The Role of Filler Modulus and Filler-Rubber Adhesion in Rubber

35 The Role of Filler Modulus and FillerRubber Adhesion in Rubber Reinforcement 1

Downloaded by UNIV QUEENSLAND on September 18, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch035

MAURICE MORTON, R. J. MURPHY,

and T. C.

2

CHENG

Institute of Polymer Science, University of Akron, Akron, Ohio 44325

Studies are made of styrene-butadiene rubber vulcanizates reinforced with different polymeric fillers that were introduced by latex blending. Experiments with polystyrene, poly(methyl methacrylate), and polyacenaphthylene, which have different moduli of rigidity, demonstrate that a higher filler modulus results in greater strength reinforcement, both tensile and tear. The changes in density of these composites under uniaxial strain reveal that dewetting of the filler-rubber interface is a linear function of the extent of strain and is inversely proportional to both particle size andfiller-rubberinterfacial energy. The effect offiller-rubberadhesion is further reflected in the reinforcement of the rubber since better adhesion leads to higher strength but greater hysteresis.

R

ecent studies i n these laboratories dealt w i t h the use of m o d e l , spherical - p o l y m e r i c fillers i n t h e r e i n f o r c e m e n t o f s t y r e n e - b u t a d i e n e r u b b e r ( S B R ) a n d p o l y b u t a d i e n e e l a s t o m e r s ( I , 2, 3, 4), w i t h b o t h fillers a n d e l a s t o m e r s p r e p a r e d b y e m u l s i o n p o l y m e r i z a t i o n . W i t h latex b l e n d i n g techniques, true d i s p e r s i o n s o f t h e filler i n t h e r u b b e r w e r e p r e p a r e d , a n d d e f i n i t i v e c o n c l u s i o n s c o u l d b e r e a c h e d a b o u t t h e effect o f t h e filler o n v u l c a n i z a t e s t r e n g t h . M o r e r e c e n t l y , w e i n v e s t i g a t e d t h e p o s s i b l e effects o f filler r i g i d i t y a n d filler-rubber adhesion o n reinforcement of the elastomer.

Table I. Variable Particle diameter, A T, 0

°C

Flex, modulus (E) at 25°C, kg/cm X 10~ 2

1 2

4

Fillers f o r Modulus S t u d y PS

PMMA

PA

570

510

410

108

110

240

3.2±0.1

2.4±0.1

8.0±0.5

Present address: Education Development Center, Newton, Mass. 02160. Present address: Air Products and Chemicals, Inc., Middlesex, N. J. 08846. 409

In Copolymers, Polyblends, and Composites; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.


410

COPOLYMERS,

—I -4.0

1 -2.0

1 0.0 LOG t

Figure 1. Filler, 25 vol %;

I 2.0 b

/ û

T

POLYBLENDS,

I 4.0

I 6.0

A N D COMPOSITES

L_ 8.0

(MIN.)

Tensile strength of filled SBR , SBR gum; Δ , PMMA;

O, PS; and •, PA

Experimental T h r e e emulsion polymers w i t h different flexural m o d u l i were prepared, i.e. p o l y s t y r e n e ( P S ) , p o l y ( m e t h y l m e t h a c r y l a t e ) ( P M M A ) , a n d p o l y a c e n a p h t h y l e n e ( P A ) ; a l l h a d v e r y s m a l l p a r t i c l e size (—500 A ) suitable f o r r u b b e r r e i n f o r c e m e n t . C h a r a c t e r i s t i c s o f t h e s e fillers a r e l i s t e d i n T a b l e I . I n a d d i t i o n , a series o f P S fillers w i t h p a r t i c l e s i z e o f 5 0 0 - 3 0 0 0 A a n d a series o f P M M A fillers w i t h p a r t i c l e s i z e o f 8 0 0 - 2 5 0 0 A w e r e p r e p a r e d . A d h e s i o n w a s s t u d i e d b y d e t e r m i n i n g t h e d e c r e a s e i n d e n s i t y o f t h e filled v u l c a n i z a t e s u n d e r u n i a x i a l strain, o n the assumption that this w a s caused b y vacuole f o r m a t i o n r e s u l t i n g f r o m d e w e t t i n g o f t h e filler. F o r these v u l c a n i z a t e s , b e n z o y l p e r o x i d e ( 2 . 5 p h r , 5 h r s at 8 0 ° C ) w a s u s e d as c r o s s l i n k i n g a g e n t i n o r d e r t o a v o i d u s i n g p a r t i c u l a t e z i n c o x i d e as is c u s t o m a r y w i t h s u l f u r - b a s e d c o m p o u n d s (1,2,3). N o r m a l techniques for preparing the peroxide vulcanizates used i n the a d h e s i o n s t u d i e s i n v o l v e d b l e n d i n g c o r r e c t p r o p o r t i o n s o f filler a n d S B R l a t e x e s to y i e l d a final c o m p o s i t e c o n t a i n i n g 2 5 v o l % filler o n a d r y b a s i s . T h e l a t e x b l e n d w a s t h e n c o a g u l a t e d i n 2 - p r o p a n o l a n d d r i e d at 4 0 ° C in vacuo to y i e l d the d r y composite. T h e peroxide ( a n d antioxidant) were then incorporated b y m i l l i n g 2 m i n o n a c o l d m i l l . V u l c a n i z a t i o n w a s at 8 0 ° C f o r 12 h r s . It w a s a s s u m e d that this c o a g u l a t i o n p r o c e d u r e r e m o v e d the a d s o r b e d soap f r o m t h e latex particles. H e n c e , a n alternative p r o c e d u r e of s i m p l e e v a p o r a t i o n w a s u s e d w i t h P S l a t e x o f 4 4 0 - A p a r t i c l e s i z e w h e n it w a s d e s i r e d t h a t t h e a d sorbed layer of soap, w h i c h c o v e r e d 6 6 % of the surface, r e m a i n o n the surface of t h e filler p a r t i c l e s a n d t h e r e b y a l t e r t h e n a t u r e o f t h e filler-rubber interface. T h i s filler w a s d e s i g n a t e d P S 4 4 0 ( 6 6 % S ) . T h i s filler w a s f u r t h e r m o d i f i e d b y a d d i n g s o a p s o l u t i o n to t h e P S l a t e x u n t i l t h e s u r f a c e w a s s a t u r a t e d w i t h soap; the product was P S 4 4 0 ( 1 0 0 % S ) . Effect of Filler

