22 Fatigue Crack Propagation in P V C : Combined Effects of Rubbery Inclusions and Molecular Weight 1
M. D.SKIBO ,J. A. MANSON, S. M. WEBLER, and R. W. HERTZBERG Materials Research Center, Lehigh University, Bethlehem, PA 18015 E. A. COLLINS B. F. Goodrich Chemical Co., Avon Lake, OH 44012 2
This paper addresses three topics of current scientific and technological interest: poly(vinyl chloride) (PVC); toughening of brittle or ductile but notch-sensitive polymers by the inclu sion of rubbery phases; and the kinetics of fatigue crack propa gation (FCP). The research described below is part of an extensive funda mental program on the effects of polymer structure, composition, and morphology on FCP in a wide range of polymers (1-11), from highly crystalline (e.g., polyamides) or mesomorphic (e.g., PVC) polymers, to typically glassy ones (e.g., acrylics). Within this program, PVC has been selected for special attention because of its interesting range of structural and compositional features (e.g., states of order, tacticity, and effects of second phases), and because of its technological importance. The first paper in this series describes results of contin ued research on the effects of molecular weight (M) on FCP re sponse (12). This paper describes and discusses results obtained in a study of the effects of a rubbery phase on the impact strength and FCP behavior of a series of PVC matrixes comprising a range of different molecular weights. It will be shown that combined effects of M with elastomer content — in this case, a methacrylate-butadiene-styrene (MBS) copolymer— lead to interest ing FCP behavior. Additional results will be discussed in future papers, as well as results of current studies on the effects of other structural and compositional factors. FCP i n P o l y m e r s . Most, i f not a l l , a r t i f a c t s f a b r i c a t e d from p l a s t i c s c o n t a i n i n h e r e n t f l a w s t h a t may, i f a p p r o p r i a t e l o a d c r i t e r i a a r e m e t , p r o p a g a t e t h r o u g h t h e s p e c i m e n and r e s u l t i n c a t a s t r o p h i c f a i l u r e ( 1 , 1 3 , 1 4 ) , o f t e n at inconvenient times and p l a c e s . C y c l i c o r r e p e t i t i v e l o a d i n g ( f a t i g u e ) can be e s p e c i a l l y i n s i d i o u s , f o r f a i l u r e can occur at loads that are w e l l 'Current address: S a n d i a L a b s , L i v e r m o r e , CA 94550 Current address: Diamond Shamrock C o r p . , P a i n e s v i l l e , OH 44077 2
0-8412-0485-3/79/47-095-311$05.00/0 © 1979 A m e r i c a n C h e m i c a l Society
DURABILITY OF
312
MACROMOLECULAR MATERIALS
below those corresponding to the s t a t i c y i e l d or f r a c t u r e s t r e s s . S i n c e many p o l y m e r s i n s e r v i c e a r e i n f a c t s u b j e c t e d t o c y c l i c l o a d s o f some k i n d , f u n d a m e n t a l u n d e r s t a n d i n g o f t h e f a t i g u e p r o c e s s i s t h u s o f e n g i n e e r i n g as w e l l as s c i e n t i f i c i n t e r e s t . In p a r t i c u l a r , i t i s b e g i n n i n g t o be r e c o g n i z e d t h a t , c o n t r a r y t o e a r l i e r b e l i e f s , much, and o f t e n m o s t , o f t h e f a t i g u e l i f e i s spent i n propagation of a p r e e x i s t e n t f l a w , r a t h e r than i n the i n i t i a t i o n o f c r a c k growth (15). To c h a r a c t e r i z e t h e r a t e o f FCP, i t i s c o n v e n i e n t t o r e l a t e t h e r a t e o f c r a c k g r o w t h p e r c y c l e , da/dN, t o AK, t h e r a n g e o f t h e s t r e s s i n t e n s i t y f a c t o r a t t h e c r a c k t i p . The f o l l o w i n g e q u a t i o n (16) has b e e n f o u n d t o be u s e f u l f o r many p o l y m e r s ( 1 ) :
