Epoxy Resin Chemistry II - American Chemical Society

12 Isothermal Cure Kinetics of an Epoxy Resin Prepreg G A R Y L. H A G N A U E R , B E R N A R D R. L A L I B E R T E , and D A V I D A. D U N N

Downloaded by FORDHAM UNIV on August 18, 2014 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch012

Polymer Research Division, Army Materials and Mechanics Research Center, Watertown, MA 02172 The isothermal cure kinetics of a glass fiber-epoxy resin prepreg was investigated over the temperature range 80 to 135°C using high performance l i q u i d chromatography (HPLC) and d i f f e r e n t i a l scanning calorimetry (DSC). Gel content was monitored gravimetrically. The prepreg resin (ca.32%by weight) consisted of three different types of epoxy resins, an accelerator (Monuron) and the curing agent dicyandiamide. Cure kinetics parameters determined by DSC correlated with compositional parameters determined by HPLC. The curing chemistry of the prepreg resin was found to be highly dependent upon the cure temperature. Both the extent of reaction at the onset of gelation and crosslink density of the reaction products compared at the same % gelation increased with increasing temperature. The magnitude of the temperature effect suggests that curing temperature could have a profound effect on the network structure of the cured resin matrix and, therefore, on the properties of composites manufactured from this prepreg. The i s o t h e r m a l c u r e k i n e t i c s o f a g l a s s f i b e r - e p o x y r e s i n prepreg i s i n v e s t i g a t e d u s i n g h i g h performance l i q u i d chromato g r a p h y (HPLC) and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC^. The p r e p r e g h a s a recommended c u r e t e m p e r a t u r e o f 121 C ( 2 5 0 F ) and i s used i n t h e m a n u f a c t u r e o f s t r u c t u r a l c o m p o s i t e s . The p r e p r e g r e s i n ( c a . 32?> by w e i g h t ) c o n s i s t s o f t h r e e d i f f e r e n t t y p e s o f epoxy r e s i n s , an a c c e l e r a t o r ( M o n u r o n ) , and t h e c u r i n g a g e n t d i c y a n d i a m i d e . HPLC i s used t o d e t e r m i n e changes i n t h e c o m p o s i t i o n o f t h e r e s i n components and DSC i s a p p l i e d t o m o n i t o r the o v e r a l l e x t e n t o f r e a c t i o n a s a f u n c t i o n o f c u r e t i m e o v e r the t e m p e r a t u r e r a n g e SO t o 135 C. A d d i t i o n a l l y , g e l c o n t e n t i s a n a l y z e d t o r e l a t e c u r e k i n e t i c s t o t h e f o r m a t i o n and s t r u c t u r e o f t h e c r o s s l i n k e d epoxy r e s i n m a t r i x . The p u r p o s e o f t h i s work i s t o g a i n an u n d e r s t a n d i n g o f t h e c u r i n g b e h a v i o r o f t h e p r e p r e g and t o d e t e r m i n e w h e t h e r v a r i a t i o n s i n c u r i n g c o n d i t i o n s may have a s i g n i f i c a n t e f f e c t on t h e p r o p e r t i e s and r e l i a b i l i t y o f t h e composite m a t e r i a l . This chapter not subject to U.S. copyright. Published 1983, American Chemical Society

