Chemical magnesium hydroxide (Mg(OH)2) is used industrially as a flame retardant. Most magnesium hydroxide used in industry is chemically synthesized. Like aluminum hydroxide, solid magnesium hydroxide has smoke-suppressing and flame-retardant properties. This property is attributed to its endothermic decomposition at 332°C (630°F):

Mg(OH)2→MgO+H 2 O

The heat absorbed by the reaction delays the fire by delaying the ignition of the materials involved. The released water dilutes the flammable gas. Common uses of magnesium hydroxide as a flame retardant include additives in cable insulation, insulating plastics, roofing and various flame retardant coatings

Fire-Retardant Fiber Reinforced Composites Fire-Resistant Textiles Brochure Mineral Filler Flame Retardants

Hydroxides or carbonates that decompose after the polymer decomposition temperature are of little use for extinguishing fires because the fire is already well spread by this time, so endothermic decomposition temperatures of 400°C or lower are the temperatures commonly used today. Metal hydroxides meet well most of the fire safety standards mentioned at the beginning of this section, with the exception of structural integrity under fire conditions, and in some cases they may have deficiencies in reducing heat release.They dilute the total amount of fuel available for combustion with non-combustible gases, which generally keeps smoke release rates low, and are therefore often used to address the smoke release deficiencies of specific polymers while maintaining other aspects of flame retardancy. However, they have a limited window of use during a fire. If a polymer containing these fillers is continuously heated, once the filler is consumed, what is left will burn as if no flame retardant was present, so in some high heat flux fires, mineral fillers are only effective in the early stages of the fire and then do nothing to mitigate the heat release later in the fire.In PMC, such high filler loading cannot be tolerated as this would compromise the mechanical properties of the material which may already have high fiber loading. Additionally, if the primary particle size of the mineral filler is too large, the flame retardant may leach out and make manufacturing difficult during processes such as resin transfer molding. Therefore, mineral fillers are not used alone in PMC, but in combination with other flame retardants. 47,48 Another disadvantage of mineral fillers is that high filling levels are usually required to obtain satisfactory flame retardant properties in regulatory tests.

For example, in wire and cable applications, it is not uncommon to use 60–80 wt% mineral fillers in jacketing materials to pass flame spread tests. 7,49–51 to address the deficiencies in fire performance. They are most commonly used to address smoke release issues and meet early flame spread/flammability standards.

Typical mineral fillers currently used include:

Magnesium hydroxide (Mg(OH)2). Also called brucite. Discharging water temperature: 320℃;

​•

Aluminum hydroxide (Al(OH)3 or Al 2 O 3·3H 2 O). Also known as alumina trihydrate. Discharging water temperature: 180℃;

​•

Boehmite (AlOOH). Discharging water temperature 320-400℃;

​•

Hydromagnesite (3MgCO 3·Mg(OH)2·3H 2 O). Water and CO2 release temperature range is 220–240°C (water release) and 320–350°C (CO2 release).