Given careful design and the right choice of resins, additives and fillers, FRP composites can be used to make structures with better fire resistance than almost any other material. As an example, phenolics are used within firewalls. FRP composites generally are poor thermal conductors, so they do not help the heat of a fire to spread in the way that can occur with metals. The smoke from fire can be a concern, and so careful choices need to be made for internal applications.

Design for fire performance

With an organic based resin there is always the potential of ignition, which can lead to flame spread on the surface of the composite and generation of heat and toxic smoke. However composites are superior to non-filled plastics in that they contain up to 70% (by weight) fibre which is non-combustible. The fibre acts to separate regions of resin, thus restricting the amount of resin (fuel) available to the fire.

With composites usage in critical applications increasing, knowledge of their fire performance becomes a safety critical issue. This particularly applies to composites in aircraft, marine and the oil and gas industries. The heat from a fire may weaken the polymer and cause eventual creep and structural failure as the temperature exceeds the glass transition temperature. Alternatively the polymer itself may ignite and spread the flame, releasing further heat and potentially toxic smoke. However, composites are by their nature inherently fire resistant. The inert fibre reinforcement displaces polymer resin during fire and thus removes fuel for the fire. When the outermost layers of a composite laminate lose their resin, they act as an insulating layer, slowing heat penetration.

Flame retardant resins are being developed that lessen the flammability of a composite. Resins such as brominated vinyl ester resist spread of flames although they do not necessarily improve ignition properties. Phenolic resins have excellent charring capabilities resisting fire ignition better than other resin systems. Coatings may also be used on composites to delay ignition, lower the rate of heat release, suppress lateral flame spread, and extend the duration of fire resistance.
Flame testing generally falls into two categories: tests to measure ignition and flame spread properties, and tests to measure fire resistance. Fire resistance is a measure of a material's ability to continue to serve its structural role during a fire.

Summary

As with all issues in design there is always a compromise to be struck between the different material properties needed for a project. If fire resistance is absolutely critical then FRP composites can often be the most cost-effective solution.