radiation curing of epoxies

Radtat Phys Chem Vol 31, Nos 4-6, pp 587-593, 1988 lnt ? Re&at Appl lnstrum Part C Printed m Great Britain

0146-5724/88 $3 00 + 0 00 Pergamon Journals Ltd

RADIATION CURING OF EPOXIES

Lawrence W. Dinkson and AJlt Singh

R a d i a t i o n A p p l l c a t i n n s R e s e a r c h Branch W h l t e s h e l l Nuc lea r R e s e a r c h E s t a b l i s h m e n t

Atomic Energy of Canada L i m i t e d R e s e a r c h Company Pinawa, Manitoba, Canada ROE ILO

ABSTRACT

The l i t e r a t u r e on r a d i a t i o n p o l y m e r i z a t i o n of epoxy compounds has been r ev i ewed to a s s e s s t h e p o t e n t l a l use of r a d i a t i o n fo r c u r i n g t h e s e i n d u s t r i a l l y impor t an t monomers. Chemical c u r i n g of e p o x i e s may proceed by e i t h e r c a t i o n i c or a n i o n i c mechanisms depend ing on the n a t u r e of the c u r i n g a g e n t , but most e p o x i e s p o l y m e r l z e by c a t i o n i c mechanisms under the i n f l u e n c e of h i g h - e n e r g y r a d i a t i o n . R a d l a t l o n - l n d u c e d c a t i o n i c p o l y m e r l z a t l o n of epoxy compounds i s i n h i b i t e d by t r a c e q u a n t l t l e s of w a t e r because of p r o t o n t r a n s f e r from the c h a l n - p r o p a g a t i n g epoxy c a t i o n to w a t e r . S e v e r a l d i f f e r e n t methods w i t h p o t e n t i a l f o r o b t a i n i n g h i g h m o l e c u l a r we igh t po lymers by c u r i n g e p o x i e s w i t h h l g h - e n e r g y r a d i a t i o n have been s t u d i e d . P o l y m e r i c p r o d u c t s w i t h e p o x y - l l k e p r o p e r t i e s have been produced by r a d i a t i o n c u r i n g of epoxy o l i g o m e r s w i t h t e r m i n a l a c r y l a t e g roups and m i x t u r e s of e p o x i e s w i t h v i n y l monomers. Both of t h e s e t y p e s of r e s i n have good p o t e n t l a l f o r I n d u s t r l a l - s c a l e c u r i n g by r a d i a t i o n t r e a t m e n t .

KEYWORDS

Radiation polymerization; radiation curing; epoxy compounds; review; chemlcal mechanisms; cationic p o l y m e r i z a t i o n mechanism; 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 mechanism; e p o x y - a c r y l a t e s .

INTRODUCTION

Typlcally, commercial epoxy resins are dlfunctinnal epoxldes produced by the base-catalyzed r e a c t i o n of e p l c h l o r o h y d r i n w i t h d l f u n c t l o n a l a l c o h o l s , such as d l p h e n y l o l propane ( b i e p h e n o l - A ) . The epox lde f u n c t i o n a l group i s a three-membered r i n g c o n t a i n i n g two carbon atoms and one oxygen atom; p o l y m e r i z a t i o n of epoxy compounds produces m o l e c u l e s w i t h p o l y e t h e r s t r u c t u r e s . Cured epoxy r e s i n s have a good c o m b i n a t i o n of t o u g h n e s s , f l e x i b i l i t y , a d h e s i o n , c h e m i c a l r e s i s t a n c e and e l e c t r l c a l i n s u l a t i n g p r o p e r t i e s and as a r e s u l t a re used in s u r f a c e c o a t i n g , f l b e r - - r e l n f o r c e d c o m p o s i t e s , c a s t i n g , p o t t i n g , e n c a p s u l a t i n n and i n d u s t r i a l f l o o r l n g . About 140 m i l l i n n kg of epoxy r e s i n s was s o l d i n the Un i t ed S t a t e s i n 1982, a t an ave rage p r i c e of $2.75 per kg (Billmeyer, 1984).

Chemically catalyzed curing of epoxy resins may proceed by cationic or anionic mechanisms depending on the nature of the catalyst (Allcock and Lampe, 1981; Billmeyer, 1984; Lee and Neville, 1967). The reactive intermediates in cationic polymerization mechanisms are carbocatinns; in anionic polymerlzatinn the intermediates are alkoxlde anions. The intermediates in chemically catalyzed ionic polymerization are probably not free ions but rather ion pairs with the counterlon always in close proximity to the growing chain end. Primary- and secondary-amlne curing agents react with the epoxlde group to give B-hydroxyemino linkages; primary and polyfunctlonal secondary amines are crossllnklng agents. Trace levels of moisture in epoxy resins do not generally inhibit the chemically catalyzed polymerlzatlon reaction. In fact, the presence of small amounts of water can promote the curing process by assisting in the formation of the initiating complex (Williams, 1968).

