Epoxy Resin Chemistry II - American Chemical Society

4 Impact Performance of Epoxy Resins with Poly(n-butyl acrylate) as the Reactive Liquid Rubber Modifier 1

Downloaded by PRINCETON UNIV on November 12, 2014 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch004

S. GAZIT and JAMES P. BELL University of Connecticut, Chemical Engineering Department and Institute of Materials Science, Storrs, CT 06268

Poly n-butyl acrylate was used as the Reactive Liquid Rubber (RLP) modifier in the investigation of the toughening of epoxy resins of the d i g l y c i d y l ether of bisphenol A type. No p l a s t i c i z i n g addi tives were used, so that the Tg remained above 100°c. Effective bonding between reactive groups on the ends of the rubber chains and the matrix was necessary for toughening. The impact strength was improved when the rubber was prereacted with the epoxy component before adding the curing agent. S o l u b i l i t y of the rubber in the epoxy resin before addition of curing agent was strongly dependent on rubber molecular weight, with a value of Μn ~4000 being desirable. The number and size of rubber particles was a function of the curing agent concen tration; the average size was approximately 1 micron. Under favorable conditions microcavitation (whitening), "butterfly pattern" formation and some times shear banding were observed. These were con current with an increase of up to 100% in tensile impact energy to break, largely attributable to increased ultimate elongation; the ultimate stress was similar or s l i g h t l y less than controls. Notched Izod results gave a smaller improvement because of the deformation by rapid propagation of a crack from a defect. The tensile impact method gives deformation over a wider area, permitting the tough ening mechanism to be effective. 1

Current address: Rogers Corporation, Lurie Research and Development Center,

Rogers, C T 06263

0097-6156/83/0221-0069$06.00/0 © 1983 American Chemical Society

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


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The i m p a c t s t r e n g t h , a s w e l l a s most o t h e r p h y s i c a l p r o p e r t i e s o f c r o s s l i n k e d epoxy r e s i n s , i s c o n t r o l l e d by t h e c h e m i c a l s t r u c t u r e and r a t i o o f t h e e p o x y r e s i n and h a r d e n e r , b y any added f i l l e r s , and b y t h e c u r i n g c o n d i t i o n s u s e d . U n f o r t u n a t e l y , c r o s s l i n k e d g l a s s y epoxy r e s i n s w i t h r e l a t i v e l y h i g h g l a s s t r a n s i t i o n t e m p e r a t u r e s (>10C°c) a r e b r i t t l e , L e e a n d N e v i l l e ( 1 ) P e r e z ( 3 ) . The p o o r i m p a c t s t r e n g t h o f h i g h g l a s s t r a n s i t i o n e p o x y r e s i n s i s a c r u c i a l l i m i t i n g f a c t o r f o r t h e usage o f e p o x i e s as s t r u c t u r a l m a t e r i a l s and i n c o m p o s i t e s . The t o u g h e n i n g o f c r o s s l i n k e d DGEBA b y R e a c t i v e L i q u i d P o l y mers ( R L P ) h a s b e e n s t u d i e d s i n c e a b o u t 1965. S e v e r a l a p p r o a c h e s h a v e b e e n t e s t e d u s i n g v a r i o u s RLP m o d i f i e r s . Generally, the t o u g h e n i n g method i s b a s e d o n t h e i n c o r p o r a t i o n o f a s m a l l p r o p o r t i o n o f a l o w Tg p o l y m e r , t y p i c a l l y b e t w e e n 5% and 2 0 % , i n t o the r i g i d epoxy r e s i n . On m i x i n g , t h e R L P , t h e e p o x y r e s i n and t h e c u r i n g a g e n t must b e c o m p a t i b l e . Then, d u r i n g t h e c u r i n g c y c l e , e l e m e n t s o f t h e RLP p r e c i p i t a t e f r o m t h e m a t r i x and f o r m a h o m o g e n e o u s l y d i s p e r s e d p a r t i c l e p h a s e , Manson and S p e r l i n g ( 2 ) . I t i s b e l i e v e d t h a t t h i s p a r t i c u l a r two p h a s e m o r p h o l o g y i s t h e key t o t h e t o u g h e n i n g m e c h a n i s m o f t h e h o s t m a t r i x ( 4 - 9 ) . The s u c c e s s o f t h e e p o x y t o u g h e n i n g p r i n c i p l e b y RLP depends v i t a l l y on t h e i n t e r a c t i o n o f t h e r u b b e r w i t h t h e e p o x y m a t r i x , and o n t h e p h a s e s e p a r a t i o n mechanism. C o n t r o l o f t h e s e two m a j o r v a r i a b l e s i s complex and o n l y p a r t i a l l y u n d e r s t o o d ( 8 , 9 ) . Experi m e n t a l r e s u l t s o f 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 RLP m o d i f i e d e p o x y r e s i n s a r e f r e q u e n t l y c o n f l i c t i n g , a n d depend o n t h e p r e p a r a t i o n techniques o f t h e polymer composites (9,10,11). S e v e r a l l o w m o l e c u l a r w e i g h t polymer m o d i f i e r s have been u s e d a s RLPs w i t h e p o x y / h a r d e n e r s y s t e m s . Noshay a n d Robeson (12) c o p o l y m e r i z e d p o l y c a p r o l a c t o n e and p o l y p r o p y l e n e o x i d e w i t h epoxy r e s i n s . The c u r i n g o f t h e e p o x y i n t h e p r e s e n c e o f t h e m o d i f i e r produced b l o c k copolymer s t r u c t u r e s o f c r o s s l i n k e d epoxy and l i n e a r m o d i f i e r s e g m e n t s . By t h i s method t h e l o w m o l e c u l a r w e i g h t p o l y m e r i c m o d i f i e r f l e x i b i l i z e d t h e e p o x y / c u r i n g agent network, and d r a s t i c a l l y l o w e r e d t h e g l a s s t r a n s i t i o n temperature o f t h e c r o s s l i n k e d p o l y m e r . The u s e o f h i g h e r m o l e c u l a r w e i g h t m o d i f i e r s r e s u l t e d i n t h e f o r m a t i o n o f a two p h a s e m o r p h o l o g y . The I z o d i m p a c t s t r e n g t h , w h i c h was u s e d a s t h e c r i t e r i o n f o r i m p r o v e d i m p a c t r e s i s t a n c e , was o n l y s l i g h t l y b e t t e r t h a n t h e controls. S p e r l i n g and F r i e d m a n (13) i n t r o d u c e d t h e S i m u l t a n e o u s I n t e r p e n e t r a t i n g Network (SIN) p r i n c i p l e . I n t h i s method t h e DGEBA r e s i n and monomers o f η-butyl a c r y l a t e w e r e s i m u l t a n e o u s l y p o l y m e r i z e d i n b u l k , one i n t h e p r e s e n c e o f t h e o t h e r . A two p h a s e m o r p h o l o g y emerged, w i t h t h e r u b b e r p h a s e d i s p e r s e d i n t h e glass matrix. The S I N c o n c e p t was f u r t h e r d e v e l o p e d b y a d d i n g s m a l l amounts o f g l y c i d y l m e t h a c r y l a t e t o t h e r e a c t i v e monomers s y s t e m , t o i m p r o v e t h e bond b e t w e e n t h e two p o l y m e r i c n e t w o r k s ,



