Colloids and Surfaces in Reprographic Technology - American

16 Surface Chemistry and the Lithographic Process ROBERT W. BASSEMIR

Downloaded by TUFTS UNIV on March 7, 2017 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch016

Sun Chemical Corporation, Carlstadt, NJ 07072

An overview of the l i t h o g r a p h i c p r o c e s s i s p r e sented with particular emphasis on the r o l e t h a t s u r f a c e and colloid c h e m i s t r y p l a y s . Recent r e s e a r c h has shown the importance o f these c h e m i s t r i e s i n f o u n t a i n solution, i n k and p l a t e i n t e r a c t i o n s and the e f f e c t o f these i n t e r a c t i o n s on the dynamic b e h a v i o r of the lithographic i n k on p r e s s . Data on the r h e o l o g i c a l behavior o f preformed f o u n t a i n solution/ink emulsions i s p r e s e n t e d a l o n g w i t h an e v a l u a t i o n o f p r i n t s made w i t h the i n k s on an a c t u a l p r e s s r u n . The importance o f pre-testing the emulsification behavior of printing inks i n predicting their printability i s demonstrated. A l a b o r a t o r y p r o c e d u r e d e s i g n e d t o measure the r a t e o f emulsification of f o u n t a i n solution i n t o ink r a t h e r than merely the amount e m u l s i f i e d i s shown to be predictive of p r e s s performance. The related t e c h n o l o g y o f w a t e r l e s s l i t h o g r a p h i c p l a t e s i s d e s c r i b e d showing the importance o f s u r f a c e e n e r g y , p l a t e c h e m i s t r y and i n k r h e o l o g y on performance. L i t h o g r a p h i c printing which was i n v e n t e d by Alois S e n e f e l d e r o v e r 180 y e a r s ago, has p r o b a b l y been the most w i d e l y s t u d i e d o f a l l the p r i n t i n g p r o c e s s e s . Desp i t e t h i s , many q u e s t i o n s remain and a t o t a l u n d e r s t a n d i n g o f the l i t h o g r a p h i c p r o c e s s s t i l l e l u d e s u s . T h i s i s due l a r g e l y t o the complex i n t e r a c t i o n s which o c c u r on p r e s s between the i n k and the dampening s o l u t i o n . In i n v e s t i g a t i n g these i n t e r a c t i o n s i t i s e v i d e n t t h a t the c h e m i s t r y o f b o t h the i n k and dampening s o l u t i o n are d i r e c t l y i n v o l v e d as independent v a r i a b l e s and t h a t the r h e o l o g y of the i n k which r e s u l t s at the i n s t a n t o f p r i n t i n g has a s t r o n g e f f e c t on the r u n n a b i l i t y o f the i n k and the f a i t h f u l r e p r o d u c t i o n o f the p l a t e and c l a r i t y of image p r o d u c e d . 0097-615 6/ 82/0200-0327$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.


