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This study evaluates flame retardant formulations in recycled poly(ethylene terephthalate) (rPET) foams, focusing on processability, performance, and structure-property relationships. Microcellular foams were produced from post-industrial rPET using a CO₂-assisted batch foaming process, incorporating aluminium-tris-(diethylphosphinate) (AlPi), melamine polyphosphate (MPP), and their combination at 5 wt% loading. Morphological, thermal, flammability, mechanical, and optical properties were comprehensively assessed. The CO₂-assisted process enabled the preparation of highly porous foams (void fraction >70 %) with fine cellular structures (average cell size ∼ 2 µm). AlPi showed superior flame retardant performance, achieving a limiting oxygen index (LOI) of 25.0 %, delayed ignition, and a 23 % reduction in peak heat release rate during cone calorimetry. Enhanced char formation containing thermally stable aluminium phosphates was confirmed via energy dispersive spectroscopy (EDS) and attenuated total reflectance infrared spectroscopy (ATR-IR) analyses. In contrast, MPP alone provided limited flame retardancy, and its combination with AlPi offered no synergistic benefit. Mechanical performance was primarily determined by foam density, with negligible influence of flame retardant additives. The foams exhibited high crystallinity (χ > 31 %) due to strain-induced crystallisation during cell growth, contributing to enhanced thermomechanical stability. Additionally, the microcellular structure enabled excellent optical reflectivity (>90 %) in the visible spectrum, which was retained even in flame-retarded formulations.
Decsov et al. (Fri,) studied this question.