Overcoming incompatibility in elastomer blends by rubber additives with tailored surface properties

Rubber is a challenging composite material, whose functionality strongly depends on the affinity of the different materials in the composite, and its morphology. One way to tailor polarity and chemistry of the filler surface is plasma coating. When using acetylene, thiophene, or pyrrole as monomers, the coating results in a reduced polarity of the filler compared to untreated silica, and unsaturated C-C bonds are formed on the surface. This improves the compatibility of the filler-polymer blends. In a SBR/EPDM blend, the filler-polymer compatibility is improved for all plasma-coated fillers compared to untreated silica. The best dispersion is achieved by plasma-pyrrole coating, as measured by the Payne effect and reinforcement parameter. The rubber-filler interaction is also highest for this blend, as measured by the bound rubber content. As expected, this results in improved tensile properties. In NBR/EPDM, the filler-filler interaction is significantly reduced by the plasma-pyrrole coating, which indicates a balanced compatibility of the pyrrole-treated silica in both polymers. The properties of the vulcanizate show the combinatorial effect of dispersion, filler-polymer interaction as well as polymer entanglements and crosslink density. All plasma-treated silica-filled NBR/EPDM materials show a considerable increase in tensile strength compared to untreated silica, with polyacetylene-treated silica resulting in the best properties. When plasma-coated curatives are used in SBR/EPDM blends, the scorch safety of the compounds is increased and the rupture energy is enhanced. In NBR/EPDM blends, all packages of the modified curatives provide an ­increased maximum torque compared to the control. Mechanical properties of the NBR/EPDM blend are improved even to a higher extent than in the case of SBR/EPDM. This indicates a more balanced distribution of crosslinks along with more homogeneous carbon black dispersion over the different rubber phases. Generally spoken, a better polarity match between additives and polymers results in improved material properties. The wide variety of monomers for the plasma polymerization allows tailoring the surface properties of the additive