Degradation Characteristics of Decommissioned Wind Turbine Blade Composites in Subcritical and Supercritical Fluids

This study investigates the degradation behavior of decommissioned wind turbine blade composites in organic fluid systems, with particular focus on the subcritical acetic acid route. Supercritical acetone and supercritical n-butanol were used as screening media, while retired-blade glass fiber-reinforced polymer (GFRP) composites and laboratory-prepared glass fiber/epoxy composites were used to compare resin removal and fiber recovery behavior. The screening results showed that supercritical acetone and supercritical n-butanol caused partial matrix degradation but left visible organic residues on recovered fibers, whereas subcritical acetic acid produced cleaner fiber surfaces under lower-pressure conditions. The effects of temperature and reaction time were then analyzed in the subcritical acetic acid system. At 280 °C for 60 min, the epoxy resin degradation rate reached 99.81%, and the recovered glass fibers retained 96.49% of their tensile strength. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that the liquid products mainly contained phenols, esters, and other oxygenated organics, with bisphenol A derivatives as representative components. These products suggest a coupled degradation process involving epoxy network swelling, bond cleavage, fragment release, and secondary acetylation in acetic acid. The boiling-point difference between acetic acid and the main degradation products, together with the product distribution obtained after recovered-acid addition, indicates the potential of acetic acid reuse. These findings support subcritical acetic acid as a promising medium for resin removal and glass-fiber recovery from decommissioned wind turbine blade composites.