across. A drop with a diameter of four pixels, with the<br />
magnifications typically used, is equivalent to a drop volume<br />
of approximately 0.01 pl. The majority of the features observed<br />
are traveling at 10 m s −1 , <strong>and</strong>, hence they will move<br />
200 nm during the 20 ns flash duration, which is<br />
0.3 pixels at the typical magnifications used here. We<br />
would not expect the feature velocity to have any significant<br />
influence on the measurements of volume.<br />
As well as the calculated data the program also outputs<br />
images on which the detected edges have been marked. In<br />
this way, the proper functioning of the program was ensured<br />
by checking the edges detected; in some cases, the volume<br />
values were also calculated manually <strong>for</strong> comparison to those<br />
calculated by the program. By flashing a second time after a<br />
short delay, two images of exactly the same drop could be<br />
captured in the same frame but with the drop moved <strong>and</strong><br />
with an evolved shape. The program correctly calculates the<br />
same volume <strong>for</strong> these second drop images.<br />
The threshold level selected to make these measurements<br />
can have some effect on the measured sizes of the<br />
objects. The correct threshold value can be determined in a<br />
number of ways. A calibration object of known size can be<br />
imaged in the system instead of the jets, <strong>and</strong> the correct<br />
threshold determined by experiment. Alternatively, an image<br />
feature of known size (such as the nozzles themselves) can<br />
be used to check the accuracy of the measured objects. It is<br />
also possible to consider the effect of optical blurring on a<br />
sharp edge <strong>and</strong> to compare edges in the images to those in<br />
computed images.<br />
Once the images have been processed, the dimensions<br />
<strong>and</strong> volume of each component of the ink stream can be<br />
computed. The camera <strong>and</strong> print head are usually arranged<br />
so that the drop <strong>and</strong> ligaments travel along the vertical axis<br />
of the image. If it is assumed that the ink stream has a<br />
circular cross section at all points, then each horizontal line<br />
of pixels in an object represents a circular slice through that<br />
object. By summing these slices, the volume of the whole<br />
stream, or of any horizontal slice through the stream, can be<br />
estimated. By comparing such measurements at various<br />
stages of drop development, a picture can be built up of how<br />
the volume of the object, or any part of it, changes over time.<br />
Particular features, such as the tip of the jet or drop or its<br />
center of mass, can be tracked, <strong>and</strong> the method thus provides<br />
a powerful tool to generate velocity <strong>and</strong> flow in<strong>for</strong>mation.<br />
The evolution of drop <strong>and</strong> satellite <strong>for</strong>mation can be<br />
tracked by dividing the ligament <strong>and</strong> drop into volume elements<br />
<strong>and</strong> processing successive images to calculate how the<br />
volume elements move over time. In the initial image of the<br />
sequence, each volume element is chosen to contain a fixed<br />
number of pixel slices (Figure 5) <strong>and</strong> the volume of each<br />
element is computed. In subsequent images, the boundaries<br />
of each fixed volume element can be determined by summing<br />
the volume of ink from each pixel slice until the volume<br />
of the element has been reached. In cases where, after a<br />
440 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Hutchings, Martin, <strong>and</strong> Hoath: High speed imaging <strong>and</strong> analysis of jet <strong>and</strong> drop <strong>for</strong>mation<br />
Figure 5. Tracking jet development.<br />
Figure 7. Raw <strong>and</strong> processed images showing satellite separation <strong>and</strong><br />
lateral fluctuations.<br />
Figure 8. Image showing tail deflection while the ligament is still attached<br />
to the nozzle; the orifices of the neighboring nozzles are visible<br />
<strong>and</strong> no other nozzles were fired.<br />
Figure 6. a Group of jets emerging from a Xaar XJ-126 print head. The<br />
distance between neighboring nozzles is 137 m. b Image at a later<br />
time, <strong>for</strong> the nozzles shown in a, showing the evolution of the jets to <strong>for</strong>m<br />
ligaments, satellites <strong>and</strong> main drops.<br />
short initial ink ejection period, the volume of ink external<br />
to the nozzle remains substantially constant, the accuracy of<br />
this determination can be improved by normalizing the total<br />
volumes measured from individual jets in successive images.<br />
RESULTS AND DISCUSSION<br />
In the present work, the print heads were operated with<br />
model UV-curable inks <strong>and</strong> the main ink drops had velocities<br />
of 6 ms −1 after 1mmflight. Ink jets emerging from<br />
an array of nozzles (spaced at 137 m), together with more<br />
fully developed ligaments, are shown in Figure 6(a). These<br />
sharp images resulting from a single 20 ns flash may be<br />
compared to those reported elsewhere (e.g., Ref. 6).<br />
During the early stages of the ejection of ink through<br />
the nozzle, the jet ligament becomes rapidly narrower, down<br />
to a diameter smaller than that of the orifice at the nozzle<br />
plate. This implies that the meniscus must lie within the<br />
nozzle, thereby allowing some air to enter the nozzle.<br />
Figure 6(b) displays jets at various later stages, showing<br />
ligament stretching, the snapping of ligaments, the <strong>for</strong>mation<br />
of satellites, <strong>and</strong> almost spherical main drops followed<br />
by trails of satellites.<br />
Example images are shown in Figures 7 <strong>and</strong> 8. Figure 7<br />
shows a raw image <strong>and</strong> corresponding processed image with<br />
four jets, one after separation of a tail satellite, with lateral<br />
fluctuations, tail ligament thinning, <strong>and</strong> beading visible. It<br />
was possible, in principle, that these tail deflections could<br />
occur through the influence of adjacent jets via aerodynamic,<br />
acoustic, or other means. Figure 8 was obtained by<br />
firing only a single nozzle <strong>and</strong> demonstrates that deflection<br />
of the tail can occur without requiring influences from jets<br />
fired from neighboring nozzles. The time after firing at<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 441
Hutchings, Martin, <strong>and</strong> Hoath: High speed imaging <strong>and</strong> analysis of jet <strong>and</strong> drop <strong>for</strong>mation<br />
Figure 9. Edge profile <strong>for</strong> a jet that has broken away from the nozzle<br />
vertical bar, showing a satellite <strong>and</strong> the center projection dotteddashed<br />
broken line of the mass flow from the nozzle. A dotted line<br />
represents the lateral deflection of the end of the tail. The lateral scale is<br />
exaggerated.<br />
which deflection occurred was similar in all images, whether<br />
<strong>for</strong> single jets or multiple groups, or after cleaning the nozzle<br />
plate. Figure 8 also shows the ink patches associated with the<br />
orifices of the two unfired neighboring nozzles.<br />
Break off occurs when the stretched ligament thins<br />
down <strong>and</strong> snaps. The two sections then follow independent<br />
histories: one will probably move backward into the nozzle<br />
orifice <strong>and</strong> the other will retract toward the main head. Either<br />
or both sections may then produce satellites, but usually<br />
the initial rupture is close to the nozzle <strong>and</strong> all satellites<br />
originate from the long ligament.<br />
Figure 9 shows the edge profile, derived by the methods<br />
described above, from a jet that had broken. The time after<br />
the first rupture was enough to allow a satellite to <strong>for</strong>m, <strong>and</strong><br />
thus, at least one more ligament break had occurred between<br />
the nozzle plane <strong>and</strong> the main ligament. The ligament width<br />
fluctuations suggest that other breaks may have been imminent.<br />
In this image, the ligament does not appear to point<br />
away from the same apparent origin (dotted-dashed line)<br />
throughout its length but shows a distinct angular deviation<br />
by 1° at 520 m. The later tail section <strong>and</strong> the satellite<br />
appear to point (dotted line) from another location<br />
10 m off center. We believe that this implies that the thin<br />
ink jet ligament became attached to the edge of the nozzle. It<br />
is possible that at some point as it thins, the ligament becomes<br />
unstable in a central position <strong>and</strong> that some small<br />
disturbance or asymmetry will then cause it to move to the<br />
edge of the nozzle.<br />
Ligament development proceeds, as described earlier,<br />
with the stretching of the material between the nozzle plane<br />
<strong>and</strong> the ink jet head. The stretched ligament has a small but<br />
finite minimum radius during the process; we have found<br />
that the ink jet ligament can be represented by a truncated<br />
cone with a specific half angle at the typical distance at<br />
which ligament rupture occurs near the nozzle, even if the<br />
ligament happens to rupture elsewhere be<strong>for</strong>e this, due to<br />
the width fluctuations in the thinning ligament.<br />
The tail width fluctuations observed in still-attached<br />
ligaments are consistent with net mass movements along a<br />
truncated cone shape fitted to the downstream ligament behind<br />
the jet head; this cone had a diameter of 6 m at the<br />
Figure 10. Independence of tail deflection <strong>and</strong> tail width fluctuations.<br />
The center of the jet head was at a position of 888 m. b Tail deflection<br />
without width fluctuations at an earlier time <strong>for</strong> the nozzle of a. The<br />
center of the head was at a distance of 826 m.<br />
position of the nozzle plane. This is illustrated in<br />
Figure 10(a), which shows part of the 0.9 mm long ligament<br />
attached to the nozzle, together with a straight line<br />
representing an equivalent undisturbed conical profile<br />
matching the ink material volume. The ink jet head is not<br />
shown because it was 50 m wide. The ligament width is<br />
clearly not constant along the length, nor is it zero near the<br />
nozzle plane: the minimum ligament width was 6.4 m.<br />
These data relate to the conditions 120 s after printing:<br />
strong width fluctuations appear at positions up to<br />
250 m from the nozzle. The trajectories of the main<br />
drop <strong>and</strong> its ligament were accurately directed away from the<br />
center of the nozzle orifice <strong>for</strong> a relatively long time, until<br />
the tail was apparently disturbed in some way. This suggests<br />
that it would be wrong to model the system as axisymmetric.<br />
At later times, the ligament tail appeared to be straight but<br />
offset, <strong>and</strong> originating from the edge of the nozzle opening.<br />
These effects were not r<strong>and</strong>om; they were consistent <strong>and</strong><br />
different <strong>for</strong> each nozzle, perhaps reflecting imperfections at<br />
the nozzle edges or some other variability, such as nozzle<br />
wetting or contamination.<br />
Figure 10(a) also shows the lateral position of a ligament<br />
tail plotted on the same scale as the width fluctuations.<br />
The center of the ligament lies accurately on the line joining<br />
the center of the head to the center of the nozzle back to a<br />
distance of 300 m from the nozzle, then shows a lateral<br />
displacement of up to 2 m be<strong>for</strong>e returning to the nozzle<br />
442 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Hutchings, Martin, <strong>and</strong> Hoath: High speed imaging <strong>and</strong> analysis of jet <strong>and</strong> drop <strong>for</strong>mation<br />
Figure 12. Ink volume passing beyond various downstream planes vs<br />
time.<br />
Figure 13. Illustration of the shapes of three long jets that have been fitted<br />
by simple empirical functions.<br />
Figure 11. a Jet tip profiles <strong>for</strong> nominally 6 m/s ink drops emerging<br />
from a 50 m diameter nozzle, at various times in microseconds following<br />
emergence. b Evolution of the diameters of the jet tip maximum<br />
width <strong>and</strong> ligament minimum width. c Percentage of total ink volume<br />
along a jet beyond the nozzle plane, <strong>for</strong> various ligament lengths <strong>and</strong><br />
ratios of length to main head width.<br />
center; the angles between the original ligament axis direction<br />
<strong>and</strong> the tail lay between −0.5° at a distance of 300 m<br />
<strong>and</strong> +1° at the nozzle plane. Figure 10(b) shows the geometry<br />
of a jet from the same nozzle as that in Fig. 10(a), but<br />
16.1 s earlier. At this earlier time, there are no appreciable<br />
width fluctuations but still a significant lateral shift. The<br />
phenomena of width fluctuations <strong>and</strong> lateral shift there<strong>for</strong>e<br />
appear to be independent. Furthermore, the timing <strong>and</strong> distance<br />
in<strong>for</strong>mation implies that the lateral shift has moved<br />
consistently with the axial stretching rate, while width fluctuations<br />
have appeared within 20 s of Fig. 10(b). The<br />
graphs in Fig. 10 were generated by averaging the images<br />
from three events captured with the same time delay. The<br />
small fluctuations derive from the digital nature of the images,<br />
which allow only certain values to be obtained <strong>for</strong> the<br />
width <strong>and</strong> position of the ligament <strong>and</strong> have an intrinsic<br />
error of ±1 pixel in the width measurement at each point.<br />
Ligament widths <strong>and</strong> tip diameters vary with time in a<br />
systematic fashion. Figure 11(a), <strong>for</strong> example, shows tip profiles<br />
of jets at several short times after emerging from the<br />
nozzle. The precision of the technique is illustrated by the<br />
linear uncertainty of only 1 pixel =0.61 m in this example,<br />
similar to the wavelength of the imaging light. Figure<br />
11(b) shows the rapid growth in the tip width of the emerging<br />
jet, followed by a rapid fall <strong>and</strong> then a rise toward the<br />
final drop size (corresponding to 100 pL printed drop volume<br />
in this example). The nozzle width is shown <strong>for</strong> comparison.<br />
The minimum ligament width falls quickly after the<br />
emergent tip starts necking, <strong>and</strong> continues to shrink as the<br />
ligament stretches.<br />
When the ligament is long <strong>and</strong> unbroken, the volume<br />
lying beyond the nozzle plane stretches as the ligament extends,<br />
while the volume fraction in the head increases. For<br />
the example shown in Fig. 11(c), the ratio of the length of<br />
the ligament to the diameter of the head reaches 20 be<strong>for</strong>e<br />
the ligament snaps, at which point there is still 30% of the<br />
total volume in the tail <strong>and</strong> 70% in the head section. The<br />
total downstream ink volume can be calculated (assuming<br />
reproducible ligaments with circular cross section) at different<br />
locations. Figure 12 shows that the ink volume beyond<br />
the nozzle plane initially overshoots the final drop volume,<br />
whereas it undershoots at downstream locations even as<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 443
Hutchings, Martin, <strong>and</strong> Hoath: High speed imaging <strong>and</strong> analysis of jet <strong>and</strong> drop <strong>for</strong>mation<br />
stretching, with additional exponential <strong>and</strong> cubic terms extending<br />
to the main drop hemisphere (see Figure 13). As<br />
noted above, the ligament behind the main drop tapers<br />
smoothly.<br />
Figure 14(a) shows a sequence of images of jets from<br />
the same nozzle as they develop over time. Two volume<br />
elements have been selected <strong>and</strong> tracked. The element nearer<br />
the nozzle exhibits considerable extension during jet development.<br />
In contrast, the element within the head shows a<br />
slight contraction, as shown in Fig. 14(b).<br />
CONCLUSIONS<br />
By using flash illumination of very short duration (ca.<br />
20 ns), high quality, single-event digital images of jets <strong>and</strong><br />
drops can be captured. A computer program, PEJET, has been<br />
written to automate the processing of such images <strong>and</strong> to<br />
generate quantitative data about the whole ink stream. From<br />
this data, it is then possible to compute the variation in fluid<br />
volume, volume flow, <strong>and</strong> velocity as a function of both<br />
position <strong>and</strong> time. The method has been shown to have high<br />
accuracy. The results can be used to study the influence of<br />
nozzle design, drive wave<strong>for</strong>m <strong>and</strong> fluid properties on jet<br />
<strong>and</strong> drop <strong>for</strong>mation, as well as to provide accurate data <strong>for</strong><br />
comparison to the results of computational modeling.<br />
The level of quantitative in<strong>for</strong>mation that can be extracted<br />
from high speed flash images allows jet profiles <strong>and</strong><br />
satellite <strong>for</strong>mation to be studied over time. As examples,<br />
tail-width fluctuations, lateral deflections, <strong>and</strong> satellite velocities<br />
have been quantitatively analyzed.<br />
It has been shown that, at a late stage, the jet ligament is<br />
unstable <strong>and</strong> tends to move away from the center of the<br />
nozzle. At similar times the ligament rapidly <strong>for</strong>ms fluctuations<br />
leading to break off <strong>and</strong> satellites. This late-stage lateral<br />
displacement of the ink jet ligament occurs in the absence of<br />
aerodynamic, acoustic, or other influences from adjacent<br />
jets.<br />
ACKNOWLEDGMENTS<br />
This work was supported by the UK Engineering <strong>and</strong> Physical<br />
<strong>Science</strong>s Research Council (EPSRC) <strong>and</strong> by a consortium<br />
of industrial partners within the Cambridge Inkjet Research<br />
Centre. Rhys Morgan is thanked <strong>for</strong> his help in constructing<br />
the experimental equipment.<br />
Figure 14. a Image sequence <strong>and</strong> b element length evolution.<br />
close as 17 m (one-third of the nozzle diameter). This indicates<br />
that some ink flows backward into the nozzle from<br />
very close range, while the rest moves <strong>for</strong>ward as a ligament.<br />
In these experiments, the ligament stretched <strong>for</strong> tens of microseconds<br />
be<strong>for</strong>e it broke off.<br />
The leading surfaces of the jets studied in this work<br />
have been found to be very closely hemispherical, once the<br />
emergent phase has passed, as would be expected if the<br />
shape is controlled by surface tension <strong>and</strong> negligible aerodynamic<br />
flattening occurs. The rear surface of the head exhibits<br />
a shape that can be approximated by constant, linear, <strong>and</strong><br />
quadratic terms linked to the nozzle plane via ligament<br />
REFERENCES<br />
1 P. Pierron, S. Allaman, <strong>and</strong> A. Soucemarianadin, “Dynamics of jetted<br />
liquid filaments”, Proc. IS&T’s, NIP17 (IS&T, Springfield, VA, 2001) p.<br />
308.<br />
2 S. Allaman <strong>and</strong> G. Desie, “Improved ink jet in situ visualization<br />
strategies”, Proc. IS&T’s, NIP20 (IS&T, Springfield, VA, 2004) p. 383.<br />
3 H. Dong, W. W. Carr, <strong>and</strong> J. F. Morris, “An experimental study of<br />
