Chemorheology of Thermosetting Polymers - American Chemical

7 Cross-linking Reaction of an Epoxy Resin with Phthalic Anhydride

Downloaded by UNIV OF ARIZONA on December 3, 2012 | http://pubs.acs.org Publication Date: August 29, 1982 | doi: 10.1021/bk-1982-0227.ch007

E L B E R T W. C R A N D A L L and WINSTON Μ Ι Η

1

Pittsburg State University, Pittsburg, KS 66762

A study of the crosslinking of an epoxy resin with phtha lic anhydride catalyzed by an imidazole catalyst has been carried out using differential scanning calorimetry, middle infrared and near infrared. The data from these methods indicate the process is a stepwise mechanism in volving first anhydride ring opening, followed by epoxy ring opening and hydroxyl formation. Ether crosslinks begin to form after epoxy ring opening and the last step is formation of ester crosslinks. A study has been undertaken t o attempt t o determine a p o s s i b l e mechanism f o r the c r o s s l i n k i n g o f an epoxy r e s i n w i t h phtha l i c anhydride u s i n g an i m i d a z o l e d e r i v a t i v e as the c a t a l y s t . I n t h i s study we have used d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) to measure the heat o f r e a c t i o n and time o f c u r i n g p r o c e s s . C r o s s l i n k e d polymers were s t u d i e d by middle and near i n f r a r e d . The m i d d l e IR c o u l d be used t o look a t the C-0 e s t e r and e t h e r bands and the secondary h y d r o x y l i n the 1000 t o 1500 cm" r e g i o n . However, more u s e f u l were the r e s u l t s from the near IR i n which the epoxy C-H, C=0, and 0-H bands can be compared t o the f i r s t overtone o f the 3.5. n M C-H band which appears a t 1.66 μ M i n the near IR. DCS and near IR data have been o b t a i n e d on samples i n which time o f c u r i n g and c o n c e n t r a t i o n s o f p h t h a l i c anhydride and i m i d a z o l e c a t a l y s t have been v a r i e d . Several authors (1-6) have proposed t h a t the c r o s s l i n k i n g r e a c t i o n s o f epoxy r e s i n s w i t h anhydrides i s a two o r t h r e e s t e p p r o c e s s : f o r m a t i o n o f the monoester, f o r m a t i o n o f the d i e s t e r and f o r m a t i o n o f e t h e r c r o s s l i n k s . I t i s hoped t h a t t h i s work may a s s i s t i n a b e t t e r u n d e r s t a n d i n g o f t h i s p r o c e s s . 1

