Unveiling the Effects of Thermal Aging on the Oxidative Stability of Biobased Low-Density Polyethylenes

The study evaluates biobased polyethylene (LDPE) structural changes induced by long-term thermal oxidation. The three grades of LDPE differentiated with structural properties were exposed to 90, 100, and 110 °C in an air atmosphere for 20 days. After aging, samples were comprehensively analyzed by Fourier transform infrared (FTIR) spectroscopy, focused on carbonyl group development, and quantified by the carbonyl index (CI). Detailed analysis, based on the deconvolution of the FTIR spectra, identifies each carbonyl compound, whose content varies depending on the structure of the LDPE. The changes in the crystallinity were investigated via X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The obtained results were supplemented by oxidation induction time (OIT), oxygen onset temperature (OOT), and an in-depth analysis of the rheological properties. The tested polymers undergo degradation with behavior similar to UV degradation of petrochemical ones, and the cross-linking process surpasses chain scission for samples degraded at 100 and 110 °C.