Sabyasachi Gaan, EMPA (Swiss Federal Laboratories for Materials Science and Technology), outlined developments in recycling of FR thermoplastics and thermosets. Thermoplastics (PET, PE), due to their linear structure, are relatively easy to recycle via mechanical means, but mixing of different classes of polymers, additives and contamination complicates the recycling process. Depolymerisation seems today feasible for polyester, polyamides, polyurethane, but difficult for other thermoplastics. The only currently feasible solution for thermosets is pyrolysis to hydrocarbon feedstock or fuels. There is very little published literature on recycling of FR polymers. Recycling of polymers containing PIN FRs has been demonstrated, however the recycling process does compromise the long term mechanical performances of the polymer. ATH (aluminium trihydroxide), a common flame retardant, has been shown to catalyse depolymerisation of PLA (poly lactic acid). In recent work, a DOPO derivative was shown to inhibit polyester degradation in a mechanical recycling process (see Goonie 2019, Bascucci 2022 in pinfa Newsletter n°136). The work by Fraunhofer LBF with pinfa also demonstrates compatibility of several PIN FR – polymer combinations with mechanical recycling. A possible route to address the difficulties of recycling thermosets may be vitrimer materials (see pinfa Newsletter n°139) and PIN flame retardancy can be built into these (inclusion of P or N).
In discussion it was noted that PIN FRs can be an obstacle to recycling if they are not durable during the recycling process. For mechanical recycling, this should not be a problem as FRs are designed to resist the extrusion or injection conditions of the polymer in which they are used. For chemical recycling or pyrolysis, this must be addressed case by case.