On the Mechanism of Physical Effects of Reinforcing Fillers in the Vulcanization Kinetics of Styrene-Butadiene-Rubber

It is demonstrated that silica and carbon black have inhibiting effect by the former and accelerating effect by the latter in the kinetics of sulfur vulcanization of rubber. It seems that in sulfur vulcanization reaction of rubber some kinetic phenomena are not systematically investigated. In this regard, due to the autocatalytic mechanism of vulcanization and the diffusional effect of its chains, it seems that immobilization of rubber chains as a result of the presence of reinforcing fillers has an essential role in changing the kinetics of sulfur vulcanization of rubber. This concept has not been explored in other researches. Kinetics measurements were performed by means of an oscillating disc rheometer. The extent of filler/filler interactions was monitored by means of dynamic-mechanical and electrical conductivity tests for silica and carbon black filled compounds, respectively. It was shown that the autocatalytic nature of the vulcanization remains unchanged regardless of the type and concentration of fillers. It was demonstrated that the vulcanization rate goes through a maximum as the loading of fillers rises, regardless of the type and surface chemistry of the fillers. Consequently, silica can also accelerate the vulcanization rate at low loading and decelerate it above a critical loading. Such critical loading exists for both silica and carbon-black, and it is related to the percolation threshold for filler network formation. Therefore, it is discussed that not only the filler surface chemistry, but also the physical phenomena originating from the filler/filler interactions can alter the vulcanization kinetics of rubbers. Such physical effect is attributed to the immobilization and lack of kinetic energy in the entrapped rubber chains which reduce the probability of reaction between the macro-radicals. Therefore, a single mechanism is introduced here to explain the effect of reinforcing fillers on the vulcanization kinetics of the filled rubber.