Cure of Intaglio Printing Inks - ACS Symposium Series (ACS

10 Cure of Intaglio Printing Inks D. L. HUNSTON and J. L. RUSHFORD Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 12, 2016 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch010

National Bureau of Standards, Polymer Division, Washington, DC 20234 W. R. NEWITT and B. A. VANDREUIL Bureau of Engraving and Printing, Research Division, Washington, DC 20228

The i n t a g l i o inks used to print currency i n the United States contain r e l a t i v e l y little solvent and dry primarily by chain extension and cross-linking reactions i n the vehicles. To obtain good performance, the initially f l u i d ink must change rheologically in the manner required to give proper transfer to the plate and then to the paper and to obtain s u f f i c i e n t hardness at the end of the process so that the printed sheets do not smear when stacked. To study these changes, the rheology of two ink formulations with very different press performances was examined, first i n the uncured state and then during curing. The uncured inks exhibited complex rheological properties including time dependence, y i e l d behavior, elasticity, and non-linearity. Curing of the inks produced an increase in both v i s c o s i t y and e l a s t i c i t y . The v i s c o s i t y change could be roughly fitted to a first order type equation. Comparisons between these results and the performance of the inks on the press show that if the rate at which properties change during cure falls outside a certain range, acceptable print quality cannot be achieved. The manufacture of paper currency i n the U n i t e d S t a t e s i s the r e s p o n s i b i l i t y of the Bureau of Engraving and P r i n t i n g (BEP) which i s the world's l a r g e s t s e c u r i t i e s manufacturing e s t a b l i s h m e n t . BEP produces, on average, 16 m i l l i o n notes per day which r e p r e sents an annual face v a l u e of 43 b i l l i o n d o l l a r s . An i n t a g l i o p r i n t i n g process i s used because i t i s the most d i f f i c u l t process to perform and thus t o c o u n t e r f e i t ( 1 ) . Other processes l a c k the f i d e l i t y of f i n e l i n e s and the d i s t i n c t i v e t h i r d - d i m e n s i o n a l e f f e c t of r a i s e d l i n e on paper i n h e r e n t i n i n t a g l i o p r i n t i n g ( 1 ) . The p l a t e s are hand-tooled by h i g h l y s k i l l e d engravers who engrave the designs u s i n g grooves of v a r y i n g depths. The currency i s then p r i n t e d on high-speed sheet-fed r o t a r y presses t h a t employ two o r f o u r p l a t e s c o n t a i n i n g 32 notes each. The presses are capable of This chapter not subject to U.S. copyright. Published 1983, American Chemical Society May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 12, 2016 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch010

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p r i n t i n g 9,000 sheets per hour. The i n k i s s u p p l i e d to the r o l l e r c o n t a i n i n g the p l a t e s from the i n k trough, or f o u n t a i n , by a s e r i e s of feed r o l l e r s . The ink i n the f o u n t a i n i s heated and kept i n slow but u s u a l l y constant motion. A f t e r the hot i n k i s t r a n s f e r r e d by the feed r o l l e r s to the p l a t e , i n k i s removed from the e l e v a t e d p o r t i o n s of the p l a t e by a s e r i e s of three wiping c l o t h s . A sheet of paper i s then f o r c e d under heavy pressure a g a i n s t the p l a t e so that i t p i c k s up most of the i n k remaining i n the f i n e engraved l i n e s . The sheets are immediately stacked by the press and on a subsequent day the reverse s i d e of the sheets are p r i n t e d i n the same way. To e f f i c i e n t l y produce a h i g h q u a l i t y product r e q u i r e s the use of s p e c i a l formula, f a s t - d r y i n g i n k s , developed i n BEP*s l a b o r a t o r i e s . The r a p i d d r y i n g e l i m i n a t e s the time-consuming need f o r t i s s u i n g or i n t e r l e a v i n g between sheets to prevent smearing when stacked. Rapid d r y i n g on the sheets alone however i s not s u f f i c i e n t t o guarantee good r e s u l t s . The proper t r a n s f e r of the i n i t i a l l y l i q u i d i n k from the f o u n t a i n to the p l a t e v i a the feed r o l l e r s , the complete removal of the i n k from the h i g h p o r t i o n s of the p l a t e by the wiping c l o t h s , and the necessary t r a n s f e r of the remaining i n k to the paper r e q u i r e that the i n k be i n the proper r h e o l o g i c a l s t a t e s a t the a p p r o p r i a t e times. As a r e s u l t the r h e o l o g i c a l changes that occur during d r y i n g must proceed both r a p i d l y and c o r r e c t l y i f proper p r i n t q u a l i t y i s to be achieved. To minimize the problems a s s o c i a t e d w i t h p o o r l y performing i n k s , a number of q u a l i t y c o n t r o l (QC) t e s t s are p r e s e n t l y being used. U n f o r t u n a t e l y , the complexity of the p r i n t i n g process i s such that undetected v a r i a t i o n s i n i n k batches are s t i l l present and t h i s leads to a higher than d e s i r a b l e percentage of i n k batches w i t h unacceptable performance. To help minimize the problem, a cooperative study was i n i t i a t e d between the N a t i o n a l Bureau of Standards and BEP w i t h the o b j e c t i v e of o b t a i n i n g a more complete p i c t u r e of the b a s i c r h e o l o g i c a l p r o p e r t i e s of the i n k and how these p r o p e r t i e s change as the i n k cures. This i n f o r m a t i o n combined w i t h the r e s u l t s of other s t u d i e s (2-6) w i l l provide both the b a s i s f o r more r e l i a b l e Q.C. techniques and the data r e q u i r e d t o help design improved i n k f o r m u l a t i o n s i n the future. Background The i n k s used i n currency p r i n t i n g are complex f o r m u l a t i o n s based on n a t u r a l d r y i n g o i l s such as tung o i l and l i n s e e d o i l . These o i l s c o n t a i n m u l t i p l e double bonds which a c t as p o l y m e r i z a t i o n s i t e s when exposed to oxygen. Although the d e t a i l s of the chemistry are not f u l l y understood, some important observations can be made. I f a bulk i n k sample i s exposed to a i r f o r s e v e r a l days a hard s k i n forms on the s u r f a c e . Underneath t h i s s k i n , however, the mechanical p r o p e r t i e s of the i n k remain v i r t u a l l y

