Preparation and performance of waterborne polyurethane coatings based on the intumescent flame retardant of boron, phosphorus, and nitrogen

P-N-B organic flame retardant was synthesized using 4-formylphenylboronic acid, 4-aminophenylthiophenol, 9,10-dihydro-9-oxa-10-phos-phaphenanthrene-10-oxide (DOPO), and then introduced into the hydroxyl-terminated polybutadiene acrylonitrile (HTBN) molecular chain to successfully prepare a macromolecular flame retardant, which was used to prepare flame retardant waterborne polyurethane. The mechanical properties, thermal properties, and flame retardancy of waterborne polyurethane (FR-WPU) were studied using thermogravimetic analysis (TGA), limiting oxygen index (LOI), scanning electron microscope (SEM), cone calorimeter, and universal testing machine, respectively, and the flame retardant mechanism of macromolecular flame retardants was explored. An LOI value of 29.76% and a UL-94 V-0 rating could be realized when the APFBH conjugation is 7.5 wt%, showing a significant improvement of melt dripping behavior and flame retardancy. It indicated that the fire resistance of FR-WPU remarkably improved and displayed both gas and condensed phase mechanism. As the content of APBDH increased, the tensile strength and the elongation at break of FR-WPU increased firstly and then decreased.