dN When A and n a r e c o n s t a n t s ( a t l e a s t o v e r a r a n g e o f A K ) . The s t r e s s i n t e n s i t y f a c t o r , AK, i s i t s e l f a f u n c t i o n o f g e o m e t r y , a p p l i e d s t r e s s , Aa, and c r a c k l e n g t h , a ( 1 7 ) : AK = YAa^a
(2)
where Y t a k e s a c c o u n t o f t h e s p e c i m e n g e o m e t r y ( s e e b e l o w ) . I n p r e v i o u s s t u d i e s by o t h e r s (18,19) and by t h i s g r o u p ( 8 , 9) i t was shown t h a t e q u a t i o n 1 s e r v e d w e l l t o d e s c r i b e t h e b e h a v i o r o f t y p i c a l PVC's. S t u d i e s by t h i s group a l s o d e m o n s t r a t e d a r e m a r k a b l e and u n e x p e c t e d e f f e c t o f M on FCP, b o t h i n p o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA) and PVC (2,_3,9) • i t was a l s o shown t h a t PVC t e n d e d t o f a i l by a d i s c o n t i n u o u s c r a c k g r o w t h mechanism. T h i s p r o c e s s c o m p r i s e s p r o g r e s s i v e g r o w t h , d u r i n g many c y c l e s , o f a c r a z e ahead o f t h e c r a c k t o a c h a r a c t e r i s t i c l e n g t h , f o l l o w e d by s t r i k i n g t h r o u g h o f t h e c r a c k i n one c y c l e , and r e i n i t i a t i o n o f the p r o g r e s s i v e c r a z e growth (4,80. The q u e s t i o n o f t o u g h e n i n g p o l y m e r s ( e s p e c i a l l y a g a i n s t i m p a c t l o a d i n g ) by t h e j u d i c i o u s i n c o r p o r a t i o n o f r u b b e r y p h a s e s has a l s o r e c e i v e d much a t t e n t i o n i n r e c e n t y e a r s ( s e e , f o r e x a m p l e , r e f e r e n c e s _20, 21_, and 2 2 ) . D e p e n d i n g on t h e c o n c e n t r a t i o n , s t a t e o f s u b d i v i s i o n , and p r o p e r t i e s o f t h e e l a s t o m e r p e r s e , v a r i o u s t o u g h e n i n g mechanisms h a v e b e e n p r o p o s e d a n d , i n some cases, demonstrated. For example, the s t i m u l a t i o n of l o c a l i z e d c r a z i n g and s h e a r y i e l d i n g i n t h e m a t r i x , and t h e r e d u c t i o n o f t r i a x i a l i t y a t a crack t i p , undoubtedly p l a y s i g n i f i c a n t r o l e s i n t o u g h e n i n g , a t l e a s t u n d e r many common l o a d i n g c o n d i t i o n s . Howe v e r , much more a t t e n t i o n has b e e n g i v e n t o t h e e f f e c t s o f s t a t i c than to c y c l i c s t r e s s e s . A l s o , p e r u s a l of the l i t e r a t u r e (see, f o r e x a m p l e , r e f e r e n c e 20, pp. 313) r e v e a l s t h a t few s t u d i e s o f the e f f e c t s of rubber have i n v o l v e d the use o f a constant-M m a t r i x when t h e r u b b e r c o n c e n t r a t i o n i s v a r i e d . Our own e a r l i e r s t u d i e s s h o w i n g an a p p a r e n t l y b e n e f i c i a l r o l e o f a r u b b e r y p h a s e (1,10,11) a r e no e x c e p t i o n .
22.
SKIBO E T A L .
PVC
Fatigue
Crack
313
Propagation
Now t h e i n c o r p o r a t i o n o f r u b b e r y p h a s e s i n PVC i s o f p a r t i c u l a r i n t e r e s t , f o r , a s p o i n t e d o u t b y B u c k n a l l ( 2 0 , p . 2 9 9 ) , maximum t o u g h e n i n g b y t h e a d d i t i o n o f r u b b e r y p h a s e s i s o b t a i n e d when t h e m a t r i x i s i t s e l f d u c t i l e , a s i s t h e c a s e w i t h PVC. Evidence suggests t h a t t h e r u b b e r y i n c l u s i o n s i n d u c e a h i g h degree of shear y i e l d i n g i n t h e m a t r i x , r e s u l t i n g i n a l e s s e n i n g o f n o t c h s e n s i t i v i t y c h a r a c t e r i s t i c o f such m a t r i x e s , and a d r a m a t i c i n c r e a s e i n f r a c t u r e energy. Certainly polyblending of elastomers w i t h b r i t t l e p o l y m e r s d o e s seem t o i m p r o v e f a t i g u e r e s i s t a n c e t o some d e g r e e , t h o u g h n o t t o a s h i g h a l e v e l a s w o u l d b e d e s i r a b l e ( 2 3 ) ; g r e a t e r improvements a r e seen w i t h c r y s t a l l i n e and