Colloids and Surfaces in Reprographic Technology - American

17

Surface

Chemistry

Control

in

Lithography

THOMAS A. FADNER

Downloaded by FUDAN UNIV on March 31, 2017 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch017

Rockwell Graphics Systems, Cicero, IL 60650

Results of recent research into ink/water interactions in lithographic printing are briefly reviewed and compared with predictions from surface chemistry principles. Contrary to popular belief, both image and nonimage areas of the printing plate act as water reservoirs during printing. And, ink is carried in both the image and nonimage areas. Viscous and dynamic mechanical forces account for most of the image/nonimage differentiation. A model is proposed that explains the advantageous effects of isopropanol and i t s substitutes as additives to the aqueous dampening solution. The lithographic printing process has been modelled mechanically, phenomenologically and by materials flow, often without reconciling the descriptions with principles of surface chemistry (1-6). Although the process may at times appear complicated, the simple concept of using o i l - l i k e inks and dilute water solutions to differentiate printing and nonprinting areas of an essentially planar printing plate, should require only simple explanations and yet remain consistent with surface science principles. Wetting of Ink by Water To account for one aspect of image differentiation by a lithographic printing plate, reference has been made to surface chemistry principles such as Statement 1. Statement 1. High energy liquids w i l l not spontaneously wet nor spread onto immiscible low energy materials. Accordingly, high surface energy, aqueous fountain solutions do not spread onto low surface energy, inked images on the printing plate. This appears to support the practical fact that water, sprayed, r o l l e d , or otherwise conveyed as fountain solution onto the printing plate in appropriate amounts, does not ordinarily get in the way of the inked image with which we are printing. 0097-6156/82/0200-0347$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.

REPROGRAPHIC


348

TECHNOLOGY

Wetting of Water by Ink The chemical converse of Statement 1, a p p l i e d to wetting of aqueous f o u n t a i n s o l u t i o n by the ink, introduces an o f t e n overlooked dilemma i n attempting to e x p l a i n l i t h o g r a p h y . Statement 2. Low energy l i q u i d s w i l l spontaneously wet and spread onto immiscible high energy m a t e r i a l s . L i t h o g r a p h i c inks are low energy m a t e r i a l s but g e n e r a l l y are not found, en masse, i n the high energy, aqueous-fountains o l u t i o n - c o v e r e d , nonimage areas of the p r i n t i n g p l a t e during printing. I f they were, d i f f e r e n t i a t i o n of image and nonimage areas would not have taken place. This i m p l i e s e i t h e r that lithography i s wrong or that surface chemistry i s wrong. Seldom do t r e a t i s e s on lithography adequately r e s o l v e t h i s apparent dilemma. In lithography we bypass the implied c o n d i t i o n s of Statement 2 by i n t r o d u c i n g a force/time dependent f a c t o r ; the ink i s formulated to have high viscous r e s i s t a n c e to flow. We can r e c o n c i l e theory with l i t h o g r a p h i c f a c t by modifying Statement 2 to read: Statement 3. Low energy, high v i s c o s i t y l i q u i d s w i l l spontaneously but not r a p i d l y wet and spread onto immiscible high energy m a t e r i a l s . Viscous Flow i n Lithography These two statements are c o n s i s t e n t with popular views of l i t h o g r a p h y , namely: the p r i n c i p l e of Statement 1 i s supposed to keep water out of the inked image areas; Statement 3 a u s t e n s i b l y e x p l a i n s keeping ink out of the aqueous, nonimage areas. As already noted, however, i n lithography we are not opera t i n g at e q u i l i b r i u m ; we are not w a i t i n g f o r spontaneous wetting a c t i o n ; we are f o r c i n g the wetting a c t i o n . The pressure and shearing f o r c e s at r o l l e r nips are purposely f a r i n excess of the ink's viscous r e s i s t a n c e to flow, to assure that the ink w i l l move a small but f i n i t e distance at the i n k - t o - i n k r o l l e r couples, w i t h i n 10"2 to 10"^ second dwell times. A low energy, high v i s c o s i t y ink that i s s u f f i c i e n t l y f o r c e d to flow w i l l of course be spread, i n the surface chemic a l sense, onto the high energy, aqueous nonimage areas of the p r i n t i n g p l a t e . Yet under normal p r i n t i n g c o n d i t i o n s , i t appears not to have done so. The image and nonimage areas remain d i f f e r e n t i a t e d . Further input i s required to r e s o l v e t h i s d i lemma. Ink and Water Mixing Analyses have repeatedly shown that 15% or more water i s found i n l i t h o g r a p h i c inks during or a f t e r normal p r i n t i n g . And, inkmakers r o u t i n e l y formulate to allow water i n t o the ink during p r i n t i n g operations (2, _3> 7_ 8). Reportedly, most of the water throughput to the substrate being p r i n t e d i s by way of the ink, not by way of the aqueous, n o n - p r i n t i n g areas of the p r i n t i n g p l a t e (1). A l s o , minute ink p a r t i c l e s are o f t e n , i f not always, found i n the n o n - p r i n t i n g areas of p l a t e s or 9