P r e v i o u s r e v i e w s of the r a d l a t l o n - l n d u c e d p o l y m e r i z a t i o n of epoxy compounds have been w r i t t e n by W i l l l a m s (1968) , Omel 'chenko and Bokalo (1978) and L a r l c h e v a (1982) . W i l l i a m s (1968) p r e s e n t e d a s h o r t d i s c u s s i o n of the r e s e a r c h on the r a d i a t i o n p o l y m e r i z a t i o n of cyc l ohexene o x i d e , e t h y l e n e o x i d e and p r o p y l e n e o x i d e . Omeltchenko and Bokalo (1978) n o t e t h a t r a d i a t i o n h o m o p o l y m e r l z a t l o n of epoxy compounds does not occur r e a d i l y , but c o p o l y m e r l z a t l o n occur s more e a s i l y . As d i s c u s s e d be low, t h i s i s l l k e l y to be due to the i n h i b i t i o n of the r a d l a t l o n - l n d u c e d f r e e ion c a t i o n i c p o l y m e r i z a t i o n mechanism by t r a c e l e v e l s of i m p u r i t i e s , such as w a t e r . In c o n t r a s t , r a d i a t i o n i s known to induce c r o s s l l n k l n g and, thereby~ i n c r e a s e the s t r e n g t h p r o p e r t i e s of cured epoxy r e s i n s bo th a l o n e and in compos i t e m a t e r l a l s (Lee and N e v i l l e , 1967; B u l l o c k , 1974; N e t r a v a l l and c o - w o r k e r s , 1984) . R a d i a t i o n c u r i n g has s e v e r a l a d v a n t a g e s over t he rma l and c a t a l y t i c c u r i n g methods , i n c l u d i n g i n c r e a s e d r a t e of p o l y m e r i z a t i o n , c u r i n g a t -mb len t t e m p e r a t u r e s and mi n i ma l t o x i c c h e m i c a l e m i s s i o n s (R ie and Be re Jka , 1986).

587

588 L W DIcg~3N and A Sn~GH

The p r e s e n t review was performed to i d e n t i f y the most promis ing a p p l i c a t i o n s fo r r a d i a t i o n c u r i n g of epoxy compounds. For t h i s rev iew, we focus on the r a d i a t i o n - i n d u c e d p o l y m e r i z a t i o n of the fo l l owing c l a s s e s of epoxy compounds: a l l c y c l i c epox ie s , s u b s t i t u t e d a l l p h a t l c e p o x i e s , a l l p h a t l c epox ies and d i f u n c t i o n a l epox ies . S tud ie s of the r a d i a t i o n po lyme r i z a t i on of epoxies in mix tu re s

CURING OF PURE EPOXY COMPOUNDS

A s y s t e m a t i c s tudy of the 7 - r a d l a t l o n - i n d u c e d po lymer l za t i nn of epoxy compounds wan performed by C o r d l s c h l , Hele and Somogyi (1967). They s tud ied the r a d i a t i o n po lymer l za t i nn of e thy lene oxide , p ropylene ox ide , 2 -bu tene oxide , e p l c h l o r o h y d r l n , s t y r e n e oxide , t e t r a f l u o r o e t h y l e n e oxide , c yc lo - pentene oxide , cyclohexene oxide , cyc loheptene oxide and cyc looc tene oxide a t s e v e r a l d i f f e r e n t t e m p e r a t u r e s . The s t r a l g h t - c h a l n a l l p h a t l c epoxldes ( e t h y l e n e , p ropylene and 2-butene ox ides ) showed no tendency to polymer lze when i r r a d i a t e d wi th ¥ - r a d l a t i o n . Ep l ch lo r ohyd r ln and s t y r e n e oxide polymerized wi th h igh G(-monomer) va lues y i e l d i n g mainly low molecu la r weight p r o d u c t s . T e t r a f l u o r o e t h y l e n e oxide polymerized only in the s o l i d s t a t e . The a l l c y c l i c epox ldes , except cyc looc tene ox ide , polymerlzed e f f i c i e n t l y in bo th the s o l i d and l l q u l d s t a t e s . Cyclooctene oxide did not y i e l d any polymeric m a t e r i a l at doses up to 150 kCy.

