Epoxy Resin Chemistry II - American Chemical Society

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11 Structure—Property Relations of Polyethertriamine-Cured Bisphenol-A-diglycidyl Ether Epoxies FUNG-MING KONG, CONNIE M. WALKUP, and ROGER J. MORGAN Lawrence Livermore National Laboratory, Livermore, CA 94550 The relations between the chemical and physical network structure, the deformation and failure pro cesses and the tensile mechanical properties of polyethertriamine cured bisphenol-A-diglycidyl ether epoxies are reported for a series of epoxy glasses prepared from a range of polyethertriamine concen trations. Near infrared spectroscopy indicates that these glasses form exclusively from epoxide-amine addition reactions. Their Tg exhibits a maximum and their swell ratio a minimum at the highest crosslink density. Stress-birefringence studies reveal that these highly crosslinked glasses are ductile and undergo necking and plastic deformation. The plastic deformation initially occurs homogen eously, but ultimately becomes inhomogeneous and shear bands develop. Tensile failure occurs i n the high strain shear band region. The ultimate tensile strain of these epoxies attains a maximum of 15% for the highest crosslinked glass. Off-stoichiometric networks fail at lower strains because such networks inherently contain more defects i n the form of unreacted ends. The density, yield stress, tensile strength, and modulus of these glasses all decrease with increasing polyethertriamine concentration as a result of increasing free volume because of the poor packing a b i l i t y of the amine molecule. A minimum is superimposed on this downtrend i n density and mechan i c a l properties with increasing amine content at the highest crosslink density because of geometric con straints imposed on segmental packing by the net work crosslinks. The a b i l i t y of these crosslinked glasses to undergo deformation i s discussed i n terms of the free volume and the crosslinked network topo graphy. Network failure i s considered i n terms of stress-induced chain scission which i s determined by the concentration and extensibility of the least extensible network segments. 0097-6156/83/0221-021l$06.00/0 © 1983 American Chemical Society


212

EPOXY

RESIN

CHEMISTRY

The i n c r e a s i n g use o f h i g h p e r f o r m a n c e , f i b r o u s c o m p o s i t e s i n c r i t i c a l s t r u c t u r a l a p p l i c a t i o n s has l e d t o a need t o p r e d i c t t h e l i f e t i m e s o f t h e s e m a t e r i a l s i n s e r v i c e e n v i r o n m e n t s . To p r e d i c t the d u r a b i l i t y o f a c o m p o s i t e i n s e r v i c e e n v i r o n m e n t r e q u i r e s a b a s i c u n d e r s t a n d i n g o f (1) t h e m i c r o s c o p i c d e f o r m a t i o n and f a i l u r e p r o c e s s e s o f the c o m p o s i t e ; (2) t h e r o l e s p l a y e d by the f i b e r , epoxy m a t r i x and f i b e r - m a t r i x i n t e r f a c i a l r e g i o n i n compo s i t e p e r f o r m a n c e ; and (3) t h e r e l a t i o n s b e t w e e n t h e s t r u c t u r e , d e f o r m a t i o n and f a i l u r e p r o c e s s e s and m e c h a n i c a l r e s p o n s e o f t h e f i b e r , epoxy m a t r i x and t h e i r i n t e r f a c e and how s u c h r e l a t i o n s a r e m o d i f i e d by e n v i r o n m e n t a l f a c t o r s . I n t h i s p a p e r we a d d r e s s t h e a r e a o f t h e 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 epoxy m a t r i c e s , w i t h s p e c i f i c e m p h a s i s on a m i n e cured epoxides. I n a t t e m p t s t o c o r r e l a t e 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 a m i n e - c u r e d e p o x i d e s , t h e r e have b e e n a number o f s t u d i e s on the m e c h a n i c a l p r o p e r t i e s o f t h e s e g l a s s e s as a f u n c t i o n o f e p o x i d e : amine r a t i o s and t h e c h e m i c a l s t r u c t u r e o f t h e c o n s t i t u e n t e p o x i d e and amine monomers.(1-15) Generally, there i s no d i r e c t c o r r e l a t i o n between t h e c h e m i s t r y o f t h e e p o x i d e and amine monomers and the m e c h a n i c a l p r o p e r t i e s o f e p o x i e s w i t h t h e e x c e p t i o n t h a t as t h e d i s t a n c e b e t w e e n c r o s s l i n k s becomes s h o r t e r , t h e s e g l a s s e s become more b r i t t l e . ( 1 , 1 0 , 1 5 ) Also, for a s p e c i f i c a m i n e - c u r e d e p o x i d e s y s t e m , t h e Tg i s a l w a y s h i g h e s t f o r the f u l l y r e a c t e d , h i g h e s t c r o s s l i n k d e n s i t y g l a s s . ( 3 , 6 , 9 , 1 2 , 16,jJ,18)

T h e r e a r e a number o f r e a s o n s why t h e r e i s n o t a s i m p l e c o r r e l a t i o n b e t w e e n t h e c h e m i s t r y o f t h e s t a r t i n g m a t e r i a l s and t h e m e c h a n i c a l p r o p e r t i e s o f t h e r e s u l t a n t c u r e d epoxy g l a s s . First, the c h e m i s t r y o f t h e epoxy n e t w o r k may n o t a l w a y s be s i m p l e . A m i n e - c u r e d epoxy n e t w o r k s a r e g e n e r a l l y assumed t o r e s u l t e x c l u s i v e l y f r o m a d d i t i o n r e a c t i o n s o f e p o x i d e g r o u p s w i t h p r i m a r y and secondary amines.(16) However, t h e s e r e a c t i o n s a r e o f t e n i n c o m p l e t e due t o 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 , and a d d i t i o n a l r e a c t i o n s s u c h as e p o x i d e h o m o p o l y m e r i z a t i o n c a n o c c u r . ( 8 , 1 2 , 1 3 , 16,19-28) The p h y s i c a l s t r u c t u r e s o f e p o x i e s a r e a l s o c o m p l e x on a m a c r o s c o p i c and m i c r o s c o p i c s c a l e and h a v e n o t b e e n c h a r a c terized in detail. M a c r o s c o p i c a l l y , the c r o s s l i n k e d networks are inhomogeneous b e c a u s e o f v o i d s and p o s s i b l e d i s t r i b u t i o n s i n t h e crosslink density. On a more m i c r o s c o p i c l e v e l , d e t a i l s o f t h e l o c a l f r e e volume, the r o t a t i o n a l i s o m e r i c c o n f i g u r a t i o n s of s e g ments b e t w e e n c r o s s l i n k s , c h a i n end d e f e c t s , and any bond a n g l e and l e n g t h d i s t o r t i o n s p r o d u c e d t o f o r m t h e c r o s s l i n k e d n e t w o r k s a r e g e n e r a l l y unknown. These c h e m i c a l and p h y s i c a l s t r u c t u r a l p a r a m e t e r s p l a y an i n t i m a t e r o l e i n t h e d e f o r m a t i o n and f a i l u r e p r o c e s s e s and m e c h a n i c a l r e s p o n s e o f the g l a s s y c r o s s l i n k e d n e t w o r k s . However, few s t u d i e s h a v e b e e n c o n d u c t e d on t h e m i c r o s c o p i c d e f o r m a t i o n and f a i l u r e p r o c e s s e s o f epoxy g l a s s e s . ( 1 3 , 22,^9,_30) T h e r e f o r e , a t p r e s e n t t h e r e a r e c o n s i d e r a b l e gaps i n our u n d e r s t a n d i n g o f 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 of e p o x i e s .


