Elucidating polymer interactions in the chemical recycling of mixed PET/PA6 via hydrothermal liquefaction

Hydrothermal liquefaction (HTL) in subcritical water is an emerging approach for the chemical recycling of plastics. In this work, the effects of polymer co-processing were investigated by focusing on polyethylene terephthalate (PET) and polyamide 6 (PA6), two major constituents of synthetic textile waste. A design of experiments approach was employed to quantify the influence of temperature (300–340 °C), reaction time (10–30 min), dry matter content (10–20 wt%), and blend composition (25–75% PET) on product yields. Co-processing revealed mutually influencing effects on PET and PA6 depolymerization, highlighting the critical role of polymer interactions. Terephthalic acid (TPA) derived from PET acidified the reaction medium and promoted the depolymerization of PA6; conversely, increasing PA6 content reduced TPA yields, attributed to incomplete PET hydrolysis and interactions between PET-derived species and nitrogen-containing reaction products. Monomer yields were strongly governed by operating conditions, with low dry matter content favoring water-soluble monomers, namely ethylene glycol (EG) and caprolactam (CL), and higher PET contents enhancing both the yield and purity of TPA. Within the modelled design space, optimal conditions allowed an overall monomer yield of 0.71 kgmon/kgfeed, while tuned conditions produced maximum experimental individual yields of 79% for TPA, 57% for EG, and 62% for CL. Finally, selected operating conditions were validated using commercial PET and PA6 textile materials, showing good agreement with results obtained from model polymers. Overall, this study provides quantitative insight into PET–PA6 interactions under HTL conditions and supports the identification of operating windows for mixed textile waste valorization toward monomer recovery.