Inhibited Oxidation of Elastomers

Abstract

Abstract 1. The oxidation of elastomers in the presence of true inhibitors, of which secondary aromatic amines are an important class, is for practical purposes an initiation reaction and primary oxidation of the chains is possible only to a slight degree. 2. Inhibited oxidation (initiation) is fundamentally distinct from autocatalytic oxidation. 3. The method of study of inhibited oxidation (initiation) consists in the study of the consumption of inhibitor, while the rate of this process is equal to either the proportional rate of initiation or the constancy of the concentration of dissolved oxygen in the entire mass of the rubber film or specimen studied. 4. In the inhibited oxidation of different types of rubber, the rate of initiation is practically constant for each temperature. The activation energy of the reaction is equal to 22.6 kcal. per mole, and is independent of the nature of the polymer in the absence of electrically negative substituents in the polymer. 5. In the case of strongly activated nitration, the diffusion of oxygen in the rubber may have a retarding influence. Then the concentration of dissolved molecular oxygen [O2] differs at different depths below the surface. The relation of the rate of consumption of inhibitor wi to the concentration of oxygen is a straight-line function, i.e., wi=A[O2]n; at 120° C, n=0.5. 6. The reactivity of rubber hydrocarbons with respect to molecular oxygen and certain other chemically active agents is determined principally by the content of double bonds in the main chains of the molecules of the polymers. Marked structural differences, such as degree of polymerization, spatial configuration, presence of aromatic rings, etc., have practically no influence on the rate of inhibited oxidation. 7. The absorption of oxygen by the double bonds of the side chains of butadiene rubbers has a much higher activation barrier (27–28 kcal. per mole); consequently, the double bonds are practically inactive during the inhibited oxidation of elastomers. 8. The oxidation of polyisobutylene in the presence of phenyl-2-naphthylamine is characterized by the presence of a temperature threshold, below which initiation does not take place. The directly related change of molecular weight and consumption of inhibitor indicate that destruction of the polymer continues until a constant molecular weight is reached. 9. The inhibited oxidation of Butyl rubber is characterized by the simultaneous progress of two rates of oxidation. The first is the rate of oxidation of the chains, which do not reach a constant weight; the second is the rate of absorption of oxygen at the double bonds. In both polyisobutylene and Butyl rubber, the ordinary bonds of the long chains are considerably weakened. Since the concentration of ordinary bonds in Butyl rubber is much higher than the concentration of double bonds, the rate of oxidation of the former is greater than the rate of oxidation of the latter.