Investigating Polyethylene Solubility for Solvent-Based Recycling: Experiments and SAFT‑γ Mie Predictions

Solvent-based dissolution/precipitation plastic recycling processes are emerging as a promising alternative to mechanical recycling, enabling the selective dissolution of polymers from contaminated plastic waste streams. Identifying suitable solvents for such processes requires an accurate assessment of polymer solubility under practical conditions; this is currently possible for very few solvent/polymer combinations. Thermodynamic modeling using molecular-scale models such as the SAFT-γ Mie group-contribution (GC) approach can significantly reduce reliance on experiments that are often time-consuming and costly, and may be integrated into process design workflows. However, polymer solubility data remain scarce, and the modeling of polymer + solvent mixtures poses several challenges, including variations in polymer melting properties and polydispersity. In this study, we present an extensive set of solubility data for four polyethylene (PE) samples, differing in molecular weight, density, and melting properties, in ten solvents and at multiple temperatures, obtained using a polythermal method. We assess the performance of SAFT-γ Mie in predicting the solubility of PE in eight solvents. A very good overall agreement is observed for both solid-liquid and solid-liquid-liquid equilibria; these results confirm the transferability of previously published parameters to complex polymer systems. Experimental and modeling results indicate that the highest solubility at a given temperature is achieved using decalin, a bicyclic solvent, and the aromatic solvents toluene, p-xylene, and mesitylene. The solubility of PE in several bioderived solvents, such as limonene, p-cymene, and α-pinene, is found to be comparable to that in conventional aromatic solvents, suggesting their suitability as environmentally benign alternatives. Furthermore, our findings indicate that the polymer melting properties have a more significant influence on PE solubility than polymer molecular weight. In this work we provide critical data and model validation, paving the way for the future integration of the SAFT-γ Mie GC approach in computer-aided molecular and process design for solvent-based dissolution/precipitation plastic recycling.