May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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unchanged f o r many months. Thus the p o l y m e r i z a t i o n r e q u i r e s d i r e c t exposure to oxygen and so the cure can be a c c u r a t e l y s t u d i e d only w i t h experiments u s i n g f i l m s . The r e a c t i o n s are a l s o g r e a t l y a c c e l e r a t e d by increased temperature. Changes which r e q u i r e many hours a t room temperature w i l l occur i n minutes or seconds a t press temperatures. I t i s the b l a c k currency i n k which has been the most t r o u b l e some and i s t h e r e f o r e the subject of t h i s study. T h i s i n k c o n t a i n s very l i t t l e s o l v e n t — d r y i n g experiments show a weight l o s s of l e s s than 1% to f u l l c u r e — a n d thus the hardening must proceed by p o l y m e r i z a t i o n and c r o s s - l i n k i n g r e a c t i o n s . In a recent study of t h i s i n k (2) i t was demonstrated t h a t cure r e a c t i o n s are a c r i t i c a l f a c t o r i n the performance of the i n k on the press. Consequently, an examination of c u r i n g behavior i s necessary to the development of an understanding of i n k performance. In the present study two d i f f e r e n t i n k formulations were examined. They are designated as BK-62 and BK-60 m o d i f i c a t i o n 17 ( h e r e a f t e r designed simply as BK-60). Formulation BK-60 was used f o r a number of years and gave acceptable performance although there was room f o r improvement. Formulation BK-62 was developed more r e c e n t l y as a t r i a l m a t e r i a l w i t h a much simpler composition. Experiments w i t h t h i s i n k on the press showed t h a t i t had unacceptable performance and t h i s has l e d to a m o d i f i c a t i o n i n the BK-62 f o r m u l a t i o n . T h i s new v e r s i o n which shows marked improvement (2) i s now being u t i l i z e d i n production and s t u d i e d i n t h i s program. For the present d i s c u s s i o n , however, i t i s u s e f u l to examine the o r i g i n a l BK-62 and BK-60 as examples of an unacceptable i n k and an acceptable, although not i d e a l , ink. I t i s a l s o h e l p f u l t o note t h a t although a number of problems were observed w i t h BK-62, a major d i f f i c u l t y (2) was "excessive d r y i n g on the p l a t e " . Experimental

Section

Various samples of the two d i f f e r e n t i n k f o r m u l a t i o n s were evaluated f o r mechanical p r o p e r t i e s p r i o r to and during cure. Both f o r m u l a t i o n s are based on n a t u r a l v e h i c l e s and are h i g h l y f i l l e d w i t h pigments and a d d i t i v e s . There are a number of d i f f e r e n c e s between the two f o r m u l a t i o n s but the most important i s t h a t the samples designated BK-60 c o n t a i n a l i n s e e d o i l v e h i c l e system w h i l e those designated BK-62 c o n t a i n a tung o i l system. D e t a i l s of the i n k ' s composition are given i n Table I . The i n k s were examined by using t r a n s i e n t , steady f l o w , and o s c i l l a t o r y t e s t s performed a t room temperature, 22±0.5 C. These experiments were performed on a s p e c i a l l y modified and computerized cone and p l a t e viscometer. The cone u t i l i z e d was 2.5 cm i n diameter and 0.5 degrees i n angle. D e t a i l s of the data a c q u i s i t i o n system f o r t h i s device have been described p r e v i o u s l y (2) and w i l l only be summarized here. The p l a t e can be d r i v e n i n e



152

Table I . Composition of Ink Formulations


Ingredients Pigments Barytes Calcium Carbonate Linseed O i l V e h i c l e Varnish Bodied Tung O i l Amorphous S i l i c a Driers Petroleum Solvent Total

