Aqueous coordination yields a thermally stable lanthanum phytate flame retardant for PET

Polyethylene terephthalate (PET) has a limiting oxygen index (LOI) of only approximately 21%, and it burns readily with severe dripping and heavy black smoke. Phytic acid, which carries six phosphate groups and up to 28 wt% phosphorus, promotes efficient dehydration and charring. Its poor thermal stability and strong acidity, however, prevent it from surviving the melt processing window of PET (260–290 °C). This constitutes the fundamental obstacle to using phytic acid as a melt-blending flame retardant for PET. To overcome this barrier, we employed aqueous coordination between phytic acid and LaCl 3 ·6H 3 O at 80 °C. The resulting lanthanum phytate complex (PALa) possesses a highly cross-linked coordination architecture that confers sufficient thermal stability for PET processing. Subsequent oxidative self-polymerisation of dopamine coated the PALa surface, yielding a PDA-coated flame retardant (PALa@PDA). The polydopamine shell improves filler-matrix compatibility and mitigates the severe embrittlement that rigid fillers ordinarily cause. The PET/5PALa@PDA composite, prepared by melt blending, retains an elongation at break of 345.9%, effectively preserving the ductility of the PET matrix. Its LOI rises to 27.2%, with immediate self-extinguishing after the first ignition, and total smoke release is substantially reduced. The char residue at 800 °C reaches 19.1%, a 59.2% increase over neat PET. This work demonstrates that constructing a thermally stable coordination architecture can enable phytic acid-based flame retardants to meet the processing requirements of high-temperature polyesters.