StructureProperty Relations of Composite Matrices - American

18 Structure-Property Relations of Composite Matrices

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 30, 2015 | http://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch018

ROGER J. MORGAN and ELENO T. MONES Lawrence Livermore Laboratory, University of California, Livermore, CA 94550

The need to conserve energy has provoked increased interest i n the use and development of high-performance, light-weight, fibrous composites for the transportation industry, e.g., for use i n a i r craft and automobiles, and i n energy-storage systems such as f l y -wheels (1). The matrices u t i l i z e d i n these high-performance composites, which can be exposed to extreme environments, have generally been epoxies. The question of the d u r a b i l i t y of the epoxy matrix and of the o v e r a l l composite i n such extreme environments i s a cause for concern (2,3). However, such d u r a b i l i t i e s cannot be predicted accurately without a knowledge of the structure, modes of deformation and f a i l u r e , mechanical response r e l a tionships of the epoxy matrices, and the possible modification of such relationships by fabrication procedures and the service environment. The structure-property relations of epoxies, however, have received little attention compared to other commonly utilized polymer glasses. The epoxy glasses, because of variation of their chemical and physical structure with fabrication conditions and because of their i n f u s i b l e , insoluble nature, are more difficult to study than noncrosslinked polymeric glasses. In this paper, we will review our structure-property studies of epoxies that are commonly utilized as composite matrices. The two systems primarily studied were: (1) diethylene triamine (Eastman)-cured bisphenol-A-diglycidyl ether (Dow, DER 332) epoxy (DGEBA-DETA); and (2) diaminodiphenyl sulfone (Ciba-Geigy, Eporal)-cured t e t r a g l y c i d y l 4,4'diaminodiphenyl methane (CibaGeigy, MY 720) epoxy (TGDDM-DDS). Physical Structure The major physical and structural parameters that control the modes of deformation and f a i l u r e as well as the mechanical responses of epoxies are their crosslinked network structures and microvoid characteristics (4-9). 0-8412-0567-l/80/47-132-233$05.00/0 © 1980 American Chemical Society In Resins for Aerospace; May, Clayton A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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RESINS F O R A E R O S P A C E

The cure process and f i n a l network s t r u c t u r e o f epoxies have been deduced from the chemistry o f the system, p r o v i d i n g the c u r i n g r e a c t i o n s are known and can be assumed to go t o completion, and by experimental techniques such as i n f r a r e d and carbon-13 nuclear magnetic resonance spectroscopy and s w e l l i n g , u l t r a s o n i c , dynamic mechanical, thermal c o n d u c t i v i t y , and d i f f e r e n t i a l scanning c a l o r i m e t r y measurements (see Ref. 8). However, i n many epoxy systems the chemical r e a c t i o n s are d i f f u s i o n c o n t r o l l e d and incomplete, and there i s a heterogeneous d i s t r i b u t i o n i n the crosslink density. High c r o s s l i n k - d e n s i t y regions from 6 t o 10^ nm i n diameter have been observed i n epoxies (8) . The most recent s t u d i e s on the network morphologies o f epoxies were c a r r i e d out by Racich and Koutsky (10) and Mason and co-workers (11). These workers s t u d i e d both etched and non-etched surfaces o f epoxies by scanning e l e c tron microscopy as w e l l as by transmission e l e c t r o n microscopy o f carbon-platinum surface r e p l i c a s . In our s t u d i e s , we a l s o s t r a i n e d f i l m s d i r e c t l y i n the e l e c t r o n microscope (6). F o r example, i n DGEBA-DETA epoxies, we observed that 6- to 9-nm-diam p a r t i c l e s remain i n t a c t and flow past one another during the flow processes. We suggested that the 6- to 9-nm-diam p a r t i c l e s were molecular domains that were i n t r a m o l e c u l a r l y c r o s s l i n k e d and t h a t formed during the i n i t i a l stages o f p o l y m e r i z a t i o n . These domains were interconnected by regions o f low c r o s s l i n k d e n s i t y , which allowed flow to occur i n t h i s network. The d u c t i l e mechanical r e sponse o f the DGEBA-DETA epoxies and the s t r a i n - r a t e and thermalh i s t o r y dependence o f t h i s response are c o n s i s t e n t with the premise that regions o f low c r o s s l i n k d e n s i t y c o n t r o l the flow processes (JL2). Hence, d e t a i l s o f the network morphology are needed f o r a b a s i c understanding o f the s t r u c t u r a l parameters that c o n t r o l the deformation processes and the mechanical response o f epoxies. More r e c e n t l y , we have observed under p o l a r i z e d l i g h t a network o f l a r g e r 1-mm-sized nodules i n B F 3 c a t a l y z e d TGDDM-DDS systems ( F i b e r i t e 934). These biréfringent networks, one o f which i s i l l u s t r a t e d i n the transmission o p t i c a l micrograph i n F i g u r e 1, break up under combined s t r e s s and temperatures o f >175°C. The b i r e f r i n g e n c e o r i g i n a t e s from the p r e f e r e n t i a l alignment o f the macromolecules w i t h i n the biréfringent network. T h i s molecular alignment only occurs i n B F 3 - c a t a l y z e d TGDDM-DDS epoxies. These networks may r e s u l t from an inhomogeneous d i s t r i b u t i o n o f the c a t a l y s t w i t h i n the TGDDM-DDS system, which would r e s u l t i n the high c a t a l y s t c o n c e n t r a t i o n regions p o l y m e r i z i n g a t a f a s t e r rate than t h e i r surroundings. The s t r e s s e s caused w i t h i n the r e s u l t a n t heterogeneous system during g e l a t i o n and g l a s s formation c o u l d produce the biréfringent network i l l u s t r a t e d i n F i g u r e 1. The microvoid c h a r a c t e r i s t i c s o f the epoxy are a l s o important i n c o n t r o l l i n g the mechanical response o f the g l a s s . M i c r o v o i d s can have a d e l e t e r i o u s e f f e c t on the mechanical p r o p e r t i e s o f epoxies by a c t i n g as s t r e s s concentrators and by s e r v i n g as a sink

