Comparative Study of Plasma-Thiophene and Acetylene Coated Silica in SBR and EPDM Reinforcement

The surface characteristics of silica were modified by plasma-thiophene and -acetylene film deposition. The plasma-coated fillers were blended with S-SBR and EPDM, and their influence on the final vulcanizate properties was compared with untreated silica and silanized silica. The change in the surface energy of plasma-acetylene (PA) and thiophene- (PTh) coated silica was characterized by immersion tests in liquids of various surface tension, water penetration measurements, Thermo Gravimetric Analysis (TGA) and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). All techniques gave evidence of a polymeric PA- and PThfilm deposition on the surface of silica. The properties of S-SBR and EPDM, filled with untreated, PA- , PTh- and silane-treated silica, were investigated by measurement of the Payne effect, bound rubber content and weight loss related to bound rubber, the reinforcement parameter, relative ranking of cross-link density and mechanical properties. The results show a lower degree of flocculation for PTh-silica filled EPDM due to a better match of the surface energies compared to untreated and PA-treated silica. EPDM filled with plasma-thiophene coated silica shows the lowest  reinforcement parameter value, thus improved dispersion compared to untreated, silane-treated and plasma-acetylene silica. However, PA-silica filled EPDM shows better mechanical properties compared to untreated and plasma-thiophene coated silica. The PTh-silica filled S-SBR shows a higher bound rubber content, which results in better mechanical properties of the S-SBR compound compared to the one with PA-coated silica. The overall results show that the compatibility and interaction of silica with different rubbers can be controlled by tailoring the surface energy of the filler by plasmapolymerization. The different functionalities on the silica surface result in different levels of compatibility and interaction, as well as final vulcanizates properties.