BK-60

BK-62

51.4

36 30 17

17.4 16.5 6.5

-

9 8

-

5.7 1.9 0.6

--

100

100

steady r o t a t i o n or o s c i l l a t o r y r o t a t i o n . The motion of the cone, which i s attached t o a t o r s i o n bar and h e l d i n l i n e by an a i r b e a r i n g , i s monitored t o provide a measure of the s t r e s s . In the o s c i l l a t o r y experiments, the d r i v e s h a f t i s attached to a s h a f t angle encoder t h a t generates 360 e q u a l l y spaced p u l s e per c y c l e . Each p u l s e t r i g g e r s the a c q u i s i t i o n of p o s i t i o n data from d i s placement transducers attached to the cone and the p l a t e . T h i s i n f o r m a t i o n i s t r a n s f e r r e d to a minicomputer f o r storage and a n a l y s i s . From t h i s data the s t r e s s and s t r a i n i n the sample can be c a l c u l a t e d and thus the mechanical p r o p e r t i e s can be d e t e r mined. In some cases i t i s u s e f u l t o examine the s t r e s s curve i t s e l f and f o r t h i s purpose the d i g i t i z e d data can be accessed. These data are i n a r b i t r a r y u n i t s but f o r the t e s t c o n d i t i o n s used here they can be converted to s t r e s s i n dynes/cm^ by m u l t i p l y i n g by 8.120. The experiments were performed a t f r e q u e n c i e s from 10 Hz t o 6x10-4 Hz and amplitudes between 4x10"^ r a d i a n s and 4xl0~2 r a d i a n s . In the i n i t i a t i o n of steady shear f l o w and the steady shear f l o w experiments, the s h a f t angle encorder was attached to a t i m i n g motor and data a c q u i s i t i o n proceeds i n the same way as i n the o s c i l l a t o r y experiments. These t e s t s were performed a t shear r a t e s between 5xl0~2 see"* and 2000 sec . A f t e r c h a r a c t e r i z i n g the b a s i c shear p r o p e r t i e s of the i n k s , t h e i r cure behavior was i n v e s t i g a t e d by p l a c i n g samples on a s p e c i a l l y designed 2 r o l l e r apparatus where one of the r o l l s was heated t o 80°C (roughly e q u i v a l e n t t o the press temperature). T h i s setup p r o v i d e s some of the important c o n d i t i o n s seen on the p r e s s , i . e . the a p p l i c a t i o n of heat, the formation of a t h i n f i l m f o r maximum exposure to oxygen, and the constant renewal of the f r e e s u r f a c e to prevent a hard c r u s t from forming. A f t e r v a r i o u s cure times a s m a l l p o r t i o n of the i n k was removed from the m i l l and c h a r a c t e r i z e d f o r shear p r o p e r t i e s u s i n g the same techniques described p r e v i o u s l y . Once c u r i n g had begun on the m i l l , i t



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continued even a f t e r the i n k was removed and c o o l e d to room temperature. N e v e r t h e l e s s , the c o o l i n g s u f f i c i e n t l y slowed the changes i n mechanical p r o p e r t i e s due t o c u r i n g so t h a t s h o r t term c h a r a c t e r i z a t i o n t e s t s c o u l d be performed. On the m i l l complete c u r i n g of the i n k f i l m r e q u i r e d more than h a l f an hour. T h i s i s much l o n g e r than r e q u i r e d on the p r e s s . The d i f f e r e n c e can be a t t r i b u t e d , a t l e a s t i n p a r t , to the f a c t that the f i l m t h i c k n e s s on the m i l l i s much l a r g e r than that on the p r e s s . Despite the d i f f e r e n c e s i n cure r a t e , however, t h i s technique was throught to be a u s e f u l method f o r comparing the cure of d i f f e r e n t i n k f o r m u l a t i o n s and f o r determining the r h e o l o g i c a l changes a s s o c i a t e d w i t h cure. R e s u l t s and D i s c u s s i o n Uncured Ink. The behavior of both i n k f o r m u l a t i o n s can be roughly d i v i d e d i n t o three r e g i o n s : i n i t i a t i o n b e h a v i o r , s h o r t term steady f l o w , and long term steady f l o w . T h i s can be i l l u s t r a t e d by experiments on the s t a r t - u p of steady shear. When the i n k i s allowed to r e s t i n the instrument f o r 15 minutes or more and then steady shear i s i n i t i a t e d , there i s a s i g n i f i c a n t s t r e s s overshoot (Figure 1 ) . Subsequently, the s t r e s s l e v e l shows a s i g n i f i c a n t time dependence f o r a p e r i o d of time t h a t depends on the experimental c o n d i t i o n s but i s g e n e r a l l y l e s s than 10 seconds. A f t e r t h i s i n i t i a l p e r i o d the s t r e s s appears to l e v e l - o f f a t what w i l l be termed the s h o r t term steady f l o w v a l u e . I f the steady shear i s maintained f o r l o n g p e r i o d s of time, however, i t i s found that the s t r e s s i s not constant but shows a s m a l l and very slow decrease. For the range of c o n d i t i o n s t e s t e d here, the s t r e s s , and t h e r e f o r e the v i s c o s i t y , drops by about 15% i n one hour (Figure 2 ) . The decrease i s approximately l i n e a r i n a l o g (η) v s l o g (time) p l o t . I f the shear f l o w i s stopped and then r e i n i t i a t e d w i t h i n a few minutes, the s t r e s s w i l l r e t u r n t o the l e v e l achieved j u s t p r i o r t o stopping the f l o w w i t h l i t t l e or no overshoot. Only i f the i n k i s allowed t o recover f o r a s i g n i f i c a n t p e r i o d of time, say 15 minutes, w i l l the response be s i m i l a r t o that observed i n i t i a l l y . A l l three aspects of the i n k b e h a v i o r — i n i t i a t i o n , s h o r t term steady f l o w , and l o n g term response—may have importance i n i n k performance and are being s t u d i e d i n t h i s program. T h i s paper, however, focuses o n l y on the s h o r t term steady f l o w regime and s m a l l amplitude o s c i l l a t o r y experiments. Steady Shear (Uncured I n k s ) . The s h o r t term steady f l o w s t r e s s l e v e l s (average v a l u e s achieved between 30 and 120 seconds) were determined as a f u n c t i o n of shear r a t e and used t o c a l c u l a t e v i s c o s i t i e s . Values of these v i s c o s i t i e s f o r samples of the two i n k f o r m u l a t i o n s are p l o t t e d a g a i n s t shear r a t e i n F i g u r e 3.