drop-on-dem<strong>and</strong> drop <strong>for</strong>mation”, Phys. Fluids 18, 072102 (2006).<br />
4 J. Etienne, E. J. Hinch, <strong>and</strong> J. Li, “A Lagrangian–Eulerian approach <strong>for</strong><br />
the numerical simulation of free-surface flow of a viscoelastic material”,<br />
J. Non-Newtonian Fluid Mech. 136, 157 (2006).<br />
5 W. T. Pimbley <strong>and</strong> H. C. Lee, “Satellite droplet <strong>for</strong>mation in a liquid jet”,<br />
IBM J. Res. Dev. 21, 21 (1977).<br />
6 C. Rembe, J. Patzer, E. P. Hofer, <strong>and</strong> P. Krehl, “Realcinematographic<br />
visualization of droplet ejection in thermal ink jets”, Recent Progress in<br />
Ink Jet Technologies II (IS&T, Springfield, VA, 1999) p. 103.<br />
7 J. C. Russ, The Image Processing H<strong>and</strong>book, 4th ed. (CRC Press, Boca<br />
Raton, FL, 2002).<br />
444 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 445–451, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Effects of Thin Film Layers on Actuating Per<strong>for</strong>mance<br />
of Microheaters<br />
Min Soo Kim, Bang Weon Lee, Yong Soo Lee, Dong Sik Shim <strong>and</strong> Keon Kuk<br />
Micro Systems Lab, Samsung Advanced Institute of <strong>Technology</strong> (SAIT), Yongin, Gyeonggi 446-712, Korea<br />
E-mail: minskim@samsung.com<br />
Abstract. We investigated effects of thin film layers on actuating<br />
per<strong>for</strong>mance of microheaters. Bubble behaviors on microheaters<br />
were observed experimentally, <strong>and</strong> heat conduction characteristics<br />
in thin film layers were analyzed numerically. Nine kinds of tantalum<br />
nitride (TaN) microheaters were prepared. Step-stress test showed<br />
that voltage limits of nonpassivated heaters were 50% of those of<br />
passivated heaters. Open pool bubble test was carried out using<br />
deionized water as a working fluid. Nonpassivated heaters produced<br />
comparable bubbles with only 20–50% of input energy required <strong>for</strong><br />
passivated heaters. However, nonpassivated heaters could be operated<br />
only in a narrow range of driving voltage. We constructed a<br />
hybrid model <strong>for</strong> bubble nucleation prediction correlating nucleation<br />
times with driving powers. Based on work of bubble <strong>for</strong>mation estimated<br />
from bubble volume evolution, actuation efficiencies of<br />
microheaters were calculated <strong>and</strong> compared. Efficiencies of<br />
nonpassivated heaters were much higher than those of passivated<br />
heaters. However, nonpassivated heaters failed to show robust actuating<br />
characteristics over a wide range of power density. As promising<br />
microactuators, nonpassivated heaters need further investigation<br />
from a viewpoint of reliability. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong><br />
<strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5445<br />
INTRODUCTION<br />
Microheaters are one of the actuators widely adopted in<br />
various microsystems. Their materials <strong>and</strong> simple structures<br />
are compatible with st<strong>and</strong>ard silicon processes. Also, large<br />
actuating <strong>for</strong>ce enables the size small compared to the other<br />
actuators, such as piezoelectric, electrostatic, electromagnetic,<br />
<strong>and</strong> acoustic. This is favorable <strong>for</strong> an array configuration<br />
of high spatial density, such as an ink jet print head.<br />
Characteristics of various types of ink jet heads have been<br />
summarized. 1<br />
Many studies of microheaters contributed underst<strong>and</strong>ing<br />
of bubble generation mechanism. Asai 2 proposed a<br />
bubble nucleation theory to guide the design of thermal<br />
inkjet heads, <strong>and</strong> Andrews 3 showed complete bubble cycles<br />
from nucleation to collapse by visualization techniques.<br />
Rembe et al. 4 visualized nonreproducible phenomena on<br />
microheaters with real high speed cine photomicrography.<br />
Kuketal. 5 studied thermal efficiencies of microheaters, especially<br />
focusing on the effects of heater size <strong>and</strong> aspect<br />
ratio.<br />
In this study, effects of thin film layers have been investigated<br />
on actuating per<strong>for</strong>mance of microheaters. Bubble<br />
Received Jan. 8, 2007; accepted <strong>for</strong> publication May 7, 2007.<br />
1062-3701/2007/515/445/7/$20.00.<br />
behaviors on microheaters were observed experimentally,<br />
<strong>and</strong> heat conduction characteristics in thin film layers were<br />
analyzed numerically. Step-stress tests (SSTs) <strong>and</strong> open pool<br />
bubble visualization were carried out. Bubble volumes were<br />
estimated from the images taken experimentally. A hybrid<br />
model was constructed <strong>for</strong> prediction of bubble nucleation.<br />
Work of bubble <strong>for</strong>mation was calculated from bubble volumes<br />
using both nucleation pressure (i.e., maximum bubble<br />
pressure) from heat conduction simulation <strong>and</strong> pressure<br />
profile following Asai’s model. 6 Finally, actuation efficiencies<br />
of microheaters with different passivation layers were obtained.<br />
Their actuating per<strong>for</strong>mances were compared, <strong>and</strong><br />
the effects of thin film layers were discussed.<br />
METHODS<br />
Preparation of Microheaters<br />
Microheaters were fabricated using conventional thin film<br />
processes (Figure 1). Thermal barrier layer of silicon dioxide<br />
SiO 2 was thermally grown up to 3 m on silicon wafer.<br />
Tantalum nitride TaN heater <strong>and</strong> aluminum (Al) electrode<br />
were consecutively deposited by sputtering <strong>and</strong> patterning.<br />
The sheet resistance of TaN film was 50 /. Silicon<br />
nitride SiN x film was deposited using the PECVD technique<br />
as a heat transfer layer. Finally, tantalum (Ta) film of<br />
3000 Å was deposited as an anticavitation layer.<br />
Nine kinds of microheaters were prepared with different<br />
thin film layers. Microheaters had a square shape <strong>and</strong> the<br />
same planar size of 22 m22 m. Three different thicknesses<br />
of SiO 2 were 1 m, 2 m, <strong>and</strong> 3 m. Three different<br />
types of passivation layers included nonpassivation, SiN x of<br />
4000 Å with Ta of 3000 Å, <strong>and</strong> SiN x of 6000 Å with Ta of<br />
3000 Å. Heaterswerelocated300 m from a chip edge<br />
<strong>for</strong> side view images of bubble. Table I lists mean values of<br />
electrical resistance of the fabricated heaters along with their<br />
st<strong>and</strong>ard deviations. The number of data points was 60 <strong>for</strong><br />
each type.<br />
A step-stress test has been per<strong>for</strong>med to compare voltage<br />
limits of different microheaters. Voltage limit means a<br />
maximum endurable voltage, defined as maximum voltage<br />
be<strong>for</strong>e causing more than 1% variation from the initial value<br />
of resistance. At a fixed pulses width, electrical pulses of a<br />
prescribed voltage level were repeatedly given to a<br />
microheater. After 100,000 cycles at a frequency of 1000 Hz,<br />
electrical resistance was measured <strong>and</strong> then the voltage level<br />
was increased with an increment of 1V. Maximum endur-<br />
445
Kim et al.: Effects of thin film layers on actuating per<strong>for</strong>mance of microheaters<br />
Table I. Values of electrical resistance of the fabricated microheaters.<br />
SiO 2<br />
Passivation Type 1 µm 2 µm 3 µm<br />
No passivation<br />
Mean a Ω 60.5 64.0 59.2<br />
St<strong>and</strong>ard deviation Ω 2.2 1.3 1.0<br />
SiN x 4000 Å <strong>and</strong> Mean Ω 60.6 66.1 65.4<br />
Ta 3000 Å<br />
St<strong>and</strong>ard deviation Ω 1.8 1.2 1.9<br />
SiN x 6000 Å <strong>and</strong> Mean Ω 61.0 67.6 68.6<br />
Ta 3000 Å<br />
St<strong>and</strong>ard deviation Ω 1.2 1.9 1.5<br />
a The number of samples is 60 <strong>for</strong> each heater.<br />
Figure 2. Experimental setup <strong>for</strong> open pool bubble visualization.<br />
Figure 1. Optical microscope images of the fabricated microheaters: a<br />
nonpassivated heater, 22 m22 m microheater with SiO 2 of 3 m<br />
<strong>and</strong> b passivated heater, 22 m22 m microheater with SiO 2 of<br />
1 m, SiN x of 6000 Å <strong>and</strong> Ta of 3000 Å.<br />
able voltage was recorded as the one causing variation more<br />
than 1% from the initial resistance.<br />
Open Pool Bubble Visualization<br />
Bubbles on the heaters were visualized with a microscope in<br />
an open pool setup shown in Figure 2. A xenon stroboscope<br />
provided sample illumination through the microscope. The<br />
bubble was synchronized with the input pulse to the heater,<br />
<strong>and</strong> bubble images were captured by high speed CCD<br />
cameras. 7 Exposure time of the CCD was 0.3 s, <strong>and</strong> frame<br />
time of consecutive images was 0.1 s. Estimation of bubble<br />
volume at a fixed time requires two synchronized bubble<br />
images, plane <strong>and</strong> side views with two CCD arrangements as<br />
shown in Figure 3. CCD1 captured bubble width, length,<br />
<strong>and</strong> height could be obtained from CCD2 image. 5<br />
Square wave electrical heating pulses were applied to<br />
microheaters with 8Hzrepetition frequency. Heating current<br />
was measured by a current probe. During pulse heating,<br />
voltage difference between electrodes was recorded using a<br />
Figure 3. Two CCDs <strong>for</strong> capturing both plane <strong>and</strong> side views of bubble.<br />
digital oscilloscope. Electrical power multiplied by pulse<br />
width represents input energy to the heater. 8<br />
Bubble volume was obtained as a function of time by<br />
image processing, using both experimental plane <strong>and</strong> side<br />
views of the bubble. 5 Work of bubble <strong>for</strong>mation was estimated<br />
following the same procedure as in our previous<br />
article. 5<br />
Hybrid Model <strong>for</strong> Bubble Nucleation Prediction<br />
In the present study, three models <strong>for</strong> bubble nucleation<br />
prediction were compared to experimental data. In the temperature<br />
model, a bubble starts to nucleate when interfacial<br />
446 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Kim et al.: Effects of thin film layers on actuating per<strong>for</strong>mance of microheaters<br />
Table II. Properties of thin film materials.<br />
Material<br />
Density<br />
kg/m 3 <br />
Heat Capacity<br />
kJ/kg<br />
Thermal<br />
Conductivity<br />
W/m K<br />
Si 2340.0 760.0 165.0<br />
SiO 2 2070.0 840.0 1.4<br />
TaN 16,600.0 150.0 57.0<br />
Al 2690.0 898.7 221.5<br />
SiN x 3,200.0 790.0 1.67<br />
Ta 16,600.0 150.0 57.0<br />
temperature T int , which is defined as maximum temperature<br />
at the interface between heater <strong>and</strong> fluid, reaches a critical<br />
temperature T cri . In the heat flux model, 9 instead of a<br />
single fixed interfacial temperature T int , the critical temperature<br />
is determined by considering heat flux through the interface<br />
as follows:<br />
T heat =230°C+L char<br />
T<br />
z ,<br />
where L char is a thermal characteristic thickness of the heat<br />
flux model.<br />
However, these two models showed agreement with experimental<br />
data only over a partial range of power density<br />
(see Bubble Nucleation). In the present study, a hybrid<br />
model was constructed from these two models to correlate<br />
bubble nucleation time with power density. Based on the<br />
results from the two models, the shorter nucleation time<br />
between two nucleation times predicted by temperature<br />
model <strong>and</strong> heat flux model in the hybrid model was adopted<br />
as follows:<br />
t hybrid = mint temp ,t heat ,<br />
where t temp represents a nucleation time determined by temperature<br />
model, <strong>and</strong> t heat by heat flux model.<br />
Numerical analyses of heat transfer in thin film layers<br />
have been per<strong>for</strong>med using CFD-ACE v2004 <strong>for</strong> seven different<br />
heaters: three nonpassivated heaters with thermal barrier<br />
SiO 2 thickness of 1 m, 2 m, <strong>and</strong> 3 m, <strong>and</strong> four<br />
passivated heaters with passivation layer SiN x thickness of<br />
4000 Å <strong>and</strong> 6000 Å, each having two different thermal barrier<br />
thicknesses of 1 m <strong>and</strong> 3 m.<br />
In numerical simulation, thermal properties of thin film<br />
layers, such as diffusivity, have a great influence on prediction<br />
of interfacial temperature. However, properties of thin<br />
film layers have been reported to vary largely, depending on<br />
measurement techniques as well as fabrication methods. In<br />
our numerical analyses, bulk properties were assumed <strong>for</strong><br />
thin film layers (Table II). Inevitably, this provided higher<br />
interfacial temperature than expected. In the present study,<br />
however, our concern was to obtain relative nucleation time<br />
as a function of power density rather than absolute value of<br />
interfacial temperature. As a critical temperature, maximum<br />
1<br />
2<br />
Figure 4. Results from step-stress test of microheaters, presenting voltage<br />
limits as a function of pulse width <strong>for</strong> three different passivation types.<br />
temperature at the liquid interface was used instead of average<br />
temperature to obtain better agreement with experimental<br />
data. In the present simulation, maximum temperature<br />
was 40°C higher than average temperature. Critical temperature<br />
<strong>and</strong> thermal characteristic thickness <strong>for</strong> the heat<br />
flux model were determined by matching numerical results<br />
with experimental data <strong>for</strong> the specific case of 4000 Å SiN x<br />
<strong>and</strong> 3 m SiO 2 . In the present study, the values were chosen<br />
as T cri =360°C <strong>and</strong> L char =0.2 m.<br />
RESULTS AND DISCUSSIONS<br />
Step-Stress Test of Microheaters<br />
Results from step-stress test are quite straight<strong>for</strong>ward <strong>and</strong><br />
intuitive. For different passivation types, Figure 4 depicts<br />
voltage limits at several selected pulse widths. Thermal barrier<br />
thickness SiO 2 was fixed with 1 m. For a specific<br />
passivation type, higher voltage could be used along with<br />
shorter pulse width. Voltage limits of passivated heaters were<br />
about two times higher than those of nonpassivated heaters.<br />
For example, at a pulse width of 1 s, voltage limit was 5V<br />
<strong>for</strong> nonpassivated heater, whereas it was 13 V <strong>for</strong> passivated<br />
heater with SiN x 6000 Å <strong>and</strong> Ta 3000 Å. This implies that,<br />
in terms of electrical power, existence of passivation layers<br />
helped the heater endure about six times higher voltage.<br />
Also, the heater with thicker passivation layers could st<strong>and</strong><br />
higher voltage. On the other h<strong>and</strong>, the nonpassivated heater<br />
had a narrow range of driving voltage as compared to passivated<br />
heaters, which means that nonpassivated heaters have<br />
a limited operating window.<br />
Table III shows voltage limits of nine microheaters at<br />
1 s pulse width. Voltage limits were higher <strong>for</strong> thicker passivation<br />
layers. On the other h<strong>and</strong>, effects of thermal barrier<br />
thickness seem to appear with increasing thickness of passivation<br />
layer. Our explanation is that thinner thermal barrier<br />
layer facilitates relaxation of thermal stresses due to more<br />
thermal dissipation through itself <strong>for</strong> the same thickness of<br />
passivation layer. In the present study, the highest voltage<br />
limit was obtained <strong>for</strong> the microheater of the thickest passivation<br />
layer with the thinnest thermal barrier layer.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 447
Kim et al.: Effects of thin film layers on actuating per<strong>for</strong>mance of microheaters<br />
Table III. Maximum voltages at 1 µs pulse width <strong>for</strong> three different passivation types,<br />
each with three different thicknesses of thermal barrier layer SiO 2 .<br />
SiO 2<br />
1µm 2µm 3µm<br />
No Passivation 5 5 5<br />
SiN x 4000 Å <strong>and</strong> Ta 3000 Å 10 9 9<br />
SiN x 6000 Å <strong>and</strong> Ta 3000 Å 13 9 10<br />
Figure 6. Input energy to microheaters of different passivation types at<br />
normal bubble creation.<br />
Figure 5. Driving conditions of microheaters of different passivation types<br />
<strong>for</strong> normal bubble creation, along with corresponding SST results in Fig.<br />
4.<br />
Driving Conditions <strong>for</strong> Bubble Actuation<br />
Figure 5 depicts relations between driving voltage <strong>and</strong> pulse<br />
width <strong>for</strong> bubble creation on microheaters with different<br />
passivation layers. Again, thermal barrier thickness SiO 2 <br />
was fixed at 1 m. In our open pool visualization a bubble<br />
that is stable <strong>and</strong> repeatable in shape as well as saturated in<br />
size was chosen as a normal bubble. A normal bubble has a<br />
consistent shape with time (dV/dt0, whereV is bubble<br />
volume <strong>and</strong> t is time) as well as a maximal size with driving<br />
energy (dV/dE0, where E is driving energy). For a<br />
nonpassivated heater, the available driving voltage margin<br />
was relatively small. Also, at a fixed pulse width, a normal<br />
bubble was observed near the voltage limit obtained from<br />
the SST. However, passivated heaters could produce normal<br />
bubbles far below the voltage limit. On the other h<strong>and</strong>, at a<br />
fixed driving voltage, longer pulse width was required <strong>for</strong> the<br />
heater with thicker passivation layer, which means that more<br />
input energy is needed <strong>for</strong> heaters with thicker passivation<br />
layers due to increased energy consumption in the passivation<br />
layer during energy transfer to fluid. There<strong>for</strong>e, more<br />
input energy should be provided so that the prescribed thermal<br />
energy can be transferred to fluid <strong>for</strong> normal bubble<br />
creation.<br />
Input energy <strong>for</strong> normal bubble creation is plotted in<br />
Figure 6. Nonpassivated heaters consumed input energy<br />
about 20–50% compared to passivated heaters. More energy<br />
was required <strong>for</strong> heaters with thicker passivation layers as<br />
stated earlier. Experiments showed that input energy decreased<br />
with increasing driving voltage, i.e., with increasing<br />
driving power. At high driving power, relatively rapid temperature<br />
elevation seemed to make nucleation occur earlier<br />
<strong>and</strong> thus reduce input energy. On the other h<strong>and</strong>, it is well<br />
known that too high a driving power causes nucleation to<br />
occur too rapidly <strong>and</strong> too small a bubble <strong>for</strong>ms. 5 There<strong>for</strong>e,<br />
this implies existence of a lower limit in driving power <strong>for</strong><br />
minimizing input energy. For passivated heaters, input energy<br />
seemed to saturate after around 9V, whereas nonpassivated<br />
heaters showed no saturation region, possibly due to<br />
a narrow range of driving voltage from the SST.<br />
Characteristics of bubble creation are presented in Figure<br />
7. Bubble nucleation time is depicted in Fig. 7(a) <strong>and</strong><br />
bubble life in Fig. 7(b). The open pool bubble test may have<br />
asystemdelayof0.1 s between trigger signal <strong>and</strong> application<br />
of the actual heating pulse to a microheater. There<strong>for</strong>e,<br />
the obtained nucleation time may have a corresponding<br />
uncertainty. From Fig. 7(a), nucleation time was largely dependent<br />
on driving power. Higher driving power resulted in<br />
earlier bubble nucleation. At the same driving power, faster<br />
