Improvement of the flame retardancy of wood-fibre/polypropylene composites with ideal mechanical properties by a novel intumescent flame retardant system

Abstract

To improve the flame retardancy and maintain the ideal mechanical properties of the widely used wood fibre (WF) reinforced polypropylene (PP) composite (WPC), a novel intumescent flame retardant (IFR) system consisting of poly[N⁴-bis(ethylenediamino)-phenyl phosphonic-N²,N⁶-bis(ethylenediamino)-1,3,5-triazine-N-phenyl phosphonate] (PTPA) and ammonium polyphosphate (APP) was developed. Addition of APP, a high performance flame retardant, improves flame retardancy but has a negative effect on flexural properties and the Notched Izod impact strength. Combining self-designed PTPA with WPC has a positive effect on the mechanical properties whereas it just slightly increases the flame retardancy. The flame retarded WPC system with both PTPA and APP, i.e. WPC/PTPA/APP, combines bilateral advantages. The selected formula has a flexural modulus of 4.4 GPa, flexural strength of 37.2 MPa and impact strength of 1.8 kJ m⁻², only the flexural strength is slightly lower than that of neat WPC. And the formula has a high limiting oxygen index (LOI) of 31.5%, which passes the UL-94 V-0 rating for a vertical burning test. The results of cone calorimetry show that the heat release rate (HRR) of the WPC/PTPA/APP system is significantly reduced compared to the neat WPC. The pyrolysis and char residue of the WPC/PTPA/APP system were investigated by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). These results reveal the flame-retardant mechanism that PTPA/APP can synergistically catalyze the etherification, dehydration and char formation of WPC.