Auto-oxidation mechanism

Auto-oxidation mechanism

Degradation of polymers by chemical reactions is a typical consecutive property and chemical changes occur due to reaction with components in the environment. The most important of these degrading reagents is oxygen. Oxidation may be induced and accelerated by radiation (photo-oxidation) or by thermal energy (thermal oxidation). Thermal oxidation, like photo-oxidation is caused by auto-oxidation. In photochemical degradation the energy of activation is supplied by sunlight. In the most ordinary chemical reactions, the activation energy changes between 60 to 270 KJ/mol ranges. This is energetically equivalent to radiation of wavelengths between 1900 and 440 nm. Above room temperature, polymers degrade in air after an induction period by thermal aging.

Oxidation phenomenon of polymers was investigated very early in connection with the aging of natural rubber. Hofmann realized the connection between aging and the absorption of oxygen. Based on the fact that hydrocarbon compounds react with molecular oxygen forming oxidation products, the auto-oxidation developed by a free-radical chain reaction. Boland and Gee showed that the oxidation of hydrocarbons proceeds autocatalytically. The reaction is slow at the start, generally associated with shorter induction period. In this period, the polymer does not show any obvious changes and there is no evidence of oxygen absorption.

This period is nevertheless important in the process of polymer oxidation, because small amounts of hydroperoxides are formed which initiate the subsequent rapid auto-oxidation of the polymer. As a rule an increase in temperature reduces the induction period and accelerates the auto-oxidation. In some cases, when the polymer contains trace amounts of peroxide impurities or catalysts such as metallic salts, the induction period is not observed at all and the process of catalytic oxidation begins immediately. The decomposition of the hydroperoxides is commonly recognized as the process for the further rapid oxidation. The free-radical initiated chain reaction of auto-oxidation is depicted schematically in Figure . The radical processes during the thermal or photo-degradation of polypropylenes are identical. The basic oxidation mechanism has generally been believed to consist of the following three steps:
(i) Initiation:
RH > R.

(ii) PropagatioO2n:
R. + O2 > ROO.
(Peroxy radical)

This reaction is very fast and quickly converted into-

ROO. + RH > R. + ROOH


ROOH is very important species (unstable) which is responsible for degradation.

Chain branching:
ROOH > RO. + .OH

Termination:
2ROO. > ROOR + O2