154

4)


C

ι

Ο α D L. •*->

F i g u r e 1. The s t r e s s a s s o c i a t e d w i t h t h e i n i t i a t i o n o f s t e a d y s h e a r f l o w i n BK-60 a t s h e a r r a t e s o f 55.7 s e e " * ( A ) and 5.6 s e c " ( B ) . 1

OJ CL

Ε

Ο

α

-η

3

sot

I ι i ι ι I 1 Log(Time)

Q-o-

I 2

'» 3

(sec)

F i g u r e 2. V i s c o s i t y o f BK-62 as a f u n c t i o n o f t i m e f o r s t e a d y s h e a r f l o w a t s h e a r r a t e s o f 1.76 s e c ~ l (Δ), 5.57 s e c " ( X ) , and 17.6 s e c " ( • ) . 1

1



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F i g u r e 3. S h o r t term s t e a d y f l o w v i s c o s i t y v s . s h e a r r a t e f o r two s a m p l e s o f BK-62: X & φ and two s a m p l e s o f BK-60: • & +.


156



1

Above shear r a t e s of 500 s e c " the data are u n r e l i a b l e because there i s o f t e n a l o s s of adhesion between the sample and the cone or p l a t e ; however, the i n d i c a t i o n s are t h a t the v i s c o s i t y may drop s i g n i f i c a n t l y a t h i g h shear r a t e s * In the r e g i o n between 80 and 500 sec ~ as w e l l as the r e g i o n between 6x10 and 0.6 sec"*, the v i s c o s i t i e s f o r the two i n k f o r m u l a t i o n s are s i m i l a r . Only a t the intermediate shear r a t e s , 0.6 to 80 sec" , i s the behavior s u b s t a n t i a l l y d i f f e r e n t . In t h i s r e g i o n the v i s c o s i t y of BK-60 reaches v a l u e s more than 3 times as l a r g e as those f o r BK-62. T h i s d i f f e r e n c e i s important because a number of the mixing steps r e q u i r e d to prepare these i n k s i n v o l v e shear r a t e s i n t h i s intermediate range. On the p r e s s , however, the i n k sees only very low shear r a t e s i n the f o u n t a i n and very h i g h shear r a t e s during p r i n t i n g . For these c o n d i t i o n s the behavior of both i n k s i s s i m i l a r i n F i g u r e 3. Since the r e s u l t s i n t h i s F i g u r e are f o r room temperature w h i l e the i n k on the press i s hot (50°C t o 80°C), a d d i t i o n a l t e s t s (2) were performed a t h i g h shear r a t e s (475 sec"*) and e l e v a t e d temperatures (60 C). Here again the behavior of the two i n k f o r m u l a t i o n s was s i m i l a r . Consequently, although the d i f f e r e n c e s i n short term steady f l o w v i s c o s i t i e s may have an e f f e c t on i n k p r e p a r a t i o n , they do not appear to be a l i k e l y cause of the poor performance f o r BK-62. e

O s c i l l a t o r y Shear (uncured I n k s ) . Because the v i s c o s i t i e s of these i n k s are h i g h l y n o n - l i n e a r the o s c i l l a t o r y behavior i s q u i t e complex. For example, F i g u r e 4 shows the s t r e s s curves a s s o c i a t e d w i t h a pure s i n e wave s t r a i n imposed on samples of BK-60 a t 3 d i f f e r e n t f r e q u e n c i e s . For the lowest frequency the s t r e s s curve i s approximately s i n u s o i d a l although s h i f t e d i n phase, but the curves become i n c r e a s i n g l y n o n - s i n u s o i d a l as the frequency i s i n c r e a s e d . Moreover, even a t the lowest frequency, i f the sample i s subjected to steady shear f o r a few minutes before the o s c i l l a t o r y t e s t , a n o n - l i n e a r response i s obtained (Figure 5 ) . A comparison between the behavior of the two i n k f o r m u l a t i o n s i n d i c a t e s a general s i m i l a r i t y i n the e f f e c t s of changing the amplitude or frequency or shearing the sample before the t e s t . I n most cases, however, the s t r e s s l e v e l s are g r e a t e r i n BK-60 w h i l e the n o n - l i n e a r i t y i s g r e a t e r i n BK-62 (Figure 6 ) . I n l i g h t of t h i s complexity only a simple a n a l y s i s of the o s c i l l a t o r y r e s u l t s was performed. N e v e r t h e l e s s , t h i s a n a l y s i s provides s e v e r a l i n t e r e s t i n g general c o n c l u s i o n s . F i r s t , the behavior of the i n k s i s v i s c o e l a s t i c i n t h a t the s t r e s s l e a d s the s t r a i n ( F i g u r e 4-6) by an angle between 0° ( e l a s t i c ) and 90° ( v i s c o u s ) . Second, a simple F o u r i e r a n a l y s i s i n d i c a t e s t h a t the s t r e s s curves can be f i t q u i t e w e l l w i t h a response i n v o l v i n g the fundamental frequency and the f i r s t two odd harmonics. For example, F i g u r e 7 shows the measured and c a l c u l a t e d curves f o r the case t h a t gave the poorest f i t of a l l the experimental c o n d i t i o n s analyzed. Even f o r t h i s case the agreement i s reason-


