Flame-retardant waterborne polyurethane based on the synergistic effect of HGB and DOPO derivatives

The inherent combustibility of waterborne polyurethane (WPU) with a limiting oxygen index (LOI) of only 18.0% has impeded its versatile applications in the automotive industry, furniture coatings, leather, and other domains. Therefore, enhancing the fire safety of WPU is imperative. This work reports the synthesis of novel reactive flame retardants and their subsequent chemical grafting onto WPU to ameliorate its flammability weakness. Using the novel flame-retardant intermediate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and para-hydroxybenzaldehyde as precursors, phosphorus-based flame retardants (DOPOs) were prepared. Hollow glass microspheres (HGB) were modified with a silane coupling agent (KH550) and incorporated into the WPU prepolymerization. Finally, chemical grafting occurred between the hydroxyl groups of DOPOs and the modified HGB to produce dual-component flame retardants involved in the prepolymerization. The addition of 2 wt% synergistic flame retardants to WPU increased its LOI to 26% and eliminated dripping during combustion.