Synergistic effect of a hypophosphorous acid-based ionic liquid and expandable graphite on the flame-retardant properties of wood-plastic composites

A hypophosphorous acid-based ionic liquid [Bmim]H2PO2 (BMP) was synthesized and characterized by FTIR, (HNMR)-H-1, (CNMR)-C-13 and (PNMR)-P-31. Moreover, a new intumescent flame-retardant system composed of BMP and expandable graphite (EG) was used to improve the flame retardancy of high-density polyethylene-based wood-plastic composites (WPC). The retardant properties and thermal decomposition behaviors were investigated by the limiting oxygen index (LOI) test, UL-94 vertical burning test, thermogravimetric analysis (TGA) and cone calorimeter test, respectively. When the mass ratio of BMP to EG was 1:2 and the total amount was kept at 30 mass%, WPC/EG-BMP composite passed UL-94 V-0 rating, and its LOI value increased to 31.5% from 21.4% for neat WPC. In addition, both the peak heat release rate and the total heat release of WPC/BMP-EG composite decreased significantly relative to WPC and WPC/EG composites according to cone calorimeter analysis. The TGA results suggested that BMP had good ability of char formation, and when combined with EG, it could greatly promote the formation of the protective char layer. The surface morphology and chemical structure of char residues were examined using scanning electron microscope-energy-dispersive spectrometer and Raman spectroscopy. It revealed that a compact and thick char layer was formed with the combination of BMP and EG, which hinder the transfer of heat flow and combustible gases in the condensed phase, thus exhibiting excellent flame retardancy. This study demonstrated that BMP and EG had a synergistic effect on the flame retardancy of WPC.