bubble nucleation was observed on nonpassivated heaters<br />
due to rapid temperature elevation with minimal energy<br />
loss. On the other h<strong>and</strong>, from Fig. 7(b), the created bubbles<br />
lasted longer on passivated heaters. Bubble life decreased<br />
with increasing driving power. All these observations support<br />
the inference that bubble life is affected mainly by heat<br />
transfer time be<strong>for</strong>e nucleation. However, effects of thickness<br />
of passivation layers seemed to be different depending on the<br />
power density level as seen from Fig. 7(b). Above a certain<br />
driving voltage (here, 8 V), bubble life became longer on a<br />
thinner passivation layer. This reversal might be explained by<br />
more energy transfer to fluid on thinner passivation layers at<br />
high power density resulting in relatively wider heated region<br />
adjacent to the heater surface. This distinct behavior,<br />
depending on power density, needs further analysis.<br />
Bubble Nucleation, Work of Bubble Formation, <strong>and</strong><br />
Actuation Efficiency<br />
With values of T cri =360°C <strong>and</strong> L char =0.2 m obtained<br />
above, our hybrid model predicted nucleation times in good<br />
agreement with experimental data as shown in Figure 8(a).<br />
The temperature model predicted later nucleation at low<br />
448 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Kim et al.: Effects of thin film layers on actuating per<strong>for</strong>mance of microheaters<br />
Figure 7. Bubble characteristics: a bubble nucleation time <strong>and</strong> b<br />
bubble life <strong>for</strong> three different passivation types.<br />
power density than experimental observations [Fig. 8(b)], as<br />
did the heat flux model at high power density [Fig. 8(c)].<br />
This means that a single critical temperature cannot cover<br />
the whole range of power density <strong>and</strong> thus it should vary,<br />
depending on the power density level. From above observation,<br />
we constructed a hybrid model where the critical temperature<br />
was determined as the one giving shorter nucleation<br />
time between the two models [Eq. (2)]. As a result, the<br />
hybrid model could fit experimental data excellently in the<br />
whole range of power density.<br />
Based on the hybrid model, input energy be<strong>for</strong>e nucleation<br />
was calculated as shown in Figure 9(a). For nonpassivated<br />
heaters, input energy decreased 50% when power<br />
density changed from 0.8 GW/m 2 to 2GW/m 2 .However,<br />
<strong>for</strong> passivated heaters, input energy was little affected by<br />
change of power density. As shown in Fig. 9(b), the energy<br />
transferred to fluid be<strong>for</strong>e nucleation was inversely proportional<br />
to the power density <strong>for</strong> both nonpassivated <strong>and</strong> passivated<br />
heaters.<br />
Estimated work of bubble <strong>for</strong>mation <strong>and</strong> efficiencies are<br />
presented in Figure 10. Bubble work decreased with increasing<br />
power density <strong>for</strong> all passivation types, as shown in Fig.<br />
10(a). This might be explained by smaller bubble size <strong>and</strong><br />
shorter bubble life, along with less heat transfer to fluid owing<br />
to earlier nucleation <strong>for</strong> higher power density. Results<br />
also showed that comparable work of bubble <strong>for</strong>mation<br />
could be obtained from nonpassivated heaters. For passivated<br />
heaters, thickness of passivation layer SiN x affected<br />
Figure 8. Prediction of nucleation time from the hybrid model along with<br />
the corresponding experimental data: a hybrid model, b temperature<br />
model, <strong>and</strong> c heat flux model.<br />
both work of bubble <strong>for</strong>mation <strong>and</strong> efficiency. Heaters with<br />
4000 Å SiN x produced more work showing higher efficiency<br />
than those with 6000 Å SiN x at the same power density.<br />
On the other h<strong>and</strong>, as seen in Fig. 10(b), in the power<br />
density range of 1–2 GW/m 2 , nonpassivated heaters<br />
showed much higher efficiencies although they produced<br />
less work. From Fig. 9, nonpassivated heaters could deliver<br />
almost half the input energy to the fluid. In the power den-<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 449
Kim et al.: Effects of thin film layers on actuating per<strong>for</strong>mance of microheaters<br />
Figure 9. Simulation results based on the hybrid model: a input energy<br />
up until the nucleation time <strong>and</strong> b effective energy transferred to fluid <strong>for</strong><br />
different thin film layers.<br />
sity of 1 GW/m 2 , the nonpassivated heater could transport<br />
12% more energy to fluid as compared to the passivated<br />
heaters, even with less input energy. There<strong>for</strong>e,<br />
nonpassivated heaters could produce comparable work [Fig.<br />
10(a)] with less input energy [Fig. 9(a)] at lower power density.<br />
This strongly supports higher efficiency of nonpassivated<br />
heaters as depicted in Fig. 10(b).<br />
Furthermore, in Fig. 10(b), efficiency of nonpassivated<br />
heaters showed an opposite trend with power density. This<br />
could be explained by a nonlinear effect of passivation layers<br />
on the heat transfer from heater to fluid. In passivated heaters,<br />
as power density increases, the temperature gradient at<br />
fluid interface is kept almost constant due to existence of<br />
passivation layers, while it increases correspondingly <strong>for</strong><br />
nonpassivated heaters. From Fig. 8, higher power density<br />
resulted in earlier bubble nucleation, showing a much larger<br />
rate of decrease <strong>for</strong> nonpassivated heaters. This implies that<br />
a relatively large decrease in input energy occurs <strong>for</strong> nonpassivated<br />
heaters, as is confirmed again in Fig. 9(a). Earlier<br />
nucleation means a decrease in heat transfer time <strong>for</strong> transporting<br />
thermal energy to fluid. However, <strong>for</strong> nonpassivated<br />
Figure 10. a Estimated work of bubble <strong>for</strong>mation <strong>and</strong> b actuation<br />
efficiency defined as the ratio of work to input energy.<br />
heaters, effective energy transferred to fluid is little affected<br />
due to absence of energy consumption in passivation layers<br />
in the passivated heaters. Thus, prescribed thermal energy<br />
<strong>for</strong> bubble creation can be delivered to the fluid with much<br />
less input energy (Fig. 9). Also, work of bubble <strong>for</strong>mation is<br />
roughly proportional to the energy input into the fluid<br />
[Figs. 10(a) <strong>and</strong> 9(b)]. There<strong>for</strong>e, the actuation efficiency,<br />
defined as a ratio of work to input energy, increases with<br />
increasing power density <strong>for</strong> nonpassivated heaters.<br />
Ink jet heads have been fabricated adopting some of<br />
heaters considered in the present study. Two kinds of passivation<br />
layers (i.e., SiN x =4000 Å or 6000 Å) <strong>and</strong> two kinds<br />
of thermal barrier layers (i.e., SiO 2 =2 m or 3 m) were<br />
tested. Heater sizes were 400 m 2 =20 m20 m or<br />
576 m 2 =24 m24 m. The fabricated ink jet heads<br />
adopted a traditional top shooting structure, <strong>and</strong> one chip<br />
had 50 heaters. Chamber sizes were 24 m24 m<br />
17 m =WLH or 28 m28 m17 m <strong>for</strong><br />
each heater. Nozzle plate thickness was 12 m, <strong>and</strong> orifice<br />
exit diameter was 12 m. Inlet flow passage was 14 m<br />
20 m17 m =WLH. Ejected droplet volume<br />
was two to three picoliters. Table IV provides droplet ejec-<br />
450 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Kim et al.: Effects of thin film layers on actuating per<strong>for</strong>mance of microheaters<br />
Table IV. Droplet ejection per<strong>for</strong>mance of the inkjet heads adopting some of the<br />
passivated heaters considered in the present study.<br />
SiN x<br />
Å<br />
SiO 2<br />
µm<br />
Heater<br />
µmµm<br />
Voltage<br />
V<br />
Pulse<br />
µs<br />
Energy<br />
µJ<br />
Speed<br />
m/s<br />
Frequency<br />
kHz<br />
4000 2 2020 8 1.3 1.35 16.9 18<br />
2424 8 1.5 1.51 17.2 15<br />
3 2020 8 1.0 1.12 15.4 15<br />
2424 8 1.1 1.21 17.2 18<br />
6000 2 2020 8 1.4 1.52 15.8 18<br />
2424 8 1.4 1.44 16.2 18<br />
3 2020 8 1.4 1.49 16.3 15<br />
2424 9 1.1 1.52 15.3 15<br />
tion per<strong>for</strong>mance of the fabricated ink jet heads under typical<br />
driving conditions. Ink jet heads with thicker passivation<br />
layers consumed more energy <strong>for</strong> stable ejection. Obviously,<br />
this is consistent with the result that more energy is required<br />
<strong>for</strong> normal bubble creation with a thicker passivation layer<br />
[Figs. 6 <strong>and</strong> 9(a)]. In Table IV a thermal barrier layer SiO 2 <br />
seemed to become significant with respect to input energy<br />
<strong>for</strong> thinner passivation layers. When the thermal barrier<br />
layer becomes too thin, undesirable heat loss may increase<br />
during the heating period.<br />
CONCLUSIONS<br />
Results from step-stress test emphasize that maximum<br />
power density level <strong>for</strong> the nonpassivated heater should be<br />
kept low compared to the passivated heaters. Open pool<br />
bubble actuation indicates that the energy margin is very<br />
small <strong>for</strong> nonpassivated heaters. On the contrary, nonpassivated<br />
heaters showed much higher actuation efficiency<br />
even with lower input energy, but only over a limited range<br />
of power density. The hybrid model could provide nucleation<br />
times in excellent agreement with experimental data.<br />
As promising microactuators, applicability of nonpassivated<br />
heaters needs further investigation from the viewpoint of<br />
reliability. Our investigation will be helpful <strong>for</strong> development<br />
of the thermal ink jet heads <strong>for</strong> more reliable <strong>and</strong> higher<br />
per<strong>for</strong>mance.<br />
REFERENCES<br />
1 K. Silverbrook, US Patent 6,338,547B1 (2002).<br />
2 A. Asai, “Application of the nucleation theory to the design of bubble jet<br />
printers”, Jpn. J. Appl. Phys., Part 1 28(5), 909 (1989).<br />
3 J. R. Andrews, “Micro-boiling <strong>and</strong> pico-jetting”, IMA Workshop: Analysis<br />
<strong>and</strong> Modeling of Industrial Jetting Processes (IMA, Cambridge, UK,<br />
2001).<br />
4 C. Rembe, S. Wiesche, M. Beuten, <strong>and</strong> E. P. Hofer, “Nonreproducible<br />
phenomena in thermal ink jets with real high-speed cine<br />
photomicrography”, Proc. SPIE 3409, 316 (1998).<br />
5 K. Kuk, J. H. Lim, M. S. Kim, M. C. Choi, C. H. Cho, <strong>and</strong> Y. S. Oh,<br />
“Research on micro heater efficiency <strong>for</strong> thermal inkjet head”, J. <strong>Imaging</strong><br />
Sci. Technol. 49(5), 545 (2005).<br />
6 A. Asai, “Bubble dynamics in boiling under high heat flux pulse<br />
heating”, J. Heat Transfer 113, 973 (1991).<br />
7 C. Rembe, J. Patzer, E. Hofer, <strong>and</strong> P. Kreh, “Real cinematographic<br />
visualization of droplet ejection in thermal ink jets”, J. <strong>Imaging</strong> Sci.<br />
Technol. 40(5), 401 (1996).<br />
8 J. H. Lim, Y. S. Lee, H. T. Lim, S. S. Baek, K. Kuk, <strong>and</strong> Y. S. Oh,<br />
“Visualization of bubbles with various current density distributions in<br />
thermal inkjet head”, Proc. IEEE MEMS Conference (IEEE, Piscataway,<br />
NJ, 2003) pp. 197–200.<br />
9 W. Runge, “Nucleation in thermal ink-jet printers”, Proc. IS&T’s NIP8<br />
(IS&T, Springfield, VA, 1992) pp. 299–302.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 451
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 452–455, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Hybrid Stacked RFID Antenna Coil Fabricated by Ink Jet<br />
Printing of Catalyst with Self-assembled Polyelectrolytes<br />
<strong>and</strong> Electroless Plating<br />
Chung-Wei Wang, Ming-Huan Yang, Yuh-Zheng Lee <strong>and</strong> Kevin Cheng<br />
DTC/Industrial <strong>Technology</strong> Research Institute, Hsinchu, Taiwan, Republic of China<br />
E-mail: wangcw@itri.org.tw<br />
Abstract. This article describes a method of <strong>for</strong>ming a stacked hybrid<br />
metal structure <strong>and</strong> pattern to enhance radio-frequency identification<br />
antenna coil inductance. The essential strategies included<br />
the use of multilayer self-assembled polyelectrolytes to modify the<br />
surface property of substrates, an ink jet printing process <strong>for</strong> a Pd<br />
containing catalyst, <strong>and</strong> a stacked hybrid metal layer <strong>for</strong>med by<br />
electroless plating in subsequent processes. The results demonstrate<br />
that the minimum line width <strong>and</strong> line spacing can reach<br />
100 m/100 m, <strong>and</strong> electrical per<strong>for</strong>mance is compared to prior<br />
research in employing different approaches. The method presented<br />
in this article enhances the capability by adapting to any substrate<br />
surface using the self-assembled polyelectrolyte technique. There<strong>for</strong>e,<br />
results were verified on different substrates, such as PI, PET,<br />
<strong>and</strong> FR-4; satisfactory electric per<strong>for</strong>mance <strong>for</strong> application was obtained.<br />
In detail, the inductance of the antenna improved from 300<br />
nH to 20 H <strong>for</strong> a monolayer coil, <strong>and</strong> 600 nH to 50 H <strong>for</strong> a double<br />
layer coil, depending on the metal thickness. © 2007 <strong>Society</strong> <strong>for</strong><br />
<strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5452<br />
INTRODUCTION<br />
In recent years, printed organic electronics have been intensively<br />
developed because of their advantages of low cost, easy<br />
<strong>and</strong> flexible processing, efficient usage of material, <strong>and</strong> large<br />
scale manufacturing compared to conventional lithography<br />
<strong>and</strong> vacuum processes. These applications 1 include polymer<br />
light-emitting diodes (PLED), organic thin-film transistors<br />
(OTFT), organic bistable memory (OBD), metal circuits, etc.<br />
According to the <strong>for</strong>ecast data from marketing research institutes,<br />
printed logic integrated circuits (ICs) will gradually<br />
attract more attention <strong>and</strong> will dominate in the future. 1<br />
Among the logic ICs, radio-frequency identification (RFID)<br />
tags seem to be one of the most promising vehicles. These<br />
low cost printed products can replace not only bar codes but<br />
also expensive inorganic RFID silicon chips used <strong>for</strong> supply<br />
chain <strong>and</strong> automated retail checking. Generally, a simple<br />
RFID system (Figure 1) consists of a reader <strong>and</strong> a tag. There<br />
are many active <strong>and</strong> passive devices in a tag, including, at the<br />
least, an antenna, a capacitor, a diode, a transistor, <strong>and</strong> some<br />
memory. The system can be operated at lower frequencies,<br />
such as 125 kHz, 13.56 MHz, or860−930 MHz, <strong>and</strong> at a<br />
higher speed of 2.45 GHz <strong>for</strong> different applications. Many<br />
companies have focused on printed RFID development with<br />
organic semiconductors; however, the operation frequency<br />
of these RFIDs is limited due to the low carrier mobility<br />
characteristic of organic semiconductors. Hence, a highly<br />
conductive coil <strong>for</strong> the tag antenna is necessary. 2,3<br />
At present, most tag antennas with good conductivity<br />
are fabricated by copper or aluminum etching. The line<br />
width <strong>and</strong> line space are 600−800 m <strong>and</strong> 200−400 m,<br />
respectively, in each common etching case. Metal paste consisting<br />
of silver can be used in screen printing or ink jet<br />
printing processes but suffers from low resolution, poor adhesion,<br />
<strong>and</strong> high cost. In addition, the resulting metal paste<br />
patterns always need a thermal curing process to sinter the<br />
metal particles in order to achieve acceptable conductivity.<br />
The curing temperature increases with silver particle size,<br />
<strong>and</strong> it represents a challenge <strong>for</strong> application to flexible<br />
substrates, 3,4 especially with the screen process.<br />
Besides the issue of conductivity, high Q value inductors<br />
<strong>and</strong> well behaved capacitors are also required <strong>for</strong> power coupling<br />
<strong>and</strong> communication. High Q inductors can be<br />
achieved via several methods, which include adding magnetic<br />
material to the coil, increasing the number of circles,<br />
<strong>and</strong> reducing the resistance of the antenna coil. Among<br />
those measures, the addition of magnetic material is the best<br />
method <strong>for</strong> enhancing Q value. 4,5<br />
Received Jan. 17, 2007; accepted <strong>for</strong> publication Jun. 20, 2007.<br />
1062-3701/2007/515/452/4/$20.00.<br />
Figure 1. An RFID system.<br />
452
Wang et al.: Hybrid stacked RFID antenna coil fabricated by ink jet printing of catalyst with self-assembled polyelectrolytes <strong>and</strong> electroless plating<br />
Because of the need <strong>for</strong> high inductance value, in this<br />
article, a hybrid circuit fabrication method combined with<br />
magnetic nickel layering onto an ink jet printed copper<br />
RFID antenna coil is presented. In this approach, the inductance<br />
of the antenna coil can be enhanced from 300 nH to<br />
20 H <strong>for</strong> monolayer cases, <strong>and</strong> from 600 nH to 55 H <strong>for</strong><br />
double-layer cases. We will discuss the details below.<br />
EXPERIMENTAL<br />
Chemical Preparation<br />
Poly(allyamine hydrochloride) (PAH) (MW55,000–<br />
65,000) <strong>and</strong> poly(acrylic acid) (PAA) (MW70,000, 25%<br />
aqueous solution) were purchased from Aldrich. All chemicals<br />
were used without further purification. Deionized water<br />
(DI) was used in all aqueous solutions <strong>and</strong> rinsing procedures.<br />
The concentration of the polyelectrolyte-dipping solutions<br />
was 10 mM, based on the molecular weight (MW) of<br />
the repeat unit. The pH values of the PAH solution <strong>and</strong> PAA<br />
solution were adjusted to 7.5 <strong>and</strong> 3.5 by the addition of 1M<br />
HCl <strong>and</strong> 1 M NaOH, respectively. Layer-by-layer assembly<br />
was carried out using an autodipping system. Be<strong>for</strong>e the<br />
layer-by-layer assembly process, double layer substrates were<br />
drilled to af<strong>for</strong>d via holes to connect the two separated antenna<br />
circuits on each substrate side.<br />
Ink Formulation <strong>and</strong> Pd-Catalyst Patterning<br />
The catalyst ink was prepared by dissolving a Pd complex<br />
compound in DI water. The ink viscosity <strong>and</strong> surface tension<br />
were about 8−20 cps <strong>and</strong> 40 dyne/cm, respectively.<br />
The printing process was conducted on a third generation<br />
print plat<strong>for</strong>m, designed <strong>and</strong> integrated <strong>for</strong> 550 mm<br />
650 mm substrates by the Industrial <strong>Technology</strong> Research<br />
Institute, Taiwan (ITRI). Polyimide (PI), polyethylene<br />
terephthalate (PET), <strong>and</strong> FR-4 substrates with different selfassembly<br />
layers were printed with the catalyst solution.<br />
While the solution was drying, the catalyst would be adsorbed<br />
on the substrate surface <strong>and</strong> would gradually diffuse<br />
into the layer of poly(allylamine hydrochloride) beneath;<br />
there<strong>for</strong>e, an ink jet defined area was af<strong>for</strong>ded. 6–8<br />
Electroless Plating of Nickel<br />