Experimental The epoxy prepolymer used was Dow Chemical's D.E.R. 331, 1

Current address: California State University, Chico, C A

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0097-6156/83/0227-0113$06.00/0 © 1983 American Chemical Society In Chemorheology of Thermosetting Polymers; May, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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m o l e c u l a r weight 390. Reagent grade p h t h a l i c anhydride was pur chased from General Chemical Company under the Baker and Adamson l a b e l . The amine c a t a l y s t used was Curezol 2E4MZ-CN 3 ( 2 - e t h y l 4 - m e t h y l i m i d a z o l y l ) p r o p a n e n i t r i l e , o b t a i n e d from Shikoku Chemi cals Corporation. The DSC data were o b t a i n e d on a P e r k i n - E l m e r DSC-1B by s e a l i n g samples weighed on a s i x p l a c e balance i n t o v o l a t i l e sample pans. The samples were prepared by weighing the proper amount o f epoxy r e s i n and f i n e l y d i v i d e d p h t h a l i c anhydride and m i x i n g . T h i s mix t u r e was p l a c e d on a c l a y p l a t e and ground w i t h a s p a t u l a u n t i l a homogeneous m i x t u r e was o b t a i n e d . The i n s t r u m e n t was operated a t 10°K per minute on range 8. The areas under the curve were determined w i t h a p l a n i m e t e r and the Δ Η o f c u r i n g were c a l c u l ated as d e s c r i b e d i n the P e r k i n - E l m e r manual ( 7 J . The samples f o r the middle IR were o b t a i n e d by c u r i n g the epoxy, p h t h a l i c anhydride m i x t u r e s i n metal p l a n c e t s w i t h covers to prevent s u b l i m a t i o n . The samples were then heated i n an oven s e t a t the proper temperature. A f t e r c u r i n g , the samples were ground t o a f i n e powder and made i n t o a window u s i n g a KBr m i n i p r e s s . Samples f o r the near IR were prepared a s g i v e n f o r the middle IR o r were cured between g l a s s p l a t e s i n an oven s e t a t the proper temperature. S p e c t r a were run on a Beckman DK-2A. Discussion The r e a c t i o n o f the epoxy r e s i n w i t h p h t h a l i c anhydride u s i n g curezol c a t a l y s t , 3(2-ethyl-4-methylimidazolyl) propanenitrile, shows an exotherm w i t h i t s minimum a t 154°C and a c u r i n g time o f f i v e t o s i x minutes depending on the c o n c e n t r a t i o n o f p h t h a l i c anhydride ( F i g u r e 1 ) . A t mole r a t i o s o f e p o x y / p h t h a l i c anhydride of 1.7-1, 2-1, and 3.5-1 a s m a l l exotherm appears a t 117°C before the main exotherm a t 154°C. T h i s exotherm decreases i n i n t e n s i t y w i t h d e c r e a s i n g p h t h a l i c a n h y d r i d e c o n c e n t r a t i o n . The heats o f r e a c t i o n f o r the c u r i n g process a r e g i v e n i n Table I . Table I . DSC D a t a Mole R a t i o Epoxy/Phthalic H (Cal/gram) Anhydride No P.A. -54. 12-1 -51.66 -49.38 5-1 -43.13 3.5-1 -32.98 2-1 -26.00 1.7-1 a

C a t a l y s t c o n c e n t r a t i o n 0.19 mole c u r a z o l / m o l e epoxy r e s i n

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


CRANDALL AND

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MIH

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Epoxy Resin Cross-linking Reaction

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425

450

Temperature (Kelvin)

F i g u r e 1.

Thermogram o f C a t a l y z e d Cure.