In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Experimental The p r e p r e g c o n s i s t s o f u n i d i r e c t i o n a l S2 g l a s s f i b e r s ( c a . by w e i g h t ) i m p r e g n a t e d w i t h a " s t a g e d " o r p a r t i a l l y r e a c t e d epoxy r e s i n f o r m u l a t i o n . B e c a u s e o f p o s s i b l e b a t c h - t o - b a t c h v a r i a t i o n s i n prepreg c o m p o s i t i o n , a l l t h e specimens f o r t h i s s t u d y were s e l e c t e d from t h e same p r e p r e g r o l l . In a d d i t i o n , e x p e r i m e n t s were c o n d u c t e d t o a s c e r t a i n t h e optimum s p e c i m e n s i z e f o r r e p r e s e n t a t i v e s a m p l i n g and f o r d e t e r m i n i n g r e s i n c o m p o s i t i o n . To p r e v e n t changes i n c o m p o s i t i o n due t o a g i n g , t h e p r e p r e g was s t o r e d i n a s e a l e d c o n t a i n e r a t -1M C and removed only f o r sampling. The p r i n c i p a l components i n t h e r e s i n were i s o l a t e d by p r e p a r a t i v e l i q u i d c h r o m a t o g r a p h y and i d e n t i f i e d u s i n g HPLC and s p e c t r o s c o p i c t e c h n i q u e s (J_ 2). The r e s i n components a r e l i s t e d i n T a b l e I . C o n c e n t r a t i o n s o f t h e components i n t h e as r e c e i v e d " s t a g e d " p r e p r e g r e s i n and i n a sample o f t h e " u n s t a g e d " r e s i n f o r m u l a t i o n a r e g i v e n . I t i s noted t h a t the staged r e s i n i s not h i g h l y a d v a n c e d . The a c t u a l e x t e n t o f r e a c t i o n based upon r e a c t e d epoxy and amine f u n c t i o n a l i t i e s i s p r o b a b l y no more t h a n 1 o r 2%. The epoxy e q u i v a l e n t w e i g h t s i n T a b l e I were o b t a i n e d from t h e m a n u f a c t u r e r ' s t e c h n i c a l l i t e r a t u r e . Monuron and d i c y were assumed t o have 1 and U e q u i v a l e n t s p e r m o l e , r e s p e c t i v e l y . The t o t a l epoxy and amine e q u i v a l e n t s i n t h e u n s t a g e d r e s i n f o r m u l a t i o n a r e c l o s e t o s t o i c h i o m e t r i c ; i . e . , 0.85 E q . amine p e r 1 Eq. e p o x i d e . P a r t i a l l y c u r e d p r e p r e g s f o r HPLC a n a l y s i s were o b t a i n e d by h e a t i n g s a m p l e s (1.7g) f o r v a r y i n g p e r i o d s o f t i m e i n a t h e r m o s t a t e d o v e n . I m m e d i a t e l y a f t e r t h e s a m p l e s were removed and c o o l e d , s o l u t i o n s were p r e p a r e d by e x t r a c t i n g t h e weighed p r e p r e g w i t h THF. S o l u t i o n c o n c e n t r a t i o n s ( c a . 1 0 y g / y l ) were c a l c u l a t e d f r o m t h e d i f f e r e n c e between t h e i n i t i a l w e i g h t o f t h e p r e p r e g and t h e f i n a l w e i g h t o f t h e f i b e r s a f t e r e x t r a c t i o n , d r y i n g , and t r e a t m e n t i n a m u f f l e f u r n a c e a t 700-800 C. The weight percentage o f i n s o l u b l e r e a c t i o n products ( g e l content) was c a l c u l a t e d from t h e w e i g h t o f t h e n o n - e x t r a c t a b l e o r g a n i c m a t e r i a l on t h e g l a s s f i b e r s . The e x t r a c t a b l e r e s i n was essentially f u l l y soluble. The s o l u t i o n s were p r e p a r e d i n v o l u m e t r i c f l a s k s and were f i l t e r e d t h r o u g h 0.2μιΜ M i l l i p o r e membrane f i l t e r s . A W a t e r s A s s o c i a t e s ALC/GPC-2144 i n s t r u m e n t w i t h 6000A s o l v e n t d e l i v e r y s y s t e m , 660 s o l v e n t programmer, 710B WISP a u t o i n j e c t i o n s y s t e m and a P e r k i n E l n e r LC75 v a r i a b l e UV a b s o r b a n c e d e t e c t o r was used f o r t h e HPLC a n a l y s e s . A l l a n a l y s e s were r u n u s i n g a W a t e r s uBondapak C ^ (30cm χ 3.4mm ID) column and a S p e c t r a P h y s i c s SP4000 d a t a s y s t e m f o r peak i n t e g r a t i o n and d a t a formatting. Reagent g r a d e w a t e r was p r e p a r e d from d i s t i l l e d water u s i n g a M i l i p o r e M i l l i - Q 2 water p u r i f i c a t i o n system. D i s t i l l e d - i n - g l a s s , UV g r a d e t e t r a h y d r o f u r a n (THF) was used as r e c e i v e d from B u r d i c k & J a c k s o n L a b s . Sample s o l u t i o n s were analyzed using the f o l l o w i n g c o n d i t i o n s : i n j e c t i o n volume t
5cal/g ( r e s i n ) b u t showed no dependence on r e a c t i o n t e m p e r a t u r e . However, t h e % r e s i d u a l h e a t d e c r e a s e d f r o m 33% a t 80 C t o l e s s t h a n 1% f o r s a m p l e s c u r e d above 130°C ( F i g u r e 5 ) · Also, i n determining q i t was n o t e d t h a t t h e t e m p e r a t u r e o f t h e e x o t h e r m peak maximum Τ increased with increasing isothermal cure temperature ( F i g u r e d ) . The i n c o m p l e t e r e a c t i o n o f t h e i s o t h e r m a l c u r e s and t h e i n c r e a s e i n t h e dynamic DSC e x o t h e r m t e m p e r a t u r e a r e most l i k e l y r e l a t e d t o c h a n g e s i n t h e g l a s s t r a n s i t i o n t e m p e r a t u r e Τ . As t h e r e s i n c u r e s and t h e Τ rises above t h e i s o t h e r m a l cur§ t e m p e r a t u r e , d i f f u s i o n t a k e s c o n t r o l fc