d u c t i l e polymers such as polyamides (24). T h i s paper, then, a d d r e s s e s two s p e c i f i c q u e s t i o n s : ( 1 ) a r e t h e d r a m a t i c a l l y h i g h v a l u e s o f i m p a c t s t r e n g t h i n t o u g h e n e d PVC c a r r i e d o v e r i n t o f a t i g u e ? a n d ( 2 ) how does t h e M o f t h e m a t r i x i n t e r a c t w i t h t h e r u b b e r added? A t h i r d q u e s i o n , t h e mechanism o f FCP i n t h i s s y s tem w i l l b e d e a l t w i t h i n d e t a i l s e p a r a t e l y . Experimental Materials. The PVC r e s i n s u s e d w e r e c h a r a c t e r i z e d b y meas u r e m e n t s o f i n t r i n s i c v i s c o s i t y ( [ n ] ) i n c y c l o h e x a n o n e a t 30°C. V a l u e s o f [n] a r e g i v e n i n T a b l e s I a n d I I , a l o n g w i t h v a l u e s o f weight-average m o l e c u l a r w e i g h t , M , e s t i m a t e d from a c a l i b r a t i o n c u r v e o f [ n ] v s . M d e v e l o p e d f o r a n e a r l i e r s e r i e s o f PVC s p e c i mens ( 9 ) . The m e t h a c r y l a t e - b u t a d i e n e - s t y r e n e (MBS) m o d i f i e r u s e d was g r a d e BTA I I I N, s u p p l i e d b y t h e M i t s u i Co., New Y o r k . Partic l e s i z e s r a n g e d m a i n l y b e t w e e n 500& a n d 750A ( 5 0 a n d 75 nm), w i t h a f r a c t i o n i n t h e r a n g e o f 2000A t o 2500& (200 t o 250 nm). F i v e d i f f e r e n t M's w e r e s e l e c t e d f o r t h e m a t r i x r e s i n s ; e a c h was b l e n d e d w i t h 0 p a r t s p e r h u n d r e d o f MBS ( p h r ) , 6 p h r , 10 p h r , a n d 14 p h r . The two h i g h e s t - M s a m p l e s w e r e m i l l e d a t 340°F (171°C); t h e o t h e r s a m p l e s w e r e m i l l e d a t 300°F (149°C). S t o c k t e m p e r a t u r e s r a n g e d b e t w e e n 360°F (182°C) a n d 390°F (199°C), d e p e n d i n g on t h e r e s i n a n d m o d i f i e r l e v e l . The m i l l e d s a m p l e s w e r e p r e s s e d a t 400°F (204°C) [ t h e h i g h e s t - M s a m p l e s ] o r 350°F (177°C) [ t h e lower-M samples). w
w
C h a r a c t e r i z a t i o n b y DMS a n d DSC. A l t h o u g h c h a r a c t e r i z a t i o n of s m a l l - s t r a i n v i s c o e l a s t i c and s t r e s s - s t r a i n b e h a v i o r i s n o t y e t c o m p l e t e , p r e l i m i n a r y d y n a m i c m e c h a n i c a l s p e c t r o s c o p y (DMS) and 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 (DSC) d a t a w e r e o b t a i n e d f o r t h e blends h a v i n g the h i g h e s t and lowest m o l e c u l a r w e i g h t s . To o b t a i n d y n a m i c m e c h a n i c a l s p e c t r a , a R h e o v i b r o n u n i t (Toyo I n s t r u m e n t C o . ) , m o d e l DDV I I , was u s e d . The i n s t r u m e n t was r u n a t a f r e q u e n c y o f 110 Hz. w i t h a h e a t i n g r a t e o f one °C per minute. G l a s s t r a n s i t i o n temperatures, T , were determined by l o c a t i n g t h e maxima o f t h e p r i n c i p a l p e a k s i n t h e l o s s m o d u l u s (E" ) as a f u n c t i o n o f temperature. DSC c u r v e s w e r e o b t a i n e d u s i n g a P e r k i n E l m e r i n s t r u m e n t , m o d e l DSC I B , a t a h e a t i n g r a t e g
m a x
314
DURABILITY OF
Table I .
MACROMOLECULAR MATERIALS
C h a r a c t e r i s t i c s o f t h e PVC
Specimen
[n]
131-1 131-2 131-3 131-4 135-1 135-2 135-3 135-4
0.54 0.54 0.54 0.54 1.09 1.09 1.09 1.09
xlO"
a
5
0.67 0.67 0.67 0.67 2.08 2.08 2.08 2.08
Polyblends
phr MBS
Av. T , °C (by D M S )
0 6 10 14 0 6 10 14
ioi
g
C
Studied Av. T , °C (by D S C ) g
d
67 66 67 67 72 71 72 72
e
— —
93 100(±3) 98(±3) 95 101(±7)
M a t r i x r e s i n i n c y c l o h e x a n o n e a t 30°C. From a c a l i b r a t i o n c u r v e o f e a r l i e r (9). Dynamic m e c h a n i c a l
[n] v s . v a l u e s o f M^
s p e c t r o s c o p y , 110
obtained
Hz.
