From mattresses to televisions, children’s toys to carpet padding, we are surrounded by products that contain flame retardants. These chemicals are added to materials or products to prevent or slow the spread of fire. They are broken down into the following 5 major categories:
1. Halogen: Within this class of flame retardants includes chlorine-based systems but perhaps the most well-known are Bromine Flame Retardants (BRFs). BFRs are commonly used by those within the electronics industry and also within textiles, construction products, and coatings. Bromine is used because it releases active bromine atoms into the gas phase before the material reaches its ignition temperature, which quenches the chemical reactions occurring within the flame. This can prevent the burning process from occurring or can slow it such that other measures can be taken to extinguish the fire. This is an example of the vapor phase inhibition approach. One major issue with this type of flame retardant is they are becoming increasingly banned within products due to safety concerns. The RoHS directive, for example, specifically limits the amount of polybrominated biphenyls and polybrominated diphenyl ethers that can be found within appliances, IT equipment, lighting equipment, medical devices, toys, and semiconductors, amongst other product categories.
2. Inorganic Flame Retardants: Many inorganic compounds are used as flame retardants or a catalyst within a flame retardant system. When it comes to flame retardants these materials often have to be used in large concentrations to achieve desired results. Alternatively, they must be used in conjunction with other types of flame retardants to be effective. For example, antimony oxides do not have flame retardant properties in and of themselves but when combined with bromine or chlorine based flame retardants they serve as a synergist. This means the antimony oxides act as a catalyst causing the bromine or chlorine to break down even faster, thus releasing active bromine atoms into the gas phase at a more rapid rate. The antimony oxides also react with the bromine or chlorine compounds to produce volatile antimony halogen compounds. While antimony oxides do not have flame retardant properties the volatile antimony halogen compounds do because they remove the high energy radicals that feed the gas phase of the fire.
Inorganic flame retardants that can be used independently include aluminum and magnesium hydroxides. These compounds interfere with the burning process through the release of inert gasses (like water vapor), creating a protective char layer, or energy apsorption (meaning the amount of energy available for the fire the spread is reduced).
3. Nitrogen Flame Retardants: Malamine-based products are the most commonly used type of nitrogen flame retardants. When melamine is in the condensed phase the molecular structures transform into cross-linked structures. This transformation promotes the formation of char, which inhibits oxygen supply. This is an example of a solid phase char flame retardant.
4. Intumescent Coatings: The aim of systems incorporating intumescent coatings is to protect materials from fire by preventing burning. They are applied to products like a layer of paint, which makes them well suited for construction materials like steel beams or wood. When exposed to heat these coatings expand to create a fire-resistant and insulating layer on the material. That layer protects the material from high temperatures, which can prevent or slow structural damage. Common components of intumescnet coatings include spumific compounds (chemicals that decompose when heated and produce large amounts of gas), a binder, an acid source and a carbon compound.
5. Phosphorous: These compounds are both chemically bound to materials and are also physically incorporated as an additive. Char is formed when the phosphorous compound is heated, thereby inhibiting the formation of combustible gas and inhibiting the pyrolysis process. What is particularly interesting about the formation of char is it hinders the release of combustible gasses while also forming a protective layer that shields the polymer from the heat of the flame.
Many products combine the various types of flame retardants within the system. This approach provides the benefits of the different prevention or mitigation modes. One such combination is phosphorus and chlorine. Phosphorus provides the solid phase char layer and chlorine provides a vapor layer inhibition approach.