In Resins for Aerospace; May, Clayton A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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f o r the accumulation o f sorbed moisture. M i c r o v o i d s can r e s u l t from trapping o f a i r i n the system during cure and from trapping low-molecular-weight m a t e r i a l , which i s subsequently e l i m i n a t e d during postcure, i n the g l a s s . This low-molecular-weight m a t e r i a l r e s u l t s e i t h e r from inhomogeneous mixing o f epoxide and c u r i n g agent or from the i n a b i l i t y o f the c o n s t i t u e n t s to r e a c t , with the r e s u l t a n t aggregation o f these c o n s t i t u e n t s . In polyamide-cured DGEBA epoxies, c r y s t a l s o f DGEBA epoxide monomer trapped i n the p a r t i a l l y cured r e s i n at room-temperature can produce microvoids by melting and v o l a t i l i z i n g during c e r t a i n postcure c o n d i t i o n s (J). A l s o , i n TGDDM-DDS epoxies, thermal anneal, moisture s o r p t i o n , and mechanical p r o p e r t y studies i n d i c a t e that the melting and v o l a t i l i z a t i o n o f unreacted DDS c r y s t a l l i t e s during cure produces microv o i d s i n these r e s i n s (9). In F i g u r e 2, voids and t h e i r t r a v e l paths during cure are i l l u s t r a t e d i n a sheet o f a commercial TGDDM-DDS epoxy (Narmco 5208). Such v o i d s only appear when the cure temperature exceeds the c r y s t a l l i n e melting p o i n t (162°C) of the DDS. Chemical S t r u c t u r e The chemical s t r u c t u r e o f epoxies can be complex. The s t r u c ture w i l l depend on s p e c i f i c cure c o n d i t i o n s , because more than one r e a c t i o n can occur and the k i n e t i c s o f each r e a c t i o n e x h i b i t s d i f f e r e n t temperature dependencies. In a d d i t i o n , the s t r u c t u r e i s a f f e c t e d by f a c t o r s such as s t e r i c and d i f f u s i o n a l r e s t r i c t i o n s of the reactants during cure (£r£r!3,1^,15,16), the presence o f i m p u r i t i e s that can a c t as c a t a l y s t s (17), the r e a c t i v i t y o f the epoxide and curing agent (18), i s o m e r i z a t i o n o f epoxide groups (18,19,20), inhomogeneous mixing o f the r e a c t a n t s {^,9) , and c y c l i c p o l y m e r i z a t i o n o f the growing chains (18). These f a c t o r s can lead t o network s t r u c t u r e s that are p h y s i c a l l y and chemically heterogeneous. In amine-cured epoxides, networks are g e n e r a l l y assumed to r e s u l t from a d d i t i o n r e a c t i o n s o f epoxide groups with primary and secondary amines (18), as i l l u s t r a t e d i n F i g u r e 3. Epoxides and amines with f u n c t i o n a l i t i e s >3 can form h i g h l y c r o s s l i n k e d network s t r u c t u r e s . However, c o n s i d e r a b l e evidence suggests that amine cured-epoxies, such as DGEBA-DETA and TGDDM-DDS systems, are not h i g h l y c r o s s l i n k e d . Such evidence includes t h e i r high temperature d u c t i l i t y (4,5,7,8,9,12), the e f f e c t s o f thermal h i s t o r y and s t r a i n rate on t h e i r mechanical response (12), and t h e i r m i c r o s c o p i c deformation and f a i l u r e processes (_4-9). Such observations suggest e i t h e r that few epoxide-secondary amine r e a c t i o n s occur, thus l i m i t i n g the number o f c r o s s l i n k s , or that polyether chains are formed by t r a n s - e t h e r i f i c a t i o n through a ring-opening homopolymerization o f the epoxide (17,18,19,21), as i l l u s t r a t e d i n F i g u r e 4. The epoxide-amine r e a c t i o n s are con t r o l l e d by the presence o f Η-bond donors, such as OH groups, which are necessary t o open the epoxide r i n g s (14,15,17). The t r a n s -