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 , a t a r a t e o f 10°C/min. Anomalously h i g h . o f 10°C/min. The T v a l u e s w e r e t a k e n t o c o r r e s p o n d p o i n t of the t r a n s i t i o n i n the curve. g
to the mid-
Impact S t r e n g t h . V a l u e s o f notched impact s t r e n g t h were d e t e r m i n e d u s i n g n o t c h e d s p e c i m e n s and an I z o d t e s t e r f o l l o w i n g t h e ASTM p r o c e d u r e D256. F a t i g u e M e a s u r e m e n t s . S p e c i m e n s f o r d e t e r m i n a t i o n o f FCP b e h a v i o r w e r e c u t i n t h e f o r m o f compact t e n s i o n (CT) c o u p o n s , w i t h d i m e n s i o n s 0.64 cm x 7.5 cm x 7.5 cm. Specimens were c u t i n t h e m a c h i n e d i r e c t i o n , w i t h t h e e x c e p t i o n o f one i n t e r m e d i a t e - M s a m p l e o f u n m o d i f i e d PVC, 1 3 3 - 1 . [The p o s s i b l e e f f e c t o f t h i s v a r i a t i o n i s u n d e r s t u d y , t h o u g h t h e FCP r a t e was c o n s i s t e n t w i t h p r e v i o u s d a t a ( 9 ) . ] Edge n o t c h e s w e r e i n t r o d u c e d by f i r s t machini n g t o a d e p t h o f M..2 cm and t h e n c u t t i n g w i t h a r a z o r b l a d e . T e s t i n g was p e r f o r m e d i n t h e t e n s i o n - t e n s i o n mode as d e s c r i b e d p r e v i o u s l y (4) u n d e r a m b i e n t c o n d i t i o n s a t 10 Hz w i t h a l o a d r a t i o R o f 0.1 (R = minimum load/maximum l o a d ) . The p r o g r e s s o f t h e c r a c k was o b s e r v e d u s i n g a t r a v e l l i n g m i c r o s c o p e and t h e c r a c k l e n g t h was o b t a i n e d as a f u n c t i o n o f l o a d c y c l e s AN. The FCP r a t e , da/dN, was t h e n computed as a f u n c t i o n o f AK, u s i n g e q u a t i o n 2 and t h e f o l l o w i n g e x p r e s s i o n f o r t h e g e o m e t r i c a l f a c t o r , Y: 2
3
Y = 29.6-185.5(a/w)+655.7(a/w) -1017(a/w) +638.9(a/w)
4
(3)
A t t e m p t s w e r e made t o m e a s u r e t h e f r a c t u r e t o u g h n e s s , K ( a meas u r e o f f r a c t u r e e n e r g y ) , by d e t e r m i n i n g A K » t h e maximum v a l u e c
m a x
22.
SKIBO E T A L .
Table I I .
Specimen 131-1 131-2
Fatigue
Matrix M^xlO-S
0.54
0.67 0.67
phr MBS 0 6
IS (IS ) 0
1.0 1.2
a
fey
(akMk )
0
C
K ,MPav^a c
0
1.0 1.2
1.0
1.0 2.5xl0"
2
3.2
131-3
0.54
0.67
10
1.9
1.9
2.2xl0"
131-4
0.54
0.67
14
4.4
4.4
2.5xl0"
2
132-1
0.67
0.95
0
1.4
1.0
1.0
2.0
132-2
0.67
0.95
6
1.5
1.1
0.2
2.1
132-3
0.67
0.95
10
3.0
2.1
0.2
132-4
0.67
0.95
14
133-1
0.78
1.14
0
1.4
1.0
1.0
133-2
0.78
1.14
6
3.8
2.7
0.5
133-3
0.78
1.14
10
50
36
0.7
133-4
0.78
1.14
14
65
46
0.9
134-1
0.95
1.69
0
134-2
0.95
1.69
6
134-3
0.95
1.69
10
53
36
0.6
134-4
0.95
1.69
14
60
40
0.8
135-1
1.09
2.08
0
1.9
1.0
1.0
135-2
1.09
2.08
6
7.4
3.9
0.2
135-3
1.09
2.08
10
1.09
2.08
14
40
29
0.2
1.5
1.0
1.0
4.8
3.2
0.36
54
28
0.2
65
34
0.2
1.4
1.4
1.8
N o t e that these values c o r r e c t a numerical e r r o r i n the data f o r t h e 131 s e r i e s i n r e f . 1 0 , and t h a t v a l u e s a r e t a k e n a s r e l a t i v e to t h e v a l u e f o r t h e lowest-M specimen, 131-1.
kImpact s t r e n g t h r e l a t i v e specimen. C
315
Propagation
I m p a c t a n d FCP R e s p o n s e o f PVC P o l y b l e n d s
[n]
0.54
Crack
2
135-4 a
PVC
t o t h e impact s t r e n g t h o f the c o n t r o l
V a l u e o f da/dN r e l a t i v e t o da/dN f o r t h e c o n t r o l s p e c i m e n ; c o m p a r i s o n a t AK = 0.9 t i m e s f o r t h e c o n t r o l specimen.
