Depth of Cure Profiling of UV-Cured Coatings - ACS Symposium

Chapter 3

Depth of Cure Profiling of UV-Cured Coatings Leslie R. Gatechair and Ann M. Tiefenthaler

Downloaded by RUTGERS UNIV on March 8, 2016 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch003

Ciba-Geigy Corporation, Ardsley, NY 10502

In this study Beer's law was utilized to determine the fraction of incident radiation absorbed (or transmitted) at various points within a UV cured coating. The calculations are carried out at each wavelength emitted by commercial curing lamps. Absorbance profiles help explain the difference in surface and through-cure effects and how cure varies with concentration, lamp selection, and coating thickness. Absorbance calculations were compared to experimental results, obtained through curing studies, with formulations containing different photoinitiator concentrations. We have found such calculations to be an effective educational tool in explaining the response of photoactive systems.

T h i s c o n t r i b u t i o n d e s c r i b e s o u r most r e c e n t e f f o r t s t o q u a n t i f y t h e complex i n t e r a c t i o n s o f p h o t o i n i t i a t o r s u s e d i n UV c u r e d c o a t i n g s w i t h commercial r a d i a t i o n s o u r c e s h a v i n g m u l t i p l e e m i s s i o n lines. The r e s u l t s o f t h e s e c a l c u l a t i o n s p r o v i d e p o t e n t i a l e x p l a n a t i o n s o f s u c h e f f e c t s as how t h e c o n c e n t r a t i o n o f a p h o t o i n i t i a t o r a f f e c t s the r a t i o o f s u r f a c e c u r e t o body c u r e , how c h a n g i n g lamps c a n a f f e c t t h e c h o i c e o f a p h o t o i n i t i a t o r , why optimum c o n c e n t r a t i o n s e x i s t f o r some i n i t i a t o r s , why some i n i t i a t o r s c o n s i s t e n t l y p e r f o r m b e t t e r under a g i v e n lamp, o r i n a g i v e n t h i c k n e s s t h a n o t h e r s , and why screening, the i n t e r n a l f i l t e r e f f e c t , can r e s u l t i n l o s s o f a d h e s i o n o r poor body c u r e . While t h e r e s u l t s o f t h e s e c a l c u l a t i o n s a r e c u r r e n t l y n o t s u f f i c i e n t t o p r o v i d e a q u a n t i t a t i v e guide f o r f o r m u l a t i n g , p e r c e n t i n c i d e n t r a d i a t i o n a b s o r b e d (PIA) c a l c u l a t i o n s do p r o v i d e an e x c e l l e n t e d u c a t i o n a l t o o l f o r understanding the principles affected by changing many formulation and p r o c e s s variables. We have r e c e n t l y d e s c r i b e d a computer program w h i c h was developed t o model the absorption o f various free radical p h o t o i n i t i a t o r s (1). Beer's law was u t i l i z e d i n t h e s e c a l c u l a t i o n s to determine the f r a c t i o n of incident radiation absorbed ( o r transmitted) a t v a r i o u s p o i n t s w i t h i n a c o a t i n g by a p h o t o i n i t i a t o r .

0097-6156/90/0417-0027$06.00/0 ο 1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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RADIATION CURING OF POLYMERIC MATERIALS

