Magnesium hydroxide (Mg(OH)₂) is an inorganic flame retardant used in selected plastic, rubber, wire and cable applications. Unlike some flame-retardant additives that rely on halogen-containing chemistry, magnesium hydroxide provides a different approach based on heat absorption and water release during thermal decomposition.
When exposed to sufficiently high temperatures, magnesium hydroxide decomposes into magnesium oxide and water vapor:
Mg(OH)₂ → MgO + H₂O
This reaction absorbs heat from the surrounding material. The released water vapor can also dilute combustible gases near the material surface, while the magnesium oxide residue may help form a protective layer. Together, these effects can help delay ignition and reduce flame propagation, depending on the formulation and test conditions.
Magnesium hydroxide is often used in polymer formulations that require halogen-free flame-retardant additives. However, achieving the required fire performance may require relatively high filler loading, which can affect flexibility, strength and processability. Manufacturers should therefore evaluate the complete formulation rather than choosing a grade based on purity alone.