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B K - 6 0 ,

FREQUENCY

ANGLE

157

DEPENDENCE

C D E G J

F i g u r e 4. The s t r e s s c u r v e s f o r BK-60 a s s o c i a t e d w i t h a s i n e wave s t r a i n a t an a m p l i t u d e o f 1.5 χ 10" r a d i a n s and f r e q u e n c i e s o f 6.0 Hz ( A ) , 0.6 Hz ( B ) , and 0.06 Hz ( C ) .

*

ω

1

'

'

F. ι . ι . ι . ι ! ι . ι . ι . ι • I • ι . ι • ι . ι , ι . ι , ι • ι • -180

- M 0

- 100

-60

-20

ANGLE

20

60

100

140

180

CDEG:

F i g u r e 5. The s t r e s s c u r v e s f o r BK-60 a s s o c i a t e d w i t h a s i n e wave s t r a i n o f a m p l i t u d e 1.5 χ 10~^ r a d i a n s and f r e q u e n c y o f 0.06 Hz b e f o r e ( A ) and a f t e r ( B ) s h e a r i n g w i t h s t e a d y s h e a r f l o w ( s h e a r r a t e 60 s e c " ) f o r 1 m i n u t e . 1



158

CHEMORHEOLOGY OF THERMOSETTING

POLYMERS

F i g u r e 6. The s t r e s s c u r v e s f o r BK-60 ( A ) and BK-62 ( B ) a s s o c i a t e d w i t h a s i n e wave s t r a i n o f a m p l i t u d e 1.5 χ 10" r a d i a n s and f r e q u e n c y 0.06 Hz.

F i g u r e 7. C o m p a r i s o n between t h e m e a s u r e d s t r e s s c u r v e ( o v e r l a p p i n g d a t a p o i n t s a p p e a r a s h e a v y l i n e ) and c a l c u l a t e d c u r v e ( t h i n l i n e ) f o r BK-60 a t f r e q u e n c y o f 6 Hz and s i n e wave s t r a i n o f a m p l i t u d e 1.5 χ 10~ r a d i a n s .


10.

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159


a b l y good. An examination of the r e s u l t s a l s o i n d i c a t e s t h a t i n the c a l c u a l t e d curves the t o t a l amplitude of the harmonics i s i n a l l cases l e s s than 14% of t h a t f o r the fundamental and i n many cases l e s s than 7%. Thus t o a l a r g e extent the fundamental frequency dominates the response. As a r e s u l t i t i s of i n t e r e s t t o examine some of the g e n e r a l c h a r a c t e r i s t i c s of t h i s component of the t o t a l response. I f the r e a l p a r t o f t h e dynamic shear v i s c o s i t y i s c a l c u l a t e d f o r t h i s component, ru » i t i s found t o be a strong f u n c t i o n of the s t r a i n amplitude and frequency. Data were obtained f o r BK-60 a t 6 d i f f e r e n t amplitudes and 9 d i f f e r e n t f r e q u e n c i e s . I n an e f f o r t t o systematize t h i s i n f o r m a t i o n , the dependence of on the shear r a t e was examined by p l o t t i n g l o g (n^) a g a i n s t the time average shear r a t e during the o s c i l l a t o r y c y c l e ( F i g u r e 8)


f

time average shear r a t e

s

^ π tan

(1)