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237

R a t i o o f M o l e s D i c y R e a c t e d P e r A v e r a g e M o l e s Epoxy R e a c t e d P l o t t e d V e r s u s I s o t h e r m a l C u r i n g Time a t D i f f e r e n t Temperatures.


EPOXY RESIN CHEMISTRY


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and c u r i n g e v e n t u a l l y c e a s e s b e f o r e a l l t h e f u n c t i o n a l g r o u p s have r e a c t e d . The d i f f e r e n c e i n q^. v a l u e s i s a t t r i b u t e d t o v a r i a t i o n s i n r e s i n c o n t e n t due t o t h e s m a l l s i z e o f t h e DSC s a m p l e s . T h e r e f o r e , i t was n e c e s s a r y t o d e t e r m i n e q^ f o r each sample i n o r d e r t o o b t a i n a c c u r a t e e x t e n t s o f r e a c t i o n a. a t t i m e t . f o r isothermal cures; i . e . , 1

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j=l i n t e g r a t e d u n d e r t h e DSC e x o t h e r m o v e r t h e t i m e i n t e r v a l t ^ O t o t^, and q i s t h e t o t a l h e a t o f r e a c t i o n o r i n t e g r a t e d a r e a . The e x t e n t o r r e a c t i o n i s assumed t o be p r o p o r t i o n a l t o t h e h e a t evolved during cure. The t i m e / t e m p e r a t u r e dependence o f t h e i s o t h e r m a l c u r e s i s i l l u s t r a t e d i n F i g u r e 6. The i s o t h e r m a l c u r e k i n e t i c s were e v a l u a t e d u s i n g t h e f o l l o w i n g equation (3) (2)

(1-α)'

where ά i s t h e r a t e o f r e a c t i o n , and are rate constants, and m and η a r e e x p o n e n t s r e l a t e d t o t h e o r d e r o f t h e r e a c t i o n . This expression describes the autocatalytic character of the epoxy c u r e where s t o i c h i o m e t r i c amounts o f amines and e p o x i e s a r e present. F u r t h e r m o r e , i t i s assumed t h a t t h e amine p r o t o n s a r e equally reactive. The k i n e t i c p a r a m e t e r s were c a l c u l a t e d a c c o r d i n g t o t h e method o f Ryan and D u t t a (£) i n w h i c h t h e r e a c t i o n i s c o n s i d e r e d t o be s e c o n d o r d e r , m+n=2, and t h e peak o f t h e i s o t h e r m a l c u r v e i s d e f i n e d by = 0