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Figure I.

Transmission optical micrograph (under pohrized light) of a network of aligned molecules in a BF -catalyzed TGDDM-DDS epoxy s

Figure 2. Transmission optical micrograph of voids and their travel paths caused by the melting and volatilization of unreacted DDS crystallites during cure in a TGDDM-DDS epoxy



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e t h e r i f i c a t i o n r e a c t i o n r e q u i r e s a t e r t i a r y amine as a c a t a l y s t and a Η-bond donor as a c o c a t a l y s t (17,19,21). Hence, the f i n a l chemical s t r u c t u r e o f the epoxy system can be complex, because i t w i l l depend on such parameters as: (1) the r e l a t i v e r a t e s o f the chemical r e a c t i o n s a t room temperature, (2) the f i n a l postcure temperature (3) the concentrations o f c a t a l y s t s such as sorbed moisture i n the system, and (4) s t e r i c r e s t r i c t i o n s that i n h i b i t r e a c t i o n s a t secondar y-ami ne s i t e s . In the case o f the TGDDM-DDS epoxy systems, there i s growing evidence that such networks do not form e x c l u s i v e l y from epoxideamine a d d i t i o n r e a c t i o n s . I n F i g u r e 5, the percentage o f epoxide groups consumed during cure, as determined by the disappearance o f the epoxide band a t 910 cm"* i n the i n f r a r e d s p e c t r a , are p l o t t e d versus cure c o n d i t i o n s f o r both a BF3~catalyzed TGDDM-DDS epoxy ( F i b e r i t e 934) and a non-catalyzed TGDDM-DDS epoxy (Narmco 5208). For a standard 177°C cure f o r 2.5 h, a l l the epoxide groups are consumed, w i t h i n experimental e r r o r , f o r the BF3-catalyzed system d e s p i t e the weight percent o f DDS i n t h i s system being w e l l below the s t o i c h i o m e t r i c q u a n t i t y necessary t o consume a l l the epoxide groups by normal epoxide-amine a d d i t i o n r e a c t i o n s . In the case o f the non-catalyzed system, ~35% o f the epoxide groups remain unreacted a f t e r c u r i n g a t 177°C f o r 2.5 h, and a l l such groups only r e a c t a f t e r exposure t o 300°C f o r 1 h. Fourier-transform infrared-spectroscopy studies u t i l i z i n g s p e c t r a l s t r i p p i n g , which r e v e a l s d i f f e r e n c e s i n the s p e c t r a recorded a t d i f f e r e n t stages o f cure, should r e v e a l information on the chemical r e a c t i o n s o c c u r r i n g that form these TGDDM-DDS epoxy networks. 1

Deformation

and F a i l u r e Processes

L o c a l i z e d p l a s t i c flow has been reported t o occur during the deformation and f a i l u r e processes o f epoxies; i n a number o f cases, the f r a c t u r e energies were two t o three times greater than the expected t h e o r e t i c a l estimate f o r p u r e l y b r i t t l e f r a c t u r e (see Ref. 0). However, no systematic s t u d i e s have been made t o e l u c i date the microscopic flow processes o c c u r r i n g during the deforma t i o n o f epoxies and t o determine the r e l a t i o n o f such flow processes to the network s t r u c t u r e . Our recent i n v e s t i g a t i o n s r e v e a l that both DGEBA-DETA and TGDDM-DDS epoxies deform and f a i l by a c r a z i n g process (4-9). Crazes were observed i n f i l m s e i t h e r s t r a i n e d d i r e c t l y i n the e l e c tron microscope or s t r a i n e d on a metal s u b s t r a t e . The f r a c t u r e topographies o f these epoxies, f r a c t u r e d as a f u n c t i o n o f temper ature and s t r a i n r a t e , are i n t e r p r e t e d i n terms o f a c r a z i n g process. The TGDDM-DDS epoxies a l s o deform t o a l i m i t e d extent by shear banding, as i n d i c a t e d by m u l t i p l e , unique r i g h t - a n g l e steps i n the fracture-topography i n i t i a t i o n r e g i o n , as i l l u s t r a t e d i n F i g u r e 6. Shear-band propagation i n these p a r t i a l l y c r o s s l i n k e d g l a s s e s produces s t r u c t u r a l l y weak planes because o f bond cleavage