Q

Ό

where ω i s the angular v e l o c i t y , A i s the amplitude of o s c i l l a t i o n ( r a d i a n s ) , and θ i s the cone angle. Although t h i s does not c o l l a p s e the data to a s i n g l e curve, i t does compress the v a r i a t i o n s to a narrow band which i s q u i t e narrow a t low shear r a t e s and broadens somewhat as the shear r a t e i s i n c r e a s e d . I f the s h o r t term steady f l o w data are added to the graph, they f a l l roughly w i t h i n t h i s same band. A much l e s s e x t e n s i v e study u s i n g f o r m u l a t i o n BK-62 gave s i m i l a r r e s u l t s . Consequently, although the o s c i l l a t o r y behavior i s q u i t e complex, there i s an i n t e r e s t i n g c o r r e l a t i o n between the o s c i l l a t o r y data and the s h o r t term steady shear r e s u l t s . Curing Study. Although the data on the mechanical p r o p e r t i e s of the uncured i n k s p r o v i d e i n f o r m a t i o n t h a t i s u s e f u l i n f a b r i c a t i n g the i n k s , no e x p l a n a t i o n f o r the poor performance of BK-62 was found. As a r e s u l t , experiments aimed a t examining the cure behavior of the i n k s were conducted. Samples of the i n k s were cured on a heated 2 r o l l e r apparatus, and, a f t e r v a r i o u s c u r i n g times, s m a l l p o r t i o n s of i n k were removed and c h a r a c t e r i z e d f o r o s c i l l a t o r y and s h o r t term steady f l o w v i s c o s i t y . In view of the complexity of the o s c i l l a t o r y b e h a v i o r , most of the emphasis i s on the steady f l o w t e s t s ; however, i t i s u s e f u l t o examine the g e n e r a l trends e x h i b i t e d i n the o s c i l l a t o r y data. Steady Shear (Curing Study). F i g u r e s 9 and 10 i l l u s t r a t e the types of changes t h a t are observed i n the short term steady f l o w v i s c o s i t y as the i n k cures. Over the e n t i r e range of shear r a t e s t e s t e d , the c u r i n g produces an i n c r e a s e i n v i s c o s i t y . For a g i v e n c u r i n g time the data f o r BK-60 shown an approximately equal i n c r e a s e i n l o g (η) a t a l l but the h i g h e s t shear r a t e s . For BK-62 the longer cure times show the same t r e n d but the 7.5 minute cure produces l a r g e r changes a t the low shear r a t e s than



160


POLYMERS

F i g u r e 8. The dependence o f l o g 07 {) on t i m e a v e r a g e s h e a r r a t e f o r BK-60 a t a m p l i t u d e s o f 4.5 χ 1 0 ~ r a d (Δ), 2.2 χ 1 0 ~ r a d ( • ) , 6.4 χ 1 0 " r a d (φ), 9.8 χ 1 0 " r a d ( +), 2.2 χ 1 0 ~ r a d ( Δ ) , and 4.5 χ 1 0 ~ r a d ( X ) . S h o r t term s t e a d y f l o w v i s c o s i t y v s . shear r a t e ( - • - ) . 4

3

3

3

2

2


HUNSTON ET AL.

161



10.

F i g u r e

10.

f o r

samples

and

30

min

S h o r t o f

term

BK-60

s t e a d y

c u r e d

f o r

f l o w 0

v i s c o s i t y

min

( X ) ,

10

v s .

shear

min

(0),

r a t e 20

(+).


min

d a t a (Q),



162

at the i n t e r m e d i a t e and h i g h shear r a t e s . One f a c t o r that c o u l d c o n t r i b u t e to t h i s d i f f e r e n c e i n behavior a t s h o r t cure times i s the p o s s i b i l i t y of an i n d u c t i o n p e r i o d . Thermal experiments (2) have demonstrated t h a t the cure r e a c t i o n s do not begin immedi a t e l y but only a f t e r the sample has been h e l d a t the cure temper a t u r e f o r a few minutes. I n the present experiment the cure temperature i s reached very r a p i d l y but as w i l l be seen l a t e r , there i s evidence f o r an i n d u c t i o n p e r i o d . Before c u r i n g b e g i n s , BK-60 has a h i g h e r v i s c o s i t y over most of the shear r a t e range than BK-62. Once c u r i n g s t a r t s however, the r a t e of i n c r e a s e i n η i s much g r e a t e r f o r BK-62. Conse q u e n t l y , w i t h i n a short time the v i s c o s i t y f o r BK-62 i s l a r g e r than that f o r BK-60. I t i s c l e a r t h e r e f o r e that the two i n k f o r m u l a t i o n s have very d i f f e r e n t c u r i n g b e h a v i o r s . O s c i l l a t o r y Shear (Curing Study). Very s i m i l a r trends are seen i n the o s c i l l a t o r y data. F i g u r e s 11 and 12 show the s t r e s s curves a s s o c i a t e d w i t h a s i n e wave s t r a i n f o r i n k samples cured v a r i o u s l e n g t h s of time. As shown p r e v i o u s l y , the i n i t i a l s t r e s s l e v e l s i n BK-62 are much lower than i n BK-60. The changes a s s o c i a t e d w i t h c u r i n g , however, are s u b s t a n t i a l l y l a r g e r f o r Bk-62 and w i t h i n a short time the s t r e s s l e v e l s i n BK-62 are l a r g e r than those i n BK-60. In view of the c o r r e l a t i o n that was found between the o s c i l l a t o r y and short term steady f l o w experiments on the uncured i n k s , the s i m i l a r trends i n the c u r i n g t e s t s are a l o g i c a l r e s u l t . With both i n k s there i s a decrease i n n o n - l i n e a r i t y as the i n k s cure; i . e . , the s t r e s s curves become more s i n u s o i d a l . The angle by which the s t r e s s l e a d s the s t r a i n ( s i n e wave) a l s o decreases w i t h c u r i n g i n d i c a t i n g that although both the e l a s t i c and v i s c o u s components of the response i n c r e a s e , the e l a s t i c i t y i n c r e a s e s more r a p i d l y a t l e a s t i n the i n i t i a l phase of cure. A n a l y s i s of Cure. A simple a n a l y s i s of the cure r e s u l t s f o r short term steady f l o w can be performed by n o t i n g t h a t f o r a number of p o l y m e r i z a t i o n r e a c t i o n s , the e a r l y stages of cure can be d e s c r i b e d by a f i r s t order type equation (9,10). I n the simplest case t h i s would mean t h a t l o g (η) would vary l i n e a r l y w i t h time. To examine t h i s p o s s i b i l i t y the data f o r v a r i o u s shear r a t e s were analyzed by p l o t t i n g l o g (η) v s . time ( F i g u r e s 13 and 14). I f the i n i t i a l p o i n t s (zero cure time data) are excluded, the data f o r each shear r a t e can be f i t , to a f i r s t approximation, w i t h a s t r a i g h t l i n e . The f a c t t h a t the zero cure time p o i n t s do not f a l l near the l i n e s suggests that the mechanical p r o p e r t y r e s u l t s show an i n i t i a t i o n time j u s t as was found p r e v i o u s l y i n thermal experiments ( 2 ) . The s l o p e s , k, of the l i n e s i n F i g u r e s 13 and 14 p r o v i d e a measure of the r a t e of change of v i s c o s i t y w i t h time d u r i n g c u r i n g . The i n i t i a t i o n p e r i o d can a l s o be c h a r a c t e r i z e d by determining the time, t , corresponding to the p o i n t on each 0



