The fire retardant behaviour of huntite and hydromagnesite – A review

Abstract

Naturally occurring mixtures of hydromagnesite and huntite have found important industrial use. Their endothermic decomposition over a temperature range similar to that of commonly used polymers and their release of water and carbon dioxide, has led to such mixtures being successfully used as fire retardants. They have replaced aluminium hydroxide and magnesium hydroxide in many applications. The current understanding of the thermal decomposition mechanism of both minerals and their combination in natural mixtures has been reviewed and related to their fire retardant action. Both minerals contribute to the reduction in flammability of polymers although the extent of these interactions has not been fully investigated. However, the fire retardant mechanism of these minerals appears more complicated than either aluminium hydroxide or magnesium hydroxide. This paper critically examines the literature on the fire retardant behaviour of mixtures of hydromagnesite and huntite, providing information of particular use to formulators of fire retardant polymers using these minerals. It also highlights the gaps in understanding of the fire retardant mechanisms of mixtures of hydromagnesite and huntite and areas that require further research.

The fire retardant effects of huntite in natural mixtures with hydromagnesite

“…Due to its high decomposition temperature it has been suggested [40,41,43] that huntite has little more influence than an inert diluent filler in terms of fire retardancy. It has been argued by the current authors [2] that the evidence in the literature does not back up this assertion. Details of recent work, presented here, demonstrates that huntite contributes significantly to the fire retardant properties of polymer compounds, filled with mixtures of huntite and hydromagnesite, in ways that had perhaps not been considered previously.…”

“…This paper forms the final part in a recent series of papers by the current authors highlighting the thermal decomposition and fire retardant behaviour of natural mixtures of huntite and hydromagnesite [1][2][3][4]. Mixtures of huntite and hydromagnesite form naturally and are commercially mined and processed as an alternative to the commonly used mineral filler fire retardants, aluminium hydroxide (ATH) and magnesium hydroxide (MDH).…”

“…Natural hydromagnesite particles have a blocky morphology [1,2] and once processed the majority of the particles are usually between 1 and 10 µm in diameter depending on the processing. It has the following chemical formula [25] and thermally decomposes [1,3], between about 220°C and 550°C in two stages, initially releasing water then carbon dioxide, leaving a solid residue of magnesium oxide.…”

“…It has the following chemical formula [26] and thermally decomposes [1,3], between about 450°C and 750°C in two stages, releasing only carbon dioxide, leaving a solid residue of magnesium oxide and calcium oxide. The thermal decomposition of mixtures of these minerals through endothermic release of carbon dioxide and water has led to several studies showing their potential applications, including fire retardant additives for polymer compounds [2,4,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46], controlling burning rates in cigarettes [47,48], and as a potential material for fighting forest fires [49][50][51][52][53]. Due to its high decomposition temperature it has been suggested [40,41,43] that huntite has little more influence than an inert diluent filler in terms of fire retardancy.…”

Fire retardant action of mineral fillers

“…In practice, the Ultracarb mixture is often found to outperform either hydromagnesite, or aluminium hydroxide as a fire retardant. This has been ascribed to the platy morphology of huntite, reinforcing the barrier properties of the residual layer 27 . The much lower energy absorbing capacity of boehmite, together with reports 28 of its successful application as an additive fire retardant, suggest that other mechanisms must also be operating.…”