Synthesis of P, N and Si-containing waterborne polyurethane with excellent flame retardant, alkali resistance and flexibility via one-step synthetic approach

To endow pristine waterborne polyurethane with excellent flame retardation without reducing the mechanical properties, a flame retardant waterborne polyurethane with phosphorus, nitrogen and silicon units (P-N-Si-WPU) with excellent alkaline resistance was successfully synthesized by one-step synthetic approach, which it is conducive to realizing industrialization and avoiding many unnecessary side-reactions. In contrast to pure waterborne polyurethane, the limiting oxygen index value of P-N-Si-WPU is demonstrated to be increased from 18 % to 27.5 %. The peak heat release rate of P-N-Si-WPU is decreased from 648 W/g to 262.4 W/g. All results demonstrate that the possible flame retardant mechanism of P-N-Si-WPU is gas phase flame retardant and condensed phase flame retardant. During the decomposition of P-N-Si-WPU, a thermally stable char layer consisting P (=O)-O-Si- structure is formed to endow the P-N-Si-WPU with better thermal insulation ability in the combustion process. At the same time, due to the flexibility of Si-O-Si bond, the flame retardant and mechanical properties of the film can be balanced in the process of synthesis of waterborne polyurethane, which provides a strategy for the preparation of outstanding flame retardant waterborne polyurethane.