10.

HUNSTON ET AL.


ANGLE

163

LDEGJ

F i g u r e 11. The s t r e s s c u r v e s f o r BK-62 a s s o c i a t e d w i t h a s i n e wave s t r a i n o f a m p l i t u d e 1.5 χ 1 0 ~ r a d i a n s and f r e q u e n c y 0.06 Hz c u r e d f o r 0 m i n ( A ) , 5 m i n ( B ) , 10 m i n ( C ) , 20 m i n ( D ) , and 30 m i n ( E ) . 3

- 1 8 0

-140

-100

-60

-20

ANGLE

20

60

100

140

180

CDEG:

F i g u r e 12. The s t r e s s c u r v e s f o r BK-60 a s s o c i a t e d w i t h a s i n e wave s t r a i n o f a m p l i t u d e 1.5 χ 10" r a d i a n s and f r e q u e n c y 0.06 Hz c u r e d f o r ô m i n ( A ) , 10 m i n ( B ) , 20 m i n ( C ) , and 30 m i n ( D ) . 3


164


POLYMERS


8.Ο1

5.51—1

Ο

1

1

1

500

1000

1500

Cure

Time

1

2000

(sec)

F i g u r e 13. S h o r t t e r m s t e a d y f l o w v i s c o s i t y o f BK-62 v s . c u r e t i m e a t s h e a r r a t e s o f 0.56 s e c ( X ) , 1.76 s e c " l ( • ) , 5.57 s e c " (φ), 17.6 s e c " ( + ), and 55.6 s e c " ( Δ ) . - 1

1

1

8.0,

—!

5.5'—ι 0

« 500

Cure

1 1000

Time

1

1

1500

2000

(sec)

F i g u r e 14. S h o r t t e r m s t e a d y f l o w v i s c o s i t y o f BK-60 v s . c u r e t i m e a t s h e a r r a t e s o f 0.56 s e c " ( X ) , 1.76 s e c " ( • ) , 5.57 s e c " (φ), 17.6 s e c " ( + ), and 55.6 s e c " ( Δ ) . 1

1

1

1


10.

HUNSTON ET AL.

165


s t r a i g h t l i n e where the v i s c o s i t y equals the zero cure time v i s c o s i t y of the i n k a t t h a t shear r a t e * Values of k and t the v a r i o u s shear r a t e s and i n k s are g i v e n i n Table I I . Table I I . Shear Rate (sec" )


1

0.56 1.76 5.56 17.60 55.60 55.60a a

Q

for

Cure Parameters

kxlO^

6.1 6.4 6.1 5.6 4.7 5.0

BK-60 t (sec) Q

330 360 410 410 420 500

BK62 kxl0«

t (sec)

9.4 9.6 9.8 8.8 6.4 7.2

150 180 270 280 340 530

Q

Does not i n c l u d e data p o i n t f o r s h o r t e s t non-zero cure time.

