Vulcanization and Stabilization Studies on Propylene Oxide Rubber

8 Vulcanization and Stabilization Studies on Propylene

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0006.ch008

Oxide Rubber C. R. BOSS Research Center, Hercules Inc., Wilmington, Del. 19899

®

Parel elastomer i s made by copolymerizing propylene oxide with a small amount o f allyl g l y c i d y l ether. T y p i c a l samples have reduced s p e c i f i c v i s c o s i t i e s (RSV) between 4 and 12, i n d i c a t i n g molecular weights o f the order o f a million. The unsatura t i o n c o n t r i b u t e d by the allyl glycidyl ether monomer u n i t s provides s i t e s f o r s u l f u r c r o s s - l i n k i n g . V u l c a n i z a t i o n can be e f f e c t e d with combinations o f s u l f u r and s u l f u r donors as used f o r other unsaturated rubbers. A t y p i c a l v u l c a n i z a t i o n formulat i o n i s given i n Table 1. Peroxides are i n e f f e c t i v e f o r c u r i n g t h i s elastomer s i n c e they tend t o degrade r a t h e r than c r o s s - l i n k it. When P a r e l elastomer i s cured with the formulation shown, f o r the recommended 30 minutes at 160°C., the p r o p e r t i e s given in Table 2 are obtained. T h i s paper will summarize some o f the s t u d i e s l e a d i n g t o s e l e c t i o n o f a number o f the components o f t h i s v u l c a n i z a t i o n formulation and t o determination o f the advantageous p r o p e r t i e s o f the v u l c a n i z e d products. Vulcanization S u l f u r cure systems are w e l l known t o be complex in t h e i r r e a c t i o n s and i n the types o f c r o s s - l i n k s they produce(1) (2). There is general agreement that i f s u l f u r alone is used to v u l canize a rubber, most o f the c r o s s - l i n k s will have more than two s u l f u r atoms i n them(1). These p o l y s u l f i d e s are l e s s s t a b l e than monosulfides or d i s u l f i d e s . T h i s phenomenon has been examined i n some v u l c a n i z e d elastomers by u s i n g chemical probes, which modify or break only c e r t a i n kinds o f cross-1inks(3) (4) (5). P a r e l e l a s tomer is more difficult to study i n t h i s f a s h i o n than styrenebutadiene rubber (SBR) or n a t u r a l rubber because many o f these chemical reagents a l s o r e a c t with the C-O bonds o f the p o l y e t h e r . © R e g i s t e r e d trademark o f Hercules Incorporated Hercules Research Center C o n t r i b u t i o n No.

1648

120

Vandenberg; Polyethers ACS Symposium Series; American Chemical Society: Washington, DC, 1975.


8.

BOSS

121

Propylene Oxide Rubber

However, triphenylphosphine i s a reagent which can be u t i l i z e d advantageously. I t reduces p o l y s u l f i d e s t o disulfides(§), but does not break the polymer chains i n P a r e l elastomer. To determine the types o f c r o s s - l i n k s which are produced i n representat i v e cure systems, two samples o f v u l c a n i z e d P a r e l elastomer were prepared. One was cured with s u l f u r alone, and the other with the formulation shown i n Table 1. The samples were then t r e a t e d with triphenylphosphine i n benzene. The r e s u l t s o f continuous s t r e s s r e l a x a t i o n measurements run on the products at 150°C. are shown i n Figure 1. The s t a b i l i z e r s were e x t r a c t e d by the benzene used i n the triphenylphosphine treatment so subsequent t e s t s i n a i r could not be compared with previous t e s t r e s u l t s . Therefore, a l l o f these s t r e s s r e l a x a t i o n measurements were made i n n i t r o g e n .

1.0-

• 3Î.

J

I

2

4

0

I 6

I 8

I

I 10

12

1 14

TIME (HRS) RUN IN N AT 150°C UNTREATED; — REACTED 24 HOURS WITH TRIPHENYLPHOSPHINE IN BENZENE. 2

Figure 1.

