Synergetic enhancement of mechanical and fire-resistance performance of waterborne polyurethane by introducing two kinds of phosphorus-nitrogen flame retardant.

In this work, a novel hydroxyl-terminated monomer containing phosphorus and nitrogen, tri(N, N-bis-(2-hydroxy-ethyl) acyloxoethyl) phosphate (TNAP), was synthesized successfully with phosphorus oxychloride, hydroxyethyl acrylate, and diethanolamine as raw materials, and then incorporated into flame-retarded waterborne polyurethanes to improve their flame retardancy, thermal behavior, and mechanical properties. Their structures were confirmed by nuclear magnetic resonance (NMR) and fourier transform infrared spectroscopy (FTIR). Besides, the thermal performance and combustion behaviors of crosslinked flame-retarded waterborne polyurethane (CFRWPU) films were evaluated through thermogravimetric analysis (TGA), thermogravimetry-FTIR, limiting oxygen index (LOI) tests, and microscale combustion calorimeter tests (MCC). Additionally, the mechanical properties were investigated by tensile stress-strain tests. These results revealed that the monomer TNAP exhibited remarkable residual char formation ability of 34.98 wt% and that TNAP-embedded FRWPU films attained an LOI value of 25.5% at a TNAP content of 4 wt%. Moreover, there was significant enhancement in tensile strength (15.81 MPa) obtained with the combined incorporation of TNAP into FRWPU. Also, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) provided the morphologies and the element distributions of char residues of CFRWPU after LOI tests. Finally, the thermal mechanical properties of CFRWPU were assessed by dynamic mechanical analysis (DMA).