Novel recycling processes for thermoset polyurethane foams

Polyurethane foams (PUFs) are widely used in many different industrial applications including automotive, construction, packaging, medical devices, thermal insulation, cushions, furniture, and so on. These wide applications of PUFs are related to their excellent mechanical, thermal, electrical properties, easy processibility, and low thermal and electrical conductivity (excellent insulator). Recycling of PUFs as thermosetting materials is not possible due to the cross-link structure and the difficult of remelting and reprocessing of the PUF wastes. With the large production and industrial applications of PUFs, the huge amount of PUFs waste is creating a serious environmental concern. Recycling of PUFs can address this concern properly. Physical recycling of PUFs is very common, and the PUFs will be grinded and used as a filler in thermoplastic polymers. Chemical recycling of PUFs is also possible; based on this process, the PUFs will be converted into their original chemical components (isocyanate and polyols) with the addition of certain catalyst. The approach of malleable thermosets or covalent adaptable networks (CANs) is a relatively new enabled reprocessing and recycling thermoset polymers via incorporation of dynamic covalent bonds in the chemical structure of polyurethane. According to the CANs, the chemically modified PUF wastes can be reconfigured and reshaped into new products through network rearrangement reaction induced by heat. In this review article, the recycling of PUFs mainly based on the covalent adaptable networks will summarized. Many different types of PUFs with CANs will be considered including thiourethane, urea, disulfide bonds, and hydroxyl–carbamate exchange reactions. In addition, a catalyst induced PUFs reprocessing through the dynamic carbamate exchange reactions will be included. Furthermore, recycling PU fine powder as a partial replacement of polyols in the new synthesis of PUFs will be reviewed.