The Science Behind Your FR Clothing
Have you ever wondered how protective flame resistant (FR) fabrics resist catching fire? We have your answers.
- By Drew Child
- Apr 01, 2020
When a flame, spark or intense heat reaches a flammable substance, a chemical reaction occurs that produces heat and light. This reaction, termed “combustion,” is the basis of fire. Have you ever wondered how protective flame resistant (FR) fabrics resist catching fire?
Arc flash, flash fire and combustible dust hazards are commonplace in many industrial settings, including the oil and gas and electric industries. Daily wear FR personal protective equipment (PPE) helps provide a final safeguard to those who may experience these short-term thermal hazards. Yet, to provide this critical protection, PPE must harness the science behind fire and FR fabrics.
To develop effective FR fabrics, textile manufacturers must first have a deep understanding of the science of fire. Manufacturers have developed different technologies to create FR textiles, each with their own merits. To help you understand how these approaches work to snuff out a flame, we explain the various technologies, explore their advantages and drawbacks, and provide some insight into how they work to provide flame resistance.
What is Fire?
We generally think of fire as requiring three separate components: oxygen, heat and fuel. Heat is required to break down the fuel in gaseous components, and then oxygen reacts with the gas that is formed. The heat produced then breaks down more fuel into gases, and this cycle continues until one of the three components—oxygen, heat or fuel—is removed or consumed.
However, the chemical reaction that results in combustion is actually very complex and requires another component that we cannot observe. In order to react with oxygen, the fuel must break apart into highly reactive molecules called “radicals”. These radicals react with oxygen and form more radicals in a chain reaction that results in fire. Therefore, scientifically, fire requires four components: fuel, oxygen, heat and this chain reaction.
The Science of Flame Resistance
By using a deep scientific understanding of fire, fabric manufacturers have developed strategies for flame resistance that are focused on removing one of the components required to sustain the fire. All of these technologies are designed to snuff out a flame and mitigate the risk of injury for the wearer, but each of them has their own set of advantages. Since each of the technologies can be made permanent for the life of the protective garment and will not wash out with laundering, the selection of the technology will depend on the specific hazard and the preference for other attributes.
There is not a technology that is best for all hazard situations. For example, an electrical worker who may be exposed to an electrical arc flash hazard, a petrochemical worker who is exposed to the risk of flash fire and a firefighter who is exposed to a longer duration fuel-fed fire may all require different technologies to their FR protective clothing. While taking into account industry requirements, work environments, comfort and employee preferences, manufacturers create FR fabrics that are both tailored to thermal hazards that may be present on the job site and that offer consistent protection day-in and day-out.
There are three technologies that are commonly used to create FR fabrics: char-forming agents, gas-phase radical scavengers and high-temperature fibers.
Char-Forming FR Agents
Char-forming agents are phosphorous-based fire retardants that can be incorporated into fibers or fabrics. They are primarily used with cellulose fibers such as cotton, rayon or lyocell. When these fabrics are exposed to fire, and the fibers begin to break down into gaseous fuel, the phosphorous agent reacts with the fuel molecules to form a stable, solid char. The char not only consumes the gaseous molecules, which would otherwise be fuel for the fire, but also creates a barrier between the flame and the fabric and prevents the fabric from further breaking down and releasing more fuel. Therefore, these FR agents form flame resistant materials by depriving the fire of one of its four necessary ingredients, in this case, fuel.
These FR agents can be incorporated into synthetic cellulosic fibers during fiber formation or engineered onto fibers after fabric formation. The engineering process involves saturating the fabric with the phosphorous-based FR agent solution, and then conducting a chemical reaction to form durable bonds that results in a permanently flame-resistant fabric. The engineering process allows the fabric to retain its original look and comfort benefits that are so desired in cotton and rayon fabrics, while providing permanent FR protection against short-term thermal hazards.
Gas-Phase Radical Scavengers
Another technology for suppressing flame is the gas-phase radical scavengers. These are special molecules, usually chlorine-based, that are part of the fiber structure and have the unique property of forming relative stable radicals when exposed to heat. Recall that the fuel must break apart into radical molecules in order to react with oxygen. As these fuel radicals are formed, they will combine with the chlorine radicals instead of oxygen and become trapped or “scavenged”. This deprives the fire of the chemical chain-reaction that is one of the required components to maintain a flame.
These chlorine atoms are typically incorporated into an acrylic fiber forming a fiber commonly known as “modacrylic” (modified acrylic). Fabrics designed with modacrylic fibers generally maintain their softness and are easy to dye for branding and visibility. Additionally, since the FR protection activates in the gas phase, which occurs above the fabric, non-FR fibers are easily blended with FR fibers to provide effective protection at a more desirable price point. Many fibers blends incorporating modacrylic and other fibers currently available to optimize the desired properties.
High Temperature Fibers
These durable fibers have a molecular structure that does break apart when exposed to heat until it reaches very high temperatures of 750 degrees Fahrenheit or higher. They provide flame resistance by depriving the fire of its fuel by simply remaining intact at the temperature of most thermal events. FR benefits are a part of the fiber’s very structure, and as such, cannot be washed out or removed.
Aramid fibers are the most widely used, high-temperature fiber, but there are many different fiber structures available. Since there are no fire-retardant chemicals to consume, these fabrics can withstand thermal events that are longer in duration. For this reason, high-temperature fibers are frequently used in military flight suits and firefighter turnout gear, where exposure times can be longer than typical flash-fires. This class of fiber makes these fabrics easily the most expensive FR technology; however, the resulting fabrics are very durable and provide excellent FR protection.
These fibers also are relatively stiff and have minimal moisture absorption, which results in a generally lower perceived comfort. But despite these limitations, fabrics made from high-temperature fibers are used extensively in many industries throughout the world due to their durability and excellent fire protection properties.
While each technology can provide excellent protection from thermal hazards, leading textile manufacturers with access to multiple technologies can develop fabrics that incorporate more than one technology into a protective textile. Combining two FR technologies into a fabric enables a manufacturer to utilize the best qualities and strengths of each technology to tailor optimized solutions for specific workplace needs in one fabric. Utilizing combinations of multiple technologies also allows manufacturers to meet industry requirements while offering performance characteristics, such as durability, lightweight, breathability or mobility.
All three FR technologies are able to help protect wearers from burns should a short-term thermal incident occur. Even though they offer critical protection, it is essential to specify the FR fabric that works best for your unique situation—not the other way around. Safety managers can specify the FR fabric that goes into PPE garments, which helps ensure that the garment will address the needs of your risk assessment. Identifying potential worksite hazards, environmental conditions and desired comfort features all contribute to selecting a durable and effective FR fabric.
Exploring the science of a flame unravels the mystery of fire into key principles that influence FR fabric creation. Both textile manufacturers and safety managers can benefit from a better understanding of fire science to help guide technology development, deployment and selection of PPE through the lens of site-specific needs. Armed with this information, the PPE industry can provide those on the job with the best-possible FR fabrics to protect them when they need it most.
This article originally appeared in the April 2020 issue of Occupational Health & Safety.