Study Of High Density Polyethylene Modified By Halogen-free Flame Retardants

High density polyethylene(HDPE) has long been used as coating materials of wires and cables owing to its excellent dielectric, corrosion-resistant and mechanical properties, however, it is flammable. In the electrical fire accidents each year, the second-time fire, caused by the poor flame retardant properties of wires and cables, takes up a big percentage. And it has heavily threatened people's lives and possessions. Thus, the coating materials must be modified by flame retardants. The traditional halogen flame retardants will produce toxic gases while burning, which limits their wide application. So the study on polyethylene materials modified by halogen-free flame retardants has become the development mainstream. At present, halogen-free flame retardants possess some issues, such as poor flame retardant effects, a lot of dosages, and low mechanical properties, etc. These problems would lead to unsatisfactory modification results. Therefore, the study on halogen-free flame retardants and high density polyethylene composites with good flame retardancy, high mechanical properties, and environmentally friendly characteristics has important practical value and theoretical significance.In this paper, the thermal stability, flame retardancy, carbon layer morphology and mechanical properties of halogen-free flame retardants and high density polyethylene composites were evaluated by thermal gravimetric analysis(TG) and limiting oxygen index(LOI), the main research contents and conclusions are as follows:(1) The intumescent flame retardants(IFR) were assembled by methyl silicate pentaerythritol ester(MSPE) and ammonium polyphosphate(coated by melamine with high degree of polymerization)(MAPP). The impacts of the ratio of MSPE and MAPP on the comprehensive performance of HDPE were evaluated. The results show that when the mass ratio of MAPP and MSPE is 2.5/1, the composites possess satisfactory flame retardant properties. According to this optimum ratio, IFR were assembled by MSPE and MAPP, and the impacts of the content of IFR in the HDPE/IFR composites on LOI values have further been investigated. It is found that the LOI values of the composites increase with the additive amount of IFR. When the IFR content is 30 wt.%, the LOI value of the composites reaches up to 26.5%, but the mechanical properties significantly decrease. The results also demonstrate that the addition of IFR expands the thermal decomposition temperature range of HDPE, increases the char residue formation, and improves the heat-resistant performance and flow properties of the melt.(2) The impacts of the addition of organic vermiculite(OVMT), vermiculite(VMT) and nylon 11(PA11) on the performance of the composites were evaluated. The carbon layer morphology and thermal stability of the composites were observed and examined, respectively. The results show that the addition of appropriate amount of OVMT and PA11 in HDPE/IFR composites can do well in flame retardant effects, and the LOI values can reach up to 31.2%. Meanwhile, the char residue formation also increases, and the carbon layers with high quality are obtained. Furthermore, the mechanical performance of the composites is also completely according with the standard of cable materials.