328

REPROGRAPHIC T E C H N O L O G Y

In o r d e r f o r a l i t h o g r a p h i c ink t o move smoothly and u n i f o r m l y t h r o u g h the p r e s s t o the s u b s t r a t e , i t i s e s s e n t i a l t h a t the i n k be a b l e t o a b s o r b a r e a s o n a b l e q u a n t i t y o f the f o u n t a i n s o l u t i o n b e i n g used* as emulsified droplets. Many a u t h o r s have o b s e r v e d t h i s f a c t (Banks, ( 1 ) , S u r l a n d (15, 16), MacPhee (18)) and have t r i e d t o p r e d i c t how the p h y s i c a l c h e m i s t r y o f the i n k s and f o u n t a i n s o l u t i o n s c o u l d be used to p r e determine i t s performance on a p r e s s . Some workers have i n t e r p r e t e d the e m u l s i f i c a t i o n o f f o u n t a i n s o l u t i o n i n an i n k from the p o i n t o f view o f s u r f a c e e n e r g e t i c s and c o l l o i d a l b e h a v i o r . Surface measurements i n the form o f c o n t a c t a n g l e s , s p r e a d i n g c o e f f i c i e n t s , i n t e r f a c i a l t e n s i o n s and s u r f a c e t e n s i o n s have been w i d e l y used t o e x p l a i n the i n t e r a c t i v e b e h a v i o r o f i n k s and f o u n t a i n s o l u t i o n s . The r o l e of i n t e r f a c i a l t e n s i o n between i n k and water i n the l i t h o p r o c e s s has been s t u d i e d by many w o r k e r s . Hansen (6) o b s e r v e d t h a t the e m u l s i f i c a t i o n t e n d e n c i e s o f the Tnk depend t o a g r e a t e x t e n t on the t e n s i o n o f the v a r i o u s components o f the i n k , because the p r e s e n c e o f even a s m a l l q u a n t i t y o f one component w i t h a low i n t e r f a c i a l t e n s i o n w i l l i n f l u e n c e the t e n s i o n o f the e n t i r e system. T h i s i s one o f the r e a s o n s t h a t c e r t a i n pigments, n o t a b l y r e d s , are more prone t o cause scumming problems than o t h e r pigments even when d i s p e r s e d i n an i d e n t i c a l v e h i c l e . The interfacial t e n s i o n o f t h e s e pigments a g a i n s t water i s shown t o be v e r y much lower than might be a n t i c i p a t e d p r i m a r i l y because o f i m p u r i t i e s t h a t are not c o m p l e t e l y removed d u r i n g the m a n u f a c t u r i n g p r o c e s s . R o s t e d (12) s t a t e s t h a t the amount o f f o u n t a i n s o l u t i o n e m u l s i f i e d i n the i n k i s r e l a t e d t o the p l a s t i c v i s c o s i t y and t a c k and t h a t i n t e r f a c i a l t e n s i o n between i n k and f o u n t a i n s o l u t i o n d e t e r m i n e s the amount o f water a d h e r i n g t o the i n k s u r f a c e . The l a t t e r o b s e r v a t i o n was c o n f i r m e d by K a r t t u n e n ( 8 ) , and he a l s o o b s e r v e s t h a t h i g h i n t e r f a c i a l t e n s i o n i s accompanied by low b o n d i n g e f f i c i e n c y o f the water t o the i n k thus r e s u l t i n g i n t h i n s u r f a c e water f i l m s on press ink r o l l e r s . Banks (1) i n a p a p e r on l i t h o f o u n t a i n s o l u t i o n s , o b s e r v e d t h a t most abnormal l i t h o b e h a v i o r such as scumming i s accompanied by the s p r e a d o f an o i l y f i l m o v e r the f o u n t a i n s o l u t i o n . He a l s o c l a i m s t h a t i f the fountain s o l u t i o n i s adjusted to a surface tension i n the range o f 36-30 dynes/cm, the s p r e a d i n g o f t h i s s e n s i t i z i n g f i l m from the i n k w i l l be i n h i b i t e d . He f e e l s t h a t the p r e v e n t i o n o f s p r e a d i n g i s not dependent on the p a r t i c u l a r c o m p o s i t i o n o f the f o u n t a i n s o l u t i o n but o n l y on i t s s u r f a c e t e n s i o n .

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


16.