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Poly(n-butyl acrylate) as Modifier

71

S c a r i t o and S p e r l i n g (14). The impact s t r e n g t h of the modified SIN system was indeed improved, but the g l a s s t r a n s i t i o n tempera t u r e of the composite decreased s u b s t a n t i a l l y r e l a t i v e to the epoxy c o n t r o l . The carboxyl terminated b u t a d i e n e - a c r y l o n i t r i l e (CTBN)/DGEBA (Epon 828) system was introduced by McGarry and W i l l n e r ( 4 ) . The compatible CTBN/Epon 828 mixture was cured i n the presence of a Lewis base c a t a l y s t , DMP-30. L a t e r , the b a s i c RLP-epoxy r e s i n system was modified and thoroughly s t u d i e d with a v a r i e t y of a d d i t i v e s and c u r i n g agents (5,6,15-19). The impact s t r e n g t h of pre sent CTBN/Epon 828 systems with Tg above 100°c i s b e t t e r than the impact s t r e n g t h observed w i t h most other RLP m o d i f i e r s . However, none of the p r e s e n t l y RLP/DGEBA formulations has improved the impact s t r e n g t h o f epoxy c o n t r o l s to the degree that has been demonstrated by RLP toughened thermoplastic polymers. The toughening mechanism o f thermoset p l a s t i c s w i t h RLP i s s t i l l not w e l l understood. The study of the toughening of DGEBA r e s i n w i t h a new RLP, Carboxyl Terminated η-Poly B u t y l A c r y l a t e (CTPnBA), i s reported here. The CTPnBA rubber was synthesized by d i l u t e s o l u t i o n p o l y m e r i z a t i o n and b u l k p o l y m e r i z a t i o n techniques (20,21). The carboxyl f u n c t i o n a l i t y of the polymers v a r i e d from f = 1.3 ( i n s o l u t i o n polymerization) to f = 1.8-2.0 ( i n bulk p o l y m e r i z a t i o n ) . Experimental The epoxy networks reported i n t h i s study were from DGEBA type monomer (EPON 828 by S h e l l ) and CTPnBA (synthesized as described by G a z i t and B e l l , 21). The molecular weight of the RLP was c h a r a c t e r i z e d by Vapor Pressure Osmometry (VPO) i n chloroform s o l u t i o n a t 37°c. The molecular weight of the CTPnBA v a r i e d between Mn = 6600 and Mn = 10,300. An acid/base p o t e n t i o m e t r i c t i t r a t i o n procedure i n organic s o l u t i o n was used to d e t e r mine the f u n c t i o n a l i t y of the CTPnBA rubber (20). The f u n c t i o n a l i t y o f the CTPnBA polymerized i n s o l u t i o n was r e l a t i v e l y low, f - 1.3, w h i l e i n the bulk p o l y m e r i z a t i o n o f the f u n c t i o n a l i t y of the l i n e a r CTPnBA reached the t h e o r e t i c a l maximum f = 2.0. One must be c a r e f u l to completely remove any r e s i d u a l carboxy1c o n t a i n i n g p o l y m e r i z a t i o n c a t a l y s t and chain t r a n s f e r agent b e f o r e f u n c t i o n a l i t y determination. Three c u r i n g agents were s t u d i e d , at v a r i o u s c u r i n g condi t i o n s : methylene d i a n i l i n e (MDA), 2.5-dimethyl-2.5-hexanediamine (DMHDA), and DMP-30. The epoxy/RLP systems were cured between separated g l a s s p l a t e s which had been t r e a t e d w i t h a f l u o r c a r b o n mold r e l e a s e agent; the hot ( u s u a l l y 80° to 120°c) l i q u i d epoxy r e s i n o r epoxy/RLP composition was thoroughly hand mixed w i t h the c u r i n g agent f o r about a minute. A i r bubbles were removed from the mixture during a few minutes i n a vacuum oven at 80°c. The c l e a r mixture was then poured between the p l a t e s and placed i n a c o n t r o l l e d oven f o r the d e s i r e d c u r i n g c y c l e . The cured p l a t e s