DURABILITY OF
316
MACROMOLECULAR MATERIALS
o b s e r v e d f o r AK p r i o r t o c a t a s t r o p h i c f r a c t u r e , b u t e x c e p t f o r t h e c o n t r o l s and f o r s p e c i m e n s h a v i n g b o t h l o w M s and l o w r u b b e r c o n t e n t s (131-2 and 132-2) b r i t t l e f a i l u r e was n o t o b s e r v e d . For t h e s p e c i m e n s t h a t d i d e x h i b i t b r i t t l e f a i l u r e , K was e s t i m a t e d as b e f o r e ( 2 , 9 ) , by d i v i d i n g ^K by 0.9 ( t o a c c o u n t f o r t h e v a l u e o f 0.1 s e l e c t e d f o r R ) . f
c
max
F r a c t u r e S u r f a c e Morphology. A l t h o u g h f u l l d e t a i l s w i l l be published separately, preliminary observations of fracture surf a c e m o r p h o l o g y w e r e made on a n ETEC s c a n n i n g e l e c t r o n m i c r o s c o p e , u s i n g s p e c i m e n s t h a t had b e e n c o a t e d w i t h g o l d and c a r b o n p r i o r to examination. R e s u l t s and D i s c u s s i o n C h a r a c t e r i z a t i o n by DMS and DSC. P r e l i m i n a r y d y n a m i c mechan i c a l s p e c t r a f o r MBS r u b b e r m o d i f i e d PVC s a m p l e s showed no c o n s i s t e n t d e p e n d e n c e o f Tg, o r o f t h e t r a n s i t i o n s l o p e , on r u b b e r c o n t e n t ( T a b l e I ) . T y p i c a l s p e c t r a f o r h i g h - M PVC w i t h 0 p h r and 14 p h r MBS m o d i f i e r a r e shown i n F i g . 1. I t may be n o t e d t h a t , i n c o n t r a s t t o p r i o r e x p e r i e n c e w i t h many p o l y m e r s , r e p r o d u c i b i l i t y o f s p e c t r a was n o t a s good a s e x p e c t e d , e s p e c i a l l y w i t h s p e c i m e n s o f s a m p l e 135-4, t h e s a m p l e h a v i n g t h e h i g h e s t m o l e c u l a r w e i g h t and t h e h i g h e s t r u b b e r c o n t e n t . T h i s may r e f l e c t g r e a t e r inhomog e n e i t y i n these specimens. A l s o , t h e Tg f o u n d f o r s p e c i m e n 131-1 a p p e a r s t o be a n o m a l o u s [ h i g h e r t h a n e x p e c t e d b a s e d on t h e v a l u e o f M a n d , a f t e r a l l o w a n c e f o r d i f f e r e n c e s i n t e s t i n g r a t e , on v a l u e s o f T f o u n d by DSC ( s e e T a b l e I ) ] . I n g e n e r a l , each s p e c i men e x h i b i t e d a b r o a d a s y m e t r i c a l s e c o n d a r y p e a k i n E spanning t h e r a n g e f r o m -100°C t o 40°C. The s h a p e o f t h i s p e a k and t h e s l o p e o f t h e Tg c u r v e d i d n o t depend on r u b b e r c o n t e n t . S i n c e t h e p e r c e n t a g e o f b u t a d i e n e i n t h e MBS m o d i f i e r i s r e l a t i v e l y l o w and s i n c e t h e Tg p r o b a b l y f a l l s w i t h i n t h e t e m p e r a t u r e r a n g e o f t h e s e c o n d a r y t r a n s i t i o n f o r PVC ( 2 0 , pp. 115 and 2 9 2 ) , i t i s n o t s u r p r i s i n g t h a t no s i g n i f i c a n t e f f e c t o f MBS c o n t e n t i s s e e n a t t h e l e v e l s p r e s e n t i n these specimens. Thus, so f a r t h e s e r e s u l t s a r e c o n s i s t e n t w i t h those expected f o r a r e l a t i v e l y incompatible s y s tem ( s e e r e f e r e n c e s 24^ and 2 5 , Ch. 3) i n w h i c h a m o d i f i e r i s p r e sent a t r e l a t i v e l y low c o n c e n t r a t i o n ( i . e . , r e l a t i v e to the conc e n t r a t i o n s o f p u r e e l a s t o m e r i n t h e c u r v e s shown i n r e f e r e n c e 24) and i n w h i c h a b r o a d s e c o n d a r y t r a n s i t i o n i s c h a r a c t e r i s t i c o f the m a t r i x . The r e l a t i v e c o n s t a n c y o f Tg was c o n f i r m e d by t h e DSC meas u r e m e n t s ( s e e T a b l e I and F i g . 2 ) . The DSC measurements a l s o i n d i c a t e d an e x p e c t e d s l i g h t d e p e n d e n c e o f Tg on m o l e c u l a r w e i g h t , w i t h T i n c r e a s i n g f r o m ^67 °C f o r M^, = 6.7x10* t o ^72°C f o r M^ = 2.08x10^. W h i l e t h e s e v a l u e s a r e l o w e r t h a n v a l u e s q u o t e d f o r s i m i l a r v a l u e s o f M ( 2 6 ) , t h e h e a t i n g r a t e was a l s o l o w e r (10°C/ m i n i n s t e a d o f 32°C/min); t h e ^5°C d i f f e r e n c e i s c o n s i s t e n t w i t h r e s u l t s o f o t h e r measurements ( 2 7 ) . g
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Figure 1. Dynamic mechanical spectra of ( ), control and ( ), MBS-modified PVC (M of matrix = 2.08 X 10 ; ( ), phr MBS = 0 and ( ), 14; frequency = 110 Hz) 5
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T,C Figure 2. DSC traces for two pairs of control and MBS-modified PVC. Pair a and b displaced from pair c and d for better display. Compositions: (a) M == w
2.08 X 10 > 0 phr MBS; (b) M = 2.08 X iO ,14 phr MBS; (c) M = 6.7 X 10\ 0 phr MBS; (d) M = 6.7 X JO ,14 phr MBS. 5
