Colloids and Surfaces in Reprographic Technology - American

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Color Ink Jet Printing: Materials Parameters

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 13, 2016 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch026

A. B. JAFFE, E. W. LUTTMAN, and W. CROOKS IBM ISD Advanced Technology, San Jose, CA 95125 High quality color reproduction is now obtainable by ink jet printing and w i l l be i l l u s t r a t e d . Once the printing parameters are selected, the quality achieved in the color reproduction depends strongly on the materials parameters of the system. Ink-paper interactions, ink formulation, and paper surface preparation all contribute to the quality of the resulting print. Archivality of the print is c o n t r o l l ed by the lightfastness of the dye, the pH of the ink, and of the waterfastness achieved. Conventional color reproduction involves many forms of printing processes which strive to reproduce a color o r i g i n a l . These can be either photographic or mechanical printing processes, depending on the number of copies to be made and the quality desired. Each one of the many mechanical processes has a unique set of requirements including equipment, inks, paper, and copy preparation that gives reproduction of differing quality and cost. For a small number of copies, photographic reproduction is the only present practical cost effective solution. Mechanical reproduction (e.g. letterpress, rotogravure, etc.) requires color separations to be made for each of the primary printing colors plus black. Even with the most modern technology, this is very time consuming and expensive, and for these reasons is only used for multiple copies. Ink jst printing has the inherent capablity to displace many of tha mecvfcnical processes since i t is an electronic technology. Data can b' printed from any digitized source such as a scanner or a computer such that plates are not necessary for multi-color printing. This makes i t equally easy for single or multiple copies With no variation of quality. Since ink jet technology has not yet been used for high quality color reproduction, the objectives of this process must be considered in terms of the materials limitations of ink jet printing. When a color reproduction is made by a mechanical process, much time is spent matching different dyes and papers with the desired effect wanted. After the materials decision is made, 0097-6156/82/0200-0531 $06.00/0 © 1982 American Chemical Society Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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p r i n t i n g then begins. With ink j e t p r i n t i n g , many t r i a l s cannot be run before the p r i n t i n g begins or any advantage t h i s technology holds would be l o s t . Reasonable q u a l i t y p r i n t must be p o s s i b l e at the f i r s t attempt. This paper w i l l d i s c u s s the parameters of the dye in the ink and the paper on which i t i s p r i n t e d that l i m i t s the c o l o r c a p a b i l i t i e s of ink j e t p r i n t i n g .


Ink The b a s i c requirements of an ink f o r i n k - j e t p r i n t i n g are numerous and sometimes c o n t r a d i c t o r y . In order to be jetted p r o p e r l y , the ink must have the c o r r e c t p h y s i c a l p r o p e r t i e s of v i s c o s i t y , s p e c i f i c g r a v i t y , surface t e n s i o n , c o n d u c t i v i t y , and pH. It should have c o m p a t i b i l i t y with a l l machine parts to which i t comes i n contact, low c o r r o s i o n , low p a r t i c u l a t e contamination, no b i o l o g i c a l growth, long s h e l f l i f e , and no chemical hazards. Once i t i s on the paper , i t should have good p r i n t q u a l i t y and should be a r c h i v a l . The ink i s composed of many materials i n order to s a t i s f y a l l these requirements but the only concerns of t h i s report are those f a c t o r s which a f f e c t the reproduction of c o l o r . Although the c o l o r a n t s i n conventional inks has t r a d i t i o n a l l y been pigments , i n s o l u b l e p a r t i c u l a t e matter i n the ink i s t o t a l l y unacceptable for j e t t e d inks due to the p o s s i b i l i t y of clogging of the small o r i f i c e s . The nozzles i n t h i s study are .75 m i l i n diameter. The i n k s , i n order to f u f i l l the operating parameters of the ink j e t process, are water based and t h e r e f o r e water s o l u b l e dyes are the chosen c o l o r a n t s . Unfortunately, t h i s makes the p r i n t s u s c e p t i b l e to problems of water-fastness a f t e r i t i s p r i n t e d . A recent study by Ohta (1) of dyes used i n photography has i n d i c a t e d the important parameters of a set of dyes f o r s u b t r a c t i v e c o l o r reproduction are: 1) Peak Wavelength 2) Breadth of the Absorption Peak 3) Unwanted Secondary Absorptions In order to reproduce a large c o l o r gamut ( t h i s can be thought as the number of c o l o r s that can be reproduced by a set of three primary i n k s ) , these dyes must be chosen i n a set that contains three peak wavelengths that corresponds to the c o l o r s cyan, magenta, and yellow. There i s a large range of each of these c o l o r s which are acceptable and the s i z e of the c o l o r gamut must be used to insure that one set of inks i s the best match f o r most c o l o r reproduction. This e n t a i l s the good reproduction of reds, blues, and greens, as w e l l as a large c o l o r gamut. These p r o p e r t i e s of a set of inks depend only on the absorption wavelength of the dyes chosen. Once a dye has been chosen, there i s l i t t l e that can be done to c o n t r o l i t s s p e c t r o s c o p i c p r o p e r t i e s except those a f f e c t e d by concentration. The example i n Figure 1 shows the spectroscopic changes that can accompany concentration v a r i a t i o n s of a dye. The o p t i c a l d e n s i t y does not n e c e s s a r i l y increase at the wavelength of i n t e r e s t as the concentration of the dye i s increased. Not only does Acid V i o l e t 19 e x h i b i t a large increase i n the width of the peak as