A l i c y c l i c Epoxies

C o r d i s c h i , Lenzi and Nele (1965) and C o r d i s c h i , Idele and Somogyi (1967) showed t h a t the r a d i a t i o n - induced p o l y m e r i z a t i o n of cyelohexene oxide is i n h i b i t e d by t r ace q u a n t i t i e s of wate r and 8mmonia, s u g g e s t i n g t h a t the po l ym er i za t i on r e a c t i o n proceeds by a c a t i o n i c mechanism.

in i t t a t ion:

p ropaga t ion:

t e r m i n a t i o n :

tnh Ib it 1on:

%~o o - - z - . , . E + + x- (1)

E + + C6H10O------, ~ ( C 6 ~ o O ) + (2 )

E(C6H100)+ n + C6H100 ----'* R(C6HI00)~+I (3)

R(C6HIO0)+ + X- - - - ' -+ R(C6HIoO)nX (4)

R(C6HIo0)+ + P-20 . . . . R(C6H100)n_IC6HgO + I'130+ (5)

R(C6HIoO)+n + ~ ----+ E(C6HI00)n_IC6H90 + NH~ (6)

H30+ + X- - - - ' -+ H20 + HX (7)

1~4 + x - ------,. t,ffl 3 + ~X (8 )

Reac t ions 1 and 2 are the i n i t i a l c r e a t i o n of an ion p a i r by a b s o r p t i o n of h l g h - e n e r g y r a d i a t i o n and the r e a c t i o n of the p o s i t i v e ion w i th cyclohexene oxide to produce an epoxlde ca t ion capable of cha in p r o p a g a t i o n . Reac t ion 3 i s the cha in p ropaga t i on r e a c t i o n . Reac t lon 4 i s the b imolecu- f a r t e r m i n a t i o n r e a c t i o n wi th the c o u n t e r i o n . Reac t ions 5 and 6 are p ro ton t r a n s f e r r e a c t i o n s of the chain p r o p a g a t i n g c a t l o n wi th wate r and ~ o n l a which are the l l k e l y causes of the observed e f f e c t s of t he se i n h i b l t o r s . The I n h l b l t o r s are r egene ra t ed by the r e a c t i o n s of the p ro tona t ed s p e c i e s wi th the c o u n t e r l o n (Reac t ions 7 and 8) . These r e a c t i o n s are the l l k e l y e x p l a u a t l o n f o r the i n h i b i t i n g e f f e c t s of E20 and NH 3.

Hiramoto and co-workers (1971 and 1972) s tud ied the r a d i a t i o n po lymer i za t ion of cyclohexene oxide in the l l q u l d , g l a s s y s o l i d and normal c r y s t a l l i n e s t a t e s . They found t h a t the p ropaga t i on r e a c t - ion i s about 40 t imes f a s t e r in the g l a s s y s t a t e than in the normal c r y s t a l l l n e s t a t e . Convers ion y i e l d s of up to 40Z at 0.1 kCy were ob ta ined In the g l a s s y s t a t e . By comparing the r a d i a t i o n chemical y l e l d s of monomer l o s s to the number-average molecu la r weights determined from v l s c o s l t y da ta , Hlramoto and co-workers (1971) a l so determined t h a t chain t r a n s f e r r e a c t i o n s are s i g n i f i c a n t in t h i s sys tem.

cha in t r a n s f e r : R(C6HIoO)+u + C6H100 - - - - + E(C6H1oO)n_lC6H90 + C6HllO+ (9)

In t h i s chain t r a n s f e r r e a c t i o n (9 ) , a p ro ton ls t r a n s f e r r e d from the chain p ropaga t i ng i n t e r - media te to the monomer, y i e l d i n g an ol igomer wi th a t e r mina l u n s a t u r a t i n n and a p ro tona t ed monomer capable of cha in p r o p a g a t i o n . H i r ~ o t o and co-workers (1971 and 1972) a l so s t ud i e d the mechanisms of the t e r m i n a t i o n r e a c t i o n s in the l l q u l d and g l a s s y s t a t e s by de te rmining the e f f e c t of dose r a t e on the r a t e of p o l y m e r i z a t i o n . In the e a r l i e r s tudy , they determined t h a t the t e r m i n a t i o n mechanism In the l i q u l d s t a t e was a comblnat lon of b lmolecu la r ( c a t i o n + anion) and un lmolecu le r ( c a t i o n + d e f e c t s , c a t i o n + i m p u r i t i e s ) r e a c t i o n s and t h a t the t e r m i n a t i o n mechanisms in the two s o l i d s t a t e s were un lmolecu la r . The l a t e r s tudy showed tha t c a r e f u l drying of the cyclohexene oxide p r i o r to I r r a d l a t l o n inc reased the p r o p o r t i o n of b imolecu la r t e r m i n a t i o n in the l i q u i d s t a t e and changed the t e r m i n a t i o n mechanism in the g l a s s y s o l i d s t a t e to a combinat ion of b lmo lecu l a r and un lmolecu le r r e a c t i o n s . Akbulut and co-werkers (1983) a l so s tud ied the r a d l a t l o n - l n d u c e d p o l y m e r l z a t i n n of cyclohexene oxide. The i r r e s u l t s confirmed tha t r a d i a t i o n p o l y m e r i z a t i o n leads to the fo rmat ion of a p o l y e t h e r s t r u c t u r e . The dry ing technique of Akbulut end co-workers (1983) (Call2) was p robab ly l e s s e f f e c t i v e than tha t of Hirsmoto and co-workers (1971) (molecula r s i eve 4A) as t h e i r r a t e s of convers ion to polymer and f ine1 molecu la r weights are lower . This obse rva - t i o n of the e f f e c t i v e n e s s of v a r i o u s dry ing agents has been noted p r e v i o u s l y ( W i l l i s ~ s , 1968). These s t u d i e s h i g h l i g h t the s i g n i f i c a n c e of t r a ce q u a n t i t i e s of ~ o t s t u r e to the r a d i a t i o n p o l y m e r i z a t i o n of epoxy compounds.

