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.

1. Threads that do not burn
2. Threads that burn in flame but self-extinguish when removed from flame
3. Threads that burn, but burn slowly and may self-extinguish

Types of Sewing Thread and their Fire Resistance

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.

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.

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.

Bullet-proof Vests

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.

Aramid Threads

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.

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