Flame retardants are reactive additives that integrate into the polymer backbone to improve combustion and fire resistance. They can be applied to materials to prevent the start or spread of fire. A variety of chemistries can often be combined with different polymer systems for overall effectiveness in suppressing the ignition process. With the rise of market demand and innovation in consumer products and industrial applications, flame retardants have become especially critical in the following end markets:
An ignition source, such as fuel or oxygen, must be present for a fire to begin. Flame and fire retardants work by interfering or reacting with the ignition source, either physically or chemically. Below are the primary mechanisms of action.
In this type, flame retardants act as a thermal sink by absorbing the ignition heat and releasing water molecules. Releasing water molecules increases the heat capacity of the product and reduces the fuel content by dilution. Examples of flame retardants that act by the physical dilution mechanism are aluminum trihydrate (ATH) and magnesium hydroxide (Mg(OH)2).
Halogenated flame retardants chemically interact with the ignition process by dissociating into halogen radicals that compete with the free radicals and interrupt the combustion reaction. Halogenated flame retardants are mainly based on bromine and chlorine compounds.
In this mechanism, flame retarding additives form a protective fluid or a carbonaceous char that acts as an insulating layer and reduces heat transfer. This type includes phosphorous- and nitrogen-based flame retardants.
Halogenated flame and fire retardants release halogen radicals that disrupt free radicals in the vapor phase and shut down the combustion process. Halogenated flame retardants include bromine, chlorine, fluorine, and iodine compounds. Brominated flame retardants are the most common, followed by chlorinated chemistries. Fluorine and Iodine flame retardants are also available but not as common. Halogenated flame retardants are often combined with synergists, such as antimony compounds, to decrease the loading rate.
The demand for halogen-free flame retardants is on the rise, driven by environmental initiatives to decrease the use of halogenated compounds. Non-halogenated flame retardants include phosphorus and nitrogen compounds. Their mode of action is forming a protective char in the condensed phase or releasing free radicals to displace oxygen in the gas phase, inhibiting the flame.
Intumescent Flame Retardants
Intumescent flame retardants are char-forming additives that form a protective barrier of carbon foam film in the condensed phase. The protective layer is formed on the polymer's surface in response to heat, which protects the product from being burnt.
This carbon char acts as a barrier or insulating layer that interrupts the combustion process from further occurring. Char-forming flame retardants offer robust protection by preventing fuel molecules from reaching the flame front, thereby protecting the polymer from further depolymerization.
Intumescent coatings do not need synergists to improve their effectiveness and do not degrade the mechanical properties of the polymer. They are often used for applications that require high levels of flame protection, such as building and construction, aerospace, military, wire and cable, automotive, and transportation applications.
Organophosphorus Flame Retardants
Organophosphorus flame retardants react chemically with free radicals in the gas or condensed phase during combustion. They act as radical scavengers that are activated by the rise of temperature. Unlike halogen compounds, they typically do not need synergists to be effective but can be combined with other halogens to enhance the protection and loading rate.
Ammonium polyphosphate is one of the most popular non-halogenated phosphorus-based flame retardants. Phosphate esters are supplied in liquid form and are highly effective in urethane foams and engineered resin systems. Although phosphorus compounds are very effective and can be used in various applications, they tend to generate smoke and carbon monoxide gas.
Endothermic Flame Retardants
Endothermic flame retardants are mineral-based additives, such as aluminum trihydrate (ATH) and magnesium hydroxide (Mg(OH)2). They release water and carbon dioxide, which dilute the fuel available for combustion. Endothermic decomposition cools down the condensation phase, slows degradation, and absorbs the fuel combustion heat.
Although mineral fillers are lower in cost and considered greener chemistries, they require higher loading rates to reach the desired protection level. The higher level of mineral fillers can also affect polymers' mechanical properties.
St. Louis Group PhosGard® APP is a fine-particle flame retardant based on modified ammonium polyphosphate. It is excellent for water-based coatings, polyurethanes, thermoset plastics, and polyolefins. Its primary chemistry is a phosphorous and nitrogen blend.
PhosGard® NH4 is a halogen-free, phosphate-based flame retardant with low scorch and volatility. It is excellent for use in flexible urethane foams, engineered resins, and PVC plastics. Its primary chemistry is phosphate ester.
CharFlam® 200-M is a micronized melamine-based intumescent flame retardant that provides superior protection to paints, coatings, and textiles. Its primary chemistry is melamine.
BroShield® 68-B is a brominated fire retardant (BFR) containing aromatic and aliphatic bromine. It provides superior protection to coatings, adhesives, and fabric back-coatings. Its primary chemistry is an oligomer of tetrabromobisphenol A.
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