6th International Meeting on Radmtion Processing 589

A i k i n s and W i 1 1 I s u (1984 and 1985) have s t u d i e d t he r a d l a t i n n - l n d u c e d p o l y m e r l z a t l o n o f l l a o n e n e o x i d e , = - p l n e n e ox ide and ~ - p l n e n e o x i d e . These monomers a re a l s o po lymer l zed by a c a t i o n i c mechanism. LiNonene ox / de was p o l y m e r l a s d w i t h a y iNld o f abou t 3 .6g pe r kGy a t 25"C. The o t h e r two epoxies were polymerlzed with lower efficiency. Chain transfer is also sIKnlflcant in these systems as the kinetic chain lengths are several hundred times larger than the number-average degree of polymerization values of the polymeric products. The liNonene oxide was polymerlzed by epoxy ring opening to a product with a 1,2-trans-polyether structure, but the plneue oxides were polymerlzed by epoxy ring opening accompanied by a significant degree of c y c l o b u t a n e r i n g open i ng .

S u b s t i t u t e d A l I p h a t l c Epoxies

Tabate (1967) studied the radlatlon-lnduced polymerlsatlon of styrene oxide in the llqu/d and solld states using both 60Co y-ray and electron-be~nn radiation sources. From the temperature dependence of the rate of the polymerlza~ion reaction, Tabata obtained apparent activation energies of 26 kJ*mol -I and 0.71 kJ*mol -I for polymerization in the liquid and solld states, respectlvely. Tabata observed that the llquld-state polymerization was inhibited by ~-benzo- qulnone and concluded from this and the relatlvely high activation energy that the polymerization proceeds by a free radical mechanism. This is in contrast with the radlatlon-lnduced polymerlzatlou of other epoxldes that proceed by cationic mechanisms. The free radical mechanism for styrene oxide polymerization would be facilltated by the stabillty of the substituted benzyl structure of the redlcel intermediate. Intrinsic viscosities of the polymer products indicate that the products ere mainly diners end low molecular weight polymers and, thus, that the chain transfer and isomerlzatlon reactions are rapid in this system. The dose-rate dependence of the polymerization rate indicates that the termination reaction is a unlmolecular one, such as radical isomerlzatlon, reaction with monomer, or reaction with impurities rather than radlcal-radlcal recombination. These results show that, although moisture does not inhibit the free radical polymerization process, irradiation of styrene oxide is not an effective means of obtaining a high molecular weight polymer.

Vlshev, Meleshevlch and Antonovskaya (1983) have studied the 7-radlatlen-lnduced polymerlzatlon of eplchlorohydrln over the temperature range from -196 to +1700C. The lar~est rates of polymerization, G(-m) - 63 molecules,(100 eV) -1 and G(-m) = 93 molecules*(100 eV)- , were observed at temperatures of about -53°C and about +170°C, respectively. The minimum polymerization rate was observed between +40 and +1200C. On the basis of these and other observations, Vishev, Meleshevlch and Antouovskaya (1983) concluded that a cationic polymerlz~tlon mechanism is operative in the solld state between -196 and -60oC (Ea_ t = 1.1 kJ~ol -~) and also in the llquld state between -60 end +40°C (E_ c. - -10.9 kJ*mol-1), bu~ a free radlcal polymerization mechanism

