Decomposition and Fire Retardancy of Naturally Occurring Mixtures of Huntite and Hydromagnesite.

Mixtures of the two minerals huntite and hydromagnesite have been successfully used as a fire retardant additive in polymers for many years. The onset of decomposition of hydromagnesite is at a higher temperature than that of aluminium hydroxide but lower than that of magnesium hydroxide, the two most commonly used mineral fire retardants. This makes it an ideal addition to the range of materials available to polymer compounders for improving fire retardant properties. In comparison to the better known mineral fire retardants there has been little published research on the fire retardant properties of huntite and hydromagnesite. What has been published has often been commercially orientated and the limited quantity of scientific literature does not fully explain the fire retardant mechanism of these blends of minerals, often dismissing huntite as having no useful fire retardant action other than diluting the solid phase fuel. Standard thermal analysis techniques (thermal gravimetric analysis, differential scanning calorimetry, Fourier transform infra-red analysis) have been used to characterise the thermal decomposition of huntite and hydromagnesite from a source in Turkey. This has lead to an understanding of the decomposition mechanism of the minerals in terms of mass loss, enthalpy of decomposition, and evolved gases between room temperature and 1000°C. Hydromagnesite endothermically decomposes between about 220°C and 500°C, initially releasing water followed by carbon dioxide. The rate of heating and partial pressure of carbon dioxide in the atmosphere can influence the mechanism of carbon dioxide release. Huntite endothermically decomposes between about 450°C and 800°C releasing carbon dioxide in two stages. The use of the cone calorimeter to study the rate of heat release during combustion of ethylene vinyl acetate based polymer compounds has lead to an understanding of how both huntite and hydromagnesite affect the burning processes at different stages of the fire. By varying the ratio of the two minerals, hydromagnesite has been shown to increase the time to ignition and reduce the initial peak in rate of heat release, while huntite has been shown to reduce the rate of heat release later in the fire. It has been shown that huntite is far from being an inactive diluent filler. The endothermic decomposition of huntite in the later stages of the fire reduces the heat reaching underlying polymer and continues to dilute the flame with inert carbon dioxide. The platy huntite particles have been shown to align themselves in such a way that they can hinder the escape of volatiles from the decomposing polymer and also physically reinforce the inorganic ash residue

Fire retardant action of mineral fillers

Endothermically decomposing mineral fillers, such as aluminium or magnesium hydroxide, magnesium carbonate, or mixed magnesium/calcium carbonates and hydroxides, such as naturally occurring mixtures of huntite and hydromagnesite are in heavy demand as sustainable, environmentally benign fire retardants. They are more difficult to deploy than the halogenated flame retardants they are replacing, as their modes of action are more complex, and are not equally effective in different polymers. In addition to their presence (at levels up to 70%), reducing the flammable content of the material, they have three quantifiable fire retardant effects: heat absorption through endothermic decomposition; increased heat capacity of the polymer residue; increased heat capacity of the gas phase through the presence of water or carbon dioxide. These three contributions have been quantified for eight of the most common fire retardant mineral fillers, and the effects on standard fire tests such as the LOI, UL 94 and cone calorimeter discussed. By quantifying these estimable contributions, more subtle effects, which they might otherwise mask, may be identified

Use of hydromagnesite & huntite in plastics as a new flame retardant

In this study, flame retardancy properties of huntite/hydromagnesite mineral in plastic compounds were investigated. Prior to production of composite materials, huntite/hydromagnesite mineral was ground at particle sizes of 10 μm, 1 μm and 0.1 μm. The ground minerals with different particle size and content levels were added to the plastic compounds to produce composite materials. After fabrication of hydromagnesite/huntite reinforced plastic composite samples, they were characterized by using DTA-TG. Flame retardancy test were performed as a main objective of this research. By this way the effects of size distribution of the additives and the content levels of ceramic in the plastic compounds on flame retardancy are measured. It was concluded that flame retardant properties of plastic composites were improved depending on particle size of hydromagnesite/huntite minerals

On the occurrence of Mg- and Fe-rich carbonate mineral assemblages hosted in the Nain ophiolite melange, Central Iran and their industrial potential

In the Nain ophiolite melange, central Iran, off-white mineral assemblages occur as nodular magnesium rich carbonates and thin veinlets disseminated within an earthy serpentinite groundmass. They are related to tectonically disturbed, strongly weathered zones of the ultramafic rocks. Combined XRD, SEM and TG/DTA analysis revealed that the mineralogy of the Mg-rich carbonate is varied. Ten distinct paragenetic assemblages containing hydromagnesite, pyroaurite, manasseite, brugnatellite, hydrotalcite, aragonite, and/or huntite were found. The mineral assemblages formed as the result of precipitation from percolating Mg-rich meteoric waters through brecciated serpentinites. The source of Mg in excess in the groundwater is attributed to the hydrolysis of Mg-rich minerals in the predominant serpentinized ultramafic rocks. Selected hydromagnesite-rich samples were tested as fire retardants. Even though hydromagnesite is the predominant mineral phase, the economic importance of the mineral assemblages in total is limited mainly because of the insufficient whiteness and the presence of Fe-rich minerals that cause undesirable thermal reactions