After the Pd-containing catalyst was patterned on the selfassembled<br />
polyelectrolyte layer, a commercial <strong>for</strong>mulation,<br />
consisting of 40 g/L nickel sulfate, 20 g/L sodium citrate,<br />
10 g/L lactic acid, <strong>and</strong> 1 g/L dimethylaminobenzaldehyde<br />
(DMAB) in DI water, <strong>for</strong> the electroless nickel plating, was<br />
used to further exchange the Ni with the Pd, <strong>and</strong> leaves the<br />
Ni at the location of the predetermined pattern. The optimum<br />
condition to obtain complete transport between Ni<br />
<strong>and</strong> Pd corresponds to pH 6.8, as determined by titration<br />
with 0.3 M ammonium hydroxide solution. Be<strong>for</strong>e deposition<br />
of nickel metal, the substrate needed to be dipped into<br />
the accelerator solution <strong>for</strong> about 3−10 s <strong>and</strong> then washed<br />
with DI water <strong>and</strong> dried. In order to keep the solution homogeneous,<br />
an air bubble generator was used in the plating<br />
bath. 6–8<br />
Electroless Plating of Copper<br />
A pure Ni circuit has good magnetic characteristics but is<br />
weak in terms of electronic per<strong>for</strong>mance, i.e., resistivity. We<br />
Figure 2. Schematic structures of the a single side antenna coil <strong>and</strong> b<br />
double side antenna coil.<br />
there<strong>for</strong>e explore how a combined layer of Cu with Ni, in a<br />
certain thickness ratio, may incorporate both the desired<br />
electrical <strong>and</strong> magnetic per<strong>for</strong>mance. Accordingly, we tried<br />
growing a copper layer on the Ni layer by the solution process.<br />
Theoretically, nickel <strong>and</strong> nickel plating solutions would<br />
induce copper ion reduction; the substrate with a nickel antenna<br />
coil, obtained from the previous plating process, then<br />
needs to be washed with DI water <strong>for</strong> 10 min to remove<br />
nonadsorbed nickel particles <strong>and</strong> nickel plating solution<br />
from the surface, <strong>and</strong> then dried prior to electroless plating<br />
of copper. All of the reagents <strong>for</strong> copper electroless plating<br />
were dissolved in DI water at room temperature with stirring.<br />
To keep the solution homogeneous, an air bubble generator<br />
was also used in the electroless plating bath; the pH<br />
value of which was controlled. The copper deposition was<br />
carried out at 40 ° C. Immersion time <strong>and</strong> temperature are<br />
two key factors with which to control the metal thickness.<br />
After being removed from the electroless plating solution,<br />
the samples were rinsed with DI water to remove residual<br />
plating solution <strong>and</strong> then set aside to dry. The structures<br />
of monolayer <strong>and</strong> double layer antenna coil are shown in<br />
Figure 2. 7,8<br />
RESULTS AND DISCUSSION<br />
Table I lists the surface treatment processes to <strong>for</strong>m selfassembled<br />
polyelectrolyte layers on a substrate. The repeating<br />
step <strong>for</strong> PAH/PAA bilayer deposition depends on the<br />
substrate. From our experience, <strong>for</strong> example, the PET substrate<br />
needs three repeats. Similarly, if this surface treatment<br />
is applied to the polyimide substrate, the number of repeating<br />
steps increases to seven. An antenna coil pattern with a<br />
line width of 75−100 m <strong>and</strong> line space of 75−100 m,<br />
was accordingly obtained. The resulting conductance can be<br />
adjusted by varying the nickel <strong>and</strong> copper electroless plating<br />
recipes. The edge of a nickel line of 2 m thickness was<br />
uni<strong>for</strong>m, as shown in Figures 3(a)–3(c). The line edge after<br />
electroless copper plating was still smooth, as shown in Figs.<br />
3(d) <strong>and</strong> 3(e), where the copper metal thickness was 5 m.<br />
The ratio of Ni film to Cu film is 0.4 2 m/5 m,<br />
which is determined by the resistivity <strong>and</strong> inductance. How-<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 453
Wang et al.: Hybrid stacked RFID antenna coil fabricated by ink jet printing of catalyst with self-assembled polyelectrolytes <strong>and</strong> electroless plating<br />
Table I. Process flow of self-assembled polyelectrolyte layers be<strong>for</strong>e ink jet printing of<br />
catalyst.<br />
Step<br />
Substrate cleaning<br />
Immerse into PAH aq<br />
Substrate flushing<br />
Immerse into PAA aq<br />
Repeating<br />
PAH/PAA bilayer<br />
deposition<br />
Immerse into PAH aq<br />
10 mM<br />
Condition<br />
Washing with toluene<br />
<strong>and</strong> DI-water in sequence<br />
PAH aq 10 mM<br />
DI-water<br />
PAA aq 10 mM<br />
Up to three pairs of PAH<br />
/PAA layers<br />
PAH aq 10 mM<br />
Figure 3. a <strong>and</strong> d are 100 m wide printed single side antenna coil<br />
structure 27 circles layout; b, c, d, <strong>and</strong>f are optical micrographs<br />
of printed inductor <strong>for</strong>med on commercial flexible substrates.<br />
ever, many unexpected particles were present within the line<br />
spaces [Fig. 3(f)], possibly from unwanted residual nickel<br />
particles <strong>and</strong> the nickel electroless solution. To measure the<br />
electrical characteristics, an RLC meter (Agilent 4294A) was<br />
employed, <strong>and</strong> the data thus recorded are shown in Table II.<br />
Table II indicated that adding Ni will dramatically increase<br />
the inductances, the L- <strong>and</strong> the Q-factor, but not reduce the<br />
resistivity. The inductances of monolayer hybrid RFID antenna<br />
coils (samples A <strong>and</strong> B) were about 50 times higher<br />
than that of the copper antenna coil (sample C), although<br />
the resistances were similar in both cases. The inductances of<br />
the double layer hybrid RFID antenna coils (samples D <strong>and</strong><br />
E) were also 90 times higher than <strong>for</strong> the double layer<br />
copper coil, in spite of the resistance of a double layer hybrid<br />
metal antenna being higher than that of the double layer<br />
copper by 12 .<br />
The measured deposition rates <strong>for</strong> electroless nickel <strong>and</strong><br />
copper were 12 m <strong>for</strong> 60 min <strong>and</strong> 5 m <strong>for</strong> 60 min, respectively.<br />
In practice, the deposition rates were dependent<br />
on temperature, metal ion concentration, pH value of the<br />
bath solution, catalyst concentration, activator, the bath stability,<br />
<strong>and</strong> finally by electroless deposition time. In this study,<br />
the electroless bath stability was very important, because it<br />
influenced both the deposition speed <strong>and</strong> quality. The maximum<br />
thickness of nickel, which can be <strong>for</strong>med is up to<br />
35 m <strong>for</strong> over 3h, <strong>and</strong> copper can be deposited up to<br />
20 m over 4h. Compared to the results of Shan et al., 8<br />
who were able to deposit 5 m over 1hat saturation, this<br />
study made an obvious improvement in both growing rate<br />
<strong>and</strong> circuit per<strong>for</strong>mance. Accordingly, we changed the <strong>for</strong>mulation<br />
<strong>for</strong> electroless plating, <strong>and</strong> have an activator process<br />
prior to electroless plating. We adopt the mist activator<br />
<strong>for</strong> 5−10 sec to coat a thin layer of activator on catalyst in<br />
order to speed up the reaction with electroless plating.<br />
The incorporation of magnetic metal into the coil structure<br />
can dramatically enhance the inductance <strong>and</strong> the Q<br />
factor. The maximum Q factor, near 0.62, was achieved from<br />
a hybrid metal structure of 2 m nickel <strong>and</strong> 5 m copper<br />
in a single side antenna coil structure. A Q factor near 1.35<br />
was achieved <strong>for</strong> the double side antenna, which is about<br />
double than that obtained in the single side case. The conductivity<br />
of the printed antennas was 90% <strong>and</strong> 80% that of<br />
bulk copper <strong>for</strong> the single side antenna <strong>and</strong> double side antenna,<br />
respectively. The higher resistivity was due to the impurities<br />
imbedded in the antenna coil by the electroless plating<br />
process. Hence, the Q factor could be further improved<br />
by resistivity reduction of the plating metal.<br />
In addition to the Q factor, the impedance between antenna<br />
coil <strong>and</strong> interconnected coaxial cable should be about<br />
50 , on the basis of the layout of Fig. 1, in order to get the<br />
desired communication response of the single side circuit<br />
<strong>and</strong> double side circuit RFID antenna coils, as presented in<br />
Table II. The resulting impedance values were 29 <strong>and</strong><br />
49 <strong>for</strong> the single side antenna <strong>and</strong> double side RFID an-<br />
454 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Wang et al.: Hybrid stacked RFID antenna coil fabricated by ink jet printing of catalyst with self-assembled polyelectrolytes <strong>and</strong> electroless plating<br />
Table II. Characteristics of the printed antenna coils in single side <strong>and</strong> double side circuits.<br />
Sample<br />
L<br />
µH<br />
R ac<br />
Ω<br />
Q<br />
Impedance<br />
Ω<br />
Impedance phase<br />
°<br />
R dc<br />
Ω<br />
Conductivity<br />
s/m Cu<br />
Single side circuit A<br />
2 µmNi+5 µmCu<br />
Single side circuit B<br />
2 µmNi+5 µmCu<br />
Single side circuit C<br />
only 5 µm Cu<br />
Double sides circuit D<br />
2 µmNi+5 µmCu<br />
Double sides circuit E<br />
2 µmNi+5 µm Cu<br />
Double sides circuit F<br />
Only 5 µm Cu<br />
16 21.1081 0.62 24.5845 32.2368 20.45 0.904<br />
17 25.8908 0.54 29.2636 27.3605 24.33 0.767<br />
0.3 23.3501 0.0093 N/A N/A N/A N/A<br />
55 40.1211 1.1 48.1572 40.2387 40.78 0.828<br />
48 44.1578 0.96 50.3541 38.2173 42.35 0.758<br />
0.6 28.5872 0.02 N/A N/A N/A N/A<br />
<strong>for</strong> different layout designs. The resulting antenna coil is<br />
flexible <strong>and</strong> passes the 3M tape peeling test <strong>for</strong> PI, PET, <strong>and</strong><br />
FR-4 substrates. The inductance of this antenna is improved<br />
from 300 nH to 20 H <strong>for</strong> the single side coil, <strong>and</strong> 600 nH<br />
to 50 H <strong>for</strong> double side coil, compared to the pure copper<br />
coil under the same conditions. Also, we found that the electroless<br />
plating recipes used <strong>for</strong> nickel <strong>and</strong> copper leads to<br />
side reactions, which resulted in impurities left in the coil,<br />
<strong>and</strong> led to a slight elevation of resistivity, which we expect to<br />
be able to change in the near future.<br />
Figure 4. Photograph of the peeling test per<strong>for</strong>med on the single side<br />
antenna with 3M TM-650 tape.<br />
tennas, respectively. Obviously, a double side antenna with<br />
this design is more suitable than the single side antenna <strong>for</strong><br />
use in RFID application. Furthermore, the double-side design<br />
can reduce the substrate size <strong>and</strong> signal interference<br />
attributed to the dense single side coil architecture.<br />
The adhesion of this printed flexible RFID hybrid antenna<br />
coil was tested using 3M tape (TM–650). As shown in<br />
Figure 4, no observed peels proved that excellent adhesion<br />
between antenna coil <strong>and</strong> flexible substrate is achieved. 6<br />
CONCLUSIONS<br />
We have successfully demonstrated that monolayer <strong>and</strong><br />
double-layer RFID antenna coils can be manufactured by the<br />
use of multilayer SAMs, ink jet printing, <strong>and</strong> electroless plating<br />
processes. Incorporation of the magnetic metal nickel<br />
could dramatically enhance the inductance value <strong>and</strong> there<strong>for</strong>e<br />
the Q value. We can employ a multilayer antenna to<br />
obtain higher inductance with respect to the requirements<br />
REFERENCES<br />
1 K. Cheng, M.-H. Yang, W. W. W. Chiu, C.-Y. Huang, J. Chang, <strong>and</strong> T.-F.<br />
Yin, “Ink-jet printing, self-assembled polyelectrolytes, <strong>and</strong> electroless<br />
plating: Low cost fabrication of circuits on flexible substrates at room<br />
temperature”, Mater. Res. Soc. Symp. Proc. 26, 247–264 (2005).<br />
2 S. Molesa, D. R. Redinger, D. C. Huang, <strong>and</strong> V. Subramanian, “Highquality<br />
ink jet-printed multilevel interconnects <strong>and</strong> inductive<br />
components on plastic <strong>for</strong> ultra-low-cost RFID applications”, Mater.<br />
Res. Soc. Symp. Proc. 769, 831-836 (2003).<br />
3 S. E. Molesa, A. de la F. Vornbrock, P. C. Chang, <strong>and</strong> V. Subramanian,<br />
“Low-voltage ink jetted organic transistor <strong>for</strong> printed RFID <strong>and</strong> display<br />
applications”, IEEE Device Research Conference 26, 742–747 (2005).<br />
4 V. Subramanian, “Towards printed low-cost RFID Tags: Device, materials<br />
<strong>and</strong> circuit technologies”, MA, (March 16–19, 2003).<br />
5 D. Redinger, R. Farshchi, <strong>and</strong> V. Subramanian, “Ink jetted passive<br />
components on plastic substrate <strong>for</strong> RFID”, IEEE Trans. Electron<br />
Devices 51, 1978 (2004).<br />
6 C.-W. Wang, M.-H. Yang, J. Chang, K. Cheng, C.-P. Yu, C. H. Yu, C.-M.<br />
Chang, <strong>and</strong> C.-M. Chang, “Surface characteristics of ink-jet printed<br />
circuits by polyelectrolyte multilayers with zone model analysis <strong>and</strong> salt<br />
solution improvement”, IS&T’s 1st Digital Fabrication Conference (IS&T,<br />
Springfield, VA, 2005) p. 86.<br />
7 T.-F. Guo, S.-C. Chang, S. Pyo, <strong>and</strong> Y. Yang, “Vertical integrated<br />
electronic circuits via a combination of self-assembled polyelectrolytes<br />
ink-jet printing <strong>and</strong> electrodeless metal plating processes”, Langmuir<br />
18(21), 8142–8147 (2002).<br />
8 P. Shan, Y. Kevrekidis, <strong>and</strong> J. Benziger, “Ink-jet printing of catalyst<br />
patterns <strong>for</strong> electroless metal deposition”, Langmuir 15, 1584–1587<br />
(1999).<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 455
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 456–460, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Top Contact Organic Thin Film Transistors with Ink Jet<br />
Printed Metal Electrodes<br />
Kuo-Tong Lin, Chia-Hsun Chen, Ming-Huan Yang, Yuh-Zheng Lee <strong>and</strong> Kevin Cheng<br />
Display <strong>Technology</strong> Center, Industrial <strong>Technology</strong> Research Institute Hsinchu, Taiwan 310,<br />
Republic of China<br />
E-mail: BillyLin@itri.org.tw<br />
Abstract. Ink jet printing conductive metal nanopastes have been<br />
studied in research on printed circuit boards <strong>and</strong> passive components.<br />
This technique provides a manufacturing method that can<br />
replace more expensive processes, such as lithography or metal<br />
evaporation. In this paper, we demonstrated a printed silver topcontact<br />
source-drain electrodes on bottom-gate organic thin film<br />
transistors (OTFTs). The soluble conjugated polymer, regioregular<br />
poly(3-hexylthiophene) (RR-P3HT), was used as the active channel<br />
material. To increase electric transport efficiency by tuning the surface<br />
hydrophibic characteristics, a buffer layer, V 2 O 5 , between organic<br />
semiconductor <strong>and</strong> printed electrodes, was introduced to resolve<br />
the incompatibility of these two layers. The results indicated<br />
the printed Ag on V 2 O 5 film is hydrophilic (contact angle 10°),<br />
different with that on P3HT only (hydrophibic, contact angle about<br />
65°). High hydrophilic surface of V 2 O 5 led to self-aligning behavior of<br />
patterned Ag nanopastes <strong>and</strong> is helpful to get a flat film. This study<br />
also compares the characteristics of OTFTs to traditionally evaporated<br />
electrodes <strong>and</strong> to ink jet printed source-drain electrodes, in<br />
order to analyze the mechanism action of the buffer layer <strong>and</strong> its<br />
related process differences. © 2007 <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong><br />
<strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5456<br />
INTRODUCTION<br />
In recent years, a trend in developing new electronic devices<br />
is to minimize the element size <strong>and</strong> to increase the density of<br />
devices, which is a solution to reducing cost <strong>and</strong> simplifying<br />
processing. 1 The major cost of fabricating an electronic device<br />
comes from the processing procedures rather than from<br />
the raw materials. Traditional photolithography <strong>and</strong> the high<br />
vacuum process can produce devices that scale down to several<br />
micrometers, but the infrastructures are expensive. Low<br />
cost process technologies <strong>for</strong> fabricating electronic devices<br />
include screen printing, 2,3 micromolding in capillaries, 4 soft<br />
lithographic stamping, 5 <strong>and</strong> ink jet printing. 6 The reduction<br />
of processing procedures through direct deposition of a material<br />
could reduce the cost <strong>and</strong> complexity of the fabrication<br />
process further. Ink jet printing is a potential alternative to<br />
the existing deposition approaches. As a maskless process, it<br />
can reduce the cost of physical molding <strong>and</strong> chemical waste<br />
of material. Moreover, ink jet printing technology is much<br />
easier extended to process on a large area substrate than are<br />
other processes.<br />
Overarching inks can be used <strong>for</strong> the process of ink jet<br />
Received Jan. 17, 2007; accepted <strong>for</strong> publication Jun. 20, 2007.<br />
1062-3701/2007/515/456/5/$20.00.<br />
printing. These inks comprise inorganic <strong>and</strong> organic electronic<br />
materials <strong>and</strong> are expected to <strong>for</strong>m wiring, electrodes,<br />
<strong>and</strong> many other kinds of components of an electric device.<br />
The application of ink jet printed electronic devices includes<br />
thin film transistors (TFTs), radio frequency identification<br />
(RFID) circuits, organic light-emitting diodes (OLED), <strong>and</strong><br />
printed circuit boards (PCBs). 7 Among various inks used in<br />
the past few years, conducting inks have been newly developed<br />
<strong>for</strong> <strong>for</strong>ming the electrodes of the devices. Poly(ethylene<br />
dioxythiophene) (PEDOT) is a good c<strong>and</strong>idate <strong>for</strong> <strong>for</strong>ming<br />
ink jet printed electrodes, but the conductivity is several orders<br />
lower than that of metals. The conductivity of the metal<br />
nanopastes is three to four orders higher than that of<br />
PEDOT. After low temperature sintering 150°C, the<br />
nanoparticles <strong>for</strong>m aggregates, <strong>and</strong> film thickness can be<br />
controlled by tuning the drop size <strong>and</strong> the solid content of<br />
the inks. The fabrication processes <strong>for</strong> flexible display devices<br />
require low processing temperatures, however.<br />
Burgietal. 8 discussed the influence of the work function<br />
<strong>for</strong> the metal on the device per<strong>for</strong>mance with poly(3–<br />
hexylthiophene) (PHT) as an active layer. The source/drain<br />
metal electrode is chosen to be Cr/Au, Ag, or Au only. Other<br />
metals <strong>for</strong>m Schottky barriers at the interface so that the<br />
resistance is high <strong>and</strong> the current is blocked therein.<br />
This study explored the effects of various printing conditions<br />
with a Dimatix SE-128 piezoelectric printhead on the<br />
quality of the printed film, <strong>and</strong> we discuss the influence of<br />
the thin transition metal buffer layer on TFT device per<strong>for</strong>mance.<br />
