BASIC MAGNESIUM CARBONATE FLAME RETARDANTS FOR POLYPROPYLENE

Magnesium hydroxide and magnesium carbonates have attracted attention as endothermic flame retardants that are sufficiently stable to be incorporated into thermoplastics without decomposition. In this survey, a basic form of magnesium carbonate, magnesium carbonate hydroxide pentahydrate [(MgCO3)4.Mg(OH)2.5H2O] was evaluated as a flame retardant for polypropylene. This filler (MCHP) has a thermal stability intermediate between that of alumina trihydrate (ATH) and magnesium hydroxide, which is sufficient to allow incorporation into polypropylene without decomposition. The MCHP is most effective at high filler concentrations near 60% where it was found to impart a Limiting Oxygen Index of 28.2 with a V-0 rating (no dripping). This is slightly more effective than the flammability ratings for ATH and Mg(OH)2 under the same conditions. The effectiveness of MCHP was attributed to the large endothermic loss of water of hydration, which also dilutes the combustion gases. This action was further aided by the formation of an intumescent char on the burning surface, which eventually extinguished the flame. Various combinations of magnesium oxide, magnesium hydroxide, magnesium carbonate, and MCHP were evaluated in order to clarify the mechanism of the flame retardant and improve the efficiency of the protective action. However, no synergism was evident, and the flame retardant results were found to be additive. The mechanical properties and processabilities of these highly filled compounds are very sensitive to the type of surface treatment. The method of Savides was employed to compare the burning temperature of the test specimens and to measure the rates of combustion.