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Flame Retardant vs. Flame Resistant Threads And Yarns

Untreated continuous multifilament polyester yarn, on the left, burns vigorously when the ignition temperature is reached, then melts, emits black smoke, and drips. FR treated continuous multifilament polyester, in the middle, shrinks away from the flame, melts, and drips, but it resists flaming once the source is removed. Therefore the smoke is greatly reduced and the retardant has done its job. Untreated aramid yarn, on the right, burns with difficulty because of its high LOI, so the flame extinguishes when the heat source is removed. Aramid does not melt but decomposes, showing signs of thermal degradation.

At Service Thread, we’ve noticed that people often use the terms “flame retardant” and “flame resistant” interchangeably. However, they are not the same thing. In fact, knowing the difference between them could impact your choice of materials for certain applications.

Here’s an introduction to what each of these terms means and how their characteristics will affect your selection of industrial yarns and threads.

Definition of Flame Retardant

When a yarn or thread is flame-retardant, it has been coated with a special chemical that expels gases and causes heat to dissipate, thereby reducing the rate of reaction. Before heat can reach the inner core of the material or catch fire, the flame will be extinguished. Most synthetic fibers with a low limiting oxygen index (LOI) can be coated to promote flame retardancy.

This includes fibers such as:

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.

However, there is something important to keep in mind about flame-retardant thread and yarn: It can still melt. Although the flame is extinguished, high heat or flame can still cause the material to warp, disfigure, become soft, and eventually begin to drip more like a liquid than a solid.

Click here to read more in our article about flame-retardant yarns and threads.

Definition of Flame Resistant

Flame resistant yarns and threads are naturally nonflammable due to their inherent structure and from the inside out, they are completely resistant to flame.  Some fibers will not melt e.g. Para-Aramids, PBI, PBO, and Modacrylic (Kaneka Modacrylic Protex®-C); other fibers such as Meta-Aramids, Kuraray Vectran™, PEEK, Carbon, PTFE, and Glass will melt. Most materials that have a Thermal Degradation Temp (TDT) with LOI values greater than 21% will not support combustion.

Flame resistant materials can be blended with other materials, depending on your needs for cost and flame resistance.

Selecting Flame Retardant vs. Flame Resistant

When choosing between flame retardant and flame resistant yarns and threads, take the following factors into consideration:

Specific use: What is the end-use? Is flammability a larger or smaller concern than strength, durability, conductivity, and other characteristics? Can you find a balance among these needs or does non-flammability outweigh them all?

Safety precautions: Will the material be used in an extremely hot environment? Will it have long-term exposure to UV rays or open flames? Choose carefully when safety is paramount. Is the retardant on the material regulated by any U.S. jurisdiction?

Processing setup: In certain facilities, there will be high abrasion and/or high temperatures during processing. There could even be airborne chemicals or particulate matter in the environment. This may affect your choice of yarn/thread.

Mil-specs: Under the Berry Amendment, there are military specifications that require non-flammable materials. Follow these guidelines if you are supplying to the U.S. government.

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

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