Characterisation of flame retarded recycled PET foams produced by batch foaming

CO2-assisted batch foaming was used to manufacture low-density (ρ = 200–350 kg/m3) microcellular foams from bottle-grade recycled poly(ethylene terephthalate) (rPET) in flame retarded form. The foamability of the PET regrind was enhanced using a reactive chain extender while the flame retardant properties of the rPET foams were improved by incorporating 6% aluminium-tris-(diethylphosphinate) flame retardant (FR). The effects of adding 1% natural montmorillonite (MMT) and polytetrafluoroethylene (PTFE) powder as potential cell nucleating agent and flame retardant synergist were also investigated. As a result of FR addition, the foam structure became less uniform, but the presence of MMT was found to improve uniformity of cellular size and distribution. The applied FR mainly acts in the gas phase as flame inhibitor, but also promotes charring of the foams, as revealed by thermogravimetric analysis (TGA) and pyrolysis combustion flow calorimetry (PCFC). The fine microcellular structure and high cell density allow homogeneous distribution of FR additives, and thus only moderate increase in flammability was observed for the high-porosity (>75%) foams compared to the corresponding bulk materials, as characterized by similar limited oxygen index (LOI) values. In cone calorimeter tests, for the flame retarded foams a 50% reduction in peak heat release rate (PHRR) and a 30% reduction in total heat release (THR) values were measured compared to the FR-free reference. The PTFE addition to the FR formulation was found to increase the time-to-ignition (TTI), reduce PHRR and effective heat of combustion (EHC) while increase charring. The mechanical performance of the flame retarded rPET foams was found to be primarily determined by the apparent density and less affected by the presence of FR additives. Due to strain-induced crystallization occurring during cell growth, the rPET foams are highly crystalline (χ > 25%) which leads to increased thermomechanical resistance compared to unfoamed references.