Fire and Heat Resistant Threads
Types of Fibres and their Fire Resistance
|Material||Flame Retardancy||Ignition Temp
°C / °F
|Cotton||Combustible, ignites and burns||252 / 485|
|Polyester||Combustible, burns slowly and may self-extinguish||421 / 790|
|Nylon||Combustible, burns slowly and may self-extinguish||449 / 840|
|Meta Aramid||Flame retardant self-extinguishing when removed from flame||-|
|Para Aramid||Flame retardant self-extinguishing when removed from flame||-|
|Fibre Glass||Flame retardant will not burn||-|
|Quartz||Flame retardant will not burn||-|
|Ceramic||Flame retardant will not burn||-|
Cotton threads, which continue to burn at relatively low temperatures, should be avoided at any time if there is a possibility of contact with fire. Standard polyester and nylon threads are combustible, but will burn slowly and may self-extinguish.
Fire resistant threads fall into three categories:
- Threads that do not burn
- Threads that burn in flame but self-extinguish when removed from flame
- Threads that burn, but burn slowly and may self-extinguish
|-||Ceramic||1||Does not burn||NFPA 701 / ASTM E84|
|-||Quartz||1||Does not burn||NFPA 701 / ASTM E84|
|Glasmo Tee||Glass fibre||1||Does not burn||NFPA 701 / ASTM E84|
|Firefly||Meta Aramid||2||Burns in flame but self-extinguishes when removed||NFPA / EN469|
|Protos||Para Aramid||2||Burns in flame but self-extinguishes when removed||NFPA|
|Flame Master||Flame retardant 100% polyester corespun||3||Will not burn for short period, then burns slowly and may self-extinguish||EN 15025|
|Epic RD||100% Polyester corespun||3||Burns slowly and may self-extinguish||CFR 1615 / BS EN 14878:2007|
|Astra RD||100% Staple spun polyester||3||Burns slowly and may self-extinguish||CFR 1615 / BS EN 14878:2007|
Products in category 1 include; glass fibre (bonded and PTFE coated), high silica glass fibre (quartz), and ceramic. These products do not ignite and are therefore useful in end products where the possibility of fire is high e.g. blast furnaces, mattresses etc.
Products in category 2 include Kevlar® and Nomex®. Kevlar® is the brand name for a para-aramid, and Nomex® is the brand name for a meta-aramid; both are special types of nylon. Kevlar® is manufactured by Du Pont. Other para-aramids include Twaron® (Acordis) and Technora (Teijin). Nomex® is also made by Du Pont; other meta-aramids are Teijinconex (Teijin). These products will normally burn in flame but will immediately self extinguish on removal from the flame. They are therefore useful in end uses where the possibility of fire is high.
Products in category 2, while not as fire resistant as category 1, have much better sewing performance. They are also much softer making them a more suitable choice for protective clothing and upholstery.
Products in category 3 include Coats Epic RD and Coats Astra RD. These are combustible threads manufactured to minimize burning and promote self extinguishing after removal of flame. These products meet fire specifications (such as CFR1615 for children’s sleep-wear).
Fire resistant testing
There is no way to test thread itself for fire resistance; it must first be sewn into fabric. In conducting any test, it is best to use the exact fabric, thread, and seam construction to be used in the finished product.
Fire Resistance Standard for Various Thread Applications
|Uniform and Building Code Standards||UBC 8-1 (42-2), 8-2(42-2), 26-1(17-2), 26-3(17-5)|
|Aviation||FAR 25.853, 25.855|
|General (Buildings, transport, interior furnishing, protective clothing)||NFPA 701 Standard Methods of Fire Tests for Flame Propagation of Textiles and Films
ASTM E84 (Standard Test Method for Surface Burning Characteristics of Building Materials)
|CFR 1615: Standard for the flammability of children's nightwear: sizes 0 through 6x (FF 3-71)
CFR 1616: Standard for the flammability of children's nightwear: sizes 7 through 14 (FF 5-74)
BS EN 14878:2007 (Nightwear Safety Regulations)
|Protective Clothing||EN 531 Protective clothing against industrial heat
EN 407 Protective gloves against thermal risks
EN 366 Protective clothing against heat and flame
Test method: Assessment of clothing materials and material assemblies exposed to a radiant heat source
EN 367 Protective clothing against heat and flame
Test method: Determination of the heat transmission on exposure to flame
EN 373 Protective clothing against heat and flame
Test method: Determination of the resistance of materials to molten metal splash
EN 469 Protective clothing - Requirements for fire fighter’s protective clothing
EN 533 Protective clothing - Protection against heat and flame - Limited flame spread materials and material assemblies
Heat resistant and fire resistant are two separate characteristics
However it is possible for a thread to be both fire resistant and heat resistant. In fact, many of the threads already mentioned as fire resistant are also heat resistant.
In general synthetic sewing threads (e.g. nylon, polyester, Kevlar® & Nomex®) are all good resistors of heat. This means that heat will not pass through them as effectively as it may pass through a substance such as silver or copper. However, as temperature increases, some polymers (such as nylon and polyester) will melt and lose strength before other polymers (such as Nomex®). Their reaction to heat is inherent in their chemical make-up.