EPOXY RESIN


72

CHEMISTRY

were s l o w l y cooled to room temperature a f t e r the c u r i n g c y c l e . Samples f o r the mechanical t e s t s were machined to standard Notched Izod Test (ASTM D256-56), and to T e n s i l e Impact T e s t dog-bone shapes. The T e n s i l e Impact T e s t e r used i n t h i s study i s a modified v e r s i o n o f the Plastechon U n i v e r s a l T e s t e r by Plas-Tech Equipment Co. I t was operated at a ram speed of 1.5 m/sec, w i t h a maximum s t r o k e of 2.45 cm. Load was measured by a Dynisco TCFTS-1.5M s o l i d - s t a t e bonded gage transducer. Displacement was measured by the Kaman M u l t i - V i t displacement measuring system, Model DK-230010CU. The Kaman transducer made non-contacting measurement of the diplacement p o s s i b l e . The readout of the two transducers was v i a an E x p l o r e r I I I d i g i t a l o s c i l l o s c o p e by N i c o l e t Instrument C o r p o r a t i o n . E x p l o r e r I I I i s a two channel, h i g h r e s o l u t i o n , storage o s c i l l o s c o p e with a c a p a b i l i t y of s t o r i n g s i g n a l waveforms on a magnetic d i s c . The o s c i l l o s c o p e recorded X/time and Y/time simultaneously and had the o p t i o n of X/Y d i s p l a y . These f e a t u r e s were used to c o n s t r u c t s t r e s s - s t r a i n curves. Ultimate s t r a i n ε and s t r e s s ou were c a l c u l a t e d d i r e c t l y from the o s c i l l o s c o p e readout. Impact s t r e n g t h was c a l c u l a t e d by measuring the area under the s t r e s s - s t r a i n curve. T y p i c a l displacement/time and force/time curves obtained by t h i s method are shown i n F i g u r e 1. Scanning E l e c t r o n Microscopy (SEM) w i t h m a g n i f i c a t i o n capabi l i t y of up to 20,000X was used f o r the study of the polymer com p o s i t e f r a c t u r e s u r f a c e s . The f r e s h l y f r a c t u r e d surfaces were coated w i t h a 25θ£ t h i c k Au-Pd conducting l a y e r . Dynamic Mechanical Spectroscopy was s t u d i e d by Rheovibron, Model DDV-II a t h e a t i n g r a t e o f 0.5°C/min. Standard samples were used; the Rheovibron was operated a t 110 Hz. R e s u l t s and D i s c u s s i o n Toughening o f DGEBA by RLPs r e s u l t s from the two phase morpho logy formed by the c o n t r o l l e d p r e c i p i t a t i o n of rubbery p a r t i c l e s from the epoxy/RLP/curing agent mixture during the c u r i n g process. T h i s phase s e p a r a t i o n o f the rubbery p a r t i c l e s from the g e l l i n g matrix s t a r t s from a homogeneous s o l u t i o n of the system compon ents. The m i s c i b i l i t y of the Epon 828/CTPnBA systems depends on the concentrations of the components, the molecular weight of the rubber and the mixture temperature. A t y p i c a l phase diagram of CTPnBA (Mn = 10,300)/Epon 828 mixture i s shown i n F i g u r e 2. The concentration/temperature r e l a t i o n s h i p o f Epon 828 and v a r i o u s CTPnBA systems can be d e s c r i b e d by a f a m i l y of p a r a l l e l curves, s h i f t i n g from l e f t to r i g h t w i t h decreasing molecular weight of the CTPnBA (20). Since the optimal rubber c o n c e n t r a t i o n i n the composite should be i n the range of 10%-20%, the example CTPnBA shown i n F i g u r e 2 i s not compatible w i t h Epon 828 at room tempera ture and the d e s i r e d c o n c e n t r a t i o n l e v e l s . In a separate study (21) i t was concluded that the l i m i t i n g molecular weight f o r the s o l u b i l i t y o f CTPnBA i n Epon 828 at low c o n c e n t r a t i o n s (£ 10%


