Poly(ethylene sulfide). III. Oxidative degradation

Abstract

Injection‐molded poly(ethylene sulfide) containing polyamines and zinc salts as thermal stabilizers has excellent physical properties at room temperature, but discolors rapidly on air aging at 150°C, with embrittlement and general deterioration of properties. Degradative breakdown under these conditions is preponderantly thermo‐oxidative in nature. The site of the polymer's thermo‐oxidative instability was sought in oxidation experiments involving the resin as well as model compounds for the polymer's organic structure. Identification studies were carried out on the oxidation products of 3,6‐dithiaoctane (a model compound). Rate of oxygen consumption by the model compound was measured by a pressure‐bomb technique and by a volumetric method. Oxidation of the model compound was found to proceed indefinitely as long as the supply of oxygen was replenished. Rate of oxidation was nonaccelerating and independent of oxygen pressure within the range 1.7–3 atm O2. The gaseous products of the oxidation of unstabilized (no polyamine) resin were found to be identical to the products from the oxidation of the model compound. A mechanism has been proposed taking into account the products isolated and identified, and the nonaccelerating nature of the oxidation. The poly(ethylene sulfide) structure is able to decompose hydroperoxides via formation of sulfoxides, preventing the acceleration of the oxidation reaction. While effecting this radical deactivation, the polymer chains are cleaved, with a resultant loss in molecular weight and physical properties. Some success has been met in the use of inorganic or metallo‐organic additives as antioxidants. Among these are dithiocarbamate salts and selenocyanates. The effectiveness of these compounds has been demonstrated in the polymer, and a suggested rationale for stabilization is given.