Single-Step Phytate Flame-Retardant Coatings for Cotton, Polyester and Cotton/Polyester Blends

Scalable halogen-free flame-retardant textile finishes remain challenging, particularly regarding laundering durability and industrially viable processing. Here, two phytate flame retardants, poly(vinylammonium) phytate (PVAmPA, partly bio-based) and chitosan phytate (ChiPA, fully bio-based), were applied to cotton (CO), polyester (PET), and a CO/PET blend by a single-step, binder-assisted coating. Both coatings suppressed surface flaming in ISO 15025 on all substrates. Although laundering at 40 °C caused systematically higher wash-off for ChiPA, surface flame suppression was retained for most coated fabrics, with the exception of ChiPA on CO and PVAmPA on PET. Thermogravimetric analysis showed earlier decomposition and increased residue formation for both systems, with the residue at 700 °C increasing from 4.5% to 18.2% for CO_PVAmPA and from 4.5% to 15.2% for CO_ChiPA. In microscale combustion calorimetry, PVAmPA reduced the heat release capacity (HRC) from 251 to 168 J/(g·K) for CO/PET, whereas ChiPA showed its strongest effect on PET, reducing HRC from 413 to 222 J/(g·K). Gas-phase analyses indicated enhanced water release for both coatings and additional NH3 evolution for PVAmPA. Overall, binder-assisted, single-step phytate coatings provide a scalable route to halogen-free flame retardancy, with PVAmPA showing the most robust overall durability and ChiPA offering a fully bio-based alternative with strong substrate-dependent performance.