Effect of polysulfides on the stress relaxation of vulcanized Parel elastomer

Both o f the unextracted samples s u f f e r from a r a p i d i n i t i a l l o s s o f modulus, but the sample cured with s u l f u r has a considerably more severe l o s s . A f t e r the c r o s s - l i n k s were converted from


POLYETHERS

TABLE 1 TYPICAL VULCANIZATION FORMULATION


100 50

FOR PAREL ELASTOMER

PAREL ELASTOMER HAF BLACK

1.0

STEARIC ACID

1.5

NICKEL DIBUTYLDITHIOCARBAMATE

5.0

ZNIC OXIDE

1.5

TETRAETHYLTHIURAM

1.5 1.25

MONOSULFIDE

MERCAPTOBENZOTHIAZOLE SULFUR

TABLE 2 TYPICAL PHYSICAL PROPERTIES OF VULCANIZED PAREL ELASTOMER

100$ MODULUS

465 P . S . I .

300% MODULUS

1740 P . S . I .

TENSILE STRENGTH

2075 P . S . I .

ELONGATION

375 %

HARDNESS (SHORE A)

68

BAYSHORE RESILIENCE

48 %



8.

BOSS

123


p o l y s u l f i d e s to d i s u l f i d e s , both samples had a much s m a l l e r l o s s o f modulus i n the f i r s t few hours o f t e s t . These s t r e s s r e l a x a t i o n r e s u l t s show that p o l y s u l f i d e s are weaker than d i s u l f i d e s . L a i has shown t h a t r e d u c t i o n o f p o l y s u l f i d e s to d i s u l f i d e s 4oes not i n t e r f e r e with the t e n s i l e p r o p e r t i e s o f n a t u r a l rubber^Z.). We d i d not determine whether t h i s was true f o r the P a r e l e l a s t o mer. The s u l f u r content o f these samples was determined a f t e r treatment with benzene alone and a f t e r treatment with t r i p h e n y l phosphine i n benzene. The samples l o s t about 1/3 o f t h e i r s u l f u r i n the benzene. About h a l f o f t h i s l o s s (about 1/6 o f the t o t a l amount o f s u l f u r ) could be a t t r i b u t e d to l o s s o f n i c k e l d i b u t y l d i t h i o c a r b o n a t e (NBC), the s t a b i l i z e r . When these samples were t r e a t e d with triphenylphosphine i n benzene, the sample cured with an a c c e l e r a t o r l o s t about 50% o f i t s o r i g i n a l s u l f u r . The one cured with s u l f u r alone l o s t 90% of i t s o r i g i n a l s u l f u r . Thus, we have f u r t h e r evidence that both cure systems r e s u l t i n p o l y s u l f i d e c r o s s - l i n k s , but they represent a s m a l l e r percentage o f the t o t a l when an a c c e l e r a t o r i s used. S t a b i l i z a t i o n and Hydroperoxide

Decomposers

When p r o p e r l y compounded, v u l c a n i z e d P a r e l elastomer i s q u i t e s t a b l e i n hot a i r . N i c k e l d i b u t y l d i t h i o c a r b a m a t e (NBC) has been found to be a very e f f e c t i v e s t a b i l i z e r f o r t h i s rubber. Results o f aging a standard formulation i n a 150°C. a i r oven are shown i n Table 3. The hardness r e s u l t s are given as a change ( i n p o i n t s ) from the o r i g i n a l value, while the percentage o f the o r i g i n a l value i s given f o r the other p r o p e r t i e s . During the f i r s t three days o f aging, the 100% modulus and hardness were higher than the o r i g i n a l v a l u e s , while the e l o n g a t i o n was lower. These changes are probably due to a d d i t i o n a l c r o s s - l i n k i n g . Then the rubber g r a d u a l l y softened and l o s t s t r e n g t h . A f t e r a week, the v u l c a n i z e d elastomer l o s t only about 10% o f i t s 100% modulus, l e s s than o n e - t h i r d o f i t s t e n s i l e s t r e n g t h , and softened very l i t t l e ; the rubber was s t i l l q u i t e s e r v i c e a b l e . A f t e r 10 days under these t e s t c o n d i t i o n s , the elastomer d e t e r i o r a t e d considerably. These heat aging t e s t s show t h a t p r o p e r l y compounded P a r e l elastomer v u l c a n i z a t e s are outstanding i n high temperature oxidation resistance. TABLE 3 CHANGE OF PROPERTIES OF PAREL ELASTOMER AGED IN AIR AT 150°C. DAYS

100£

% OF ORIGINAL ELÔNÊ. MOD. T.S.

