RECOVERY AND SEPARATION PROCESSES IN THE RECYCLING CHAIN OF POLYAMIDE PLASTIC MATERIALS TO OBTAINECO-SUSTAINABLE RAW MATERIALS

The rise of fast fashion and the growing accumulation of mixed textile and plastic waste are creating serious environmental and economic challenges. In particular, polyamides, commonly used in technical textiles and plastics, are often blended with other polymers, making recycling difficult. Traditional methods such as mechanical processing or depolymerization often fail when dealing with these complex mixtures, due to degradation, contamination, or the need for extensive purification. This PhD research, carried out in collaboration with Radici InNova s.c.a r.l. (Radici Group), focused on developing and optimizing a solvent-based recycling process to selectively recover polyamide from post-industrial and post-consumer textile and plastic waste. The process, known as selective dissolution, involves the use of a specific solvent or solvent mixture to dissolve only the target polymer (polyamide) from a complex material. The other components, such as polyurethane or polyester, remain undissolved and are removed by filtration. The dissolved polyamide is then recovered from the solution by changing the solvent conditions (for example, by adding water as an anti-solvent or by evaporating the solvent), allowing the polymer to re-precipitate in a purified form. Two solvent systems (solvents A and B) were selected for in-depth study based on a literature review and preliminary experimental screening. These solvents were chosen for their ability to dissolve PA while maintaining selectivity over other polymers. Experimental work showed that both solvents were effective, but solvent B allowed better separation, reducing contamination and preserving the possibility to reuse PU. Moreover, by adjusting the composition of the solvent mixture, it was possible to tune the selectivity of the process, either favouring the dissolution of polyamide or the extraction of polyurethane. This tunable selectivity was demonstrated through systematic tests on PA6, PA66, and PU fibers, using controlled solvent B/water ratios and analysing the dissolution temperatures of each sample. The solubility behaviour was mapped in relation to solvent composition, helping to define process windows for each target polymer. Different types of waste were tested. Various methods were tested to recover PA from the solution, including anti-solvent crystallization, direct evaporation, and spray drying. Anti-solvent precipitation produced a purer polyamide, with limited molecular degradation and mechanical properties comparable to those required for textile applications. Evaporation resulted higher polymer degradation and in a more flexible materials, likely due to the presence of residual low-molecular-weight compounds acting as plasticizers. Spray drying offered rapid solvent removal but posed challenges in terms of polymer degradation and system clogging, especially with PA-rich solutions. All samples were extensively characterized to assess their chemical integrity and applicability: intrinsic viscosity, end-group analysis (–NH2 and –COOH), infrared spectroscopy, and differential scanning calorimetry (DSC) were used to determine degradation and purity. In selected cases, the recovered PA was also tested for mechanical performance through tensile strength and impact resistance measurements on injection-moulded specimens, without blending with virgin polymer. The results confirmed that the recycled PA showed good performance. Special attention was given to solvent recovery, which is essential for the environmental and economic sustainability of the process. For the water–solvent A mixture, both simple and multi-stage distillation were tested, showing good efficiency and the possibility of solvent reuse. Both the impurities accumulating in the reboiler and in the distillate were characterized by GC-MS and TOC analyses. Tests were also carried out to evaluate both the direct reuse of the distillation bottom and the complete purification of the glycol through total solvent evaporation. For solvent B, azeotropic distillation using an entrainer was developed to improve separation and reduce losses. Process simulations using AVEVA Pro/II helped design the distillation systems and confirm the experimental results, especially in terms of tray number, reflux ratio, and entrainer recovery. The use of azeotropic distillation proved advantageous over traditional binary separation, both in terms of energy consumption and solvent purity. Finally, a Life Cycle Assessment (LCA) was conducted to evaluate the environmental performance of the process. Two process options were compared: anti-solvent precipitation and direct evaporation. Both showed much lower impacts than virgin PA production, with the evaporation route performing slightly better in most categories. The potential reuse of polyurethane and the efficient solvent recovery helped improve the overall sustainability. Contribution analysis highlighted the influence of steam consumption and solvent losses, underlining the importance of process optimization for full-scale implementation. This research shows that selective dissolution is a promising method for recycling nylon-based textiles and plastics. It allows recovery of high-quality PA and offers a feasible path towards more circular and sustainable production. A patent application has been filed, and scale-up activities are currently underway, including pilot plant development studies and additional research on solvent recovery under the RE-POLY.AI project.