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

17.

FADNER

Surface Chemistry Control in Lithography

349


blankets during normal p r i n t i n g operations. These observations imply that water and ink are not completely d i f f e r e n t i a t e d by the l i t h o g r a p h i c process. Apparently, surface chemistry p r i n c i p l e s that r e q u i r e e x c l u s i v e ink/water d i f f e r e n t i a t i o n are not c o n t r o l l i n g t h i s process. Isopropanol i n Lithography The f i r s t i n a s e r i e s of continuous-dampener patents by Dahlgren d i s c l o s e d a d i r e c t - t o - p l a t e device ( 9 ) . A p r a c t i c a l way to bypass p r i n t i n g press manufacturers, who at the time were d i s i n t e r e s t e d i n t h i s continuous-dampening innovation, was to r e t r o f i t the device onto e x i s t i n g presses i n the f i e l d . E x i s t i n g press c o n f i g u r a t i o n s required using inked f o r m - r o l l e r s . Dahlgren found that using inked f o r m - r o l l e r s to convey aqueous f o u n t a i n s o l u t i o n s to the p r i n t i n g p l a t e , required generous amounts, 10 to 50%, of a water-soluble, ink-compatible, f o u n t a i n s o l u t i o n a d d i t i v e , as described i n h i s subsequent patent (10)• Isopropanol was cheap, r e l a t i v e l y safe, a v a i l a b l e , and d i d not adversely a f f e c t image d i f f e r e n t i a t i o n . Isopropanol has s i n c e become a widely-used i n d u s t r y standard. The prevalent explanation f o r isopropanol*s r o l e i n t h i s k i n d of dampening system i s that i t s surface t e n s i o n , about 29 dynes/cm, s u f f i c i e n t l y lowers the aqueous f o u n t a i n s o l u t i o n surface tension to allow wetting of the inked f o r m - r o l l e r by that s o l u t i o n , as i l l u s t r a t e d i n Figure 1. That i s , the fount a i n s o l u t i o n wets and spreads onto, and i s c a r r i e d by the i n k f i l m on the f o r m - r o l l e r to the p r i n t i n g p l a t e , as a r e l a t i v e l y t h i n , uniform f i l m . However, the most important f a c t about isopropanol, i n understanding lithography, i s not the advent and success of continuous dampening that ensued. Rather, i t i s the f a c t that pressmen found d i s t i n c t and very important advantages i n the use of isopropanol; advantages g e n e r a l l y lumped together and termed " b e t t e r ink/water balance c o n t r o l " . These are seen over and over as the f o l l o w i n g q u a l i t a t i v e observations: 1. F a s t e r acceptable copy. This means f a s t e r a t t a i n ment of steady s t a t e c o n d i t i o n s ; the point i n the operation where ink and water feed r a t e s no longer need c r i t i c a l adjustment to obtain d e s i r e d q u a l i t y , i l l u s t r a t e d schematically i n F i g u r e 2. 2. Less v a r i a t i o n . Q u a l i t y v a r i a t i o n s are l e s s extreme and l e s s frequent, as depicted i n Figure 2. Response to water or ink feed r a t e s changes i s faster. 3. Wider ink/water l a t i t u d e . Pressmen f i n d l e s s v a r i a t i o n i n p r i n t q u a l i t y due to form d i f f e r ences, and ink or press s e t t i n g d i f f e r e n c e s . 4. Less work. With isopropanol, l e s s a t t e n t i o n i s r e q u i r e d . The ink/water balance i s more automatic. An acceptable model of the l i t h o g r a p h i c process must ade-


