Synergistic Condensed-Phase Flame-Retardant Mechanism of Zinc Phosphinate and Zirconium Hydrogen Phosphate in Recycled PET

Abstract

This study investigates the synergistic condensed-phase flame-retardant mechanism of recycled polyethylene terephthalate (rPET) modified with zinc phosphinate (ZnPi) and zirconium hydrogen phosphate (ZrP). Three formulations were prepared: rPET with 4.5wt% ZnPi (ZnPi45), 4.5 wt% ZrP (ZrP45), and a hybrid containing 2.25wt%ZnPi+2.25wt%ZrP (BL225). Rheological analysis confirmed that no premature crosslinking occurs at processing temperature (265°C), indicating that char formation originates from thermally activated degradation chemistry. Thermogravimetric analysis (TGA) under air revealed that ZnPi promotes phosphorus-assisted charring during the primary degradation window (370–470°C), while ZrP enhances residue persistence above 600°C. The hybrid BL225 exhibited an extended degradation width (ΔT₁₀–₉₀ = 182.07°C) and positive synergy indices of approximately 20% at 500°C and 800°C, confirming cooperative interactions. Raman spectroscopy, analyzed using three independent crystallite size calculation methods (Tuinstra–Koenig, Cancado, and Ferrari–Robertson), consistently ranked the structural order as ZnPi45>BL225>ZrP45>rPET. ZnPi45 achieved the largest crystallite size (La = 59.9 nm by Cancado method), while BL225 retained over 85% of this efficiency at half ZnPi loading. These results quantitatively support a cooperative condensed-phase mechanism in which ZnPi generates phosphorus-rich crosslinked networks and ZrP provides a thermally stable inorganic scaffold, collectively forming a dense, oxidation-resistant protective char.