A stream of atomic oxygen, produced by passing oxygen at low pressure through a radio‐frequency coil, was allowed to impinge on films prepared from several dozen different polymers. The flow of oxygen radicals was regulated so that the reaction temperatures were between 40 and 70°C. The rapid reactions which occurred at the polymer film–oxygen radical interface were essentially unaffected by the presence of phenolic antioxidants over a wide range of concentrations but rate of reaction was greatly affected by the structure of the polymer. Bulk properties of the polymers were unchanged because the attack by atomic oxygen is limited to the surface of the polymer. In many instances a simple ablation of the surface was observed, but in some cases, especially polyethylene and polypropylene, a highly oxidized surface layer was created. These oxidized surface layers had remarkably low contact angles with water and should be of great interest in improving adhesion and other surface‐dependent properties of polymers.

SynopsisThermal oxidation and copper-catalyzed thermal oxidation of polypropylene were studied over a range of elevated temperatures. The apparent activation energy for both of these processes, based on tlo induction periods, was 27 kcal./mole. Measurement of the time ( t l 0 ) required for interaction of 1 g. of polymer with 10 cc. of oxygen was found to be a convenient and expedient technique for estimating €he induction period, since this amount of oxygen is sufficient to cause deterioration of physical and dielectric properties of the polymer. In the absence of copper, stabilization of polypropylene comparable to that achieved with polyethylene was observed when antioxidant concentration was increased proportionately to compensate for the larger number of oxidation-susceptible tertiary carbon atoms in polypropylene. However, it was found that thermal antioxidants, even in high concentration, were ineffective protectants for polypropylene when copper was present. Conventional copper chelators and metal deactivators were, a t best, only slightly effective in suppressing oxidation catalyzed by copper and usually were unsatisfactory for a variety of other reasons. Oxamide, however, and particularly its less volatile, high melting, disubstituted derivatives, functioned cooperatively with a wide variety of antioxidant systems and were found to comprise a highly effective and useful family of inhibitors of the copper-catalyzed oxidation of polypropylene.

The effectiveness of polyproylene antioxidant systems in other than a simple oxidative environment has been studied. For example, it has been found that contact of stabilized propylene polymers with a number of materials, particularly with plasticized poly (vinyl chloride) and with copper and copper salts, markedly decreases the resistance of the composition toward oxidative degradation. The use of antioxidants having higher molecular weights results in improved retention of stabilization when polypropylene is used in contact with materials such as plasticized poly (vinyl chloride), presumably because partition of the antioxidant from the polypropylene composition is decreased. Antioxidant effectiveness in polypropylene compositions in contact with copper can be greatly increased by the addition of new and more effective copper inbibitors. The usefulness of copper compleming agents such as hydrazides, triazoles, tetrazoles, and derivatives of malonamide and oxamide are compared in polypropylene compositions containing copper dust and antioxidants.