BASSF.MIR

Surface Chemistry & the Lithographic Process

329

Hansen (6) has o b s e r v e d t h a t the i n t e r f a c i a l t e n s i o n o f an o f f s e t i n k w i t h f o u n t a i n s o l u t i o n s h o u l d be n e i t h e r too h i g h , which would p r e v e n t s u f f i c i e n t emuls i f i c a t i o n , nor too low which would cause scumming. He s t a t e s t h a t the normal range f o r a good i n k s h o u l d be between 15-25 dynes/cm. Padday (11) p r e s e n t e d a t h e o r e t i c a l e x p l a n a t i o n of the l i t h o p r o c e s s i n which he used the c o n c e p t o f c o h e s i v e and a d h e s i v e f o r c e s a c t i n g i n and a c r o s s s u r f a c e s t o e x p l a i n the a c t i o n o f the i n k i n g o f image a r e a s on a l i t h o p l a t e . H i s e x p l a n a t i o n r e s t s e n t i r e l y on the r u p t u r e o f l a y e r s a t the weakest p o i n t . Kartunnen (9) d i s c u s s e s the p o l a r b a l a n c e and the s u r f a c e roughness o f the v a r i o u s l a y e r s i n the l i t h o p r o c e s s and c o n c l u d e s t h a t the e m u l s i f i c a t i o n o f water i n the i n k depends not o n l y on the s u r f a c e c h e m i s t r y o f t h e s e f l u i d s but a l s o on the t h i c k n e s s and s u r f a c e s t r u c t u r e o f the i n k and water f i l m s when they e n t e r the v a r i o u s n i p s i n the p r e s s . S e v e r a l workers have a l s o commented on the p o s s i b i l i t y o f u s i n g dynamic s u r f a c e t e n s i o n measurements on the f o u n t a i n s o l u t i o n as b e i n g more r e a l i s t i c i n view o f the f a s t p r i n t i n g speeds used. T h i s might a l s o make measurements o f dampening s o l u t i o n s c o n t a i n i n g s u r f a c e a c t i v e agents c o r r e l a t e b e t t e r w i t h a c t u a l p r e s s performance due t o the r a t e o f d i f f u s i o n o f m a t e r i a l s t o the s u r f a c e s of these f l u i d s . F r e d Shubert (13) o f our l a b o r a t o r y has made some s t u d i e s u s i n g commercial photopolymer p l a t e s and e x p e r m e n t a l i n k s w h i c h were a l s o s t u d i e d f o r t h e i r e m u l s i f i c a t i o n and r h e o l o g i c a l b e h a v i o r and are r e p o r t e d on l a t e r i n t h i s paper. The image and non-image a r e a s o f a f r e s h l y exposed and d e v e l o p e d Kodak LN p l a t e were used t o determine c o n t a c t a n g l e s o f water and methylene i o d i d e i n a c o n t r o l l e d atmosphere. The Rame -Hart c o n t a c t angle i n s t r u ment was used i n these e x p e r i m e n t s a t r e l a t i v e humidi t i e s o f 100% and 0%. An S.E.M. photo o f the v i r g i n image a r e a i s shown i n F i g u r e 1, r e v e a l i n g the rough t e x t u r e o f the s u r f a c e . C o n t a c t a n g l e measurements and S.E.M. p h o t o g r a p h s were a l s o made on the image and non-image areas o f the same p l a t e a f t e r r u n n i n g i t on the p r e s s f o r 1000 imp r e s s i o n s w i t h i n k #C-4. A p o r t i o n o f the image a r e a was measured w i t h the i n k s t i l l a d h e r i n g t o the image (see F i g u r e 2 ) . A n o t h e r p o r t i o n o f the p l a t e was washed f r e e o f i n k w i t h s o l v e n t (naphtha) and measurements t a k e n on b o t h image and non-image a r e a s (see F i g u r e 3 ) . Some wear o f the image i s s e e n . f


330


REPROGRAPHIC

Figure 3.

Figure 1.

New plate image area (1200 X )>

Figure 2.

Ink film on image area (1200 X)-

TECHNOLOGY

Used plate after cleaning (1200 X , image area on right side of photo).


16.

BASSEMIR

331


The c o n t a c t angle measurements were used i n a computer program t o o b t a i n v a l u e s o f the s u r f a c e energy o f the p l a t e and t h i s v a l u e r e s o l v e d i n t o d i s p e r s i o n and p o l a r components. T h i s program was w r i t t e n u s i n g the e q u a t i o n s p r o p o s e d by Wu ( 1 9 ) . T a b l e #1 l i s t s these v a l u e s on the new p l a t e TJefore r u n n i n g on p r e s s . Table

I, S u r f a c e E n e r g i e s o f V i r g i n Dynes/cm @25°C

gdisp

S


Plate

s

polar

Image Area

0% 100%

Rel.Hum. Rel.Hum.

44.9 44.2

34.7 31.3

10.2 12.9

Non-Image Area

0% Rel.Hum. 100% Rel.Hum.

76.5 72.8

34.7 22.2

41.8 50.6

In b o t h a r e a s o f the p l a t e , i t i s e v i d e n t t h a t a d s o r p t i o n o f water v a p o r from the s a t u r a t e d atmosphere c a u s e s an i n c r e a s e i n the c a l c u l a t e d p o l a r component. The i n c r e a s e i n the non-image a r e a p o l a r component i s g r e a t e r , as might be e x p e c t e d from the chemic a l n a t u r e o f the two a r e a s . A f t e r r u n n i n g on the p r e s s , the p l a t e was removed w i t h o u t c l e a n i n g o f the image, and measurements o f c o n t a c t angle made on the i n k e d s u r f a c e . The s u r f a c e e n e r g i e s o f the v i r g i n i n k were a l s o d e t e r m i n e d by measurement o f a s e r i e s o f d i l u t i o n s i n Bromonaphthalene and the c o n t a c t a n g l e s on p o l y e t h y l e n e p e l l e t s . The r e s u l t s are shown i n T a b l e #11. Table

I I , Surface Energies of Dynes/cm @25°C s

Inked Image A r e a o f P l a t e Run on P r e s s 100% R.H. Virgin

Ink,

C-4,

100%

R.H.

