Accelerating the nitric acid degradation of high density polyethylene through a radiation oxidation pre-treatment

The chemical inertness of saturated C-C bonds in polyethylene makes it highly resistant to degradation into low molecular weight products. In this work, rapid nitric acid oxidation degradation of pre-treating high-density polyethylene was achieved through controlled radiation oxidation pre-treatment. During pre-treatment, high-density polyethylene films were irradiated by γ-rays in an oxygen environment, leading to the incorporation of carbonyl groups into the polymer's main chains. The number of carbonyl groups was proportional to the absorbed dose within the range of 0-1000 kGy. Elemental analysis revealed that approximately 0.6 oxygen per 100 carbon atoms could be introduced to HDPE for every 100 kGy of γ-rays irradiation in an oxygen atmosphere. During the subsequent nitric acid oxidation process, the pre-oxidized high-density polyethylene degraded significantly faster than that of original high-density polyethylene. Under the optimized conditions of 0.15 g/ml nitric acid at 180°C for 3 h, 100 % conversion was achieved using pre-oxidized high-density polyethylene at 500 kGy as the feedstock, while it was only 55 % for original high-density polyethylene. Additionally, the conversion of pre-oxidized high-density polyethylene was related to the absorbed dose. A possible mechanism suggested that radiation oxidation generated a large number of ketone carbonyl groups on the high-density polyethylene molecular chains, which acted as active sites to initiate nitric acid oxidation degradation, thereby accelerating the entire degradation reaction. This work proposed a new strategy to fulfill 100 % conversion of high-density polyethylene, demonstrating significant potential for application in polyolefin recycling technologies.