11.

KONG ET AL.

Polyetherîriamine-Cured

Resins

213

To f u r t h e r o u r u n d e r s t a n d i n g o f c r o s s l i n k e d e p o x i e s , i n t h i s p a p e r we r e p o r t s t r u c t u r e - p r o p e r t y s t u d i e s o f a s e r i e s o f p o l y e t h e r t r i a m i n e c u r e d b i s p h e n o l - A - d i g l y c i d y l e t h e r (DGEBA) e p o x i e s whose c h e m i c a l s t r u c t u r e s a r e w e l l c h a r a c t e r i z e d . Different epoxy n e t w o r k s were p r o d u c e d by v a r y i n g t h e e p o x i d e : amine ratios. The p h y s i c a l s t r u c t u r e , d e f o r m a t i o n and f a i l u r e p r o c e s s e s and m e c h a n i c a l p r o p e r t i e s o f t h e s e g l a s s e s a r e r e p o r t e d and a r e c o r r e l a t e d w i t h t h e s y s t e m a t i c c h e m i c a l s t r u c t u r a l changes i n t h i s epoxy s e r i e s . The h i g h d u c t i l i t y o f t h e s e e p o x i e s and t h e i r r e l a t i v e i n s e n s i t i v i t y t o i n h e r e n t f a b r i c a t i o n f l a w s were a d v a n t a g e o u s i n c o r r e l a t i n g t h e i r n e t w o r k s t r u c t u r e w i t h t h e i r mechanical response. Experimental Materials. Pure DGEBA, DER 332 (Dow) e p o x i d e monomer was used i n t h i s s t u d y and was c u r e d w i t h an a l i p h a t i c p o l y e t h e r t r i a m i n e , J e f f a m i n e T403 ( J e f f e r s o n ) . The c h e m i c a l s t r u c t u r e s o f t h e amine and e p o x i d e monomers a r e shown i n F i g u r e 1. The e p o x i d e and amine e q u i v a l e n t w e i g h t s d e t e r m i n e d by c h e m i c a l t i t r a t i o n were 175.15 and 82.92 grams, r e s p e c t i v e l y . I f these s y s t e m s form e x c l u s i v e l y f r o m e p o x i d e - a m i n e a d d i t i o n r e a c t i o n s , a b o u t 47 p h r ( p a r t s p e r h u n d r e d ) T403 a r e r e q u i r e d f o r c o m p l e t e c u r e . A range o f e p o x i e s w i t h 25 t o 75 p h r T403 was p r e p a r e d . The DGEBA e p o x i d e monomer was p r e h e a t e d t o 60°C t o m e l t any c r y s t a l s p r e s e n t ( J S ) p r i o r t o m i x i n g w i t h t h e T403. A f t e r m i x i n g t h e two monomers, t h e m i x t u r e s were vacuum d e g a s s e d and c a s t between g l a s s p l a t e s s e p a r a t e d by 0.3 cm t h i c k T e f l o n s p a c e r s . A t h i n l a y e r o f r e l e a s e a g e n t (DC-20 Dow) was b a k e d o n t o each p l a t e p r i o r to c a s t i n g . The epoxy s h e e t s were c u r e d between t h e g l a s s p l a t e s a t room t e m p e r a t u r e f o r 24 h o u r s , p o s t - c u r e d a t 85°C f o r 16 h o u r s and t h e n s l o w l y c o o l e d a t a b o u t 2°C/min t o room t e m p e r a t u r e . A l l s p e c i m e n s used f o r p h y s i c a l s t r u c t u r a l c h a r a c t e r i z a t i o n and m e c h a n i c a l p r o p e r t y t e s t s were f a b r i c a t e d from t h e s e epoxy s h e e t s . E x p e r i m e n t a l . Near i n f r a r e d s p e c t r o s c o p y ( C a r y 14 s p e c t r o p h o t o m e t e r ) was used t o s t u d y the c u r e r e a c t i o n s and t h e r e s u l t a n t chemical s t r u c t u r e of the e p o x i e s . A c o r r e c t p o r t i o n of e p o x i d e and amine were m i x e d , d e g a s s e d , and p o u r e d i n t o a g l a s s cell. To r e d u c e t h e p a t h l e n g t h , a t h i c k g l a s s p l a t e was i n s e r t e d . A c o p p e r c o n s t a n t a n t h e r m o c o u p l e was used t o c o n t r o l t h e t e s t temperature. Each epoxy m i x t u r e was scanned a t room t e m p e r a t u r e f o r 24 h o u r s and a t 85°C f o r 16 h o u r s . The a b s o r b a n c e s o f t h e e p o x i d e ( A ) and p h e n y l (Ap) g r o u p s were m o n i t o r e d a t 2.205 and 2.160 ym, r e s p e c t i v e l y . S i n c e t h e p h e n y l g r o u p does n o t p a r t i c i p a t e i n any o f t h e c h e m i c a l r e a c t i o n s , t h i s g r o u p ' s a b s o r b a n c e was u t i l i z e d as an i n t e r n a l s t a n d a r d . The e p o x i d e g r o u p c o n s u m p t i o n was d e t e r m i n e d by m o n i t o r i n g t h e changes i n t h e A :Ap r a t i o w i t h cure time. e

e


214

EPOXY

Ο

CH,

RESIN

Ο

C H - C H - C H - 0-@>-C--0—CH — C H - C H 2

2

2

CH

CHEMISTRY

2

3

Diglycidyl ether of bisphenol A DER 332

H C -f Ο CH CH (CH )4^ NH 2

2

3

2

CH CH CCH -[0 C H CH (CH )^-NH 3

2

2

2

3

2

χ + y + ζ ~ 5.3

H C -+0 C H CH (CHgH^-NHj 2

F i g u r e 1.

