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Hot and cold

Features | December 10, 2013 | By:

New technology and materials offer advanced thermal control and protection.

For most of us it’s simple enough to dress properly for the weather and our working conditions, but for workers in extreme conditions—both hot and cold—it gets more complex. In a cold environment, layers of sometimes bulky clothing will keep the worker warm, but if he or she is active and sweating, being bundled up can be burdensome and uncomfortable. In an environment with exposure to extreme heat, protective clothing is a requirement, but extra clothing and gear can cause overheating.

Whether in average temperatures, extreme heat or a subfreezing environment, the same principles apply: how much and what type of clothing is necessary for the level of exposure?  The advanced textiles industry is addressing the question of balancing safety and comfort with new materials that make responding to the dilemma considerably easier.

Measuring effectiveness

Thermal comfort is determined by a number of factors, mainly how one’s apparel and metabolic heat production during activities responds under four environmental factors: air temperature, water vapor pressure, mean radiant temperature and air velocity. Technically, the measurement of these variables gives an insulation value, called “clo,” guiding us on how effectively the clothing maintains body temperature equilibrium. Simply stated, clo is the standard amount of insulation required to keep a resting person warm in a windless room at 70 degrees F (21.1 degrees C).

In extreme heat, such as in firefighting, thermal protection performance (TPP) is the relevant measurement, based on an algorithm that determines the time it takes for heat to penetrate to cause second-degree burns—the higher the TPP, the more heat protection. A second factor, Total Heat Loss (THL), has an inverse relationship with TPP because TPP is concerned with heat loss through the material/ensemble and THL is concerned with the heat and perspiration being dissipated from the skin to the outside.

Extreme heat

More fire service personnel die of heat stroke each year than of direct exposure to fire. The usual heavy, multilayer ensemble of a firefighter can create problems. The ensemble normally consists of five layers:

  1. A base layer worn next to the skin (underwear).
  2. The secondary layer, which is the firefighter’s station wear (pants and shirt).
  3. An aramid needle felt (typically) thermal barrier to block heat from penetrating.
  4. A vapor barrier (such as GORE-TEX® PTFE film) to keep moisture out in the generally wet environment of firefighting.
  5. The outer layer or turnout gear, the first line of defense when a firefighter is exposed to the heat, which is designed to not burn and to keep heat from reaching the body.

The outer layer(s) keeping the heat and moisture out and the inner layers trying to pass moisture (perspiration) out to cool the body collide, resulting in the body’s internal cooling mechanism not working, which can limit the amount of time a person may be able to work before having to rest and cool down.

Numerous suppliers are working to create lighter, more abrasion-resistant and effective heat-barrier materials for the outer layers and heat and moisture barriers that are more comfortable to wear. Nomex®, Kevlar® and Twaron® aramids, and aramid/PBI (polybenzimidazole) blends are widely used.

Other companies are working to improve the performance of the base layer and thermal barrier. Cotton is the material of choice for underwear for most firefighters; it is inexpensive but is slow to dry and absorbs odors. Other materials, among them FR modacrylic blends, may wick a bit better, and phase change materials (PCM) that absorb and release heat at certain levels may be effective in certain scenarios.

Several studies have shown that specialty fabric underwear, while effective for athletes, when worn under turnout gear in strenuous conditions of active firefighting may help a little, at best, so cooling off periods are still required. PCM materials may be most effective in cooling the body quicker during the less strenuous period.

Extreme cold

Like protection from heat, protection from extreme cold also requires layering:

  1. A base layer of soft material worn next to the skin to provide some insulation and made with synthetic materials that wick moisture away. PCMs are used in a variety of formats and are generally effective in this area.
  2. Mid-insulating layers may be combinations of materials—sweaters (wool or otherwise) fleece such as Polartec®, down or other insulating material—depending on the activity.
  3. The outer layer is a rugged, windproof material that is also breathable. It may or may not have insulation, depending on the circumstance and intended use. Hoods, gloves and footwear must also be considered.

Military influence

Many protective materials in everyday use had their beginnings in the military, a circumstance that continues. Typical of this is the Fire Resistant Environmental Ensemble (FREE) developed for the U.S. Army, a multifunctional, multilayered system of garments that fit over the current Army issue duty service uniforms for aircraft personnel, as well as those in mounted vehicles (tanks). FREE consists of seven FR layers:

  1. FR-blend underwear next to skin.
  2. A base layer of a long sleeved top and drawers composed of Nomex/spandex mesh.
  3. Midweight layer of long sleeved top and bottom of Nomex/FR polyester fleece.
  4. Duty service uniform layer with FR options for air and mounted personnel.
  5. Lightweight outer layer of FR wind-resistant nylon/cotton/Lycra® blend.
  6. Intermediate weather outer layer of fleece jacket, pants and vest of the nylon/cotton/Lycra blend.
  7. For extreme weather, an outer layer of a shell jacket and pants of FR rayon/para-aramid/nylon outer shell, PTFE membrane and Nomex knit jersey backing.

