Effect of tetramethylthiuram disulphide on bis(triethoxysilylpropyl)tetrasulphane as coupling agent for silica in polyisoprene

Polyisoprene was compounded with 30 phr unmodified silica and with silica in the presence of modifiers bis(triethoxysilyrpropyl)tetrasulphane (TESPT) and the unsymmetric polysulphide formed on reaction of TESPT with tetramethylthiuram disulphide. The modifiers improved bound rubber formation and the green strength characteristics of compounds compared with compounds with unmodified silica. However, the use of the unsymmetric polysulphide, part of which could react with hydroxy groups on the silica surface and part of which contains an accelerator moiety, gave only a marginal improvement in bound rubber formation and green strength, compared with compounds in which silica is modified with TESPT alone. Heating at 150 degrees C is essential to improve bound rubber Formation and it ensures that essentially all of the bound rubber, formed in the presence of modifiers, is firmly held to the surface, swelling in an ammonia atmosphere giving no further solublisation of rubber compared with normal swelling. Chain end radicals, formed during mixing, play no role in bound rubber formation. PRC/1604.