Copolymers, Polyblends, and Composites

33 Fundamental Considerations for the

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch033

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.


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


common

388

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES


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


33.

WOODS A N D MASS

Covulcanization


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


390

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES


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

Copolymers, Polyblends, and Composites

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