33 Fundamental Considerations for the
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Covulcanization of Elastomer Blends MARTIN E. WOODS and THOMAS R. MASS B. F. Goodrich Chemical Co., Avon Lake Technical Center, Avon Lake, Ohio 44012
It has been generally reported that elastomer blends do not covulcanize, i.e., the blend properties are inferior to those predicted from the properties of the component elastomers. We demonstrate that the lack of covulcanization results from diffusion of the vulcanization accelerators from one phase into the other. Furthermore, this diffusion is the result of two driving forces: thermodynamic, i.e., preference of the accelerators for the more polar elastomer, and kinetic, i.e., depletion of the curatives in the phase with the greater cure rate and subsequent diffusion of more curatives into that phase. The design of cure systems to overcome the two driving forces is discussed, and their ability to covulcanize a series of elastomer blends is demonstrated.
E
l a s t o m e r b l e n d t e c h n o l o g y is w i d e l y u s e d i n t h e r u b b e r i n d u s t r y t o d a y as it has b e e n f o r m a n y y e a r s . T y p i c a l l y , t h e s c i e n c e o f e l a s t o m e r b l e n d s has e v o l v e d m u c h m o r e s l o w l y t h a n t h e t e c h n o l o g y . T h i s is e v i d e n c e d b y t h e f a c t t h a t i t is w i d e l y k n o w n a n d a c c e p t e d t h a t e l a s t o m e r b l e n d s a r e s e l d o m as g o o d as o n e w o u l d e x p e c t e m p i r i c a l l y b y i n t e r p o l a t i n g b e t w e e n t h e p r o p e r ties of t h e b a s e e l a s t o m e r s u s e d i n t h e b l e n d ( J , 2 ) . T h i s o b t a i n s e v e n w h e n similar elastomers are b l e n d e d ( 3 ) . T h e science of elastomer blends has b e g u n to d e v e l o p more r a p i d l y . T h e m o s t n o t a b l e e x a m p l e s are t h e w o r k p u b l i s h e d b y G a r d i n e r ( 4 , 5 ) a n d Z a p p {6,7) since 1 9 6 8 . G a r d i n e r s t u d i e d t h e p r o b l e m of c u r a t i v e m i g r a t i o n w h e r e a s Z a p p m e a s u r e d t h e i n t e r f a c i a l a d h e s i o n i n e l a s t o m e r b l e n d s . T h e o b j e c t i v e of t h i s p a p e r is t o b u i l d o n t h e w o r k of these a u t h o r s a n d to d e m o n s t r a t e t h a t c u r a t i v e d i f f u s i o n o c c u r s b y t w o m e c h a n i s m s . W e w i l l also s h o w t h a t t r u e c o v u l c a n i z a t i o n c a n b e o b t a i n e d , i.e., t h e b l e n d p r o p e r t i e s w i l l b e a l i n e a r f u n c t i o n of b l e n d c o m p o s i t i o n , w h e n b o t h curative diffusion mechanisms are counterbalanced b y proper curative selection. Strategy W e s t u d i e d a series of b l e n d s b a s e d o n t h e r e l a t i v e l y n o n p o l a r e t h y l e n e p r o p y l e n e - d i e n e m o n o m e r ( E P D M ) elastomer w h i c h has a solubility parameter 386
In Copolymers, Polyblends, and Composites; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
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33.
WOODS A N D MASS
of 8 . 3 ( 8 ) .
