On the oxidation mechanism of rubbers in the presence of inhibitors

Abstract

Abstract Free oxidation of rubbers represents a chain process which, at moderate temperatures, consists of two stages. The first stage, constant oxidation rate, is characterized predominantly by the development of an unbranched chain reaction leading to the accumulation of an intermediate compound of low stability. The second stage, autocatalytic oxidation, represents a degenerated branched process, resulting from the decomposition of intermediate products. Inhibitors of the secondary aromatic amine type lower the oxidation rate in the first stage and sharply increase its duration. The inhibited process activation energy is 21.3 kcal./mole. In the inhibited oxidation, the major part of the combined oxygen is contained in the aldehyde groups. In contrast to free oxidation, in which crosslinking of the chains is the dominant process, in inhibited oxidation, the process of destruction of polymer molecules is predominant at first. The inhibited rubber oxidation rate depends on the concentration of the inhibitor. This dependence is described by a curve with a minimum. During the process of the oxidation of rubber, some of the inhibitor combines with the molecules of the polymer, and the rest is partially subjected to the action of molecular oxygen. There is an increase in the rate of rubber oxidation, at inhibitor concentrations above a given level, which is attributable to the initiating action of intermediate radical products of inhibitor oxidation. The inhibitors of the secondary amine type do not interact with stable rubber peroxides and with hydrocarbon radicals. It was shown, that the NH bond in the amine molecule is above all responsible for the inhibition effect. It was found that the inhibiting power of amines is a direct function of the mobility of the hydrogen of the NH group. The increase in mobility of this atom is attained by: ( 1 ) an increase of the conjugation effect in the inhibitor radical, which results from an increase in the number of aromatic nuclei in the amine molecule; and ( 2 ) introduction of electron donor substituents in the position para to the secondary amine in the benzene nucleus. The experimental data obtained and also the contemporary theory of the mechanism of oxidation of low molecular weight hydrocarbons made it possible to suggest a chemical scheme of the rubber oxidation mechanism in the presence of inhibitors of the secondary aromatic amine type.