Hair and Croucher; Colloids and Surfaces in Reprographic Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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Spectral changes of Acid Violet 19 when the concentration is varied from 1% to 8%.


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the concentration i s increased, but , more importantly, there i s a s h i f t i n the wavelength of the p r i n c i p a l absorption. This shift a f f e c t s the hue of the d e s i r e d c o l o r and the broadening of the peak a f f e c t s both the s a t u r a t i o n and l i g h t n e s s . The perceived c o l o r s at higher concentration are darker, have s h i f t e d t h e i r hues, and are ' d i r t y . In t h i s case, the s h i f t i n p r i n c i p a l wavelength can be a t t r i b u t e d to the formation of a dimeric species (2) which absorbs l i g h t very s t r o n g l y . Not a l l dyes behave i n t h i s manner, but the use of water based systems does increase the p o s s i b i l i t y of t h i s happening. Since many water s o l u b l e dyes are p r i m a r i l y composed of hydrophobic, organic molecules which have a few s p e c i f i c s i t e s for water s o l u b i l i z a t i o n , as the concentration of the dye i s increased to the l e v e l s needed f o r an ink, d i m e r i z a t i o n can occur. Any dye that i s being considered f o r a s u b t r a c t i v e set of inks must be carefully screened to examine the change in spectroscopic parameters i n the concentration range of i n t e r e s t . Since many water-soluble dyes are pH i n d i c a t o r s , t h i s becomes an important parameter to consider when formulating an ink. For a continuous ink j e t system i n which the unused ink supply i s r e c i r c u l a t e d back to the main ink supply and r e c o n s t i t u t e d , c o n t r o l l i n g the pH becomes a serious problem. A c i d V i o l e t 14 ( f i g u r e 2) shows a strong dependence of the peak wavelength of absorption as the pH i s v a r i e d . For a p r a c t i c a l ink, a dye such as that i n Figure 3 i s more s u i t a b l e as i t does not vary g r e a t l y over a wide pH range. I f the pH d r i f t s , s h i f t s i n o p t i c a l d e n s i t y w i l l be n o t i c a b l e to the eye i n the c o l o r reproduction.


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Although l i g h t f a s t n e s s of any dye does not immediately a f f e c t the c o l o r c a p a b i l i t i e s of an ink, the long range p o t e n t i a l of the c o l o r can be s e v e r e l y r e s t r i c t e d . Any dye that would be considered f o r an ink should have s u f f i c i e n t a r c h i v a l p r o p e r t i e s to s u s t a i n an image a f t e r exposure to l i g h t . Figures 4, 5, and 6 show three dyes with v a r y i n g l i g h t f a s t n e s s c a p a b i l i t i e s . Patent Blue A would not be very u s e f u l s i n c e i t i s t o t a l l y decomposed by exposure to 40 hours i n a fadeometer. The other dyes are more r e s i s t a n t to fading and could be considered p r a c t i c a l i n an ink. Paper Important parameters a f f e c t i n g the q u a l i t y of any printing process i n v o l v i n g the d e p o s i t i o n of ink onto a surface are the p r o p e r t i e s of the ink and the p r o p e r t i e s of the surface to which the ink i s a p p l i e d . A paper, or a paper modified s u r f a c e , i s the most common surface on which inks are deposited. Most p r i n t i n g papers have been formulated to accept ink which are not water-based and to r e s i s t the p e n e t r a t i o n of water. To o b t a i n high q u a l i t y images from water-based inks a p p l i e d by ink j e t , i t i s necessary that the p r i n t i n g surface has p r o p e r t i e s which allow c o n t r o l l e d p e n e t r a t i o n of the water based inks. This i s complicated by the f a c t that the d r y i n g of the ink i s accomplished by absorption i n t o the paper. Instead of spreading on the paper i n a regular f a s h i o n , the ink can


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Variation of the extinction coefficient of Acid Violet 19 at the peak wavelength of absorption with pH.