Using a transition metal oxide as the hole injection<br />
layer is an effective way to improve the characteristics of<br />
organic thin film transistors (OTFTs). 9 At the same time, the<br />
contact angle of the nanopaste on the transition metal oxide<br />
is smaller than that on P3HT; thus, the printing quality of<br />
the metal line is better. Furthermore, the thickness of this<br />
buffer layer is so thin that device per<strong>for</strong>mance is not significantly<br />
influenced.<br />
TOP CONTACT–BOTTOM GATE OTFT STRUCTURE<br />
AND PROCESSES<br />
The structure of the organic thin film transistor with ink jet<br />
printed silver electrodes is shown in Figure 1. The bottom<br />
gate with heavily doped Si is selected, <strong>and</strong> SiO 2 with thickness<br />
of 300 nm is used as the gate dielectric. The Si/SiO 2<br />
substrate was cleaned sequentially with acetone <strong>and</strong> isopro-<br />
456
Lin et al.: Top contact organic thin film transistors with ink jet printed metal electrodes<br />
Figure 1. The structure of the polymer thin film transistor device PTFT<br />
with a silver top electrode.<br />
Figure 3. Profiles of the sintered silver nanopaste dot <strong>for</strong>med by ink jet<br />
printing on the different substrates: a P3HT <strong>and</strong> b V 2 O 5 .<br />
Figure 2. Observation of the break-off behavior of the printed drop beneath<br />
an arbitrary nozzle.<br />
Table I. Contact angles <strong>for</strong> silver nanopaste on the P3HT <strong>and</strong> V 2 O 5 .<br />
Substrate<br />
Contact Angle<br />
P3HT 65°<br />
V 2 O 5 10°<br />
panol in an ultrasonic bath. After 150 W, 450 mtorr, <strong>and</strong><br />
5 min, oxygen plasma treatment, a monolayer of OTS was<br />
self-assembled onto the SiO 2 layer by dipping the substrate<br />
(with SiO 2 layer) into the 60°C OTS dilute toluene solution<br />
<strong>for</strong> 20 min. Then, regioregular poly(3-hexylthiophene)<br />
(P3HT) (purchased from Rieke) was used as the organic<br />
active layer. P3HT dissolved in xylene 0.5 wt. % was spin<br />
coated on the substrate <strong>and</strong> baked at 90°C <strong>for</strong> 90 mins. The<br />
thickness of this P3HT film was about 50 nm. The transition<br />
metal buffer layer comprised of dielectric material, vanadium<br />
oxide V 2 O 5 ,was then thermally evaporated by<br />
shadow mask on the P3HT; its thickness was 3nm. The<br />
shape of V 2 O 5 layer is defined by a metal mask, <strong>and</strong> Ag is<br />
ink jet printed approximately within the area of V 2 O 5 . The<br />
printing position need not to be correctly defined, since the<br />
surface energy behavior self-aligns this Ag ink on the low<br />
contact angle surface, i.e., V 2 O 5 . To compare the per<strong>for</strong>mance<br />
of this device with ink jet printed silver nanopaste top<br />
electrodes, we made a device with evaporated silver film top<br />
electrodes.<br />
The ink jet printing plat<strong>for</strong>m, developed by the Display<br />
<strong>Technology</strong> Center (DTC) in the Industrial <strong>Technology</strong><br />
Research Institutes (ITRI), equipped with Dimatix SE-128<br />
piezoelectric printhead, was used to discharge the silver<br />
nanoparticle inks (AG-IJ-G-100-S1), purchased from Cabot.<br />
The low resistivity patterned silver film was obtained after<br />
postbaking at 150°C <strong>for</strong> 30 min in a glove box to sinter the<br />
Figure 4. Morphology of the sintered silver nanopaste dot <strong>for</strong>med by ink<br />
jet printing on the P3HT substrate with different drop densities.<br />
Figure 5. Morphology of the sintered silver nanopaste dot <strong>for</strong>med by ink<br />
jet printing on the V 2 O 5 /P3HT substrate with different drop densities: a<br />
100 dpi, b 200 dpi, c 300 dpi, <strong>and</strong> d 400 dpi.<br />
predeposited precursor material. The thickness of the metal<br />
layer was 200 nm. In contrast, the pressure of the chamber<br />
was maintained at about 510 −6 torr during evaporation of<br />
silver electrodes. Finally, measurements of V D versus I D <strong>and</strong><br />
V G versus I D <strong>for</strong> devices were conducted using a Keithley<br />
4200 semiconductor parameter analyzer.<br />
The field effect mobility was calculated in the saturation<br />
regions from the following equation:<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 457
Lin et al.: Top contact organic thin film transistors with ink jet printed metal electrodes<br />
Figure 6. I d -V ds characteristics of the PTFT, wherein the source-drain electrodes are: a evaporated Ag, b<br />
evaporated V 2 O 5 /Ag, c evaporated V 2 O 5 /Ag with postannealing, <strong>and</strong> d evaporated V 2 O 5 /IJP Ag with<br />
postannealing.<br />
I DS = WC i /2LV G − V T 2 ,<br />
where W is the channel width, L is the channel length, C i is<br />
the capacitance per unit area of the insulator, <strong>and</strong> V T is the<br />
threshold voltage.<br />
RESULTS AND DISCUSSION<br />
Stability of the Ink Jet Printed Ink<br />
This study improved the jetting behavior of piezoelectric<br />
print heads by controlling three parameters, i.e., pulse wave<strong>for</strong>m,<br />
pulse frequency, <strong>and</strong> driving voltage. Suitable range of<br />
wave<strong>for</strong>m modulation <strong>and</strong> pulse frequency are defined corresponding<br />
to given driving voltages <strong>for</strong> appropriate ink discharging.<br />
The jetting behavior can be observed by using a<br />
strobe image capturing system integrated in the printing<br />
plat<strong>for</strong>m. The break-off behavior of the jettable silver<br />
nanoparticles ink was captured within 60 s <strong>and</strong> shown in<br />
Figure 2. The velocity of the jetted drop was about 6.4 m/s<br />
<strong>and</strong> stable. With a st<strong>and</strong>-off of 1mm, we successfully modulated<br />
the ink jet printing parameters of Dimatix SE-128 <strong>for</strong><br />
silver nanoparticles of ink, <strong>and</strong> obtained uni<strong>for</strong>m <strong>and</strong> continuous<br />
silver paste lines.<br />
1<br />
S-D<br />
Electrodes<br />
Table II. Electrical characteristics of the various PTFTs.<br />
Mobility<br />
cm 2 / V.s<br />
Threshold<br />
Voltage<br />
V<br />
On/Off<br />
Ratio<br />
Evaporate Au 3.48 10 −3 −9.49 14268<br />
Evaporate Ag 1.88 10 −3 −11.17 629<br />
Evaporate V 2 O 5 / Ag 2.23 10 −3 −10.63 1398<br />
Evaporate V 2 O 5 /Ag<br />
1.19 10 −3 −9.95 3107<br />
with postannealing<br />
Evaporate V 2 O 5 / IJP Ag<br />
with postannealing<br />
2.98 10 −4 −15.27 138<br />
Morphology of the Ink Jet Printed Nanopaste<br />
The contact angles of the silver nanopaste between the P3HT<br />
film <strong>and</strong> the V 2 O 5 buffer film are shown in Table I. The<br />
contact angle on P3HT film is 65°, which is larger than that<br />
on the V 2 O 5 film. The morphology of printed nanopaste is<br />
related to the contact angle on the surface. The morphologies<br />
of the nanopaste on these two substrates are shown in<br />
Figure 3. As the nanopaste ink jet printed on the P3HT film<br />
was postannealed, the dot diameter shrunk to about 30 m<br />
458 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lin et al.: Top contact organic thin film transistors with ink jet printed metal electrodes<br />
Figure 7. I d -V gs characteristics of the PTFT, wherein the source-drain electrodes are: a evaporated Ag, b<br />
evaporated V 2 O 5 /Ag, c evaporated V 2 O 5 /Ag with postannealing, <strong>and</strong> d evaporated V 2 O 5 /IJP Ag with<br />
postannealing.<br />
<strong>and</strong> <strong>for</strong>med a coffee-ring structure. The thickness of the ring<br />
edge was 2.5 m. The drop morphology on P3HT alone<br />
was so rough that it was difficult to <strong>for</strong>m a flat film <strong>and</strong><br />
pattern uni<strong>for</strong>m metal electrodes. On the other h<strong>and</strong>, it was<br />
a good choice to print nanopaste on V 2 O 5 /P3HT film. The<br />
dot size was 73 m <strong>and</strong> did not shrink. A coffee ring still<br />
<strong>for</strong>med, but the morphology of the dot was flatter, <strong>and</strong> it<br />
was easier to construct lines <strong>and</strong> patterns of metal electrodes.<br />
The ring thickness was 0.5 m. In addition, the<br />
more hydrophilic surface of V 2 O 5 led to self-aligning behavior<br />
of patterned Ag nanopastes. After sintering the patterned<br />
Ag nanopaste at 150°C <strong>for</strong> 30 min, the resultant resistivity of<br />
the ink jet printed electrode was about 310 −8 m, which<br />
was two to three times that <strong>for</strong> the normal bulk silver.<br />
Currently, no OTFT-related article has reported how to<br />
ink jet print Ag ink onto the semiconductor layer <strong>and</strong> discussed<br />
the resulting interface. All of the previously reported<br />
devices were bottom-contact devices, <strong>and</strong> Ag ink was printed<br />
onto the dielectric layer. However, the contact angle of Ag<br />
ink to hydrophilic dielectric layer was small, <strong>and</strong> the printed<br />
pattern was easy to fix in position. On the other h<strong>and</strong>, the<br />
contact angle of Ag ink to the hydrophibic organic semiconductor<br />
was large, which makes the printed ink deposits<br />
shrink relative to each other, <strong>and</strong> it is accordingly hard to<br />
<strong>for</strong>m a uni<strong>for</strong>m film, as shown in Figure 4. In this article an<br />
approach is proposed to improve the morphology of the<br />
electrode ink on the semiconductor by inserting an inorganic<br />
buffer layer. The morphology of the nanopaste on<br />
V 2 O 5 /P3HT film is shown in Figure 5. As the printing density<br />
increased to 400 dpi, the drops began to aggregate <strong>and</strong><br />
<strong>for</strong>med uni<strong>for</strong>m lines or patterns. The best printing density<br />
was found to be 400 dpi, with the width of the line<br />
62 m. Further increase in the printing density led to<br />
thicker <strong>and</strong> wider conductive Ag lines. If we discharged<br />
denser inks, it would be harder to control the morphology<br />
because more ink would lead to unstable boundaries of the<br />
Ag lines.<br />
DEVICE PERFORMANCE<br />
The source-drain (S-D) electrodes are bilayer electrodes<br />
<strong>for</strong>med on the organic active layer. The channel width <strong>and</strong><br />
length of each device were 2000 m <strong>and</strong> 140 m, respectively.<br />
The device per<strong>for</strong>mance is stable, on the basis of the<br />
averaged results obtained with three devices. Basically, no<br />
obvious difference was found in device per<strong>for</strong>mance between<br />
evaporated silver <strong>and</strong> bilayer V 2 O 5 /Ag electrodes. The<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 459
Lin et al.: Top contact organic thin film transistors with ink jet printed metal electrodes<br />
results are shown in Figures 6(a) <strong>and</strong> 6(b). The device with<br />
the Ag S-D electrode had an on-off ratio I on /I off <strong>and</strong> mobility<br />
similar to the device with V 2 O 5 /Ag S-D electrode. The<br />
on-off ratio was near 10 3 , <strong>and</strong> the mobility was<br />
10 −3 cm 2 /V s. However, the current of the V 2 O 5 /Ag S-D<br />
electrode was about twofold larger than that of the device<br />
with Ag S-D electrode, indicating better hole injection after<br />
inserting the V 2 O 5 , which can also be a hole injection layer<br />
as well as providing surface energy modification. In the case<br />
of ink jet printed silver electrode, the patterned modification<br />
of the V 2 O 5 buffer layer was necessary <strong>for</strong> the correct definition<br />
of the S-D boundary. The Ag nanopaste ink was selfaligned<br />
onto the patterned S-D V 2 O 5 buffer layer <strong>and</strong> did<br />
not deposit onto the P3HT due to the differences in contact<br />
angle, i.e., the surface energy. The device per<strong>for</strong>mance after<br />
postannealing is shown in Figs. 6(c) <strong>and</strong> 6(d). We observed<br />
that there is no decrease in the per<strong>for</strong>mance of the device<br />
with evaporated S-D electrodes after postannealing at 150°C<br />
<strong>for</strong> 30 min. The per<strong>for</strong>mance of the device with ink jet<br />
printed Ag electrode was not as good. The mobility <strong>and</strong><br />
current dropped one order of magnitude, which may be due<br />
to the conductivity decrease of the ink jet printed Ag electrode<br />
<strong>and</strong> organic residuals from the nanopastes being<br />
present at the metal/semiconductor interface. These results<br />
are summarized in Table II. It is worth noting that the choice<br />
of S-D electrode <strong>and</strong> interface engineering are important.<br />
The I d -V gs characteristics of these four devices are shown in<br />
Fig. 7 <strong>for</strong> comparison. There are some leakage currents in<br />
our newly developed device, which we will discuss <strong>and</strong> try to<br />
improve in the future.<br />
CONCLUSION<br />
In this paper, we successfully demonstrated a top contact–<br />
bottom gate OTFT device fabricated by ink jet printing silver<br />
nanopaste electrode, along with thermally evaporation <strong>and</strong><br />
spin coating. By modulating the ink jet printing parameters,<br />
a well-sintered silver electrode was <strong>for</strong>med to fabricate a TFT<br />
device with a channel width of 2000 m <strong>and</strong> channel<br />
length of 140 m. The morphology of the nanopaste drop<br />
on the P3HT was rough, <strong>and</strong> it was hard to <strong>for</strong>m flat lines<br />
<strong>and</strong> patterns of the metal electrodes; thus, we introduced a<br />
thin transition metal oxide as the buffer layer. The more<br />
hydrophilic surface of V 2 O 5 led to self-aligning behavior of<br />
patterned Ag nanopaste, <strong>and</strong> a continuous conductive line<br />
could be <strong>for</strong>med. The current of the V 2 O 5 /Ag S-D electrode<br />
was about twofold larger than that of a device with an<br />
evaporated Ag S-D electrode, indicating better hole injection<br />
after inserting the V 2 O 5 , which can also be a hole injection<br />
layer, as well as providing surface energy modification. The<br />
device per<strong>for</strong>mance of the ink jet printed Ag electrode was<br />
inferior to that of the evaporated one after annealing. The<br />
mobility <strong>and</strong> current dropped one order of magnitude,<br />
which may be due to the conductivity decrease of the ink jet<br />
printed Ag electrode post annealing <strong>and</strong> the organic residuals<br />
from the nanopaste present in the metal/semiconductor<br />
interface. In the future, we expect to improve the interface<br />
between the S-D electrodes <strong>and</strong> the semiconductor layer of<br />
the printed OTFT device to achieve comparable per<strong>for</strong>mance<br />
to an evaporated or spin-coated counterpart.<br />
REFERENCES<br />
1 P. Calvert, Chem. Mater. 13, 3299 (2001).<br />
2 C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson, M. G.<br />
Kane, I. G. Hill, M. S. Hammond, J. Campi, B. K. Greening, J. Francl,<br />
<strong>and</strong> J. West, Appl. Phys. Lett. 80, 1088 (2002).<br />
3 H. Klauk, D. J. Gundlach, <strong>and</strong> T. N. Jackson, IEEE Trans. Electron<br />
Devices 20, 289 (1999).<br />
4 J. A. Rogers, Z. Bao, <strong>and</strong> V. R. Raju, Appl. Phys. Lett. 72, 2716 (1998).<br />
5 J. A. Rogers, Z. Bao, A. Makhija, P. Braun, Adv. Mater. (Weinheim, Ger.)<br />
11, 741 (1999).<br />
6 H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W.<br />
Wu, <strong>and</strong> E. P. Woo, <strong>Science</strong> 290, 2123 (2000).<br />
7 K. Cheng, M. Yang, W. Chiu, C. Huang, J. Chang, T. Ying, <strong>and</strong> Y. Yang,<br />
Macromol. Rapid Commun. 26(4), 247 (2005).<br />
8 L. Burgi, T. J. Richards, R. H. Friend, <strong>and</strong> H. Sirringhaus, Appl. Phys.<br />
Lett. 94, 6129 (2003).<br />
9 C. W. Chu, S. H. Li, C. W. Chen, V. Shrotriya, <strong>and</strong> Y. Yang, Appl. Phys.<br />
Lett. 87, 193508 (2005).<br />
460 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 461–464, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Ring Edge in Film Morphology: Benefit or Obstacle<br />
<strong>for</strong> Ink Jet Fabrication of Organic Thin Film Transistors<br />
Jhih-Ping Lu<br />
Dept. of Photonics <strong>and</strong> Display Institute, National Chiao Tung University, Hsinchu, Taiwan, 30010,<br />
Republic of China <strong>and</strong> Display <strong>Technology</strong> Center, Industrial <strong>Technology</strong> Research Institute, Hsinchu, Taiwan<br />
310, Republic of China<br />
E-mail: Anthony_lu@itri.org.tw<br />
Ying-pin Chen<br />
Dept. of Photonics <strong>and</strong> Display Institute, National Chiao Tung University, Hsinchu, Taiwan, 30010,<br />
Republic of China<br />
Yuh-Zheng Lee <strong>and</strong> Kevin Cheng<br />
Display <strong>Technology</strong> Center, Industrial <strong>Technology</strong> Research Institute, Hsinchu, Taiwan 310,<br />
Republic of China<br />
Fang-Chung Chen<br />
Dept. of Photonics <strong>and</strong> Display Institute, National Chiao Tung University, Hsinchu, Taiwan, 30010,<br />
Republic of China<br />
Abstract. In this study, a novel process using ink jet printing with<br />
dilute PMMA [poly(methylmethacrylate)] solution to <strong>for</strong>m fine separation<br />
banks as confined boundaries <strong>for</strong> the subsequent depositing<br />
poly(3,4–ethylene dioxythiophene) (PEDOT) was proposed. The<br />
PEDOT comprised the source <strong>and</strong> drained electrodes with a gap of<br />
several micrometers due to the innate characteristic of the ring edge<br />
of ink jetted PMMA. Using this technique, the organic device was<br />
designed with a bottom gate <strong>and</strong> evaporated pentacene as the<br />
channel material. The device mobility <strong>and</strong> on/off ratio were about<br />
3.610 −4 <strong>and</strong> 210 2 cm 2 /V sec, respectively. © 2007 <strong>Society</strong> <strong>for</strong><br />
<strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5461<br />
INTRODUCTION<br />
Inorganic semiconductor technology <strong>and</strong> related applications<br />
have already attained a mature stage in the past decades.<br />
Because of this revolutionary advancement, science<br />
<strong>and</strong> technology have grown vigorously. The dem<strong>and</strong> <strong>for</strong><br />
lightweight <strong>and</strong> flexible electronic products is gradually increasing.<br />
However, high temperature processing of inorganic<br />
semiconductors is difficult to apply to this dem<strong>and</strong>. Low<br />
temperature direct patterning technology <strong>and</strong> organic electronics<br />
have accordingly been intensively developed. 1 With<br />
respect to organic electronics, pentacene <strong>and</strong> regioregular<br />
poly(3-hexylthiophene) (RR-P3HT) are the most popular<br />
materials used in fabrication of organic thin film transistors<br />
(OTFT). The electrical per<strong>for</strong>mance of pentacene is superior<br />
among the present organic materials, but it is processed by<br />
vacuum evaporation, e.g., thermal evaporation 2 or organic<br />
Received Jan. 17, 2007; accepted <strong>for</strong> publication Jun. 20, 2007.<br />
1062-3701/2007/515/461/4/$20.00.<br />
vapor phase deposition. 3 Despite of the lower mobility of the<br />
polymeric material, the solubility of P3HT, compared to<br />
pentacene, makes it compatible with the printing process,<br />
which has the advantages of low temperature processing,<br />
large area manufacturing capability, etc.<br />
Among patterning technologies, ink jet printing is considered<br />
the most promising potential c<strong>and</strong>idate <strong>for</strong> industrial<br />
mass production. In recent years, ink jet printing is used not<br />
only on paper <strong>and</strong> wide <strong>for</strong>mat media, but also widely applied<br />