o 1 is operative in the superheate~ ~lquld at temperatures greeter than +120 C (E_ct - 64.9 kJ-mol- ). The apparent activation energy is negative in the liquid state between -60 an~ +40"C because moisture becomes a more effective inhibitor as the temperature increases. The molecular weights of the polyeplchlorohydrln produced by irradiation were between 7,000 and 11,000 Daltons, indicating that chain transfer is probably not significant in this system. The polymeric product was e viscous resin. As observed prevlously for cyclohexene oxide, it was necessary to dry the eplchlorohydrln carefully prior to cationic radiation polymerization but, since the polymerization reaction proceeds by a free radical mechanism at the higher temperatures, such precautions should not be necessary for radiation polymerization st temperatures over +120°C. Some dechlorlnatlon of the polymeric product was observed as a result of irradiation, but the authors noted that the dechlorlnatlon process appeared to be independent of the polymerization reaction; the polymer had a polyether backbone. It is also possible that the production of chloride ions by the reaction of free electrons with the chlorine atoms in the eplchlorohydrln molecule reduces the rate of the Ion-lon neutralization chain termination reaction and, thereby, increases the polymer production in this system. In contrast to free radical polymerization processes, radlatlon-lnduced cationic polymerizations could be promoted in the presence of air since molecular oxygen acts as an electron scavenger.

In contrast to most other allphatlc epoxldes, both styrene oxide and eplchlorohydrln polymerlze under the influence of ionizing radiation. This is likely to be due to the relatlve stsbillty imparted to the propagating intermediates by their substltuent groups (phenyl and chloro).

AlIphat ic Epoxies

Early studies (Cordlshl, Melt and Somogyl, 1967; Ueno and co-workers, 1967; Tabata and Sobue, 1967) showed that radiation is not a very effective initiator for polymerization of allphatlc e xy compounds. Recently, Kichlglna and co-workers (1983) have demonstrated that irradiation (~Co 7- rays at a dose rate of 8.3 Gy.s -I) of ethylene oxide frozen in a glassy matrix of chlorobutaue at -273°C leads to polymer formation. The polymerization reaction occurs as the irradiated mixture is warmed. Any moisture present in the system does not inhibit the cationic polymerlzatlou reaction because it crystallizes as a separate phase during the solidification of the ethylene oxide - chlorobutaue mixture. The vlscoslty-average molecular weight of the poly(ethylene oxide) was 2,000 Daltons for a 5Z solutlon of ethylene oxide in chlorobutaue irradiated to 130 _k~y. The polymer yleld under these conditions was about 16Z, G(-N) = 182 molecules*(100 eV)- ~ based on the energy absorbed in the ethylene oxide and G(-m) = 9.1 molecules*(100 eV) -I based on the energy absorbed in the mixture. It may be that the free electrons produced during radlolysls are reacting with the chlorobutane to produce chloride ions, thus reducing the rate of the blmolecular termination reaction and promoting the formation of a

590 L W DicgsoN and A SINOH

h i g h polymer i n t h i s s y s t e m . I r r a d i a t i o n of e t h y l e n e o x i d e in m a t r i c e s of d i m e t h y l f o r m s m i d e and e t h a n o l ( c a t i o n s c a v e n g e r s ) d i d no t r e s u l t in polymer f o r m a t i o n . Th i s t e c h n i q u e of p o e t - i r r a d i a - t i o n d u r i n g d e v L t r i f i c a t i o n of a c h l o r o b u t a n e m a t r i x o f f e r s good p o t e n t i a l f o r i n d u c i n g c a t i o n i c p o l y m e r i z a t i o n of e p o x i e s w i t h o u t e x t e n s i v e d r y i n g of the monomer p r i o r to i r r a d i a t i o n .

D i f u n c t i o n a l Epox i e s

Onal and c o - w o r k e r s (1983) s t u d i e d t he 7 - r a d i a t i o n - i n d u c e d p o l y m e r i z ~ t t o n of 1 , 2 - e p o x y - 4 - e p o x y - e t h y l c y c l o h e x a n e (EECH) a t 25°C and a f i x e d dose r a t e of 0.461 G y * s - ' . They d e t e r m i n e d by i n f r a - r ed and n u c l e a r m a g n e t i c r e s o n a n c e s p e c t r o s c o p y t h a t the a l i c y c l i c epoxy group p o l y m e r i z e d a t doses up to 66 kGy and c o n t i n u e d i r r a d i a t i o n r e s u l t e d in c r o s s l i n k i n g by r e a c t i o n of the a l i p h a t i c epoxy group. The c r o s s l i n k e d polymer was i n s o l u b l e in a l l of t he common s o l v e n t s t e s t e d and had a s o f t e n i n g t e m p e r a t u r e g r e a t e r t han 330°C. The polymer y i e l d was 7.23Z a t a dose of 39 .8 kGy, G(-m) - 125 m o l e c u l e s ° ( l O 0 e V ) - l ; a 90 .9~ y i e l d of c r o s s l L n k e d polymer was o b t a i n e d a t a dose of 254 kGy, G(-m) - 247 m e l e c u l e 8 , ( l O 0 eV) -~ . When a sample c o n t a i n i n g O.05Z w a t e r was i r r a d i a t e d t o 254 kGy, a 7.9Z polymer y i e l d was o b t a i n e d . Th i s o b s e r v a t i o n i n d i c a t e s t h a t some p o r t l o n of the p o l y m e r i z a t i o n of EECH p roceeds by an i o n i c mechanism. Al though the dose r e q u i r e d f o r p o l y m e r i z a - t i o n of EECE i s r a t h e r h i g h , i t would appear t h a t r a d i a t i o n - i n d u c e d p o l y m e r i z a t i o n l e a d s to a p o l y m e r i c s o l i d w i t h p o t e n t i a l l y u s e f u l p r o p e r t i e s .

