Fiber Reinforced Polymer (FRP) is a composite material combining a polymer matrix with embedded reinforcing fibers, typically glass, carbon, or aramid. The polymer, often a thermoset plastic like epoxy or vinyl ester, binds the fibers. Whether FRP is fire-resistant depends entirely on the chemical composition of the polymer matrix. Standard FRP formulations are organic, meaning they are inherently combustible and will burn when exposed to a sustained heat source.
Baseline Flammability of Standard FRP
Unmodified FRP composites, especially those using common polyester or vinyl ester resins, readily support combustion because they are made from organic polymers. When exposed to fire, the polymer matrix ignites, contributing fuel, releasing heat, and producing substantial smoke. Although reinforcing fibers like glass are inorganic and do not burn, the matrix decomposes under thermal load, quickly compromising the composite’s structural integrity.
The polymer matrix softens significantly above its glass transition temperature, which can be as low as 150°C. As the matrix softens, melts, or ablates, the FRP loses its ability to transfer load between the fibers, resulting in a rapid loss of structural capacity. The combustion of standard resins also generates dense, black smoke, which obscures visibility and poses a significant hazard in enclosed spaces.
How Fire-Resistant FRP is Achieved and Classified
Manufacturers achieve fire-resistant FRP by altering the material’s chemistry and combustion behavior. The first method uses specialized, inherently fire-resistant resin systems, such as phenolic resins, which have lower flammability and heat release rates. The second, more common method incorporates fire-retardant additives into standard resins.
These additives interrupt the combustion cycle. Halogenated compounds interfere with flame chemistry, suppressing the fire. Mineral fillers, such as aluminum trihydrate, release water when heated, cooling the material and creating a protective char layer. These modifications allow fire-retardant FRP to resist ignition and burn less vigorously, often becoming self-extinguishing once the external flame source is removed.
Fire resistance is defined by standardized testing procedures used in construction and materials science. The most recognized standard for classifying surface burning characteristics in North America is the ASTM E-84 test, often referred to as the Steiner Tunnel Test. This test measures two key performance indicators: the Flame Spread Index (FSI) and the Smoke Developed Index (SDI).
Based on the results of the ASTM E-84 test, materials are assigned a classification, which is crucial for determining where a product can be legally installed:
- Class A materials must achieve an FSI between 0 and 25, indicating the highest level of fire resistance.
- Class B materials fall within an FSI range of 26 to 75, representing moderate performance.
- Class C materials, the lowest classification, have an FSI between 76 and 200.
For all three classes, the maximum acceptable Smoke Developed Index is typically 450.
Practical Factors Affecting Fire Performance
The actual fire performance of an FRP component in a real-world setting is influenced by several practical application variables beyond laboratory classifications. Material thickness plays a role, as a thicker section requires more time for heat to penetrate and raise the polymer matrix temperature to its decomposition point. This delay provides a temporary barrier, even in an unmodified material.
External protective systems are commonly used to significantly enhance the fire safety of FRP installations. Applying intumescent coatings, which are specialized paints that swell and char when exposed to heat, creates an insulating layer. This barrier effectively delays the heat transfer to the underlying FRP and helps structures meet stringent Class A flame spread requirements.
It is important to differentiate between fire resistance and fire endurance. Resistance focuses on the material’s reaction to fire—its ability to resist ignition and slow flame spread. Endurance refers to the material’s ability to maintain its structural load-bearing capacity over a specified period during a fire. Since the polymer matrix softens at relatively low temperatures, even fire-resistant FRP may quickly fail structurally if not protected.