to patterning deposition of organic <strong>and</strong> inorganic electronics.<br />
There are many investigations of ink jet printing<br />
patterning, such as deposition of insulators, organic<br />
semiconductors, 4 or <strong>for</strong>mation of conductive paste circuits, 5<br />
<strong>and</strong> microlens array. 6 Fabrication of organic TFT (OTFT), a<br />
stacked device, needs to integrate several printing steps; thus,<br />
it is more difficult <strong>and</strong> complicated than a single material<br />
deposition process. In spite of these challenges, the use of<br />
ink jet printing to make good polymer TFT devices has already<br />
been discussed in many papers. 7,8<br />
Most research on OTFT has been concerned with new<br />
material development, surface modification or structural design<br />
<strong>for</strong> improving the electric characteristics. 9 As is well<br />
known, the current from drain to source is inversely proportional<br />
to the channel length. It is favorable to shorten channel<br />
length instead of increasing channel width W <strong>for</strong> better<br />
TFT array resolution. However, the printing feature is limited<br />
by the drop size <strong>and</strong> directionality of the ink jet droplet.<br />
The printed line width is usually greater than 80 m with<br />
the use of a print head with 35 pl droplet, such as a Spectra<br />
SE-128. The variation of ink jet printing directionality also<br />
makes it difficult to obtain small <strong>and</strong> uni<strong>for</strong>m gate channel<br />
461
Lu et al.: Ring edge in film morphology: Benefit or obstacle <strong>for</strong> ink jet fabrication of organic TFTs<br />
Figure 1. Structure of the organic thin film transistor OTFT device with a<br />
ridge of PMMA ring edge.<br />
length over the whole printing area. In this study, we separated<br />
the organic electrodes by a poly(methyl methacrylate)<br />
(PMMA) ridge, <strong>for</strong>med by the natural phenomenon of capillary<br />
flow to obtain gate lengths of 10 m using an ink jet<br />
printing method.<br />
PMMA PARTITION TO SEPARATE CHANNELS<br />
The phenomenon of ring shaped patterns from dried solution<br />
is common in our daily life. When a liquid droplet l<strong>and</strong>s<br />
on a substrate, there are three interface interactions governing<br />
the droplet drying behavior. These interactions occur<br />
between the individual interface of solid to liquid, liquid to<br />
gas, or gas to solid. When the solvent gradually evaporates,<br />
liquid droplets are dried to the solid state. Finally, the solute<br />
accumulates around the peripheral boundary as ridges,<br />
much thicker than that of the center area <strong>and</strong> is called the<br />
“coffee ring shape.”<br />
Much ef<strong>for</strong>t has been devoted to explain the coffee ring<br />
phenomenon. Deegan et al.’s hypothesis 10 is the most acceptable<br />
to describe the ring shaped <strong>for</strong>mation behavior. He<br />
assumed that the liquid evaporates faster around the periphery<br />
than at the center part. This makes the peripheral<br />
boundary easier to dry than the center area <strong>and</strong>, there<strong>for</strong>e, to<br />
<strong>for</strong>m a contact line around the liquid peripheral. This results<br />
in a concentration gradient of dissolving solute, <strong>and</strong> the solution<br />
accompanying the solute begins to diffuse from the<br />
center to the edge. When the drying process is completed,<br />
most solute in the liquid has been carried to the edge <strong>and</strong><br />
<strong>for</strong>med into a ring shaped profile. The natural behavior of<br />
ridge <strong>for</strong>mation is an obstacle <strong>for</strong> obtaining smooth films <strong>for</strong><br />
organic electronics, such as polymer light emitting diodes<br />
(PLED) <strong>and</strong> organic thin film transistors. As an alternative,<br />
this study exploited this behavior as a beneficial micropatterning<br />
method to define the gate length of OTFT. Through<br />
this method, OTFTs with a several micrometer channel<br />
length are fabricated.<br />
EXPERIMENTAL<br />
The structure of the organic thin film transistor (OTFT)<br />
used in this study is shown in Figure 1. The bottom gate is<br />
heavily doped Si on which thermal oxide, SiO 2 , 300 nm<br />
thick is <strong>for</strong>med. Sputtered Au 70 nm thick <strong>and</strong> Cr 30 nm,as<br />
an adhesive layer, was used to <strong>for</strong>m the interconnection<br />
tracks with spacing of 100 m, as defined by a stainless<br />
shadow mask. Poly(methylmethacrylate) (PMMA) ridge several<br />
micrometers wide was <strong>for</strong>med by the coffee-ring edge<br />
effect. The statistical data <strong>for</strong> the ring width distribution<br />
Figure 2. Side <strong>and</strong> top views of the process flow schematic illustration of<br />
ring ridge patterned organic thin film transistors.<br />
yield a mean value of 7.88 m <strong>and</strong> a st<strong>and</strong>ard deviation of<br />
1.68 m <strong>for</strong> 25 devices. After suitable plasma treatment to<br />
remove the thinner PMMA film from the surface, the diluted<br />
poly(ethylene dioxythiophene) (PEDOT 4071, purchased<br />
from Bayer) was ink jet printed on both sides of the ridge as<br />
source <strong>and</strong> drain electrodes, using a thermal bubble print<br />
head capable of 80 pl droplets as developed by our institute<br />
(ITRI).<br />
The fabricating procedures <strong>for</strong> ring ridges, electrodes,<br />
<strong>and</strong> the pentacene active layer are shown in Figure 2. As<br />
shown in Fig. 2(a), the Si/SiO 2 substrate with Au/Cr interconnection<br />
pads were treated with O 2 plasma at 200 W, 800<br />
SCCM (st<strong>and</strong>ard cubic centimeter per minute) <strong>for</strong> 1 min to<br />
improve the surface wettability. In Fig. 2(b), 1 wt. % PMMA<br />
solution dissolved in anisole was printed <strong>and</strong> there was one<br />
ridge located between the two Au/Cr pads. An ion-coupled<br />
plasma was used to etch the thinner part inside the ring<br />
ridge by treating 50 sec O 2 plasma with etching rate<br />
6.78 Å/sec, <strong>and</strong> there<strong>for</strong>e, two thicker ridges were left on<br />
both sides. Sequentially, carbon tetrafluoride plasma was applied<br />
to make the PMMA surface liquid repellent as shown<br />
in Fig. 2(c). The residual ridge was treated with CF 4 plasma<br />
<strong>for</strong> 100 sec under a pressure of 1 torr with a small etching<br />
rate of 0.3 Å/sec. The treated PMMA surface then had contact<br />
angles of 106° <strong>and</strong> 60° <strong>for</strong> water <strong>and</strong> anisole, respectively.<br />
There<strong>for</strong>e, after the dry etching <strong>and</strong> repellent treatment,<br />
a ridge with width of 5.37 m was made as shown in<br />
Figure 3. Then PEDOT solution was ink jet printed on both<br />
sides of the ridge to connect to each of the Au/Cr pads,<br />
462 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Lu et al.: Ring edge in film morphology: Benefit or obstacle <strong>for</strong> ink jet fabrication of organic TFTs<br />
Figure 3. Microscopy of a PMMA residual ridge after plasma etching.<br />
Figure 5. Organic thin film transistor characteristics of a ring ridge patterned<br />
with pentacene as the active layer.<br />
Figure 4. Profile of printed PEDOT by ink jet printing on both sides of the<br />
etched repellant PMMA ridge.<br />
which were used as a source or drain electrode as shown in<br />
Fig. 2(d). Figure 4 shows the profile of PEDOT, which was<br />
printed on both sides of the etched PMMA ridge. This profile<br />
was obtained by XP-1 profilometer (AMBIOS technology).<br />
From the profile, we can see that the printed PEDOT<br />
can be adequately confined between ridges, <strong>and</strong> the height of<br />
PEDOT domains <strong>and</strong> ridges are around 70 nm <strong>and</strong> 110 nm,<br />
respectively. Hexamethyldisilazane (HMDS) <strong>and</strong> pentacene<br />
were thermally evaporated in sequence onto the above substrate<br />
to complete the OTFT devices, as shown in Fig. 2(e).<br />
In order to prevent source/drain electrodes from interconnecting,<br />
it is better to make the PMMA ridge as repellent to<br />
the PEDOT solution as possible.<br />
RESULTS AND DISCUSSION<br />
According to the above procedures, utilizing the ink jet<br />
printing technique incorporated with ring edge effect we<br />
were able to produce a narrow line of 10 m width, <strong>for</strong><br />
which the mean <strong>and</strong> st<strong>and</strong>ard deviation are 5.16 m <strong>and</strong><br />
0.31 m <strong>for</strong> 25 samples. Applying this patterning method to<br />
fabricate an OTFT device, better MOS (metal-oxide semiconductor)<br />
characteristics could be obtained. The I d versus<br />
V d <strong>and</strong> I d versus V g measured using a Keithley 4200 semiconductor<br />
parameter analyzer are shown in Figure 5. The<br />
field effect mobility can be calculated at the saturation region<br />
from the following equation:<br />
IDS = WC i<br />
2L<br />
V G − V T 2 ,<br />
where C i is the capacitance per unit area of the insulator, <strong>and</strong><br />
V T is the threshold voltage.<br />
1<br />
Figure 6. Capacitance comparison of plasma treatment on the oxide<br />
layer.<br />
The mobility, on/off ratio <strong>and</strong> threshold voltage are<br />
about 3.610 −4 cm 2 /V sec, 210 2 , <strong>and</strong> 22 V, respectively.<br />
It is found that the off-current <strong>and</strong> threshold voltage are<br />
relatively high in this device, which may be attributed to the<br />
negative charging into the PMMA ridge after the plasma<br />
treatment. In addition, pentacene cannot <strong>for</strong>m a better<br />
alignment on the HMDS/PMMA ring ridge owing to the<br />
poor mobility of these devices. Further study is needed to<br />
clarify the above problems.<br />
Besides the patterning process, the plasma charging effect<br />
on an oxide layer should be considered because the insulator<br />
layer was also treated during the etching <strong>and</strong> repellent<br />
treatment. This issue was addressed in terms of the<br />
capacitance variation of devices with MIM structure, doped<br />
Si/oxide/Al, with or without plasma treatment on the oxide<br />
layer. After examination with an HP 4194 impedance ana-<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 463
Lu et al.: Ring edge in film morphology: Benefit or obstacle <strong>for</strong> ink jet fabrication of organic TFTs<br />
lyzer, we found the capacitance to be without dramatic<br />
change after plasma treatment under the conditions used in<br />
the patterning process, as shown in Figure 6. Most devices<br />
have similar capacitance, even though some have a maximum<br />
variation of 5%. 11<br />
CONCLUSION<br />
In this paper, we overcame the restriction of line width by<br />
using the direct ink jet printing technique (with droplets of<br />
35 pl the line width is generally around 80–150 m) to<br />
obtain fine lines of 5 m using the ring-edge effect. We<br />
successfully applied this novel method to fabricate OTFT<br />
devices with small channel lengths of several micrometers.<br />
This patterning method was proven feasible. Devices with<br />
common characteristics of transistors were proposed,<br />
though the results have yet to be optimized, where the mobility<br />
<strong>and</strong> on/off ratio were about 3.610 −4 cm 2 /V sec <strong>and</strong><br />
210 2 cm 2 /V sec, respectively. It is evident that this patterning<br />
method of ink jet printing can be widely applied<br />
after additional studies in the future.<br />
REFERENCES<br />
1 S. R. Forrest, Nature (London) 428, 911-918 (2004).<br />
2 Y.-Y. Lin, D. J. Gundlach, S. F. Nelson, <strong>and</strong> T. N. Jackson, IEEE Trans.<br />
Electron Devices 44, 1325–1331 (1997).<br />
3 M. Baldo, M. Deutsch, P. Burrows, H. Gossenberger, M. Gerstenberg, V.<br />
Ban, <strong>and</strong> S. Forrest, Adv. Mater. (Weinheim, Ger.) 10, 1505-1514 (1998).<br />
4 K. F. Teng <strong>and</strong> R. W. Vest, IEEE Electron Device Lett. 9, 591 (1988).<br />
5 B.-J. de Gans, P. C. Duineveld, U. S. Schubert, Adv. Mater. (Weinheim,<br />
Ger.) 16, 203 (2004).<br />
6 K. Cheng, M. Yang, W. Chiu, C. Huang, J. Chang, T. Ying, <strong>and</strong> Y. Yang,<br />
Macromol. Rapid Commun. 26, 247 (2005).<br />
7 W. R. Cox <strong>and</strong> T. Chen, Opt. Photonics News 12, 32–35 (2005).<br />
8 H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran, W.<br />
Wu, <strong>and</strong> E. P. Woo, <strong>Science</strong> 290, 2123 (2000).<br />
9 C. W. Sele, T. von Werne, R. H. Friend, <strong>and</strong> H. Sirringhaus, Adv. Mater.<br />
(Weinheim, Ger.) 17(8), 997 (2005).<br />
10 R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, <strong>and</strong> T. A,<br />
Nature (London) 389, 827-829 (1997).<br />
11 J. P. Lu <strong>and</strong> F. C. Chen (unpublished).<br />
464 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>® 51(5): 465–472, 2007.<br />
© <strong>Society</strong> <strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 2007<br />
Digital Fabrication Using High-resolution Liquid Toner<br />
Electrophotography<br />
Atsuko Iida, Koichi Ishii, Yasushi Shinjo, Hitoshi Yagi <strong>and</strong> Masahiro Hosoya <br />
Advanced Electron Devices Laboratory, Corporate Research & Development Center, Toshiba Corporation, 1,<br />
Komukai-Toshiba-cho, Saiwai-ku, Kawasaki, 212-8582, Japan<br />
E-mail: atsuko.iida@toshiba.co.jp<br />
Abstract. We have developed a digital fabrication process using<br />
high-resolution liquid toner electrophotography, consisting of fine liquid<br />
toner, a high-resolution exposure system, <strong>and</strong> nonelectrical<br />
transfer. Fine pitch multiline patterns of Cu wiring can be obtained<br />
by printing fine lines with seed toners <strong>and</strong> by electroless plating<br />
deposited on lines. Submicron-diameter seed toners have superfine<br />
conductive particles on their surfaces. Adhesion between the seed<br />
toner layer <strong>and</strong> Cu layer was increased by applying surface modification.<br />
Multiline patterns of 1 pixel line width (21.6 m) with the<br />
volume resistivity of 2.110 −6 Ωcm were realized by using a 1200<br />
dpi resolution light-emitting diode. Furthermore, the development<br />
process of multiline patterns with 2540 dpi resolution was examined<br />
by numerical simulations based on the electrophoretic characteristics<br />
of liquid toner <strong>and</strong> on the electrostatic <strong>for</strong>ces. The capability of<br />
multiline-pattern <strong>for</strong>mation of line <strong>and</strong> space (L/S)10/10 m was<br />
confirmed. The actual toner images of L/S10/10 m multiline pattern<br />
were obtained by using a 2540 dpi resolution luster scanning<br />
unit (LSU). Theoretical <strong>and</strong> experimental results confirm that the<br />
fabrication process using liquid toner electrophotography is available<br />
<strong>for</strong> realizing high-resolution multiline patterns. © 2007 <strong>Society</strong><br />
<strong>for</strong> <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong>.<br />
DOI: 10.2352/J.<strong>Imaging</strong>Sci.Technol.200751:5465<br />
INTRODUCTION<br />
Fine-pitch-pattern <strong>for</strong>mation of wiring circuit boards <strong>and</strong><br />
reduction in the cost of manufacturing processes are critically<br />
important requirements <strong>for</strong> electronic components <strong>and</strong><br />
semiconductor packages. Recently, digital fabrication of electronic<br />
components using printing technology has been<br />
reported. 1–4 Digital fabrication of circuit boards offers several<br />
advantages, notably simple mask-less process, <strong>and</strong> reduced<br />
costs.<br />
We have been developing a high resolution liquid toner<br />
electrophotographic imaging technology with the potential<br />
to produce fine images equal in quality to those produced by<br />
offset printing. 5–13 Extremely high resolution images have<br />
been realized by our technology that includes a full color<br />
Image-On-Image (IOI) development process, highresolution<br />
LSU with 2540 dpi, <strong>and</strong> the non-electric transfer<br />
process called “shearing transfer”.<br />
In this paper, we applied our imaging technology to a<br />
new digital fabrication process <strong>for</strong> an electronic circuit<br />
board. 14 The capability of fine line-pattern <strong>for</strong>mation by using<br />
liquid toner electrophotographic technology was verified<br />
theoretically <strong>and</strong> experimentally.<br />
HIGH-RESOLUTION LIQUID TONER IMAGING<br />
PROCESS<br />
Figure 1 shows a typical configuration of our liquid toner<br />
electrophotographic imaging system. It includes a photoreceptor<br />
drum, a scorotron charger, an exposure unit, a development<br />
unit, a dryer, <strong>and</strong> a transfer unit at the periphery of<br />
the photoreceptor drum. The toner image is dried properly<br />
by the dryer on the photoreceptor <strong>and</strong> is transferred to the<br />
intermediate transfer roller by the non-electric transfer process,<br />
<strong>and</strong> then secondary transferred to <strong>and</strong> thermally fixed<br />
on a substrate. The highly efficient per<strong>for</strong>mance of the<br />
shearing transfer is realized <strong>for</strong> the drying states in which the<br />
toner layer includes the solvent in the range of<br />
5–15 wt %. 7,8<br />
The searing transfer method is the nonelectrostatic process,<br />
which is used in the newly developed liquid toner<br />
electrophotograpy. 5–10 The intermediate transfer roller consists<br />
of a metal roller with 0.2 mm thick elastic layer of<br />
silicone rubber on the surface. The shearing transfer does<br />
not depend on an adhesive <strong>for</strong>ce of the elastic layer of the<br />
intermediate transfer roller. Figure 2 shows a schematic description<br />
of the mechanism of this method. In this method,<br />
the velocity of the surface of the intermediate transfer roller<br />
is slower than that of the surface of the photoreceptor drum<br />
by several percent. A distortion occurs in the nip of the<br />
elastic layer due to the difference of the velocities, <strong>and</strong> a<br />
<br />
IS&T Member<br />
Received Mar. 5, 2007; accepted <strong>for</strong> publication Jun. 7, 2007.<br />
1062-3701/2007/515/465/8/$20.00.<br />
Figure 1. Configuration of liquid toner electrophotographic technology.<br />
465
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
Figure 2. A schematic of the mechanism of shearing transfer: a Distortion<br />
in the nip area <strong>and</strong> restoration in the backward of the nip occur by<br />
the difference of the velocities. b The restoration <strong>and</strong> friction cause a<br />
shearing stress. Shearing stress slides the toner layer on the photoreceptor<br />
<strong>and</strong> transfers it to the transfer roller.<br />
restoration occurs behind the nip [Fig. 2(a)]. The restoration<br />
<strong>and</strong> friction cause a shearing stress between the toner layer<br />
<strong>and</strong> the photoreceptor drum, which slides the toner layer on<br />
the photoreceptor <strong>and</strong> transfers it to the intermediate roller<br />
[Fig. 2(b)]. The toner layer moves in unity by the capillary<br />
condensation because the toner particles are covered with a<br />
thin film of the solvent during the transfer per<strong>for</strong>mance. 7,8,15<br />
The method is effective <strong>and</strong> well directed <strong>for</strong> a highresolution<br />
image because there is no degradation in quality<br />
without toner scattering in an electric transfer method.<br />
Figure 3 shows the comparison of the toner images be<strong>for</strong>e<br />
<strong>and</strong> after the transfer process observed by a stereoscopic<br />