Murata (1972) s t u d i e d t he 7 - r a d i a t i o n - i n d u c e d p o l y m e r i z a t i o n of 1 , 2 - e p o x y - 3 - b u t e n e and i t s g r a f t c o p o l y m e r i z a t i o n to p o l y e t h y l e n e . Murata obse rved polymer y i e l d s up to 36Z a t 960 kGy; t h l s c o r r e s p o n d s t o G(-m) = 52 m o l e c u l e 8 ° ( l O 0 eV)- . There was no s i g n i f i c a n t e f f e c t of dose r a t e on polymer y i e l d ove r the d o s e - r a t e r ange s t u d i e d (0 .083 to 0 .694 Gy-8-1) . I n f r a r e d s p e c t r a of t h e polymer showed t h a t t h e epoxy f u n c t i o n a l groups po l ymer t zed w i t h r a d i a t i o n but t he v i n y l groups d id n o t . C o n f i r m a t i o n of t h i s c o n c l u s i o n was p r o v i d e d by the o b s e r v a t i o n t h a t the r a d i a t i o n - c u r e d po lymer c r o s s l t n k e d on s t a n d i n g in t he p r e s e n c e of a t~nospheric oxygen. The epoxy group p o l y m e r i - z a t i o n i8 l i k e l y to be f avoured ove r v i n y l p o l y m e r i z a t i o n f o r 1 , 2 - e p o x y - 3 - b u t e n e because of the r e l a t i v e s t a b i l i t y of the a l l y l i n t e r m e d i a t e produced by epoxy r i n S open in S. Though some degree of i n h i b i t i o n was no t ed in the p r e s e n c e of f r e e r a d i c a l s c a v e n g e r s , a d d i t i o n a l work would be r e q u i r e d to d e t e r m i n e w he the r the p o l y m e r i z a t i o n p r o c e s s p roceeds by a c a t i o n i c or f r e e r a d i c a l mechanism. The paper d id not d i s c u s s whe the r 1 , 2 - o r 1 , 4 - p o l y e t h e r l i n k a g e s p redomina t e in t he p o l y m e r i c p r o d u c t .

S l d y a k t n , Karpov and Egorov (1972) h a ~ s t u d i e d the p o l y m e r i z a t i o n of epoxy r e s i n s based on b i s p h e n o l - A ~nd e p i c h l o r o h y d r i n w i t h v Co ¥ - r a y s a t doses between 0 .5 and 80 MGy and a dose r a t e of 3 .9 Gy ° s - . The major obse rved r e a c t i o n s were d e g r a d a t i o n r e a c t i o n s but r a d i a t i o n - i n d u c e d c r o s s l t n k i n S be tween h y d r o x y l groups on one polymer c h a i n and epoxy groups on a n o t h e r was a l s o n o t e d . The c r o s s l i n k t n g r e a c t i o n was most i m p o r t a n t when the r a t i o of h y d r o x y l to epoxy groups in the r e s i n was 1 : 1 .

CURING OF EPOXIES IN MIXTURES

Since radiation alone 18 not a particularly effective initiator of polymerization for many epoxy compounds, several different techniques for promoting the radiation polymerization of epoxies have been i n v e s t i g a t e d . The f o l l o w i n g s e c t i o n s summarize t he l i t e r a t u r e on r a d i a t i o n c u r i n g of e p o x y - c a t a l y s t m i x t u r e s , e p o x y - v i n y l monomer m i x t u r e s and a c r y l a t e d epoxy compounds.