2

Polyether triamine T403 C h e m i c a l s t r u c t u r e s o f DGEBA e p o x i d e and T403

p o l y e t h e r t r i a m i n e monomers.

11.

KONG E T A L .

Polyethertriamine-Cured

Resins

215

A 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 e r (DSC) (Du P o n t 910) was used t o m o n i t o r t h e Tg's o f t h e e p o x i e s . The h e a t i n g r a t e was 10°C/min. The dynamic m e c h a n i c a l p r o p e r t i e s o f t h e e p o x i e s were m o n i t o r e d by a R h e o m e t r i c s m e c h a n i c a l s p e c t r o m e t e r ( M o d e l RMS7200). Epoxy s p e c i m e n s w i t h d i m e n s i o n s o f 6.4x1.3x0.3 cm were t o r q u e d a t an o s c i l l a t i n g f r e q u e n c y o f 2.0 Hz. The s h e a r s t o r a g e ( G ) and l o s s (G") m o d u l i and t a n 6 were d e t e r m i n e d f r o m -160°C t o +140°C. The d e n s i t i e s o f t h e e p o x i e s were p e r f o r m e d on 1x1x0.3 cm epoxy s p e c i m e n s t h a t were immersed i n m e t h y l e t h y l k e t o n e f o r s e v e n d a y s . The w e i g h t s (w) and v o l u m e s ( v ) o f t h e s p e c i m e n s were measured p r i o r t o (w^ and V^) and a f t e r (wf and v ^ ) i m m e r s i o n and t h e s w e l l i n g r a t i o Vf/v^ d e t e r m i n e d . F o r t h e m e c h a n i c a l t e n s i l e t e s t s , dogbone-shaped s p e c i m e n s w i t h a gauge l e n g t h 7.5 cm and a w i d t h o f 1.3 cm were c u t f r o m t h e c a s t epoxy s h e e t s . A f t e r p o l i s h i n g t h e s p e c i m e n edges a l o n g t h e gauge l e n g t h , t h e s p e c i m e n s were a n n e a l e d a t 85°C u n d e r vacuum f o r 1.5 h o u r s t o remove any s o r b e d m o i s t u r e and r e l e a s e any f a b r i c a t i o n s t r a i n s . The s p e c i m e n s were s t o r e d i n a d e s s i cator prior to testing. A l l t e n s i l e t e s t s were c o n d u c t e d a t room t e m p e r a t u r e on a m e c h a n i c a l t e s t e r ( i n s t r o n , TTDM) a t a c r o s s h e a d speed o f 0.5 cm/min. The f r a c t u r e t o p o g r a p h i e s o f t h e f a i l e d s p e c i m e n s were documented by o p t i c a l m i c r o s c o p y ( Z e i s s ) . The d e f o r m a t i o n modes were m o n i t o r e d w i t h a p o l a r i s c o p e ( P h o t o e l a s t i c I n c . ) as a f u n c t i o n o f s t r e s s f o r e p o x i e s w i t h 3 7 , 47, and 57 p h r T403. An a t t a c h e d camera was used t o r e c o r d t h e birefringence patterns.


f

R e s u l t s and D i s c u s s i o n C h e m i c a l S t r u c t u r e . A m i n e - c u r e d epoxy n e t w o r k s g e n e r a l l y form e x c l u s i v e l y from epoxide-amine a d d i t i o n r e a c t i o n s ; i . e . , R CH-CH ^0 1

2

+ R NH 2

2

-R CH(0H)CH NHR x

2

2

F o r a l l t h e amine h y d r o g e n s t o r e a c t w i t h e p o x i d e g r o u p s i n t h e DGEBA-T403 s y s t e m e x c l u s i v e l y by e p o x i d e - a m i n e a d d i t i o n r e a c t i o n s w o u l d r e q u i r e a s t o i c h i o m e t r i c q u a n t i t y o f 47 p h r T403 b a s e d o n t h e c h e m i c a l l y d e t e r m i n e d e q u i v a l e n t w e i g h t s o f t h e DGEBA and T403 r e a c t a n t s . The e p o x i d e c o n s u m p t i o n d u r i n g c u r e was m o n i t o r e d by n e a r IR as a f u n c t i o n o f T403 c o n c e n t r a t i o n (25-75 p h r r a n g e ) f o r a 24 h o u r c u r e a t 23°C f o l l o w e d by a p o s t c u r e a t 85°C. The IR mea surements were t r u n c a t e d when e i t h e r a 100% e p o x i d e c o n s u m p t i o n o c c u r r e d o r t h e e p o x i d e c o n s u m p t i o n c e a s e d t o change w i t h t i m e a t 85°C. The u l t i m a t e p e r c e n t a g e e p o x i d e c o n s u m p t i o n s a t 85°C a r e p l o t t e d a s a f u n c t i o n o f T403 c o n c e n t r a t i o n i n F i g u r e 2. A l l e p o x i d e g r o u p s a r e consumed a t ">45 p h r T403. The l o w e s t T403 c o n c e n t r a t i o n o f 45 p h r f o r f u l l e p o x i d e c o n s u m p t i o n i s s l i g h t l y l o w e r t h a n t h e 47 p h r T403 p r e d i c t e d from c h e m i c a l a n a l y s i s o f


216

EPOXY

0

10

20

30

40

50

60

RESIN

70

CHEMISTRY

80

T403 concentration (phr) F i g u r e 2. U l t i m a t e p e r c e n t e p o x i d e c o n s u m p t i o n v e r s u s T403 c o n c e n t r a t i o n f o r 24-hour c u r e a t 23°C f o l l o w e d by 16-hour 85°C p o s t c u r e f o r DGEBA-T403 e p o x i e s .


