Structural and thermal interpretation of the synergy and interactions between the fire retardants magnesium hydroxide and zinc borate

The mechanism of cooperative action of commercial fire retardants is interpreted as resulting from specific chemical reaction and phase changes. This investigation focuses on the thermally initiated interactions between two forms of commercially available fire retardant compounds. The fire performance of a polyolefin with a metal hydroxide fire retardant, magnesium hydroxide, can significantly reduce the heat release rate through absorption of heat during conversion to its metal oxide. Formation of water, followed by vaporisation, decreases heat and dilutes volatiles from polymer degradation. The second form of fire retardant compounds are zinc borates (2ZnO center dot 3B(2)O(3)center dot 3H(2)O and 4ZnO center dot B2O3 center dot H2O), that undergo dehydration with increasing temperature. Differential thermal analysis and wide-angle X-ray spectroscopy indicated that various structural changes occurred during heating. Endothermic transitions were observed for all components, while zinc borate (2ZnO center dot 3B(2)O(3)center dot 3H(2)O) showed an exothermic crystallisation transition at relatively high temperature. The exotherm was modified by the development of a new crystalline phase, magnesium orthoborate (3MgO center dot B2O3) that formed on reaction with magnesium oxide (MgO) at temperatures greater than 500 degrees C. Formation of crystalline zinc oxide (ZnO) was also detected. From zinc borate (4ZnO center dot B2O3 center dot H2O), ZnO was primarily formed. No new crystalline phases were observed in the presence of MgO over the temperature range investigated. (c) 2006 Elsevier Ltd. All rights reserved.