Fire-Resistant Materials: Nomex, CarbonX, Hocotex

Most drivers’ worst fear isn’t a wreck: it’s being unable to get out of a burning car. Let’s look at the fire-resistant materials that keep drivers from getting burned.

If you’re interested in the chemistry of fire or fire-suppression systems, we’ve got you covered there, too.

A Brief History of Fire-Resistant Materials

The need for fire-resistant materials came about as soon as humans discovered fire. In 83 BCE, the Romans coated their siege towers with alum (aluminum potassium sulfate) to protect them from fire. Scientists explored a lot of chemical formulations, but in the 1820’s, the chemist Gay-Lussac came up with a composite formula. One set of chemicals formed a glassy layer around the flammable material to keep it away from the fuel. A second chemical produced a non-flammable vapor that displaces oxygen. That takes care of two out of the three elements in the fire triangle.

Fire-Resistant Coatings

Coating clothing with chemicals cam make it fire resistant. This was state-of-the-art in NASCAR circa 1960s, but was revisited when Fireball Roberts died of burns sustained in a crash. (Roberts, an asthmatic, skipped the chemicals because they caused him respiratory problems.)

The problem with coating something flammable to make it non-flammable is that the fire-resistance comes off the next time you wash it. You could try making chemicals that are harder to remove, but starting with a flammable fiber already puts you at a disadvantage.

Almost all natural fibers (from animals, plants and minerals) are flammable. A few are not, but they are things like asbestos, which pose other problems.

Since Mother Nature didn’t give us a solution, we had to make one ourselves. The first human-made fiber was created in the early 1880s from cellulose, a component of wood. The first entirely synthetic fiber was nylon, introduced in the 1930’s.

That advantage of crafting a fiber from scratch, is that we can customize the fiber properties. For motorsports, we want a fiber that will minimize the effects of being close to a 1500 ºF gasoline fire

Polymers = Repeating Molecules

The prefix “poly” means many. Polygon means many sides. Polymer means many units. The unit in this case is a particular arrangement of atoms into a molecule.

For example, linking up a bunch of ethylene molecules makes polyethylene. (The problem with recycling many plastics is figuring out how to make the monomers let go of each other!)

A graphic showing an ethylene molecule and the polymer polyethylene

We call the single unit a monomer, or just a ‘mer’. You can make the polymer long or short by how many monomers you put together.

Kevlar and Nomex: First Cousin Polymers

These two polymers play an important role in motorsports, not to mention the tires on your car. If you’re a first responder, they may save your life.

Kevlar

DuPont chemist Stephanie Kwolek, discovered Kevlar in the early 1960’s. Kevlar (shown below) is five times stronger than steel by weight.

Schematic of a Kevlar molecule
By convention, a bunch of hydrogen atoms aren’t shown in this schematic, even though they’re a very important part of why Kevlar is so strong.

A bunch of these molecules join together to make the Kevlar polymer.

A graphic showing how long and straight the Kevlar polymer is.

The way the molecule links up to itself make the polymer very straight. That straightness affects of the chains link together. For Kevlar, that’s an important part of its strength. Kevlar makes bulletproof vests and reinforces tires and carbon fiber pieces.

But Kevlar isn’t a miracle material. It has a few limitations. A primary limitation is that Kevlar disintegrates at about 900 ºF.

Nomex

Nomex (aka poly (m-phenylenediamine isophthalamide)) came on the market in 1967 and is a close relation of Kevlar. Compare the Kevlar and Nomex molecules shown below. They’re very similar, but the positions of some of the atoms in Nomex make it, well, a little kinky.

Comparing the molecular structure of Nomex and Kevlar

That difference in shape – straight vs. kinked – makes a huge difference in properties. Nomex is nowhere near as strong as Kevlar; however, when you heat Nomex, it doesn’t melt and it doesn’t burn.

It chars. Char is the black residue left over from heating something. (For example, charcoal is made by slowly, partially burning an animal or vegetable fuel source. 

Nomex doesn't burn when heated: It chars, forming an insulating layer of carbon around the fiber.

The char does three things. First, the fiber is thicker and char is not a good thermal conductor. That gives you a little more insuation.

Secondly, the char prevents the fire from getting any more fuel.

Finally, forming the char closes up the holes in the fabric, which shuts out oxygen and prevents hot air from getting in to your skin.