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Variation of the extinction coefficient of Dye A at the peak wavelength of absorption with pH.



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Lightfastness behavior of Patent Blue A as exposed to an Atlas Fadeometer.

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Lightfastness behavior of Dye A as exposed to an Atlas Fadeometer.


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t r a v e l i n a c e l l u l o s e f i b e r . This causes a problem i n which the drop spreads out i n an i r r e g u l a r shape which i s c a l l e d f e a t h e r i n g . The p r i n t q u a l i t y i s degraded. I f the ink p e n e t r a t i o n i s c o n t r o l l e d , then spots o f s p e c i f i c dimensions can be p r i n t e d . The spot diameter i s r e l a t e d not only t o the paper surface c h a r a c t e r i s t i c s such as i t s water a b s o r p t i v i t y and surface smoothness, but a l s o to the ink j e t nozzle diameter, ink pressure and the p h y s i c a l p r o p e r t i e s o f the ink. To obtain optimum c o l o r q u a l i t y or maximum c o l o r gamut, an opaque paper with a white, b r i g h t surface i s required. Bond paper, which has a rough surface, absorbs the ink i n t o the bulk. The r e s u l t i n g d i f f u s e r e f l e c t i o n form the f i b e r / a i r i n t e r f a c e l i m i t s the maximum o p t i c a l d e n s i t y obtainable ( Figure 7 ). T h i s i s due t o the f a c t that the eye receives white l i g h t from the source that has not been through the ink l a y e r . Furthermore, because o f the u n c o n t r o l l e d absorption o f the ink i n t o the paper, ink spreading or f e a t h e r i n g occurs and r e s o l u t i o n i s adversely a f f e c t e d . In Figure 8, the SEM shows the drop shape i r r e g u l a r i t y on bond paper. I f a c o a t i n g such as p o l y v i n y l a l c o h o l i s a p p l i e d to the paper, the water-based inks remain on the surface a f t e r p r i n t i n g . The f i r s t surface r e f l e c t i o n becomes specular ( Figure 7 ) and r e s u l t s i n increased o p t i c a l d e n s i t y and b e t t e r s a t u r a t i o n . The l i g h t going through the ink layer a l s o must be r e f l e c t e d from the paper base. Very high q u a l i t y reproduction can be obtained i f a commercially manufactured photobase paper i s used. (3) The barium s u l f a t e c o a t i n g on t h i s paper i s extremely smooth and has high optical r e f l e c t i v i t y . A p o l y v i n y l a l c o h o l c o a t i n g i s a p p l i e d on top to ensure that the water-based inks remain on the surface. Figure 9 shows the e x c e l l e n t drop formation on t h i s type of paper.

Conclusion High q u a l i t y c o l o r p r i n t i n g has been achieved using digitized ink j e t p r i n t i n g at a r e s o l u t i o n up t o 189 drops/cm. Materials parameters of importance are paper surface preparation and inks. E x c e l l e n t q u a l i t y has been obtained on poly v i n y l a l c o h o l coated photographic base paper with a c a r e f u l choice o f dyes and dye concentration. When dyes with appropriate s p e c t r a l p r o p e r t i e s have been s e l e c t e d , then the dye concentration l e v e l s must be chosen i n which dye aggregation e f f e c t s are minimized. The n e c e s s i t y o f using dyes which are r e l a t i v e l y i n s e n s i t i v e to pH changes has a l s o been discussed. Dyes must a l s o be s e l e c t e d t o have maximum fastness t o l i g h t i f a r c h i v a l p r o p e r t i e s are r e q u i r e d .


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Figure 8. Scanning Electron Micrograph of bond paper with and without a drop of ink.



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Scanning Electron Micrograph of polyvinyl alcohol coated barytes photobase paper with and without ink.


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Acknowlegments The authors would like to thank J . P . Montoya, T. Niweigha, and G.S. Keller.


Literature Cited 1. N. Ohta: Color Reproduct ion in Reflection-Type Color Prints , J . Appl. Photog. Eng., 2, 75-81, (1976) 2. E. Coates: 'Aggregation of Dyes in Aqueous Solutions , J . Soc. Dye Color. , 355 - 368, (1969) 3. This paper was obtained from Schoeller Technical Papers, Inc., Pulaski, New York. 1

RECEIVED June 18, 1982


Colloids and Surfaces in Reprographic Technology - American

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