The influence of mixed conventional sulfur/peroxide vulcanization systems on the mechanical and thermal properties of natural rubber/polypropylene blends

The role of mixed conventional sulfur/peroxide vulcanization systems on the mechanical and thermal properties of natural rubber/polypropylene thermoplastic vulcanizate (TPV) was investigated. Various proportions of peroxide in the mixed vulcanization system were used to dynamically vulcanize the natural rubber phase in the blend. The results show that using a mixed vulcanization system results in TPVs with superior properties in all areas studied, to those of TPVs cured by individual sulfur or peroxide systems. However, the peroxide content in the mixed vulcanization system has a strong effect on the variation in properties. A system containing 30 parts of peroxide to 70 parts of sulfur, results in a TPV with the best overall properties due to a balance of the effects of the sulfur and the peroxide curing agents in the blend. This ratio promotes optimum cross-link density, cross-link patterns, and low competing reactions of the peroxide and co-agents. As a result, improvements in the mechanical and thermal properties of the TPV are obtained with 30 parts of peroxide in the mixed system. Any further increase in the peroxide content results in a deterioration of those properties due to inefficient cross-linking with a high degree of competing reactions. Thus, close attention should be paid to the ratio of sulfur to peroxide or more specifically to the content of peroxide when selecting a mixed vulcanization system as a cross-link agent.