Hydromagnesite and Huntite Mixtures
Naturally occurring mixtures of hydromagnesite (Mg5(CO3)4(OH)2.4H2O) and huntite (Mg3Ca(CO3)4), some of which are sold as UltraCarb®, have similar potential as FRs to ATH or MDH. Commercially extracted natural reserves of hydromagnesite and huntite (HMH) mixtures are already sufficiently pure and do not need chemical processing or precipitation of the final product. This makes production of fine particle size mixtures of huntite and hydromagnesite much less energy intensive. The only by-product is a small quantity of dolomite, MgCa(CO3)2, a mineral closely related to huntite, Mg3Ca(CO3)4, that occurs naturally in the mixture and is removed during the grinding process.
Nesquehonite (MgCO3.3H2O) has the properties of an effective fire retardant mineral filler but is not commercially produced for this purpose due to its low decomposition temperature (70-100 °C); it would decompose during processing.
Hydromagnesite (basic magnesium carbonate) can be formed from nesquehonite and has greater thermal stability (decomposition onset at approximately 220-240 °C). It finds its main fire retardant and smoke suppressant applications in elastomers, although its use is limited by its platy morphology and its increased cost compared to ATH (Rothon 2003). Magnesium carbonate subhydrate (MgO.CO2 (0.96)H2O(0.30)) has higher decomposition temperatures still (340-350 °C) and is capable of withstanding processing temperatures of up to 300 °C. Its application as a fire retardant is, however, limited: despite showing oxygen index results equivalent to ATH, its decomposition is too slow to perform effectively in ignitability tests (Rothon 2003).
Boehmite AlO(OH) and nanoboehmite have recently shown (Zhang et al. 2010) potential as fire retardant additives, although nanoboehmite is also believed to act in the gas phase as a free radical trap, since its endotherm and heat capacity contributions are too small to justify its wider use as an absorber of heat. Boehmite is an intermediate decomposition product of ATH. ATH undergoes its largest endothermic stage in the formation of boehmite, while the endotherm of the final decomposition stage of boehmite to alumina is smaller; thus, endothermic decomposition is unlikely to be the principal fire retardant action of boehmite.