Mastication and Rate of Set-Up. Part II

Abstract

Abstract In the earlier communication it was shown that the length of time of milling of a mixed rubber exerted a marked effect on the rate at which it commenced to scorch. The longer the time of milling, the slower was found to be its rate of set-up. It is intended in this paper to give a brief account of some experiments which were undertaken with a view to deciding which factor or factors in the process of mastication are responsible for this retardation of the initial stages of vulcanization. During the milling of rubber only a limited number of factors can be considered as having any effect on its subsequent behavior as far as its rate of set-up is concerned. These are, decomposition of the accelerating components due to heat and pressure, softening of the rubber due to mechanical working, disaggregation or other causes, breakdown of the ultimate rubber particles, depolymerization, or, finally progressive oxidation of the rubber throughout the milling and mixing process. The possibility of decomposition of the accelerator during milling was examined by adding the accelerator to one batch at the beginning of the mixing process, adding it to another at the end and comparing the results of the rates of set-up on the two rubbers. Softening of the rubber without resorting to mechanical means was carried out by adding various softeners to the rubber while in solution and dispersing these by gentle stirring. It is of interest to note that the addition of softeners to the mixing without simultaneous mechanical working resulted in only a slight lowering of the plasticity reading (Y value), which seems to indicate that, for the effective development of softening action, mechanical working or the catalytic effects induced during mechanical working are essential.