4.

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DISPLACEMENT/ TIME

FORCE/TIME

F i g u r e 1.

A t y p i c a l output o f the TIT; displacement/time, force/time.

160

120

single phase

ο

/

/

80 Μη · 10,300

ο φ

V

two phase

k.

"δ 40 k_ α>

-

β

Μη > 4,400

CL

ε

0

J .2

I .4

ι .6

I .8

1.0

Epon 828, Weight fraction F i g u r e 2. T y p i c a l P h a s e D i a g r a m f o r a m i x t u r e o f Epon 828 a n d PnBA r u b b e r .




74

CTPnBA), a t 40°C, was about Mn = 4400. T h i s i s shown as p o i n t A i n Figure 2. I t was a l s o observed that CTPnBA rubbers o f higher molecular weights, up t o Mn = 6600, became m i s c i b l e w i t h Epon 828 at room temperature i n the presence o f s e l e c t e d c u r i n g agents; t h i s study w i l l be presented l a t e r . The c u r i n g agent plays an important r o l e i n c o n t r o l l i n g the mechanical p r o p e r t i e s o f epoxy r e s i n s . Most c u r i n g agents p a r t i c i p a t e i n the c o n s t r u c t i o n o f the c r o s s l i n k e d epoxy network. Therefore, the nature o f the c u r i n g agent and the epoxy r e s i n / c u r i n g agent r a t i o have a major e f f e c t i n shaping the p r o p e r t i e s and the morphology o f the thermoset polymer. Although DMP-30 a c t s only as a c a t a l y s t i n the c u r i n g process o f Epon 828, the concentration o f t h i s t e r t i a r y amine has a s u b s t a n t i a l e f f e c t on the c r o s s l i n k d e n s i t y and t h e r e f o r e on the impact s t r e n g t h of the c r o s s l i n k e d epoxy. CTPnBA/EPon 828 systems cured w i t h v a r i o u s concentrations of DMP-30 were s t u d i e d by notched Izod impact t e s t , Table I. The CTPnBA used i n t h i s study had a molecular

TABLE I .

#

a-1 a-2 a-3 a-4 a-5 a-6 a-7

Izod impacts s t r e n g t h o f Epon 828/PnBA/DMP-30 systems and c o n t r o l s : Low f u n c t i o n a c i t y RLP

Epon 828 parts 100 100 100 100 100 100 100

CTPnBA parts

DMP-30 parts

10 10 10 / / 10 /

1.0 2.3 5.0 5.0 7.2 10.0 10.0

Impact strength Izod ( l b - f t / i n ) 1.13±0.07 0.91±0.06 0.77±0.01 1.37±0.27 1.18±0.14 0.79±0.09 0.86±0.20

