Hydrothermal Processing of Multilayer Plastic Film for Cascaded Valorization of Nonrecyclable Waste

Mixed polymer plastics cannot undergo mechanical recycling, resulting in heterogeneous low-quality recycled products. Multilayered mulch film, used in agriculture, is an example of mixed polymeric product, as it usually contains a gas barrier polymer layer, which is different in composition compared to the outer layers. Thus, multilayered mulch films are considered nonrecyclable and landfilled. Hydrothermal processing (HTP) is an alternative solution for mixed plastic waste management. HTP is used here as a combined separation and treatment of a multilayer film. Low-density polyethylene (LDPE) film with a 20–25 wt % barrier layer of polyamide (PA) was treated with HTP, and analyses were performed for products in all phases (solid, liquid, and gas). After HTP, the inner PA layer had decomposed, and 22 ± 3 wt % of the original film was recovered as caprolactam monomer and similar compounds in the liquid phase. The remaining solid product, 80 ± 2 wt % of the original film, mainly comprises the LDPE fraction. Evaluation of its suitability for subsequent processing with pyrolysis and mechanical recycling was also demonstrated. This work indicates that HTP can valorize landfill-designated mixed polymer plastics and should be considered as a beneficial alternative for a circular plastic economy.