Contribution to the role of aldehydes and peracids in polyolefin oxidation 1. Photolysis and photooxidation of aldehydes in polyethylene

摘要

The fate of a model aldehyde on UV exposure has been examined in PE-HD. It is found that the rate of disappearance of the aldehyde is not significantly affected by the presence of air. Photolysis of the aldehyde is very fast on exposure to the light of a filtered xenon arc. Then, only the subsequent reactions of the radicals formed are influenced by oxygen. This leads to the deduction that, usually, no significant concentrations of aldehyde accumulate under photooxidative conditions. In addition, photolysis according to the Norrish type I reaction leads essentially to the formation of a low molecular mass acyl peroxy radical and a pri- mary peroxy radical. Both these radicals will favor chain termination reactions. As a consequence, aldehydes can hardly be of much importance for chain propagation under photooxidative conditions. This is even more unlikely with stabilized samples. The results explain easily the findings that added aldehydes have no significant effect on the photooxidative stability of polypropylene. From the results it is also deduced that aldehydes cannot be seen as the precursors of long chain carboxylic acids under photooxidative conditions. However, photooxidation of aldehydes can account for the formation of volatile formic acid. Photolysis of the aldehyde is compared to that of two ketones. Photolysis of the ketones is slower by almost an order of magnitude. Furthermore, photolysis of ketones occurs mainly according to the Norrish type II reaction, the Norrish type I reaction being much less important than with aldehydes.