weight of Mn = 3850, and a low f u n c t i o n a l i t y of f = 1.15-1.30. The r e s i n s were cured a t 120°C/60 min and post cured a t 150°C/180 min. The r e s u l t s i n d i c a t e that the impact s t r e n g t h o f the p o l y mers c o n t a i n i n g the low f u n c t i o n a l CTPnBA rubber were i n general lower than the c o n t r o l s , an observation which i s i n agreement w i t h s i m i l a r s t u d i e s i n the f i e l d (4,14). This behavior may be expected s i n c e the rubber i s not bonded to the r e s i n , and the rubber p a r t i c l e s e f f e c t i v e l y a c t as " h o l e s " i n the matrix and reduce the s t r e n g t h . The e f f e c t of the c u r i n g agent concentration on the morphology of the polymer-polymer composites was s t u d i e d by SEM. Micrographs o f the f r a c t u r e d surfaces o f samples a-1, a-2, a-3 and a-4 are shown i n Figures 3, 4, 5 and 6 r e s p e c t i v e l y . The average d e n s i t y and diameter o f the rubber p a r t i c l e s a r e given i n Table I I . The number o f p a r t i c l e s per u n i t area increased w i t h



GAZIT

A N D

B E L L


F i g u r e 3.

SEM m i c r o g r a p h o f c o m p o s i t e a - 1 , 1700X.

F i g u r e h.

SEM m i c r o g r a p h o f c o m p o s i t e a-2, 1500X.




F i g u r e 5.

SEM m i c r o g r a p h o f c o m p o s i t e a - 3 , 1750X.

F i g u r e 6.

SEM m i c r o g r a p h o f c o n t r o l a-4, 750X.


4.

GAZIT AND BELL

TABLE I I .

A v e r a g e number a n d d i a m e t e r o f t h e r u b b e r p a r t i c l e s i n Epon 828/CTPnBA/DMP-30 c o m p o s i t e s T a b l e 1 g i v e s the compositions. average diameter (cm)

# of particles p e r cm

#

1.18 1.48 2.24

a-1 a-2 a-3


77


χ χ χ

10* 107 10*

1.36

χ

9.18 4.37

χ χ

4

10" 10~^ 10"°

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 t h e c u r i n g agent, whereas t h e average d i a m e t e r o f t h e p a r t i c l e s showed a n o p p o s i t e t r e n d . The o b s e r v a t i o n s c o u l d be e x p l a i n e d by t h e d i f f e r e n c e i n t h e degree o f s o l u b i l i t y o f t h e CTPnBA r u b b e r i n t h e e p o x y r e s i n , a s a f f e c t e d b y the v a r i o u s c o n c e n t r a t i o n s o f t h e c u r i n g agent. This hypothesis i s based on t h e enhanced m i s c i b i l i t y o f t h e rubber/epoxy system w i t h t h e 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 DMP-30. A n o t h e r h y p o t h e s i s w h i c h might e x p l a i n t h e o b s e r v a t i o n i s based on t h e d i f f e r e n c e i n the p r e c i p i t a t i o n r a t e o f t h e rubber p a r t i c l e s , during the c u r i n g p r o c e s s , a t t h e v a r i o u s DMP-30 c o n c e n t r a t i o n s . This theory assumes t h a t t h e c r o s s l i n k i n g d e n s i t y o f t h e e p o x y r e s i n i s a f u n c t i o n o f t h e c u r i n g agent c o n c e n t r a t i o n . From t h e d a t a i n T a b l e I we s e e t h a t t h e p r e s e n c e o f a l o w f u n c t i o n a l i t y RLP i n t h e epoxy n e t w o r k h a s r e d u c e d t h e i m p a c t s t r e n g t h o f t h e c r o s s l i n k e d Epon 828. The e f f e c t o f a h i g h e r r u b b e r f u n c t i o n a l i t y o n t h e i m p a c t s t r e n g t h o f t h e epoxy/RLP s y s t e m was s t u d i e d w i t h CTPnBA w h i c j i was p o l y m e r i z e d i n b u l k ( 2 1 ) , a n d h a d a f u n c t i o n a l i t y o f f - 1.8. The c o m p o s i t i o n o f Epon 828/CTPnBA s y s t e m s c u r e d w i t h t h r e e d i f f e r e n t c u r i n g a g e n t s a r e g i v e n i n T a b l e I I I . T h e CTPnBA r u b b e r h a d a m o l e c u l a r w e i g h t

TABLE I I I .

Composite and c o n t r o l c o m p o s i t i o n s : f u n c t i o n a l i t y RLP's.

