142

CHAPTER  V RESULTS AND DISCUSSION In the rubber industry, Mooney viscosity is used for specification of flow property of rubbers(1). The study carried out by us on samples of the S1552 type rubber showed that molecular weight parameters i.e. number average and weight average molecular weights and molecular weight distribution of the rubber had a significant effect on its mill behaviour, ability to mix with the compounding ingredients, delta Mooney viscosity, compound Mooney viscosity, minimum viscosity, scorch time, cure time, cure index, tensile strength, 300% modulus, elongation at break and aging characteristics both at varying and constant Mooney viscosity. Further the study showed that Mooney viscosity itself depended on molecular weight and molecular weight distribution. Some of the above properties were also found to depend on the organic acid content of the rubber. Thus, molecular weight and molecular weight distribution of the rubber were found to be by far the most important factors which determined various properties of S1552 rubber. The following discussion of the results will show that data of significant industrial importance was generated during the course of the study. (i) Processability of S1552 Rubber: Mill behaviour of rubber and its ability to mix with the compounding ingredients is an import determines its processability(24) during preparation of useful and products from the raw rubber. However, due to inherent difficulties in quantifying the results, it is impractical to specify values in terms of mill behaviour of rubber.

143

In the case of S1712 rubber (an oil extended cold polymerized styrene butadiene rubber), the delta Mooney test provides a general indication of its processing characterisatics(5,6). The delta Mooney test for non oil extended rubbers such as S1552 does not seem to be reported by any worker. The results of Mooney viscosity, delta Mooney viscosities (ML17 and ML115), compound Mooney viscosity, minimum Mooney viscosity, overall processability and molecular weight parameters of the different samples of S1552 rubber with constant Mooney viscosity and the unblended samples of S1552 rubber with varying Mooney viscosity have been reproduced in Table 1 and 2 respectively. Variation of delta Mooney viscosities, ML115 and

ML17 with

polydispersity index of the samples of S1552 rubber with constant Mooney viscosity (Samples 3, 6, 7, 8 and 9) and variation of the same with raw rubber Mooney viscosity, weight average molecular weight and number average molecular weight of the unblended samples of S1552 rubber (Samples, 1, 2, 3, 4 & 5) have been graphical represented in Figs. 1, 2, 3 and 4 respectively. It can be clearly seen from the results that overall procesability (as determined by mill behaviour and mixing behaviour) was found to progressively deteriorate with increasing polydispersity index i.e. broadness of the molecular weight distribution of the samples of S1552 rubber i.e. 50 ML1+4. Processability of the rubber samples was found to be good upto a polydispersity index value of 4.84 and to progressively deteriorate as the polydispersity index was morethan 5.15.

144

Table 1 Effect of Molecular weight parameters on Mooney viscosity, Delta Mooney Viscosity, Compound Mooney Viscosity, minimum viscosity & overall processability of S1552 Rubber with constant Mooney Viscosity Properties

Sample No. 3

8

6

9

7

Mooney Viscosity, ML1+4 at 100C

50

50

49.5

51

49

Delta Mooney Viscosity, ML115 at 100C

20

21

20

18

17

Delta Mooney Viscosity, ML17 at 100C

16

17

16

13.5

12

Compound Mooney Viscosity, ML1+4 at 126C

67

63

63

62

62

Minimum Mooney Viscosity at 126C

66

60

63

61

62

Good

Good

Good

Fair to

Poor

Overall Processability**

Poor Molecular Weight Parameters: Weight Average Molecular Weight, M w

5,63,356

6,23,646

6,30,058

6,77,435

7,23,609

Number Average Molecular Weight, M n

1,28,177

1,29,281

1,30,082

1,31,621

1,33,464

4.39

4.80

4.84

5.15

5.42

Polydispersity Index, D **Details given in Table 2, Chapter 4.

145

Table 2 Effect of Molecular weight parameters and Mooney viscosity on Delta Mooney Viscosity, Compound Mooney Viscosity, minimum viscosity & overall processability of unblended samples of S1552 Rubber Properties

Sample No. 1

2

3

4

5

33

41

50

60

85

Delta Mooney Viscosity, ML17 at 100C

18.5

16.5

16

14

5

Delta Mooney Viscosity, ML115 at 100C

22.5

21

20

18

16

Compound Mooney Viscosity, ML1+4 at 126C

56

63

67

70

72

Minimum Mooney Viscosity at 126C

54

60

66

65

67

Good

Good

Good

Fair

Poor

Weight Average Molecular Weight, M w

5,15,457

5,30,413

5,63,356

6,86,488

9,96,848

Number Average Molecular Weight, M n

1,23,226

1,25,226

1,28,177

1,36,409

1,60,650

4.17

4.24

4.39

5.03

6.20

Mooney Viscosity, ML1+4 at 100C

Overall Processability** Molecular Weight Parameters:

Polydispersity Index, D **Details given in Table 2, Chapter 4.