11.

KONG E T AL.


217

Resins

the s t a r t i n g m a t e r i a l s and a s s u m i n g e x c l u s i v e l y e p o x i d e - a m i n e a d dition reactions. T h i s s l i g h t d i s c r e p a n c y c o u l d be due t o i n s u f f i c i e n t a c c u r a c i e s i n (1) the chemical t i t r a t i o n procedures to d e t e r m i n e t h e DGEBA and T403 e q u i v a l e n t w e i g h t s a n d / o r ( 2 ) i n t h e n e a r IR t o d e t e c t s m a l l d i f f e r e n c e s i n e p o x i d e c o n c e n t r a t i o n s . I n t h e 25-45 p h r T403 range i n F i g u r e 2, t h e e p o x i d e c o n s u m p t i o n l e v e l s f a l l on a s t r a i g h t l i n e w h i c h , when e x t r a p o l a t e d , go through the o r i g i n . T h i s d a t a i n d i c a t e s t h a t t h e DGE3A-T403 e p o x i e s form e x c l u s i v e l y from epoxide-amine a d d i t i o n r e a c t i o n s w h i c h p r o c e e d t o c o m p l e t i o n w i t h o u t any a d d i t i o n a l e p o x i d e c o n s u m p t i o n o c c u r r i n g as a r e s u l t o f s i d e r e a c t i o n s . Network S t r u c t u r e . The p h y s i c a l n e t w o r k s t r u c t u r e s and t h e a v e r a g e m o l e c u l a r w e i g h t between c r o s s l i n k s , M , f o r DGEBAT403 e p o x i e s as a f u n c t i o n o f T403 c o n c e n t r a t i o n were d e t e r m i n e d and a r e i l l u s t r a t e d i n T a b l e I . These s t r u c t u r e s were a s c e r t a i n e d based on the c h e m i c a l and p h y s i c a l s t r u c t u r e o f t h e DGEBA and T403 monomers and t h e f o l l o w i n g a s s u m p t i o n s : ( 1 ) t h e c u r e r e a c t i o n s and n e t w o r k f o r m a t i o n r e s u l t e x c l u s i v e l y f r o m e p o x i d e - a m i n e a d d i t i o n r e a c t i o n s ; ( 2 ) a l l p r i m a r y amine h y d r o g e n s r e a c t w i t h epoxide groups p r i o r to the onset o f secondary amine-epoxide a d d i t i o n r e a c t i o n s . The f u n c t i o n a l i t i e s and f l e x i b i l i t i e s o f the DGEBA and T403 s t r u c t u r e s a l l o w a v a r i e t y o f r i n g s t r u c t u r e s to form i n the f u l l y c r o s s l i n k e d DGEBA-T403 0^47 p h r T403) epoxy r e s u l t i n g i n an i r r e g u l a r n e t w o r k . M o l e c u l a r models i n d i c a t e such r i n g s a r e r e l a t i v e l y d e f o r m a b l e . However, b e c a u s e o f t h e i r r e g u l a r n e t work, c o o p e r a t i v e r i n g d e f o r m a t i o n s w i l l be s e v e r e l y r e s t r i c t e d and i n d e e d l e a d t o n e t w o r k c h a i n s c i s s i o n s . The n e t w o r k t o p o g r a p h y i s even more i r r e g u l a r t h a n i l l u s t r a t e d i n T a b l e I b e c a u s e o f t h e d i f f e r e n t l e n g t h s o f t h e t h r e e arms o f t h e T403 m o l e c u l e i n w h i c h 23% have an M v a l u e o f 200 and t h e r e m a i n d e r an M v a l u e o f 158. These M v a l u e s a r e d e t e r m i n e d by a s s u m i n g x, y, o r ζ = 1 o r 2 i n t h e T403 c h e m i c a l s t r u c t u r e i l l u s t r a t e d i n F i g u r e 1. M o l e c u l a r models a l s o i n d i c a t e t h a t t h e p a c k i n g e f f i c i e n c y o f t h e n e t w o r k segments i s t h e most s e v e r e l y r e s t r i c t e d f o r the h i g h e s t c r o s s l i n k d e n s i t y network. A p l o t o f M v s T403 c o n c e n t r a t i o n , d e t e r m i n e d f r o m t h e c u r e r e a c t i o n s and e p o x i d e and amine e q u i v a l e n t w e i g h t s , i s i l l u s t r a t e d i n F i g u r e 3 as the f u l l l i n e . The e x p e r i m e n t a l d a t a p o i n t s i n t h i s f i g u r e were f r o m N i e l s e n ' s (31) e m p i r i c a l e q u a t i o n c

c

c

c

c

log

1 0

G' * 7.0 + 293d/M

c

where G' i s t h e s h e a r modulus above Tg i n dynes/cm and d i s the p o l y m e r d e n s i t y . The G' v a l u e s were d e t e r m i n e d f r o m dynamic m e c h a n i c a l measurements above T i n t h e 100-120°C r a n g e . The e x p e r i m e n t a l d a t a p o i n t s f o l l o w t h e same g e n e r a l t r e n d as t h e c h e m i c a l l y d e t e r m i n e d M -T403 c o n c e n t r a t i o n p l o t b u t e x h i b i t on the a v e r a g e ~ 3 0 % l a r g e r M v a l u e s . The agreement between g

c

c


218

EPOXY

RESIN CHEMISTRY

Table I. The Network Structures and Molecular weight, Mc, for DGEBA-T403 Epoxies T403 Concentration phr

Structure

(a)

(b)

Comments

M

Al I-NHL and -NH groups o.

react; (a) one -c'— cgroup reacts on each DGEBA molecule; (b) In some DGEBA molecules both — c^-^cgroups reach whereas in others no —c — c—groups react

23.7

c

OO

All -NH , -NH and 2

— c ' ^ c —groups reacted completely A network of interconnected rings

47.3

258

94.7

All -NH and — ( R e groups react; no -NH groups react, (a) Ring and (b) branched structures

682

142.0

.o. All—c — c— groups react; 2/3 N H groups react; no NH groups react; (a) branched and (b) linear structures

CO

2

2

τ4ο3

Λ

DGEBA


11.

