Some materials are inherently flame retardant and do not need the addition of flame retardant additives. Some examples of naturally flame retardant materials are wool, some cotton fibers, eucalyptus, and polyimides. There are several reasons that a polymer might be considered inherently flame retardant. One reason could be the presence of a significant amount of aromatic content or a significant amount of nitrogen.

In a previous post, I talked about how a side effect of the intumescents used to create a thermal shield is the production of hydrates that then degrade and through an endothermic reaction further contribute to the flame retarding process. When some materials start to be consumed in combustion reactions, the inert gases that are produced such as carbon dioxide and water dilute the combustible gases in the area and as a result slow the combustion reaction.

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Dilution of Fuel

An inexpensive category of flame retardant is actually introduced to the plastic for a different reason. Many plastics are manufactured with as much as twenty percent filler. The most common filler is calcium carbonate and another is talc. The calcium carbonate is added to make the production of the plastic less expensive, but it does have the added benefit of providing some flame retardant properties.

The way that the dilution agent works as a flame retardant is that is lowers the percentage of combustible material in the finished product. If there is less combustible material then there is less fuel for the fire. Less fuel means it could burn itself out more quickly. Hence and unsurprisingly, the industry calls this method of fire protection dilution of fuel.

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The most common type of flame retardant used in products today is the halogen/antimony oxide combination. The antimony trioxide is a synergist acting with a halogenated compound to use radical quenching as a means to slow the spread of fire. Brominated and chlorinated compounds are used most often. When using a brominated compound it undergoes thermal degradation when exposed to high temperatures and releases hydrogen bromide. The hydrogen bromide reacts with the hydrogen and hydroxyl radicals that are present in the flame resulting in water and bromine radicals. Bromine radicals are far less reactive than hydrogen and hydroxyl radicals and have much less potential to feed the combustion reaction.

This type of flame retardant is the most effective in the market place today, but there is one downside. The hydrogen bromide or hydrogen chloride released during the degradation process is corrosive. That means that this type of flame retardant might not be the best choice for electronics or other applications that would be sensitive to corrosion. However, electronics do not tolerate being engulfed in flames very well either.

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Some flame retardants work under the concept of thermal shielding. In this process, a thermal barrier is formed between the burning part of material and the unaffected part. The thermal barrier insulated the unburned material from the flames. Intumescents are classic examples of the type of chemical that achieves thermal shielding. Intumescents are substances that swell when exposed to heat. In plastics, the intumescent additive causes carbonized foam to form which acts as a shield.

There are two main types of intumescents, soft char producers and hard expanding char producers. The soft char producers are poor heat conductors and often contain hydrates. In the previous post, I talked about how hydrates can be flame retardants in their own right due to their endothermic degradation. Once the hydrates have decomposed, the soft intumescent insulates the good material from the open flame. Hard expanding producers are just what they sound like. They produce a very hard and substantial char that will literally not let the fire through.

Soft chars are appropriate for spray fireproofing or firestop pillows that are stuffed into holes in wall or floor assemblies. Hard expanding chars are used for plastic pipe firestops and fireproofing of exterior steel structures. They are used here because you need a firestop that is capable of handling expansion pressure.

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One type of flame retardant is a chemical that degrades endothermically. Examples of these types of chemicals are magnesium hydroxide, aluminum hydroxide and many different hydrates. An endothermic reaction is one where heat is absorbed. Systems in which an endothermic reaction is taking actually undergo a relative decrease in temperature. Hence, endothermic degradation is the breakdown of a chemical while absorbing the heat from the surrounding materials and environment. For example, while a magnesium hydroxide degradation reaction is absorbing heat, the surrounding material is cooling down. This inhibits the combustion process.

The hydroxides and hydrates used as flame-retardants only degrade once they reach their decomposition temperature. For magnesium hydroxide, the decomposition temperature ranges from 250 to 340 degrees Celsius. For aluminum hydroxide, the decomposition temperature is much lower at approximately 230 degrees Celsius. Hydrates usually have similar decomposition temperatures. These temperatures are relatively low when you think about the operating temperatures of plastic manufacturing lines. Molten plastic often reaches temperatures anywhere from 200 to 275 degrees Celsius in an extruder. Some of these chemicals would start to degrade in the extruder and that could be a big problem so you must be careful to consider your operating parameters when choosing a flame retardant.

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