Upcycling Polyurethane Plastics via Thermochemical Conversion Pathways: A Comparison of Hydrothermal Liquefaction and Pyrolysis Processes

Polyurethane is a plastic used in everyday applications in the form of foams, adhesives, coatings, or sealants and is predominantly landfilled. This study examined the liquid products formed through the thermochemical depolymerization of linear thermoplastic polyurethane pellets via both pyrolysis and hydrothermal liquefaction processes. The reaction temperatures investigated ranged from 250 to 550 °C and were based on thermogravimetric analysis of the feedstock. The pyrolysis of polyurethane at 550 °C produced the highest liquid yields (63.08 ± 2.46 wt %) and also had the greatest energy recovery of 59.25%. Conversely, the highest oil yields for the hydrothermal liquefaction process occurred at 400 °C with 40.09 ± 4.06 wt %. The hydrothermal liquefaction oil produced at 400 °C had the highest carbon content (70.37 ± 4.00 wt %) and the lowest oxygen content (16.45%). The most valuable chemicals detected were aniline and p-aminotoluene, found predominantly in hydrothermal liquefaction oils. The highest aniline concentration (94.25 ± 11.39 mg of aniline/g of sample) was measured for the 400 °C hydrothermal liquefaction oils. Hydrothermal liquefaction of thermoplastic polyurethane at 400 °C produced oil with the highest yield, carbon content, and aniline and p-aminotoluene concentrations. The extraction of aniline and p-aminotoluene for producing diisocyantes in conjunction with using biopolyols to produce a sustainable polyurethane polymer should be investigated to improve the circularity of the plastic industry.