Flame-retardant and solvent-free polyurethane adhesive via synergistic phosphorus-containing polyol and ammonium dihydrogen phosphate

To meet the demands for fire safety and environmental sustainability, developing solvent-free, flame-retardant polyurethane adhesives with high bonding strength is crucial. In this work, a novel intrinsically flame-retardant phosphorus-containing polyester polyol (ADHE) is synthesized and combined with the flame retardant aluminum diethylphosphinate (ADP) to enhance the flame-retardant properties of the bio-based polyurethane adhesive APUx (where x denotes the mass fraction of ADP). The results show excellent synergistic flame-retardant effects. When the mass fraction of ADP is 10 %, APU10 exhibits good anti-dripping and self-extinguishing performance. It achieves a vertical combustion test (UL-94) V-0 rating, with a limiting oxygen index (LOI) increasing from 20 % for the pure sample to 29 %, and a 50.4 % reduction in total heat release (THR). Hydrogen bonding in the matrix improves the interface bond to the substrate, resulting in a shear strength of 21.7 MPa on the aluminum substrate. This study proposes an integrated strategy for developing flame-retardant polyurethane adhesives with high adhesion performance.