Vulcanization of Latex. Differences in the Behavior of Fresh, Old, and Purified Latex

Patents granted to Schidrowitz show that when latex is vulcanized and then dried at room temperature, the product has the properties of vulcanized rubber. Films produced in this way show tensile strengths and elasticity which correspond to those of latex films vulcanized in the dry state. It is apparent, however, that when fresh latex is vulcanized under certain definite conditions, the product has fair tensile strength and elasticity only at a relative humidity of zero, but which under ordinary atmospheric conditions is brittle, seems to be overcured, and is practically without tensile strength. This tensile strength, however, is increased by additional dry vulcanization, so there can be no question of overcure. Just as soon as vulcanization has proceeded to a point where brittle films without tensile strength are obtained, the latex, when treated with acid, does not coagulate, but merely flocculates. Nor can such vulcanized fresh latex at this stage be made to coagulate coherently by other means. This form of latex is not sticky. The flocculate referred to can be obtained only by vulcanizing fresh latex in the presence of zinc oxide, and under conditions such that hydrolysis of the nonrubber substances is a minimum. It is, therefore, desirable to have recourse to ultra-accelerators and to be sure that the vulcanization temperature is not too high. By keeping fresh latex alkaline, or by purifying it, it will not flocculate. Latex that has been purified or aged may occasionally, under similar conditions, give a brittle and incoherent coagulum, whereas in other cases a normally coherent but somewhat brittle coagulum results. The nature of the coagulum is governed by the degree of purification and hydrolysis of the nonrubber substances; hence all transition stages between a flocculate and a completely coherent coagulum may occur. By adding serum from fresh latex to purified latex, the behavior of such purified latex changes in the sense that it behaves more like fresh latex. In studying experimentally the difference in behavior of fresh latex and purified latex, the first thing considered was the combination of sulfur. It was found that sulfur first dissolves in the serum, after which it dissolves in the rubber itself. Only then does vulcanization take place. This became evident from the definite acceleration of the combination of sulfur in the latex stage, when before vulcanization, latex was heated with sulfur alone. By this preparatory treatment too, dry vulcanization at room temperature was accelerated, but there was no noticeable effect on dry vulcanization at 80° and 110° C. At 30° C, about 1 per cent of the sulfur dissolved in the rubber particles, in the form of free sulfur. From this it was concluded that it is not possible to remove by mechanical means (as by clarification) excess free sulfur from vulcanized latex. No essential difference could be found between the combined sulfur of fresh latex and that of purified old latex.