Towards sustainable polyamide recycling: exploring the impact of carboxylic acid on polyamide 6 acidolysis

This work investigates the acidolysis recycling of PA6 using several carboxylic acids (adipic, sebacic, succinic, aspartic, terephthalic, glutaric, and gallic) at 200 °C for 4 h under atmospheric pressure (PA/CA = 1), yielding recycled products (RPs) that were subsequently characterized. Adipic and sebacic acids, and to a lesser extent succinic and glutaric acids, were most effective in depolymerizing PA6 into lower-molecular-weight species, with RP properties (acidity, molecular weight distribution, and phase behavior) strongly dependent on the acid structure and correlated with polarity, melting point, and solubility; spectroscopic analysis provided insight into the main reaction products and mechanisms. In a second stage, 10 wt% of selected RPs were incorporated into PA6 and extruded into filaments, while R-adipic was also evaluated at 10–30 wt%. The results show that 10 wt% RP addition allows tuning of mechanical and rheological properties by reducing ductility and modifying stiffness and melt flow depending on RP structure, without affecting the semicrystalline nature and thermal stability of PA6. Increasing R-adipic content led to higher melt flow index, minor mechanical changes, and improved thermal stability while preserving the degradation profile. Overall, the recycled products showed good compatibility with PA6, with no significant deterioration in performance.