A closer look at how composites are constructed
By Todd Herring
Firefighters spend a lot of time in their turnout gear from completing academy training to serving in the line of duty. They know what makes a good-fitting garment, but how many know what process goes into coats and pants that can withstand harsh conditions and wear? As a manufacturer of head-to-toe protection apparel. Keep reading to get acquainted with the materials and methods that matter in an emergency—factors that allow firefighters to lunge, climb, or crawl smoothly through the fireground while eliminating their exposure.
Three main layers
Turnout coats and pants are constructed with three main layers: an outer shell, a moisture barrier, and a thermal liner (this three-layer combination is known as a “composite”). The various properties of these layers work together to protect firefighters from heat, flames, and other occupational hazards. Garment design and performance requirements are set by the National Fire Protection Agency (NFPA).
The outer shell is the first layer of protection. Turnout gear is constructed using several textile layers, each being engineered with a blend of different fibers and yarn to provide the desired performance attributes.
A typical flame-resistant outer shell is constructed using spun yarns or a combination of spun and filament yarns. Spun yarns are short fibers crimped and twisted together, whereas filament yarns are composed of continuous, cable-like fibers. Spun yarns tend to provide softer, more flexible fabrics while filament yarn, being one continuous strand, is typically stronger and smoother to the touch.
A mill then weaves fibers into the finished outer shell fabric. To enhance the fabric’s tensile and tear strength, many products on the market today use filament yarns in novel ways to provide robustness and abrasion resistance. Other fabrics utilize special filaments such as KEVLAR® in a twill weave (a pattern of diagonal parallel ribs) to give the fabric a softer feel and improved flexibility.
Each fiber brings different characteristics to the equation be it enhanced thermal stability, improved strength, and resilience, or better maneuverability and comfort. Manufacturers engineer fabrics by blending the fibers to achieve the desired result. For the end-user, it is important to identify what characteristics are most important as garment weight, comfort, protection, durability, and cost are all impacted by the fibers and yarns used in its construction.
Notable is that the outer shell also provides limited protection against water penetration. Fabric mills add a durable, water-resistant finish to the woven outer shell materials. The NFPA 1971 standard requires all outer shell fabrics to maintain a minimum performance level after five launderings of no more than 15% absorption of water (formerly 30% absorption or less; American Association of Textile Chemists and Colorists (AATCC) 42-Spray Rating). Most outer shells far exceed this requirement.
The moisture barrier is an integral part of the turnout composite as it serves two purposes: 1) to prevent liquid penetration of water, blood, body fluids, and common fireground chemicals and 2) to alleviate heat stress allowing moisture vapor to escape from the garment. A moisture barrier both protects the firefighter from extreme hazards and keeps the wearer as cool and comfortable as possible.
Moisture barriers are engineered using a membrane film laminated to a lightweight, flame-resistant fabric. Various fabric substrates give strength to the barrier and may contribute to the thermal protection of the composite. The membrane component provides a breathable and liquid-resistant property while allowing moisture vapor to escape.
The moisture barrier also protects against liquid penetration. The NFPA requires moisture barrier fabric and seams to be tested for liquid penetration resistance to common chemicals identified as battery acid, gasoline, hydraulic fluid, aqueous film-forming foam (AFFF), and a swimming pool chlorine solution. The moisture barrier and seams cannot show any penetration of the test chemicals for at least one hour.
Seam construction is also critical in maintaining the durability and protection offered by the moisture barrier. These barriers require the second step of manufacturing construction—seam sealing—that is critical in preventing liquid penetration through the seam stitch holes. Seams are sealed using a sealing tape designed specifically for the barrier itself. The seam tapes are applied using barrier manufacturer-provided seam-sealing equipment.
The third layer, the thermal liner, provides both insulation and moisture management. The layer closest to the wearer is the facecloth. This is made from a thin but durable layer of flame-resistant cloth— either an aramid fiber like NOMEX (a class of strong, heat-resistant synthetic fibers) or a para-aramid fiber such as KEVLAR (offering even higher strength than aramid fibers). Facecloths may also use a blend of NOMEX, KEVLAR, and other fibers that enhance the strength or moisture management capabilities of the fabric. The facecloth is then quilt stitched to a batting material, similar to a blanket. The batt is also made from nonwoven, spunlaced, flame-resistant fibers (Basofil or Polybenzimidazole (PBI) for example) and can vary in thickness or have multiple thin layers that provide varying degrees of thermal resistance.
The batting creates a layer of air that functions as a heat barrier. Thermal liner facecloths can also provide a wicking property that draws perspiration away from the wearer, allowing sweat to be absorbed and spread out into and through the batt where it can be transported out by the moisture barrier. Wicking not only allows the wearer to cool down but keeps liquid sweat from building up on the body, thereby reducing the chance for steam generation and a resulting burn.
Some thermal liner facecloths are woven with filament yarns and may be referred to as high-lubricity facecloths. These facecloths are smoother and therefore easier to don and doff or to maneuver in. However, they may not wick perspiration as easily as nonfilament facecloths. Nonfilament facecloths and filament facecloths provide different properties when used with different batting.
Also, pay attention to high-compression areas
A high-compression area is anywhere constant pressure is applied to a composite. For instance, shoulders — when wearing a Self-Contained Breathing Apparatus (SCBA) and knees (when crawling) — become high-compression areas. Compression reduces the protective qualities of a garment and as a result, NFPA 1971 requires these areas to be evaluated for Conductive Compressive Heat Resistance (CCHR).
CCHR testing ensures that when compressed, the turnout gear will provide adequate heat and thermal protection. The test requires that the shoulder reinforcement areas be tested under a pressure of two psi, which simulates a firefighter wearing an SCBA. Knee reinforcement areas are tested under a pressure of eight psi, representing the amount of force applied by a 180-pound firefighter when kneeling.
To provide maximum protection in these potential danger zones, extra layers may be added to the inside of the thermal liner:
• Beneath the trim at the cuff of each sleeve, reducing the risk of burn due to moisture under the trim.
• To the elbow area, helping to ensure blockage of liquid penetration due to compression.
• At the knee area (an extra layer of thermal liner and moisture barrier), providing additional heat and moisture protection when kneeling or crawling.
• At the shoulders, reducing heat transfer when compressed by the straps of an SCBA.
Find your fit
Behind every filament and fiber that firefighters wear into the heat of the action is an industry striving to ensure that the next generation of coat and pant composites works even harder for them. If you have questions about turnout gear, ask your equipment supplier to arrange a look at the latest technologies including how garments are designed and manufactured.
Todd Herring is V.P. of Product Innovation and Strategy at Fire-Dex. FireDex.com offers more information about turnout gear composites built for today’s changing needs.