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-R,—CH—CH„


Figure

3.

Epoxide-amine action

+H,NR

OH

addition re

I _R

η

_ C H — C H

0

—

NH—R

(—R—CH—CH ) 2

OH 1 H R—C—C

2

I Ο i H _ C _ C Η I Ο I H R__C—C Η I Ο H I 2 R — C — C Η I Ο 2

R

2

Figure 4.

Homopolymerization of epoxides

R cΗ

Cure conditions

Figure 5.

Percentage of reacted epoxide groups as a function of cure conditions for (TGDDM-DDS)-based epoxies



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caused during molecular flow. H u l l (22) and M i l l s (23) have both noted that the i n t e r s e c t i o n o f shear bands, which occurs a t r i g h t angles, causes a s t r e s s c o n c e n t r a t i o n . T h i s s t r e s s c o n c e n t r a t i o n i s s u f f i c i e n t to cause a crack to propagate through the s t r u c t u r a l l y weak planes caused by shear band propagation. These phenomena w i l l produce the m u l t i p l e r i g h t - a n g l e steps i n the f r a c t u r e topography. Mixed modes o f deformation that i n v o l v e both c r a z i n g and shear banding were a l s o observed i n the f r a c t u r e topography o f TGDDM-DDS epoxies. The type o f m i c r o s c o p i c deformat i o n that occurs i n epoxy matrices can p l a y a d i r e c t r o l e i n the environmental s e n s i t i v i t y and mechanical response o f the composite. Durability The d u r a b i l i t y o f epoxy matrices depends on many complex i n t e r a c t i n g phenomena. The f a c t o r s that c o n t r o l the c r i t i c a l path to u l t i m a t e f a i l u r e or unacceptable damage depend s p e c i f i c a l l y on the p a r t i c u l a r p r e v a i l i n g environmental c o n d i t i o n s . These environmental f a c t o r s include s e r v i c e s t r e s s e s , humidity, temperature, and s o l a r r a d i a t i o n . The combined e f f e c t s o f thermal h i s t o r y , moisture exposure and s t r e s s have a d e l e t e r i o u s e f f e c t on the p h y s i c a l and mechanical i n t e g r i t y o f epoxies. We have r e c e n t l y studied the e f f e c t o f s p e c i f i c combinations of moisture, heat, and s t r e s s on the p h y s i c a l s t r u c t u r e , f a i l u r e modes, and t e n s i l e mechanical p r o p e r t i e s o f TGDDM-DDS epoxies. Our main f i n d i n g s from these studies were as f o l l o w s . Sorbed moisture p l a s t i c i z e s TGDDM-DDS epoxies and lowers t h e i r t e n s i l e strengths, u l t i m a t e e l o n g a t i o n s , and moduli. The f r a c t u r e topographies o f the i n i t i a t i o n c a v i t y and mirror regions of these epoxies i n d i c a t e that sorbed moisture enhances the craze i n i t i a t i o n and propagation processes. The c r a z i n g process i s more s u s c e p t i b l e to sorbed moisture than to the g l a s s t r a n s i t i o n temperature (Tg), which can be explained i n terms o f l o c a l moisture concentrations enhancing the l o c a l c a v i t a t i o n and flow processes. Hence, m o d i f i c a t i o n o f Tg by sorbed moisture cannot be u t i l i z e d alone as a s e n s i t i v e guide to p r e d i c t d e t e r i o r a t i o n i n the mechani c a l response and, hence, the d u r a b i l i t y o f epoxies. We a l s o i n v e s t i g a t e d the e f f e c t o f i n c r e a s i n g s t r e s s l e v e l s on the subsequent moisture s o r p t i o n c h a r a c t e r i s t i c s o f i n i t i a l l y dry TGDDM-DDS epoxies. I n F i g u r e 7, the e q u i l i b r i u m moisture s o r p t i o n l e v e l s a f t e r ~40 days exposure t o 100% r e l a t i v e humidity (RH) at room temperature are p l o t t e d versus the s t r e s s l e v e l s t h a t were a p p l i e d to the epoxies p r i o r t o moisture s o r p t i o n . A l l data p o i n t s f a l l w i t h i n the shaded areas. S t r e s s e s i n the 0- t o 38-MPa range had no d e t e c t a b l e i n f l u e n c e on the subsequent moisture sorpt i o n l e v e l s . However, moisture s o r p t i o n l e v e l s i n c r e a s e s h a r p l y by up to -11% i n the 38- t o 43-MPa s t r e s s range. A t higher s t r e s s l e v e l s i n the 43- to 65-MPa range, i n which a few specimens a c t u a l l y broke, there i s o n l y a s l i g h t trend towards higher moisture s o r p t i o n l e v e l s with i n c r e a s i n g s t r e s s .




