Crosslinking of Rubbers by Fillers

Oxygen containing chemical groups on the carbon black surface can react with the carboxyl groups of XNBR, or epoxy groups of ENR, or chlorosulfonated groups of CSM, during high temperature molding of the rubber-filler mixtures. This leads to crosslinking of the rubber phase. The extent of crosslinking increases if the carbon black surface is oxidized, the concentration of the reactive groups of the rubber increases, or a suitable silane coupling agent is incorporated in the rubber-filler mixtures. Similarly, high temperature molding of the XNBR-precipitated silica, ENR-precipitated silica, CSM-precipitated silica, and CR-ferrite mixtures leads to crosslinking of the rubber phases, even in the absence of conventional rubber vulcanizing agents. XNBR-ZnO mixture on high temperature molding also produces crosslinked rubbers consisting of ionic crosslinks. During mixing stage, the reactive fillers interact with the polar rubbers leading to formation of high bound rubber, presumably through hydrogen bond formation. During high temperature molding of the rubber-filler mixtures for a prolonged time, the reactive groups on the filler surface chemically react with the functional groups of the rubber chains leading to crosslinks of the rubber phase. This is evident from the rise in the rheometric torque of the rubber-filler mixture and marked changes in properties of the mixture on high temperature molding. The properties in many instances are similar to that obtained in the case of rubbers crosslinked by conventional vulcanizing agents. Infrared spectroscopy has been used to identify the chemical structures at the filler-rubber interface formed during crosslinking of the rubber by the filler.