Magnesium hydroxide decomposes starting at 330°C, this decomposition is endothermic. Applications: When magnesium hydroxide is present in a burning polymer then it will act in a number of ways as a flame retardant.

Firstly, this absorption of heat acts to lower the temperature of the material and hence reduce flame spread and can lead to an extinguishing of the flame.
The presence of water vapor near the surface of the polymer will lead to oxygen depletion in that area and hence reducing the burning rate of the gases.

Magnesium hydroxide will assist in forming a carbonaceous char, which is a hard carbon containing ash formed during the combustion process.
This char will prevent heat from being returned back to the underlying polymer and protect it from further combustion. It has been suggested that the oxide of magnesium that is formed in the char layer may also form an insulation layer that protects the underlying polymer.

In addition, magnesium hydroxide acts as a smoke suppressant during the burning process. It is unclear how this happens but it is suggested that the effect is largely due to the high surface area oxides that are formed as a result of the decomposition.

These absorb combustion products that could lead to soot formation and also promote the oxidation of carbon to non-sooty oxides. In addition the endothermic action of the filler acts to reduce combustion rates and this favors cross-linking and char formation to soot production.