146

In case of the unblended samples of S1552 rubber, processability of the rubber samples having Mooney viscosity less than 60ML1+4 was good. The rubber samples having 60 and higher Mooney viscosity exhibited progressively poor processability. However, it was clear from the foregoing paragraph that Mooney viscosity alone could not be used as a measure of processability of rubber since processability was found to be influenced by the polydispersity index of the rubber samples even though their Mooney viscosity was the same. A comparison of delta Mooney viscosities, ML115 and ML17 with process ability of S1552 rubber clearly indicated that the samples of the rubber which had delta Mooney viscosity, ML115 of more than 18 units and delta Mooney viscosity, ML17 of more than 14 units exhibited good overall processability. Samples of the S1552 rubber having delta Mooney viscosities less than these values exhibited fair to poor overall processability. It could, therefore, be concluded that delta Mooney viscosity test could be satisfactorily used as a measure of processability of S1552 rubber. It can be clearly seen from Fig. 1 that delta Mooney viscosities, ML115 and ML17, remained more or less constant upto a polydispersity index value of 4.8 and decreased rapidly thereafter. ML17 value was more sensitive than the ML115 value. Delta Mooney viscosities, ML17 and ML115 both decreased with increasing Mooney viscosity, ML1+4, increasing weight average molecular weight and increasing number average molecular weight of the unblended samples of S1552 rubber (cf. Figs. 2, 3 and 4). The drop in ML17 value was more sharp as the Mooney viscosity increased beyond 50 ML1+4 value. This was attributed to the fact that polydispersity index increased sharply for the samples with more than 50 Mooney viscosity.

147

No worker seems to have reported on the influence of molecular weight parameters on compound Mooney viscosity and minimum viscosity of the rubber. Based on our study, variation of compound Mooney viscosity, ML1+4 at 126C and Minimum viscosity at 126C with polydispersity index of Samples of S1552 rubber with constant Mooney viscosity has been represented graphically in Fig. 5. It is clear from the graph that both compound Mooney viscosity and Minimum viscosity decreased with increased polydispersity index but became more or less constant as the polydispersity index increased beyond 5. However, compound Mooney viscosity and Minimum viscosity increased with increasing raw rubber Mooney viscosity and increasing weight average and number average molecular weights of the unblended samples of S1552 rubber upto a certain extent and then tapered off (Fig. 6, 7 and 8). Compound Mooney viscosity was, in general, higher than the raw rubber Mooney viscosity below about 70 Mooney viscosity. However, it was less than the raw rubber Mooney viscosity beyond 70 ML1+4. This was attributed to the possibility that the rubber with higher Mooney viscosity (due to its higher molecular weight) suffered higher chain scission reactions during mastication and mixing of the rubber with the compounding ingredients than the rubber with lower Mooney viscosity and thus relatively lower molecular weight. A comparison of compound Mooney viscosity of Sample 3 (which was an unblended sample) with Samples 6, 7, 8 and 9 (which were all blended samples) indicated that compound Mooney viscosity of the blended rubber was, in general, lower than that of the unblended rubber at the same Mooney viscosity. This was attributed to the presence of lower molecular weight fraction in the blended samples, which acted as a lubricant in reducing its Mooney viscosity.

