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
Properties Sample No.
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:
Weight Average Molecular Weight, wM 5,15,457 5,30,413 5,63,356 6,86,488 9,96,848
Number Average Molecular Weight, nM 1,23,226 1,25,226 1,28,177 1,36,409 1,60,650
Polydispersity Index, D 4.17 4.24 4.39 5.03 6.20
**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
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, 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
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
Properties Sample No.
1 2 3 4 5
Mooney Viscosity, ML1+4 at 100C 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, wM 5,15,457 5,30,413 5,63,356 6,86,488 6,96,848
Number Average Molecular Weight, nM 1,23,697 1,25,226 1,28,177 1,36,409 1,60,650
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
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, 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
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
Properties Sample No.
1 2 3 4 5
Mooney Viscosity, ML1+4 at 100C 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, wM 5,15,457 5,30,413 5,63,356 6,86,488 6,96,848
Number Average Molecular Weight, nM 1,23,697 1,25,226 1,28,177 1,36,409 1,60,650
Polydispersity Index, D 4.17 4.24 4.39 5.03 6.20
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
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, 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
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.
Properties Sample No.
1 2 3 4 5
Mooney Viscosity, ML1+4 at 100C 33 41 50 60 85
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
Weight Average Molecular Weight, wM 5,15,457 5,30,413 5,63,356 6,86,488 6,96,848
Number Average Molecular Weight, nM 1,23,697 1,25,226 1,28,177 1,36,409 1,60,650
Polydispersity Index, D 4.17 4.24 4.39 5.03 6.20
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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