3
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Kinetics of Acetylene-Terminated Resin Cure and Its Effects on Dynamic Mechanical Properties I. J. GOLDFARB, C. Y - C . L E E , and C. C. K U O
1
Materials Laboratory (AFWAL/MLBP), Air Force Wright Aeronautical Laboratories, Wright-Patterson Air Force Base, O H 45433
Kinetics of the cure of acetylene terminated resins can be obtained by differential scanning calorimetry (DSC) of the reaction exotherm and by disappearance of the ethynyl band in the infrared spectrum. Multiple scan DSC curves at various heating rates were utilized to obtain kinetic parameters for an acetylene terminated sulfone (ATS) which were compared with isothermal reaction rates. The comparison between scanning and isothermal reaction kinetics illustrated the retarding effect of incipient vitrification of the ATS on the reaction rate. Glass transition temperatures (Tg) were determined for partially cured resins as a function of the extent of cure by dynamic mechanical analysis. This allowed the determination of the reduced parameter (T-Tg) during cure of the resin under a variety of time-temperature profiles. The dynamic mechanical properties were then estimated as a function of (T-Tg). This estimate is compared with the results of torsion impregnated cloth analysis of the curing of the ATS resin. Recent advances i n a c e t y l e n e terminated r e s i n technology have provided a f a m i l y o f new high performance s t r u c t u r a l mater i a l s (JU2). One member o f t h i s f a m i l y , t h e a c e t y l e n e terminated s u l f o n e (ÏÏTS) (3) has been made a v a i l a b l e i n s u f f i c i e n t quant i t i e s t o a l l o w research e f f o r t s on c h a r a c t e r i z a t i o n and d e t e r m i n a t i o n o f t h e e f f e c t s o f s t r u c t u r e on p h y s i c a l and mechanical p r o p e r t i e s (4-(>). In a companion paper {7) t h e e f f e c t s o f environment on t h e cure o f ATS was r e p o r t e d . This paper r e p o r t s 1
Current address: University of Dayton Research Institute, Dayton, O H 45469
This chapter not subject to U.S. copyright. Published 1983, American Chemical Society May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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CHEMORHEOLOGY OF THERMOSETTING
POLYMERS
on t h e e f f e c t s o f t h e k i n e t i c s o f t h e ATS cure and t h e r e s u l t a n t Tg on f u r t h e r c u r e . EXPERIMENTAL Material
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The uncured r e s i n used i n t h i s study has t h e b a s i c s t r u c t u r e
J
n
where t h e monomer ( n = l ) , 4 , 4 ' b i s ( 3 - e t h y n y l p h e n o x y ) d i p h e n y l s u l f o n e , represents^77% o f t h e r e s i n . The o t h e r components a r e p r i m a r i l y n=0,2,3 o l i g o m e r s . An average v a l u e o f n=1.23 i s computed based on heat o f r e a c t i o n compared t o pure monomer. Extent o f Cure The e x t e n t s o f cure were determined u t i l i z i n g DSC and FT-IR. DSC experiments were conducted on a P e r k i n - E l m e r DSC-2 instrument. The method f o r o b t a i n i n g k i n e t i c parameters from scanning DSC has been d e s c r i b e d elsewhere ( 8 ) . Isothermal e x t e n t s o f cure were determined by c u r i n g i n t h e DSC f o r a predetermined time under n i t r o g e n and then scanning a t 10°K/min to determine t h e r e s i d u a l exothermic heat o f r e a c t i o n as compared w i t h a scan o f uncured r e s i n . I n f r a r e d measurements o f e x t e n t o f cure under c o n d i t i o n s s i m i l a r t o t h e TICA experiments were conducted. ATS was c a s t from methylene c h l o r i d e s o l u t i o n onto KBr windows and, a f t e r vacuum e v a p o r a t i o n o f a l l s o l v e n t , t h e KBr windows were put i n t o the Rheometrics RMS environmental chamber and were s u b j e c t e d t o a temperature p r o f i l e under n i t r o g e n i d e n t i c a l t o t h e mechanical measurement experiments (2°C/min). The windows were removed one a t a time a t v a r i o u s temperatures and IR s p e c t r a were taken a t room temperature. TICA Experiments The T o r s i o n Impregnated C l o t h A n a l y s i s (TICA) measurements u t i l i z e d here were conducted under n i t r o g e n w i t h a Rheometrics Mechanical Spectrometer (RMS) a c c o r d i n g t o t h e procedures des c r i b e d i n t h e p r e v i o u s paper (_7).
May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
3.
GOLDFARB E T A L .