148

A comparison of compound Mooney viscosity of the samples 1, 2, 3, 4 and 5 with their minimum viscosity vide the results summarized in Table 2 and Figs. 6, 7 and 8 indicated that the difference between the compound Mooney viscosity (4 minutes value) and the minimum viscosity increased with increasing raw rubber Mooney viscosity and increasing weight average and number average molecular weights. Based on the results obtained, the compound Mooney viscosity and the minimum viscosity did not, however, seem to correlate well with the processability of rubber. The compound Mooney viscosity and the minimum viscosity could, therefore, not be used as a measure of processability of the rubber. Variation of Mooney viscosity with weight average and number average molecular weights of Samples, 1, 2, 3, 4 and 5 of the unblended samples of S1552 rubber has been graphically represented in Fig. 9. It is clear from the figure that Mooney viscosity increases with both weight average molecular weight and number average molecular weight. In this respect, effect of number average molecular weight. In this respect, effect of number average molecular weight was more pronounced than the effect of weight average molecular weight since a relatively smaller change in number average molecular weight caused a greater change in Mooney viscosity of the rubber. Further, the fact that Mooney viscosity of the rubber Samples, 3, 6, 7, 8 and 9 was more or less same, even though the weight average and number average molecular weights of these samples differed significantly from each other, lead us to the conclusion that Mooney viscosity of the rubber decreased with increasing polydispersity index or broadness of the molecular weight distribution (Table 1). This was further confirmed by the results summarized in Table 2 and Fig. 9 which slowly shaped that the rate of increase in

149

Mooney viscosity decreased at higher molecular weight which could be attributed to increase in the polydispersity index of samples having higher Mooney viscosity. These observations were thus more or less in agreement with the findings of White(7) who found that the 4 minutes Mooney viscosity increase much slower with molecular weight for other polymers with broader molecular weight distribution than for narrow molecular weight distribution. (ii)

Scorch time, cure time and cure index of S1552 Rubber: Scorch is premature vulcanization which may take place during processing

of the rubber compound due to accumulated effects of heat and time. As premature vulcanization of the compound will make it improcessable any further and will result in ruining of the compound, it is necessary that scorch time of the compound should be more than the maximum heat history accumulated during entire processing of the compound.(8) Very little or no information on influence of molecular weight distribution on scorch time of rubber compounds seems to be available in literature. The results of the study conducted by us on scorch time, cure time and cure index and molecular weight data of the samples of S1552 rubber (Samples 3, 6, 7, 8 and 9) with constant Mooney viscosity and the unblended samples of S1552 rubber (Samples 1, 2, 3, 4 and 5) with varying Mooney viscosity have been reproduced in Table 3 & 4 respectively. Variation of scorch time, cure time and cure index with polydispersity index of the samples of S1552 rubber having constant Mooney viscosity and variation of the same with the raw rubber Mooney viscosity and weight average and number average molecular weights of the unblended samples of S1552 rubber having varying Mooney viscosity have been graphically represented in Figs. 10, 11, 12 and 13 respectively.

150

Table 3 Effect of Molecular weight parameters on Scorch Time, Cure time and Cure Index of Samples of S1552 Rubber with constant Mooney Viscosity Properties

Sample No. 3

8

6

9

7

Scorch Time, Minutes

22.0

22.8

23.2

16.6

16.8

Cure Time, Minutes

28.7

29.7

31.2

22.9

22.8

Cure Index

6.7

6.9

8.0

6.3

6.0

Weight Average Molecular Weight, M w

5,63,356

6,23,646

6,30,058

6,77,435

7,23,609

Number Average Molecular Weight, M n

1,28,177

1,29,281

1,30,082

1,31,621

1,33,464

4.39

4.80

4.84

5.15

5.42

Polydispersity Index, D

151

Table 4 Effect of Molecular weight parameters and Mooney Viscosity on Scorch Time, Cure Time and Cure Index of Unblended Samples of S1552 Rubber Properties

Sample No. 1

2

3

4

5

33

41

50

60

85

Scorch Time, Minutes

24.8

23.8

22.0

21.0

16.2

Cure Time, Minutes

33.2

30.8

28.7

27.2

22.0

Cure Index

8.4

7.0

6.7

6.2

5.8

Weight Average Molecular Weight, M w

5,15,457

5,30,413

5,63,356

6,86,488

6,96,848

Number Average Molecular Weight, M n

1,23,697

1,25,226

1,28,177

1,36,409

1,60,650

4.17

4.24

4.39

5.03

6.20

Mooney Viscosity, ML1+4 at 100C

Polydispersity Index

152

It can be seen from Fig. 10 that while scorch time and cure time tended to reduce with increasing polydispersity index, cure index which was a measure of rate of cure was only marginally effected. Figures 11, 12 and 13 showed that scorch time, cure time and cure index all decreased with increasing Mooney viscosity and increasing weight and number average molecular weights of the unblended samples of S1552 rubber. However, the influence of Mooney viscosity and weight and number average molecular weights particularly at higher values of these was less marked on cure index than on scorch time and cure time. These observations lead us to the conclusion that scorch time and cure time of S1552 rubber were more pronouncedly influenced by its Mooney viscosity, molecular weights and broadness of the molecular weight distribution than its cure rate index was. Thus, the rubber with narrow molecular weight distribution offered better scorch safety than the rubber with broader molecular weight distribution. Blending of lattices with widely varying Mooney viscosity (as in Samples 7 and 9) which gave broader molecular weight distribution and thus poor scorch safety was, therefore, not desirable. (iii)