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Figure 6. Scanning electron micrographs illustrating multiple right-angle steps in the fracture-topography initiation region of a TGDDM-DDS epoxy

1-h room -tern peratu re stress (MPa)

Figure 7. Equilibrium weight percent moisture sorbed by a TGDDM-DDS epoxy at rehtive humidity of 100% and at 23°C vs. 1-hr constant-stress levels that were applied prior to moisture exposure



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The data i n F i g u r e 7 i n d i c a t e that the i n i t i a l stages o f craze-crack growth enhance the a c c e s s i b i l i t y o f moisture t o s o r p t i o n s i t e s t o a greater extent than the l a t e r stages o f growth. (The primary s o r p t i o n s i t e s w i t h i n the TGDDM-DDS epoxy are the hydroxyl, s u l f o n y l , and primary and secondary amine groups, a l l o f which are capable o f forming hydrogen bonds with water molecules.) The TGDDM-DDS epoxy specimens that f r a c t u r e d under constant l o a d were found to e x h i b i t s i m i l a r f r a c t u r e topographies as p r e v i o u s l y s t u d i e d specimens that f r a c t u r e d i n shorter times i n the 10"" t o 1 0 m i n " s t r a i n - r a t e r e g i o n (9 ). Such topographies have been i n t e r p r e t e d i n terms o f a craze-crack growth process (9) with c r a z i n g , followed by crack propagation, predominating i n the i n i t i a l stages o f f a i l u r e and crack propagat i o n alone predominating during the l a t e r stages o f f a i l u r e . The d i l a t a t i o n a l changes produced i n the epoxy g l a s s by the c r a z i n g process w i l l enhance the a c c e s s i b i l i t y o f moisture t o s o r p t i o n s i t e s w i t h i n the epoxy to a g r e a t e r extent than w i l l crack propagat i o n alone. Hence, the i n i t i a l stages o f f a i l u r e i n TGDDM-DDS epoxies w i l l enhance the a c c e s s i b i l i t y o f moisture to s o r p t i o n s i t e s to a greater extent than i n the l a t e r stages o f f a i l u r e . 2

1

1

One o f the more extreme environmental c o n d i t i o n s experienced by an epoxy composite matrix on a f i g h t e r a i r c r a f t occurs during a supersonic dash. The a i r c r a f t d i v e s from high a l t i t u d e s (outer surface temperature -20 t o -55°C) i n t o a supersonic, lowa l t i t u d e run during which the s u r f a c e temperature r i s e s i n minutes to 100 t o 150°C as a r e s u l t o f aerodynamic h e a t i n g . On reduct i o n o f speed, the outer s u r f a c e temperature drops extremely r a p i d l y a t r a t e s up to ~500°C/min, thus exposing the epoxy composite to a thermal s p i k e . S i m u l a t i o n o f such thermal s p i k e s has been shown t o i n c r e a s e the amount o f moisture sorbed by the epoxy or epoxy composite (24-30). However, a f t e r a c e r t a i n number of consecutive thermal s p i k e s , the amount o f moisture sorbed ceases to i n c r e a s e . Browning (26,29) has suggested such i n c r e a s e s r e s u l t from microcracks caused by the moisture and temperature g r a d i e n t s present during the thermal s p i k e . McKague (28) has r e c e n t l y noted that damage does not occur unless the thermal-spike maximum temperature exceeds the Tg o f the moist epoxy. We observed that the amount o f moisture sorbed by TGDDM-DDS epoxies i s enhanced by ~1.6 wt% a f t e r exposure to a 150°C thermal s p i k e . The s u r f a c e s o f the t h e r m a l l y - s p i k e d epoxies were examined by scanning e l e c t r o n microscopy f o r the presence o f surface microcracks. No s i g n i f i c a n t areas o f microcracking were observed i n any o f the specimens when examined under m a g n i f i c a t i o n s o f up to 30,000x. Hence, the a d d i t i o n a l moisture sorbed by the epoxies a f t e r exposure to thermal spikes i s not p r i m a r i l y caused by m i c r o c r a c k i n g . The primary mechanism by which t h e r m a l l y - s p i k e d epoxies sorb a d d i t i o n a l amounts o f moisture can be e x p l a i n e d i n terms o f moisture-induced free-volume changes. The molecular m o b i l i t y o f the epoxy i s enhanced as the T o f the epoxy-moisture system i s q


