Free-Radical Activity of Reinforcing Rubber Fillers

In our studies of the mechanism of rubber reinforcement, we were led to assume that primary valency bonds are likely to form between an “active” filler and the rubber-matrix via free-radical intermediates. The hypothesis was based on the following considerations. (1) Although a strong school of thought still maintains that the strength of bonding depends mainly on van der Waals forces, and therefore on particle size, a number of individual observations suggest the presence of “reactive sites” and formation of strong bonds of a chemical nature between filler and rubber. Foremost among these is the phenomenon that the incorporation of active pigments such as carbon blacks, silicas and the like into rubber results in the insolubilization of a part of the rubber. This part is conveniently called “bound rubber” or “carbon-gel”. (2) Farmer and his school have shown that the vulcanization process is based on a free-radical mechanism, wherein a number of the rubber double-bonds and α-methylene groups, sulfur and accelerator are involved, resulting in the formation of three-dimensional giant networks. The mechanism of mastication is also known to consist of the production of free radicals in the presence of suitable ‘radical acceptors’, such as oxygen. (3) There are some common features in both reinforcement and vulcanization, such as crystallization effects. (4) Little or no reinforcing action of otherwise active pigments is observed with saturated polymers, for example, polyisobutylene.