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R e s u l t s o f t h e more d e t a i l e d c h a r a c t e r i z a t i o n i n p r o g r e s s w i l l be r e p o r t e d s e p a r a t e l y , a l o n g w i t h r e s u l t s o f s t r e s s - s t r a i n and c r e e p b e h a v i o r . Impact S t r e n g t h . As shown i n F i g . 3 and T a b l e I I , i m p a c t s t r e n g t h i s i n c r e a s e d by t h e i n c o r p o r a t i o n o f MBS a t a l l l e v e l s o f m o l e c u l a r w e i g h t . However, t h e r e l a t i v e e f f e c t s depend s t r o n g l y on b o t h M and MBS c o n t e n t . Thus w i t h 0 p h r and 6 p h r MBS, v a l u e s o f i m p a c t s t r e n g t h a r e i n c r e a s e d t w o f o l d and s i x f o l d , r e s p e c t i v e l y a s M i n c r e a s e s f r o m 0.67x10^ t o 2.08x10^ ( s e e v a l u e s o f I S / ( I S ) i n T a b l e I I ) . A t h i g h e r MBS c o n t e n t s , t h e e f f e c t s a r e much more d r a m a t i c , w i t h v a l u e s o f i m p a c t s t r e n g t h f o r s p e c i m e n s c o n t a i n i n g 10 p h r and 14 p h r MBS i n c r e a s i n g by f a c t o r s o f 28 and 34, r e s p e c t i v e l y , o v e r t h e same r a n g e o f M. It i s interesting t h a t the g e n e r a l trend of impact s t r e n g t h w i t h respect t o m o d i f i e r c o n c e n t r a t i o n i s c o n s i s t e n t w i t h t h a t n o t e d by P e t r i c h (28) f o r M B S - m o d i f i e d PVC a s a f u n c t i o n o f t e m p e r a t u r e . In h i s case, the a d d i t i o n o f m o d i f i e r had t h e same e f f e c t as r a i s i n g t h e t e m p e r a t u r e ; t h e MBS i n d u c e d a b r i t t l e - d u c t i l e t r a n s i t i o n . I n our case, at a m o d i f i e r l e v e l o f 6 phr, the lower-M m a t e r i a l s are b r i t t l e ( c f . o b s e r v a t i o n s o f K b e h a v i o r n o t e d a b o v e ) ; as M i s i n c r e a s e d , f a i l u r e becomes more d u c t i l e . A t a m o d i f i e r l e v e l o f ^10 p h r , however, the temperature o f the b r i t t l e - d u c t i l e t r a n s i t i o n i s c l e a r l y >20°C (28) f o r a l l s p e c i m e n s , r e g a r d l e s s o f M. 0
0
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A t t h e same t i m e ( s e e F i g . 4 ) , t h e i m p a c t s t r e n g t h ( a t a g i v e n M) r e l a t i v e t o t h a t o f t h e u n m o d i f i e d c o n t r o l r e s i n ( I S / I S ) app e a r s t o p a s s t h r o u g h a s l i g h t maximum a t l x l 0 < M < V L . 5 x l 0 5 f o r s p e c i m e n s w i t h MBS c o n t e n t s £10 p h r . ( V a l u e s o f I S / I S do, howe v e r , i n c r e a s e w i t h m o d i f i e r c o n t e n t a t a l l v a l u e s o f M, a s ment i o n e d above.) Thus a t some c r i t i c a l v a l u e o f MBS c o n t e n t (>6 p h r ) , t h e i m pact strength e x h i b i t s a s i g n i f i c a n t increase. This i s c o n s i s t e n t w i t h a r g u m e n t s a d v a n c e d by P e t r i c h ( 2 8 ) , B u c k n a l l ( 2 0 , c h . 10) and D e a n i n and M o s h a r ( 2 9 ) : t h a t t h e i n c l u s i o n o f an e l a s t o m e r i c p h a s e i n PVC r e d u c e s PVC's i n h e r e n t n o t c h s e n s i t i v i t y and p e r m i t s t h e t a k i n g a d v a n t a g e o f PVC's i n h e r e n t d u c t i l i t y by s t i m u l a t i n g a h i g h degree of shear y i e l d i n g . I t i s a l s o s u g g e s t e d ( 2 0 , c h . 10) t h a t c r a z i n g i s induced a t the h i g h r a t e s of s t r a i n i n v o l v e d i n i m p a c t l o a d i n g , t h o u g h c r a z i n g may n o t c o n t r i b u t e much t o d e f o r m a t i o n e n e r g i e s a t l o w e r s t r a i n r a t e s ( 2 0 , c h . 7; 28, 30, 3 1 ) . With r e s p e c t t o t h e r o l e o f M, one w o u l d c e r t a i n l y e x p e c t t h a t t h e i m p a c t f r a c t u r e e n e r g y s h o u l d r i s e as M i s i n c r e a s e d t o a l i m i t i n g v a l u e , as i s t h e c a s e w i t h f r a c t u r e e n e r g i e s d e t e r m i n e d u n d e r o t h e r c o n d i t i o n s of l o a d i n g ( 3 ) . With r e s p e c t to the s m a l l but apparent tendency f o r I S / I S to decrease s l i g h t l y at h i g h v a l u e s o f M, one may s u p p o s e t h a t t h e h i g h e s t - M m o l e c u l e s may be r e l a t i v e l y l e s s mobile at the high s t r a i n r a t e s i n v o l v e d . Q
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F r a c t u r e Toughness. V a l u e s o f K f o r the neat r e s i n agree r e a s o n a b l y w e l l w i t h those r e p o r t e d p r e v i o u s l y ( 9 ) , though the c
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Effect of MBS content and M of matrix on notched impact strength of " PVC blends
M x10"5 w
Figure 4.
