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

Abstract

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.