Abstract
There are indications that the chemical reaction involved in bound rubber formation proceeds via a free‐radical mechanism for compounds filled with reinforcing hydrated fine‐particle silica, as well as in the case of carbon black as filler. The high potential of silica as a rubber reinforcer is indicated by its bound rubber formation. This potential was realized by the extent of filler‐polymer interaction developed in silica‐filled vulcanizates cured by peroxide and by high‐energy radiation, respectively. Both these relatively simple curing systems are postulated to effect crosslink formation via a free‐radical mechanism. The value, after cure, of the ratio of the crosslink density of the filled compound to the crosslink density of the respective unfilled compound, η/η~0~, was taken to be a logical measure of the ability to reinforce. In natural rubber compounds filled with silica (30 vol.), the values of η/η~0~ obtained were 1.6–1.9 for peroxide‐cured material, and 3.6 for radiation‐cured material. Comparisons are made with similarly cured HAF black‐filled stocks. Sulfur‐cured natural rubber stocks filled with HAF black (30 vol.) generally exhibit η/η~0~ values in the range of 1.5 to 2.0. Comparative effects of antioxidants are described.