| Abstract: | In an attempt to eradicate many of the problems associated with outdoor and accelerated testing of polymeric formulations, an apparatus was constructed for the sensitive measurement of oxygen uptake into a polymer during its incipient stages of photooxidation. The photooxidation curves of certain HDPE and LDPE formulations demonstrate a first order asymptotic approach to a limiting value which corresponds to the initial number of reactive centers that are available in the material for oxygen attack. An equation for oxygen uptake as a function of time is derived which incorporates the asymptotic value together with a constant whose value reflects the rate at which the asymptote is approached. These parameters are used to calculate the value of the initial quantum yield for oxygen uptake, a quantity which is indicative of the relative photostability of a given formulation. The kinetics parameters obtained from analyses of certain photooxidative curves confirm that the initial rate of photooxidation is proportional to the concentration of polymer hydroperoxide species which are present in the material as a result of oxidation during processing. The removal of these by thermal treatment of the material in an inert atmosphere results in the appearance of an induction period in the photooxidative profile. The experimental results suggest that this method of determining and interpreting photooxidative curves has certain potential for assessing the relative photostabilities of polymeric formulations. |
