This review provides relevant background information about the vulcanization process, as well as the chemistry of thiuram- and sulfenamide-accelerated sulfur vulcanization with emphasis on the role of activators, to lay a base for further research. It commences with an introduction of sulfur vulcanization and a summary of the reaction mechanisms as described in literature, followed by the role of activators, particularly ZnO. The various possibilities to reduce ZnO levels in rubber compounding, that have been proposed in literature, are reviewed. A totally different approach to reduce ZnO is described in the paragraphs about the various possible roles of multifunctional additives (MFA) in rubber vulcanization. Another paragraph is dedicated to the role of amines in rubber vulcanization, in order to provide some insight in the underlying chemical mechanisms of MFA systems. Furthermore, an overview of Model Compound Vulcanization (MCV) with respect to different models and activator/accelerator systems is given. In the last part of this review, the various functions of ZnO in rubber are summarized. It clearly reveals that the role of ZnO and zinc compounds is very complex and still deserves further clarification.
The dump temperature and mixing interval between rubber, silica and silane coupling agent for silica-filled natural rubber (NR) tire tread compounds using bis-triethoxysilylpropyl tetrasulfide (TESPT) as silane were optimized. The dump temperature turns out to be the key parameter governing the properties of the silica-filled NR compounds. The increase in
The addition of a coupling agent to silica-rubber compounds enhances the filler-matrix compatibility. Under certain mixing conditions the surface of the filler may be only partly activated, which may have an adverse effect on the properties in the final product. Some coupling agents may also act as sulfur donor. The dump temperature employed during mixing and the length of time the compound is exposed to that temperature govern the reaction mechanisms of the coupling agent and determine whether the agent leads to the formation of a silica-rubber bond or acts as a curing agent. A temperature of at least 130 °C is necessary to ensure that the reaction between the coupling agent and the silica proceeds, whereas the coupling agent starts to react with the rubber or to donate sulfur, resulting in scorching, at temperatures above 160 °C. An increase in the 300% modulus and/or G' at 100% strain above 150 °C is an indication of scorching caused by the sulfur in the coupling agent. No scorching is observed when a coupling agent without sulfur is used. Another important parameter is the mixing time. It was observed that the coupling agent must be mixed with the silica for at least 10 minutes at 150 °C to obtain a sufficient degree of coupling.
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