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f r o m K and t h e r a t e ά and e x t e n t o f r e a c t i o n α a t t h e peak o f r e a c t i o n r a t e c u r v e α %s cl ( F i g u r e 7). S i g n i f i c a n t r e s u l t s a r e i l l u s t r a t e d i n F i g u r e s 5 and 7· As the i s o t h e r m a l c u r i n g temperature i s r a i s e d , the onset o f g e l a t i o n t a k e s p l a c e a t h i g h e r e x t e n t s o f r e a c t i o n and t h e f o r m a t i o n o f an i n s o l u b l e g e l n e t w o r k o c c u r s o v e r a n a r r o w e r c u r i n g range. The e x t e n t o f r e a c t i o n a t t h e e x o t h e r m peak α d e c r e a s e s f r o m 0.35 a t 90°C t o 0.30 a t 130°C and e x h i b i t s a marked change between 110° and 120°C. The k i n e t i c e x p o n e n t m u n d e r g o e s a s i m i l a r c h a n g e . I n F i g u r e 5, t h e r a t i o s o f t h e mole % o f Monuron and d i c y r e a c t e d t o t h e mole % o f epoxy components r e a c t e d a t t h e o n s e t o f g e l a t i o n i s p l o t t e d vs_ t h e i s o t h e r m a l curing temperature. B o t h r a t i o s i n c r e a s e w i t h t e m p e r a t u r e and show a l a r g e change ( t r a n s i t i o n ) o v e r a r e l a t i v e l y n a r r o w temperature range. The s h a r p i n c r e a s e i n t h e r e a c t i o n o f d i c y corresponds t o the decrease noted f o r α and m between 110 and 120°C. _ The r a t e o f r e a c t i o n à and t i m e t t o t h e e x o t h e r m peak and t h e c a l c u l a t e d r a t e c o n s t a n t s K^ and K ^ f o l l o w an A r r h e n i u s dependence on t e m p e r a t u r e o v e r t h e r a n g e 90-130 C ( F i g u r e 8 ) . The a c t i v a t i o n e n e r g i e s a r e 2 3 . 8 , 2 2 . 1 , 23.2 and 2 0 . 9 K c a l / m o l f o r ά , t , K^ and K^, r e s p e c t i v e l y , and a r e i n e x c e l l e n t a g r e e m e n t w i t h t h e v a l u e 22.0 K c a l / m o l c a l c u l a t e d f r o m t h e t i m e t o t h e o n s e t o f g e l a t i o n d a t a . The d a t a p o i n t s f o r are connected w i t h a curved l i n e t o i l l u s t r a t e the c o m p l e x i t y of the t e m p e r a t u r e dependence o f t h e c u r i n g r e a c t i o n . The a c t i v a t i o n e n e r g y i n c r e a s e s f r o m 13.1 K c a l / m o l between 9 0 t o 100 C, t o 27.7 K c a l / m o l between 100 and 115°C, t o 35.2 K c a l / m o l e between 120 and 130 C. The c h a n g e s i n a c t i v a t i o n e n e r g y r e l a t e t o t e m p e r a t u r e i n t e r v a l s b e f o r e , d u r i n g and a f t e r t h e s h a r p i n c r e a s e i n t h e r a t i o s Monuron/epoxy and d i c y / e p o x y . 1

P


P

P

P

Conclusions ( 1 ) The c u r i n g c h e m i s t r y o f t h e p r e p r e g r e s i n i s h i g h l y d e p e n d e n t upon t h e c u r e t e m p e r a t u r e . ( 2 ) The c r o s s l i n k d e n s i t y o f r e a c t i o n p r o d u c t s compared a t t h e same * g e l a t i o n i n c r e a s e w i t h i n c r e a s i n g c u r e t e m p e r a t u r e . ( 3 ) B o t h HPLC and DSC show t h a t t h e e x t e n t o f r e a c t i o n a t the onset of g e l a t i o n i n c r e a s e s with i n c r e a s i n g cure temperature. ( 4 ) The e x t e n t o f r e a c t i o n o v e r w h i c h an i n s o l u b l e g e l p r o d u c t forms becomes n a r r o w e r w i t h i n c r e a s i n g c u r e t e m p e r a t u r e . ( 5 ) T h e r e a r e c o r r e l a t i o n s between i s o t h e r m a l c u r e k i n e t i c s p a r a m e t e r s d e t e r m i n e d by DSC and c o m p o s i t i o n a l p a r a m e t e r s d e t e r m i n e d by HPLC. ( 6 ) DSC measurements a r e s e n s i t i v e t o c h a n g e s i n t h e c u r i n g chemistry of the prepreg r e s i n . ( 7 ) P r e c a u t i o n s a r e a d v i s e d when a t t e m p t i n g t o i n t e r p r e t t h e DSC measurements. Since the c o m p o s i t i o n of the r e a c t i o n product i s temperature dependent, t h e r e are problems i n d e f i n i n g α and c o m p a r i n g DSC d a t a o b t a i n e d a t d i f f e r e n t t e m p e r a t u r e s .




