Abstract
The vulcanization of natural rubber with sulfur, using mercaptobenzothiazole as accelerator in the presence of zinc stearate, was investigated. The results were as follows: 1) When natural rubber is cured with sulfur in the presence of zinc oxide and mercaptobenzothiazole, as well as zinc stearate, one observes, with the first order diminution of sulfur concentration, an induction period that grows longer as the cure temperature falls. 2) For the disappearance of sulfur there is calculated, from the temperature dependence of the rate constants, an activation energy of 19.5 kcal/mole. This value is considerably smaller than that found for the mercaptobenzothiazole-accelerated sulfur cure when no zinc stearate is present. 3) The percentage loss in mercaptobenzothiazole during vulcanization is, in the presence of zinc stearate, independent of the temperature; there is a reaction, independent of the amount of stearate added of 5 molecules of S8 for each molecule of mercaptobenzothiazole. 4) The diminution in accelerator can also be explained as a first order reaction, and it becomes evident that disappearance of accelerator and sulfur are equivalent-rate processes. 5) The crosslinking, measured by the reciprocal equilibrium swelling, goes according to the first order, yet sulfur disappearance and the crosslinking reaction are not equal rate processes, which was always true in the absence of zinc stearate. In each case the crosslinking rate constants are greater than those for the decrease in sulfur. 6) For the activation energy of network formation, we calculated, from the temperature dependence of the rate constants of the reciprocal equilibrium swelling, 20.5 kcal/mole. 7) When the stearate content is constant, there is a linear relationship between kSI, the rate constant for sulfur diminution, and the given amount of mercaptobenzothiazole, up to an initial molar ratio of mercaptobenzothiazole: stearate = 4:1. Increases in accelerator proportion beyond this initial ratio cause no further rise in the rate constants. 8) The rate constants of the crosslinking reaction also increase with increasing proportion of mercaptobenzothiazole (stearate portion remaining constant), until the initial molar ratio of mercaptobenzothiazole:zinc stearate = 4:1 is reached. Nevertheless, the relationship is not linear. 9) There is a linear relationship between the amount of zinc stearate in the vulcanizate and the maximum sulfur-loss rate constants kSI (max). 10) The maximum rate constants of the network forming reaction kQI (max) are proportional to the square root of the stearate content. 11) The number of crosslinkage points formed is independent of the quantity of mercaptobenzothiazole, when the stearate content is constant. 12) The number of crosslinking points formed increases, however, along with increasing stearate content, and is proportional to the square root of the stearate content. The investigations will be continued.