Ink

Surfaces s

d

i

s

P

sP°

41.2

29.8

11.4

33.5

27.7

5.8

l

a

r

The i n k on the image area o f the p l a t e has i n c r e a s e d i n the p o l a r component. T h i s i s l i k e l y due t o the e m u l s i f i c a t i o n o f f o u n t a i n s o l u t i o n i n the i n k as w e l l as s u r f a c e f o u n t a i n water b o t h o f which may leave a h y d r o p h i l i c r e s i d u e upon e v a p o r a t i o n .


332

REPROGRAPHIC

TECHNOLOGY

The used p l a t e was then washed f r e e o f i n k w i t h naphtha and c o n t a c t a n g l e s measured i n b o t h image and non-image a r e a s . The r e s u l t s are g i v e n i n T a b l e I I I . Table

I I I , Surface Energies of Plate Dynes/cm @25°C s

Image Area @100%

d

i

s

p

s P °

l

a

r

R.H.

Virgin Plate Used P l a t e Downloaded by TUFTS UNIV on March 7, 2017 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch016

s

Areas

Non-Image Area @100%

44.2 48.8

31.3 28.3

72.8 72.6

22.2 18.2

12.9 20.5

R.H.

Virgin Plate Used P l a t e

50.6 54.4

The d a t a i n d i c a t e t h a t the most s i g n i f i c a n t change on r u n n i n g i s an i n c r e a s e i n the p o l a r component o f the image a r e a . I t i s p o s s i b l e t h a t some abs o r p t i o n o f i n k e m u l s i o n i n the i n t e r s t i c e s o f the photopolymer image a r e a may have o c c u r r e d d u r i n g r u n ning. The c l e a n i n g p r o c e s s w i t h h y d r o c a r b o n s o l v e n t may have s e l e c t i v e l y removed the more h y d r o p h o b i c cons t i t u e n t s o f the i n k . The non-image a r e a shows the e x p e c t e d i n c r e a s e i n p o l a r component due t o a d s o r p t i o n of fountain s o l u t i o n ingredients. Work i s c o n t i n u i n g i n an attempt t o r e l a t e s u r f a c e e n e r g e t i c s t o the p r e c i s i o n of i n k i n g the p l a t e and p r i n t a b i l i t y o f the i n k s . Ink E m u l s i f i c a t i o n

Tests

Many l a b o r a t o r y t e s t s t o f i n d the e m u l s i f i c a t i o n b e h a v i o r o f i n k s have been p r o p o s e d d u r i n g the p a s t s e v e r a l decades, an e a r l y one b e i n g Bowles and R e i c h ( 3 ) , b u t o n l y one, S u r l a n d (15, 16) s u g g e s t e d t h a t measurement o f the r a t e was more important than the amount e m u l s i f i e d . S i n c e the i n k / w a t e r i n t e r a c t i o n on the p r e s s i s a dynamic one, t h i s approach seems more r a t i o n a l than s i n g l e p o i n t d e t e r m i n a t i o n s o f the p e r c e n t e m u l s i f i e d , which are w i d e l y used i n i n d u s t r y . The e s s e n t i a l f e a t u r e s o f h i s method are a m i x e r w i t h s p e c i a l l y d e s i g n e d b l a d e s and bowl, good c o n t r o l o f speed and temperature and g r a v i m e t r i c d e t e r m i n a t i o n o f the p e r c e n t water e m u l s i f i e d each minute f o r t e n c o n s e c u t i v e m i n u t e s . The d a t a o b t a i n e d i s p l o t t e d as shown i n F i g u r e #4. T h i s c h a r t shows a s e r i e s of r a t e


16.

BASSEMIR


p

333


Water Balance Range

A

zero

B

narrow

C

wide

D

narrow

E

zero

F

narrow

Figure 4.

Damp. Sol'n. Effect

Image Area: Print

Ink Lay On Print

Rel. Print Density

OO

flotation

tint

>1 ~1

Colloids and Surfaces in Reprographic Technology - American

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