350

TECHNOLOGY


REPROGRAPHIC

Figure 1.

Surface tensions of propanol solutions. Key: O, n-propanol; and • , isopropanol.



17.

FADNER


351

CONTROL VALUE

1

T NUMBER

Figure 2.

1

1

r

O F IMPRESSIONS

Effects of isopropanol.


352

REPROGRAPHIC

TECHNOLOGY


quately e x p l a i n these observations. I t must a l s o e x p l a i n an a d d i t i o n a l dilemma. Based on surface science p r i n c i p l e s , we can accept that a low surface tension, aqueous s o l u t i o n i n cont a c t with a low energy, inked, f o r m - r o l l e r surface w i l l r a p i d l y wet and spread as a t h i n f i l m onto the ink, when forced at the nip. But, then, we must a l s o admit that t h i s same s o l u t i o n w i l l r e a d i l y wet and spread over any inked s u r f a c e , i n c l u d i n g image areas of the p r i n t i n g p l a t e . O v e r a l l wetting of the p r i n t i n g p l a t e by the aqueous f o u n t a i n s o l u t i o n appears as a necessary c o r o l l a r y to the use of isopropanol. How, then, do we e x p l a i n that, during p r i n t i n g , t h i s aqueous f o u n t a i n s o l u t i o n f i l m does not prevent i n k - t o - i n k t r a n s f e r ? Why does isopropanol not thwart lithography? Role of Isopropanol i n Lithography In a previous paper 05), we have shown that with t h i s addi t i v e , an 84% isopropanol concentration e x i s t s i n both phases at the v a p o r / l i q u i d - s o l u t i o n i n t e r f a c e . This high surface c o n c e n t r a t i o n i s maintained at the press by automatic, r e c i r c u l a t i n g c o n t r o l systems. Under these c o n d i t i o n s , the h i g h l y v o l a t i l e isopropanol has a c o n t i n u a l , p r e f e r e n t i a l surface-energylowering e f f e c t . I t i s a surface a c t i v e m a t e r i a l because of a c o l l i g a t i v e mechanism, r a t h e r than because of more f a m i l i a r , slow-to-form, i n t e r f a c i a l s t r u c t u r e mechanisms (11). This bulkr e i n f o r c e d , s u r f a c e a c t i v i t y i s important i n understanding lithographic p r i n t i n g . Since dampening f o r m - r o l l e r s cannot d i f f e r e n t i a t e between image and nonimage areas of a p r i n t i n g p l a t e , the low-energy, i s o p r o p a n o l - c o n t a i n i n g , aqueous f o u n t a i n s o l u t i o n spreads r a p i d l y at the n i p over a l l areas of the p l a t e , independent of image and nonimage d e t a i l s . At the nip e x i t , t h i s mechanical spreading f o r c e i s r e l e a s e d ; there are no other s i g n i f i c a n t f o r c e s a c t i n g at the i n t e r f a c e ; f i l m s p l i t t i n g occurs withrn the aqueous phase; and a t h i n f i l m of aqueous fountain remains i n the image as w e l l as the inked, nonimage areas. It follows that the uppermost surfaces of the inked, image areas on the p r i n t i n g p l a t e , l i k e the aqueous, nonimage areas, w i l l be p r i m a r i l y isopropanol, not ink! Lithography S i m p l i f i e d with Isopropanol Within a few r e s o l u t i o n s of the press, when isopropanol i s used, an i s o p r o p a n o l - r i c h l a y e r of f o u n t a i n s o l u t i o n forms and i s continuously maintained at a l l i n k / a i r and fountain s o l u t i o n / a i r i n t e r f a c e s . Consequently, f o r m - r o l l e r to p r i n t i n g p l a t e contact at the nip entrances involves two l a y e r s of aqueous i s o propanol f i r s t coming i n t o contact, not ink f i l m s and/or fountain solution films! Since the incoming surfaces are v i r t u a l l y i d e n t i c a l and the press i s a l s o f o r c i n g intimate contact, i n stantaneous wetting and spreading i n t o an extensive, t h i n f i l m i s assured, independent of whether the area on the p r i n t i n g p l a t e w i t h i n the nip i s an image or a nonimage area or any m u l t i p l e combination that might e x i s t because of the format



