Peroxide cure is an important and widely used cure system for rubber. Several properties obtained via peroxide vulcanization are superior and not achievable with sulfur vulcanization, e.g.: aging resistance, no reversion and low compression set. However, other properties such as tensile strength and dynamic properties, are inferior to those of sulfur vulcanizates. The use of co-agents in peroxide cure leads to a certain extent to improvement in mechanical properties such as tensile strength. Nevertheless the properties are still inferior with respect to mechanical/dynamical properties of sulfur-cured articles. If these properties can be improved, the range of applications of peroxide cure in the rubber industry can be significantly broadened.Scorch is a common problem in peroxide cure, especially for injection molding and extrusion applications. Several additives can help to improve scorch safety, however, they always result in a lower peroxide efficiency, thus inferior vulcanizate properties.In the present study a new concept of co-agents for peroxide vulcanization is introduced. This new concept consists of the use of a combination of a bismaleimide type co-agent, like N,N'-m-phenylenedimaleimide (BMI-MP), and a sulfur containing compound, like dipentamethylenethiuram tetrasulfide (DPTT). This combination provides scorch safety and at the same time improves the mechanical properties of the vulcanizates. Within the bismaleimide type co-agents N,N'-p-phenylenedimaleimide (BMI-PP) provides better mechanical properties than BMI-MP. The concentration of coagent and sulfur containing compound have a big influence on the scorch time and on the mechanical properties. Optimal properties are reached with 4 phr of co-agent and 0.7 to 0.96 phr of sulfur containing compound. * Corresponding

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In previous studies a new concept of a peroxide cure system for rubber has been reported where scorch safety and mechanical properties are improved at the same time. This new system consists of the use of a combination of a peroxide, a bismaleimide type co-agent and a sulfur spender for the cure of saturated or low-unsaturated rubber. Several mechanical properties have been studied and were found to be improved by using this curing system, such as tensile strength, moduli, compression set and aging. The present paper deals with the understanding of the reported phenomena. Mechanistic studies have been performed with the aim of comprehending the chemistry that takes place during peroxide vulcanization in the presence of a maleimide co-agent and a sulfur spender. From Nuclear Magnetic Resonance and Infrared analysis the loss of the maleimide unsaturation from the co-agent was observed, due to reaction with the sulfur spender. This reaction already takes place at room temperature, thus previous to peroxide decomposition. Further, Model Compound Vulcanization studies were performed using 3-methylpentane as representative model for the saturated rubber backbone. The reaction products of the peroxide vulcanization of 3-methylpentane in the presence of a maleimide type co-agent and a sulfur donor have been analyzed by means of Liquid Chromatography-Mass Spectrometry. Several reaction products have been identified and a reaction mechanism that explains the scorch delay and the property improvement by the new cure system is proposed.