spopsisFurther studies of a new and highly effective method for the surface treatment of low surface energy polymers for adhesive bonding are reported. Mechanisms are suggested for the increase in the cohesive strength in the surface region of polyethylene when it is exposed to activated species of inert gases. The technique is unique because, in contrast with results obtained with other methods, bulk properties of the polymer such as color or tensile strength and elongation are unaffected and surface properties such as wettability and dielectric properties such as surface conductivity are essentially unchanged.

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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.

About 3% by weight of carbon black adequately protects polyethylene against photo‐oxidation and, under accelerated test conditions, slightly inhibits thermal oxidation. As a rule small amounts of organic antioxidants are also added to the polymer for optimum protection. Now many of the common phenolic and amine additives have been found to function much less effectively in polyethylene containing carbon black than in clear polymer. Loss of effectiveness is attributed to adsorption and/or decomposition of the antioxidant by both basic and acidic carbon black.

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.