The c a l c u l a t i o n s were c a r r i e d o u t a t each w a v e l e n g t h e m i t t e d b y a s e l e c t e d lamp. Knowing t h e p e r c e n t o f i n c i d e n t r a d i a t i o n a b s o r b e d (PIA) a t v a r i o u s p o i n t s w i t h i n t h e c o a t i n g a l l o w e d t h e e s t i m a t i o n o f the r e l a t i v e c o n c e n t r a t i o n o f i n i t i a t i n g r a d i c a l s . I t s h o u l d be noted that although these c a l c u l a t i o n s apply to both f r e e r a d i c a l and c a t i o n i c UV c u r e d c o a t i n g s , many o f t h e d i s c u s s i o n s i n this m a n u s c r i p t may a p p l y o n l y t o c o a t i n g s c u r e d f r e e r a d i c a l l y i n t h e p r e s e n c e o f oxygen. A v a r i e t y of free r a d i c a l p h o t o i n i t i a t o r s are available f o r c o n v e r t i n g t h e energy o f t h e r a d i a t i o n s o u r c e i n t o c h e m i c a l energy f o r i n i t i a t i n g the c u r i n g process (2-7). An understanding o f the photochemistry o f these i n i t i a t o r s i s necessary to a i d i n proper s e l e c t i o n and f o r m u l a t i n g f o r a g i v e n a p p l i c a t i o n . I n t h i s s t u d y we have u t i l i z e d a d e v e l o p m e n t a l i n i t i a t o r t h a t i s d e s c r i b e d later. The mechanism o f p h o t o l y s i s , subsequent g e n e r a t i o n o f r a d i c a l s and a p p l i c a t i o n r e s u l t s a r e d e s c r i b e d i n t h e c o n t r i b u t i o n b y Desobry and co-workers i n t h i s same symposium s e r i e s ( 8 ) . THEORY A common m i s c o n c e p t i o n i s t h a t t h e r a t e o f p h o t o p o l y m e r i z a t i o n i s p r o p o r t i o n a l t o the c o n c e n t r a t i o n o f the i n i t i a t o r . More c o r r e c t l y , the r a t e o f p h o t o p o l y m e r i z a t i o n w i l l be a f u n c t i o n o f t h e i n t e n s i t y o f a b s o r b e d r a d i a t i o n (9. 1 0 ) . U n f o r t u n a t e l y , t h i s v a l u e i s n o t e a s i l y a v a i l a b l e f o r measurement by common i n s t r u m e n t s . In ideal systems, t h e o r y s u g g e s t s t h a t the rate o f a photopolymerization s h o u l d be p r o p o r t i o n a l t o t h e square r o o t o f t h e p h o t o i n i t i a t o r c o n c e n t r a t i o n ( 1 1 ) . However, f o r m u l a t e d c o a t i n g s behave f a r from i d e a l i n t h i s respect. A v a r i e t y o f other factors also a f f e c t the p o l y m e r i z a t i o n rate: t h e average f u n c t i o n a l i t y o f t h e f o r m u l a t i o n , t h e v i s c o s i t y , the quantum y i e l d o f t h e p h o t o i n i t i a t o r , the f r a c t i o n o f f r e e r a d i c a l s p r o d u c e d by t h e i n i t i a t o r which r e a c t w i t h monomer, t h e p r e s e n c e o f i n h i b i t i n g a g e n t s s u c h as oxygen, and t h e p r e s e n c e o f c o i n i t i a t o r s such as t e r t i a r y amines. I n t h e c o n t e x t o f t h i s paper these f a c t o r s a r e n e g l e c t e d . F o r many comparisons, t h e b u l k o f t h e s e f a c t o r s may be h e l d constant i f only the p h o t o i n i t i a t o r concentration i s varied. Such s i m p l i f i c a t i o n s a r e l e s s v a l i d f o r t h e c o m p a r i s o n o f different photoinitiators. However, e x p e r i e n c e has s u g g e s t e d that calculations o f PIA p r o v i d e a r e a s o n a b l e g u i d e t o u n d e r s t a n d i n g p h o t o i n i t i a t o r performance. The power o f c u r i n g lamps, r e f l e c t o r geometry, sample p o s i t i o n and b e l t speed a r e a l s o i m p o r t a n t . In t h i s s t u d y we have m a i n t a i n e d t h e s e a t f i x e d l e v e l s . A number of publications have proposed that an optimum c o n c e n t r a t i o n o f a p h o t o i n i t i a t o r e x i s t s , such t h a t the o p t i c a l density ( a b s o r b a n c e ) o f t h e r e s u l t i n g c o a t i n g i s 0.434 (9. 12-14). T h i s i s t h e o p t i c a l d e n s i t y a t which t h e maximum r a d i a t i o n w i l l be absorbed i n the lowest r e g i o n o f the c o a t i n g . I n p r i n c i p l e , such a guideline only has meaning for ideal systems cured using monochromatic r a d i a t i o n . I f t h e r a d i a t i o n s o u r c e h a s two o r more l i n e s s e p a r a t e d by a t l e a s t 10-20 nm, t h e o p t i c a l d e n s i t y w i l l be d i f f e r e n t , t h e l i n e i n t e n s i t i e s w i l l l i k e l y be d i f f e r e n t , and t h e system i s c l e a r l y n o n i d e a l . I n commercial UV c u r i n g applications, i n d u s t r i a l lamps may have 20 t o 40 e m i s s i o n l i n e s . More ' i d e a l '

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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a p p l i c a t i o n s would be i n imaging systems such as p r i n t i n g p l a t e s o r photoresists f o r p r i n t e d w i r i n g boards. In these a p p l i c a t i o n s the radiation from a mercury b u l b i s often filtered by layers of p o l y e s t e r f i l m and g l a s s . For m o d e l i n g the i n t e r a c t i o n o f r a d i a t i o n with UV cured coatings, one must consider the intensity and w a v e l e n g t h o f each l i n e i n the r a d i a t i o n s o u r c e , as w e l l as the o p t i c a l d e n s i t y o f the c o a t i n g (due t o the p h o t o i n i t i a t o r ) a t each wavelength. Since many of the photo i n i t i a t o r s used in these a p p l i c a t i o n s do n o t absorb beyond 380 nm, o n l y a few l i n e s between 330 and 366 nm a r e u t i l i z e d . As w i l l be d e s c r i b e d below, u s i n g the o p t i c a l d e n s i t y o f 0.434 ( o r below) r e s u l t s i n a r e l a t i v e l y u n i f o r m a b s o r p t i o n of r a d i a t i o n t h r o u g h o u t the film. Assuming t h a t each p h o t o n a b s o r b e d (or a c o n s t a n t f r a c t i o n t h e r e o f ) r e s u l t s i n the f o r m a t i o n o f initiating radicals, one may a n t i c i p a t e uniform c r o s s l i n k i n g throughout the volume o f the c o a t i n g . I n p r a c t i c e , however, c o a t i n g s c u r e d by f r e e r a d i c a l p o l y m e r i z a t i o n a r e s t r o n g l y i n h i b i t e d by oxygen. Under such conditions, a high concentration o f r a d i c a l s a t the s u r f a c e o f f s e t the d i f f u s i o n o f oxygen i n t o the c o a t i n g d u r i n g the c u r i n g p r o c e s s and r e s u l t i n improved f i l m p r o p e r t i e s . Alternate solutions to oxygen i n h i b i t i o n are w e l l known t o the UV curing industry and include the use o f i n e r t gases, oxygen b a r r i e r f i l m s , and amine s y n e r g i s t s (2) . Calculations. The Beer-Lambert law d e s c r i b e s the dépendance o f the a b s o r p t i o n o f a compound (A) on the e x t i n c t i o n c o e f f i c i e n t (E) the concentration (C) and the p a t h l e n g t h o f the sample (D) and p r o v i d e s the key t o c a l c u l a t i n g the d i s t r i b u t i o n o f a b s o r b e d r a d i a t i o n . A