KONG E T A L .


I, oo I

219

Resins

I

1

1

1

Ijoc

2000 1600 s

°

1200

/

1° \o

800

1

1 o

1

s/

400 0

l

0

20

1

40

1

60

1

80

1

100

ι

120

i !

140

160

T403 concentration (phr) F i g u r e 3. determined determined

v

i i v e r s u s T403 c o n c e n t r a t i o n f o r DGEBA-T403 e p o x i e s , from cure r e a c t i o n s ( E x p e r i m e n t a l d a t a p o i n t s f r o m G v a l u e s above Τ ) . g 1

EPOXY

220

RESIN

CHEMISTRY

t h e d a t a i s r e l a t i v e l y good i n t h e l i g h t o f t h e e m p i r i c a l n a t u r e of the M - l o g i 0 relationship. A more r a p i d i n c r e a s e i n M w i t h T403 c o n c e n t r a t i o n o c c u r s on t h e e x c e s s e p o x i d e o f s t o i c h i o m e t r y r e l a t i v e t o t h e e x c e s s T403 s i d e . G §


c

c

P h y s i c a l P r o p e r t i e s . The p h y s i c a l p r o p e r t i e s o f t h e DGEBA-T403 e p o x i e s as a f u n c t i o n o f T403 c o n c e n t r a t i o n c a n be u n d e r s t o o d i n terms o f t h e n e t w o r k s t r u c t u r e s d i s c u s s e d i n t h e previous s e c t i o n . I n F i g u r e 4, t h e T ' s o f t h e DGEBA-T403 e p o x i e s , d e t e r m i n e d f r o m 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 measurements, a r e p l o t t e d v e r s u s T403 c o n c e n t r a t i o n . The h i g h e s t T , 93°C, i s e x h i b i t e d by t h e epoxy w i t h t h e h i g h e s t c r o s s l i n k d e n s i t y a t ^-45 p h r T403. The Tg d e c r e a s e s l e s s s h a r p l y w i t h T403 c o n c e n t r a t i o n on t h e e x c e s s T403 s i d e o f s t o i c h i o m e t r y b e c a u s e M i n c r e a s e s l e s s s h a r p l y on t h i s s i d e o f s t o i c h i o m e t r y . The s w e l l r a t i o as e x p e c t e d e x h i b i t s a minimum f o r t h e h i g h e s t c r o s s l i n k e d g l a s s . A p l o t o f t h e 23°C d e n s i t y o f DGEBA-T403 e p o x i e s v s T403 c o n c e n t r a t i o n , i n F i g u r e 5, r e v e a l s t h e d e n s i t y p r o g r e s s i v e l y d e c r e a s e s w i t h i n c r e a s i n g T403 c o n c e n t r a t i o n w i t h a minimum s u p e r i m p o s e d on t h i s t r e n d n e a r t h e T403 s t o i c h i o m e t r i c c o n c e n tration. The i n a b i l i t y o f t h e t h r e e - a r m e d T403 m o l e c u l e t o p a c k as w e l l as t h e more l i n e a r DGEBA m o l e c u l e c a u s e s a p r o g r e s s i v e d e n s i t y d e c r e a s e w i t h i n c r e a s i n g T403 c o n c e n t r a t i o n . A minimum i n t h e d e n s i t y i s s u p e r i m p o s e d on t h i s d o w n t r e n d a t t h e h i g h e s t c r o s s l i n k d e n s i t y b e c a u s e o f t h e g e o m e t r i c c o n s t r a i n t s imposed on s e g m e n t a l p a c k i n g by t h e c r o s s l i n k e d n e t w o r k g e o m e t r y . g

g

c

M e c h a n i c a l P r o p e r t i e s and D e f o r m a t i o n and F a i l u r e P r o c e s s e s . The DGEBA-T403 e p o x i e s a r e d u c t i l e i n t e n s i o n a t 23°C and e x h i b i t a m a c r o s c o p i c y i e l d p o i n t , n e c k i n g and c o l d d r a w i n g . The u l t i m a t e s t r a i n o f t h e DGEBA-T403 e p o x i e s e x h i b i t s a maximum o f 15% a t t h e s t o i c h i o m e t r i c T403 c o n c e n t r a t i o n as shown i n F i g u r e 6. B e l o w ^-35 p h r T403 t h e u l t i m a t e s t r a i n i n c r e a s e s r a p i d l y w i t h d e c r e a s i n g T403 c o n t e n t as a r e s u l t o f i n c r e a s i n g M , d e c r e a s i n g Tg and p l a s t i c i z a t i o n o f t h e g l a s s e s by u n r e a c t e d DGEBA m o l e c u l e s . Above 35 p h r T403 c o n c e n t r a t i o n , Young's modulus and t h e t e n s i l e m a c r o s c o p i c y i e l d s t r e s s and s t r e n g t h a l l d e c r e a s e w i t h i n c r e a s i n g T403 c o n c e n t r a t i o n . T h i s downtrend i s i l l u s t r a t e d i n F i g u r e 7 f o r t h e m a c r o s c o p i c y i e l d s t r e s s . A minimum i s s u p e r imposed on t h i s d o w n t r e n d a t t h e s t o i c h i o m e t r i c T403 c o n c e n t r a t i o n , w h i c h i s most e v i d e n t i n t h e modulus d a t a . B e l o w 35 p h r T403 c o n c e n t r a t i o n , t h e s e m e c h a n i c a l p r o p e r t i e s a l l r a p i d l y d e c r e a s e w i t h d e c r e a s i n g T403 c o n c e n t r a t i o n as a r e s u l t o f i n c r e a s i n g s o f t e n i n g o f t h e epoxy g l a s s e s . The d e f o r m a t i o n p r o c e s s e s o f DGEBA-T403 e p o x i e s were m o n i t o r e d u n d e r p o l a r i z e d l i g h t as a f u n c t i o n o f s t r a i n l e v e l . The biréfringent p a t t e r n s o f DGEBA-T403 ( 4 7 p h r T403) epoxy as a f u n c t i o n o f s t r a i n i s i l l u s t r a t e d i n F i g u r e 8. A t s t r a i n s c


11.