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Two trends are observed from these d a t a . The heat o f r e a c t i o n de creases w i t h i n c r e a s i n g c o n c e n t r a t i o n o f p h t h a l i c anhydride and i n c r e a s e s as c a t a l y s t c o n c e n t r a t i o n i s i n c r e a s e d . The values ob t a i n e d are i n l i n e w i t h those given by Fava (8) and M a l a v a s i c e t a l . ( 9 ) . Using somewhat d i f f e r e n t systems, Fava g i v e s values o f 54-87 cal/gram, w h i l e M a l a v a s i c g i v e s exothermic values o f 74-80 cal/gram. When no p h t h a l i c anhydride i s p r e s e n t , u s i n g o n l y the i m i d a z o l e c a t a l y s t , an exothermic v a l u e o f 54 cal/gram i s ob t a i n e d . Since the l a t t e r i n v o l v e s epoxy r i n g opening and forma t i o n of e t h e r c r o s s l i n k s , i t must be assumed t h a t these processes are exothermic. The lower exotherm w i t h the anhydride systems i n d i c a t e s t h a t the heat o f formation o f the e s t e r l i n k a g e s i s lower than t h a t r e s u l t i n g from e t h e r f o r m a t i o n . The small exotherm a t 117°C preceding the main exotherm a t 154°C i s dependent on the c o n c e n t r a t i o n o f p h t h a l i c anhydride i t appearing a t h i g h e r c o n c e n t r a t i o n s o f p h t h a l i c anhydride. A m i x t u r e o f p h t h a l i c anhydride and i m i d a z o l e c a t a l y s t w i t h no epoxy r e s i n present shows a broad, low exotherm s t a r t i n g a t 125°C. T h i s suggests t h a t anhydride r i n g opening occurs as the f i r s t s t e p i n the process. The middle i n f r a r e d r e g i o n o f the cured c r o s s l i n k e d polymers shows bands i n the 3480 cm" , the 1200-1300 cm" and the 1000-1100 cm-1 r e g i o n , the l a t t e r two i n c r e a s e w i t h p h t h a l i c anhydride con c e n t r a t i o n . A band a t 1420 cm" along w i t h the 3480 cm" 0-H s t r e t c h band appears i n the s p e c t r a o f the epoxy r e s i n cured w i t h c u r e z o l o n l y . T h i s band i s a weak band i n the uncured epoxy r e s i n and i s i n t e r p r e t e d t o be a s s o c i a t e d w i t h a secondary hydroxyl group a r i s i n g from epoxy r i n g opening (10). Bands a t 1080 cm" and 1290 cm" a l s o are present and are probably the C-0 s t r e t c h bands o f e t h e r c r o s s l i n k s . Upon the a d d i t i o n o f p h t h a l i c anhy d r i d e new bands appear a t 1110 cm" , 1160" and 1280" , which are assumed t o be the C-0 s t r e t c h i n g o f e s t e r groups. Whereas the e t h e r and e s t e r bands i n c r e a s e w i t h i n c r e a s i n g p h t h a l i c anhydride c o n c e n t r a t i o n the 1420 cm" and 3480 cm" hydroxyl bands remain constant. The near i n f r a r e d r e g i o n has been shown t o be very u s e f u l i n s t u d y i n g polymer systems (11^, 12). A m i x t u r e o f epoxy r e s i n and p h t h a l i c anhydride before a d d i t i o n o f c a t a l y s t and heat shows a very s t r o n g C-H band i n the 2.19 Ρ M r e g i o n which has been shown t o be a hydrogen attached to an epoxy group (13). In the 2.16. μ M r e g i o n the C-H bands o f the a r y l r i n g s appear. The 2.0 - 2.5. μΜ r e g i o n i s the r e g i o n o f a l k y l and a r y l C-H combi n a t i o n bands. The second overtone o f the 5.7. μ M C=0 shows up a t 1.91. μ M which i n the m i x t u r e i s very weak. In a d d i t i o n the f i r s t overtone o f the 3.5. M C-H s t r e t c h appears a t 1.66. μ M, and the f i r s t overtone of the 2.8. Ρ M 0-H s t r e t c h i s seen a t 1.42 yM. 1

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Upon a d d i t i o n o f c a t a l y s t and heat, t h r e e changes appear i n the near IR s p e c t r a . The epoxy band decreases r a p i d l y , t h e 0-H band i n c r e a s e s and t h e C=0 band a t 1.91 shows a l a r g e i n c r e a s e (Tables I I and I I I ) . Table I I shows t h e e f f e c t s o f p h t h a l i c an hydride c o n c e n t r a t i o n . Using t h e 1.66 y M band as an i n t e r n a l standard a c c o r d i n g t o t h e method o f Henniker ( 1 4 ) , t h e 1.91 y M carbonyl and t h e 1.42 y M hydroxyl were compared t o t h e 1.66 y M C-H band. I n c r e a s i n g t h e c o n c e n t r a t i o n o f p h t h a l i c anhydride causes an i n c r e a s e i n t h e i n t e n s i t y o f t h e carbonyl band which becomes a maximum a t a mole r a t i o o f 1.7 t o 1. T h i s carbonyl c o u l d be due t o monoester o r d i e s t e r i n t h e f i n a l product. The hydroxyl band a t 1.42 y M on t h e o t h e r hand remains f a i r l y con stant. Table I I . Near I n f r a r e d Band R a t i o s : E f f e c t o f C o n c e n t r a t i o n Epoxy/P.A. Catalyst . 0-H C=0 Mole R a t i o C-H C-H Mole χ 1 0 " 4

12-1

0.26 0.39 0.29 0.31 0.29 0.33 0.35 0.33 0.38 0.33

5-1 3.5-1 2-1 1.7-1

a

0.34 0.46 0.38 0.44 0.58 0.54 0.65 0.43 0.61 0.64

8 12 8 12 24 24 28 20 32 40

A l l samples cured a t 200°C f o r 8 min.

