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Thermal stability of structural turnout gear’s component materials is an important factor when selecting new gear. In general, thermal stability describes a material’s ability to remain unchanged when exposed to heat. In relation to firefighting gear, thermal stability is often used to explain changes in the performance of turnout gear after exposure to heat. Materials used in structural turnouts can be compromised by a multitude of thermal conditions – from sudden flashover to extended heat exposure in sub-flashover conditions (i.e., extremely high temperatures for short periods or less extreme temperatures for longer periods). Thermal stability is an important indicator of the durability of the protection provided by your gear.
Structural turnout gear does much more than protect you from extreme heat. Each layer in the ensemble’s composite – the outer shell, thermal liner and moisture barrier – provides different aspects of protection. The outer shell is your first line of defense for many hazards, including protection against cuts, tears, punctures and direct flame. The thermal liner provides the greatest resistance against heat transfer through the garment to your skin.
The moisture barrier serves two roles:
• It keeps the thermal liner dry by blocking liquid penetration from water, blood, body fluid and the five common chemicals listed in National Fire Protection Association (NFPA) 1971, Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting, 2007 edition.
• It helps reduce heat stress by allowing internal moisture vapor from perspiration to escape from the ensemble.
Because of the specific types of protection these three components offer, they require different materials and constructions. Today’s outer shells and thermal liners are made primarily with fibers that are inherently flame and heat resistant. Moisture barriers, however, are constructed with more than just inherently flame- and heat-resistant fibers. They generally include a polyurethane layer, which tends to be more susceptible to change from thermal exposure. Therefore, the technology used to engineer moisture barriers is very important to their thermal stability. Thermally stable moisture barriers are more likely to maintain their performance properties, especially breathability, after being exposed to heat.
The NFPA 1971 Standard specifies the minimum performance requirements for new turnout gear. Within this standard, certain performance requirements specify pre-conditioning of the test samples as an attempt to address some durability issues. Although you can specify turnout gear with performance requirements that exceed the standard, you may also want to ensure that your gear maintains its performance throughout its useful life. Change in components due to thermal exposure can affect the overall performance or durability of your structural turnout gear. Therefore, when selecting the most appropriate gear for you or your department, it is important to consider the performance of each component before and after heat exposure.
Thermal stability of the outer shell
The outer shell is engineered to provide protection against flames as well as cuts and abrasions from objects you may encounter on the fireground. Today’s NFPA-certified outer shells are made with inherently flame- and heat-resistant fibers, such as Nomex®, Kevlar® and PBI™ fibers. These fibers are spun into yarns and then woven into strong flame- and heat-resistant fabrics. After being exposed to high heat, they still maintain their flame-resistant properties; however, over time, some of these fabrics can become brittle and stiff from heat exposure, which decreases their ability to resist cuts and tears.
Most outer shells will change color when exposed to high heat. A color change in the outer shell can be used as an indicator that the turnout gear has been exposed, which may warrant an examination to determine if it should be repaired or replaced.