0.20

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0.4

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T e m p e r a t u r e Dependence o f t h e S /q*. ), t h e R e s i d u a l res t E x o t h e r m Peak Maximum (T ) , t h e K i n e t i c Exponent max (m), t h e E x t e n t o f R e a c t i o n a t t h e I s o t h e r m a l Peak Maximum ( a ) and t h e R a t i o s o f Mole 1 Components Reacted. e

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243


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Figure 8.

Arrhenius P l o t s . Température Dependence o f t h e I s o t h e r m a l C u r i n g Time ( t ) , t h e R a t e o f R e a c t i o n (ά ) Ρ a t t h e E x o t h e r m Maximum, and o f t h e K i n e t i c s R a t e C o n s t a n t s ( K , and K ) . p

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EPOXY RESIN

244

CHEMISTRY

(8) The magnitude of the temperature e f f e c t on c u r i n g chemistry and the observation that the e f f e c t p e r s i s t s to r e l a t i v e l y high extents of r e a c t i o n suggests that c u r i n g temperature may have a profound e f f e c t on the network s t r u c t u r e of the cured r e s i n matrix and, t h e r e f o r e , on the p r o p e r t i e s of the composite. Likewise, v a r i a t i o n s i n the cure c y c l e or heating r a t e could a f f e c t the r e l i a b i l i t y o f the composite. Comments


The isothermal cure k i n e t i c s of the prepreg has c h a r a c t e r i s t i c s s i m i l a r to those reported f o r the neat r e s i n ( 5 ) . A u t o c a t a l y s i s and d i f f u s i o n contro? are observed and s i m i l a r heats of r e a c t i o n and a c t i v a t i o n energies are determined. The extents of r e a c t i o n at the gel point estimated by DSC and t o r s i o n a l b r a i d a n a l y s i s of the neat r e s i n ( s ) correspond to values determined g r a v i m e t r i c a l l y i n t h i s study. The low value of the r e a c t i o n r a t i o dicy/epoxy at the lower c u r i n g temperatures would e x p l a i n the unreacted d i c y noted by Schneider e t . a l ( 5 ) . Eventhough the prepreg has a complex, heterogeneous r e s i n formulation and t^ere are questions concerning the equivalency of the amine protons and the d i c y c u r i n g mechanism (£,9), s u r p r i s i n g l y c o n s i s t e n t r e s u l t s are obtained using Equation 2 and assuming second order k i n e t i c s . The temperature dependence o f α , m, , dicy/epoxy and Monuron/epoxy i n d i c a t e s a complicated c 8 r i n g mechanism and suggests that the formation and s t r u c t u r e of the c r o s s l i n k e d r e s i n matrix are h i g h l y dependent on c u r i n g c o n d i t i o n s . Hence v a r i a t i o n s i n c u r i n g c o n d i t i o n s may be expected to have a s i g n i f i c a n t e f f e c t on the p r o p e r t i e s and r e l i a b i l i t y of the composite m a t e r i a l .

Literature Cited

A-1,

1. Hagnauer, G. L.; Setton, I. J. Liq. Chromatogr., 1, 55 (1978). 2. Hagnauer, G. L.; Dunn, D. A. "Materials 1980", 12th National SAMPE Technical Conference, Seattle, WA, 1980, Vol. 12, p. 648. 3. Sourour, S.; Kamal, M. R. Thermochimica Acta, 14, 41 (1976). 4. Ryan, M. E.; Dutta, A. Polymer, 20, 203 (1979). 5. Schneider, N. S.; Sprouse, J. F.; Hagnauer, G. L.; Gillham, J. K. Polym. Eng. Sci., 19, 304 (1979). 6. Saunders, T. F.; Levy, M. F.; Seriono, J. F. J. Polym. Sci., 5, 1609 (1967). 7. Son, P.; Weber, C. D. J. Appl. Polym. Sci., 17, 1305 (1973). 8. Zahir, S. A. "Proc. 6th Intern. Conf. in Organic Coatings Science and Technology", July 14-18, 1980, Athens, Greece, p. 745. 9. LaLiberte, B. R.; Bornstein, J., Army Materials and Mechanics Research Center (1981), TR 81-34. RECEIVED December 16, 1982


Epoxy Resin Chemistry II - American Chemical Society

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