FROM WASTE TO RESOURCES: CHEMICAL RECYCLING OF POLYURETHANES

The growing environmental concerns about global plastic consumption have intensified interest in plastic recycling. Polyurethane, widely used for its versatility, is mostly a thermosetting polymer with a crosslinked structure that limits recyclability. Consequently, end-of-life products are often incinerated or landfilled. Chemical recycling offers a solution by breaking down polyurethane into reusable chemicals, promoting resource recovery and supporting circular economy objectives. The aim of this study was to investigate the application of chemical recycling processes with potential for an industrial scale-up to two complex polyurethane-based waste. The waste (shoe insoles) consisting of a flexible polyurethane foam, polyethylene terephthalate fibres, and a polyurethane adhesive was chemically recycled to produce both rigid and flexible polyurethane foams. In contrast, the polyisocyanurate foam filled with a phospho-halogenated flame retardant was recycled to obtain products suitable to produce new polyurethane and polyisocyanurate rigid foams. One of the main challenges in the chemical recycling of polyurethanes is the occurrence of secondary reactions that lead to the formation of free aromatic diamines. These compounds (i.e., 4,4’-methylenedianiline (MDA)) present carcinogenic potential, they derive from the isocyanate component degradation. To address this issue, several attempts were made to minimise the presence of the respective diamine in recycled polyols below a labelling limit of 1000 ppm. For the flexible foam-based material, several polyurethane glycolysis catalysts (potassium hydroxide and acetate, titanium butoxide and tin octoate) were tested to evaluate their effectiveness in the presence of polyethylene terephthalate, at standard glycolysis conditions (210 °C, dipropylene glycol (DPG) as cleavage agent, 10 mmol per 100 g of waste as catalyst concentration, 4 hours and waste to DPG mass ratio of 1.5/1). The optimal product with appropriate physio-chemical properties was achieved via potassium acetate. MDA was verified reactive toward the polyester part of the waste, however, 18.2 phr of 2-ethylhexyl glycidyl ether (2-EHGE) were still required as deaminating agent to reduce its concentration in the recycled polyol till the desired range. Rigid polyurethane foams were formulated with up to 75 % of this deaminated recycled polyol substitution to the virgin polyol. With this substitution the foam highlighted a stable thermal conductivity, with 22 % lower compressive strength due to reduced apparent density and less uniform cell morphology. The same shoe insole waste was processed to obtain a recycled polyol suitable for new flexible PU foams. Two routes were assessed: acidolysis and glycolysis. Acidolysis under succinic acid and PM 6000 as medium resulted an interesting approach to recover a high-quality polyol by the precipitation of the PU isocyanate part in form of imides, however, it was not suitable to depolymerise both PU and PET. This goal was instead achieved through a glycolysis process performed at high temperatures (240 °C), with potassium acetate and in presence of higher molecular weight glycol (polypropylene glycol 2000). After the deamination treatment with 2-EHGE of the recycled polyol, new flexible polyurethane foams were synthesised with up to 30% of recycled polyol substitution; for this system it was possible to validate a 50% constant-deflection compression set test....