Enhanced Flame-retardant Performance of Undervalued Polyethylene Terephthalate Waste as a Potential use in Foamed Materials

Nowadays, the post-consumer polyethylene terephthalate (PET) waste from the packaging industry is one of the largest plastic waste streams worldwide. While clear PET waste is commonly recycled and is reused for textile and packaging applications (even with food contact), coloured PET waste`s degraded state limits its reusing potential. This highlights the urgent need to upgrade low-value PET waste. This study focuses on enhancing coloured recycled PET (rPET) quality by introducing an epoxide chain extender (CE) from 0 to 1 wt%, to improve rheological behaviour. Simultaneously, upcycling opportunities are explored by incorporating an eco-friendly phosphorous-based flame retardant (FR) from 0 to 10 wt%, to reduce flammability and thus enabling electrical and electronic applications, among others. The impact of each additive, as well as their combination, is evaluated on the chemical structure, thermal, rheological and burning behaviour of undervalued rPET. The optimal CE content is determined at 0.8 wt%, promoting branched and higher molecular weight polymer chains. Regarding FR, 6, 8 and 10 wt% highly enhance the fire resistance. Furthermore, the combination CE/FR enables a synergistic effect, notably improving burning behaviour. Additionally, the foaming potential of the resulting high-value rPET is assessed for the first time through one-step batch foaming using supercritical CO₂ as foaming agent, aiming to develop lightweight materials endowed with superior burning behaviour. The material containing 0.8 wt% CE reaches the lowest density (200 kg/m³) and a closed cellular structure with smaller cell diameters (8 ± 3 μm). Meanwhile, the combination of 0.8 wt% CE and 6 wt% FR gives rise to a foamed material with density of 659 kg/m³ and cell diameter of 7 μm. Thus, this batch procedure in one-step enables the formation of microcellular foams based on coloured rPET (cell size below 10 μm).