Characterization of the aging behavior of polyethylene by photoluminescence spectroscopy

Two commercial polyethylene grades were oven-aged at 95 °C and 115 °C for more than 1000 h. Aging characterization was performed by laser-induced photoluminescence spectroscopy (λEx = 375 nm), laser confocal microscopy, infrared (IR) spectroscopy, high performance liquid chromatography, differential scanning calorimetry and tensile testing. Photoluminescence increased significantly upon oven-aging, especially at 115 °C. The maximum of emission was around 450 nm. Decreasing oxidation onset temperatures (DSC) and stabilizer concentrations (HPLC) indicate oxidation-induced changes in the materials that are in agreement with increasing photoluminescence. IR spectroscopy and tensile testing results showed no global oxidative degradation and no premature failure. Hence, the materials were still in the induction period of oxidative degradation. The evolution of photoluminescence is presumably related to the formation of oxygenated groups like α,β-unsaturated carbonyl groups. The investigations clearly indicated that photoluminescence spectroscopy is a sensitive method to monitor aging-induced changes within the induction period.