Epon 828 + TBAI parts 3-a 3-b 3-c 3-d 3-e 3-f 3-g 3-h

100 100 100 100 100 100 100 100

CTPnBA parts 9.9 25.0 9.9 10.0 10.0

/ / /

Higher

curing type

agent parts

DMHDA DMHDA DMP-30 DMP-30 MDA DMHDA DMP-30 MDA


27.0 27.0 1.4 4.9 37.9 27.7 5.0 39.2


78


average o f Mh • 6600, and a f u n c t i o n a l i t y o f f = 1.91. The l i q u i d rubber was i n i t i a l l y mixed w i t h the Epon 828, which con t a i n e d 1% by weight o f d i s s o l v e d tetra-butylammonium i o d i d e (TBAI). The TBAI c a t a l y s t was added to the mixture to enhance the r e a c t i o n between the terminal carboxyl groups and the epoxide r i n g s . The epoxy and the CTPnBA mixtures were not compatible a t room temperature, but the a d d i t i o n of 2,5-dimethyl-2,5 hexanediamine (DMHDA) c u r i n g agent r e s u l t e d i n c l e a r i n g of the epoxy/rubber s o l u t i o n ; the "hazy" mixture became u s u a l l y c l e a r . The a d d i t i o n of the DMP-30 and the methylene d i a n i l i n e (MDA) to the Epon 828/CTPnBA d i d not c o n s i d e r a b l y change the m i s c i b i l i t y of the mixture components. The systems were cured a t 120°C/60 min followed by 150°C/180 min. Table IV summarizes the Izod and the t e n s i l e impact strength r e s u l t s o f the polymer-polymer composites and the r e s p e c t i v e c o n t r o l s . Four s i g n i f i c a n t r e s u l t s were observed.

TABLE IV.

//

Impact s t r e n g t h o f the composites and c o n t r o l s . ± Values represent standard d e v i a t i o n s .

Izod Impact

Strength

(lb-ft/in) 3-a 3-b 3-c 3-d 3-e 3-f 3-g 3-h

1.43 1.51 0.89 1.01 1.13 1.40 1.73 1.68

+ 0.10 ± 0.12 + 0.07 + 0.12 + 0.13 + 0.22 + 0.27 + 0.22

T e n s i l e Impact Strength σ

(pa χ 10~ )

U

8.68 9.15 7.32 6.36 6.87 10.36 8.88 9.60

ε

(%)

U

± ± ± ± ± ± ± ±

0.77 0.63 0.15 1.33 1.36 0.63 0.49 0.32

15.7 18.6 12.9 10.5 15.8 9.7 9.2 9.1

Impact Strength 3

3

(J/m χ 1 0 ) + 1.4 + 1.3 + 3.6 + 1.1

± 2.8 + 2.5 + 1.9 + 0.7

7.77 8.62 5.75 3.76 6.15 5.45 5.47 6.03

± ± ± ± ± ± ± ±

1.57 1.02 1.09 0.75 0.65 2.55 1.48 0.31

1. The Izod impact s t r e n g t h o f the composites remain i n general lower than the c o n t r o l s . 2. The t e n s i l e impact energies o f a l l composites (except f o r 3-d) a r e h i g h e r than the c o n t r o l s . 3. The high-speed u l t i m a t e s t r a i n s o f a l l the composites are l a r g e r than the c o n t r o l , up to 90% i n the extreme case. 4. The u l t i m a t e s t r e s s e s o f the composites a r e s l i g h t l y lower than the c o n t r o l s . These f a c t s , without f u r t h e r examining the d e t a i l s (type of rubber and c u r i n g agents) suggest that a. Based on the T e n s i l e Impact T e s t e r r e s u l t s , the CTPnBA rubber has toughened the epoxy somewhat, p r i m a r i l y through i n c r e a s i n g the u l t i m a t e t e n s i l e s t r a i n .



4.

