Improving the flame retardancy and mechanical properties of high-density polyethylene-g-maleic anhydride with a novel organic metal phosphonate

Zinc N,N′-piperazinebis(methylenephosphonic acid) (PPMPA-Zn) was melt blended with high-density polyethylene-g-maleic anhydride (HDPE-MA) to fabricate flame-retardant HDPE-MA/PPMPA-Zn composites. The thermal stability and flame retardancy of the composites were investigated by thermogravimetric analysis (TGA) and cone calorimetry. The incorporation of PPMPA-Zn did not appreciably increase the thermal stability of HDPE-MA, but the residual char content increased to 18.7% when the content of PPMPA-Zn reached 25 wt%. X-ray diffraction and Raman spectroscopy tests showed that the residual char was composed mainly of Zn(PO3)2, Zn11(HPO3)8(OH)6 and amorphous graphite. Scanning electron microscopy (SEM) images of samples collected after cone calorimetric tests also confirmed the formation of char in the sample with 25 wt% PPMPA-Zn loading in HDPE-MA, and the char layer acted as a compact and intact physical barrier in the condensed phase. Even at a loading as low as 5 wt% PPMPA-Zn, the peak value of the heat release rate (PHRR) was reduced by 38% compared with that of pure HDPE-MA. In contrast to other intumescent flame retardants, the introduction of PPMPA-Zn enhanced not only the flame retardancy but also the mechanical properties of HDPE-MA. For example, the yield strength and Youngʹs modulus of HDPE-MA with 5 wt% PPMPA-Zn were 125% and 128% higher, respectively, than those of the pure HDPE-MA matrix.