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 (ΔT ₁₀ – 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.