Covulcanization
Blends were made
of Elastomer
w i t h highly polar acrylonitrile-butadiene co-
polymer ( N B R ) , w i t h slightly polar styrene-butadiene rubber
387
Blends
elastomer
( S N ) . T h e s e elastomers have s o l u b i l i t y parameters of 9.4, 8.5, 8 . 1 , (8).
synthetic
polyisoprene
Since G a r d i n e r h a d shown that polar
s u c h as t e t r a m e t h y l t h i u r a m diffuse into the more
nonpolar
with
natural
a n d 8.1 r e s p e c t i v e l y
( N R ) , and with
copolymer ( S B R ) ,
nonpolar
disulfide ( T M T D )
polar elastomer
than
accelerators
w a s five t i m e s m o r e l i k e l y t o
other
curatives,
w e studied the
effects o f v a r i o u s d e g r e e s of a l k y l a t i o n of s i m i l a r a c c e l e r a t o r s o n t h e p r o p e r t i e s of t h e e l a s t o m e r
blends.
T h i s is d e p i c t e d i n F i g u r e
represents the solubility parameter
tors as a f u n c t i o n o f t h e d e g r e e o f a l k y l a t i o n . using Smalls method
(9).
1 where
the solid
of the t e t r a a l k y l t h i u r a m disulfide T h e values
were
line
acceleracalculated
F i g u r e 1 clearly indicates that the more
In Copolymers, Polyblends, and Composites; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
common
388
COPOLYMERS,
POLYBLENDS,
A N D COMPOSITES
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m e t h y l a t e d ( T M T D ) a n d ethylated ( T E T D ) accelerators w o u l d have a strong tendency to diffuse into the most polar elastomer. This tendency should definitely decrease w i t h h i g h e r degrees of a l k y l a t i o n . W e d e s i g n e d the experim e n t s t o d e m o n s t r a t e t h a t , w h e n t h e p o l a r i t y o f t h e a c c e l e r a t o r is r e d u c e d sufficiently, the t h e r m o d y n a m i c a l l y i n d u c e d diffusion w i l l be overcome a n d covulcanization w i l l be obtained. W e i n t e r p r e t e d o u r results i n t e r m s o f t h e v u l c a n i z a t i o n d i a g r a m s h o w n i n F i g u r e 2 ; t h i s is a t h r e e - d i m e n s i o n a l v e r s i o n o f t h e v u l c a n i z a t i o n p o s s i b i l i t i e s r e p o r t e d b y G a r d i n e r ( 5 ) . O n t h e basis o f t h i s m o d e l , w e f e l t t h a t c o v u l c a n i zation w o u l d occur only w h e n the f o l l o w i n g conditions were satisfied: (a) the c o n t i n u o u s p h a s e w a s v u l c a n i z e d t o n e a r its o p t i m u m l e v e l , ( b ) t h e d i s c o n t i n u o u s p h a s e w a s v u l c a n i z e d t o n e a r its o p t i m u m l e v e l , a n d ( c ) t h e r e w a s excellent adhesion b e t w e e n the dispersed a n d continuous phases. T h e exact d e g r e e o f c u r i n g d e p e n d s o n t h e m a i n p r o p e r t y t h a t is d e s i r e d b e c a u s e s o m e p r o p e r t i e s a r e m a x i m i z e d b y s l i g h t l y u n d e r c u r i n g t h e e l a s t o m e r s , others m a x i m i z e at o p t i m u m c u r e , a n d s t i l l others m a x i m i z e w h e n t h e r e is s o m e o v e r c u r i n g (10). Experimental
Procedure
D e s c r i p t i o n o f M a t e r i a l s . T h e accelerators e v a l u a t e d i n this s t u d y are l i s t e d i n T a b l e I . T h e first t h r e e a c c e l e r a t o r s a r e a v a i l a b l e c o m m e r c i a l l y ; t h e r e m a i n d e r w e r e synthesized i n o u r laboratory. T h e elastomers are d e s c r i b e d in Table II.
Figure 2.
Vulcanization cube
In Copolymers, Polyblends, and Composites; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
33.
