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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