microscope. The toner image on the paper is in good agreement<br />
with that on the photoreceptor surface. The difference<br />
of their line widths was observed, but image destruction did<br />
not occur due to the transfer. The observation confirms that<br />
the shearing transfer method is an excellent method.<br />
Figure 3. Comparison of the images be<strong>for</strong>e <strong>and</strong> after the transfer process<br />
2540 dpi, 1 point Chinese character: a Toner image be<strong>for</strong>e the transfer<br />
on the photoreceptor drum <strong>and</strong> b toner image after the transfer on<br />
the paper.<br />
Figure 4. Printed image obtained by liquid toner electrophotography: a<br />
Comparison of the printed images on the paper <strong>and</strong> b toner particle.<br />
A printed image realized by our technology is shown in<br />
Figure 4(a). Comparison to a printed image realized by dry<br />
toner reveals that a finer pitch line was obtained by our<br />
technology. Figure 4(b) shows an image of a toner particle<br />
observed by a field emission scanning electron microscope<br />
(FE-SEM). The average diameter of toner particles was<br />
200 nm.<br />
DIGITAL FABRICATION PROCESS<br />
The digital fabrication process shown in Figure 5 proceeds as<br />
follows: first, fine-pitch patterns are printed on the substrate<br />
by using seed toners [Fig. 5(a)], <strong>and</strong> then a surface modification<br />
is added on the printed pattern [Fig. 5(b)]. Finally,<br />
conductive layers are deposited on the substrate by electroless<br />
plating [Fig. 5(c)]. The same electrophotographic system<br />
<strong>for</strong> imaging technology can be used <strong>for</strong> digital fabrication.<br />
Patterns <strong>for</strong>med on the photoreceptor can be transferred to<br />
the substrate, which is almost or partly covered with metallic<br />
layers, by using shearing transfer. Silicon wafers, glass, <strong>and</strong><br />
resin films (such as polyimide, glass epoxy, or polyethylene<br />
terephthalate) are used <strong>for</strong> the substrate.<br />
Image Formation of Fine-pitch Patterns<br />
The basic particle of the seed toner was composed of acrylic<br />
thermoplastic resin. The superfine conductive particles were<br />
chosen from the metal particles of Ag, Cu, Pd, or any other<br />
materials with the catalytic ability to per<strong>for</strong>m as seed <strong>for</strong><br />
electroless plating. Their average diameters were 20–80 nm.<br />
Toner particles prepared by adding a charge controlling<br />
agent were dispersed in an insulative solvent, Isopar ® manufactured<br />
by ExxonMobil Chemical Japan Pte. Ltd.<br />
A positively charged organic photoreceptor with a single<br />
coated layer of the phthalocyanine pigment was used. The<br />
466 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
Figure 6. Line/space=1/1 pixel multiline pattern with 1200 dpi exposure<br />
system.<br />
Figure 5. Digital fabrication process: a Printing seed toner, b surface<br />
modification, c electroless plating.<br />
photoreceptor drum rotating at a speed of 100 mm/sec was<br />
charged uni<strong>for</strong>mly at +700 V by a charger, <strong>and</strong> a latent image<br />
was <strong>for</strong>med on the photoreceptor by a 1200 dpi LED<br />
exposure system. The exposure time was 1.6 s. Then, the<br />
latent image was visualized by a development unit. The voltage<br />
applied to the development roller was +550 V. Excess<br />
liquid included in the image was removed by a squeeze<br />
roller, <strong>and</strong> the image was dried by an air dryer. Next, the<br />
image was transferred to an intermediate transfer roller under<br />
pressure of 0.74 MPa at the 100°C without electrostatic<br />
<strong>for</strong>ces. The velocity of the surface of the intermediate transfer<br />
roller was slower than that of the surface of the photoreceptor<br />
drum by 2.5%. Finally, the image was transferred to a<br />
substrate under pressure of 0.74 MPa by a backup roller at<br />
100°C.<br />
Fine-pitch pattern of L/S=1 pixel/1 pixel<br />
=21.6 m/21.6 m with 1200 dpi resolution was printed<br />
on polyimide film as shown in Figure 6. The fine-pitch pattern<br />
<strong>for</strong>med on the photoreceptor surface was transferred to<br />
the substrate without deterioration during our transfer<br />
process.<br />
In the case of the dry toner electrophotographic system,<br />
the toner particle size is 4−8 m in diameter. 3,4 The limit of<br />
the L/S using dry toner is about 80/80 m <strong>and</strong> the line<br />
edge shows a ragged shape in the range of several tens of<br />
m. Compared with the dry toner electrophotography, it is<br />
possible to achieve higher resolution patterns owing to edge<br />
sharpness <strong>and</strong> to provide higher per<strong>for</strong>mance in electroless<br />
plating owing to uni<strong>for</strong>m dispersion of conductive particles<br />
over the printed pattern. Additionaly, the amount of scattered<br />
particles in the vicinity of the line, in the case of using<br />
the shearing transfer process, is considerably less than that in<br />
the case of using the electric transfer process.<br />
Surface Modification after Printing Pattern<br />
Liquid toner imaging technology is advantageous <strong>for</strong> realizing<br />
a flat <strong>and</strong> smooth surface <strong>for</strong> printed patterns. However,<br />
a surface modification is required in order to successfully use<br />
the electroless plating process. The dry etching process was<br />
applied to the patterns after printing to <strong>for</strong>m a surface with<br />
irregularities. Plasma etch system TE-7500M of Tokyo<br />
Electron Ltd. was used <strong>for</strong> plasma etching with fluorocarbon<br />
<strong>and</strong> oxygen mixture gases. The rate of mixture gases was<br />
1 sccm <strong>for</strong> C 4 F 8 <strong>and</strong> 10 sccm <strong>for</strong> O 2 , <strong>and</strong> the total gas pressure<br />
was 7.3 Pa. The applied power was 50 W, <strong>and</strong> etching<br />
time was 10 sec. The fluoride deposition was removed by<br />
cleaning with HCl solution after plasma etching.<br />
Figure 7 shows the SEM images of the pattern surface,<br />
both of as printed [Fig. 7(a)] <strong>and</strong> after adding surface modification<br />
[Fig. 7(b)]. The pattern was printed on the silicon<br />
wafer with an insulating coating of epoxy resin film. A<br />
slightly rough surface after surface modification is observed,<br />
as shown in Fig. 7(b). The resin of the surface was selectively<br />
decomposed, <strong>and</strong> the superfine conductive particles remained<br />
after the dry eching process. The weight of the resin<br />
decreased by 5%, <strong>and</strong> the amount of exposed superfine conductive<br />
particles increased at the surface.<br />
In the case of the as-printed pattern without any surface<br />
modification, however, the Cu film was deposited on it at the<br />
initial state of the electroless plating process <strong>and</strong> the adhesion<br />
of the Cu film was insufficient. The deposited Cu film<br />
was removed during the electroless plating process. It was<br />
not necessary to add the special etching process whenever<br />
the line width was more than 50 m using conductive particles<br />
of several hundred nanometers in diameter, the Cu<br />
film was deposited successfully without the plasma etching<br />
in that case.<br />
The failure of the electroless plating was considered to<br />
be attributable to two factors, namely, only a small area of<br />
the superfine conductive particles was exposed <strong>for</strong> electroless<br />
plating solution <strong>and</strong> the “anchor effect” was not obtained<br />
sufficiently <strong>for</strong> the flat surface of the as-printed pattern.<br />
There<strong>for</strong>e, surface modification as described above was<br />
needed be<strong>for</strong>e electroless plating.<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 467
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
Figure 7. SEM images of pattern surface of seed toner layer: a Printed<br />
pattern <strong>and</strong> b after surface modification.<br />
Figure 9. SEM cross-sectional view of Cu conductive line.<br />
Figure 8. SEM images of L/S=1/1 pixel Cu conductive lines. Substrate:<br />
Si wafer with insulation film of epoxy resin. a Cu conductive<br />
pattern <strong>and</strong> b edge of Cu line.<br />
Electroless Plating<br />
Samples were prepared by cleaning in low-concentration<br />
acid be<strong>for</strong>e electroless plating. The Cu layer was deposited<br />
on the printed surface by using a plating solution based on<br />
ethylenediamine-tetraacetic acid. Samples were dipped in<br />
Thru-Cup ELC-SP ® ,providedbyC.Uyemura&Co.,Ltd.,at<br />
60°C.<br />
SEM images of Cu conductive pattern are shown in<br />
Figure 8. The thickness of the toner layer was 1 m <strong>and</strong><br />
that of Cu layer was 3 m. Figure 8(b) shows an edge of<br />
the Cu line. The raggedness of the line edge was 2 m. Cu<br />
lines were clearly isolated from one another. The amount of<br />
scattered particles between the printed lines was decreased<br />
by applying the surface modification, as shown in Fig. 6. A<br />
high-resolution multiline pattern was obtained by using our<br />
fabrication process.<br />
Figure 9 shows the SEM cross-sectional view of the Cu<br />
line. The cross section of the seed toner layer has fine irregularities<br />
whose maximum height (Rz) per reference length c<br />
(where c=1 m) is300 nm. The very fine irregularities<br />
were obtained by the plasma etching. Adhesion between the<br />
seed toner layer <strong>and</strong> Cu layer was increased dramatically by<br />
applying surface modification. No peeling off of the layers<br />
was observed during the electroless plating.<br />
The volume resistivity of the Cu line was<br />
2.110 −6 cm, which is 1.2 times as high as that of bulk<br />
Figure 10. Reliability test: a Test pattern L/S=2/4 pixel, b reflow<br />
oven condition, <strong>and</strong> c reflow test.<br />
Cu. The volume resistivity was slightly higher than that of<br />
bulk Cu because the fabrication process was insufficiently<br />
optimized in the present study. That is an issue to be tackled<br />
in the next phase of this research.<br />
Reliability Test<br />
A reliability test was carried out. Change of the resistance of<br />
the wiring patterns <strong>for</strong> L/S=2 pixel/4 pixel shown in<br />
Figure 10(a) was measured after applying the reflow test.<br />
Samples were covered with Au t=50 nm/Ni t=4 m<br />
layers by additional electroless plating. The reflow condition<br />
was set up <strong>for</strong> a peak temperature of 260°C, as shown in<br />
Fig. 10(b), which is the usual condition <strong>for</strong> a lead-free solder<br />
bump. The reflow test is shown schematically in Fig. 10(c).<br />
Results of the reflow test are shown in Figure 11. The<br />
rates of resistance change were 10% <strong>for</strong> every sample after<br />
the reflow test had been per<strong>for</strong>med eight times. The reflow<br />
test is severe in the case of the samples including the resin<br />
layer. Though the surface of the insulating film was partly<br />
decomposed after the reflow test had been per<strong>for</strong>med a few<br />
times, the resistance values were stable. The circuits will accordingly<br />
be compatible with the conventional bump assembly<br />
process.<br />
468 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
Figure 11. Results of the resistance change after the reflow test.<br />
Figure 13. Surface potential distributions V p of 2540 dpi multiline pattern:<br />
a L/S=10/10 m <strong>and</strong>b L/S=10/20 m.<br />
Figure 12. Analysis model of the development area: V p x, surface potential<br />
distribution of photoreceptor; V d , development bias; E x , E y , electric<br />
field in the development area; <strong>and</strong> tx,y, potential distribution in the<br />
development area.<br />
Furthermore, <strong>for</strong> the purpose of examining the capability<br />
of multiline-pattern <strong>for</strong>mation using liquid development<br />
with a 2450 dpi resolution exposure system, a theoretical<br />
analysis is discussed in the next section.<br />
THEORETICAL ANALYSIS OF MULTILINE-PATTERN<br />
FORMATION USING LIQUID TONER<br />
DEVELOPMENT<br />
In our previous study, 9–13 the high-resolution liquid toner<br />
development process of a very fine single dot with 2540 dpi<br />
resolution was analyzed theoretically <strong>and</strong> experimentally. In<br />
the present study, the development processes of<br />
L/S=1 pixel/1 pixel multiline patterns <strong>for</strong>med with the<br />
2540 dpi resolution exposure system are analyzed.<br />
Analysis Models<br />
An analyzed model is a multiline pattern including ten lines,<br />
whose line width is 1 pixel =10 m <strong>and</strong> space is 10 m<br />
<strong>and</strong> 20 m in each case. The development area illustrated in<br />
Figure 12 is defined as the rectangle of 300 m <strong>for</strong><br />
L/S=10/10 m <strong>and</strong> 380 m <strong>for</strong> L/S=10/20 m, respectively,<br />
on the x-axis <strong>and</strong> 150 m on the y-axis. The surface<br />
potential distributions on the photoreceptor V p determined<br />
from the exposure energy distribution <strong>and</strong> the photoinduced<br />
discharge curve (PIDC) of the positively charged<br />
photoreceptor are shown in Figure 13 (V 0 =700 V,<br />
V d =400 V). 9–13,16 However, V p <strong>for</strong> L/S=10/20 m were<br />
sufficiently isolated from each other, but those <strong>for</strong><br />
L/S=10/10 m were lower because of the narrow space.<br />
Two-Dimensional Liquid Development Model<br />
The toner particles migrate toward the latent image due to<br />
the effect of the electric field E originated from the surface<br />
potential on the photoreceptor V p <strong>and</strong> the development bias<br />
V d . The space <strong>and</strong> time distributions of the charge density<br />
are brought about by the continuity equations [Eqs. (1) <strong>and</strong><br />
(2)] <strong>and</strong> Poisson’s equation [Eq. (3)]. 9–13,17–19 The calculation<br />
was per<strong>for</strong>med using differential equations. The charge<br />
density of the toner particles p is positive, <strong>and</strong> the charge<br />
density of the counter ions n is negative. In the initial state<br />
of the development process, both the charge densities, p<br />
<strong>and</strong> n , are homogeneous in the area of analysis <strong>and</strong> their<br />
absolute values P 0 are the same. The values used in the<br />
analysis are listed in Table I<br />
2 t<br />
x 2<br />
p<br />
t =− p p E x <br />
x<br />
n<br />
t<br />
+ 2 t<br />
y 2<br />
=+ n n E x <br />
x<br />
− p p E y <br />
, 1<br />
y<br />
+ n n E y <br />
, 2<br />
y<br />
=− E x<br />
x + E y<br />
=−<br />
y p + n <br />
. 3<br />
t<br />
Table I. Parameters <strong>for</strong> numerical simulation.<br />
Parameters Symbol Value Unit<br />
Development time T d 48 msec<br />
Initial charge density of liquid toner P 0 1.54 C/m 3<br />
Relative dielectric constant of liquid toner t 2.03 -<br />
Development gap dt 150 µm<br />
Mobility of toner particle µ p 4.00 10 −10 m 2 / Vsec<br />
Mobility of counter ion µ n 2.00 10 −10 m 2 / Vsec<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 469
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
Figure 15. Time dependence of toner distributions <strong>for</strong> L/S<br />
=10/10 m: a 1 msec, b 5 msec, <strong>and</strong> c 48 msec.<br />
toner particles. Toner particles deposited almost equally on<br />
each line in the early stage of the development process<br />
t=1 msec. However, toner density of the area outside the<br />
multiline pattern decreased rapidly <strong>and</strong> the depletion area<br />
became wider as the development process proceeded. The<br />
amounts of deposited toner on both outer lines were less<br />
than those of deposited toner on inner lines in the last stage<br />
of the development process t=48 msec.<br />
Multiline Formation<br />
Figure 16 shows the L/S dependence of toner distribution in<br />
the case of L/S=10/10 m <strong>and</strong> 10/20 m, respectively. Although<br />
the surface potential V p <strong>for</strong> L/S=10/10 m becomes<br />
lower, the lines <strong>for</strong>med on the photoreceptor are sufficiently<br />
isolated from one another.<br />
In this result, the above mentioned “edge effects” were<br />
also observed. In particular, the edge effects are rather significant<br />
<strong>for</strong> the fine-pitch mode.<br />
Figure 14. Calculated potential distributions in the development area:<br />
a L/S=10/10 m <strong>and</strong>b L/S=10/20 m.<br />
Potential <strong>and</strong> Charge Density Distribution of Multiline<br />
Pattern<br />
The calculated potential distributions in the development<br />
area <strong>for</strong> L/S=10/10 m <strong>and</strong> 10/20 m, respectively, are<br />
shown three-dimensionally in Figure 14. The x-axis indicates<br />
the position in subscanning direction on the photoreceptor<br />
surface, <strong>and</strong> the y-axis indicates the development gap between<br />
the photoreceptor <strong>and</strong> the development roller. It indicates<br />
that the potential gradient in the positive direction of<br />
y-axis is low except in the vicinity of the photoreceptor surface.<br />
Figure 15 shows the time dependence of toner distribution<br />
<strong>for</strong> L/S=10/10 m. Clear contrast of the charge density<br />
was observed in the vicinity of the photoreceptor surface.<br />
The multiline pattern <strong>for</strong>med gradually by migration of<br />
Figure 16. L/S dependence of toner distributions <strong>for</strong> multiline pattern:<br />
a L/S=10/10 m <strong>and</strong> b L/S=10/20 m.<br />
470 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
In order to obtain fine multiline patterns with a uni<strong>for</strong>m line<br />
width, image processing will be required.<br />
The numerical analysis of fine-pitch multiline patterns<br />
agrees well with that of the actual toner images obtained<br />
experimentally. We infer that the simulation results precisely<br />
describe the actual toner development system.<br />
Figure 17. L/S=10/10 m multiline pattern on the Si substrate.<br />
EXPERIMENTAL RESULTS<br />
The seed toner images of L/S=10/10 m multiline pattern<br />
were realized by using the real 2540 dpi resolution LSU.<br />
Figure 17 shows the lines printed on Si wafer with the insulating<br />
coating. Figure 18(a) shows the lines on the polyimide<br />
film. Width <strong>and</strong> height of the lines were measured by laser<br />
microscope [Fig. 18(b)]. The edge effect is observed; the<br />
amount of deposited toner <strong>for</strong> the outer line (line No. 1) was<br />
less than that of deposited toner <strong>for</strong> inner lines (Nos. 2–4).<br />
Figure 18. L/S=10/10 m multiline pattern on the polyimide film: a<br />
Printed pattern on the polyimide film <strong>and</strong> b line shape data measured by<br />
laser microscope.<br />
CONCLUSION<br />
We have developed a digital fabrication process using highresolution<br />
liquid toner electrophotography. The multiline<br />
pattern of L/S=21.6/21.6 m was <strong>for</strong>med by printing the<br />
seed toner with a 1200 dpi resolution LED exposure system,<br />
<strong>and</strong> the Cu layer was deposited on the printed lines by electroless<br />
plating after surface modification. The liquid toner<br />
development of multiline patterns <strong>for</strong>med with 2540 dpi<br />
resolution LSU was analyzed theoretically. The numerical<br />
analysis agrees well with the experimental results. These results<br />
confirm that the fabrication process using liquid toner<br />
electrophotography is available <strong>for</strong> <strong>for</strong>ming high-resolution<br />
multiline patterns.<br />
