Recycling of flame retardant polymers: Current technologies and future perspectives

Polymers are indispensable to humans in different applications due to their ease of manufacturing and overall performance. However, after a material lifetime, there is a large amount of polymer-based waste, which greatly contributes to the loss of valuable resources and environmental pollution. Thermoplastics may be readily recycled, but because of their flammability, large amounts of flame retardant (FR) additives are required for many applications. This results in a significant volume of FR polymeric wastes too, particularly halogenated plastics, which are subject to severe recycling regulations. In general, thermoplastics containing FRs are raising concerns, as their effective recycling is strongly influenced by the chemical composition, additive content, and physicochemical characteristics of the waste stream. The recycling of FR thermosets is even more challenging due to their crosslinked and cured nature, which makes them resistant to melting and reprocessing. In many cases, traditional mechanical recycling methods, such as grinding and melting, are not applicable to thermosetting polymers. Current recycling methods do not always consider the recovery of the thermosetting/thermoplastic matrix and the presence of toxic FRs in the polymer network. Sorting and solvent washing treatment are important steps, which are usually performed before recycling the FR polymeric waste to reduce contamination in the following steps. Considering all the technical difficulties during recycling, the high cost of sorting and solvent washing, and the increasing demand for more sustainable procedures, the scientific community is fostering the transformation toward more feasible and energy-efficient recycling strategies. Also, many directives are imposing strict disposal and sorting rules, limiting the use of FR halogen-based compounds, and promoting the commercialization of more recyclable polymers. This review aims to provide a general overview of currently applied approaches for recycling FR thermoplastics and thermosets, and possible approaches for designing the next generation of FR polymer-based materials. The existing recycling strategies for FR polymers are summarized. Developments in the manufacturing of covalent adaptable networks as an outlook towards circularity in polymers are also addressed in this review.