A study of some accelerator combinations in the vulcanization of rubber

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Date
1975-10
Authors
K. C., Rasaratnam
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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.
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Keywords
Sulphur vulcanization
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