ACKNOWLEDGMENTS<br />
The authors wish to thank H. Aoki <strong>and</strong> N. Yamaguchi of<br />
Semiconductor Company, Toshiba Corporation, <strong>for</strong> advice<br />
on the assembly technology. They also wish to thank T.<br />
Yasumoto <strong>and</strong> N. Yanase of Semiconductor Company,<br />
Toshiba Corporation, <strong>for</strong> advice on the surface modification.<br />
They are grateful to S. Sakamoto <strong>and</strong> Y. Yamamoto of C.<br />
Uyemura & Co., Ltd., <strong>for</strong> supporting the electroless plating<br />
process.<br />
REFERENCES<br />
1 P. H. Kydd <strong>and</strong> D. L. Richard, “Electrostatic printing of Parmod<br />
electrical conductors”, Proc. IS&T’s NIP14 (IS&T, Springfield, VA, 1998)<br />
pp. 222–225.<br />
2 P. Calvert, “Inkjet printing <strong>for</strong> materials <strong>and</strong> devices”, Chem. Mater. 13,<br />
3299 (2001).<br />
3 N. Yamaguchi, H. Aoki, C. Takubo, K. Imamiya, T. Yamauchi, <strong>and</strong> H.<br />
Hashizume, “New process using electrophotographic technology <strong>for</strong><br />
manufacturing printed circuit board”, Proc. ISJ’s Japan Hardcopy 2004<br />
(The <strong>Imaging</strong> <strong>Society</strong> of Japan, Tokyo, 2004) pp. 121–124.<br />
4 H. Aoki, N. Yamaguchi, <strong>and</strong> C. Takubo, “A study of electrophotography<br />
process <strong>for</strong> manufacturing printed circuit board”, Proc. IS&T’s NIP20<br />
(IS&T, Springfield, VA, 2004) pp. 241–245.<br />
5 H. Yagi, Y. Shinjo, H. Oh-oka, M. Saito, K. Ishii, I. Takasu, <strong>and</strong> M.<br />
Hosoya, “Image-on-image color process using liquid toner”, Proc.<br />
IS&T’s NIP16 (IS&T, Springfield, VA, 2000) pp. 246–250.<br />
6 S. Hirahara, T. Watanabe, M. Saito, A. Iida, K. Ishii, <strong>and</strong> M. Hosoya,<br />
“Liquid toner image transfer using shearing stress [I]”, J. Imag. Soc. Jpn.<br />
42, 17 (2003).<br />
7 A. Iida, Y. Shinjo, H. Nukada, S. Hirahara, N. Yoshikawa, <strong>and</strong> M.<br />
Hosoya, “A study of the relationship between drying state of toner <strong>and</strong><br />
transfer per<strong>for</strong>mance in the IOI color process using liquid toner”, Proc.<br />
International Congress of <strong>Imaging</strong> <strong>Science</strong> 2002 (The <strong>Imaging</strong> <strong>Society</strong> of<br />
Japan, Tokyo, 2002) pp. 596–597.<br />
8 A. Iida, Y. Shinjo, H. Nukada, N. Yoshikawa, S. Hirahara, <strong>and</strong> M.<br />
Hosoya, “Relationship between drying state of toner image <strong>and</strong> transfer<br />
per<strong>for</strong>mance in the IOI color process using liquid toner”, J. Imag. Soc.<br />
Jpn. 42, 24 (2003).<br />
9 K. Ishii, M. Takahashi, H. Nagato, K. Higuchi, M. Hosoya, <strong>and</strong> K.<br />
Komata, “2540 dpi full color imaging creation with a liquid<br />
electrophotography system”, Proc. IS&T’s NIP19 (IS&T, Springfield, VA,<br />
2003) pp. 9–12.<br />
10 K. Ishii, M. Takahashi, H. Nagato, K. Higuchi, M. Hosoya, <strong>and</strong> K.<br />
Komata, “Liquid toner electrophotography <strong>for</strong> 2540 dpi high resolution<br />
image”, J. Imag. Soc. Jpn. 44, 437 (2005).<br />
J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007 471
Iida et al.: Digital fabrication using high-resolution liquid toner electrophotography<br />
11 H. Yagi, Y. Shinjo, H. Oh-oka, M. Saito, K. Ishii, H. Nukada, <strong>and</strong> M.<br />
Hosoya, “Image-on-image color process using liquid toner”, J. Imag.<br />
Soc. Jpn. 44, 416 (2005).<br />
12 Y. Shinjo, H. Yagi, M. Takahashi, K. Ishii, I. Takasu, <strong>and</strong> M. Hosoya,<br />
“Theoretical analysis <strong>and</strong> experiment of high-resolution development<br />
using liquid toner”, J. Imag. Soc. Jpn. 44, 429 (2005).<br />
13 I. Takasu, H. Yagi, Y. Shinjo, M. Takahashi, <strong>and</strong> M. Hosoya, “Analysis of<br />
2540 dpi dot reproduction by liquid development”, Proc. IS&T’s NIP20<br />
(IS&T, Springfield, VA, 2004) pp. 1027–1031.<br />
14 A. Iida, K. Ishii, Y. Shinjo, H. Yagi, M. Saito, H. Nukada, M. Takahashi,<br />
<strong>and</strong> M. Hosoya, “A study of digital fabrication using high-resolution<br />
liquid toner electrophotography”, Proc. IS&T’s Digital Fabrication 2005<br />
(IS&T, Springfield, VA, 2005) pp. 26–29.<br />
15 J. N. Israelachivili, Intermolecular <strong>and</strong> Surface Forces (Academic Press,<br />
New York, 1992).<br />
16 Y. Kishi, M. Kadonaga, <strong>and</strong> Y. Watanabe, “Numerical simulation of toner<br />
motion in magnetic brush development system”, Proc. ISJ’s Japan<br />
Hardcopy ’99 (The <strong>Imaging</strong> <strong>Society</strong> of Japan, Tokyo, 1999) pp. 177–180.<br />
17 I. Chen, “A charge transport model of liquid development <strong>for</strong><br />
electrography”, J. <strong>Imaging</strong> Sci. Technol. 39(6), 473 (1995).<br />
18 F. J. Wang, G. A. Demoto, H. R. Till, <strong>and</strong> J. F. Knapp, “Electrophoretic<br />
deposition of liquid toners in a plate-out cell—A numerical analysis”,<br />
Proc. IS&T’s NIP15 (IS&T, Springfield, VA, 1999) pp. 623–626.<br />
19 S. Aoki, H. Ishida, <strong>and</strong> G. Takano, “3D toner image simulation<br />
considering the latent image <strong>for</strong>mation with laser exposure”, Proc. ISJ’s<br />
Japan Hardcopy 2003 (The <strong>Imaging</strong> <strong>Society</strong> of Japan, Tokyo, 2003) pp.<br />
273–276.<br />
472 J. <strong>Imaging</strong> Sci. Technol. 515/Sep.-Oct. 2007
COMING SOON!... to the IS&T Wiley Series<br />
COLOR CONSTANCY<br />
Marc Ebner<br />
Julius-Maximilians-Universitat Wurzburg, Germany<br />
ISBN: 978-0-470-05829-9<br />
Hardcover<br />
416 pages<br />
June 2007<br />
List price: $135.00<br />
IS&T price<br />
Member $115.00 Non-member $125.00<br />
This book provides a comprehensive introduction to the field of color constancy, describing all the<br />
major color constancy algorithms, as well as presenting cutting edge research in the area of color<br />
image processing. Beginning with an in-depth look at the human visual system, Ebner goes on to:<br />
• examine the theory of color image <strong>for</strong>mation, color reproduction <strong>and</strong> different<br />
color spaces;<br />
• discuss algorithms <strong>for</strong> color constancy under both uni<strong>for</strong>m <strong>and</strong> non-uni<strong>for</strong>ms<br />
illuminants;<br />
• describe methods <strong>for</strong> shadow removal <strong>and</strong> shadow attenuation in digital images;<br />
• evaluate the various algorithms <strong>for</strong> object recognition <strong>and</strong> color constancy <strong>and</strong><br />
compare this to data obtained from experimental psychology;<br />
• set out the different algorithms as pseudo code in an appendix at the end of the book.<br />
Color Constancy is an ideal reference <strong>for</strong> practising engineers, computer scientists <strong>and</strong> researchers<br />
working in the area of digital color image processing. It may also be useful <strong>for</strong> biologists or scientists<br />
in general who are interested in computational theories of the visual brain <strong>and</strong> bio-inspired<br />
engineering systems.<br />
For more in<strong>for</strong>mation please visit:<br />
http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470058293.html
Journal of the <strong>Imaging</strong> <strong>Society</strong> of Japan VOL.46 NO.4<br />
2007<br />
CONTENTS<br />
Original Papers<br />
Analysis of the Penetration Action to the Ink-Jet Media of Pigment Ink-Jet Ink<br />
K. MURAYAMA, K. INOUE, M. YATAKE <strong>and</strong> K. KOROGI ...2362<br />
An Action Model of Silica Additives on the Electrification <strong>and</strong> the Adhesion of Toner<br />
H. OKADA, M. TAKEUCHI <strong>and</strong> J.M. EUN ...2417<br />
Investigation of Color Electrophoretic Display Utilizing Electrophoretic Colored Particles<br />
S. SUNOHARA <strong>and</strong> T. KITAMURA ...24713<br />
<strong>Imaging</strong> Today<br />
Latest Toner Technologies<br />
IntroductionH. YAMAZAKI, K. NAGATO, N. KOBAYASHI <strong>and</strong> H. NAITO ...25420<br />
Current Technologies of Suspension Polymerization Toner M. UCHIYAMA ...25521<br />
<strong>Technology</strong> Development of EA-HG Toner F. KIYONO ...26127<br />
Development of KONICA MINOLTA HD(High Definition) Digital Toner<br />
Produced by Emulsion Polymerization <strong>and</strong> Coagulation Method T. YAMANOUCHI ...26632<br />
Ester Elongation Polymerized Toner<br />
H. YAMADA, J. AWAMURA, H. NAKAJIMA, A. KOTSUGAI, F. SASAKI <strong>and</strong> T. NANYA ...27137<br />
Toner Manufacturing Using a Chemical Milling Method E.J. CHOI, C.H. KIM <strong>and</strong> H.N. YOON ...27743<br />
The Powder Processing Machines to Do with Grinding Toner Production <strong>and</strong> a New <strong>Technology</strong><br />
<strong>for</strong> Toner Particle Production M. YOSHIKAWA ...28349<br />
The Manufacture of the Small Particle Size Toner in a Dry Process <strong>for</strong> High Speed Printing,<br />
Fine Quality <strong>and</strong> Oil-less Fusing S. OMATSU ...28955<br />
Lectures in <strong>Science</strong><br />
Introduction of OpticsII<br />
The Behavior of a Beam of Light upon Reflection or Refraction at a Single Spherical Surface <br />
H. MUROTANI ....29561<br />
ISJ’s Awards 2006 30268<br />
In<strong>for</strong>mation Materials <strong>for</strong> the 50th ISJ General Meeting 31278<br />
Meeting Reports 340106<br />
Announcements 342108<br />
Guide <strong>for</strong> Authors 346112<br />
Contents of J. Photographic <strong>Society</strong> of Japan347113<br />
Contents of J. Printing <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> of Japan 348114<br />
Contents of J. Inst. Image Electronics Engineers of Japan 349115<br />
Contents of Journal of <strong>Imaging</strong> <strong>Science</strong> <strong>and</strong> <strong>Technology</strong> 350116<br />
Essays on <strong>Imaging</strong><br />
The <strong>Imaging</strong> <strong>Society</strong> of Japan<br />
c/o Tokyo Polytechnic University, 2-9-5, Honcho, Nakano-ku, Tokyo, 1648768 Japan<br />
Phone :033373-9576 Fax :033372-4414 E-mail :
imaging.org<br />
IS&T<br />
your source <strong>for</strong> imaging science in<strong>for</strong>mation<br />
CHECK OUT THESE NEW OFFERINGS FROM THE IS&T BOOKSTORE<br />
CLOSE RANGE PHOTOGRAMMETRY<br />
Principles, Methods <strong>and</strong> Applications<br />
Thomas Luhmann, Stuart Robson, Stephen Kyle, <strong>and</strong> Ian Harley, published by John Wiley & Sons, 2007, 528 pp,<br />
ISBN: 978-0-470-10633-4, 2 lbs.<br />
English-language edition of a book originally published in 2003 in German. The translation was undertaken with cooperation of<br />
the original author <strong>and</strong> a team of three researchers in the UK. The book describes the fundamentals of photogrammetry, the<br />
equipment used in both hardware <strong>and</strong> software, analytical methods <strong>and</strong> various real-world applications where the technique has<br />
found broad use over the past 20 years. Photogrammetry is a very strong field in Germany, <strong>and</strong> many of the best researchers,<br />
leading companies <strong>and</strong> equipment manufacturers are based there. Because of this, the book is authoritative, <strong>and</strong> its translation<br />
into English will be appreciated by users in North America who often don’t have access to the latest writing on the topic.<br />
Members $100 / Non-Members $110 (list price $120)<br />
DIGITAL VIDEO AND HDTV<br />
Algorithms <strong>and</strong> Interfaces, First Edition<br />
Charles Poynton, published by Morgan Kauffman, 2002, 736 pp., ISBN: 1-55860-792-7, 4 lbs.<br />
Rapidly evolving computer <strong>and</strong> communications technologies have achieved data transmission rates <strong>and</strong> data storage<br />
capacities high enough <strong>for</strong> digital video. But video involves much more than just pushing bits! Achieving the best possible<br />
image quality, accurate color, <strong>and</strong> smooth motion requires underst<strong>and</strong>ing many aspects of image acquisition, coding,<br />
processing, <strong>and</strong> display that are beyond the usual realm of computer graphics. At the same time, video system designers are<br />
facing new dem<strong>and</strong>s to interface with film <strong>and</strong> computer system that require techniques outside conventional video engineering.<br />
Member: $58.00 / Non-Member: $63.00 (list price $67.95)<br />
DIFFRACTION, FOURIER OPTICS AND IMAGING<br />
Okan K. Erosy, published by John Wiley & Sons, 2006, 413 pp, ISBN: 978-0-23816-4, 2 lbs.<br />
This book presents current theories of diffraction, imaging, <strong>and</strong> related topics based on Fourier analysis <strong>and</strong> synthesis<br />
techniques, which are essential <strong>for</strong> underst<strong>and</strong>ing, analyzing, <strong>and</strong> synthesizing modern imaging, optical communications<br />
<strong>and</strong> networking, as well as micro/nano systems. Applications covered include tomography; magnetic resonance imaging;<br />
synthetic aperture radar (SAR) <strong>and</strong> interferometric SAR; opticalcommunications <strong>and</strong> networking devices; computer-generated<br />
analog holograms; <strong>and</strong> wireless systems using EM waves.<br />
Members $95.00 / Non-Members $105.00 (list price $110.00)<br />
DIGITAL IMAGE PROCESSING<br />
PIKS Scientific Inside, 4th Edition (includes CD Rom)<br />
William K. Pratt, published by John Wiley & Sons, 2007, 782 pp., ISBN: 0-471-76777-0, 3 lbs.<br />
The Fourth Edition of Digital Image Processing provides a complete introduction to the field <strong>and</strong> includes new in<strong>for</strong>mation that<br />
updates the state of the art. The text offers coverage of new topics <strong>and</strong> includes interactive computer display imaging examples<br />
<strong>and</strong> computer programming exercises that illustrate the theoretical content of the book. These exercises can be implemented using<br />
the Programmer’s <strong>Imaging</strong> Kernel System (PIKS) application program interface included on the accompanying CD.<br />
Members $105.00 / Non-Members $115.00 (list price $125.00)
IS&T Corporate Members<br />
IS&T Corporate Members provide significant financial support, thereby assisting the <strong>Society</strong> in achieving its goals of<br />
disseminating in<strong>for</strong>mation <strong>and</strong> providing professional services to imaging scientists <strong>and</strong> engineers. In turn, the <strong>Society</strong><br />
provides a number of material benefits to its Corporate Members. For complete in<strong>for</strong>mation on the Corporate<br />
Membership program, contact IS&T at info@imaging.org.<br />
Sustaining Corporate Members<br />
Adobe Systems Inc.<br />
345 Park Avenue<br />
San Jose, CA 95110-2704<br />
Eastman Kodak Company<br />
343 State Street<br />
Rochester, NY 14650<br />
Hewlett-Packard Company<br />
1501 Page Mill Road<br />
Palo Alto, CA 94304<br />
Lexmark International, Inc.<br />
740 New Circle Road NW<br />
Lexington, KY 40511<br />
Xerox Corporation<br />
Wilson Center <strong>for</strong> Research <strong>and</strong><br />
<strong>Technology</strong><br />
800 Phillips Road<br />
Webster, NY 14580<br />
Supporting Corporate Members<br />
Fuji Photo Film Company, Ltd.<br />
210 Nakanuma, Minami-ashigara<br />
Kanagawa 250-0193 Japan<br />
Konica Minolta Holdings Inc.<br />
No. 1 Sakura-machi<br />
Hino-shi, Tokyo 191-8511 Japan<br />
TREK, Inc./TREK Japan KK<br />
11601 Maple Ridge Road<br />
Medina, NY 14103-0728<br />
Pitney Bowes<br />
35 Waterview Drive<br />
Shelton, CT 06484<br />
Donor Corporate Members<br />
ABBY USA Software House, Inc.<br />
47221 Fremont Blvd.<br />
Fremont, CA 94538<br />
Axis Communications AB<br />
Embdalavägen 14<br />
SE-223 69 Lund, Sweden<br />
Ball Packaging Europe GmbH<br />
Technical Center Bonn<br />
Friedrich-Woehler-Strasse 51<br />
D-53117 Bonn, Germany<br />
Cheran Digital <strong>Imaging</strong> & Consulting, Inc.<br />
798 Burnt Gin Road<br />
Gaffney, SC 29340<br />
Clariant Produkte GmbH<br />
Division Pigments & Additives<br />
65926 Frankfurt am Main Germany<br />
Felix Schoeller Jr. GmbH & Co. KG<br />
Postfach 3667<br />
D-49026 Osnabruck, Germany<br />
Hallmark Cards, Inc.<br />
Chemistry R & D<br />
2501 McGee, #359<br />
Kansas City, MO 64141-6580<br />
ILFORD <strong>Imaging</strong> Switzerl<strong>and</strong> GmbH<br />
Route de l’Ancienne Papeterie 1<br />
CH-1723 Marly, Switzerl<strong>and</strong><br />
MediaTek Inc.<br />
No. 1 Dusing Rd., 1<br />
Hsinchu 300 R.O.C, Taiwan<br />
Pantone, Inc.<br />
590 Commerce Blvd.<br />
Carlstadt, NJ 07072-3098<br />
Quality Engineering Associates (QEA), Inc.<br />
99 South Bed<strong>for</strong>d Street, #4<br />
Burlington, MA 01803<br />
The Ricoh Company, Ltd.<br />
16-1 Shinei-cho, Tsuzuki-ku<br />
Yokohama 224-0035 Japan<br />
Sharp Corporation<br />
492 Minosho-cho, Yamatokoriyama<br />
Nara 639-1186 Japan<br />
Sony Corporation/<br />
Sony Research Center<br />
6-7-35 Kita-shinagawa<br />
Shinagawa, Tokyo 141 Japan<br />
Torrey Pines<br />
5973 Avenida Encinas, Suite 140<br />
Carlsbad, CA 92008<br />
*as 9/1/07
Advanced Measurement Systems <strong>for</strong> All R&D <strong>and</strong><br />
Quality Control Needs in Electrophotography,<br />
Inkjet <strong>and</strong> Other Printing Technologies<br />
PDT ® -2000 series<br />
Electrophotographic characterization,<br />
uni<strong>for</strong>mity mapping, <strong>and</strong> defect<br />
detection <strong>for</strong> large <strong>and</strong> small <strong>for</strong>mat<br />
OPC drums<br />
PDT ® -1000L<br />
PDT ® -1000<br />
ECT-100 TM<br />
OPC drum coating thickness<br />
gauge<br />
Electrophotographic<br />
Component Testing<br />
MFA-2000 TM<br />
Magnetic field distribution<br />
analysis in mag roller magnets<br />
DRA-2000 TM<br />
Semi-insulating components testing<br />
including charge rollers, mag rollers,<br />
transfer rollers, transfer belts, <strong>and</strong><br />
print media<br />
TFS-1000 TM<br />
Toner fusing latitude testing<br />
Objective Print Quality<br />
Analysis <strong>for</strong> All Digital<br />
Printing Technologies<br />
IAS ® -1000<br />
Fully-automated high volume print<br />
quality testing<br />
Scanner-based high speed print<br />
quality analysis<br />
Scanner IAS ®<br />
Personal IAS ®<br />
H<strong>and</strong>held series <strong>for</strong> print quality,<br />
distinctness of image (DOI), <strong>and</strong><br />
color measurements. Truly portable;<br />
no PC connection required<br />
PocketSpec TM<br />
DIAS TM<br />
Quality Engineering Associates, Inc.<br />
99 South Bed<strong>for</strong>d Street #4, Burlington, MA 01803 USA<br />
Tel: +1 (781) 221-0080 • Fax: +1 (781) 221-7107 • info@qea.com • www.qea.com
imaging.org<br />
your source <strong>for</strong> imaging technology conferences<br />
2008 MEETING CALENDAR<br />
4th European<br />
Conference<br />
on Colour in<br />
Graphics, <strong>Imaging</strong>,<br />
<strong>and</strong> Vision<br />
CGIV 2008<br />
Archiving 2008<br />
June 24-27, 2008<br />
Bern, Switzerl<strong>and</strong><br />
June 9-13, 2008<br />
Terassa, Spain<br />
CALL FOR PAPERS<br />
Abstract deadline: November 15, 2007<br />
www.imaging.org/conferences/cgiv2008/<br />
CALL FOR PAPERS<br />
Abstract deadline: November 26, 2007<br />
www.imaging.org/conferences/archiving2008/<br />
www.imaging.org/conferences/NIP24/<br />
NIP24<br />
24th International Conference on<br />
Digital Printing Technologies<br />
Abstract deadline: February 11, 2008<br />
September 7-12, 2008<br />
Pittsburgh, Pennsylvania<br />
Digital<br />
Fabrication<br />
2008<br />
www.imaging.org/conferences/DF2008/<br />
CIC16<br />
16th Color <strong>Imaging</strong> Conference<br />
November 10-14, 2008<br />
Portl<strong>and</strong>, Oregon<br />
CALL FOR PAPERS<br />
Abstract deadline: April 15, 2008<br />
www.imaging.org/conferences/CIC16/<br />
Visit www.imaging.org to download programs <strong>and</strong> general in<strong>for</strong>mation on these <strong>and</strong> other IS&T-sponsored conferences.