Stability of the vulcanized cross link in butyl rubber: Theory and application

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Abstract
It has been determined analytically that, when a Butyl vulcanizate softens during exposure to high temperature, the disulfide cross links between polymer chains break down to thiol groups. Of all the accelerator types, tellurium derivatives of dithiocarbamic acids impart the greatest inherent resistance to this reversion. Such a mechanism of breakdown, however, suggested the use of oxidizing agents to reform the disulfide cross linkage, and four metallic peroxides or dioxides have been found effective. These are CaO2, PbO2, MnO2, and SrO2. Barium peroxide is too stable to be an effective oxidizing agent while magnesium peroxide expels oxygen too rapidly for an effective retarder. Other types of organic and inorganic oxidizing agents have not proved beneficial. The fact that an oxidizing action is responsible for retardation of reversion has been demonstrated by the fact that the corresponding mono‐oxides of calcium, strontium, lead, and manganese do not perform in the same manner. In rubber technology the stabilization of vulcanized materials by oxidizing agents is novel, and it appears applicable only in the case of Butyl whose low unsaturation offers greater resistance to oxidation of the chain proper. When CaO2 is added to diene type rubbers such as Buna S, Buna N, and natural rubber the common oxidative effects are accelerated. With the Buna rubbers the hardening action is more rapid, while with natural rubber a softening followed by a hardening action is promoted.A practical illustration of this principle of retarding reversion of Butyl vulcanizates by oxidizing agents has been presented. It has been demonstrated in the laboratory that the service life of automotive tire curing bags made from Butyl can be prolonged by the addition of these metallic dioxides or peroxides to the compound.