Magnesium Hydroxide (Flame Retardant)

Magnesium hydroxide (Mg(OH)₂, also marketed as MDH — magnesium dihydroxide) is a halogen-free flame retardant and smoke suppressant that operates through the same endothermic decomposition mechanism as ATH but at a higher activation temperature of approximately 300–320°C. This higher decomposition temperature makes Mg(OH)₂ the preferred mineral flame retardant for thermoplastics processing at temperatures above 200°C and for high-temperature cure thermoset systems.

When exposed to fire temperatures, Mg(OH)₂ absorbs approximately 1.3 kJ/g of heat during decomposition to MgO + H₂O, releasing 31% of its weight as water vapor. The residual MgO layer forms a protective char barrier on the burning surface, contributing to the flame retardant mechanism. Compared to ATH, Mg(OH)₂ provides slightly better smoke suppression due to the basic MgO layer quenching acidic combustion gases.

In composites, Mg(OH)₂ is used in applications where the process or service temperature exceeds the decomposition range of ATH. Epoxy systems with high-temperature cure cycles (>180°C), PEEK composites, and phenolic composites benefit from Mg(OH)₂'s higher thermal stability. In UPR systems processed at ambient temperature, both ATH and Mg(OH)₂ can be used, but Mg(OH)₂ is typically more expensive than ATH for equivalent FR performance. Surface-treated grades (silane, stearate) improve resin compatibility and reduce compound viscosity.