17.

FADNFR


353

being p r i n t e d . As already noted, these t h i n , i s o p r o p a n o l - r i c h , aqueous l a y e r s r e a d i l y wet e i t h e r fountain s o l u t i o n or i n k , consequently, under n i p pressure, they are e a s i l y forced to d i f fuse back and f o r t h i n t o or across the r a p i d l y changing i n t e r face, to o r from whichever p o r t i o n of the p r i n t i n g p l a t e o r type of f o r m - r o l l e r happens to be at the n i p . That i s , under these low-surface-energy, mechanically-forced wetting c o n d i t i o n s , a r a p i d , d i f f u s i o n a l displacement may occur i n the f i l m - t h i c k n e s s direction. Transport of t h i n l a y e r s i n t h i s d i r e c t i o n w i l l be much f a s t e r than l a t e r a l or c i r c u m f e r e n t i a l bulk flow away from the n i p would be. Since a r e a l d i f f e r e n t i a t i o n by the f o u n t a i n s o l u t i o n i s not involved, d i f f u s i o n a l mixing w i l l be l a r g e l y independent of the p r i n t i n g p l a t e format, independent of dimensions or l o c a t i o n of h a l f t o n e s or l i n e s o l i d s . The r a p i d , d i f f u s i o n a l displacement of f o u n t a i n s o l u t i o n i n t o or out of the ink, allows extremely r a p i d , intimate, i n k t o - i n k contact necessary f o r r e p l e n i s h i n g the i n k i n the image areas that was used i n the p r i n t i n g process. I t a l s o allows r a p i d recovery when ink or water feed rates are changed. There i s a place f o r spurious water to go. Lithography i s not thwarted by t h i s omni-present, o v e r a l l t h i n , aqueous, a l c o h o l i c f i l m of f o u n t a i n s o l u t i o n because i t a u t o m a t i c a l l y gets out of the way of i n k t r a n s f e r as r e q u i r e d , by i n s t a n t l y disappearing i n t o whichever l a y e r , i n k or f o u n t a i n solution, i s relatively moisture-starved. This view f i t s with the well-known f a c t that inks formul a t e d to not accept water are i n o p e r a t i v e i n high-speed l i t h o graphy. With nowhere to go, fountain s o l u t i o n that i s continua l l y forced i n t o intimate contact with the whole p r i n t i n g p l a t e w i l l , sooner o r l a t e r , i n t e r f e r e with normal i n k t r a n s f e r and d i f f e r e n t i a t i o n . Modern, r o t a r y , l i t h o g r a p h i c p r i n t i n g r e q u i r e s a f o u n t a i n s o l u t i o n sink to counterbalance the mechanicallyinduced ink/water mixing. That sink i s the i n k i t s e l f ! And, there i s an i n t e r e s t i n g c o r o l l a r y from t h i s p r i n t i n g - w i t h isopropanol model; the p r i n t i n g p l a t e does not need to d i f f e r e n t i a t e acceptance of f o u n t a i n s o l u t i o n based on i t s image/ nonimage content! Lithography without Isopropanol Without fountain s o l u t i o n a d d i t i v e s , c o n t r o l of image d i f f e r e n t i a t i o n has been found nearly impossible. Water feed rates must be s i g n i f i c a n t l y increased to keep nonimage areas f r e e of ink. This may r e s u l t a t times i n too much i n k t r a n s f e r and at other times, too l i t t l e . To avoid these extremes, t y p i c a l l i t h o g r a p h i c f o u n t a i n s o l u t i o n s contain small amounts of n a t u r a l or s y n t h e t i c weakly a c i d i c or n e u t r a l , w a t e r - d i s p e r s i b l e gums. During operation, these gums concentrate i n nonimage areas o f the p r i n t i n g p l a t e , a c t i n g as t h i n - f i l m , water-absorptive r e s e r v o i r s . Phosphoric o r s i m i l a r l y weak acids are added to help c o n t r o l gum s o l u b i l i t y . A l s o , not p e r t i n e n t to t h i s d i s c u s s i o n , are f u n g i c i d e s , b u f f e r and c o l o r a n t s . With these minimal ad-



