Intumescent flame retardant behavior of triazine group and ammonium polyphosphate in waterborne polyurethane

In this work, a novel triazine-based flame retardant, namely (4,6-dimethoxy-1,3,5- triazine)-2-methyl-propane-1,3-diol (TMPD) was synthesized and used as a chain extender to prepare flame retardant waterborne polyurethane (FRWPU). The structure of TMPD was characterized by fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR) and mass spectrometry (MS). To enhance the flame retardancy, ammonium polyphosphate (APP) was simultaneously introduced into FRWPU, and the synergistic flame retardant effects of both TMPD and APP were investigated in detail via the limited oxygen index (LOI), vertical burning test, cone calorimetry, and thermogravimetric analysis (TG). Compared with pure waterborne polyurethane (WPU), the flame retardancy of FRWPU with different TMPD loadings were all found to be increased, and LOI an value of 27 % and a UL-94 V-0 rating were attained when 5 wt% APP and 6 wt% TMPD were employed (APP/FRWPU-6). Moreover, the peak heat release rate (pk-HRR), total heat release (THR), and peak of smoke production rate (pk-SPR) of APP/FRWPU-6 were respectively found to be decreased by 43 %, 10 %, and 24 % compared with APP/WPU (with 5 wt% APP), respectively, thereby exhibiting an obvious synergistic flame retardant effect. In addition, the char residues of TMPD and APP in WPU after the cone calorimetry test were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), laser Raman spectroscopy (LRS), and x-ray photoelectron spectroscopy (XPS), and the results indicate that the combination of TMPD and APP can form a more compact and graphitized char layer and a nitrogen-containing gas isolation layer. Moreover, it can produce PO• radicals in the flame that combine with H• and OH• radicals. These factors are all believed to shield the underlying polyurethane from the further influence of heat flux. Therefore, this work presents a novel and efficient method by which to form FRWPU.