Solvent-Based Separation of PA6 and PET Using Solubility Data from Differential Scanning Calorimetry

To minimize plastic pollution and prevent further extraction of fossil feedstock, the circular economy for polymers offers a promising solution. As an integral part of the circular economy to close material loops, solvent-based recycling enables the separation of mixed plastic waste into pure component streams without altering the polymer structure but requires reliable solubility data of polymers in solvents. We present a standardized differential scanning calorimetry (DSC) protocol to determine decrystallization and precipitation temperatures of polymers in various solvents and apply it to the challenging separation of polyamide 6 (PA6) and polyethylene terephthalate (PET). The DSC method enables quantitative solubility analysis even above solvent boiling points and shows good agreement with a visual polythermal cloud point method while offering higher objectivity and additional phase transition enthalpies. Temperature-dependent solubility data for nine solvents reveal that methanol, ethanol, propylene glycol, dimethyl sulfoxide, and water are selective solvents for PA6 over PET. A proof-of-concept PA6-PET separation using methanol yields recycled polymers with near-virgin properties under mild conditions (109 °C, ∼4 bar), demonstrating the process relevance of the obtained DSC data for solvent selection in low-energy, solvent-based recycling processes.