RESINS FOR


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approached at the high temperatures experienced during the thermal s p i k e . T h i s molecular m o b i l i t y i s s u f f i c i e n t to enhance the d i s s o c i a t i o n o f Η-bonds between the water molecules and a c t i v e s i t e s w i t h i n the epoxy. Although the ruptured Η-bonds can reform at a c t i v e s i t e s , there i s an o v e r a l l decrease i n the amount o f hydrogen bonding and a corresponding increase i n the m o b i l i t y o f the water molecules. The more mobile water molecules with fewer Η-bonds r e q u i r e a greater f r e e volume because hydrogen bonding g e n e r a l l y causes a volume decrease. The molecular m o b i l i t y o f the epoxy-moisture system during a thermal spike i s s u f f i c i e n t to allow c o n f i g u r a t i o n a l changes to occur w i t h i n the epoxy network that accommodates the greater f r e e volume r e q u i r e d by both the more mobile water molecules and the normal moisture-induced s w e l l i n g s t r e s s e s imposed on the epoxy. Such f r e e volume i n creases, which i n v o l v e permanent r o t a t i o n - i s o m e r i c p o p u l a t i o n changes w i t h i n the epoxy network, are frozen i n t o the epoxy g l a s s during the r a p i d cool-down p o r t i o n of the thermal s p i k e . The a d d i t i o n a l f r e e volume allows water molecules access to p r e v i o u s l y i n a c c e s s i b l e a c t i v e s i t e s w i t h i n the epoxy. To a l e s s e r extent, the rupture of c r o s s l i n k s , c r a z i n g and/or c r a c k i n g , and the l o s s of unreacted m a t e r i a l can a l s o c o n t r i b u t e to enhanced moisture s o r p t i o n a f t e r thermal-spike exposure. Thermal-spike exposure can cause s u r f a c e c r a z i n g and/or c r a c k i n g o f epoxies i f the moisture-induced s w e l l i n g s t r e s s e s , together with those s t r e s s e s that r e s u l t from temperature g r a d i e n t s and r e l a x a t i o n o f f a b r i c a t i o n s t r e s s e s , exceed the c r a z e - i n i t i a t i o n s t r e s s at the maximum thermal-spike temperature. Thicker epoxy specimens are more s u s c e p t i b l e to the growth o f permanent damage regions during thermal-spike exposure, because they are exposed to l a r g e r temperature gradients and shrinkage s t r e s s e s during cure, which i n turn produce l a r g e r f a b r i c a t i o n s t r e s s e s and s t r a i n s . Conclusions The s t r u c t u r e - p r o p e r t y r e l a t i o n s o f amine-cured epoxies and the m o d i f i c a t i o n o f such r e l a t i o n s by f a b r i c a t i o n and e n v i r o n mental f a c t o r s have been reviewed and our primary f i n d i n g s are as follows: (1) The p h y s i c a l s t r u c t u r a l parameters that c o n t r o l the mechanical response of the epoxies are the network topography and m i c r o v o i d c h a r a c t e r i s t i c s . The network s t r u c t u r e s can be hetero geneous because o f v a r i a t i o n s i n the c r o s s l i n k d e n s i t y i n the 5to 10-nm range and, a l s o , from the alignment o f macromolecules i n regions ~ 1 mm i n s i z e . Microvoids can be produced i n these epoxies as a r e s u l t o f the e l i m i n a t i o n o f unreacted m a t e r i a l . (2) The chemical r e a c t i o n s that produce the amine-cured epoxies can be complex and do not e x c l u s i v e l y occur by epoxideamine a d d i t i o n r e a c t i o n s . (3) M i c r o s c o p i c deformation occurs i n amine-cured epoxies by e i t h e r c r a z i n g and/or shear banding.



18.