WOODS A N D MASS
Covulcanization
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T a b l e I.
of Elastomer
Blends
389
Accelerators E v a l u a t e d
Code
Chemical Description
CI C2 C4 C8 CIO C12 C20
zinc dimethyldithiocarbamate zinc diethyldithiocarbamate zinc dibutyldithiocarbamate zinc diethylhexyldithiocarbamate zinc didecyldithiocarbamate zinc didodecyldithiocarbamate zinc diarachidicdithiocarbamate
C o m p o u n d i n g . T h e c o m p o u n d s s t u d i e d were a l l m a d e b y t h e base recipe i n T a b l e I I I . T h e c o n c e n t r a t i o n o f 1.6 Χ 1 0 " m o l e s a c c e l e r a t o r / 1 0 0 g r u b b e r was chosen because i t represents a n o r m a l l e v e l f o r t h e l o w m o l e c u l a r w e i g h t accelerators of this t y p e that are b e i n g u s e d i n t h e elastomer i n d u s t r y . B l e n d ratios w e r e 100/0, 7 5 / 2 5 , 5 0 / 5 0 , 2 5 / 7 5 , a n d 0 / 1 0 0 i n o r d e r to o b t a i n a c o m p l e t e p i c t u r e as a f u n c t i o n o f t h e b l e n d c o m p o s i t i o n . 3
T a b l e II. Rubber Type
Description of P o l y m e r i c Materials
Trade Name
Mooney'
Supplier B.F. Goodrich Chemical Co. B.F. Goodrich Chemical Co. B.F. Goodrich Chemical Co. Rubber Trade Association of New York Dealers B.F. Goodrich Chemical Co.
1
EPDM NBR SBR NR
Epcar 585 Hvcar 1032 Ameripol 1502' RSS # 1
50 55 50 —
SN
Ameripol SN600
80
b
« Raw Mooney M L - 4 @ 2 1 2 ° F . Nominal 33 % acrylonitrile. 23.5% bound styrene.
b
c
The compounds were prepared using a normal Banbury milling procedure. T h e rubbers were charged to the B a n b u r y a n d a l l o w e d to m i x f o r 2 m i n ; then the r e m a i n d e r of t h e ingredients w e r e a d d e d . T h e b a t c h w a s d u m p e d after 5 m i n total m i x i n g time. Curatives were a d d e d o n a standard two-roll labora t o r y m i l l b y the n o r m a l m i l l i n g p r o c e d u r e . M o n s a n t o r h e o m e t e r curves at 3 2 0 ° F w e r e o b t a i n e d f o r t h e c o m p o u n d s . T h e c o m p o u n d s w e r e t h e n press c u r e d f o r o p t i m u m c u r e t i m e s e x c e p t t h a t flexometer s p e c i m e n s w e r e c u r e d a n a d d i t i o n a l 10 m i n b e c a u s e o f t h e i r t h i c k n e s s . P h y s i c a l Testing. T h e physical properties were measured i n accordance w i t h the applicable A S T M procedures. T a b l e III.
Base C o m p o u n d Recipe
Substance
Content
Rubber Stearic acid N550 (FEF) ZnO Sp sulfur Accelerator Results and Blends
100 parts 2 parts 55 parts 5 parts 1.5 parts 1.6 X 10" moles 3
Discussion of E P D M
parameters of E P D M
with
NBR.
Because
of differences
i n the solubility
a n d N B R e l a s t o m e r s , t h e s e b l e n d s s h o u l d h a v e t h e least
t e n d e n c y t o c o v u l c a n i z e . T h i s is c l e a r i n F i g u r e s 3 , 4 , a n d 5 f o r t h e e t h y l a t e d
In Copolymers, Polyblends, and Composites; Platzer, Norbert A. J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
390
COPOLYMERS,
POLYBLENDS,
A N D COMPOSITES
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3000
Ο Ζ 2000 Lu
R _ —
rr
IN
in UJ ζ
O
1000
R -
C H 2
r
Δ
R
•
R = C
S - C - N
2
25
J
5
CH CH(C H )C H 2 0
v
2
5
4
g
H4|
_L
_L
50
75
100
WT
