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Lee, K. O. (1951) Studies on the autoxidation of rubber under the influence of mechanical processing. Masters thesis, Northern Polytechnic.
Reaction with a fraction of 1% oxygen is essential to the softening of rubber during mastication; but the chemical mechanisms proposed have been conjectural. For information concerning the process a detailed investigation has been made of the acetone soluble materials produced by autoxidation under the influence of mechanical processing and heat ageing respectively.
The acetone soluble products recovered in an atmosphere of nitrogen after mastication are leas than hitherto reported as the result of extraction exposed to air in conventional apparatus. The extract has bean divided by chromatographic analysis into three fractions. Infra-red spectra disclosed trisubstituted ethylenic double bonds, alcoholic hydroxyl and carbonyl groups in the acetone extracts from both milled and heat aged raw rubbers, previously deprived of naturally occurring constituents soluble in this solvent. Some of the oxygen absorbed during the milling of rubber passes into these acetone soluble products and simultaneously the chain molecules of rubber hydrocarbon add oxygen (in a typical Instance 0.27%) as determined by micro analysis.
The products of oxygen attack during milling and thermal ageing of rubber appear to be similar. Mechanical kneading increases the effective rate of reaction by exposing fresh rubber surfaces and easing replenishment of oxygen, usually restricted by slow diffusion. The mechanism of the milling process is discussed. Autoxidation and mechanical rupture may be complementary. The evidence does not permit precise discrimination.
Volatile products, undetected in cold mastication of rubber, are produced during mastication at 150-170°C. They have been fractionated. Infra-red spectra show the presence of vinyl, alcoholic hydroxyl and carbonyl groups. Mastication at these high temperatures causes thermal scission of rubber molecules, followed by autoxidation of the exposed end groups.
The minimum rate of breakdown at 120°C, the effect of peptising agents and antioxidants on mastication, and the ready crystallisation of masticated rubber on cooling are considered in the light of possible mechanisms of mastication.
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