GAZIT AND BELL


79

b. The toughening e f f e c t o f the rubber was not evident from the Izod impact t e s t . The toughening mechanism i s thought to r e s u l t from a m i c r o c a v i t a t i o n process. This phenomenon occurs throughout the gage l e n g t h o f the dogbone sample i n the t e n s i l e t e s t e r , w h i l e the Izod f r a c t u r e i s l i m i t e d to deformations i n a short r e g i o n o f crack propagation i n the sample. I t appears that the rubber toughening mechanism cannot be f u l l y appreciated by the nature o f the f r a c t u r e imposed on the notched sample i n the Izod t e s t . A t y p i c a l s t r e s s - s t r a i n curve o f a CTPnBA/Epon 828 composite i s shown i n F i g u r e 7. The e l a s t i c t e n s i l e s t r a i n was followed by the " p l a s t i c " s t r a i n which was a c t u a l l y a deformation caused by a m i c r o c a v i t a t i o n mechanism throughout the whole gage l e n g t h . The composites t e s t e d on the T e n s i l e Impact Tester have " s t r e s s whitened" i n the gage s e c t i o n , Figure 8. T h i s whitening i s an evidence f o r m i c r o c a v i t a t i o n i n the composite, a phenomenon not observed i n the c o n t r o l samples. O v e r a l l , the s t r a i n o f the polymer-polymer composites appears to r e s u l t from a combination of e l a s t i c deformation and m i c r o c a v i t a t i o n / p l a s t i c deformation. A " b u t t e r f l y " p a t t e r n i s observed upon bending or t e n s i l e d e f o r mation of the composites ( f i g . 9 ) . The s t r e s s whitening and ' b u t t e r f l y ' p a t t e r n both disappear upon m i l d h e a t i n g ; the sample a f t e r h e a t i n g appears the same as b e f o r e deformation. Some composite samples (cured w i t h DMHDA) have a l s o experienced shear y i e l d i n g during the t e n s i l e impact t e s t , i n a d d i t i o n to the m i c r o c a v i t a t i o n deformation, Figure 10. Shear y i e l d i n g was observed, although t o a l e s s e r degree, w i t h the c o n t r o l s of the DGEBA/DMHDA systems. The e f f e c t o f the c u r i n g c y c l e on the impact s t r e n g t h of the CTPnBA/Epon 828 system was found to be s i g n i f i c a n t . The e f f e c t was demonstrated i n a two step c u r i n g c y c l e procedure. I n the f i r s t step o f the procedure 40 p a r t s o f CTPnBA rubber (îîn ^ 6600, f = 1.91) were mixed w i t h 60 p a r t s of Epon 828, c a t a l y z e d by 1% TBAI. The mixture was reacted a t 120°C f o r 2 h r s . During t h i s stage the carboxyl-terminated η-butyl a c r y l a t e reacted w i t h the Epon 828 to near completion; t i t r a t i o n of the mixture a f t e r the 2 h r s . r e a c t i o n showed e s s e n t i a l l y no c a r b o x y l groups remaining i n the s o l u t i o n . The epoxy r e s i n , i n l a r g e excess, had reacted w i t h the CTPnBA to form an epoxy/rubber/epoxy intermediate compound. In the second step, the i n t e r m e d i a t e compound was d i s s o l v e d i n more epoxy to form a mixture o f 10 p a r t s CTPnBA i n 100 p a r t s Epon 828. Then, the c u r i n g agent (DMHDA) was added and the mixture was cured a t 120°C/60 min and then 150°C/120 min. The two step c u r i n g c y c l e r e s u l t e d i n a tougher composite having an Izod impact s t r e n g t h o f 1.70±0.20 f t - l b / i n o f notch, an improvement of 21% over the c o n t r o l sample 3f, and a t e n s i l e impact s t r e n g t h o f (10.80±2.2) χ 10" (J/m ) , an improvement o f 98% over the c o n t r o l . These r e s u l t s suggest the two step c u r i n g c y c l e was b e t t e r than the s i n g l e step procedure. Samples made by the two step c u r i n g c y c l e procedure showed a p p r e c i a b l e s t r e s s whitening i n the t e n s i l e impact t e s t .




80

F i g u r e 8.

S t r e s s w h i t e n i n g o f c o m p o s i t e 39-b f o r m e d b y s t r e s s i n the t e n s i l e impact t e s t .


tensile


4.

GAZIT

A N D B E L L


F i g u r e 9. M i c r o c a v i t a t i o n i n a " b u t t e r f l y " p a t t e r n formed i n Epon 828/CTPnBA c u r e d b y DMHDA i n t h e two s t e p p r o c e d u r e .

F i g u r e 1 0 . Shear band f o r m a t i o n i n a t e n s i l e t e s t o f t h e DGEBA/DMHDA s y s t e m .


81

82


The g l a s s t r a n s i t i o n t e m p e r a t u r e o f t h e PnBA c o m p o s i t e c u r e d u s i n g t h e two s t e p p r o c e d u r e was o n l y a b o u t 3°C l o w e r t h a n t h e 3 f c o n t r o l w i t h o u t r u b b e r , F i g u r e 1 1 . T h i s shows t h a t t h e m e c h a n i s m o f r u b b e r t o u g h e n i n g was n o t s i m p l e p l a s t i c i z a t i o n . Interest i n g l y , t h e 3 - t r a n s i t i o n o f t h e c o m p o s i t e a p p e a r e d t o be s l i g h t l y h i g h e r t h a n t h e Epon 828-(DMHDA) c o n t r o l .