A similar non-FR cold weather ensemble, the Army’s Extended Cold Weather Clothing System (ECWCS) is similar to FREE but is intended for cold weather wear where fire resistance may not be needed.

Industry consultant Moe Larrivee, a retired administrator with U.S. Army Natick’s Individual Protection Directorate, points out that “with the winding down in Afghanistan, the Army is contemplating combining the better features of FREE and ECWCS and making it one uniform for the future.” The Marines and Air Force have their own systems and may also be looking to consolidate and replace their extreme cold weather systems.

Military technology often filters to the civilian sector for protection in various activities. Likewise, some civilian technology has been adopted for use by the military, reducing substantial development costs.

Moisture management

There have been a number of materials developed for moisture management and “wicking,” primarily for active wear garments. Polypropylene and polyester fibers—some recycled—are used in constructions that wick the moisture from the body.

Phase-change materials (PCMs) are not altogether new, but there have been improvements. Outlast, a pioneer in the field, has developed techniques and fabrics for high heat and extreme cold conditions. Heather Manuel, director of sales and marketing for Outlast Technologies LLC, says she “wants people to rethink the term ‘moisture management.’” Instead of thinking only about reactive wicking technologies, think proactive solutions, she says. She explains that PCMs cool by absorbing the heat from the body, reducing the potential to perspire—a proactive rather than reactive behavior.

Traditional wicking materials require the body to sweat before cooling by moisture transfer can begin, Manuel says. Less sweating, by using PCMs, results in cooler body temperatures. Releasing that heat provides a warmer garment in cooler weather.

George Kelnhofer, vice president of Gehring-Tricot Corp. has worked with several garment manufacturers to utilize PCMs in more unusual ways. Combining PCMs with FR modacrylic yarns and/or para-aramid blends provides the benefits of PCMs but enhances thermal performance as well. Working with Kraton Performance Polymers Inc. and Kenyon Consumer Products, they have jointly developed knit fabrics with proprietary modacrylic yarn blends that have antistatic and other properties, concentrating the PCMs in a special way on the side worn next to the skin. The PCM side provides greater heat absorption while the modacrylic fabric allows increased wicking of any moisture trapped in the fabric.

Andy Curtis, president of Kenyon Consumer Products, works with Gehring to incorporate the new materials into garments, including an extensive line of FR, camping, military and thermal products. Kenyon uses the proprietary modacrylic blend fabrics that include PCM materials for their FlameShield™ and other garments (consumer and military) for various levels of protection, meeting the ASTM D-6413 standard for no melt/no drip FR protection. A similar product is produced in a grid fleece, said to be the first commercial FR grid fleece for no melt/no drip standards. The fabric is included in the U.S. Marines FR Cold Weather system.

The future

One major possible future development in heat insulation is an old one with new legs: aerogels, the lightest solid material, with exceptional insulation properties and able to resist temperatures up to 3,000 degrees F. Aerogels are silica-based nano products composed of 90 percent air. The material resists compression, retaining 85–90 percent of its height under pressure and providing the highest level of insulation.

Aerotherm, marketer of aerogels, says they are 2–8 times more effective than other insulation, and without loft. Produced by Aspen Aerogels Inc. in thin (2mm–3mm) sheets or shapes, the product offers great potential for lightweight outdoor garments and for use in high-heat protection applications.

In a garment, when the knee or elbow is bent, conventional insulation can collapse and lose effectiveness. Aerogels can be made into specifically-cut shapes and, when used in conjunction with other materials, can provide uninterrupted insulation in those spots. Due to its high cost, it is limited for now to niches, such as footwear and inserts, but expanded use of aerogels in more applications is something to look for.

Thermal regulation is an area of intense interest to technical textile producers. Many textile, chemical and garment companies are heavily involved in developing new products, not only for the markets and applications where high-level protection is required, but also products for athletes and consumer leisure activity use. Even routine work wear manufacturers are moving toward more high performance materials, such as increased breathability and fire retardancy. Eventually, some of these new products may become mainstream, offering wider market opportunities.

William C. Smith is a technical textile consultant.

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