—

ECD

—

log

I /I 0

(1)

t

In these calculations I represents the intensity of r a d i a t i o n i n c i d e n t on the s u r f a c e . I f we wish to c a l c u l a t e the i n t e n s i t y o f a b s o r b e d r a d i a t i o n a t v a r i o u s p o i n t s i n the c o a t i n g , we d i v i d e the c o a t i n g i n t o a s e r i e s o f η u n i f o r m segments h a v i n g a thickness t - D/n. The i n t e n s i t y of r a d i a t i o n absorbed (I ) or t r a n s m i t t e d ( I ) by a segment i s d e s c r i b e d by the e q u a t i o n s below: Q

a

t

I I

E C t

I' 10- I' (1-10-ECt)

t

t

a

t

(2) ) ( 3

Here, I ' i s the i n t e n s i t y t r a n s m i t t e d by the p r e c e d i n g segment. As the e x t i n c t i o n c o e f f i c i e n t i s dependent on w a v e l e n g t h , these c a l c u l a t i o n s must be p e r f o r m e d f o r each w a v e l e n g t h e m i t t e d by the r a d i a t i o n s o u r c e and f o r each segment i n the c o a t i n g . Rubin has p u b l i s h e d a more d e t a i l e d d e s c r i p t i o n o f s i m i l a r c a l c u l a t i o n s (15). To o b t a i n a parameter r e l a t i v e t o the i n t e n s i t y i n c i d e n t on the s u r f a c e o f the c o a t i n g , we d e f i n e PIA as the p e r c e n t o f i n c i d e n t r a d i a t i o n absorbed: t

PIA

- la/Ιο χ 100

Our Additive Absorbance c a l c u l a t i o n s f o r each w a v e l e n g t h To simplify the analysis, we

(4) analysis program performs PIA and each segment o f the coating. have also made the following




30

assumption: i f a p h o t o n o f any w a v e l e n g t h i s a b s o r b e d we assume t h a t p h o t o c h e m i c a l e v e n t s o c c u r w i t h the same e f f i c i e n c y . This i s e q u i v a l e n t t o assuming t h a t the quantum y i e l d o f i n i t i a t i n g r a d i c a l s i s independent o f w a v e l e n g t h . Although photochemical r e a c t i o n s are known h a v i n g a quantum y i e l d dependent on wavelength, i n f o r m a t i o n on the dependence o f quantum y i e l d o f i n i t i a t i n g r a d i c a l s on w a v e l e n g t h i s n o t a v a i l a b l e f o r most p h o t o i n i t i a t o r s . We a l s o assume t h a t the same r e a c t i v e species i s formed from photons absorbed at any wavelength. I t s h o u l d be n o t e d t h a t e f f e c t s o f r e f l e c t a n c e , s c a t t e r i n g and diffraction have been neglected. Calculations also assume ( i n c o r r e c t l y ) t h a t the polymer i s t r a n s p a r e n t t o a l l w a v e l e n g t h s , and t h a t t h e r e a r e no o t h e r a b s o r b i n g s p e c i e s competing w i t h the additive. C a l c u l a t i o n s of PIA which include such competitive a b s o r p t i o n a r e n o t t r i v i a l (10. 16-18). We have a l s o assumed t h a t the absorbance o f the a d d i t i v e does n o t change s i g n i f i c a n t l y d u r i n g the i r r a d i a t i o n . T h i s assumption i s r e a s o n a b l e c o n s i d e r i n g the low degree of photolysis of the photoinitiator during most cure conditions. EXPERIMENTAL D e v e l o p m e n t a l p h o t o i n i t i a t o r CGI-369 ( S t r u c t u r e I ) was o b t a i n e d from the CIBA-GEIGY A d d i t i v e s D i v i s i o n as a y e l l o w c r y s t a l l i n e powder and u s e d as r e c e i v e d . Absorbance v a l u e s were t a k e n from a b s o r p t i o n s p e c t r a r u n i n e t h a n o l a t f o u r c o n c e n t r a t i o n s r a n g i n g from .01 t o 10.0 g/1. E x t i n c t i o n c o e f f i c i e n t s were c a l c u l a t e d from E q u a t i o n 1, a t a p p r o x i m a t e l y 10 nm i n c r e m e n t s i n u n i t s o f l i t e r / ( c m χ m o l e ) . Where e x t i n c t i o n c o e f f i c i e n t s c o u l d be c a l c u l a t e d a t more t h a n one c o n c e n t r a t i o n , the v a l u e s were averaged. Extinction coefficients needed f o r s p e c i f i c wavelengths i n the program were i n t e r p o l a t e d from n e a r e s t n e i g h b o r i n g e x p e r i m e n t a l v a l u e s . Curing Conditions. 1 m i l (25.4 urn) c o a t i n g s were c u r e d a t 200 f/m under 1 f o c u s e d F u s i o n Η lamp o p e r a t i n g a t 300 W/in. C o a t i n g s were a p p l i e d u s i n g a w i r e wound r o d t o Form N2A o p a c i t y cards (The L e n e t t a Company). S u r f a c e c u r e was measured as t h e minimum number of passes to achieve light s c r a t c h r e s i s t a n c e over the black substrate region. The f o r m u l a t i o n below was u s e d f o r the d a t a i n Table I I I . Model Urethane A c r y l a t e F o r m u l a t i o n E b e c r y l 8800-20R TMPTA TRPGDA Ν-vinyl p y r r o l i d o n e

