CLOTHING

Introduction
As we mentioned in the previous section, bunker gear is a three-layer system consisting of an outer shell, a moisture barrier and a thermal barrier. The intention of this article is to give you an overview of moisture barriers: what are they?; how do they work?; what types are there?

Moisture Barriers
Moisture barriers are typically the second layer in bunker gear. They consist of a film or coating applied to a substrate. The film or coating is either semi-permeable (breathable) or impermeable (non breathable). The substrate is either a woven fabric, e.g. Nomex ripstop, or a non-woven fabric, e.g. E-89 aramid spunlace. Examples of breathable moisture barriers are Crosstech®, Vapro^TM, Breathe-Tex^TM, and CrossTech®. An example of a non breathable moisture barrier is neoprene on polyester/cotton.

The most common orientation of breathable moisture barriers is with the substrate toward the outer shell and the breathable film toward the thermal barrier and the body of the wearer. This orientation provides better protection for the fragile moisture barrier film as well as increases the moisture vapor (i.e. perspiration) transmission potential of the moisture barrier film.

The moisture barrier is the second line of defense in the firefighter garment. It provides some burn protection because of the insulation value of the substrate as well as the ability of the film/coating to reduce the passage of hot gases or liquids. However, its principal function is to increase firefighter comfort and protection by preventing fire ground liquids from reaching the skin of the firefighter.

Breathable vs. Non-Breathable Moisture Barriers
Breathable moisture barriers offer some clear advantages over non breathable moisture barriers:

1. Breathable moisture barriers are lighter than non breathable ones -- approx. 4.0- 4.5 oz/yd˛ for Crosstech, Breathe-Tex and Vapro vs. approx. 11.0 oz/yd˛ for neoprene on poly/cotton. This translates into a lighter garment that is more comfortable and that reduces heat stress.
2. Gear with a breathable moisture barrier is cooler than gear with a non breathable barrier. The escape of perspiration vapor through the breathable moisture barrier allows heat to escape from the firefighter's body thereby reducing the rate of metabolic heat build-up.
3. Our interpretation of the research is that breathable moisture barriers reduce the incidence of burn injury. By allowing perspiration to escape from inside the gear, the inner liner stays drier and the risk of burn injury may be reduced. Experiments conducted for NFPA to study the behavior of bunker gear materials exposed to moderate temperatures and low heat fluxes for extended periods of time have shown that the time to pain and second degree burn was longer when the composite of outer shell, moisture barrier and thermal liner included a breathable moisture barrier rather than a non breathable one.

How Do Breathable Moisture Barriers Work and How Effective are They?
The most commonly used breathable moisture barriers are micro-porous semi permeable membranes. The membrane or film is perforated with thousands of microscopic passageways (micro-pores) which are too small to permit the passage of liquids but are just large enough to permit the passage of vapors. On the side of the moisture barrier closer to the body the vapor pressure (i.e. the relative humidity) and the temperature are higher than they are on side of the barrier further from the body. This temperature and pressure differential between the two sides of the membrane is the force that drives the perspiration through it.

(There are also hydrophilic (water-loving) and casted moisture barrier films which act slightly differently but whose underlying principle is the same. Furthermore, some moisture barriers are hybrids having both micro-porous and hydrophilic properties, e.g. Crosstech)

Breathable moisture barriers can handle a finite amount of moisture vapor - of the order of three to four litres per square meter of surface area per 24 hours. When this limit is reached the moisture barrier "wets out", that is moisture vapor begins to condense on the inner surface of the moisture barrier . This condensed moisture vapor also increases the resistance of the moisture barrier to the transport of moisture vapor across the membrane. pass the excess perspiration will begin to accumulate in the thermal barrier.

A firefighter in a working fire will generate more perspiration than the moisture barrier can pass with the result that the moisture barrier will soon "wet-out". At this point, the firefighter's turnout gear is probably no drier than if he were wearing gear with a neoprene moisture barrier. However, the gear will still be lighter and more supple, and will dissipate more heat than if it had a non breathable moisture. Furthermore, the recovery time of the firefighter will be shorter if his gear has a breathable moisture barrier.

Principal Moisture Barriers in Use in the Fire Service
Moisture barriers fall into two categories, those based on a polyurethane film technology - e.g. Vapro, Breathe-Tex and, Aquatech; and those based on a polytetrafluorethylene (PTFE) film technology - e.g. Gore-Tex, CrossTech.

Both categories of moisture barriers have similar weights, appearance and breathability. Both categories of films can pass the test requirement for resistance to blood-borne pathogens, e.g. HIV, Hepatitis, as prescribed in NFPA 1971.

The PTFE films have, by nature, chemical resistance to a wider range of substances and a higher melting point than do the polyurethane films. However, the relevance to firefighter safety of this wider chemical resistance and higher melting point is not immediately obvious in the context of bunker gear (the melting points of moisture barriers is not a test requirement of the current edition of NFPA 1971).

The most significant difference between the polyurethane and PTFE moisture barriers is price with the PTFE barriers being the more expensive.

There are many good moisture barriers available for the fire service; discuss with your bunker gear supplier which one best meets your concerns and budget.

More information...

A. Turnout Gear Selection
C. About Thermal Liners

• CROSSTECH is a registered trademark of W. L. Gore.
• VAPRO is a trademark of Dominion Textiles
• BREATHE-TEX and AQUATECH are trademarks of Aldan Rubber.

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