Tensile Strength, 300% Modulus and Elongation at Break Tensile strength, 300% modulus and elongation at break are important

properties of a given rubber vulcanisate.(9) Data on these properties of the sample 3, 6, 7, 8 and 9 of S1552 rubber having constant Mooney viscosity and the same of Samples 1, 2, 3, 4 and 5 having increasing Mooney viscosity together with their weight and number average molecular weights and polydispersity index have been given in Tables 5 and 6. Fig. 14 shows variation of tensile strength, 300% modulus and elongation at break with polydispersity index of the samples of S1552 rubber prepared by

153

blending lattices with varying Mooney viscosity so that the final Mooney viscosity of the blended latex was same in all cases as that of the unblended sample 3. It is clear from the figure that tensile strength decreased with increasing polydispersity index less steeply in the beginning and more steeply later indicating thereby that the rate of drop in tensile strength was more marked above polydispersity index value of 4.8 or so. The 300% modulus of S1552 rubber increased with increasing polydispersity index upto about 5.2 and decreased thereafter as the polydispersity was increased further. The drop in elongation at break with increasing polydispersity index was most pronounced and was almost linear in the range of polydispersity index (4.39 to 5.42) studied. Figures 15, 16 and 17 show that tensile strength increased with increasing Mooney viscosity and weight and number average molecular weights of the samples of unblended S1552 rubber. The rise in tensile strength was found to be more rapid in the beginning and tapering off later as the Mooney viscosity and weight and number average molecular weights increased further beyond 60 ML1+4, 7  105 M w and 1.35  105 M n respectively. This tapering of tensile strength was attributed to the increase in the polydispersity index of the samples of S1552 rubber with 60 and 65 Mooney viscosity which must have tended to reduce their tensile strength. Similarly, 300% modulus was also found to increase with increasing Mooney viscosity and weight and number average molecular weights of the samples of S1552 rubber.

154

Table 5 Effect of Molecular Weight parameters on Tensile Strength, 300% Modulus and Elongation at Break of Samples of S1552 Rubber with constant Mooney Viscosity Properties

Sample No. 3

8

6

9

7

Tensile Strength, kg/cm2

245

228

225

200

165

300% Modulus, kg/cm2

162

172

170

174

165

Elongation at Break, %

440

390

380

340

300

Weight Average Molecular Weight, M w

5,63,356

6,23,646

6,30,058

6,77,435

7,23,609

Number Average Molecular Weight, M n

1,28,177

1,29,281

1,30,082

1,31,621

1,33,464

4.39

4.80

4.84

5.15

5.42

Polydispersity Index, D

155

Table 6 Effect of Molecular weight Data and Mooney Viscosity on Tensile Strength, 300% Modulus and Elongation at Break of Unblended Samples of S1552 Rubber Properties

Sample No. 1

2

3

4

5

Mooney Viscosity, ML1+4 at 100C

33

41

50

60

85

Tensile Strength, kg/cm2

210

232

245

254

251

300% Modulus, kg/cm2

146

152

162

195

200

Elongation at Break, %

470

460

440

400

380

Weight Average Molecular Weight, M w

5,15,457

5,30,413

5,63,356

6,86,488

6,96,848

Number Average Molecular Weight, M n

1,23,697

1,25,226

1,28,177

1,36,409

1,60,650

4.17

4.24

4.39

5.03

6.20

Polydispersity Index, D

156

Elongation at break was found to reduce with increasing Mooney viscosity and weight average and number average molecular weights of S1552 rubber, the drop being more steep in the beginning of the curve than towards the end. (iv)

