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What is Flame Retardant Yarn and Thread?

The video above features NeC 8/4 staple spun polyester. The untreated thread on the left burns vigorously once ignition temperature is reached, melts, emits black smoke and drips. The treated thread on the right melts and drips, however, resists combustion and flaming, therefore, smoke is greatly reduced, retardant has done its job.

Flame retardant threads and yarns are not fireproof, but they have a special treatment that makes them slow-burning. This means when they are exposed to flame, the treatment will slow down the spread of fire or even self-extinguish.

Rather than a treatment, flame retardant properties can come from chemicals that are added to the chip during extrusion. Most PET flame retardant yarns are produced either using a polymer, which contains the flame retardant chemistry or by using a standard PET polymer and adding the flame retardant chemistry using a masterbatch.

For many years, Trevira CS resin has been the go-to resin for this purpose. The chemistry is typically phosphorus-based and a non-halogen. These additives are typically non-migrating, which means they do not lose their impact over time.

Do Flame Retardant Yarns and Threads Melt?

Although flame-retardant materials may extinguish the flame, they won’t completely prevent melting. Synthetic polymers can melt, run and drip if exposed to temperatures near or above their melting point.

Still, flame-retardant yarns are far less likely to ignite and burst into flame than many other types of threads and yarns. This makes them a good choice for applications where you need to control the possibility of ignition.

Some yarns are inherently designed for smoke prevention and resistance to dripping. This can be important if the materials will be subject to fire safety tests.

Specific synthetic yarns may vary in their characteristics, but here is a list of basic melting points for some common synthetic yarns:

Flammable Synthetic Fibers

Inherently Non-Flammable Synthetic Fibers



Melting Point

( C )

LOI (%)


Melting Point

( C )

Thermal Degradation Temp ( C )

LOI (%)


> 260


Para-Aramids (DuPont™ Kevlar®)

Does Not Melt

> 500

28 - 30


224 - 280


Meta -Aramid (DuPont™ Nomex®)

> 370

> 350

29 - 32

Nylon 6





> 450

28 - 30

Nylon 6,12

217 - 227


PEEK (Zyex)


> 500


Nylon 6,6

254 - 267


PBI (Celanese)

Does Not Melt

> 580

> 41


140 - 155

17 - 19

PBO (Zylon)

Does Not Melt

> 600



256 - 268

20 - 23



2000 - 3500

45 - 60


152 - 175 18.6 PTFE (Teflon) 327 508 95
      Glass 1225 - 1360 850 *
      Modacrylic Does Not Melt > 670 29 - 32

* Sizing promotes flaming, once burnt-off glass does not support combustion
Limiting Oxygen Index (LOI) - Values greater than 21 percent will likely NOT support combustion.

Applications of Flame Retardant Yarn and Thread

Common applications of flame retardant treated yarns include clothing, accessories, and equipment where fire resistance is extremely important. This includes gear for firefighters, police, industrial workers and many other heavy-duty applications.

Military specifications regarding flame-retardant yarns are much stricter than typical standards for ordinary textiles and apparel. Here are two strict mil-specs required for flame retardant materials:

  • MIL-STD-3020 establishes the requirement that fire protection materials, including acoustic insulation and thermal insulation, must be resistant to very high temperatures. However, this mil-spec does not specify flame spread, ignitability, and some other design requirements.
  • The Department of Defense requires that all military apparel be 100 percent made in the U.S. under Berry Amendment standards.

Give careful consideration when choosing flame retardant yarns and threads. Your selection depends on your specific use, safety precautions and environmental setup.

If you need help selecting the ideal material, contact Service Thread for specs and advice.

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