A study of some accelerator combinations in the vulcanization of rubber
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Date
1975-10
Authors
K. C., Rasaratnam
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
Continuous research and development towards
the improvement of sulphur vulcanization since its
discovery in 1839, has resulted in useful modifications
such as the use of single accelerator and activators
in the conventional sulphur s.ystems, the E.V,.
(efficient vulcanization) systems, and the applications
of accelerator combinationso The use of combinations
of organic accelerators started as early as 1930s.
Certain organic accelerators when used in combinations
were found to give boosted effects or synergistic
effects in the rate of cure and physical properties
of the vulcanized rubber~ The mode of their action
and mechanisms have not been satisfactorily explained
hitherto; and thus there is no theoretical basis
available to predict the favourable ratio (of the
components of any accelerator combination), which
could give the maximum synergistic effects.
Early opinion of this subject was that the
comb~nations of accelerators gnve increased activity
(or boosting effect) due to the formation of a salt
like molecular complex compound by the two accelerators,
and this complex was more soluble in. rubber thus showing
the increased 98-100 L'lC ti vi ty. · · A:r.10ther exp-.lanation
o:t: th:i.:_s_ synergistic actio:n, is based on a free radical mech2
i b D dk . 101
n s~ Y ogq_ 1n ~ He has stated that the accelerated
sulphur vulcanization reactions proceed through a
free radical mechanism and these free radicals are
formed generally by thermal dissociation. In the
case of accelerator combinations the free radicals
are formed by chemical action between accelerators
(refer section 2.16) which probably occurs with less
energy than that by thermal dissociation and thus
provides a larger concentration of free radicals
leading to increased activityo
A free radical mechanism does not prove to be
acceptable as reasoned and elaborated in section
4o2.1. A polar mechanism appears to be more feasible
in explaining the results and observations of this
study. The rates of cure of systems containing fixed
amount of natural rubber, sulphur, zinc oxide and
stearic acid and variable molar ratios (but the same
total weight) of four selected accelerator combinations
were measured at five different temperatures using
a Monsanto rheometer 100. The results of this study, showing maximun
synergistic effects in the rate of cure, activation
energy, scorch time, and the height of the rheoaeter
curve, around a critical .(weight) .ratio of the component
accelerators in each of the combinations studied,
point to a critical molar ratio giving the highest aotivity
thus throwing some light o~ the nature of the probable
structures of the accelerator-zinc ion-complexes,
involved in these systems, as explained in section
4.2.2. This idea,opens a path :for one to select,
on a rational basis the favourable weight ratios
o:f the component accelerators, to obtain the maximu,-n
benefits, from an accelerator combination exhibiting
synergistic effects, for sulphur vulcanization of
rubber.
It is envisaged that these results and
observations may arouse Gore active interest in
deeper research in the future, on this important
subject in rubber technology.
Description
Keywords
Sulphur vulcanization