An Experimental and Computational Study of Thermomechanical Recycling of Flame-Retardant PET*

Polyethylene terephthalate (PET), well known for its excellent recyclability via thermomechanical recycling, has a wide variety of applications in packaging and textiles due to its outstanding properties. In many PET products, flame retardants (FRs) are blended in as additives by extrusion to extend the time-of-escape from fires. However, PET/FR products often are not designed for recycling and downgrade during thermomechanical recycling. A crucial step toward improving recyclability is to understand the physio-chemical interactions between FRs and PET during melt (re-)processing. To address this issue, a series of rheological, thermal, and chemical experiments were performed on various PET/FR compounds containing a DOPO derivative (DOPO-PEPA, shortened DP) or a phosphonate compound (Aflammit PCO 900, shortened AF). Furthermore, thermomechanical recycling of PET/FR compounds was simulated by multiple extrusion and injection molding cycles. To study the synergistic properties of DP, the chemical degradation mechanism of DP was investigated by molecular dynamics (MD) simulations using the reactive force field ReaxFF, combined with detailed chemical analysis. The results of this study make it possible to improve the recyclability of PET/FR materials by designing new phosphorus FRs.