Comparison of the effect of montmorillonite, magnesium hydroxide and a mixture of both on the flammability properties and mechanism of char formation of HDPE composites

The effects of pristine (MMT), organo-modified montmorillonite (OMMT), magnesium hydroxide (MDH) and combination of montmorillonites and magnesium hydroxide on the fire retardancy of high density polyethylene were assessed using TGA-FTIR analysis, limiting oxygen index (LOI), UL-94 test and the cone calorimeter. The results indicated that the addition of organo-modified montmorillonite to high density polyethylene HDPE/MDH composites allowed achievement of higher limiting oxygen index values at lower total flame retardants content while fulfilling the fire retardancy requirements. High density polyethylene composites with pristine (MMT) or organo-modified (OMMT) montmorillonite were prepared by melt-blending in a twin-screw co-rotating extruder introducing two methods. In the first method montmorillonites or magnesium hydroxide was directly provided to high density polyethylene matrix to obtain HDPE/MMT, HDPE/OMMT and HDPE/MDH composites. The second method consists of two stages. The extrusion of master batches of ethylene-vinyl acetate copolymer (EVA) with pristine or organo-modified montmorillonite (ratio 1:1) in first stage and dilution of obtained premix in high density polyethylene matrix in second stage. The thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR) results indicate that pristine MMT, OMMT, MDH and their combination have an effect on the thermal degradation of HDPE in oxidative atmosphere. Cone calorimetry showed that, montmorillonites shorten times of ignition and reduce peak heat release rates (peak HRRs). Magnesium hydroxide, however delayed ignition owing to loss of water at lower temperatures, reduced the peak HRR. The reduced peak HRRs for OMMT and MDH together can be attributed to the formation of a surface layer which acts as mass and heat barrier to protect the underlying material from further burning. The flame retardant efficiency of magnesium hydroxide in high density polyethylene matrix was improved by application of MMT or OMMT introduced in ethylene-vinyl acetate copolymer.