1. Description
The thermal liner is the third of the three principal layers of turnout gear (the other two being the outer shell and the moisture barrier) and is usually the layer closest to the body. It traditionally consists of a non-woven felt or batting quilted or laminated to a woven lining fabric, usually called the face cloth.
2. Function
It is the thermal liner, more than any other component of the turnout gear, that blocks the transfer of heat from the fire fighting environment to the body of the wearer (i.e. the thermal barrier contributes the most to the TPP rating of a bunker suit).
3. Composition
3.1 Face Cloth Fabrics
3.1.1 Spun Nomex®
The face cloth has for approximately 20 years been a spun fabric weighing a nominal 3.3 oz./yd². This fabric has the advantage of being moderately priced but has the disadvantages of absorbing a lot of perspiration and of being slow to dry and of not being slippery (i.e. the bunker gear does not readily slide over the clothing the firefighter wears underneath with the result that movement is impaired and donning and doffing of the gear is difficult.
3.1.2 Multi-filament Nomex
To overcome the disadvantages of the traditional face cloth material, Securitex developed and patented a new light weight, high strength, slippery face cloth made of 100% multi-filament Nomex weighing approximately 2.5 oz./yd². This new fabric makes donning and doffing easier, enhances mobility and comfort, and results in a thermal liner that holds on to less perspiration and that dries quicker. Securitex's trademark for this fabric when used in a thermal barrier is UltraflexTM
(Pat'd).
3.1.3 Blends of spun and multi-filament Nomex
Southern Mills produces a 4.1 oz./yd² thermal liner face cloth called CalduraTM, which consists of spun Nomex yarns in one direction and multi-filament yarns in the other. Although Caldura appears to have been an attempt to capture some of the benefits of a 100% filament Nomex face cloth it is not apparent that Southern has succeeded: Caldura weighs significantly more than a 100% filament
Ultraflex; it tends to form pill balls; it absorbs more water and takes longer to dry than 100% filament; and most importantly it does not provide the same low coefficient of friction as 100% filament. Southern has attempted to enhance the slipperiness of Caldura by treating it with a special finish but it is not clear if this finish will remain effective after repeated washing. Similarly, Southern has announced that it is switching from a twill weave to plain weave in order to reduce fabric pilling.
3.1.4 FR Cotton
Some manufacturers offer a thermal barrier with a FR cotton facecloth in an effort to reduce the cost of turnout gear. Although this particular face cloth meets the performance requirements of NFPA 1971, it is significantly heavier than the Nomex facecloth fabric. As well, it does not have the same wear life as either a spun or filament Nomex facecloth.
3.2 Thermal Insulation Materials 3.2.1 Non Woven Batting
The majority of thermal barriers employ a non woven felt or batting to provide the bulk of the thermal resistance. The thermal resistance is partly a result of the type of fiber and partly a result of the dead air spaces in the batting.
The batting is usually constructed of Nomex fibers (also know as an aramid or a meta aramid) and/or Kevlar® fibers (a.k.a. aramid or para-aramid). It comes in a variety of weights and a variety of proportions of virgin or reprocessed meta and para aramids.
Another non woven thermal insulating material is called E-89® spunlace and consists of 67% Nomex and 33% Kevlar quilted to a face cloth. ( E-89 is the most common substrate for Crosstech®, Breathe-TexTM, and VaproTM breathable moisture barriers.) E-89 is the most expensive of the commonly used thermal insulation materials. Its principal advantages are light weight and a high rate of total heat loss (as measured by the Guarded Sweating Hot Plate test). It is usually used as two or three layers of 1.5 oz./yd² each, quilted to a face cloth.
3.2.2 Closed Cell Foam
One bunker gear manufacturer has introduced a closed cell foam thermal insulation material, made of polyvinyl chloride and nitrile rubber (PVC/NBR). Since closed cell foams are impermeable to perspiration vapor (think wet suit!) the manufacturer has perforated or apertured the foam in order to permit the transfer of perspiration across the insulation. The claimed benefits of the closed cell foam are reduced water absorption and more rapid drying time; not surprising since the foam itself is impermeable. Some critics claim that the foam is less flexible than traditional liners and that it will degrade after exposure to heat levels under 466 ºF. whereas traditional liners show no degradation below 700 ºF. The closed cell foam does, however, meet all the requirements of NFPA 1971 Ed. 1997.