354

REPROGRAPHIC

TECHNOLOGY

d i t i v e s , l i t h o g r a p h i c p r i n t i n g becomes t o l e r a b l e , but r e q u i r e s considerable a t t e n t i o n to obtain and maintain c o n s i s t e n t q u a l ity. Long before the advent of isopropanol, inkmakers found that the inks serve as a r e s e r v o i r or s i n k f o r fountain s o l u t i o n . This i n f e r s that some f o u n t a i n s o l u t i o n may be forced i n t o the ink at the n i p s , even without isopropanol present, as discussed by Bassemir (3). Schaeffer (_5) has pointed out, p a r t i c u l a r l y when using high energy f o u n t a i n s o l u t i o n s , that most of the mixing may take place at n i p e x i t s where c a v i t a t i o n occurs duri n g s p l i t t i n g of the ink f i l m ; an e x c e l l e n t c o n d i t i o n f o r t r a p ping or e m u l s i f y i n g spurious or f r e e fountain s o l u t i o n i n t o the ink. Whichever mechanism a p p l i e s , under these high-energyf o u n t a i n - s o l u t i o n c o n d i t i o n s , we are expecting the press system to work against surface chemical p r i n c i p l e s of wetting and spreading, r a t h e r than i n concert with them. Consequently, the necessary mixing of water i n t o ink w i l l be slow, r e q u i r i n g many press r e v o l u t i o n s to achieve a n a t u r a l ink/water steady-state c o n d i t i o n , or r e q u i r i n g higher energy input to reach t h i s steady s t a t e c o n d i t i o n i n reasonable time, perhaps both. T h i s i s equivalent to the converse of the f i r s t isopropanol advantage statement; without isopropanol, approach to steady-state opera t i o n i s slower. Despite e a r l y r e c o g n i t i o n by inkmakers that l i t h o g r a p h i c inks must accept some water, temporarily excess f o u n t a i n s o l u t i o n e x i s t s at the ink f o r m - r o l l e r / p l a t e image area couples, more o f t e n than not, and must be squeezed out of the way. As i n d i c a t e d , the i n k cannot r a p i d l y absorb t h i s high-energy fount a i n s o l u t i o n because the system does not s a t i s f y molecular wetting p r e r e q u i s i t e s . The excess fountain s o l u t i o n w i l l have to flow l a t e r a l l y or c i r c u m f e r e n t i a l l y out of the way. I f t h i s bulk flow i s not completed during each r e v o l u t i o n of the press, i t can lead to p r i n t i n g v a r i a n c e s , such as sncwflaking or washed-out images. Most modern, commercial f o u n t a i n s o l u t i o n s a l s o contain 1% or l e s s of a low-to-moderate surface energy, non-foaming surf a c t a n t o r a s o l u b l e ink/water coupling agent, such as hydroxyethers o r polyoxyethylene g l y c o l s . These a d d i t i v e s cannot counteract the ever-present, format-dependent, excess f o u n t a i n s o l u t i o n c o n d i t i o n s during p r i n t i n g as e f f e c t i v e l y as a bulk r e i n f o r c e d , surface a c t i v e compound l i k e i s o p r o p a n o l . Using these w i l l i n v o l v e poor-to-incomplete wetting of the i n k by the f o u n t a i n s o l u t i o n ; a wetting e f f e c t i v e n e s s that i s dependent upon a f o u n t a i n s o l u t i o n a d d i t i v e with poor a b i l i t y to c o n t i n uously maintain low surface tension under dynamic, high-speed, p r i n t i n g c o n d i t i o n s . Consequently, p r i n t i n g i s h i g h l y dependent upon the a d d i t i v e ' s concentration and chemical nature. The extent of aqueous phase f i l m - s p l i t t i n g versus i n t e r f a c i a l f i l m s p l i t t i n g at the n i p e x i t s , and therefore the completeness of r e t a i n i n g a t h i n , uniform f o u n t a i n s o l u t i o n on the p r i n t i n g



