High molecular weight poly(ethylene sulfide) undergoes severe thermal degradation at the high temperatures (220–260°C) required for processing in injection‐molding equipment. Thermal degradation of the polymer is accompanied by gas evolution and a decrease in melt viscosity. Stabilization of poly(ethylene sulfide) can be effectively accomplished by addition of small concentrations of certain 1,2‐polyamines, preferably together with certain zinc salts as coadditives. Use of this stabilizer system inhibits thermal degradation to a remarkable extent, making it possible to mold the polymer at these high temperatures and obtain excellent physical and mechanical properties. Investigation of the thermal degradation process was carried out. The rate at which gases evolved from unstabilized poly(ethylene sulfide) resins of various molecular weights and preparative histories and from model compounds of the same organic backbone structure was measured at temperatures ranging from 220 to 260°C. Rate of gas evolution from the resins, irrespective of chain length or preparation, was found to be constant at 230°C. The evolved gases, analyzed by infrared spectroscopy and gas chromatography, contained ethylene. Nearly identical apparent activation energies were found for the gas evolution reaction from the resin and model compounds. The ΔE* values were in good agreement with ΔE* determined by other techniques, 58 ± 2 kcal/mole. This is about the energy requirement expected for the homolytic cleavage of a carbon–sulfur bond of the type present in a poly(ethylene sulfide) structure. The rate and analytical data indicate that the degradative mechanism at processing (molding) temperatures is primarily due to the organic structure of the polymer. A mechanism of thermal stabilization is proposed in which the polyamine and zinc salt, in presence of molten polymer at processing temperatures, form a two‐centered electron transfer complex, capable of reacting with both radicals of the homolytically cleaved bond, “healing” the scission, so to speak.

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.