Solvolysis-based chemical recycling of flexible polyurethane foams

The increasing generation of polyurethane waste and the thermoset nature of flexible polyurethane foams make chemical recycling an important strategy for improving material circularity. This work investigates the acidolysis-based recycling of flexible polyurethane foam waste, focusing on the recovery of reusable polyols and the reduction of residual toluenediamine (TDA), a critical aromatic amine generated during the depolymerization of toluene diisocyanate (TDI)-based polyurethane. Two types of flexible polyurethane foam were investigated: a high-resilience foam without flame retardants and a conventional flexible foam in two formulations, one without flame retardants and one containing chlorine- and phosphorus-based flame retardants. Different recycling strategies based on succinic acid, succinic anhydride and combinations of the two reagents were investigated. Since succinic acid alone generally did not reduce residual TDA below the target value of 1000 ppm, an additional epoxy-based deamination treatment was evaluated. The bifunctional epoxy compound D.E.R.™ 734 showed greater deamination efficiency than the monofunctional D.E.R.™ 721. Model-compound experiments also demonstrated the much higher reactivity of succinic anhydride toward TDA, with approximately 95.9% of the initial TDA consumed within the first 5 min at 150 °C. The optimized succinic acid/succinic anhydride process produced recycled polyols with residual TDA concentrations below the target limit of 1000 ppm without requiring a separate deamination step. The process was effective for polyurethane foams both without and with flame retardants. Alternatively, acidolysis with succinic acid followed by deamination using D.E.R.™ 734 also provided an effective reduction in residual TDA, including for recycled polyols obtained from flame-retardant-containing foams. Fourier-Transform Infrared Spectroscopy (FTIR), Gel Permeation Chromatography (GPC), viscosity, hydroxyl value and acid value measurements confirmed polyurethane depolymerization and showed that the recovered products retained molecular characteristics compatible with polyol reuse. Selected recycled polyols were incorporated into new flexible polyurethane foams at replacement levels of 15 and 30 wt.%. Environmental Scanning Electron Microscopy (ESEM) analysis confirmed that the characteristic open-cell morphology was generally preserved. At 50% compression, the best performance was obtained with the foam containing 15 wt.% non-flame-retardant recycled polyol produced through the succinic acid/deamination route, which showed a markedly lower compression set than the virgin reference. Higher recycled-polyol contents generally increased permanent deformation, while Differential Scanning Calorimetry (DSC) analysis showed no major changes in the principal thermal transitions. Overall, the results demonstrate that acidolysis combined either with epoxy-based deamination or with succinic anhydride is a promising route for recovering reusable polyols from flexible polyurethane foam waste, including flame-retardant-containing materials.