chapter v results and discussion -...

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 S1552 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 S1552 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 S1552 Rubber:

Mill behaviour of rubber and its ability to mix with the compounding

ingredients is an import determines its processability(24) 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 S1712 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 S1552 does not seem to be reported by any worker.

The results of Mooney viscosity, delta Mooney viscosities (ML17 and

ML115), compound Mooney viscosity, minimum Mooney viscosity, overall

processability and molecular weight parameters of the different samples of S1552

rubber with constant Mooney viscosity and the unblended samples of S1552

rubber with varying Mooney viscosity have been reproduced in Table 1 and 2

respectively.

Variation of delta Mooney viscosities, ML115 and ML17 with

polydispersity index of the samples of S1552 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 unblended samples of S1552 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 S1552 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 S1552 Rubber with constant Mooney Viscosity

Properties Sample No.

3 8 6 9 7

Mooney Viscosity, ML1+4 at 100C 50 50 49.5 51 49

Delta Mooney Viscosity, ML115 at 100C 20 21 20 18 17

Delta Mooney Viscosity, ML17 at 100C 16 17 16 13.5 12

Compound Mooney Viscosity, ML1+4 at 126C 67 63 63 62 62

Minimum Mooney Viscosity at 126C 66 60 63 61 62

Overall Processability** Good Good Good Fair to

Poor

Poor

Molecular Weight Parameters:

Weight Average Molecular Weight, wM 5,63,356 6,23,646 6,30,058 6,77,435 7,23,609

Number Average Molecular Weight, nM 1,28,177 1,29,281 1,30,082 1,31,621 1,33,464

Polydispersity Index, D 4.39 4.80 4.84 5.15 5.42

**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 S1552 Rubber


1 2 3 4 5

Mooney Viscosity, ML1+4 at 100C 33 41 50 60 85

Delta Mooney Viscosity, ML17 at 100C 18.5 16.5 16 14 5

Delta Mooney Viscosity, ML115 at 100C 22.5 21 20 18 16

Compound Mooney Viscosity, ML1+4 at 126C 56 63 67 70 72

Minimum Mooney Viscosity at 126C 54 60 66 65 67

Overall Processability** Good Good Good Fair Poor

Molecular Weight Parameters:




**Details given in Table 2, Chapter 4.

146

In case of the unblended samples of S1552 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, ML115 and ML17 with process

ability of S1552 rubber clearly indicated that the samples of the rubber which had

delta Mooney viscosity, ML115 of more than 18 units and delta Mooney viscosity,

ML17 of more than 14 units exhibited good overall processability. Samples of the

S1552 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

S1552 rubber.

It can be clearly seen from Fig. 1 that delta Mooney viscosities, ML115 and

ML17, remained more or less constant upto a polydispersity index value of 4.8 and

decreased rapidly thereafter. ML17 value was more sensitive than the ML115

value. Delta Mooney viscosities, ML17 and ML115 both decreased with increasing

Mooney viscosity, ML1+4, increasing weight average molecular weight and

increasing number average molecular weight of the unblended samples of

S1552 rubber (cf. Figs. 2, 3 and 4). The drop in ML17 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 126C and

Minimum viscosity at 126C with polydispersity index of Samples of S1552

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 unblended samples of S1552 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

unblended 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 unblended samples of

S1552 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 S1552 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 S1552 rubber (Samples 3, 6, 7, 8 and 9)

with constant Mooney viscosity and the unblended samples of S1552 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 S1552 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 unblended samples

of S1552 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 S1552 Rubber with

constant Mooney Viscosity


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




151

Table 4

Effect of Molecular weight parameters and Mooney Viscosity on Scorch Time, Cure Time and Cure Index of

Unblended Samples of S1552 Rubber


1 2 3 4 5


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



Polydispersity Index 4.17 4.24 4.39 5.03 6.20

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

unblended samples of S1552 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

S1552 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 S1552 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 S1552 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 unblended 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 S1552 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 unblended S1552 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 wM and 1.35 105 nM respectively. This tapering of tensile strength was

attributed to the increase in the polydispersity index of the samples of S1552

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 S1552 rubber.

