A phosphorus- and nitrogen-containing aromatic Schiff base derivative in waterborne polyurethane backbone for excellent flame retardant, UV-shielding and mechanical properties

Waterborne polyurethane (WPU) is widely used in the fields of coatings, adhesives and treatment agents. However, the WPU is highly flammable and has a high demand for UV-shielding property in practical application. To date, it remains a challenge to effectively improve the flame retardant and UV-shielding properties without compromising the inherent mechanical property. Here, we report a novel flame retardant WPU (FRWPU) with 9, 10-dihydro-9-oxo-10-phosphophenanthrene-10 oxide (DOPO)-based aromatic Schiff base derivative containing phenolic hydroxyl (DOPO-HAMB) in the backbone. Compared to the pristine WPU, the FRWPU reaches the limiting oxygen index (LOI) value of 27.9 %, and achieves a V-0 rating in the UL-94 test with only 3 % of DOPO-HAMB. Meanwhile, in the cone calorimetry test, the peak heat release rate (pHRR) and total heat release (THR) of FRWPU are reduced by 37.7 and 29.3 %, respectively. In addition, the FRWPU can suppress melt dripping and smoke release during combustion effectively. These enhancements are attributed to the cooperation between DOPO and aromatic Schiff base derivatives with both gas-phase and condensed-phase flame retardant mechanisms. More interestingly, the FRWPU exhibits excellent UV shielding property while maintaining the inherent mechanical property of polyurethane, which provides a useful reference for the design and fabrication of flame retardant waterborne polyurethane.