Organic Curing Agents for Polysulfide Sealants. III. A Mechanistic Interpretation of the Thermal Degradation of Nitrile Oxide-Cured Polysulfide Sealants

Sealants obtained by cure of polysulfide liquid polymers with aryl bis ( nitrile oxides) possess the thiohydroximic acid ester structural feature. Unexpectedly these materials were found to exhibit poor thermal stability: when heated at 60° they soften within days and liquefy in 3 weeks. Products obtained with excess nitrile oxide degraded faster than those produced with equimolar amounts of reagents. Interpretations considered included (A) dissociation of the thiohydroximic acid group, (B) reaction between nitrile oxide and a reactive centre in the polysulfide structure, and (c) addition of nitrile oxide to the side chain. It was shown that neither of the first two possibilities was applicable. Moreover, spectroscopic studies demonstrated that, after an initial rapid addition between nitrile oxide and thiol , a second slower reaction occurred which consumed additional nitrile oxide. Although aldehyde oximes and a- chloro oximes do not participate in such processes unless catalysts are present, thiohydroximic acid derivatives were shown to react with nitrile oxides at ambient temperature to form 1,2,4-oxadiazole 4-oxides and alkyl thiol . For the case of a polysulfide sealant the equivalent rupture of a C-S bond to form the thiol involves cleavage of the polymer backbone. Continuation of the process would lead to degradation of the sealant. These observations were supported by thermal analysis studies on the polysulfide sealants and model polymers.