17.

FADNER


355

p l a t e a f t e r the n i p s , w i l l t h e r e f o r e a l s o be dependent on a d d i t i v e chemistry, and h i g h l y dependent upon the p r i n t i n g p l a t e format. As a consequence of t h i s l i m i t e d - w e t t i n g c o n d i t i o n , and of the 10-50 m i l l i s e c o n d time r e s t r a i n t s due t o press design, and depending on the p l a t e format, r e t r a c t i v e formation of beads or d i s j o i n t e d , r e l a t i v e l y - t h i c k i s l a n d s of f o u n t a i n s o l u t i o n occurs to v a r y i n g degrees i n the inked areas of the p r i n t i n g p l a t e . Thus, we encounter i n t e r m i t t e n t , sporadic, or retarded d i f f u s i o n a l transport of f o u n t a i n s o l u t i o n across i n t e r f a c e s i n t o the ink, i n s t e a d of smooth, instantaneous d i f f u s i o n . We encounter bulk l a t e r a l flow of t h i c k e r f o u n t a i n s o l u t i o n f i l m s , f i l m s of s o l u t i o n that could not r a d i a l l y d i f f u s e i n t o the i n k r a p i d l y enough. We encounter a behavior that w i l l be more s t r o n g l y dependent upon f o u n t a i n s o l u t i o n feed r a t e s and p r i n t ing p l a t e format, press design and r o l l e r i n t e r f e r e n c e s e t t i n g s . The p r a c t i c a l r e s u l t s are converse to the second and t h i r d i s o propanol advantages p r e v i o u s l y l i s t e d . When we f o s t e r ink/water contact with a d d i t i v e s l i k e i s o propanol, we overcome chemical b a r r i e r s to water-in-ink mixing. The mixing process may become as r a p i d as the p r i n t i n g process; i t becomes l e s s dependent on d e t a i l s of ink and f o u n t a i n s o l u t i o n chemistry, press s e t t i n g s and p r i n t i n g format. The process i s e a s i e r to operate. We have introduced a measure of chemical automation. I t i s apparent, then, that r e t r a c t i o n of f o u n t a i n s o l u t i o n from ink image areas, the formation of beads on ink areas, does not need to occur f o r image d i f f e r e n t i a t i o n . The bulk r e t r a c t i o n mechanism appears only as an a r t i f a c t i n a broader concept of l i t h o g r a p h i c p r i n t i n g , i t s r e l a t i v e importance during p r i n t ing being dependent upon chemical and image format f a c t o r s . I t i s not an e s s e n t i a l mechanism i n l i t h o g r a p h y . S i m i l a r conclusions can be drawn regarding the f i l m s p l i t t i n g mechanism f o r water uptake by the ink at i n k - t o - i n k nip e x i t s . I t may occur, but i t i s an a r t i f a c t not a r e q u i r e ment i n l i t h o g r a p h i c p r i n t i n g . Lithography with Isopropanol S u b s t i t u t e s There i s one way, other than evaporation, to ensure high s u r f a c e c o n c e n t r a t i o n of a s o l u b l e f o u n t a i n s o l u t i o n a d d i t i v e . I f we s e l e c t a low-surface-energy compound having p a r t i a l or l i m i t e d water s o l u b i l i t y , we w i l l expect i t s s a t u r a t e d aqueous s o l u t i o n to r e a d i l y separate i n t o two phases, one r i c h i n the a d d i t i v e , the other r i c h i n water. Using the a d d i t i v e a t or near i t s s o l u b i l i t y l i m i t w i l l assure that some of the a d d i t i v e i n concentrated form w i l l always be a v a i l a b l e a t the f o u n t a i n solution interfaces. With these c r i t e r i a , and the d e s i r e to uncover a low-vaporpressure, low-use-rate a d d i t i v e , a number of candidates were evaluated (11-14). One c l e a r l y r e p e t i t i v e c h a r a c t e r i s t i c of p a r t i a l l y - w a t e r - s o l u b l e compounds i s that t h e i r q u a l i t a t i v e



