Flame retardants are substances added to manufactured materials to delay ignition and slow the spread of fire. For many years, flame retardants containing halogen elements like bromine and chlorine were common due to their effectiveness in interrupting the combustion process. However, environmental and health concerns regarding these persistent, halogenated chemicals have driven a significant shift toward safer, mineral-based alternatives. This newer generation of non-halogenated compounds, derived from naturally occurring mineral sources, functions primarily through physical mechanisms to suppress fire.
The Cooling and Dilution Effect
The primary fire-suppression mechanism employed by the most common mineral flame retardants is a dual action known as the cooling and dilution effect. This process begins when the material is exposed to the intense heat of a developing fire. The mineral compound undergoes an endothermic decomposition, which means it absorbs a substantial amount of heat from the surrounding environment.
For example, aluminum and magnesium hydroxides release water molecules bound within their crystal structure when heated. This chemical reaction consumes heat energy that would otherwise fuel the fire.
The second part of this mechanism is the dilution effect, caused by the release of non-flammable water vapor. As the mineral decomposes, the steam generated mixes with the volatile, flammable gases released by the burning material. This vapor dilutes the concentration of combustible gases and oxygen in the flame zone. The decomposition products, which are stable metal oxides, also remain on the surface to form a protective char-like layer.
Aluminum Hydroxide and Magnesium Hydroxide
The two most widely used mineral flame retardants are aluminum hydroxide (ATH) and magnesium hydroxide (MDH). Both are prized for their non-toxic nature, low smoke generation, and effectiveness in a wide range of polymers. Aluminum hydroxide, also known as alumina trihydrate, begins its endothermic decomposition at a relatively low temperature, typically between 180°C and 220°C. This lower decomposition temperature makes ATH suitable for polymers that are processed at lower temperatures, such as certain rubbers and thermoset resins.
During its decomposition, ATH consumes approximately 1050 Joules of heat per gram of material. The resulting aluminum oxide residue is a stable, inert solid that contributes to the protective barrier on the material’s surface. ATH is widely incorporated into wire and cable insulation, carpet backing, and construction materials due to its high efficiency in these applications.
Magnesium hydroxide offers a significant advantage with its higher decomposition temperature, around 340°C to 490°C. This thermal stability allows MDH to be used as a flame retardant in engineering plastics that require high processing temperatures, such as polyamides and polycarbonates. MDH consumes a greater amount of heat during decomposition, registering approximately 1316 Joules per gram, making it a slightly more efficient heat sink.
In addition to its cooling and dilution effects, MDH is particularly noted for its superior smoke suppression capabilities. The magnesium oxide residue left behind can also neutralize acidic, corrosive gases that might be produced by other components of the burning material. These two hydroxides are often used together to achieve synergistic effects, broadening the range of materials and processing conditions in which they can be used.
Beyond Hydroxides: Borates and Other Mineral Solutions
Other mineral solutions function primarily by forming a protective surface barrier. Zinc borate is a mineral compound often used synergistically with the hydroxide flame retardants. When exposed to heat, zinc borate dehydrates, but its primary function is to melt and form a glassy, vitreous layer of boron oxide on the material’s surface. This barrier also prevents the escape of flammable gases, promoting the formation of a stable carbonaceous char beneath the surface.
Zinc borate is also an effective smoke suppressant, reducing the emission of volatile organic compounds that contribute to smoke generation during a fire. Other mineral-derived flame retardants include nanoclays, huntite, and hydromagnesite. Huntite and hydromagnesite are naturally occurring carbonates that decompose endothermically, releasing both water and carbon dioxide gases.