R a d i a t i o n Cur ing of Epoxy - C a t a l y s t M i x t u r e s

S e v e r a l d i f f e r e n t i n v e s t i g a t i o n s of the e f f e c t s of p o t e n t i a l l y c a t a l y t i c a d d i t i v e s on t h e r a d i a t i o n - i n d u c e d p o l y m e r i z a t i o n of epoxy compounds have been per formed. The use of c a t a l y t i c a d d i t i v e s w i t h r a d i a t i o n t r e a t m e n t cou ld i n c r e a s e the r a t e of the p o l y m e r i z a t i o n p r o c e s s or r educe t h e t e m p e r a t u r e r e q u i r e d compared to the c o n d i t i o n s fo r c h e m i c a l l y c a t a l y z e d c u r i n g . Omel~chenko and c o - w o r k e r s (1977) showed t h a t p r im a r y amines d e c r e a s e t he dose r e q u i r e d fo r r a d i a t i o n - i n d u c e d c r o s s l i n k i n g of epoxy r e s i n s w i t h u n s a t u r a t e d o l t g o e s t e r s . P r i s h c h e p a , F a J z i and S h t r y a e v a (1980) showed t h a t 5Z d i m e t h y l a m t n o e t h y l m e t h a c r y l a t e r e duces the dose r e q u i r e d fo r s o l i d i f i c a t i o n of EHD-20 r e s i n , a p a r t i a l l y m e t h e c r y l a t e d epoxy r e s i n , from 50-70 kCy to 20 kCy. Kozlov , Doroshenko and M e l e s h e v i c h (1983) and M e l e s h e v i c h , Kozlov and Doroshenko (1984) showed t h a t f e r r o c e n e , t r t - p h e n y l s u l f o n l u m b o r o f l u o r L d e , pheny ld i azon tum b o r o f l u o r i d e and dLphenyl iedonium b o r o f l u o r i d e promote the r a d i a t i o n p o l y m e r i z a t i o n of e p i c h l o r o h y d r t n . T e r e n t ' e v , Duvaktna and N i k o l a e v (1984) studied the mechanism of the catalysis of epoxy radiation polymerization by malelc anhydride. These studies show that there i8 potential for the promotion of radiation-induced polymerization of epoxy compounds with catalytic additives, but the additives studied to date do not appear to be u n i v e r s a l l y a p p l l c a b l e .

R a d i a t i o n Cur ing of Epoxy - V i n y l Monomer M i x t u r e s

S e v e r a l s t u d i e s of the c u r i n g of e p o x i e s i n t he p r e s e n c e of u n s a t u r a t e d monomers have been conduc ted . Kuzne teova and co -worke r s (1978a) showed t h a t r a d i a t i o n c u r i n g of a m i x t u r e of epoxy olLgomer, a c r y l a t e and s t y r e n e gave a h i g h e r g e l c o n t e n t t han c h e m l c a l l y i n i t i a t e d c u r i n g of t h e same m i x t u r e and t h a t p o s t - l r r a d i a t l o n h e a t t r e a t m e n t f u r t h e r i n c r e a s e s the Eel c o n t e n t to 92- 98Z. Kuzne t sova and co -worke r s (1978c) a l s o showed t h a t a dose of i 00 t o 120 kGy i s r e q u i r e d f o r c o m p l e t e c u r i n g of an epoxy o l l g o m e r - a c r y l i c a c i d m i x t u r e . P e r s l n e n (1982) showed t h a t , in m i x t u r e s of epoxy r e s i n s w i t h a c r y l i c a c i d , t h e r e i s some r eac tLon of the epoxy w i t h t he a c i d p r o d u c i n g t he u n s a t u r a t e d e s t e r p r i o r to i r r a d i a t i o n , the r a d i a t i o n c h e m i c a l y l e l d of C-C r e a c t i o n

6th International Meeting on Radmtmn Processing 591

i s 400 t imes t h a t of e pox ide r e a c t i o n , and c r o s s l i n k i n g i s enhanced by p o s t - i r r a d i a t i o n h e a t i n g o r by i r r a d i a t i o n a t e l e v a t e d t e m p e r a t u r e s . Vinhev and co -worke r s (1983) d e t e r m i n e d t h a t a p o s t - i r r a d i a t i o n t h e r m a l t r e a t m e n t of an epoxy o l i g o m e r - a c r y l a t e - po lyemide - p l a s t i c i z e r m i x t u r e gave b e t t e r p r o p e r t i e s t o the f i n a l p r oduc t than t h e r m a l c u r i n g of the same m i x t u r e . Barun in and c o - w o r k e r s (1983) used a c o m b i n a t i o n of i r r a d i a t i o n end h e a t t r e a t m e n t to produce a p roduc t w i t h h i g h s t r e n g t h and t h e r m a l s t a b i l i t y from a m i x t u r e of a b i s p h e n o l - A epoxy r e s i n , p o l y i m i d e and a c r y l i c a c i d or a c r y l a u i d e . Ivanov and c o - w o r k e r s (1985) showed t h a t the cu re t ime f o r an epoxy - a c r y l a t e - amine ~ i x t u r e i s 100 t i m e s s h o r t e r in the p r e s e n c e of e l e c t r o n - b e e m r a d i a t i o n (dose r a t e = 1.6 kGy. s - ~ , t o t a l dose - 70 to 100 kGy) than fo r t h e r m a l c u r i n g . S e v e r a l s t u d i e s of the use of r a d i a t i o n f o r c r o s s l l n k l n g of e p o x i e s w l t h u n s a t u r a t e d o l l g o e s t e r s have a l s o been per formed (Kuzne t sova and c o - w o r k e r s , 1978b; 0m e l ' c he nko , Ponomarenko and V l d e n l n a , 1978; B r z o s t o w s k l and Pietrzak, 1982; Furman and co-workers, 1983). These studies of the use of radiation for the curing of mixtures of epoxies with unsaturated compounds show that it is possible to obtain polymeric products with potentlally useful properties by this technique.