A study o f c u r i n g times i s recorded i n Table I I I . Three changes a r e o c c u r r i n g d u r i n g t h e c u r i n g process. I n i t i a l l y t h e epoxy band goes t o a minimum w i t h i n t h e f i r s t minute. The c a r bonyl i n c r e a s e s s l o w l y and a t t a i n s a maximum a f t e r s i x minutes, w h i l e t h e hydroxyl band i n c r e a s e s f o r one minute and then r e mains f a i r l y c o n s t a n t . While the c u r i n g process i s complex and i n v o l v e s s e v e r a l s t e p s , some o f t h e steps can be deduced from t h e data d e s c r i b e d here. The appearance o f t h e exotherm preceding t h e main exo therm i n t h e DSC suggests t h a t anhydride r i n g opening occurs as the f i r s t step t o g i v e a carboxy anion as suggested by F i s c h e r (15). The r a p i d f o r m a t i o n o f hydroxyl and decrease o f epoxy bands i n d i c a t e s t h a t i n the e a r l y stages o f the c u r i n g p r o c e s s , the carboxy anion i s l e a d i n g t o epoxy r i n g opening and the f o r mation o f pendant hydroxyl groups. While t h i s i s o c c u r r i n g , e t h e r c r o s s l i n k s a r e f o r m i n g , t h e l a s t p a r t o f t h e c u r i n g curve appears t o be f o r m a t i o n o f e s t e r l i n k s . The data suggests t h a t the pendant hydroxyl groups a r e probably n o t i n v o l v e d i n e s t e r f o r m a t i o n t o any e x t e n t .



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Conclusions


DSC, middle IR and near IR data suggest t h a t the c r o s s l i n k i n g r e a c t i o n o f an epoxy r e s i n w i t h p h t h a l i c anhydride i s a s t e p w i s e process as f o l l o w s . 1. anhydride r i n g opening 2. epoxy r i n g opening and f o r m a t i o n o f pendant hydroxyl groups 3. e t h e r l i n k s begin t o form 4. f o r m a t i o n o f e s t e r c r o s s l i n k s Table I I I . Near I n f r a r e d Data: E f f e c t o f Time Minutes 160°C 1 2 6 0. 0. 0.09 0.54 Ί3Γ C-H 0.26 0.51 0.37 0.41 0-H C-H 0.17 0.11 0.13 0.47 Epoxy C-H 200°C 0. 0.14 0.15 0.50 C°0 C-H 0.26 0.31 0.32 0.44 0-H C-H 0.47 0.10 0.10 0.08 Epoxy C-H

Literature Cited 1. Fisch, W.; Hofmann, W. Makromol. Chem. 1961,44,8. 2. Shechter, L.; Wynstra, J. Ind. Eng. Chem. 1956,48,86. 3. Kannebley, G. Kunstoffe. 1957,47,693. 4. Fischer, R.F. J. Polym. Sci. 1960,44,155. 5. Arnold, R.J. Mod. Plast. 1964,41,149. 6. Batzer, H.; Porret, D.; Lohse, F. Makromol. Chem. 1966,91,195. 7. Instructions for Differential Scanning Calorimeter, Perkin-Elmer, Norwalk, Conn. April 1974. 8. Fava, R.A. Polymer. 1968,9,137. 9. Malavasic, T.; Moze, Α.; Vizovisek, I.; Lapanje, S. Makromol. Chem. 1975,44,89. 10. Meloan, C.E. "Elementary Infrared Spectroscopy". Macmillan Company. New York, NY 1963. 11. Crandall, E.W.; Johnson, E.L.; Smith, C.H. J. Appl. Polym. Sci. 1975,19, 897.


7.

12. 13. 14. 15.

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Crandall, E.W.; Jagtap, A.N. J. Appl. Polym. Sci. 1977,21,449. Goddu, R.F.; Delker, D.A. Anal. Chem. 1953,30,2013. Henniker, J.C. "Infrared Spectroscopy of Industrial Polymers", Academic Press, London and New York, 1967, p. 153. Fischer, F.R. Ind. Eng. Chem. 1960,52,321.


RECEIVED M a y 10, 1983


Chemorheology of Thermosetting Polymers - American Chemical

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