Conclusions The p o t e n t i a l o f CTPnBA a s a n RLP f o r e p o x y r e s i n s was shown t o b e p r o m i s i n g . F o r e f f e c t i v e t o u g h e n i n g , t h e p o l y η-butyl a c r y l a t e must b o n d t o t h e m a t r i x . C a r b o x y l t e r m i n a l g r o u p s on PnBA were found e f f i c i e n t f o r t h i s purpose. The t y p e o f c u r i n g a g e n t and t h e c u r i n g p r o c e d u r e w e r e f o u n d t o be p r i m a r y f a c t o r s i n c o n t r o l l i n g t h e t o u g h e n i n g o f t h e c o m p o s i t e ; DMHDA was f o u n d most e f f e c t i v e c u r i n g a g e n t f o r t h e DGEBA/CTPnBA s y s t e m . The two s t e p c u r i n g p r o c e s s gave b e s t impact s t r e n g t h . I t was o b s r v e d t h a t t h e t o u g h e n i n g mechanism was m a i n l y b y a m i c r o c a v i t a t i o n p r o c e s s , h o w e v e r , some s h e a r d e f o r m a t i o n was e v i d e n t i n a f e w o f t h e t e s t e d samples.

Uo

2

i_

Temperature, F i g u r e 11.

°C

The m e c h a n i c a l damping o f Epon 828/DMHDA s y s t e m v s . t e m p e r a t u r e a t v a r i o u s amine e x c e s s f o r m u l a t i o n s , 0 - 0%, θ - 2 5 % , © - 5 0 % , β - 7 5 % , 0 - 1 0 0 % , Q - CTPnBA m o d i f i e d s y s t e m , c u r e d i n t h e two s t e p p r o c e d u r e .


4.

GAZIT AND BELL


83

Acknowledgment The authors express t h e i r a p p r e c i a t i o n to S h e l l Development Company f o r support of t h i s r e s e a r c h .


Literature Cited 1. Lee, H.; Neville, K. "Handbook of Epoxy resins," McGrawH i l l , New York, 1967. 2. Manson, J. Α.; Sperling, L. H. "Polymer Blends and Compos ites," Plenum Press, New York, 1976. 3. Perez, R. J. in "Epoxy Resin Technology," P. S. Bruins, Ed., Interscience, New York, 1968, p. 45. 4. McGarry, F. J.; Willner, A. M. "Toughening of Epoxy Resin by an Elastomeric Second Phase," R68-8, March, MIT, 1968. 5. Manzione, L. T.; Gillham, J. K. American Chemical Society Organic Coatings and Plastics Chemistry Preprints, 1979, 41, 364. 6. Manzione, L. T.; Gillham, J. K. American Chemical Society Organic Coatings and Plastics Chemistry Preprints, 1979, 41, 371. 7. Bucknall, C. B.; Yoshii, T. The British Poly J., 1978, 10, 53. 8. Manzione, L. T.; Gillham, J. K.; McPherson, C. A. J. Appl. Polym. Sci. 1981, 26, 889. 9. Manzione, L. T.; Gillham, J. K.; McPherson, C. A. J. Appl. Polym. Sci. 1981, 26, 907. 10. Meeks, A. C.; Polymer, 1974, 15, 675. 11. Walker, J. W.; Richardson, W. E.; Smith, C. H. American Chemical Society Organic and Plastics Chemistry Preprints, 1975, 35, 333. 12. Noshay, Α.; Robeson, L. M. J. Polym. Sci., Chem. Ed., 1974, 12, 689. 13. Sperling, L. H.; Friedman, D. W. J. Polym. Sci., part A-2, 1969, 7, 425. 14. Scarito, P. R.; Sperling, L. H. Poly. Eng. and Sci., 1979, 19, 297. 15. Rowe, Ε. H. 26th Annu. Tech. Conf., Reinforced Plastics/Com posites Div., SPI, Sec. 12-E, 1, 1971. 16. Riew, C. K.; Rowe, Ε. H.; Siebert, A. R. in "Toughness and Brittleness of Plastics", ADVANCES IN CHEM. SERIES, No. 154, American Chemical Society, Washington, D. C., 1976, p. 326. 17. Rowe, Ε. H. 24th Annu. Tech. Conf. Reinforced Plastics/Com posites Div., SPI, Sec. 11-A, 1, 1969. 18. Rowe, Ε. H.; Siebert, A. R.; Drake, R. S. Modern Plastics, 1970, 47, 110. 19. Drake, R.; Siebert, A. SAMPE Quarterly, July, 1975. 20. Gazit, S. "Toughening of Epoxy Resin by Acrylic Elastomer," Ph.D. Dissertation, University of Connecticut, Sept. 1980. 21. Gazit, S.; Bell, J. P. (another paper in this volume) RECEIVED

December 16, 1982 In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Epoxy Resin Chemistry II - American Chemical Society

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