Oxidation and heat aging of carboxyl‐terminated polybutadiene

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Abstract
Liquid carboxyl‐terminated polybutadiene (CTPB) on aging in air increased in viscosity and molecular weight and eventually gelled. Simultaneously, the acid content decreased without formation of anhydride, the changes being a function of the hydrocarbon portion of the molecule. Cured stocks of CTPB underwent changes on heating in air which were largely resinification due to oxidation. However, chain scission or hydrolysis also occurred, resulting in reversion of cure which could be observed in vacuum‐aged material or in the interior of the stocks. Commercial resins were compared, and a difference noted between CTPB prepared by ionic and free‐radical methods. The idea was presented that the greatest depth of oxidation was determined by the plane benetath the surface where the rate of oxidation equalled the rate of arrival of oxygen by diffusion. This idea was devloped mathematically from Fick's diffusion law and an equation derived which related the depth of oxidation to the temperature, the energies of activation for reaction and diffusion, and the rate constants for reaction and diffusion. By using the equation, relative rates of oxidation were determined and the energy of activation for diffusion of oxygen in CTPB was found to be 13.6 kcal./mole.

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