Development of innovative chemical processes for the recycling of combined synthetic textiles

This research explores the chemical recycling of blended fabrics through a glycolysis process in order to face one of the biggest environmental pollution aspect of these years: the textile waste. Combined textiles waste come from different fields such as fast fashion, automotive, upholstery, furniture and technical clothes and many of them are made of different materials, difficult to be treated together: the final goal is to obtain, with a single process, a liquid product that can be useful in the production of new polyurethane foams. Glycolysis is a well-known technique that exploits a glycol as a medium in which the polymer chain of the plastic material, in this case polyurethane (PU) and polyethylene terephthalate (PET) are broken, usually under the action of a catalyst and the temperature. Once the individual materials (PET and PU) were preliminary tested, the focus was on their combinations, increasing the degree of complexity by changing the process variables involved. The application of a DoE was necessary to optimize the different operating conditions (ratio waste/glycol, PET/PU ratio, reaction temperature and time, source of waste), keeping two variables, known from literature and laboratory tests, unchanged: the type of glycol (DPG) and catalyst (potassium acetate). The resulting polyols from the glycolysis processes were then characterized in terms of viscosity and hydroxyl number (HV). Both of them are important because the product must be compatible with the virgin polyol used to produce new foams: the viscosity must be lower than almost 10000 cPoise and the hydroxyl number must stay between a specific range, depending on the kind of the final foam; for rigid foams, this range is 350-550 mgKOH/g while for PIR this is 200-300 mgKOH/g. Furthermore, the free aromatic amine content in the polyol must be evaluated using high-performance liquid chromatography (HPLC). They are by-products of the PU glycolysis and their value must stay below the regulatory limit of 0.1% wt due to their cancerogenic nature. To solve this problem, the addition of a post-glycolysis deaminating agent was proposed, investigating the action of different compounds. The most effective deaminating agent was an epoxy resin diluent: 2-ethylhexyl glycidyl ether (2-EHGE). The combination of PU and PET waste also help to reduce the content of free aromatic amines, not only by dilution but also for the interaction between the ester groups with amine groups giving amides. Finally, the polyols were tested into many formulations to produce new PU foams with ever-increasing quantity of recycled polyol. The new foams, characterized in terms of morphological, mechanical and thermal properties, shown good results even with a percentage of recycled polyol up to 75% in the formulation. Due to the good results obtained and the upcoming laws (Extended Producer Responsibility, EPR), the industrial scale up of this process is already underway in collaboration with some industrial companies. To complete the scenario of the most common synthetic textile waste, a third component was added: polyamide (PA). Almost completely inert to glycolysis, the maximum percentage of PA that the system can support without altering its physical characteristics was studied. Beyond this limit, a hot filtration step is required to remove the unreacted percentage of PA. The separated product is not discarded, but an additional route for its recycling is proposed: PA acidolysis. This innovative technique would allow the depolymerization of the polyamide chains, reproducing the monomers that can then be reused to form new PA.