No abstract

Divinylbenzene (48%)/ethylvinylbenzene copolymer was converted to a series of pyrolytic derivatives. Enough of the carbon bond network remained intact throughout thermal rearrangement and condensation to retain the original gross shape of the copolymer in the final polymer carbon. Although a carbon residue (6% by weight) was obtained by direct heating of the copolymer, yields were increased eightfold by preoxidation or prechlorination. Such an alteration of thermal degradation is obviously a complex process involving both “inhibition,” in the ordinary sense, and a considerable contribution toward an increased valence network density. As a consequence, the average molecular weight of volatile fragments evolved during carbonization is inversely proportional to the oxygen content of the original hydrocarbon polymer. Pyrolysis of divinylbenzene copolymer, containing 18% oxygen, resulted in a 50% volume shrinkage, 50% weight loss, and a 100% density gain. Unless these data are attributable to extensive microporosity, it is difficult to account for the sorption of up to 3 cc. of helium gas per gram of polymer carbon at 30°C. and 600 mm. pressure; and calculated surface areas as large as 1400 sq.m./g. Abrupt changes in the progress of polymer carbon formation occurred between 600 and 700°C. The residue became rigid; vigorous evolution of volatile products, principally hydrogen, suddenly diminished; and paramagnetic resonance absorption (unpaired electron concentration) dropped about tenfold while d.c. resistivity decreased 106 ohm cm. In this region, also, x‐ray patterns were most diffuse, exhibiting no maxima characteristic of carbon scattering in either hydrocarbons or condensed rings. Finally, the x‐ray patterns of polymer carbon intermediates became less diffuse in samples prepared at 700°C. or above. Yet, scattering indicated the presence of crosslinked graphitic layers of such stability that reordering or true graphitization did not occur at 2400°C.

Oxidation rates of several uninhibited polyolefins were determined in the temperature range 40–140°C. The logerithm of the induction period increased linearly with 1/T, no changes in slope being noted in the melting range with polythylene. At the lower temperatures branched polythylenes were more susceptible to oxidation than the linear types. In the solid statem, oxygen uptake of both modifications were substantially less than that observed for the molten polymer. The amount of oxygen reacting in the solid state was shown to be inversely proportional to the per cent crystallinity, which indicates that reaction with oxygen takes place only in the amorphous region of these semicrystalline polymers. Isotactic polypropylene was only slightly less reactive than the atactic form, and both were more reactive than polyethylene. The effect of temperature on the activity of several moderately effective anotioxidants in branched polythylene was measured. These data also conform to linear Arrhenius‐type plots. Activation energies were specific, however, for each antioxidant and varied considerably; thus, an antioxidant rated as relatively ineffective under accelerated testing conditions could be quite effective within the temperature range over which it would normally be used as a polythylene protectant. Carbon black, which is comparable with some to the less effective thermal antioxidants under accelerated test conditions, provided an unexpected high order of protection for polyethylene below the melting point. This increased protection in the solid state was attributed to the impermeability of the crystalline phase to oxygen and increased concentration of the bulky carbon (antioxidant) particles in the amorphous region.

Thermal oxidation in linear polyethylene is mainly confined to disordered regions in which scission reactions cause crystallization and eventual deterioration of mechanical properties. Gel formation is negligible at 100°C. As degradation proceeds, comparable changes occur in the intrinsic viscosities of melt and solution‐crystallized liner polymers, indicating that chain folds are regularly arranged and are resistant to oxidative scission. Breakdown is much more extensive in branched and crosslinked polymers since crosslinking retards oxidative crystallization and branching increases the volume fraction of substrate ultimately accessible to oxygen.

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.