356

REPROGRAPHIC

TECHNOLOGY

water-transport c a p a b i l i t y as t h i n f i l m s on inked r o l l e r s was i n v a r i a b l y greatest when the compound was used at or j u s t below i t s water s o l u b i l i t y l i m i t ; the c o n d i t i o n where a separate l i q u i d phase i s l i k e l y to be present. P r e d i c t i o n s based on these laboratory evaluations l e d to s u c c e s s f u l pressroom t r i a l s and then to marketing of a f o u n t a i n s o l u t i o n a d d i t i v e product to r i v a l isopropanol, 2-ethyl-1,3-hexanediol (15). The compound i s used at 1/2% to 1% of the f o u n t a i n s o l u t i o n ; 10 to 6 0 - f o l d l e s s than isoporpanol. I t i s not as f o r g i v i n g of mechanical press v a r i a b l e s as isopropanol, a not s u r p r i s i n g r e s u l t , i n view of the low concentrations that are used. I t has had general success i n meeting most of the isopropanol advantages i n the f i e l d , most of the time. This success i s strong support f o r the b u l k - r e i n f o r c e d surface a c t i v i t y l i t h o g r a p h i c model and the l i m i t e d s o l u b i l i t y concepts presented here. Driography i s Lithography V a l i d as the present model may be, the p r a c t i c a l f a c t i s that water i n lithography i s now an h i s t o r i c a l , a l b e i t very useful, artifact. A number of p r i n t e r s are operating without water, using f o r instance, polymeric s i l i c o n e s as the n o n - p r i n t i n g , nonimage areas of s o - c a l l e d d r i o g r a p h i c p l a t e s . In the i n k i n g formr o l l e r / p l a t e n i p s , the low surface energy, viscous ink w i l l be f o r c e d i n t o i n t i m a t e , wetting, contact with the low surface energy, nonimage areas, as always i n l i t h o g r a p h y . Rapid, cohesive r e l a x a t i o n of t h i s f o r c e a b l y spread ink at the nip e x i t s , precludes r e t e n t i o n of an ink l a y e r on the s i l i c o n e surfaces by comparatively weak, i n t e r f a c i a l f o r c e s , even i f the f o r c e a b l y spread c o n d i t i o n i s thermodynamically f a v o r a b l e . The system i s f u r t h e r biased against an ink l a y e r adhering i n nonimage areas, by the absence of adhesional p e n e t r a t i o n of the ink i n t o the smoother, lower-surface-energy s i l i c o n e . The e n e r g e t i c a l l y r e c e p t i v e , d i f f u s i o n a l l y - r e c e p t i v e image areas on the p r i n t i n g p l a t e assure ink t r a n s f e r and image d i f f e r e n t i a t i o n . Image area s u r f a c e energy i s r e l a t i v e l y unimportant s i n c e f o u n t a i n s o l u t i o n i s not present, tending to d i s p l a c e the ink. In the nonimage areas of a l i t h o g r a p h i c p r i n t i n g p l a t e , a t h i n , m o l e c u l a r l y smooth, incompressible water l a y e r , with i s o propanol at i t s surface, i s chemically and p h y s i c a l l y analogous to a t h i n , m o l e c u l a r l y uniform, incompressible, polymeric s i l i cone f i l m . Thus, driography i s e n t i r e l y analogous to l i t h o graphy. Water was a v a i l a b l e long before polymeric s i l i c o n e s ; a f a c t that has confounded attempts to analyze the system. When we adequately help water to not i n t e r f e r e with the process, that i s , when we f o s t e r r a p i d , on-press mixing of water i n t o the ink with an instantaneously responsive, surface-tension-lowering a d d i t i v e l i k e isopropanol or 2-ethy1-1,3-hexanediol, we optimize image d i f f e r e n t i a t i o n by the p r i n t i n g p l a t e . Summary L i t h o g r a p h i c presses f o r c e water to mix with the ink at the p r i n t i n g p l a t e , g e n e r a l l y working against well-known p r i n c i p l e s