CHANGE IN HARDNESS

1

130

95

65

+4

3

145

75

60

+3

7

90

70

65

-2

10

60

30

55

-7


124

POLYETHERS


Figure 2 compares the e f f e c t i v e n e s s o f two s t a b i l i z e r s i n P a r e l elastomer. Since the elastomer contains an in-process a n t i o x i d a n t , o n l y a hydroperoxide decomposer was added. The s u p e r i o r i t y o f NBC i n maintaining the p h y s i c a l p r o p e r t i e s o f P a r e l elastomer i s obvious.

Figure 2.

Effect of stabilizer on vulcanized Parel elastomer aged at 150°C

A model system has been used t o e x p l a i n the e f f e c t i v e n e s s o f dithiocarbamates as hydroperoxide decomposers. Table 4 summarizes r e s u l t s o f decomposing t - b u t y l hydroperoxide (TBHP) with d i t h i o carbamates o r d i l a u r y l t h i o d i p r o p i o n a t e (LTDP). The d i t h i o c a r b a mates were t e s t e d at room temperature and at 6 0 ° C , while the experiment with LTDP was run at 100°C. LTDP i s an e f f e c t i v e hydroperoxide decomposer and i s o f t e n used f o r t h i s purpose i n polypropylene and other polymers. It decomposes hydroperoxides c a t a l y t i c a l l y and q u i c k l y at 100-150°C, but not at room temperature Each mole o f NBC r e a c t s with about 6 moles o f hydroperoxide i n 15 minutes at 25°C. The z i n c compound r e q u i r e s s e v e r a l hours at 25° or about 20 minutes at 60°C. to e f f e c t i v e l y decompose TBHP. Undoubtedly more important than the r a t e o f decomposition o f hydroperoxides i s the type o f product obtained. Table 5 l i s t s the major products obtained by completely decomposing TBHP with v a r i o u s s u l f u r compounds. With n i c k e l and z i n c dithiocarbamates


8.

BOSS

125


TABLE 4 DECOMPOSITION OF TBHP BY SULFUR COMPOUNDS

NBC (25°)


TIME (MIN)

TBHP CONCENTRATION NBC ZEC (60°) (25°)

(MOLAR) ZEC (60°)

LTDP (100°)

0

0.205

0.203

0.201

0.201

0.188

15

0.060

-

0.020

0.188

0.162

30

0.030

0.198-

0.016

0.075

0.108

60

0.029

0.195

0.014

0.026

0.014

0.179

-

-

-

0.019

180 ALL

SOLUTIONS I N I T I A L L Y 0.02M

STALILIZER

NBC= n i c k e l d i b u t y l d i t h i o c a r b a m a t e ZEC= z i n c d i e t h y l d i t h i o c a r b a m a t e LTDP= d i l a u r y l t h i o d i p r o p i o n a t e TBHP= t - b u t y l h y d r o p e r o x i d e

TABLE 5 PRODUCTS OF TBHP DECOMPOSITION

ΓΤΒΗΡΙ

STABILIZER NBC

L > s

% TBHP TO TBA DTBP

5

70

5

ZBC

4

80

5

LTDP

5

0

40

(C^HgS)2

5

0

40

10

0

90

so TBA=

2

t-butanol

DTBP- di-t-butylperoxide


126

POLYETHERS

most o f the hydroperoxide went t o the a l c o h o l , and only a small amount t o the peroxide; LTDP formed the peroxide from TBHP, but not the a l c o h o l . About h a l f o f the TBHP was unaccounted f o r , but n e i t h e r methanol n o r acetone was detected. The products formed from TBHP may e x p l a i n why LTDP does not work w e l l i n P a r e l elastomer. The peroxides formed i n p o l y e t h e r s would not be s t a b l e , so t h e i r decomposition would cause continua t i o n o f the o x i d a t i v e chain r e a c t i o n .


Heat Aging Comparison with Other Elastomers P a r e l elastomer, n a t u r a l rubber, and neoprene, compounded with standard recommended formulas f o r each, were heat aged i n a 125°C. f o r c e d d r a f t oven. Figure 3 i s a p l o t o f the change i n t e n s i l e strength and hardness o f the v u l c a n i z e d P a r e l and neoprene elastomers. Neoprene maintained i t s p r o p e r t i e s f o r 3 days, but a f t e r a week a t 1 2 5 ° C , i t was q u i t e hard and b r i t t l e . Results with n a t u r a l rubber are not i n c l u d e d i n the f i g u r e because i t f a i l e d so q u i c k l y a t 125°C. I t was s e r i o u s l y d e t e r i o r a t e d i n about 3 days a t 100°C.