Many threads will resist high temperatures and continue to function well in the seam, e.g. aramids, co-polyimide (P84™), glass fibre (bonded and PTFE-coated), quartz, and ceramic. These products remain functional (in the case of ceramic or quartz) up to temperatures in excess of 2000°F (see Table 4).
Nylon and polyester are thermoplastic and start to soften well below their melting point. This means that nylon and polyester seams will begin to weaken around 350°F, while aramid seams will remain as at room temperature, even in excess of 500°F.
As shown in Table 4, the trade-off for better heat resistance is sewing performance.
Maximum Operating Temperature for Various Fibres
|Material||Max. Operating Limit °C / °F||Constant Operating Limit °C / °F||Sewing Ability|
|Quartz||1093 / 2000||1093 / 2000||Slow speeds, fibres may break|
|Ceramic||1371 / 2500||1371 / 2500||Slow speeds, fibres may break|
|Glass Fibre||538 / 1000||538 / 1000||Breaks easily. PTFE coating used for difficult operations|
|P84® Polyimide Fibres||260 / 500||260 / 500||Good|
|Meta Aramid e.g. Nomex®||371 / 700||204 / 400||Good|
|Para Aramid e.g. Kevlar®||427 / 800||204 / 400||Good|
|Nylon||177 / 350||121 / 250||Excellent|
|Polyester||177 / 350||121 / 250||Excellent|
The Coats Technology Centre is experimenting with nylon threads for automotive applications. High amounts of copper in these threads will act as a heat sink and improve the thread's resistance to heat, for instance in the deployment of an airbag.
Nowadays, bullet-proof vest manufacturers use Kevlar®. This allows for lighter-weight, more wearable body armour. There are other materials available for soft-body armour that are cheaper than Kevlar®, but which require a trade-off in weight, comfort and/or bulk.
To understand how Kevlar® works, imagine several wooden sticks. You can break them one at a time with your fist. However, if these sticks are bundled, none can be broken. When Kevlar® is woven into cloth and layered; a bullet encounters many threads at once. The denser the weave (the more threads per unit area), the more bullet resistant it is.
A small bullet fired at Kevlar® encounters fewer threads than a large bullet. A fast small bullet (with the same total energy of a large slow bullet) will penetrate body armour to a greater extent. A .357magnum fired from a revolver is therefore easier to stop than a .22 magnum from a rifle.
Additionally, harder bullets do not deform as much on impact, and penetrate to a greater extent than soft ones. A deformed bullet will encounter more Kevlar® and is more likely to be defeated.
There are two main aramid threads - para-aramids (e.g., Kevlar®), and meta-aramids (e.g., Nomex®). While both products are widely used for heat and fire resistance, they have slightly different properties in terms of thread performance.
The table below describes these products' advantages and disadvantages.
|Meta Aramid||Flame and heat retardant
Sewing performance and physical properties similar to nylon
Good UV resistance
|Lower tenacity than nylon
Much lower than para aramid
|Para Aramid||Flame and heat retardant
Very high tenacity
|Poor UV resistance
Poor abrasion resistance
Low loop strength
Low stretch can sometimes be a disadvantage
Aramids come in a number of forms:
- Continuous filament -- generally the heavier the yarn, the lower the cost, e.g., 138 (1500/1) bonded aramid is significantly lower in cost than a 138 (400/4) bonded aramid
- Spun staple
- Stretch broken - a hybrid form consisting of six-inch length staples, which have been made by stretching and breaking continuous filament Kevlar®, and then spinning these staples together.
Glass Fibre Thread
As well as having excellent heat resistance, glass fibre (particularly the PTFE-coated form) has excellent chemical resistance. This makes it especially suitable for high-temperature filtration. Glass fibre thread also has a high tensile strength. The trade-off for excellent heat resistance is brittleness and difficult sewability. Finishes, such as PTFE, improve the sewing performance.
Table of Products
|Coats Brand||Material||Product||End Use|
|Protos Steel||Steel||Kevlar® coated special steel||High temperature insulation up to 1093°C / 2000°F|
|Helios||Steel||Cotton or Kevlar® coated stainless steel||Ducting|
|Glasmo Tee||Glass Fibre||Bonded||Filtration, blast furnaces, mattress, ducting|
|Glasmo PTFE||Glass Fibre||PTFE Coated||Filtration, blast furnaces, mattress, ducting|
|Firefly Metaspun||Meta Aramid e.g. Nomex®||Staple spun or stretch broken||Protective clothing|
|Firefly Metafil||Meta Aramid e.g. Nomex®||Bonded continuous filament||Filtration, airbags, upholstery, protective gloves / footwear, electrical motors|
|Firefly Paraspun||Para Aramid e.g. Kevlar®||Staple spun or stretch broken||Airbags, gauntlets, bullet proof vests|
|Firefly PF||Para Aramid e.g. Kevlar®||Bonded continuous filament||Airbags|
|HTF||P84® Polyimide Fibres||Bonded continuous filament||Filtration|
|Epic RD||100% Polyester corespun||Lubricated||Children's nightwear|
|Astra RD||100% Staple spun polyester||Lubricated||Children's nightwear|