KONG E T A L .


0

20

40

221

Resins

60

80

100

T403 concentration (phr)-^ F i g u r e 4.

T

v e r s u s T403 c o n c e n t r a t i o n f o r DGEBA-T403 e p o x i e s g

10

20

30

40

50

60

T403 concentration (phr)^* F i g u r e 5. D e n s i t y v e r s u s T403 c o n c e n t r a t i o n f o r DGEBA-T403 e p o x i e s , a t 23°C.

222

EPOXY

RESIN CHEMISTRY

80 70


60 50

-

40 -Stoichiometric ratio

30 20

U

/ Scatter limits ^ ^ a t 10% strain

10

40

_i_

JL 80

120

160

T403 concentration (phr) -> F i g u r e 6. The u l t i m a t e t e n s i l e s t r a i n v e r s u s T403 c o n c e n t r a t i o n f o r DGEBA-T403 e p o x i e s , a t 23°C.

£

70 60

2

50

> υ α

40

8 S a-

30

Ε

20

υ CO

10

K

- Scatter limits Stoichiometric ratio I

I

0 20 40 60 80 100 T403 concentration (phr) F i g u r e 7. T e n s i l e m a c r o s c o p i c y i e l d s t r e s s o f DGEBA-T403 e p o x i e s as a f u n c t i o n o f T403 c o n c e n t r a t i o n , a t 23°C.

light.

polarized

Biréfringent d e f o r m a t i o n p r o c e s s e s i n DGEBA-T403 (47

p h r T403) epoxy as a f u n c t i o n o f s t r a i n a t 23°C, u n d e r

F i g u r e 8.



224

EPOXY

RESIN

CHEMISTRY

5*2·5%, o n l y e l a s t i c d e f o r m a t i o n s o c c u r and homogeneous c o l o r changes p r o d u c e d u n d e r p o l a r i z e d l i g h t i n t h i s s t r a i n r e g i o n d i s a p p e a r i n s t a n t a n e o u s l y upon r e m o v a l o f the l o a d . I n the 2.5 t o 4% s t r a i n r e g i o n , homogeneous e l a s t i c and p l a s t i c deformations o c c u r ; and upon r e m o v a l o f the l o a d the homogeneous p l a s t i c d e f o r m a t i o n does n o t r e l a x o u t , w h i c h r e s u l t s i n a permanent homogeneous c o l o r change i n t h e u n s t r a i n e d epoxy when v i e w e d under p o l a r i z e d l i g h t . Above 4% s t r a i n the p l a s t i c d e f o r m a t i o n becomes i n c r e a s i n g l y inhomogeneous w i t h i n c r e a s i n g s t r a i n as i n d i c a t e d by t h e d e v e l o p m e n t o f biréfringent f r i n g e s . U l t i m a t e l y , a l o c a l r e g i o n o f h i g h s t r a i n d e v e l o p s i n t h e sample i n t h e f o r m o f a d i f f u s e s h e a r band and a neck d e v e l o p s i n t h i s region. The biréfringent f r i n g e s o r i e n t a t ~45° t o the d i r e c t i o n o f t h e a p p l i e d l o a d i n the s h e a r band. F r a c t u r e o c c u r s i n t h e h i g h s t r a i n , s h e a r band r e g i o n . F r a c t u r e t o p o g r a p h y s t u d i e s i n d i c a t e f a i l u r e o c c u r s by a c r u d e c r a z e p r o c e s s , as has been observed i n other epoxies.(30) F o r t h e o f f - s t o i c h i o m e t r i c DGEBA-T403 e p o x i e s t h a t a r e i n the 30-75 p h r T403 r a n g e , b i r e f r i n g e n t - s t r a i n s t u d i e s i n d i c a t e the f o l l o w i n g t r e n d s i n the d e f o r m a t i o n p r o c e s s e s f o r e p o x i e s t h a t a r e i n c r e a s i n g l y f u r t h e r removed f r o m t h e s t o i c h i o m e t r i c T403 concentration: ( 1 ) p l a s t i c f l o w i s i n i t i a t e d and becomes inhomogeneous a t l o w e r s t r a i n s ; ( 2 ) the r e g i o n s o f h i g h s t r a i n a r e l e s s o r i e n t e d and t h e i r a s s o c i a t e d bands a r e l e s s w e l l d e v e l o p e d upon c r a c k p r o p a g a t i o n t h r o u g h s u c h r e g i o n s . DGEBA-T403 e p o x i e s i n the 20-30 p h r T403 range t h a t e x h i b i t u l t i m a t e e l o n g a t i o n s i n the 20-70% r a n g e d e f o r m o n l y i n a homogeneous p l a s t i c f a s h i o n . S t r u c t u r a l F a c t o r s C o n t r o l l i n g M e c h a n i c a l P r o p e r t i e s and D e f o r m a t i o n and F a i l u r e P r o c e s s e s . The u l t i m a t e e l o n g a t i o n o f the DGEBA-T403 e p o x i e s i s c o n t r o l l e d by the c o n c e n t r a t i o n o f i n h e r e n t a n d / o r s t r e s s - i n d u c e d d e f e c t s i n t h e most h i g h l y s t r a i n e d r e g i o n o f the epoxy n e t w o r k . A maximum i n the f a i l u r e s t r a i n o c c u r s a t s t o i c h i o m e t r y b e c a u s e the most h i g h l y c r o s s l i n k e d n e t w o r k c o n t a i n s the l e a s t number o f d e f e c t s . In f u l l y r e a c t e d n e t w o r k s , n e t w o r k f a i l u r e i s i n i t i a t e d by c h a i n s c i s s i o n i n o v e r s t r a i n e d segments. Such s c i s s i o n s r e s u l t i n the n e i g h b o r i n g segments c a r r y i n g more o f t h e a p p l i e d l o a d . Furthermore, the c h e m i c a l l y a c t i v e f r e e r a d i c a l s t h a t f o r m upon s c i s s i o n can r e a c t w i t h n e i g h b o r i n g segments t o p r o d u c e f u r t h e r s c i s s i o n s . B o t h t h e s e phenomena r e s u l t i n a u t o c a t a l y t i c n e t w o r k d e g r a d a tion. The s c i s s i o n p r o c e s s w i l l depend d i r e c t l y on t h e n e t w o r k e x t e n s i b i l i t y and, t h e r e f o r e , on the n e t w o r k t o p o g r a p h y . The e x t e n s i b i l i t y o f t h e n e t w o r k segments i n the DGEBA-T403 epoxy n e t w o r k w i l l e x h i b i t t h r e e d i s c r e t e v a l u e s as a r e s u l t o f t h e r e being three d i s c r e t e M values. The DGEBA segments w h i c h make up 50% o f t h e t o t a l n e t w o r k segments e x h i b i t t h e l a r g e s t M v a l u e o f 342. The T403 segments have M v a l u e s o f 200 and 258 and a r e a s s o c i a t e d w i t h the arms o f the T403 m o l e c u l e i n w h i c h c