Aging Behaviour of S1552 Rubber Properties of rubber are known to deteriorate on storage due to the effect of

heat and time. Air aging at higher temperature gives an accelerated way of estimating the lide of various products made from the rubber. Usually air aging of styrenebutadiene rubber vulcanisates is carried out at 100C.(10,11) While it is known that the properties such as tensile strength and elongation at break of unaged and aged rubber vulcanisates depend to a large extent on the number and type of the chemical crosslinks, the dependence of these properties on the molecular weight and the molecular weight distribution of the rubber does not seem to be systematically reported by any workers. Percent deterioration in tensile strength and elongation at break, based on original values of the unaged samples, 3, 6, 7, 8 and 9 of S1552 rubber with constant Mooney viscosity, and the same of Samples 1, 2, 3, 4 and 5 with different Mooney viscosity, together with their weight and number average molecular weights and polydispersity(1216) index have been reproduced in Table 7 and 8. The dependence of deterioration in tensile strength and elongation at break (due to air aging for 120 hours at 100  1C) of the samples of S1552 rubber with constant Mooney viscosity vulcanized under identical conditions on polydispersity index of the rubber samples is graphically shown in Fig. 18. It can be clearly seen from the figure that both deterioration in tensile strength and deterioration in elongation at break increased with increasing polydispersity index of the rubber or in other words aging characteristics of S1552 rubber were observed to be poorer

157

Table 7 Effect of Molecular Weight parameters on Deterioration of Tensile Strength and Elongation at Break on Aging of Samples of S1552 Rubber with Constant Mooney Viscosity for 120 Hours at 100  1C Properties

Sample No. 3

8

6

9

7

Determination in Tensile Strength, % of original value.

35.1

32.0

34.7

39.5

38.8

Deterioration in Elongation at Break, % of original value

61.4

61.5

60.5

61.8

63.3

Weight Average Molecular Weight, M w

5,63,356

6,23,646

6,30,058

6,77,435

7,23,609

Number Average Molecular Weight, M n

1,28,177

1,29,281

1,30,082

1,31,621

1,33,464

4.39

4.80

4.84

5.15

5.42

Polydispersity Index, D

158

Table 8 Effect of Molecular weight parameters and Mooney Viscosity on Deterioration of Tensile Strength and Elongation at Break on Aging of Unblended Samples of S1552 Rubber for 120 Hours at 100  1C. Properties

Sample No. 1

2

3

4

5

33

41

50

60

85

20.0

26.3

35.1

39.4

44.2

66.0

65.2

61.4

60.0

57.9

Weight Average Molecular Weight, M w

5,15,457

5,30,413

5,63,356

6,86,488

6,96,848

Number Average Molecular Weight, M n

1,23,697

1,25,226

1,28,177

1,36,409

1,60,650

4.17

4.24

4.39

5.03

6.20

Mooney Viscosity, ML1+4 at 100C Deterioration in Tensile Strength % of original value. Deterioration in Elongation at break, % of original value.

Polydispersity Index, D

159

for rubber samples with broader molecular weight distribution than with narrower molecular weight distribution. Deterioration in tensile strength with increasing polydispersity index beyond 4.84 was more pronounced than deterioration in elongation at break.(1720) The effect of Mooney viscosity, weight average molecular weight and number average molecular weight on deterioration in tensile strength and elongation at break of S1552 rubber samples 1, 2, 3, 4 and 5 has been shown in Figs. 19, 20 and 21 respectively. One can see that the shape of the curves in these figures remained more or less same indicating thereby that the basic nature of the effect of Mooney viscosity and weight and number average molecular weights on aging characteristics of S1552 rubber was nearly the same. It is clear from the figures that whereas deterioration in tensile strength on aging increased with increasing Mooney viscosity, increasing weight average and number average molecular weights, deterioration in elongation at break(2125) decreased i.e. improved. (v)

Influence of Organic Acids on Properties of S1552 Rubber Properties of the five unblended samples of S1552 rubber viz. 1, 2, 3, 4

and 5 containing about 5% organic acid (mixed rosin and fatty acids) and the corresponding five unblended samples of S1552 viz. 1A, 2A, 3A, 4A and 5A containing about 0.4% organic acid have been summarized in Table 9. Each set of samples that is 1 and 1A, 2 and 2A, 3 and 3A, 4 and 4A & 5 and 5A were prepared from the same base latex and differed only with respect to their organic acid contents. A comparison of the weight average and number average molecular weights and the polydispersity indices of each set of the samples clearly indicated that these values did not summarized in change as a result of reduction of organic acid by alkali washing of various samples of S1552 rubber and that any difference in properties was mainly due to difference in organic(2627) acids content only.

