Influence of Vulcanization on the Oxidizability of Rubber. II. Vulcanizing Agents Other Than Sulfur
Authors: Le Bras Jean 
Published: February 2011

If it is attempted to sum up the important results obtained in the present study, the following points seem to be well established. First of all, with respect to selenium. Whether the oxidizabilities of products vulcanized by selenium alone or of products vulcanized by a mixture of selenium and sulfur are measured, they show the same general behavior as do mixtures vulcanized by sulfur alone. It is the percentage of vulcanizing agent in combined form that governs the oxidizability, for the addition of selenium to sulfur increases this oxidizability. If part of the sulfur is replaced by selenium, there is a decrease in the oxidizability, but this can be explained by the lesser reactivity of the selenium, as a result of which the product is vulcanized to a less extent. It is not true, then, that, for a given state of cure, selenium improves aging. The case of diazoaminobenzene is a more complicated one. This compound decomposes during vulcanization, with liberation of nitrogen and probable formation of antioxygenic substances such as diphenylamine. Diazoaminobenzene itself is a proöxygenic substance, and mixtures containing it which have been heated only a short time have a strong tendency to oxidize. With longer heating, complete decomposition of the diazoaminobenzene inevitably takes place, and the oxidizability becomes established at a relatively small value. In the same way, with chloranil and with dichloroiminoquinone, the reaction leads to the formation of antioxygenic substances, and when vulcanization is carried to a sufficient point, the oxidizability decreases. It should be mentioned that such a prolongation of the time of heating does not have any harmful effect on the mechanical properties of the vulcanizate. Actually all the agents gave a remarkably broad plateau, and no appreciable differences in mechanical properties were found over a wide range of heating times, e.g., 30 minutes to 8 hours. As for the joint use of an amine and an oxidizing agent, such as N, N′-di-β-naphthyl-p-phenylenediamine and lead peroxide, it will be found that, here too, with increase in the time of vulcanization the tendency to oxidize decreases to a minimum, probably as a result of progressive decomposition of the peroxide. if it is attempted to counteract this proöxygenic action directly by the addition of a large amount of the amine, it will be found that, on the contrary, vulcanization becomes less and less effective and the oxidizability increases. However, by adjusting the proportions of amine and of oxidizing agent judiciously, it is possible to obtain products the resistance of which to oxidation is remarkably good. In a special experiment, a direct comparison was made of samples vulcanized with the agents described in the present work and a simple rubber-sulfur vulcanizate, both unprotected and protected by different antioxygenic substances, including diphenylamine, hydroquinone and N, N′-di-β-naphthyl-p-phenylenediamine. By plotting the results, the curves shown in Figure 13 were obtained. These curves make particularly clear the fact that it is possible, by means of certain vulcanizing agents, e.g., diazoaminobenzene or an amine plus an oxidizing agent, to obtain vulcanizates which have as much resistance to oxidation as that of rubber-sulfur vulcanizates containing an antioxygenic substance. This is not true of nitro compounds, with which even ordinary antioxygenic agents have only a relatively slight protective action. A thorough investigation of these vulcanizing agents is then of importance, not only in helping to explain the mechanism of vulcanization itself but also because of the possibilities of its leading to various practical applications.