c

Q

11.

KONG E T A L .


Resins

225

χ, y, o r ζ = 1 o r 2, r e s p e c t i v e l y , i n F i g u r e 2. Those T403 s e g ments w i t h t h e l o w e s t M v a l u e o f 200 a r e 11.5% o f t h e t o t a l n e t w o r k segments. These l e a s t e x t e n s i b l e segments c a r r y a s i g n i f i c a n t p o r t i o n o f t h e l o a d , u n d e r s t r e s s , and w i l l t h e r e f o r e be t h e f i r s t segment t o u n d e r g o c h a i n s c i s s i o n . In a d d i t i o n t o the segmental e x t e n s i b i l i t y , t h e network topography w i l l a l s o play a s i g n i f i c a n t r o l e i n the u l t i m a t e e l a s t o m e r i c n e t w o r k p r o p e r t i e s . The a b i l i t y o f t h e b a s i c r i n g s t r u c t u r e s o f t h e epoxy n e t w o r k t o u n d e r g o d e f o r m a t i o n c o n t r o l s the n e t w o r k e x t e n s i b i l i t y . The d e f o r m a t i o n o f t h e s e r i n g s d e pends on ( 1 ) t h e e x t e n s i b i l i t y o f t h e i r s i d e s ; ( 2 ) t h e f l e x i b i l i t y o f t h e i r i n t e r n a l a n g l e s ; and ( 3 ) t h e i r a b i l i t y t o u n d e r g o cooperative deformation with t h e i r interconnected neighbors. T h e i r c o o p e r a t i v e d e f o r m a t i o n i s c o n t r o l l e d by t h e r e g u l a r i t y o f t h e n e t w o r k t o p o g r a p h y w h i c h i s d e t e r m i n e d by t h e d i f f e r e n t g e o m e t r i e s o f t h e b a s i c r i n g s and t h e i r o r i e n t a t i o n r e l a t i v e t o one a n o t h e r . The more r e g u l a r n e t w o r k s c o n s i s t o f i n t e r c o n n e c t e d r i n g s o f s i m i l a r s i z e and shape. However, t h e more i r r e g u l a r networks c o n s i s t o f r i n g s w i t h a v a r i e t y o f geometries that w i l l d e v e l o p o v e r s t r a i n e d segments a t l o w e r e x t e n s i b i l i t i e s . The f u l l y c r o s s l i n k e d DGEBA-T403 epoxy n e t w o r k i s r e l a t i v e l y i r r e g u l a r as i l l u s t r a t e d i n T a b l e I . The m a g n i t u d e s o f t h e m a c r o s c o p i c and m i c r o s c o p i c y i e l d s t r e s s e s , t h e t e n s i l e s t r e n g t h ( w h i c h i s c o n t r o l l e d by t h e f l o w p r o p e r t i e s o f t h e e p o x y ) and Young's modulus Ε o f DGEBA-T403 e p o x i e s a r e d e t e r m i n e d by t h e g l a s s y - s t a t e p a c k i n g and f r e e v o l u m e . These m e c h a n i c a l p r o p e r t i e s w i l l f o l l o w t h e same t r e n d s as t h e d e n s i t y as a f u n c t i o n o f T403 c o n c e n t r a t i o n . The d e n s i t y o f t h e DGEBA-T403 e p o x i e s d e c r e a s e s w i t h i n c r e a s i n g T403 c o n t e n t w i t h a minimum s u p e r i m p o s e d on s u c h a d o w n t r e n d a t s t o i c h i o m e t r y ( F i g u r e 5 ) . Above ~ 3 5 p h r T403 t h e y i e l d s t r e s s e s , t e n s i l e s t r e n g t h , and Young's modulus a l l f o l l o w t h e same t r e n d as t h e d e n s i t y , and d e c r e a s e w i t h i n c r e a s i n g T403 c o n c e n t r a t i o n . The minimum i n t h e d e n s i t y a t s t o i c h i o m e t r y t h a t i s s u p e r i m p o s e d o n the d o w n t r e n d w i t h i n c r e a s i n g T403 c o n c e n t r a t i o n i s r e v e a l e d i n t h e modulus d a t a b u t i s n o t c l e a r l y e v i d e n t i n t h e y i e l d s t r e s s and t e n s i l e s t r e n g t h d a t a . B e l o w ~ 3 5 p h r T403, t h e DGEBA-T403 e p o x i e s become i n c r e a s i n g l y s o f t w i t h d e c r e a s i n g T403 c o n c e n t r a t i o n s which causes sharp decreases i n the y i e l d s t r e s s e s , t e n s i l e s t r e n g t h and modulus w i t h d e c r e a s i n g T403 c o n t e n t , r e s u l t i n g i n a maximum i n s u c h p r o p e r t i e s n e a r 35 p h r T403.


c

Conclusions DGEBA-T403 epoxy g l a s s e s f o r m e x c l u s i v e l y f r o m e p o x i d e - a m i n e a d d i t i o n r e a c t i o n s . T h e i r Tg e x h i b i t s a maximum and t h e i r s w e l l r a t i o a minimum a t t h e h i g h e s t c r o s s l i n k d e n s i t y . These h i g h l y c r o s s l i n k e d g l a s s e s a r e d u c t i l e and undergo n e c k i n g and p l a s t i c deformation. The p l a s t i c d e f o r m a t i o n i n i t i a l l y o c c u r s homogeneously, b u t u l t i m a t e l y develops inhomogeneously i n t h e form o f shear bands. F a i l u r e o c c u r s i n t h e h i g h s t r a i n , shear