160

Table 9 Effect of Presence of Organic Acid on various properties of Unblended Samples of S1552 Rubber Properties Weight Average Molecular Weight, M w

Sample No. 1 1A 2 2A 3 3A 4 4A 5 5A 515457 514151 530413 537074 563356 555789 686488 699431 996848 990743

Number Average Molecular Weight M n

123697 129476 125226 129589 128177 130762 136409 138584 160650 161438

Polydispersity Index, D

4.17

3.97

4.24

4.14

4.39

4.25

5.03

5.05

6.20

6.14

Organic Acid Content, %

5.25

0.38

4.95

0.40

5.15

0.41

5.30

0.42

5.20

0.39

33

38

41

46

50

56

60

67

85

91

ML115 at 100C

22.5

21

21

20

20

21

18

17

16

16

ML17 at 100C

18.5

18

16.5

17

16

17

14

12

5

4

Overall Processability

Good

Good

Good

Good

Good

Good

Fair

Fair

Poor

Poor

56

59

63

64

67

69

70

74

72

81

54

55

60

62

66

67

65

74

67

81

Raw Rubber Mooney Viscosity, ML1+4 at 100C Delta Mooney Viscosity:

Compound Mooney Viscosity, ML1+4 at 126C Minimum Mooney Viscosity at 126C

161

Table 9 Contd… Effect of Presence of Organic Acid on various properties of Unblended Samples of S1552 Rubber Properties

Sample No. 3 3A 22 25

Scorch Time, Minutes

1 28.8

1A 25

2 23.8

2A 24.8

4 21

4A 22

5 16.2

5A 17

Cure Time, Minutes

33.2

33

30.8

31.4

28.7

31.7

27.2

28

22

23.6

Cure Index, Minutes

8.4

8

7

6.6

6.7

6.7

6.2

6

5.8

6.6

Tensile Strength, kg/cm2

210

210

232

230

245

241

254

246

251

233

300% Modulus, kg/cm2

146

154

152

162

162

172

185

190

200

198

Elongation at break, %

470

460

460

450

440

430

400

380

380

360

Deterioration in Tensile Strength on

20.0

21.9

26.3

27.0

35.1

34.8

39.4

39.8

44.2

43.8

66.0

60.9

65.2

57.8

61.4

55.8

60.0

55.3

57.9

52.8

aging for 120 Hours at 100  1C, % of orginal value. Deterioration in Elongation at break on aging for 120 Hours at 100  1C, % of original value.

162

A comparison of raw rubber Mooney viscosity of S1552 rubber samples with about 5% organic acids with that of the corresponding rubber samples with about 0.4% organic acids indicated that as expected Mooney viscosity of the samples with less amount of organic acids was higher than that of the samples with higher amount of organic acids. The difference in the Mooney viscosity increased with increasing Mooney viscosity or molecular weight. This could be attributed to the lubricating effect of the organic acids on the rubber molecules causing reduction in Mooney viscosity the rubber containing it.(28) There was no significant effect of organic acids observed on the delta Mooney visocisities of S1552 rubber in the entire range of the Mooney viscosity studied. This was further confirmed by the fact that overall processability of each set of the rubber samples with higher and lower amounts of organic acids was found to be nearly the same. Effect of organic acids on compound Mooney viscosity, ML1+4 at 126C, and minimum viscosity at 126C was found to be only marginal at lower Mooney viscosities. However, as the Mooney viscosity of the rubber samples increased beyond 60, the effect on compound Mooney viscosity became more significant. There was no significant effect of organic acids noticed on the cure characteristics such as scorch time, cure time and cure index of S1552 rubber. Tensile strength of S1552 rubber samples containing higher amount of organic acids was found to be generally higher than the samples containing lower amount of organic acids. The difference in tensile strength was found to increase with increasing Mooney viscosity.(29,30) The samples of S1552 rubber having higher amount of organic acids exhibited lower 300% modulus than the samples containing lower amount of

163

organic acids. However, for samples with lower Mooney viscosity the difference observed was more significant. Amount of organic acids did not seem to effect deterioration in tensile strength on air aging of the samples of S1552 rubber for 120 hours at 100  1C. However, a comparison of values of % deterioration in elongation at break on aging of S1552 rubber samples for 120 hours at 100  1C clearly indicated that the samples of S1552 containing higher amount of organic acids exhibited higher deterioration in elongation at break on aging than the samples containing lower amount of organic acids.

164

CHAPTERV REFERENCES 1.

Tokita, N., and Pliskin, I., Rubb. Chem. & Tech., 46, 1166, 1973.

2.

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