Modulus

T h e effect o f t h e t h r e e fillers w i t h d i f f e r e n t m o d u l i , i.e. P S , P M M A , a n d P A , o n t h e t e n s i l e s t r e n g t h o f t h e S B R v u l c a n i z a t e s is d e p i c t e d i n F i g u r e 1 . A s is c u s t o m a r y , t e n s i l e s t r e n g t h is e x p r e s s e d as a f u n c t i o n o f t i m e a n d t e r n -


35.

Rubber

MORTON ET A L .

411

Reinforcement

perature o n master curves obtained b y using a n e m p i r i c a l shift factor (1) w h i c h is g e n e r a l l y l a r g e r f o r f i l l e d v u l c a n i z a t e s t h a n t h a t p r e d i c t e d b y t h e W i l l i a m s - L a n d e l - F e r r y ( W L F ) equation. Fillers w i t h higher m o d u l u s gave v u l c a n i z a t e s w i t h g r e a t e r s t r e n g t h . T h e s e findings c o n f i r m e d t h e p r e l i m i n a r y findings r e p o r t e d f o r a n o t h e r series of p o l y m e r i c fillers ( 4 ) a n d i n d i c a t e v e r y s t r o n g l y t h a t filler r i g i d i t y does i n d e e d affect r e i n f o r c e m e n t of t h e e l a s t o m e r .


Filler-Rubber

Adhesion

and

Reinforcement

V o l u m e d i l a t i o n o f these v u l c a n i z a t e s a p p e a r e d t o b e a l i n e a r f u n c t i o n of t h e u n i a x i a l s t r a i n ; h e n c e a p a r a m e t e r AV/VX could be derived for each filler, w h e r e AV/V is t h e f r a c t i o n a l v o l u m e c h a n g e at a n y s t r a i n λ . I n F i g u r e 2 , this d i l a t i o n p a r a m e t e r (AV/VX) is p l o t t e d as a f u n c t i o n of filler p a r t i c l e d i a m eter ( o r o f 1 / A , t h e r e c i p r o c a l o f t h e s u r f a c e a r e a ) f o r t h e v a r i o u s fillers u s e d . It w a s n o t s u r p r i s i n g t h a t d e w e t t i n g w a s a l i n e a r f u n c t i o n o f 1 / A s i n c e i t w a s expected to d e p e n d d i r e c t l y o n t h e w o r k r e q u i r e d to create a n e w surface. H o w e v e r , a l l t h e d a t a f o r t h e s o a p - f r e e P S fillers w e r e o n o n e l i n e w h e r e a s t h o s e f o r t h e s o a p - m o d i f i e d P S fillers w e r e o n a n o t h e r l i n e t o g e t h e r w i t h those f o r t h e PMMA fillers. T h e c o i n c i d e n c e o f t h e l a t t e r t w o fillers m a y b e f o r t u i t o u s , b u t i t i n d i c a t e s a n a p p a r e n t g r e a t e r a d h e s i o n b e t w e e n t h e s e fillers a n d t h e rubber w h i c h m i g h t be expected f r o m the k n o w n higher surface energy ( 5 ) of P M M A ( 3 9 d y n e s / c m ) t h a n o f u n m o d i f i e d P S filler ( 3 3 d y n e s / c m ) . 2

2

T h e effect of these a d h e s i o n differences o n r e i n f o r c e m e n t of t h e v u l c a n i zates w a s d e t e r m i n e d b y m e a s u r i n g u n i a x i a l s t r e s s - s t r a i n h y s t e r e s i s as w e l l as t e a r s t r e n g t h . T h e s e t w o c h a r a c t e r i s t i c s are p r e s u m a b l y d i r e c t l y r e l a t e d (6, 7) so t h e i r r e l a t i o n t o i n t e r f a c i a l a d h e s i o n i n filled v u l c a n i z a t e s w a s c o n s i d e r e d

Figure 2.

Dilation as a function of particle size and surface

Filler, 25 vol %; O, PS; Δ , PS (66% S); and •,

PMMA



Λ

CTQ*

• 3.

s-% a

a.*. 3

Β Ο Co

ρ s-

7

Ν)

δ

RELATIVE

HYSTERESIS

ENERGY LOSS ( k g . c m . / c m


3

)

*

The Role of Filler Modulus and Filler-Rubber Adhesion in Rubber

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