Are FRP composites fire retardant?
FRP composites are very good thermal insulators, and panels to protect steel structures from jet fire on oil platforms were developed after the Piper Alpha disaster in the 1990s. There are several ways to protect FRP composites from fire, such as:
- Adding a fire retardant in the resin. The most used are halogen or phosphorus compounds, or ATH (aluminium tri-hydroxide) fillers
- Using a char-forming resin such as phenolic or furan
- Coating with an intumescent (swells to form an insulating layer when heated), ablative or protective coating (e.g. ceramic), or a layer of insulation
The polymers used as FRP matrices are combustible, but the commonly used thermoset matrices do not melt and typically burn less easily than thermoplastic materials. All composites will weaken and lose strength above their heat deflection temperature (HDT). Metals lose strength as temperature rises, too. HDT for a standard boatbuilding resin may be 80˚C, but some high performance resins are over 200˚C.
Design for Fire Performance
With composites usage in demanding 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, leading to structural failure. Alternatively the polymer itself may ignite and spread the flame, releasing further heat and potentially toxic smoke, but this can be mitigated by the inclusion of fire-retardant additives. Moreover, composites, as mentioned, 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. The highest performance passive fire protection coatings for steelwork are effectively composite systems, and can be used to protect critical systems from even extreme evnts such as jet-fire.
Flame retardant resins are available that significantly lessen the flammability of a composite. Some resins (e.g. brominated vinylester) resist the 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. Additives can be passive e.g. inert fillers reducing organic flammable content and hence smoke; active, releasing flame suppressing or cooling gases,;or even intumescent, expanding with heat to provide addition heat inulation thickness. Coatings may also be used on or within a composite product 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.
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.