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Thermal stability of the thermal liner
The primary purpose of the thermal liner is to provide insulation against the heat you encounter on the fireground. Thermal liners are often made with fibers similar to those used in outer shells. Thermal liners typically include a non-woven structure made by entangling and interlocking the fibers. This type of construction gives the material sufficient loft, allowing it to trap a significant amount of air. The amount of trapped air is a key in providing thermal protection – the more air trapped in the liner, the better the thermal protection, with the air acting as an insulator against heat transfer.
Today, the concern with thermal liners is more often the mechanical stability of the non-woven construction than its thermal stability. Heavy wear, abrasion and compression over the life of the turnout gear can reduce the loft of the thermal liner, which affects its performance. Therefore, it is important to choose a high-quality, well-constructed thermal liner. However, when a thermal liner gets wet, its insulating properties can decline significantly. Keeping the thermal liner dry provides more consistent thermal protection, and the moisture barrier plays a key role in keeping the thermal liner dry.
Thermal stability of the moisture barrier
The moisture barrier prevents water and other fireground liquids from entering the gear, keeping the thermal liner and your body protected from these outside elements. In addition, the moisture barrier’s breathability enables perspiration vapor from your body to escape through the ensemble. This is important because it helps your body manage heat stress, and it helps keep the thermal liner dry so it can continue to provide the appropriate thermal insulation. To maintain consistent breathability throughout the life of your gear, moisture barriers should be thermally stable.
Today’s moisture barriers are usually constructed by combining poly-tetrafluoroethylene (PTFE) membranes, polyurethane layers and inherently flame-resistant textiles. PTFE is an inert, stable material that resists change caused by most types of exposure, including relatively high heat. Polyurethanes, on the other hand, vary in their make-up and can be susceptible to change when exposed to high heat, and some are not stable even when exposed to moderate amounts of heat.
These changes can affect both the breathability and the durability of the moisture barrier. For this reason, it is critical that moisture barriers are engineered to maximize their thermal stability so that these properties do not change over time and/or after use. The specific PTFE and polyurethane materials selected – as well as the technology used to construct the barrier – have a direct impact on the moisture barrier’s thermal stability and its ability to maintain protection and breathability after exposure to heat.
Testing for thermal stability
There are many test methods and specifications that address the performance of new turnout gear; several of these test methods can also be used to evaluate the performance of the individual composite layers after heat exposure. For outer shells, ASTM D5587, Standard Test Method for the Tearing Strength of Fabrics by Trapezoid Procedure, is one method that can be used to compare its strength before and after various types of heat exposure. In this test method, the shell fabric’s strength is determined by measuring the amount of force required to tear the fabric. A decrease in tear strength after heat exposure indicates that the performance of the outer shell has been affected (see Figure 1 on page 92).
For a thermal liner, one of the major causes of reduced performance is change to its physical structure. Visual inspection is one of the best ways to determine if the thermal liner has been compromised. Any sign of pilling or tearing is an indication that the mechanical stability of the thermal liner has been affected.