1050

65

700

60

350

55

10 DAYS

Figure 3.

50

10 DAYS

Physical properties of Parel and neoprene elastomers aged at 125°C

Compression s e t i s the i n a b i l i t y o f rubber t o r e t u r n t o i t s o r i g i n a l s i z e a f t e r being squeezed. Standard t e s t s u s u a l l y compress the v u l c a n i z e d rubber t o 75% o f i t s o r i g i n a l s i z e f o r 1 o r


8.

BOSS


127


3 days a t 70°, 100°, o r 125°C. Figure 4 i s a p l o t o f the compress i o n set o f v u l c a n i z e d P a r e l elastomer t e s t e d f o r up t o 7 days a t temperatures from 70° t o 150°C. While r a t h e r h i g h compression s e t was a t t a i n e d i n i t i a l l y there was not much change as the time and temperature increased. F o r example, there was n e a r l y 50% s e t a f t e r 1 day a t 100°, but s t i l l l e s s than 70% a f t e r a week a t 150°C. The r e s u l t s obtained a t 125° and 150°C. are w i t h i n e x p e r i mental e r r o r o f each other.

100r-

90 -

80[-

DAYS Figure 4. Effect of time and temperature on the compression set of vulcanized Parel elastomer

Figures 5 and 6 compare the compression s e t o f v u l c a n i z e d P a r e l , n a t u r a l rubber, and neoprene elastomers a t 100° and 1 5 0 ° C , r e s p e c t i v e l y . At 1 0 0 ° C , the compression s e t o f P a r e l elastomer i s w i t h i n experimental e r r o r o f that o f n a t u r a l rubber; neoprene i s c l e a r l y s u p e r i o r . When the compression s e t t e s t was run f o r a long time at 150°C. (Figure 7), P a r e l elastomer was the best o f these three rubbers. Natural rubber had 100% compression s e t


128

POLYETHERS

100

90

80


70

S oc

50

40

30

20 10

0 0

1

2

3

4

5

6

_ 7

-

DAYS

Figure 5.

Compression set at

100°C

w i t h i n three days, probably as a r e s u l t o f o x i d a t i v e degradation. Neoprene had a lower compression set than the P a r e l elastomer v u l c a n i z a t e f o r about f i v e days, but then i t became worse. S t r e s s r e l a x a t i o n measurements were made on cured Parel and neoprene elastomers. The samples were h e l d at 25% e l o n g a t i o n i n our t e s t s . T h i s technique measures chain o r c r o s s - l i n k s c i s s i o n , but does not show the e f f e c t o f any recombination or a d d i t i o n a l c u r i n g that occurs. Such new bonds would be load-bearing i f the sample were p u l l e d f u r t h e r as happens i n an i n t e r m i t t e n t run where the sample i s extended f o r a few seconds, then returned t o i t s o r i g i n a l length f o r about 10 minutes. Since the sample spends about 95% o f i t s time i n the r e l a x e d s t a t e , new bonds are loadbearing the next time the sample i s elongated. Many rubbers undergo a r a p i d l o s s o f modulus i n continuous experiments, but not i n i n t e r m i t t e n t runs. At 70°C. n a t u r a l rubber underwent l e s s r a p i d i n i t i a l l o s s than P a r e l elastomer or neoprene. Since oxidat i v e degradation i s not s i g n i f i c a n t i n 10-20 hours at 7 0 ° C , n a t u r a l rubber was s u p e r i o r to P a r e l elastomer or neoprene i n t h i s t e s t . At 1 0 0 ° C , however, the o x i d a t i v e chain s c i s s i o n or n a t u r a l



8.

BOSS


129

10

01 5

I 1

I

I

2

3

I 4

I

1

L

5

6

7

DAYS

Figure 6.