Acetylene-Terminated Resin Cure Kinetics
RESULTS AND DISCUSSION K i n e t i c s o f ATS Cure
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The k i n e t i c s o f the cure o f ATS a t 130°C i n n i t r o g e n de termined by v a r i o u s methods are shown i n F i g u r e 1. The dashed curve r e p r e s e n t s the p r e d i c t i o n based on k i n e t i c parameters determined by a n a l y s i s o f scanning DSC data by the method o f m u l t i p l e h e a t i n g r a t e s d e s c r i b e d p r e v i o u s l y ( 8 ) . T h i s method u t i l i z e s a r a t e e q u a t i o n o f the form: A f ( a ) e
dt "
(1)
where a = degree o f c o n v e r s i o n and f ( a ) i s a r a t e f u n c t i o n a l i t y determined by the a n a l y s i s a t l e a s t i n t a b u l a r form. To produce an i s o t h e r m a l c u r v e , g
(
a
)
=
« f _ * ^ 0
or
t
( 2 )
J Af(a')
t = g( )-e a
-e-ί/ΚΓ.
E / R T
(3)
The f u n c t i o n g(a) o f e q u a t i o n (3) i s determined by numer i c a l l y i n t e g r a t i n g 1/Af(a) vs cr and i s t a b u l a t e d a l o n g w i t h A f ( a ) d u r i n g the computer a n a l y s i s o f the DSC d a t a . Because o f the scanning r a t e s i n the k i n e t i c a n a l y s i s (5,10,20,40 and 80°K/min) the p r e d i c t e d curve i n F i g u r e 1 i s seen t o e f f e c t i v e l y proceed t o completion (a=l) even though 130°C i s much below the Τ o f the completely cured r e s i n (366°C). T h i s k i n e t i c a n a l y s i s i s thus i n s e n s i t i v e t o the v i t r i f i c a t i o n which we s h a l l subsequently show does indeed occur d u r i n g such a low temperature c u r i n g r e a c t i o n . The s o l i d curve i s the a c t u a l 130°C i s o t h e r m a l r e a c t i o n curve. The c i r c l e s show measurements o f α determined by the d i s a p p e a r ance o f the a c e t y l e n e IR band a t 941 cm ( 6 ) . The squares show the DSC r e s i d u a l heat measurements. I t should be noted t h a t a problem occurs i n the a n a l y s i s o f the DSC data s i n c e another exotherm i s observed i n a c e t y l e n e t e r m i n a t e d s u l f o n e s a t h i g h e r temperatures than the r e a c t i o n exotherm but s u f f i c i e n t l y low so as t o o v e r l a p w i t h the end o f the r e a c t i o n exotherm. The r e s i d u a l heat data were determined by e s t i m a t i n g the c o n t r i b u t i o n o f the second exotherm and g r a p h i c a l l y s u b t r a c t i n g i t from the t o t a l heat e v o l v e d . While t h i s approximate method produces c o n s i d e r a b l e s c a t t e r i n the d a t a , i t appears t o agree q u i t e w e l l w i t h the i n f r a r e d d a t a . With t h i s data the e f f e c t o f i n c i p i e n t v i t r i f i c a t i o n and subsequent l o s s of m o b i l i t y i s q u i t e e v i d e n t . The r e a c t i o n was c a r r i e d out f o r 89 hours t o make sure t h a t the r e a c t i o n had 9
May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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CHEMORHEOLOGY OF THERMOSETTING
May; Chemorheology of Thermosetting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
POLYMERS
3.
GOLDFARB ET A L .
Acetylene-Terminated Resin Cure Kinetics
indeed ceased and a f i n a l c o n v e r s i o n α (130°) * .72 i s observed. A s i m i l a r isothermal r e a c t i o n a t 180°C°can be p l o t t e d on t h e same graph by a p p l y i n g a time s h i f t f a c t o r e-E/RU/Tj-l/Tg) which i n t h i s case equals 18.8 where 1\ = 403°K, T = 453°K and Ε = 21.3 Kcal/mole. T h i s i s shown by t h e d o t t e d l i n e where the t r i a n g l e s r e p r e s e n t t h e FT-IR c o n v e r s i o n data ( 6 ) . As would be expected t h e two curves superimpose a t low e x t e n t s o f conver s i o n b u t a t h i g h e r c o n v e r s i o n s t h e 180° curve c o n t i n u e s t o r i s e beyond t h e 130°C curve t o a f i n a l c o n v e r s i o n , a J l S O ) - .80. Glass T r a n s i t i o n Temperatures ( T ) β
2
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0
g
Τ o f ATS r e s i n as a f u n c t i o n o f t h e e x t e n t o f cure were determined u t i l i z i n g t h e c r i t e r i o n o f t h e maximum o f t h e l o s s modulus i n TICA experiments on t h e RMS a t 1.6 Η (10 r a d / s e c ) . Several methods were u t i l i z e d t o i d e n t i f y t h e r and α v a l u e s . The values along w i t h t h e methods o f measurements a r e given i n Table I . P o i n t s No. 2,3,4,6, and 7 r e p r e s e n t Τ as d e s c r i b e d i n t h e p r e v i o u s paper (7) which a r e a t low enough temperatures t o be t h e instantaneous Τ f o r t h e r e a c t i o n time given i n t h e l a s t column o f Table I . By comparison w i t h F i g u r e 1, one o b t a i n s t h e value o f