MORGAN AND MONES

Composite

Matrices

243

(4) S o r b e d m o i s t u r e e n h a n c e s t h e c r a z e c a v i t a t i o n a n d p r o p a g a t i o n p r o c e s s e s i n e p o x i e s and d e t e r i o r a t e s t h e i r m e c h a n i c a l properties. (5) The i n i t i a l s t a g e s o f f a i l u r e , w h i c h i n v o l v e b o t h d i l a t a t i o n a l c r a z e - a n d s u b s e q u e n t c r a c k - p r o p a g a t i o n , enhance t h e a c c e s s i b i l i t y o f m o i s t u r e t o s o r p t i o n s i t e s w i t h i n t h e epoxy t o a greater e x t e n t than i n the l a t t e r stages o f f a i l u r e , which i n v o l v e crack propagation alone. (6) The amount o f m o i s t u r e s o r b e d by e p o x i e s i s e n h a n c e d a f t e r exposure t o a thermal s p i k e as a r e s u l t o f moisture-induced f r e e volume i n c r e a s e s t h a t i n v o l v e r o t a t i o n a l - i s o m e r i c p o p u l a t i o n changes. Acknowledgment T h i s work was p e r f o r m e d under t h e a u s p i c e s o f t h e U.S. D e p a r t ment o f E n e r g y by L a w r e n c e L i v e r m o r e L a b o r a t o r y under c o n t r a c t No. W-7405-Eng-48. R e f e r e n c e t o a company o r p r o d u c t name does n o t i m p l y a p p r o v a l o r r e c o m m e n d a t i o n o f t h e p r o d u c t by t h e U n i v e r s i t y o f C a l i f o r n i a o r the Department o f Energy t o t h e e x c l u s i o n o f o t h e r s t h a t may be s u i t a b l e . Abstract The s t r u c t u r e , d e f o r m a t i o n a n d f a i l u r e p r o c e s s e s , a n d m e c h a n i c a l p r o p e r t y r e l a t i o n s o f composite m a t r i c e s , and the m o d i f i c a t i o n o f s u c h r e l a t i o n s by f a b r i c a t i o n a n d e n v i r o n m e n t a l f a c t o r s a r e p r e s e n t e d . The p r i m a r y c o m p o s i t e m a t r i c e s c o n s i d e r e d a r e e p o x i e s . The p h y s i c a l s t r u c t u r e o f e p o x i e s i s d i s c u s s e d i n terms o f the network topography and m i c r o v o i d c h a r a c t e r i s t i c s . Such parameters d i r e c t l y c o n t r o l the m e c h a n i c a l response o f these g l a s s e s . The c h e m i c a l r e a c t i o n s t h a t p r o d u c e e p o x y n e t w o r k s c a n be c o m p l e x a n d do n o t e x c l u s i v e l y o c c u r by e p o x i d e - a m i n e a d d i t i o n reactions. The c o m p l e x n a t u r e o f t h e s e r e a c t i o n s a n d t h e f a c t o r s t h a t c o n t r o l them a r e d i s c u s s e d . E p o x i e s d e f o r m m i c r o s c o p i c a l l y and f a i l by e i t h e r c r a z i n g and/or s h e a r b a n d i n g . The d u r a b i l i t y of these r e s i n s i s d i s c u s s e d i n terms o f the e f f e c t o f s p e c i f i c c o m b i n a t i o n s o f m o i s t u r e , h e a t , and s t r e s s on t h e i r p h y s i c a l a n d mechanical i n t e g r i t y .


