14 Cationic Photoinitiation Efficiency L. R. GATECHAIR CIBA-GEIGY Corporation, Plastics and Additives Division, Ardsley, ΝY 10502
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S. P. PAPPAS North Dakota State University, Polymers and Coatings Department, Fargo, ND 58105 The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photo chemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was in vestigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. Sensitized photolysis of both salts resulted in the formation of the same major products as was observed in direct irradiation, iodobenzene, or phenylsulfide and acid. Quantum yields of organic and acidic products were measured using nine sensitizers. Product quantum yields were relatively high (< 1) under conditions where electron transfer sensitiza tion was expected to occur. Quantum yields > 2 were obtained in solvents likely to be substrates for hydrogen abstraction, indicating that a chain reaction was involved. Triplet energy sensitiza tion resulted in low product quantum yields.
Many recent p u b l i c a t i o n s and patents give evidence of the growing i n t e r e s t i n p h o t o i n i t i a t e d c a t i o n i c polymerization using onium s a l t s . ( 1 - 7 ) The o b j e c t i v e of t h i s study was to b e t t e r un derstand the photochemistry of onium s a l t s i n order to improve t h e i r e f f i c i e n c y as photocuring agents. The r e s u l t s of these studies form g u i d e l i n e s for the choice of s e n s i t i z e r s , onium s a l t s and other a d d i t i v e s p o t e n t i a l l y useful i n the c a t i o n i c curing pro cess. S e n s i t i z e r s which produce e a s i l y o x i d i z a b l e free r a d i c a l s r e s u l t e d in the highest quantum e f f i c i e n c i e s for Diphenyliodonium hexafluoroarsenate s a l t s (φ> 3 ) . Electron transfer sensitization 0097-6156/83/0212-0173$06.00/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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was the most e f f i c i e n t process with t r i p h e n y l s u l f o n i u m hexafluoroarsenate (φ »
+ Epoxy Resin
^
Η
Crossi inked Polymer
Figure 1. Many p h o t o i n i t i a t o r systems have been developed which are capable of producing a c i d i c products. Most of these c a t i o n i c p h o t o i n i t i a t o r s are based on some form of onium s a l t which ( i n the absence of l i g h t ) i s s t a b l e i n the presence of epoxy f u n c t i o n a l r e s i n s . Following i r r a d i a t i o n , an a c i d i c c a t a l y s t i s produced which can i n i t i a t e the formation of polymer. Photochemistry of Aryldiazonium S a l t s . Aryldiazonium s a l t s have long been known to be a c l a s s of photoactive chemicals (see Figure 2 ) . When aryldiazonium s a l t s having n u c l e o p h i l i c anions (such as c h l o r i n e or bromine) are i r r a d i a t e d , the r e s u l t i n g pro duct i s n o n a c i d i c . A c i d i c products may, however, be formed during p h o t o l y s i s i n the presence of water.(17) Such chemistry i s o f t e n u s e f u l i n the area of diazo copying, more commonly known as b l u e p r i n t s . _ I f the diazonium s a l t has a nonnucleophilic anion such as P F , BF^, A s F , PC1 , the r e s u l t i n g product ( i . e . , PF ) i s a Lewis acid capable of c a t a l y z i n g the polymerization or epoxy func t i o n a l r e s i n s . This chemistry was patented by the American Can Company f o r use i n various kinds of coatings. The c l a s s of diazonium s a l t s , i n general, have the advantage of very strong absorption curves which extend into the v i s i b l e region of the spectrum. Thus, some diazonium s a l t s are s e n s i t i v e to a wide range of wavelengths. They a l s o have disadvantages, i n that many o f the compounds are h i g h l y c o l o r e d , and are, to some degree, thermally unstable, l i m i t i n g the package s t a b i l i t y o f a formulated coating. 6
6
6
Photochemistry of Diaryliodonium S a l t s . The photochemistry of diaryliodonium s a l t s having n u c l e o p h i l i c anions i s i l l u s t r a t e d
Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
14.
GATECHAIR AND PAPPAS
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in Figure 3.(19) T h i s photochemistry a l s o r e s u l t s i n the forma t i o n of nonacidic products. P h o t o l y s i s in a l c o h o l , however, was reported to form acid.(13) C r i v e l l o and Lam, at the General E l e c t r i c Company, found that the p h o t o l y s i s of d i a r y l i o d i u m s a l t s having nonnucleophilic anions r e s u l t s i n the formation of a c i d i c species capable of polymerizing epoxy r e s i n s . ( 1 8 ) T h e i r evidence, based on product a n a l y s i s , sup ported the formation of protons as the i n i t i a t i n g species, r a t h e r than the formation of a Lewis acid as i n the photochemistry of diazonium s a l t s (see Figure 4 ) . The p h o t o l y s i s quantum e f f i c i e n c y (φ) ranges from 0.2-0.4 f o r the formation of organic products. (15,18) Photochemistry of T r i a r y l s u l f o n i u m S a l t s . A s i m i l a r mecha nism was proposed for the p h o t o l y s i s of t r i a r y l s u l f o n i u m s a l t s having nonnucleophilic anions. The a r y l s u l f o n i u m and to a l e s s e r extent, the aryliodonium s a l t s have improved thermal s t a b i l i t y as compared to the diazonium s a l t s , however, t h e i r absorption maximum occurs at a much shorter wavelength since the aromatic r i n g s are i s o l a t e d by the heteroatom, and therefore not conjugated. Recognizing t h i s d e f i c i e n c y , the workers at General E l e c t r i c , and separately, Smith at 3M (7), discovered that iodonium and sulfonium s a l t s could be s e n s i t i z e d to longer wavelengths by v a r i ous c l a s s e s of aromatic hydrocarbons, aromatic carbonyl compounds, and some c l a s s e s of dyes. S e n s i t i z a t i o n of Onium S a l t s In the curing of coatings, the use of p h o t o s e n s i t i z e r s o f f e r several advantages over d i r e c t e x c i t a t i o n of the onium s a l t s . S e n s i t i z e r s added to onium s a l t s may d r a m a t i c a l l y increase the polymerization rate of r e a c t i v e monomers as compared to unsensit i z e d experiments. (_5) The choice of s e n s i t i z e r w i l l l a r g e l y de termine what p o r t i o n of the a v a i l a b l e r a d i a t i o n w i l l be u t i l i z e d i n the curing process. Some s e n s i t i z e r s may r e s u l t i n a dramatic increase i n cure rate simply because they have greater a b s o r p t i v i t y than the onium s a l t at e x c i t a t i o n wavelengths. A l t e r n a t i v e l y , the absorbed l i g h t may be used more e f f i c i e n t l y i n the s e n s i t i z e d system. There are many commercially a v a i l a b l e p h o t o s e n s i t i z e r s . The o v e r a l l cost of coating formulation may be reduced by the use of small amounts of a h i g h l y absorbing p h o t o s e n s i t i z e r i n combination with a reduced concentration of the onium s a l t . In order to choose appropriate s e n s i t i z e r s f o r onium s a l t s , i t i s necessary to understand the p o s s i b l e mechanisms by which s e n s i t i z a t i o n can occur. Energy T r a n s f e r S e n s i t i z a t i o n . Energy t r a n s f e r i s a process where an e x c i t e d state donor molecule i s returned to the ground s t a t e , with simultaneous promotion of an acceptor molecule to i t s
Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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2
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(NONACIDIC )
(LEWIS
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Figure 2. Photolysis of aryldiazonium salts. Photochemistry of these salts is dependent on the nucleophilicity of the anion.
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