When woven Nomex expands, the holes in the fabric get smaller, which prevent oxygen and hot air from getting in through the suit.

New Fire-Resistant Materials

Most stock car drivers use Nomex, but new fire-resistant materials are making inroads. Breaking into the market is difficult: You have to prove your material is better, and it has to be comparable in price with Nomex.

Racing isn’t a huge market. If it were just racing, companies wouldn’t spend huge amounts of money on R&D; however, there is great demand for fire-resistant clothing in firefighting, welding, utility work, oil/gas industries and in the military.

CarbonX

CarbonX is a blend of oxidized polyacrylonitrile and other strengthening fibers and is inherently non-flammable. Polyacrylonitrile is the precursor for 90% of carbon fiber production. These fibers are basically already-burnt Nomex. Carbon-X products come black, dark gray, navy blue and a dark olive green. To get colors, you have to blend CarbonX fibers with other fire-resistant fibers (usually Nomex).

Where CarbonX excels is its very high LOI (Limiting Oxygen Index). That’s the percentage of oxygen that has to be present before the material will combust. CarbonX won’t combust unless 55% of the air around it is oxygen. Oxygen only makes up about 21% of air, so that’s perfect for motorsports.

You can hold it at 2600 ºF for two minutes and it won’t ignite or burn. The video below compares CarbonX (the black material) with Nomex baselayers.

You can get a CarbonX sport bra for about $80, and a pair of SFI 3.3 certified socks for $45 and a CarbonX balaclava for about $65 now. A good Nomex balaclava will cost you almost the same.

The choice of CarbonX vs Nomex comes down to comfort, since both will protect you in a fire. Drivers have different opinions about a suit’s weight, mobility and breathability. The people I know who have tried both feel like CarbonX suits are heavier, but more breathable and less scratchy.

CarbonX is almost overkill for stock-car racing, but if you’re in the military or work in a metal foundry, CarbonX might be a superior fire-resistant material — and it doesn’t matter that you can’t get it in red.

Hocotex

Sparco Racing has a new firesuit made from Hocotex, which it turns out isn’t technically a new material, but a new twist on Nomex. It optimizes the Nomex’s fire-resistance, but creates the material differently.

All woven materials all start with thread. From Sparco’s promotional materials (and this patent), it looks like they’ve created a fabric that has air cells within the fabric. Normally, the protective air pockets in a firesuit are created by quilting together layers of fabric. There are about 300 air pockets in a normal suit.

Sparco is using a weaving technique that first makes three layers, then interweaves those layers with each other. This makes a single piece of fabric, but with many air pockets. It takes 7 miles of Nomex thread to make one square meter (about a square yard) of HOCOTEX.

I imagine it to be something like the batting you’d use for a quilt, but thinner. In traditional firesuit construction, you never see more than three-layer suits because they become very bulky and uncomfortable. The Hocotex patent claims they can make up to seven layers, with different patterns in each. The middle layers could be knit, giving lateral air gaps, so there’s huge flexibility in this method. They claim their Extrema RS-10 suit has “60,000 microcells” in a fabric that is 0.6 mm thick, with a 10-12% increase in heat protection and two to three times the breathability.

Because, ultimately, it’s made of Nomex, it can be screen printed and made in many different colors. So you can make pretty much any design you could make out of regular Nomex, but with the advantage of the suit being lighter and easier to breathe in. They can also apply the method using other types of fibers

Sparco is an Italian company; most of their suits are FIA homologated (meaning “approved”), but they claim their Victory RS-4 suit is the only single-layer suit to pass the SFI 3.2A/5 spec.

Nomex Nano

DuPont is looking at improving Nomex as well, with Nomex Nano, a more breathable version of the Nomex fabric they already offer. Drivers wear suits for a couple hours; if you’re in the military or a factory job, you might be wearing the suit for 8-12 hours at a time, so breathability becomes even more important.

For More on Firesuits

Take a look at the video we made a number of years back with the National Science Foundation

Note: This blog was revised and updated on May 7th, 2020to add new details and streamline the explanations.

3 Trackbacks / Pingbacks

  1. Fire: A Motorsports Safety Challenge - The Building Speed Blog
  2. In-Car Fire Suppression Systems : Building Speed
  3. The Science of Firesuits : Building Speed

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