COMPARISON OF UV DEGRADATION CHEMISTRY IN ACCELERATED XENON) AGING TESTS AND OUTDOOR TESTS .2.

Compared with outdoor weathering, accelerated weathering of polyethylene (using a filtered xenon lamp) leads to a more rapid decrease in elongation at break, higher oxygen uptake and a higher CO/CO2 formation rate, a faster decrease in the concentration of UV stabilizer (2-hydroxy-4-octoxybenzophenone) and faster changes in the IR spectra. The accelerated degradation rate depends on the type of filter used to filter the xenon lamp. With an UV filter (wavelengths > 290 nm) degradation is faster than with a glass filter (wavelengths > 310 nm). A comparison of the data for accelerated (with a glass filter and an UV filter) and outdoor aging at the same degree of oxidation shows unexpected differences, especially in the IR spectra. These differences are explained by assuming a change in the ratio between oxygen uptake by initiation due to charge transfer complexes (CTCs) and oxygen uptake due to normal autooxidation. An increase in the cut-off wavelength of the filter from > 290 nm to > 310 nm causes a higher conversion of oxygen to carbonyl. This is because the absorption of the CTCs above 290 nm decreases with increasing wavelength, which means that when a glass filter is used (wavelength > 310 nm) oxygen uptake via the CTC mechanism becomes less important and normal oxidation becomes more important. The conversion of oxygen to carbonyl during outdoor weathering is even lower than that during accelerated weathering using an UV filter. This is probably because outdoor weathering takes place at a lower temperature than accelerated weathering, which results in a higher stability of the CTCs, so that during outdoor weathering more oxygen is converted via the CTCs and less through normal oxidation.