Improved properties of dissimilar rubber-rubber blends using plasma polymer encapsulated curatives : a novel surface modification method to improve Co-vulcanization

Abstract

In industrial applications, different rubber types are often blended to fine-tune or optimize the property portfolio required for successful performance of articles. Considering the complexity of a rubber blend compound, wherein numerous additives are involved, vulcanization or cure mismatch often occurs as an outcome of inhomogeneous distribution of the curatives or imbalanced reactivities of the different rubber phases towards the curatives. The solubility study of various curatives in different rubbers was measured with an easy and effective method based on weight up-take by the migration of curative molecules into slightly pre-crosslinked rubbers. The experimental results correlate well with calculated solubility parameter differences. The study provides valuable data for predicting the distribution of curatives in the different phases of dissimilar rubber blends. A mixing study was carried out by applying the solubility knowledge of curatives in rubbers to improve the properties of unreinforced 50/50 w/w blends of SBR/EPDM (SE) and NBR/EPDM (NE). Different mixing procedures are performed based on the same overall recipe for each rubber blend. Large improvements are achieved for the SE blends, not for the NE blends. The surface modification of both sulfur and CBS through plasma polymerization with acetylene, perfluorohexane, and acrylic acid are described. As a result of the plasma surface treatment, the surface energies of sulfur and CBS are decreased and brought closer to the range of the rubber polymers involved in this study. A better co-vulcanisation is achieved in the SE blends with the plasma polyacetylene micro-encapsulated sulfur and CBS, however, appreciable improvements were obtained in the NE blends with only acrylic acid. Several combinations of various plasma coated sulfur and CBS, in carbon black reinforced dissimilar rubber blends. The improvements achieved in the unreinforced rubber blends are again found by using both plasma coated sulfur and plasma coated CBS in carbon black filled blends. Interestingly, a bigger increase in properties is achieved in NE blends instead of in SE blends. The blooming behavior of plasma polyacetylene and polyperfluorohexane treated sulfur was determined in carbon black reinforced natural rubber and compared to soluble and insoluble sulfur.