154

Table 5

Effect of Molecular Weight parameters on Tensile Strength, 300% Modulus and Elongation at Break of Samples of

S1552 Rubber with constant Mooney Viscosity


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




155

Table 6

Effect of Molecular weight Data and Mooney Viscosity on Tensile Strength, 300% Modulus and Elongation at Break

of Unblended Samples of S1552 Rubber


1 2 3 4 5


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




156

Elongation at break was found to reduce with increasing Mooney viscosity

and weight average and number average molecular weights of S1552 rubber, the

drop being more steep in the beginning of the curve than towards the end.

(iv) Aging Behaviour of S1552 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 100C.(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 S1552 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(1216) 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 1C) of the samples of S1552 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 S1552 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 S1552 Rubber with Constant Mooney Viscosity for 120 Hours at 100 1C


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




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 S1552 Rubber for 120 Hours at 100 1C.


1 2 3 4 5


Deterioration in Tensile Strength

% of original value.

20.0 26.3 35.1 39.4 44.2

Deterioration in Elongation at break,

% of original value.

66.0 65.2 61.4 60.0 57.9




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.(1720)

The effect of Mooney viscosity, weight average molecular weight and

number average molecular weight on deterioration in tensile strength and

elongation at break of S1552 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 S1552 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(2125) decreased i.e.

improved.

(v) Influence of Organic Acids on Properties of S1552 Rubber

Properties of the five unblended samples of S1552 rubber viz. 1, 2, 3, 4

and 5 containing about 5% organic acid (mixed rosin and fatty acids) and the

corresponding five unblended samples of S1552 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 S1552 rubber and that any

difference in properties was mainly due to difference in organic(2627) acids content

only.

160

Table 9

Effect of Presence of Organic Acid on various properties of Unblended Samples of S1552 Rubber

Properties Sample No. 1 1A 2 2A 3 3A 4 4A 5 5A

Weight Average Molecular Weight, wM 515457 514151 530413 537074 563356 555789 686488 699431 996848 990743

Number Average Molecular Weight nM 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

Raw Rubber Mooney Viscosity, ML1+4

at 100C

33 38 41 46 50 56 60 67 85 91

Delta Mooney Viscosity:

ML115 at 100C 22.5 21 21 20 20 21 18 17 16 16

ML17 at 100C 18.5 18 16.5 17 16 17 14 12 5 4

Overall Processability Good Good Good Good Good Good Fair Fair Poor Poor

Compound Mooney Viscosity, ML1+4 at

126C

56 59 63 64 67 69 70 74 72 81

Minimum Mooney Viscosity at 126C 54 55 60 62 66 67 65 74 67 81

161

Table 9 Contd…

Effect of Presence of Organic Acid on various properties of Unblended Samples of S1552 Rubber

Properties Sample No. 1 1A 2 2A 3 3A 4 4A 5 5A

Scorch Time, Minutes 28.8 25 23.8 24.8 22 25 21 22 16.2 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

aging for 120 Hours at 100 1C, % of

orginal value.

20.0 21.9 26.3 27.0 35.1 34.8 39.4 39.8 44.2 43.8

Deterioration in Elongation at break on

aging for 120 Hours at 100 1C, % of

original value.

66.0 60.9 65.2 57.8 61.4 55.8 60.0 55.3 57.9 52.8

162

A comparison of raw rubber Mooney viscosity of S1552 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 S1552 rubber in the entire range of the Mooney viscosity

studied. This was further confirmed by the fact that overall 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 126C,

and minimum viscosity at 126C 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 S1552 rubber.

Tensile strength of S1552 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 S1552 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 S1552 rubber for 120 hours at 100 1C.

However, a comparison of values of % deterioration in elongation at break on

aging of S1552 rubber samples for 120 hours at 100 1C clearly indicated that

the samples of S1552 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

CHAPTERV

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chapter v results and discussion -...

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