Radiation Curln~ of Acr71ated Epoxies

One method for imparting good radiation curing properties to an epoxy resin is to acrylate the terminal epoxy groups of an epoxy ollgomer. Gotoda and co-workers (1974a and b) and Gotoda, Miyashlta and Takeysma (1975) studied the radlatlon-lnduced polymerlsatlon of epoxy-acrylate prepolymers in the presence and absence of several vinyl monomers; they found that an acrylic acid based epoxy acrylate cured at a dose of g i0 kG~. Kumanotanl and co-workers (1977) determined that electron-beam curing (dose rate - 7 kGyos -~) o i epoxy acrylates gave a larger amount of crossllnklng than UV curing (dose rate = 0.2 kGy°s- ). Pyun and co-workers (1977) studied the use of electron beams for curing of a methacryllc ester of a low molecular weight dlglycldyl ether epoxy resin. Vasil'evs and co-workers (1977) found that a modified epoxy resin (NEO 20A) could be crossllnked in the presence of styrene at a dose of 5 kGy. Rot, Shlryaeva and Chernyakov (1978) investigated the use of a radiation hardened epoxy acrylate as s coating for wood. Omel'chenko and co-workers (1982) showed that an acryllc acid modified epoxy ollgomer cured by irradiation in the presence of a sensitizer to a dose of 200 kGy had a gel fraction of 97.2%. Videnlna and Bratslavskaya (1982) determined that epoxy acrylate mixtures containing 20 to 50% allphatlc epoxy a c r y l a t e s ( r e m a i n d e r a r o m a t i c epoxy a c r y l a t e s ) r a d i a t i o n cured w i t h e l e c t r o n beams (dose r a t e - 8 k G y ° s - ' ) gave t he b e s t p h y s i c a l and m e c h a n i c a l p r o p e r t i e s . Yakov leva and co -worke r s (1984) s t u d i e d the e l e c t r o n - b e a m c u r i n g of s e v e r a l r e s i n s , i n c l u d i n g an epoxy a c r y l a t e . T h a l a c k e r and B o e t t c h e r (1985) d e t e r m i n e d t h a t the t he r m a l s t a b i l i t y of e l e c t r o n beam cured a c r y l a t e d e p o x i e s i n c r e a s e s w i t h the number of a c r y l a t e d epox ide groups p r e s e n t in the p a r e n t compound. Thompson, Song and Wilkes (1987) have found that electron-beam curing of the dlglycldyl methacrylate of blsphenol-A to T dose of 100 kGy gives a polymer with a gel fraction of about 98%. Since acrylated epoxies are now being promoted for use in structural automotive composites (Babblngton and co-workers, 1987), radiation processing may have potential for appllcetlon in this growing market.

CONCLUSIONS

Most epoxy compounds polymerlze with hlgh-energy radiation by a cationic mechanism. Cationic polymerlzatlon reactions are inhibited by water even at trace Impurlty levels and, thus, most epoxies are not currently good candidates for industrlal-scale radiation polymerization. Addi- tional research would be required to identify conditions under which radlatlon-lnltlated cationic polymerlzatlon is not inhibited by water. The radlatlon-lnduced polymerlzatlons of styrene oxide and eplchlorohydrln (at temperatures over its boiling point) are not inhibited by water since their radiation curing reactions occur by free radical mechanisms. However, the radiation poly- merlzstlon of styrene oxide produces mainly low molecular weight products and special vessels would be required to polymerlze eplchlorohydrln with radiation at temperatures higher than its hoillng point. The dlfunctlonal epoxies 1,2-epoxy-4-epoxyethylcyclohexane and 1,2-epoxy-3-butene appear to produce useful polymeric products on radiation treatment. Acrylated derivatives of epoxy ollgomers and mixtures of epoxy compounds with vinyl monomers also cure satlsfactorily with radiation. These types of resin may be the most suitable for commercial production of epoxy polymers with radiation.

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radiation curing of epoxies

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