Effect of MBS content and M of matrix on notched impact strength of PVC blends relative to that of unmodified matrix
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v a l u e s t e n d t o be l o w e r a t h i g h v a l u e s o f M. This lower trend i s b e l i e v e d t o be more r e a l i s t i c t h a n t h e e a r l i e r one. In the e a r l i e r s p e c i m e n s i n t e r g r a n u l a r f r a c t u r e a l o n g PVC p a r t i c l e b o u n d a r i e s was o b s e r v e d a t h i g h v a l u e s o f AK; s u c h a phenomenon w o u l d be e x p e c t e d t o i n d u c e an i n c r e a s e i n t h e a p p a r e n t v a l u e o f K i n comp a r i s o n t o t h e c a s e o f i n t r a g r a n u l a r (and h e n c e r e l a t i v e l y smooth) f r a c t u r e . As m e n t i o n e d a b o v e , h o w e v e r , v a l u e s o f Kc c o u l d o n l y be e s t i m a t e d f o r s p e c i m e n s h a v i n g b o t h l o w M and l o w MBS content— n a m e l y , s p e c i m e n s 131-2 and 132-2. C l e a r l y a b r i t t l e - d u c t i l e t r a n s i t i o n i s i n v o l v e d (see a l s o d i s c u s s i o n above). In contrast t o o b s e r v a t i o n s o f i m p a c t s t r e n g t h , t h e a d d i t i o n o f 6 p h r MBS t o t h e l o w e s t - M r e s i n i n c r e a s e d t h e f r a c t u r e e n e r g y ( a s i m p l i e d by K ) t h r e e f o l d ; l i t t l e e f f e c t was n o t e d a t t h e n e x t l e v e l o f M. Hence a t t h e l o w e r s t r a i n r a t e s o p e r a t i v e i n FCP i n c o m p a r i s o n t o i m p a c t f r a c t u r e , t h e r u b b e r y p h a s e does s i g n i f i c a n t l y i m p r o v e t h e a b i l i t y t o d i s s i p a t e e n e r g y i n low-M PVC, a s r e f l e c t e d b o t h i n K and i n t h e r e d u c t i o n i n FCP r a t e s ( s e e b e l o w ) . As M i s i n c r e a s e d , f a i l u r e becomes d u c t i l e a t a l l l e v e l s o f MBS; s u c h s p e c i m e n s d i d n o t u n d e r g o r u p t u r e b u t r a t h e r became t o o c o m p l i a n t t o c o n t i n u e testing. In t h i s r e s p e c t , the q u e s t i o n of p o s s i b l e temperature r i s e w o u l d be w o r t h y o f i n v e s t i g a t i o n , t h o u g h so f a r g r o s s i n c r e a s e s i n t e m p e r a t u r e due t o h y s t e r e t i c h e a t i n g h a v e n o t b e e n observed (see a l s o d i s c u s s i o n below). c
c
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F a t i g u e C r a c k P r o p a g a t i o n R e s p o n s e . T y p i c a l FCP responses a r e s u m m a r i z e d i n T a b l e I I and i n F i g s . 5-9. As may be s e e n by c o m p a r i s o n o f t h e d a t a w i t h t h o s e r e p o r t e d i n r e f e r e n c e J9, t h e behavior of the c o n t r o l specimens agrees w e l l w i t h the behavior found w i t h the e a r l i e r s e r i e s . A l t h o u g h t h e c u r v e s e x h i b i t some c u r v a t u r e not p r e v i o u s l y observed, the p o s i t i o n s of the curves f i t w e l l w i t h t h o s e o f t h e p r e v i o u s o n e s , and t h e v a l u e s o f da/dN a t AK = 0.6 MPav^n f a l l c l o s e l y on t h e c u r v e a s a f u n c t i o n o f M ( s e e F i g . 2, r e f e r e n c e 9 ) . As shown i n T a b l e I I and F i g . 5 ( t h e c u r v e s i n F i g . 5 a l s o h o l d f o r 6 p h r MBS), on a d d i n g 6 p h r o f MBS, FCP r a t e s a r e s i g n i f i c a n t l y reduced. The e f f e c t i s d e s c r i b e d q u a n t i t a t i v e l y by t h e r e l a t i v e v a l u e s o f c r a c k g r o w t h r a t e s a t c o n s t a n t AK ( s e e T a b l e I I , i n w h i c h a and a r e p r e s e n t v a l u e s o f da/dN ( a t A K = 0 . 