Green Intrinsic Flame-Retardant Modification of Waterborne Polyurethane with a Phosphorus–Nitrogen–Silicon-Containing Biomass-Based Flame Retardant

Waterborne polyurethane (WPU) is a highly flammable polymer that is unsuitable for use in fire-prone environments. To address this limitation, an innovative biomass-derived intrinsic flame retardant containing phosphorus, nitrogen, and silicon (hereafter referred to as DETS) was synthesized via a one-pot reaction involving 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, protocatechualdehyde, and diethyl phosphite. Because its molecular structure includes four terminal hydroxyl groups, DETS enables covalent cross-linking with WPU during curing to construct an internally cross-linked network. The resulting WPU/DETS composites exhibited outstanding flame retardancy. Notably, the limiting oxygen index increased up to 27.7%, and vertical burning tests demonstrated self-extinguishing behavior. Cone calorimetry tests revealed reductions of 46.6% and 57.1% in peak heat release rate and smoke production rate, respectively. Furthermore, thermogravimetric analyses confirmed enhanced char yield, and TG-IR analyses showed suppressed emission of toxic volatiles. The tensile strength and elongation at break of WPU/DETS-5 were significantly improved relative to pristine WPU. The study provides a promising strategy for developing sustainable and intrinsically flame-retardant WPU materials based on renewable chemistry.