FADNER


357

of molecular wetting. The printing press is easiest to control when this mixing can occur at press speeds. Overall wetting of the printing plate by fountain solution, both image and nonimage areas, allows formation of a uniform, thin film of fountain solution on the plate; i t allows rapid, diffusional transport of fountain solution into and out of ink or fountain solution areas at the printing plate couples. Water in ink mixing at pressure nips becomes nearly instantaneous. Improved control of and/or improved latitude in ink/water balance at the printing plate by this mechanism is accomplished best with isopropanol because of i t s bulk-reinforced surface activity. Limited-solubility additives, such as 2-ethy1-1, 3-hexanediol, function similarly but are slightly less effective. Commercial fountain concentrates using soluble, lowsurf ace- tension additives provide some of this improved control but remain more dependent on printing format, ink and additive variables. Commercial concentrates having high surface tension provide none of this enhanced control and with no fountain solution additives the system is v i r t u a l l y inoperable. Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

Kartunen, S. and Lindquist, U. 15th IARGAI Conference, Lillihammer, Norway, (1979). MacPhee, J., TAGA Proc, 237 (1979). Bassemir, R.W., Amer. Ink Maker, February 1981, 33ff. Anon, Dampening Basics, Heidlberger Druckmachine AG, (March 1980). Fadner, T . A . , Schaeffer, W.D. and Smith, D.E. 1978 Annual Research Department Report, Graphic Arts Technical Foundation, Pittsburgh, PA., 103-111 (1979). Schlapfer, K . , 13th IARIGAI Conference, Wildhaus, Switzerland (1975). Albrecht, J. and Wirz, B . , 9th IARIGAI Conference, Rome, 99-114 (1967). Lehtonen, T . , 13th IARIGAI Conference, Wildhaus, Switzerland, 269-298 (1975). Dahlgren, H.P., U.S. 3,168,037, February 2, 1965. Dahlgren, H.P., U.S. 3,259,062, July 5, 1966; U.S. 3,343,484, September 26, 1967. Fadner, T . A . , White, M.G. and Hayden, R,H, 1975 Annual Research Department Report, Graphic Arts Technical Foundation, Pittsburgh, PA., 103-124 (1976). Smith, D.E. i b i d , 35-44. Smith, D.E. 1977 Annual Research Department Report, Graphic Arts Technical Foundation, Pittsburgh, PA., 135-144 (1978). Fadner, T . A . , i b i d , 119-126. Fadner, T . A . , U.S. 4,178,467, July 14, 1981.

RECEIVED February 3,

1982


Colloids and Surfaces in Reprographic Technology - American

Recommend Documents