62.5 22.9 4.2 10.4

parts

Pendulum h a r d n e s s (Koenig) was measured on 1 m i l (25.4 um) c o a t i n g s a p p l i e d t o gray p r i m e d aluminum p a n e l s . A l l p a n e l s were exposed t o 5 p a s s e s a t 200 f/m under the F u s i o n Η b u l b . Dark s u b s t r a t e s have been u t i l i z e d t o m i n i m i z e r e f l e c t i o n .


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DISCUSSION F i g u r e 1 shows the e x t i n c t i o n c o e f f i c i e n t spectrum o f developmental p h o t o i n i t i a t o r CGI-369. Such s p e c t r a a r e e q u i v a l e n t t o a b s o r p t i o n s p e c t r a b u t a r e independent o f c o n c e n t r a t i o n and t h i c k n e s s . Lamp E m i s s i o n S p e c t r a . F i g u r e 2 shows the e m i s s i o n l i n e s from 200-600 nm o f a F u s i o n H medium p r e s s u r e mercury a r c lamp o p e r a t i n g a t 300 W/inch. A l t h o u g h e m i s s i o n s p e c t r a a r e n o r m a l l y p l o t t e d as Energy v s wavelength, we have u t i l i z e d the S t a r k E i n s t e i n r e l a t i o n to c o n v e r t energy t o i n t e n s i t y , which i s p r o p o r t i o n a l t o the number o f photons e m i t t e d a t each w a v e l e n g t h .


m - Ε Y/hc,

(5)