9 A K f o r t h e c o n t r o l ) f o r m o d i f i e d and n e a t r e s i n s , r e s p e c t i v e l y ) . T h u s , w i t h t h e l o w e s t - M m a t r i x , a s t r i k i n g r e d u c t i o n i n da/dN i s o b s e r v e d a t a l l AK l e v e l s , a c c o m p a n i e d by a t h r e e f o l d i n c r e a s e i n K (Table I I ) . Indeed, at a v a l u e of AK=0.9AK (of the c o n t r o l ) da/dN i s r e d u c e d 4 0 - f o l d . On t h e o t h e r h a n d , w i t h t h e h i g h e s t - M m a t r i x da/dN i s r e d u c e d t o a r e l a t i v e l y l e s s e r e x t e n t , o n l y f i v e f o l d ( t h e c u r v e s i n F i g . 6 a l s o h o l d f o r 6 p h r MBS). A t t h e same t i m e , i n t h e l a t t e r c a s e , t h e f a i l u r e becomes d u c t i l e i n n a t u r e (see above). A t i n t e r m e d i a t e v a l u e s o f M, r e d u c t i o n s o f da/dN a t 6 p h r o f MBS r a n g e b e t w e e n t w o f o l d and f i v e f o l d . Thus i n c o n t r a s t to t h e c a s e o f i m p a c t s t r e n g t h , e v e n a l o w c o n c e n t r a t i o n o f MBS (6 p h r ) i n d u c e s a s i g n i f i c a n t t o u g h e n i n g i n f a t i g u e e v e n a t a l o w K
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Figure 5. Effect of MBS addition (14 phr) on FCP behavior of PVC (M — 6.7 X 10 ). The curve for 14 phr also represents data for 6 phr and 10 phr of MBS. w
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Figure 6. Effect of MBS addition (14 phr) on FCP behavior of PVC (M = 2.08 X 10 ). The curve for 14 phr also represents data for 6 phr and 10 phr of MBS. w
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AK,MPAv7n Figure 8.
Effect of M on FCP behavior of PVC modified with 10 phr of MBS
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v a l u e o f M ( 6 . 7 x 1 0 * ) . A t h i g h e r v a l u e s o f M, t h e r e l a t i v e f a t i g u e i m p r o v e m e n t s a t 6 p h r o f MBS a r e o f t h e same o r d e r o f m a g n i t u d e as t h e i m p r o v e m e n t s i n i m p a c t s t r e n g t h ( T a b l e I I ) . I n t e r e s t i n g l y , w i t h b o t h t h e two l o w e s t - M and t h e h i g h e s t - M m a t r i x e s ( s p e c i m e n s 1 3 1 , 132, and 135, r e s p e c t i v e l y ) , t h e a d d i t i o n o f >6 p h r o f MBS has no s i g n i f i c a n t e f f e c t on FCP r a t e s . This b e h a v i o r m i g h t s p e c u l a t i v e l y be a t t r i b u t e d t o t h e c o n s e q u e n c e s o f a d y n a m i c b a l a n c e b e t w e e n (1) r u b b e r - i n d u c e d t o u g h n e s s due t o t h e a b i l i t y o f t h e r u b b e r t o i n d u c e s h e a r y i e l d i n g and c r a z i n g and (2) r u b b e r - i n d u c e d s o f t e n i n g ( i . e . , r e d u c t i o n i n s t i f f n e s s ) . Now s t i f f e n i n g c a n be r e d u c e d by two f a c t o r s , a c t i n g a l o n e o r t o g e t h e r : the presence o f the rubber per se (see, f o r example, r e f e r e n c e s 28 and 3 2 ) , and by h y s t e r e t i c h e a t i n g due t o i n h e r e n t m e c h a n i c a l damping ( 1 , 6 ) . W h i l e t h e p r e s e n t s t u d y c a n n o t r e s o l v e these p o i n t s , techniques are b e i n g developed i n our l a b o r a t o r y to d e t e r m i n e t h e v a l u e o f E u n d e r a c t u a l t e s t c o n d i t i o n s , and an i m p r o v e d DMS i n s t r u m e n t on o r d e r s h o u l d p e r m i t more p r e c i s e v a l u e s of damping, (Note t h a t , d e p e n d i n g on t h e s y s t e m and p r e s u m a b l y f r e q u e n c y , E" o r t a n