Compression set at

150°C

rubber was much more r a p i d than that o f e i t h e r o f the s y n t h e t i c rubbers. Therefore, i t s modulus dropped r a p i d l y a f t e r a few hours. Figure 7 shows s t r e s s r e l a x a t i o n r e s u l t s with P a r e l e l a s tomer and neoprene at 1 2 5 ° C ; t h e i r r a t e s o f chain s c i s s i o n are similar. In t h i s t e s t , neoprene maintained a l a r g e r f r a c t i o n o f i t s i n i t i a l modulus than the v u l c a n i z e d Parel elastomer. However, Figure 8 shows s t r e s s r e l a x a t i o n r e s u l t s with these two rubbers at 150°C. Under these c o n d i t i o n s , the g r e a t e r o x i d a t i v e s t a b i l i t y o f the P a r e l elastomer i s more s i g n i f i c a n t than i t s somewhat greater l o s s o f modulus i n the f i r s t few hours o f t e s t . Dynamic

Properties

The dynamic p r o p e r t i e s o f t y p i c a l formulations o f P a r e l elastomer, neoprene, n i t r i l e , and n a t u r a l rubber were measured from -55°C. t o +80°C. u s i n g a v i b r a t i n g reed apparatus. These



POLYETHERS

I

•31 0

PAREL ELASTOMER

I 2

ι 4

I 6

I 8

I

I

1 0 1 2

1 14

TIME (HRS)

Figure 7.

Stress relaxation at

125°C

measurements were made a t a frequency o f about 500 h e r t z a t temperatures below the g l a s s t r a n s i t i o n , and a t about 30 h e r t z a t higher temperatures. The dynamic modulus o f the four rubbers i s p l o t t e d against temperature i n Figure 9. Although the samples were s t r a i n e d l e s s than 0.5%, dynamic modulus r e s u l t s are c a l c u l a t e d by e x t r a p o l a t i n g t o 100% s t r a i n . Therefore, dynamic t e s t s gave h i g h e r modulus values than s t a t i c t e s t s . The g l a s s t r a n s i t i o n temperatures are very obvious, as shown by the r a p i d changes i n modulus. They v a r i e d from about -55°C. f o r P a r e l elastomer t o about 0°C. f o r neoprene. In many a p p l i c a t i o n s , the region o f importance i s from about -25° t o +40°C. The dynamic modulus o f these f o u r rubbers changed l i t t l e as the temperature was increased above 30°C. A t w e l l below room temperature, the dynamic modulus o f P a r e l elastomer and n a t u r a l rubber remained d e s i r a b l y constant. The other two rubbers are so c l o s e t o t h e i r glass t r a n s i t i o n temperatures that the p r o p e r t i e s o f a r t i c l e s prepared from them



8.

BOSS


.31

0

1

I

2

4

131

I

»

I

6

8

10

t

12

ι

14

TIME (HRS) Figure 8.

Stress relaxation at

150°C

would vary c o n s i d e r a b l y a t temperatures encountered i n the winter. Decreasing the l e v e l o f carbon b l a c k o r adding p l a s t i c i z e r s h i f t e d these curves t o lower modulus v a l u e s , and adding p l a s t i c i z e r s lowered the Tg. However, p l a s t i c i z e r s can be l o s t by e x t r a c t i o n or v o l a t i l i t y , and so should be s e l e c t e d t o avoid such l o s s . In a rubber, there i s a time l a g between an a p p l i e d s t r e s s and the r e s u l t i n g s t r a i n . In a dynamic t e s t , the s t r e s s i s ap p l i e d i n a s i n u s o i d a l f a s h i o n so t h i s delay can be t r e a t e d as a phase angle - d e l t a . Dynamic r e s u l t s are o f t e n given as the tangent o f d e l t a . Tan β i s a measure o f the r a t i o o f energy absorbed t o energy put i n p e r c y c l e . Heat b u i l d u p i n a f l e x i n g rubber i s dependent upon the energy r e q u i r e d t o s t r a i n the sample (the dynamic modulus), and on tan δ, which i s a measure o f the f r a c t i o n o f t h i s energy r e t a i n e d by the sample as heat. Small values are d e s i r a b l e where heat b u i l d u p may be a problem, while l a r g e r values mean the m a t e r i a l w i l l absorb a l a r g e r f r a c t i o n o f v i b r a t i o n a l energy. Tan

Vulcanization and Stabilization Studies on Propylene Oxide Rubber

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