New Silane Coupling Agents for NR/Truck Tire Application

A series of organofunctional silanes was evaluated as coupling agents in silica reinforced elastomers. The reactivity of silanes with silica dispersed in natural rubber (NR) containing typical additives and curatives was measured for two silanes, (4- chloromethyl-phenyl)-trimethoxy-silane (1) and [2-(4-chloromethyl-phenyl)-ethyl]- trimethoxy-silane (2). The ratio of moduli at 300% and 100% strains, referred to as reinforcing index, was used to assess the extent of interactions between the silanes and silica. The steric hindrance and electron withdrawing effects of the 4-chloromethylphenyl group inhibited the interactions of silane (1) with silica. Similarly, the reactivity of natural rubber matrix with methylphenylethyl-, chloropropyl-, chloropropenyl- and chloromethylphenylethyl-functional silanes was determined. The reinforcing indices for filled elastomers containing triethoxy-(2-p-tolyl-ethyl)-silane (3) and [2-(4-chloromethylphenyl)- ethyl]-triethoxy-silane (4), increased from 4.09 to 7.36 respectively. The trend tracks the ability of the hydrocarbon group linking the silicon atom and the chloride atom to stabilize carbocations. The effects of these chloro-containing silanes on the other rheological, physical and dynamic properties were compared to traditional carbon black- and silica-reinforced elastomers using polysulfide functional silanes. The elastomer containing silane (4) provided the highest reinforcing index and amount of bound rubber and the lowest abrasion loss and tan δ at 60°C. These properties indicate strong chemical interactions between silane (4) and the silica and rubber matrix. These measured physical and dynamic properties for silica-filled natural rubber elastomers containing silane (4) indicate significant improvements in rolling resistance and wear properties for truck tires.