Discolouration of polypropylene-based compounds containing magnesium hydroxide

Magnesium hydroxide is extensively used in the plastics industry as a flame retardant and smoke suppressant in various polymeric matrices (polyolefins, nylons etc.). However, it was found that compounding Mg(OH) 2 with polymers leads to the formation of various colours ranging from light grey to rather dark beige. The effect of the temperature and processing technology, and of various properties of Mg(OH) 2 (impurities, particle size and morphology) on colour formation was investigated in this work. One of the causes for the discolouration appears to be the interaction between the Mg(OH) 2 and antioxidants containing phenolic groups. This reaction takes place rapidly when the former is added to the polymer melt containing the antioxidant. The amount and type of coating, and its continuity over the particles surface (dependent on the powder morphology and coating technology) may represent a way to prevent the chemical interaction between the filler and plastics components and avoid discolouration. A method for the quantitative evaluation of coating quality and continuity was developed. Based on this research, ways to improve the quality of magnesium hydroxide as a fire retardant were suggested. One of these ways, namely the addition of TiO 2, is particularly interesting. We found that besides its pigmentation effect, TiO 2 is synergistic with magnesium hydroxide in terms of flame retardancy and improves the thermal stability of polypropylene.