The three stages in degradation of polymers—polyethylene as a model substance

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Abstract
Data relating to the degradative conversion of 14C present in low density polyethylene (LDPE) film to respiratory 14CO2 during a 10‐year aerated cultivation with soil are presented. The degradation was performed with two sets of LDPE samples, one without additive (PE) and the other containing UV sensitizer (NDPE). Samples were exposed to UV irradiation for 0, 7, 26, and 42 days. The degradation is characterized by three stages: (I) a constant degradation rate, (II) a parabolic decline in the rate of degradation, and (III) a subsequent final increase in the rate of degradation. The first step (I) is probably dependent on the environment. The material changes rapidly until some kind of equilibrium with the environment has been achieved. CO2 is evolved, oxygen uptake is rapid, and a rapid change in mechanical properties is also observed. The second step (II) is characterized by low oxygen uptake, a low evolution of CO2 and slow changes in the mechanical properties, crystallinity, and molecular weight. The changes in mechanical properties are not necessarily synchronous with the decrease in molecular weight. Step III, finally, is a rapid deterioration of the structure. The degradation rate increases again, and all the mechanical properties are more or less lost due to the final collapse of the structure. For an inert material such as PE, 10 years is a short time, so that only small indications of step III and a coming mineralization point can be observed. The changes are more evident for NDPE. The use of degradable materials, for example polypropiolactone, however, means that it is possible during a 2‐year period to study all three stages. An understanding of the mechanisms in each step will give a better base for lifetime predictions.