where Ε - energy, Y - wavelength, h - P l a n c k ' s c o n s t a n t , c - the speed o f l i g h t , and m - the number o f p h o t o n s . To facilitate relative comparisons, the total emission intensity has been normalized t o 100. Thus, each l i n e r e p r e s e n t s the p e r c e n t of e m i t t e d photons a t a g i v e n wavelength. Since photochemical events a r e p r o p o r t i o n a l t o the number o f photons a b s o r b e d (not energy absorbed) t h i s c o r r e c t i o n makes the e m i s s i o n s p e c t r a more u s e f u l f o r PIA c a l c u l a t i o n s . F i g u r e 3 shows the dependence o f PIA t h r o u g h o u t a coating, h a v i n g 10 t h e o r e t i c a l segments, on the t o t a l o p t i c a l d e n s i t y o f the f i l m f o r the i d e a l c a s e o f monochromatic r a d i a t i o n . Below OD 0.25, PIA i s l e s s t h a n 6% and u n i f o r m l y a b s o r b e d throughout the coating. As the OD approaches 0.434 the PIA increases at a l l p o i n t s , b u t i s almost t w i c e as h i g h n e a r the s u r f a c e as a t the substrate. I n c r e a s i n g the OD beyond 0.434 r e s u l t s i n s i g n i f i c a n t l y i n c r e a s e d PIA n e a r the s u r f a c e , a t the expense o f r e d u c e d PIA deeper in the c o a t i n g due t o the i n t e r n a l f i l t e r effect i n the upper segments. T h i s extreme would p r o b a b l y f a v o r improved s u r f a c e c u r e at the p o s s i b l e expense o f a d h e s i o n . The lower OD model would probably r e s u l t in s l o w e r , b u t more u n i f o r m polymer formation. T h i s model would be most u s e f u l under inert atmosphere c u r i n g conditions. The optimum PIA p r o f i l e f o r a g i v e n p r o p e r t y p r o b a b l y depends on what p o r t i o n o f the c o a t i n g ( i e . s u r f a c e , body or s u b s t r a t e i n t e r f a c e ) i s most d i r e c t l y a s s o c i a t e d w i t h the p r o p e r t y . We have r e c e n t l y d e m o n s t r a t e d how d i f f e r e n t e m i s s i o n w a v e l e n g t h s and v a r i a t i o n o f i n i t i a t o r s e l e c t i o n can cause s i m i l a r e f f e c t s ( 1 ) . The first law o f p h o t o c h e m i s t r y states that " i n order f o r photochemistry t o o c c u r , r a d i a t i o n must be absorbed." A useful c o r o l l a r y t o t h i s law f o r UV c u r i n g i s t h a t the o n l y wavelengths w h i c h need be considered are those emitted by the radiation source. F o r the d a t a i n T a b l e I we have u s e d t h i s c o r o l l a r y t o s e l e c t the 18 wavelengths o f a F u s i o n H b u l b mercury lamp. The molar extinction coefficients (molar e p s i l o n ) f o r CGI-369 were c a l c u l a t e d a t each wavelength. O p t i c a l D e n s i t y (OD) was calculated u s i n g E q u a t i o n 1. C a l c u l a t i o n s o f % t r a n s m i t t e d and % a b s o r b e d i n Table I do not i n c l u d e the lamp emission intensity at each wavelength. At a level of 3% (typical f o r many commercial applications at 1 mil thickness) this photoinitiator absorbs radiation efficiently. The a b s o r p t i v i t y i n the b l u e r e g i o n a l l o w s the use o f the e m i s s i o n l i n e s a t 405 and 434 nm. CGI-369 a l s o






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Depth of Cure Profling

20 19 18 17 16 15


141 13 12i 11 10 9i 8 7 6 5 4 3 • 2 1• 0

200

300

Figure 2 - Emission spectra to r e l a t i v e p h o t o n f l u x .

of

600

500

400 WAVELENGTH

(nm)

a F u s i o n H c u r i n g lamp

adjusted

COATING DEPTH IN MILS F i g u r e 3 - The i n f l u e n c e o f o p t i c a l d e n s i t y percent of incident radiation absorbed (monochromatic r a d i a t i o n ) .

of at

a coating various


on the depths

34 n t - i 1 i 7 P e

RADIATION CURING OF POLYMERIC MATERIALS t-hp

IfiS

nm

line

much

more

efficiently

than

the

p h o t o i n i t i a t o r s d e s c r i b e d i n our e a r l i e r s t u d i e s (1). The amount ot o h o t o c h e m i s t r v o c c u r r i n g a t a g i v e n p o i n t w i c n i n

a

coating w i l l be a f u n c t i o n ot the intensity or tne raaiation r e a c h i n g t h a t p o i n t , and the e f f i c i e n c y a t which i t i s a b s o r b e d . Usine e a u a t i o n s 2 and 3. the D e r c e n t of i n c i d e n t r a d i a t i o n t r a n s m i t t e d and a b s o r b e d was c a l c u l a t e d f o r each w a v e l e n g t h , i n each segment. T a b l e II shows the r e s u l t s o f t h e s e c a l c u l a t i o n s t o r a 1 m i l c o a t i n g d i v i d e d i n t o 10 segments c u r e d w i t h 3% CGI-369. These c a l c u l a t i o n s i n c l u d e the lamp e m i s s i o n i n t e n s i t y . The a b s o r b e d r a d i a t i o n i n a g i v e n segment was summed o v e r an w a v e l e n g t h s to determine the t o t a l p e r c e n t or i n c i d e n t r a d i a t i o n

absorbed in each segment.

Plots ot PIA vs. depth w i n

D e utilized


i n the d i s c u s s i o n s below. By c o n v e n t i o n , PIA c a l c u l a t e d a t d e p t h 0 9 m i l s r e n r e s e n t s the a v e r a e e PIA t h r o u g h o u t the s e c o n d segment, i . e . between 0.1

and

0.2

mils.

EFFECTS OF CONCENTRATION. Analyses of cured films have often i n d i c a t e d t h a t most o f the p h o t o i n i t i a t o r remains u n r e a c t e d (19. 20}. S u m r i s i n g l v . the use o f lower c o n c e n t r a t i o n s o f t e n r e s u l t i n i n s u f f i c i e n t cure. The c o n c e n t r a t i o n o f a p h o t o i n i t i a t o r i s n o t simnlv or d i r e c t l y r e l a t e d to c u r i n g e f f i c i e n c y . I f the s e l e c t i o n o f i n i t i a t o r and lamD a r e h e l d c o n s t a n t , the n e t e f f e c t o f v a r y i n g the c o n c e n t r a t i o n o f the p h o t o i n i t i a t o r w i l l be t o d e t e r m i n e the d i s t r i b u t i o n o f the i n i t i a t i n g r a d i c a l s t h r o u g h o u t the c o a t i n g . F i g u r e 4 shows the PIA c a l c u l a t i o n s f o r 1 m i l f i l m s c u r e d under