244

RESINS FOR AEROSPACE

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"Proceedings o f 1977 Flywheel Technology Symposium"; Conf.771053, U.S. Dept. o f Energy: Washington, D.C., March 1978. "U.S. A i r Force D u r a b i l i t y Workshop"; Battelle-Columbus: Columbus, Ohio, September 1975. "U.S. A i r Force Conference on the E f f e c t s o f R e l a t i v e Humidity and Temperature on Composite S t r u c t u r e s " , U n i v e r s i t y o f Delaware; AFOSR-TR-77-0030, U.S. A i r F o r c e : Washington, D.C., 1977. Morgan, R. J . ; O'Neal, J. E. i n "Toughness and B r i t t l e n e s s o f Advances i n Chemistry", S e r i e s 154, American Chemical S o c i e t y : Washington, D.C., 1976; Chap. 2. Morgan, R. J . ; O'Neal, J. E. in "Chemistry and P r o p e r t i e s o f C r o s s l i n k e d Polymers", S. S. Labana, Ed., Academic Press: New York, 1977; p. 289. Morgan, R. J . ; O'Neal, J. E. J. Mater. S c i . , 1977, 12, 1966. Morgan, R. J . ; O'Neal, J. E. J. Macromol. Sci. Phys., 1978, B15 (1), 139. Morgan, R. J . ; O'Neal, J. E. Polym. P l a s t . Technol. Eng. 1978, 10(1), 49. Morgan, R. J . ; O'Neal, J. E.; Miller, D. B. J. Mater. S c i . , 1979, 14, 109. Racich, J. L.; Koutsky, J. A. J. Appl. Polym. S c i . , 1976, 20, 2111. Manson, J. A.; S p e r l i n g , L. H.; Kim, S. L. "Influence o f C r o s s l i n k i n g on the Mechanical P r o p e r t i e s o f High T Polymers", AFML-TR-77-109, U.S. A i r Force M a t e r i a l s Lab.: Dayton, Ohio, 1977. Morgan, R. J. J. A p p l . Polym. S c i . , 1979, 23, 2711. French, D. M.; S t r e c k e r , R. A. H.; Tompa, A. S. J. Appl. Polym. S c i , 1970, 14, 599. H o r i e , K.; H i u r a , H.; Sawada, M.; M i t a , I . ; Kambe, H. J . Polym. Sci, 1970, A - l ( 9 ) , 1357. Acitelli, M. A.; Prime, R. B.; Sacher, E. Polymer, 1971, 12, 335. Prime, R. B.; Sacher, E. Polymer, 1972, 13, 455. Whiting, D. Α.; K l i n e , D. E. J. A p p l . Polym. S c i . , 1974, 18, 1043. Lee, H.; N e v i l l e , K. "Handbook o f Epoxy Resins"; McGraw-Hill: New York, 1967. S i d y a k i n , P. V. Vysokomol. Soyed, 1972, A14, 979. B e l l , J. P.; M c C a v i l l , W. T. J. Appl. Polym. S c i . , 1974, 18, 2243. N a r r a c o t t , E. Brit. P l a s t . , 1953, 26, 120. H u l l , D. A c t a . Met., 1960, 8, 11. M i l l s , N. J. J. Mater. S c i . , 1976, 11, 363. V e r e t t e , R. M. "Temperature/Humidity E f f e c t s on the Strength o f Graphite/Epoxy Laminates"; AIAA Paper No. 75-1011, 1975. g

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McKague, E. L., J r . ; H a l k i a s , J. E.; Reynolds, J. D. J . Compos. Mater., 1 9 7 5 , 9, 2. Browning, C. Ε., Ph.D. T h e s i s , U n i v e r s i t y o f Dayton: Dayton, Ohio, 1976. Hedrick, I . G.; Whiteside, J. B. E f f e c t s o f Environment on Advanced Composite S t r u c t u r e s , in "AIAA Conference on Air craft Composites: The Emerging Methodology o f S t r u c t u r a l Assurance", San Diego, California, Paper No. 77-463, 1977. McKague, E. L. in "Proceedings o f Conference Environmental Degradation o f Engineering M a t e r i a l s " , Louthan, M. R. and McNitt, R. P., Eds., Virginia P o l y t e c h n i c I n s t . P r i n t i n g Dept.: Blacksburg, Virginia, 1977; Chap. V, p. 353. Browning, C. E. The Mechanism o f E l e v a t e d Temperature Property Losses in High Performance S t r u c t u r a l Epoxy Resin M a t r i x M a t e r i a l s A f t e r Exposure to High Humidity Environments, 22nd N a t i o n a l SAMPE Symposium and E x h i b i t i o n , San Diego, CA, 1 9 7 7 , 2 2 , 3 6 5 . "Advanced Composite M a t e r i a l s — Environmental E f f e c t s " , ASTM STP 658, Vinson, J. P., Ed. American S o c i e t y f o r T e s t i n g and M a t e r i a l s , 1978.

26. 27.


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28.

29.

30.

T h i s report w a s p r e p a r e d as a n account of w o r k sponsored b y the U n i t e d States Government.

N e i t h e r the U n i t e d States n o r the U n i t e d States D e p a r t m e n t of E n e r g y ,

n o r a n y of t h e i r employees, n o r a n y of t h e i r contractors,

subcontractors,

or t h e i r e m

ployees, makes a n y w a r r a n t y , express or i m p l i e d , or assumes a n y l e g a l l i a b i l i t y or respon s i b i l i t y f o r the accuracy, completeness or usefulness of a n y i n f o r m a t i o n , apparatus, p r o d u c t or process disclosed, or represents t h a t its use w o u l d not i n f r i n g e p r i v a t e l y - o w n e d rights.

RECEIVED May

23,

1980.


StructureProperty Relations of Composite Matrices - American

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