Chemical recycling of high-density polyethylene (HDPE) wastes by oxidative degradation to dicarboxylic acids and their use as value-added curing agents for acrylate-based materials

Plastic waste has become a serious pollution problem due to the material's high consumption volume and durability. Therefore, effective waste treatment and conversion technology are urgently required. In this work, a chemical recycling process has been developed to convert high-density polyethylene (HDPE) from post-consumer milk bottles to value-added chemicals, especially mixed diacids with high succinic acid content, via hydrothermal nitric acid oxidation. The results reveal that HDPE can be completely recycled into water-soluble compounds with a yield ranging from 15 to 70% (∼8–36% carbon recovery) at 180–200 °C for 2–4 h. The oxidized products examined by 1H NMR spectroscopy are a mixture of succinic acid (C4), glutaric acid (C5), and adipic acid (C6), whose compositions are strongly governed by the reaction temperature, reaction time, filling capacity, nitric acid concentration and the HDPE/nitric acid ratio. With 20 ml of 0.18 g/ml nitric acid solution (1/7.2 HDPE/nitric acid ratio), a maximum product yield (%carbon recovery) of 69% (35%) with C4:C5:C6+ ratio of 55:30:15 (mol%) is achieved at 180 °C for 3 h. When the filling capacity was increased from 40 to 60% by adding extra water at the same HDPE: nitric acid ratio, prolonging the reaction time for 1 h resulted in the products with a high succinic acid composition (>94%) and 75% yield (36% carbon recovery). The benefits of employing dilute nitric acid solutions with the same HDPE: nitric acid ratio on the selectivity of succinic acid is also confirmed by a reaction at 200 °C for 2 h. As oxidation typically occurs at a higher rate at a higher temperature, the maximum product yield (∼53–56%) is received at 200 °C for 2 h. This lower optimum yield obtained at higher reaction temperature and higher nitric acid concentration implies heterogeneous degradation due to limited diffusion of reactive molecules. By adjusting the HDPE: nitric acid ratio (1:3.6 to 1:10.8), the optimum content of nitric acid is in a range of 3.6x–5.4x. The as-received products are successfully developed as UV-curable acrylate precursors by esterifying their carboxylic groups with hydroxyethyl acrylate, using 5 mol% p-toluene sulfonic acid as a catalyst. UV-cured sheets obtained after irradiating under a UV lamp for 10 min have a gel fraction of 80–90% tested in toluene, chloroform, and ethanol.