Over most of the measured shear r a t e range, the s l o p e s are q u i t e s i m i l a r . At the two h i g h e s t shear r a t e s the s l o p e s are l e s s as would be suggested by the data i n F i g u r e s 9 and 10. I t should be noted, however, that the i n i t i a t i o n times are g r e a t e r a t these shear r a t e s , and as a r e s u l t the data p o i n t s a t the s h o r t e s t non-zero cure time may be i n the i n i t i a t i o n r e g i o n . I f s t r a i g h t l i n e s are f i t t o the data f o r the shear r a t e of 55.6 s e c " w i t h out i n c l u d i n g the r e s u l t s f o r e i t h e r zero cure time or the s h o r t e s t non-zero cure time, the s l o p e s are g r e a t e r although s t i l l not as l a r g e as the v a l u e s obtained a t lower shear r a t e s (Table I I ) . Consequently, although the data here are l i m i t e d , the changes do appear t o be somewhat slower a t the h i g h e r shear rates. The r e s u l t s i n Table I I h e l p q u a n t i f y the d i f f e r e n c e s i n cure behavior between BK-60 and BK-62. P r e v i o u s experiments (2) u s i n g thermal a n a l y s i s techniques have found t h a t the i n i t i a t i o n p e r i o d f o r BK-62 i s s h o r t e r than that f o r BK-60. The same t r e n d i s seen i n the mechanical p r o p e r t i e s data. Moreover, the r a t e a t which the p r o p e r t i e s change once c u r i n g has begun i s approximately 50% g r e a t e r f o r BK-62 than f o r BK-60. When these r e s u l t s are combined w i t h the o b s e r v a t i o n that a major problem w i t h the performance of BK-62 on the press i s e x c e s s i v e d r y i n g on the p l a t e , the inescapable c o n c l u s i o n i s t h a t the d i f f e r e n c e s i n c u r i n g behavior are a major source of the problems w i t h BK-62. I t i s easy to see how these d i f f e r e n c e s i n c u r i n g c o u l d l e a d to e x c e s s i v e d r y i n g on the p l a t e . Even under c o n d i t i o n s where e x c e s s i v e d r y i n g on the p l a t e i s not observed, however, there w i l l s t i l l be s i g n i f i c a n t d i f f e r e n c e s i n the mechanical p r o p e r t i e s of the two i n k f o r m u l a t i o n s due t o the c u r i n g behavior. T h i s would a f f e c t t r a n s f e r on the feed r o l l e r s , w i p i n g of the



166

p l a t e , and pick-up of i n k by the paper and thus other problems w i t h BK-62 may a l s o be r e l a t e d to the cure behavior. In general i t would appear t h a t the changes i n i n k p r o p e r t i e s during cure must f a l l w i t h i n a given range i f proper performance on the press i s to be achieved. U n f o r t u n a t e l y , the o r i g i n a l BK-62 i n k f a i l s to meet t h i s c r i t e r i o n .


Conclusion The rheology of two i n t a g l i o i n k f o r m u l a t i o n s w i t h very d i f f e r e n t performance on the press was examined p r i o r to and during the e a r l y phases of cure. The uncured i n k s e x h i b i t complex v i s c o e l a s t i c p r o p e r t i e s w i t h s t r e s s overshoot, time dependence, and n o n - l i n e a r i t y . A major aspect of the nonl i n e a r i t y i s a l a r g e shear r a t e dependence of the v i s c o s i t y . Curing of the i n k s produces changes i n both the v i s c o s i t y and e l a s t i c i t y of the i n k s . A f t e r a short i n i t i a t i o n p e r i o d , the i n c r e a s e i n the l o g a r i t h m of the short term, steady flow v i s c o s i t y i s l i n e a r w i t h respect to time so the slopes of the b e s t - f i t s t r a i g h t l i n e s a t v a r i o u s shear r a t e s can be used t o c h a r a c t e r i z e the r a t e of change i n p r o p e r t i e s . Over a wide range of shear r a t e s the slopes of these l i n e s show only a s m a l l v a r i a t i o n . A comparison between the two f o r m u l a t i o n s , however, r e v e a l s s i g n i f i c a n t d i f f e r e n c e s : BK-62 has a s h o r t e r i n i t i a t i o n p e r i o d than BK-60 and a r a t e of i n c r e a s e i n v i s c o s i t y during c u r i n g that i s 50% g r e a t e r than BK-60. These d i f f e r e n c e s i n c u r i n g behavior p l a y a major r o l e i n the poor performance of BK-62. Acknowledgment The equipment u t i l i z e d i n t h i s work was developed and used i n cooperation w i t h the Naval Research Laboratory. Without t h e i r a s s i s t a n c e t h i s work would not have been p o s s i b l e . Literature 1. 2.

3. 4. 5. 6. 7.

Cited

"Production of Government Securities," Bureau of Engraving and P r i n t i n g , Public Brochure, P.15 - REV 9-80. Newitt, W. R.; Lipman, B.; Vandreuil, Β. Α.; Bureau of Engraving and Printing Research Report BEP-81-013, February, 1982. Hirayama, K.; Research B u l l e t i n Printing Bureau, Ministry of Finance, Japan 1979, 47, 1. Shurz, J . ; Kashmoula, T.; Papier 1976, 30, V52. Oittinen, P.; Acto Polytechnica Scandinavica 1976, 131, 1. Amari, T.; Watanabe, K.; Rep. Prog. Polym. Phys. i n Japan 1979, 22, 101 and 105. Huston, D. L.; Bascon, W. D.; Wells, E. E.; Fahey, J . D.; Bitner, J. L.; "Adhesion and Adsorption of Polymers,," Lee, L.H., Ed; Plenum Press: New York, 1980, Part A, p 321.


10.

HUNSTON ET AL.

8.

Hollands, K. M.; Kalnin, I. L.; "Epoxy Resins," Advances i n Chemistry Series No. 9 2 , ACS: Washington, D.C., 1970; p.60. Roller, M. B.; Poly. Eng. S c i . 1 9 7 5 , 1 5 , 4 0 6 .

9.

11,

1983


RECEIVED May



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Cure of Intaglio Printing Inks - ACS Symposium Series (ACS

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