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The Facts Behind the Performance of Nomex® when Exposed to Intense Heat

A recent Round Table on personal protective equipment (PPE), attended by recognized experts such as Chief Fire Officers, garment manufacturers and weavers revealed the importance of time related to the protective performance of garments in the most feared hazard, the flashover.

Time plays a crucial role in the protection cycle of PPE and so it is important that firefighters consider how their garments behave and interact with the wearer during and after these events to ensure they take the correct course of action and are properly protected.

It is broadly accepted that the risk of being totally engulfed by a flashover or backdraft during a fire fighter’s career is minimal but regardless of this fact, fabric and garment manufacturers are still expected to engineer and manufacture materials and garments that would protect against this unlikely event.

However, the result of developing such protective garments tends to produce clothing that brings other issues such as heat stress, low breathability and low abrasion resistance leading to higher repair costs and faster aesthetic deterioration. George Farenden, former fire-fighter and Consultant with DuPont explains that one of the key enablers to ensuring these two issues can sit comfortably together is the right blend of para-aramid and meta-aramid within the production of the fabric, that produces a material that both protects, even in extreme conditions, but is also comfortable to wear and long lasting. George added, “It is important to remember that fabrics and garments can of course be developed to withstand the most extreme conditions but it should also be remembered that, regardless of the protection, the fire fighter must not be forgotten and that there is a natural limit that the human body can withstand.“

However, technical developments over the last few years have resulted in fabrics like Hainsworth® TITAN, containing a blend of Nomex® and Kevlar® being produced that will not only protect the fire fighter in case of a potential flashover or backdraft but provide breathability and comfort on a daily basis. The Nomex® or meta aramid content of the fabric provides the heat and flame resistance. There may be some misunderstanding about how Nomex® performs within fabrics and how it is designed to absorb the heat. When Nomex® carbonizes, it’s absorbing the heat, translating that heat into a protective char barrier, preventing that heat from entering the garment and the fire fighter’s body. If a char barrier isn’t formed, where does the heat go?

Phases in the performance of the garment

There are three distinct phases in the performance of the garment, happening over a relatively short period of time, that are all designed to protect the fire fighter if exposed to a flash-over or backdraft. In the first phase - FLASHOVER - the garment is exposed to and absorbs significant heat, potentially 800 - 1000°c. This absorption causes the Nomex® fibres to swell and thicken, helping to eliminate conductive heat transfer and ultimately reducing heat transfer to the body. The distinctive performance of the Nomex® fibres reacting to the heat provides extra, valuable seconds of heat protection. During this process the outershell fabric and the level of protection remain intact and flexible enabling the fire fighter to escape and move away from the source of heat.

The most important thing for the fire fighter if exposed to a flashover, regardless of what material the garment is made of, is to get out of the area to a place of safety as quickly as possible and remove the garment (both coat and trouser). During this second phase – ESCAPE - Nomex® remains supple and flexible until it cools, providing extra, crucial seconds of protection. In this phase break open does not occur. But all fire garments, regardless of their construction, will continue to hold the heat and so removal is critical to avoid burns that could be caused due to gradual conductive heat transfer.

During the last phase - RECOVERY - the firefighter removes the outer garments so that he or she can cool down and recover from the incident. Similarly, the garment continues to gradually cool down and release the remaining, still significant amount of heat. Where the heat has been absorbed by the Nomex® fibres the cooling process results in the Nomex® fibres solidifying and then if subsequently flexed they may fracture or crumble, which is sometimes referred to as ‘break open’ – a principle similar to that of a modern car designed to absorb the impact and crumple. For example, a car is not designed to stay intact and look pretty after an accident. It is designed to absorb the energy and crumple, thereby protecting the driver. This reaction is very similar to a garment made with DuPont™ Nomex®. In certain cases it will crumble when cooling down after it has done its job of absorbing energy and protecting the fire fighter: in short, it is designed to protect and not to look pretty after a flash over. The crumbling appearance is a clear sign that the heat has been absorbed and the garment has performed as expected. Most importantly, this characteristic only occurs once the garment is out of the intense heat and the fabric has cooled down.

At the Round Table event, the question of whether a garment that had deteriorated during an actual flashover or backdraft had ever been seen was posed to the panel. It was agreed that this characteristic had never been seen other than in controlled manikin tests.

There are many things to consider when purchasing EN469 compliant garments such as what are the most important and likely hazards fire fighters could face and therefore how much protection is required compared to the importance of comfort, protection against heat-stress, ease of movement and of course, in today’s budget restricted world, durability.

Nomex® is a fibre of choice for fire fighters garments around the globe, as it has been designed to protect. DuPont continues to conduct extensive research to develop fabrics and garments in conjunction with its Nomex® Partners that will protect fire fighters in their multiple requirements so that they can do their job safely and comfortably.

Hainsworth® is a registered trademark of A W Hainsworth & Sons Ltd.