The Nature of Optimum Vulcanization in Natural Rubber

The vulcanization of natural rubber is accompanied by a considerable change in mechanical properties as the vulcanizate is formed. With sulfur as the vulcanizing agent an optimum is observed in the change in tensile strength and other physical properties. In many studies the presence of an optimum vulcanization has been considered as the result of the superimposition of two processes: (1) the structure-forming action of sulfur and (2) the degrading action of the oxygen, which causes a reversion of vulcanizates. We have established2 that the oxidative degradation of natural rubber in the vulcanization process proceeds extremely slowly and cannot have any substantial effect on the tensile strength of vulcanizates. To resolve the question as to the true causes of the reversion of vulcanizates in “overvulcanization”, several series of experiments were set up. In the first series the mobility of sulfur bonds was studied in natural rubber vulcanizates containing various accelerators by the use of the sulfur radio-isotope S35 with a method described earlier. The results of these experiments shown in Figure 1 indicate the “concentration” of mobile links in the vulcanizate to change proportionately with the change in tensile strength and the maximum density of mobile polysulfide links thus corresponds to optimum vulcanization according to tensile strength. The modulus likewise changes during the course of vulcanization, proportionately to the density of the intermolecular crosslinks formed. The formation of a complex three-dimensional network proceeds continually through all the various stages of the process up to the optimum. The increase in network density in the vulcanizates being formed, up to the optimum, is accompanied by a steady substantial rise in tensile strength. Further vulcanization, however, causes a decrease in network density and a drop in tensile strength. In a second series of experiments with vulcanizates at various stages of the process the polysulfide bonds were removed by extraction with sodium sulfite. When this occurred a decrease in the tensile strength of the vulcanizates was observed, which was especially marked at optimum vulcanization (Figure 2, Curve 2). In a third series of experiments, vulcanization was achieved with tetramethylthiuram disulfide (TMTD) without added sulfur. It is known that TMTD forms monosulfide bonds in rubber which are stable under the usual vulcanization conditions (a temperature of 143° C). Figure 2 (Curve 3) shows that no reversion is observed in vulcanization with TMTD.