a Fusion H lamo havine concentrations or CGI-Jby ranging rrom u.D to 7.5%. These c u r v e s i n d i c a t e t h a t i n the h i g h e r c o n c e n t r a t i o n s D-1U f o l d h i g h e r PIA i s o b t a i n e d i n the f i r s t 0.1 m i l as compared t o the t y p i c a l PIA found i n the lower h a l f o f the c o a t i n g . T h i s h i g h PIA, r e s u l t i n g i n a h i g h c o n c e n t r a t i o n o f i n i t i a t i n g r a d i c a l s , i s needed t o o f f s e t the r e d u c e d c u r i n g e f f i c i e n c y n e a r the s u r t a c e c a u s e d oy oxygen i n h i b i t i o n . Lower p h o t o i n i t i a t o r c o n c e n t r a t i o n s w h i c h would c r e a t e more u n i f o r m PIA t h r o u g h o u t the c o a t i n g a r e known t o r e s u l t i n poor surrace cure i n a i r . In a D r e v i o u s s t u d y o f a b s o r b a n c e D r o f i l e s f o r a h i g h e x t i n c t i o n coefficient photoinitiator, aminoacetophenone (ΑΑΡ), we observed s i g n i f i c a n t c r o s s o v e r o f the PIA p l o t s s i m i l a r t o t h o s e o b s e r v e d i n Figure 3 (1). ΑΑΡ i s known t o need c a r e f u l f o r m u l a t i n g t o a v o i c s u r f a c e s k i n c u r e o r w r i n k l i n g i n t h i c k o r pigmented a p p l i c a t i o n s (1). The b r o a d e r absorbance o f CGI-369, c o u p l e d w i t h the e m i s s i o n o f the F u s i o n Η b u l b , r e d u c e s the i n t e r n a l t i l t e r e t t e c t . AS a result. one can formulate fairly high concentrations of this photoinitiator with no surface wrinkling. This benefit is e s p e c i a l l y u s e f u l i n pigmented a p p l i c a t i o n s ( 8 ) . CORRELATION OF PIA CALCULATIONS WITH CURING RESULTS. Table III shows the r e s u l t s o f t e s t i n g f i v e c o n c e n t r a t i o n s o f CGI-369 f o r s c r a t c h r e s i s t a n c e and pendulum h a r d n e s s i n c o a t i n g s c u r e d under the Fusion Η bulb. These t e s t r e s u l t s a r e p l o t t e d i n F i g u r e 5. Both surface cure (scratch resistance) and through cure (pendulum h a r d n e s s ) were found to D l a t e a u a t the h i g h e r c o n c e n t r a t i o n s or CGI-369 t e s t e d .



Table I.

Absorbance and Transmittance C a l c u l a t i o n s For Each Wavelength of a Fusion H Bulb Medium Pressure Mercury Arc Lamp

Photoinitiator:


Depth of Cure Profiling

WaveLength (nm ) 210 220 230 240 250 254 270 280 290 300 310 313 340 365 405 434 545 580

Molar Eusilon 1637 1637 1642 4169 4766 3847 2627 4478 7393 11905 13131 13252 11580 858 120 25 0 0

Table I I .

3% CGI-369

1 mil Thickness

OD % Trans. 45 6 0. 3 45 6 0. 3 45 5 0. 3 13 5 0. 8 10 1 0. 9 15 8 0. 8 28 4 0 5 11 7 0 9 2 8 1 5 0 3 2 4 0 1 2 7 0 1 2 7 0 3 2 4 66 2 0 1 94 3 0 024 98 .7 0 0053 100 .0 0 0000 Ρ 0000 100,0

% Absorbed 54.3 54.3 54.4 86.4 89.8 84.1 71.5 88.2 97.1 99.6 99.8 99.8 99.6 33.7 5.6 1.2 0.0 0.0

C a l c u l a t i o n s of Absorbance and Transmittance by Thickness

P h o t o i n i t i a t o r : 3% CGI-369 1 m i l Thickness Segment Thickness - 0.1 mils Lamp - Fusion Η

Segment No. 0 1 2 3 4 5 6 7 8 9 10

Note: Segment absorbed.

0

Coating Depth (mils} 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

represents

% Trans. bv See. 100 91 85 81 78 76 74 73 72 70 69

the

PIA % Incident Absorbed 0.0 (Surface) 8.4 5.6 3.9 2.9 2.2 1.8 1.5 1.2 1.1 0.9 (Substrate)

surface,

thus

no

radiation


is


36


0 H

1

0.0

1

1

1

1

1

0.4

0.2

1

0.6

1

1

1

0.8

1.0

COATING DEPTH IN MILS F i g u r e 4 - Percent o f i n c i d e n t r a d i a t i o n absorbed a t depths i n a c o a t i n g c o n t a i n i n g c o n c e n t r a t i o n s o f CGI-369 from 0.5 t o 7.5% ( F u s i o n H b u l b r a d i a t i o n ) .

various ranging

35

0 Η 0

1

1

2

1

1

r—

4

1

1

6

1 8

CONCENTRATION (WT %) F i g u r e 5 - The dependence o f S u r f a c e Cure ( s c r a t c h r e s i s t a n c e ) and Through Cure (pendulum h a r d n e s s ) on concentration of d e v e l o p m e n t a l p h o t o i n i t i a t o r CGI-369.


