The influence of magnesium hydroxide morphology on the mechanical properties of polypropylene

Magnesium hydroxide is of growing interest as a halogen-free flame retardant, and has a high thermal stability, so it can be melt processed into most thermoplastics without premature decomposition. The hydrated mineral filler commences endothermic decomposition at approximately 340°C, withdrawing heat from the polymer substrate, releasing water of crystallisation to dilute the fuel supply. However, to be an effective flame retardant filler, high loadings up to 66% are required. This may cause detrimental effects on both rheological and mechanical properties of composites produced. Two synthetic seawater magnesium hydroxide fillers (DP393 and DP390s), of different morphologies but similar surface area, were selected as flame retardant fillers for polypropylene. These were coated with stearate, oleate, rosin and vinyl silane and compounded at 60% by weight into an injection moulding grade ofpolypropylene. The effect of filler morphology, coating agent, coating type and coating level on the crystallisation, rheological and mechanical properties of polypropylene were studied. DP393 is a one-micron platey structure which was found (by XRD) to be orientated parallel to the flow direction, whereas DP390s is a five-micron porous sphere residing isotropically within the matrix. Incorporation of uncoated magnesium hydroxide increased the overall crystallisation rate of polypropylene, causing orientation of the polymer b axis parallel to the flow direction, this being induced by the 001 crystal face of magnesium hydroxide....