Knowledge Bank

Polyethylene can be heat-oxidized in air when temperatures in excess of $125\ast C$. are employed or photo-oxidized when exposed to ultraviolet radiation during weathering. Infrared spectrophotometric studies show that during these types of oxidation carbonyl groups are formed and the concentrations of the three types of carbon-to-carbon double bond groups present are altered. Grating spectra in the 5.6 to 6.0 micron region reveal that the carbonyl groups formed by heat-oxidation are mainly ketonic although some aldehyde and acid groups are also present. In photo-oxidized samples, the carbonyl groups are rather well distributed among the aldehyde, acid, and ketone forms. For oxidized samples showing moderately intense carbonyl absorptions, there is a small increase in the concentration of RRC = CHR groups for both types of oxidation. Although both heat- and photo-oxidation produce decided decreases in the concentration of $RHC=CH2$ groups, this change is more pronounced in the photo-oxidized samples. Finally, there is a small decrease in the concentration of $RHC=CH2$ groups in the heat-oxidized samples which stands in contrast to a large increase in concentration of $RHC=CH2$ groups produced by photo-oxidation. Limited results obtained in following the heat-oxidation of polyethylene indicate the formation of unstable hydroperoxide groups as the oxidation progresses.