3. GATECHAIR & TIEFENTHALER TABLE I I I .

% CGI-369 0.5 1 3 4.6


7.5

Depth ofCure Profting

P h o t o i n i t i a t o r Concentration on Cure Response Scratch Resistance (passes) 25. 21. 6. 2.

L

Effect

Pendulum Hardness (seconds) 83. 129. 167. 174.

im-

Pendulum h a r d n e s s r e s u l t s were f o u n d t o c o r r e l a t e b e s t w i t h PIA v a l u e s i n t h e m i d d l e r e g i o n (0.3-0.5 m i l s depth) o f t h e c o a t i n g s (see F i g u r e 6) . T h i s c o r r e s p o n d s t o t h e common i n t e r p r e t a t i o n o f pendulum hardness t o be a measure o f 'body cure'. Scratch r e s i s t a n c e was f o u n d t o c o r r e l a t e b e s t w i t h PIA i n t h e second segment (0.2 m i l s ) , c l o s e r t o the surface (see Figure 7 ) . We i n t e r p r e t these r e s u l t s t o i n d i c a t e t h a t the m a j o r i t y o f r a d i c a l s g e n e r a t e d i n t h e f i r s t segment a r e u s e d t o o f f s e t t h e d i f f u s i o n o f oxygen. T h i s s u r f a c e c u r e r e g i o n i s about 0.1 m i l s (2.5 urn) t h i c k , below w h i c h oxygen d i f f u s i o n i s s i g n i f i c a n t l y r e d u c e d and b e t t e r cure i s achieved. This could also indicate that our s c r a t c h r e s i s t a n c e t e s t i s more a n i n d i c a t i o n o f c u r e j u s t below t h e s u r f a c e than r i g h t a t the surface. I n a s e c o n d s t u d y we have compared t h e p e r f o r m a n c e o f t h r e e commercial p h o t o i n i t i a t o r s f o r s u r f a c e c u r e i n a commercial u r e t h a n e acrylate formulation. The f o l l o w i n g p h o t o i n i t i a t o r s were u t i l i z e d in this study: HCPK - a l p h a - h y d r o x y - a l p h a - e y e l o h e x y l p h e n y I k e t o n e ( S t r u c t u r e I I ) , BDMK - b e n z i l - d i m e t h y l - k e t a l ( S t r u c t u r e I I I ) , and ΑΑΡ ( S t r u c t u r e I V ) . The e x t i n c t i o n s p e c t r a o f t h e s e i n i t i a t o r s a r e i n F i g u r e 8. T a b l e IV shows c u r i n g r e s u l t s f o r a commercial u r e t h a n e - a c r y l a t e formulation cured under the conditions of Figure 9 (0.5 mil t h i c k n e s s c o a t i n g c o n t a i n i n g 3% p h o t o i n i t i a t o r ) .

Table

Structure IV III II

IV.

Cure Comparison U s i n g

Photoinitiator ΑΑΡ BDMK HCPK

a M e r c u r y Lamp Scratch Resistance (nasses) 8 13 14

F i g u r e 9 shows a c o m p a r i s o n o f PIA c a l c u l a t i o n s f o r t h e t h r e e p h o t o i n i t i a t o r s i n t e g r a t i n g o v e r t h e 18 l i n e s o f a medium p r e s s u r e mercury a r c lamp. I n t h i s c a s e , a l l t h r e e p h o t o i n i t i a t o r s have a higher PIA n e a r t h e s u r f a c e t h a n c l o s e t o t h e s u b s t r a t e . ΑΑΡ generates the highest PIA a t a l l p o i n t s i n the coating. BDMK r e s u l t s i n i n t e r m e d i a t e l e v e l s o f PIA, w h i l e HCPK i s p r e d i c t e d t o have t h e l o w e s t c o n c e n t r a t i o n o f r a d i c a l s a t any d e p t h w i t h i n t h e coating. F i g u r e 9 s u g g e s t s t h a t under t h e s e c o n d i t i o n s ΑΑΡ w i l l c u r e s i g n i f i c a n t l y f a s t e r t h a n BDMK, which w i l l c u r e s l i g h t l y f a s t e r




38

8

1 H 0

1

1

4

1

1

8

1

1

12

1

1

16

1

1

20

1

"

24

1

1

1

28

SURFACE CURE (PASSES) F i g u r e 7 - C o r r e l a t i o n o f S u r f a c e Cure ( s c r a t c h r e s i s t a n c e ) w i PIA c a l c u l a t e d a t depth » 0.2 m i l s c o n c e n t r a t i o n s o f CGI-3 range from 0.5 t o 7.5%.