3.2.3 Basofil/aramid batting
There exists a batting consisting of a blend of Basofil and aramid (e.g. Nomex or Kevlar). The data we have seen suggests that this batting provides thermal protection performance similar to that of the 100% aramid thermal battings. For the most part, Basofil/aramid batting has been used with the FR cotton facecloth.
4. Examples of Thermal Barriers
4.1 Nomex Aramid (Example: Southern Mills' Q-9TM
The generic description of this product is a 3.3 oz./yd² Nomex face cloth quilted to 6.0 oz./yd² Nomex aramid batting. The batting consists of a high percentage of reprocessed Nomex and Kevlar.
The advantages of this type of thermal barrier are low cost and good TPP. Disadvantages are higher weight, higher water absorption and higher resistance to the transfer of metabolic heat.
4.2 Kevlar Aramid (Example: Southern Mills' AraliteTM)
The generic description of this product is a 3.3 oz./yd² Nomex face cloth quilted to 3.8 oz./yd² Nomex aramid batting. The batting consists mostly of virgin Nomex and Kevlar with the possibility of a small percentage of reprocessed aramid fiber. (N.B. Although Southern Mills states that the nominal weight of their Aralite barrier is 7.1 oz./yd² the product as received by Securitex typically weighs between 7.3 and 7.5 oz./yd².)
The advantages of this type of thermal barrier are a good TPP to weight ratio, durability and moderate cost.
(Southern Mills offers its Caldura face cloth fabric with either a Nomex aramid batting, a Kevlar aramid batting or multi-layer E-89 thermal insulation.)
4.3 Ultraflex
Ultraflex is the trademarked name that Securitex applies to a thermal barrier consisting of a Kevlar aramid batting laminated to 100% multifilament Nomex face cloth. This thermal barrier combines the thermal and cost advantages of a Kevlar aramid batting with the enhanced comfort and mobility of a multifilament Nomex face cloth. The multi-filament Nomex face cloth is laminated rather than quilted to the Kevlar aramid batting in order to maintain a uniform thickness (quilt stitching compresses the batting ) and hence a uniform thermal resistance.
4.4 SRS ReboundTM
SRS Rebound is Lion Apparel's name for their proprietary closed cell foam (PVC/NBR) thermal barrier. However, rather than being the layer closest to the body it is usually installed between the outer shell and the moisture barrier (see our technical review of moisture barriers). A lightweight Nomex scrim is laminated to the foam so that it can pass the tear strength requirements of NFPA.
4.5 Nomex Omega® Turnout Systems
For its new Nomex Omega Turnout System Dupont does not specify specific materials to be used in any of the layers of the gear, rather it specifies performance requirements for each layer. Our interpretation of these performance requirements leads us to believe that the thermal must be constructed of two or three layers of 1.5 oz./yd² E-89 quilted to a 100% multifilament Nomex face cloth.
5. Trends
For over a decade there had been very little change in thermal barrier design in spite of the recognition of the changing nature of fire fighting and the menace of heat stress. Securitex was the first to make a major innovation when it introduced in 1993 the Ultraflex thermal barrier with its slippery face cloth. The advantages of this lighter, more comfortable liner have not been lost on firefighters or manufacturers. We anticipate that the overwhelming trend of the next ten years will be toward thermal liners that are lighter, that retain less water and dry more quickly, that are comfortable, and that enhance mobility.
More information...
A. Turnout Gear Selection
B. About Moisture Barriers In Turnout Gear
- Nomex, Kevlar, Nomex Omega Turnout System and E-89 are Dupont trademarks for its aramid fibres.
- Ultraflex is a trademark of Securitex Inc.
- SRS Rebound is a trademark of Lion Apparel
- Caldura, Aralite and Q-9 are trademarks of Southern Mills
- Vapro is trademark of Dominion Industrial Fabrics Co.
- Crosstech is a registered trademark of W.L. Gore & Assoc. Inc.
- Breathe-Tex is a trademark of Aldan Rubber
- Basofil is a registered trademark of BASF AG
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