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ABSTRACT
Because of environmental concerns, the zinc content in rubber compounds has come under scrutiny; therefore it is necessary to explore possibilities to reduce this zinc content. In this article the application of several zinc complexes as activator for sulfur vulcanization are discussed, in order to find alternatives for the conventionally used ZnO and fatty acid activator system. The effects of different zinc complexes on the cure and physical properties of two widely different rubbers, viz. EPDM and s-SBR, are studied.
It can be concluded that zinc-m-glycerolate is a good substitute for ZnO as activator for sulfur vulcanization, in EPDM as well as in s-SBR rubber, without detrimental effects on the cure and physical properties. Furthermore, the results indicate that, dependent on the intended applications, zinc-2-ethylhexanoate represents a substitute for the commonly used ZnO. Zinc stearate is considerably less active as activator in sulfur vulcanization.
INTRODUCTION
The zinc content in rubber products has corne under increased scrutiny due to environmental concerns. The trend in rubber industry is therefore to reduce or even completely eliminate the zinc content, although to date no viable alternative for ZnO and zinc-containing species for vulcanization purposes has been found. Also zinc-containing processing additives, e.g. zinc soaps of fatty acids, have not been matched in their improvement of processing properties, although recently a promising new zinc-free processing additive has been introduced.1
There is general agreement, that zinc cations from ZnO and/or zinc compounds react with organic accelerators to give active zinc-accelerator complexes, which is one of the main steps in the vulcanization scheme.2 The complexes interact with sulfur, a sulfur donor and other activators to generate the active sulfurating agent. It has been suggested in many different studies that these active complexes of Zn^sup 2+^-ions with accelerators are more reactive than the free accelerator.3, 4 This active sulfurating agent reacts at the allylic sites of the rubber polymer unsaturations to form a rubber bound intermediate, which reacts with another rubber bound intermediate or with another polymer chain to generate a crosslink. The exact activator role of ZnO is highly dependent on the type of accelerator present in the initial vulcanization system.
The acceleration system often needs some proportion of fatty acid for cure activation. In general, fatty acids as...