226

EPOXY

RESIN

CHEMISTRY

band r e g i o n . The u l t i m a t e s t r a i n o f t h e s e c r o s s l i n k e d g l a s s y n e t w o r k s i s d e t e r m i n e d by t h e c o n c e n t r a t i o n o f u n r e a c t e d a n d / o r b r o k e n n e t w o r k segments. Stress-induced chain scission of net work segments i s d e t e r m i n e d by t h e c o n c e n t r a t i o n and e x t e n s i b i l i t y o f t h e l e a s t e x t e n s i b l e segments. The a b i l i t y o f t h e s e e p o x i e s t o u n d e r g o p l a s t i c f l o w depends on ( 1 ) t h e d e f o r m a b i l i t y o f t h e b a s i c n e t w o r k r i n g s t r u c t u r e s and t h e i r a b i l i t y t o u n d e r g o c o o p e r a t i v e m o t i o n ; and ( 2 ) t h e g l a s s y - s t a t e f r e e v o l u m e . The f r e e volume depends on t h e m o l e c u l a r shape and p a c k i n g a b i l i t y o f the c o n s t i t u e n t e p o x i d e and amine p o r t i o n o f t h e n e t w o r k and t h e g e o m e t r i c c o n s t r a i n t s imposed on s e g m e n t a l p a c k i n g by t h e n e t w o r k geometry. Acknowledgments T h i s work was p e r f o r m e d u n d e r t h e a u s p i c e s o f t h e U. S. Department o f E n e r g y by Lawrence L i v e r m o r e N a t i o n a l L a b o r a t o r y u n d e r C o n t r a c t No. W7405-Eng-48.

Literature Cited 1. Lee, H. and Neville, Κ., "Handbook of Epoxy Resins," McGraw-Hill, New York, 1967, Ch. 6. 2. Busso, C. J . , Newey, Η. Α., and Holler, Η. V., Proc. 25th Conf. of Reinforced Plastics/Composites Division of SPI, Washington, D.C., 1970. 3. Chiao, T. T., Jessop, E. S., and Newey, Η. Α., "A Moderate Temperature Curable Epoxy for Advanced Composites," Lawrence Livermore Laboratory Report UCRL-76126, 1974. 4. Chiao, T. T. and Moore, R. L., Proc. 29th Conf. of Reinforced Plastics/Composites Division of SPI, Washington, D.C., Section 16-B, 1, 1974. 5. Chiao, T. T., Jessop, E. S., and Newey, Η. Α., SAMPE Quarterly 1974, 6, No. 1. 6. Selby, K. and M i l l e r , L. Ε., J. Mater. Sci. 1975, 10, 12. 7. Pritchard, G. and Rhoades, G. V., Mater. Sci. and Eng. 1976, 26, 1. 8. Morgan, R. J. and O'Neal, J. Ε., J. Macromol. Sci. Phys. 1978, 15, 139. 9. Kim, S. L., Skibo, M. D., Manson, J. Α., Hertzberg, R. W., and Janiszewski, J., Polym. Eng. Sci. 1978, 18, 1093. 10. P h i l l i p s , D. C., Scott, J. Μ., and Jones, Μ., J. Mater. S c i . 1978, 13, 311. 11. Young, R. J. i n "Developments i n Polymer Fracture-1," Ed. E. H. Andrews, Appl. Sci. Publishers, London, 1979. 12. Morgan, R. J., J. Appl. Polym. Sci. 1979, 23, 2711. 13. Morgan, R. J., O'Neal, J. E., and M i l l e r , D. B., J. Mater. Sci. 1979, 14, 109. 14. Yamini, S. and Young, R. J . , J. Mater. Sci. 1980, 15, 1814. 15. Scott, J. M., Wells, G. Μ., and P h i l l i p s , D.C., J. Mater. Sci. 1980, 15, 1436.


11.

KONG E T A L .

Polyetherîriamine-Cured

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16. Lee, H. and Neville, Κ., "Handbook of Epoxy Resins," McGraw-Hill, New York, 1967, Ch. 7. 17. Kreahling, R. P. and Kline, D. E., J. Appl. Polym. Sci. 1969, 13, 2411. 18. Murayama, T. and Bell, J. P., J. Polym. Sci. 1970, A-2, 8, 437. 19. French, D. M., Strecker, R. A. H., and Tompa, A. S., J. Appl. Polym. S c i . 1970, 14, 599. 20. Acitelli, M. Α., Prime, P. Β., and Sacher, Ε., Polymer 1971, 12, 335. 21. Prime, R. B. and Sacher, Ε., Polymer 1972, 13, 455. 22. Whiting, D. A. and Kline, D. E., J. Appl. Polym. Sci. 1974, 18, 1043. 23. Sidyakin, P.V., Vysokomol. Soyed 1972, A14, 979. 24. Tanaka, Y. and Mika, T. F., "Epoxy Resins," C. A. May and Y. Tanaka, Eds., Marcel Dekker, Inc., New York, 1973, Ch. 3. 25. B e l l , J. P. and McCavill, W. T., J. Appl. Polym. Sci. 1974, 18, 2243. 26. Dusek, Κ., Bleha, M., and Lunak, S., J. Polym. Sci. (Polym. Chem. Ed.) 1977, 15, 2393. 27. Morgan, R. J. and O'Neal, J. Ε., Polym. Plast. Tech. Eng. 1978, 10, 49. 28. Schneider, N. S., Sprouse, J. F., Hagnauer, G. L., and Gillham, J. K., Polym. Eng. and Sci. 1979, 19, 304. 29. Morgan, R. J . and O'Neal, J. E., J. Mater. Sci. 1977, 12, 1966. 30. Morgan, R. J . , Mones, E. T., and Steele, W. J., Polymer 1982, 23, 295. 31. Nielsen, L. E., J. Macromol. Sci. 1969, C3, 69. RECEIVED January 7,

1983

Epoxy Resin Chemistry II - American Chemical Society

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