Depth of Cure Profling



Structure

IV




200

240

280 •

HCPK

320

360

# 6Bdfc >

WAVH

NG

m

400 ·

440

480

ΑΑΡ

Figure 8 - Extinction coefficient spectra of HCPK, BDMK and ΑΑΡ.


3.


41

Depth of Cure Profting

t h a n HCPK. The s c r a t c h r e s i s t a n c e t e s t with this prediction.

results

i n Table

IV

agree


CONCLUSIONS C a l c u l a t i o n s o f the p e r c e n t o f i n c i d e n t r a d i a t i o n a b s o r b e d a i d i n understanding the i n t e r a c t i o n between p h o t o i n i t a t o r c o n c e n t r a t i o n and the s e l e c t i o n o f a c u r i n g lamp. V a r y i n g the c o n c e n t r a t i o n o f a p h o t o i n i t i a t o r was d e m o n s t r a t e d t o have a d r a m a t i c e f f e c t on where the r a d i a t i o n i s a b s o r b e d w i t h i n a c o a t i n g , and hence where the i n i t i a t i n g r a d i c a l s are generated. The t r a d i t i o n a l b e l i e f o f an optimum o p t i c a l d e n s i t y o f 0.434 does n o t a p p l y t o p h o t o s e n s i t i v e systems c u r e d i n a i r o r under a r a d i a t i o n s o u r c e h a v i n g m u l t i p l e wavelengths. Although dependent on the test method, fair correlation exists between how model and commercial f o r m u l a t i o n s cure, and the calculated PIA values below the surface of the coating. Formulators s h o u l d be aware t h a t changes i n any o f the variables which affect the PIA will result i n the need for r e o p t i m i z a t i o n o f the p h o t o i n i t i a t o r l e v e l . ACKNOWLEDGEMENTS The a u t h o r s thank Dr. M. Rembold, D. W o s t r a t z k y , and Dr. H. A n g e r e r for discussions and critique during the development of the absorbance algorithms, and to J. Parchment and H. Evers for assistance with the experimental curing data. M. Synder is g r a t e f u l l y acknowledged f o r h i s e f f o r t s i n r e f i n i n g the programming code. We thank the CIBA-GEIGY C o r p o r a t i o n f o r p e r m i s s i o n t o p r e s e n t t h i s paper. LITERATURE CITED 1. Gatechair, L. R. Proceedings of Radtech '88, April 26, 1988. 2. Gatechair, L. R. and Wostratzky, D., a) J. Radiation Curing, 10(3) 4-19,(1983). b) Proc. Radiation Curing VI, Sept. 20-23, 1-24,(1982). 3. Berner, G. and Sitek, F. Proceedings of the Conference of Dutch Paint Association, Woerden, Netherlands, December 17,(1985). 4. Pappas, S. P., "UV Curing Science and Technology", (S. P. Pappas, Ed.) Technology Marketing Corp. Norwalk, CT. USA, pp 1-25 (1985). 5. Vesley, G. F., J. Radiation Curing 13(1) 4-10 (1986). 6. Phan, X. T., J. Radiation Curing, 13(1) 11-17,1986. 7. Berner, G., Puglisi, J., Kirchmayr, R., Rist, G., J. Radiation Curing, April, 2-9(1979). 8. Desobry, V., Dietliker, K., Husler, R., Misev, L. Rembold, Μ., Rutsch, W. in this symposium. 9. Bush, R. W., Ketley, A. D., Morgan, C. R. and Whitt, D. G., J. Radiation Curing 7 (2): 20 (1980). 10. Shultz, A. R. and Joshi, M. G., J. Polym. Sci.: Polym. Phys, Ed. 22 (1984) 1753-1771. 11. Reference 4, p. 62. 12. Reiser, A. and Pitts, Photographic Sci. Eng. 20 (1976) 229. 13. Gutierrez, A. R., and Cox, R. J., Polymer Photochemistry 7 (1986) 517-521.


42



14. Thommes, G. A. and Webers, V. J., J. Imaging Sci., 29 (1985) 112-116. 15. Rubin, Η., a) TAGA Proceedings, 279-301 (1976) b) pp 187-201 in reference 3. 16. Heller, H. J., European Polymer Journal-Supplement, 5(5) 105-132 (1969). 17. Shultz, A. R. and Andrady, A. L., J. Appl. Polym. Sci., 33 (1987) 2249-2252. 18. Guillory, J. P. and Cook, C. F., J. Polym. Sci.: Port A-1, 9 (1971) 1529-1536. 19. Huit, Α., Yuan, Y.Y., Ranby, Β. Polymer Degradation and Stability, 8(1984) 241-258. 20. Huit, Α., Ranby, Β., Polym. Degradation and Stability 8(1984) 89-105. RECEIVED September 13, 1989


Depth of Cure Profiling of UV-Cured Coatings - ACS Symposium

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