Flame resistant (FR) garments are a foundational piece of safety equipment for workers in hazardous environments. The FR fabrics that are used to construct those garments are specially engineered to resist ignition and self-extinguish. But how does that all actually work? It’s important for safety professionals and end users alike to understand the science of FR fabrics, including understanding the limitations of flame resistance.
The Science Behind FR Fabrics
FR fabrics work by incorporating specific chemical and physical properties that inhibit combustion. The two primary categories of FR fabrics—inherent and treated—achieve flame resistance through different mechanisms.
Inherent Flame Resistance
Inherent FR fabrics are made from fibers with intrinsic flame resistant properties. These fibers are engineered at a molecular level to resist ignition and prevent flame spread.
Mechanisms of Inherent FR Fabrics:
- Chemical Structure: Inherent FR fibers, such as aramids (e.g., Nomex®) and modacrylics, have chemical structures that naturally resist combustion. Aromatic rings and nitrogen atoms in the polymer backbone enhance thermal stability and char formation.
- Blends: When modacrylic fibers are blended with cotton or lyocell, there is a synergistic effect that improves the LOI (limiting oxygen index), which is the minimum concentration of oxygen that will support combustion of a polymer. The blend with cellulosic fibers also provides extra comfort. Blending modacrylic with other fibers imparts the fabric with additional thermal properties, as well as increases abrasion resistance, static charge dispersion, strength, durability, and shape retention.
- Char Formation: When exposed to high temperatures, instead of melting, inherent FR fabrics form a stable char layer that acts as a protective layer.
Example: Tecasafe® Plus by TenCate Protective Fabrics is an inherently flame resistant fabric with a unique fiber blend that offers permanent protection.
Treated Flame Resistance
Treated FR fabrics are made by chemically treating fabrics to impart flame resistant properties. The treatment is applied to the surface or integrated into the fabric during manufacturing.
Mechanisms of Treated FR Fabrics:
- Chemical Additives: These fabrics are treated with flame retardants, such as phosphorus-based compounds, that promote char formation and inhibit flame spread. These additives decompose to release non-flammable gases that dilute the combustible gases.
- What About Surface Treatments?: Fabrics that are treated with a flame-retardant surface treatment are called flame retardant rather than flame resistant (FR) and are used in home furniture, carpets, and curtains. They are not appropriate for use in protective garments.
Example: FR treated cotton fabrics used in industrial safety garments, where the flame-retardant treatment ensures compliance with safety standards.
Measuring Flame Resistance
- Heat Transfer Performance (HTP): This measures the time before a second-degree burn occurs and is a requirement for the NFPA 2112 standard. HTP needs to be > 6 cal cm2 with a spacer or > 3 cal/cm2 without.
- Burn Injury Prediction: This measures % body burn (for both second and third-degree burns) after a three-second flash fire exposure and is tested with a Mannikin Burn test.
- Arc Thermal Performance Value (ATPV): This value indicates the level of protection from electrical arc flashes. Higher ATPV values mean greater resistance to arc flash energy.
- Char Length and Afterflame: In standard tests like ASTM D6413, the char length and afterflame time are measured to evaluate the fabric's flame resistance. Afterflame is the time a test fabric continues to flame after the flame source has been removed. Shorter char lengths and afterflame times signify better performance.
Knowledge Is Power
Understanding how FR fabrics resist ignition and burn-through is essential for understanding their protection capabilities and limits. They do not render the wearer invincible to fire, however, they do buy the wearer critical time to get away from the fire source where their FR garment can self-extinguish.
Remember, the fabric only self-extinguishes after the flame source has been removed. It’s important for both safety professionals and wearers to understand this aspect of FR workwear so that they can respond appropriately in the event of a fire—get away from the fire as quickly as possible.
Whether made with inherent or treated FR fabrics, flame resistant protective garments are critical in protecting workers, enhancing safety, and ensuring compliance with industry standards.
For more information on selecting the right FR fabrics for your industry, explore our comprehensive range of industrial safety solutions here.
Get in touch
Get in touch with a TenCate Protective Fabrics expert to discuss your safety needs:
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