In one of his Holiday Inn commercials, Jeff Burton is asked whether his firesuit is made of “100% fireproof nylon”, to which he replies, ‘yes’.

Firesuits, while probably overkill in terms of hotel lobby safety, are a critically important part of a driver’s safety gear. During a 2007 race at California, Michael Waltrip went to pretty extreme lengths (even for Michael) to get his sponsor some TV time. A cut left-front tire took out the oil lines in his NAPA car. Good thing Mikey got his firesuit at a different place than Jeff Burton got his.

You may have heard of Kevlar^®, which is a material used to make (among other things) bulletproof clothing. Kevlar has a first cousin named Nomex.^® Both are polymers, which means that they are made of a unit, called a mer, that is repeated over and over to form a long chain. There are natural polymers, like spider silk and natural rubber, as well as polymers like Kevlar and Nomex that are made in the lab. In the case of Kevlar and Nomex, the lab was that of DuPont, sponsors of the No. 24 car.

The repeating units for Nomex and Kevlar are shown below. The hexagons with rings in them represent phenyl groups, which in these molecules, have a carbon atom at each vertex (six carbons total) and a hydrogen attached each of to the four vertices that aren’t connected to the rest of the repeating unit. Phenyl groups are used so frequently that chemists use this shorthand to keep from having to write out all of the C’s and H’s.

If you look at these molecules for a few moments, you’ll notice that they are almost identical. They have the same atoms arranged in the same order. The big difference is that the carbon (C) and nitrogen (N) atoms attached to the phenyl groups are attached directly opposite each other (at a 180 degree angle) in Kevlar, while they are at an angle of 120 degrees in Nomex. That difference gives Nomex gives slightly different – but very important – properties than Kevlar.

Spider silk is an incredibly strong material, as a strand of spider silk is stronger than a piece of steel the same weight. Kevlar is a similarly strong molecule. Because Kevlar molecules are straight, they pack very nicely, as I’ve shown in the drawing below. The phenyl groups are so large that there aren’t a lot of ways the polymer chains can stack with each other. The chains link by hydrogen bonding, which is sharing electrons between the hydrogen and the oxygen of adjacent chains. Hydrogen bonding is much stronger than van der Waals bonding, which holds Nomex together. The combination of regular stacking and hydrogen bonding make Kevlar five times stronger than an equal weight of steel.

On the negative side, Kevlar starts to melt at about 900 °F. The bonds between the atoms weaken and the solid polymer turns into a liquid. Because gasoline fires can burn around 1800 °F - 2100 °F (982 °C – 1149 °C) and methanol fires can reach almost 3500 °F (1927 °C), Kevlar doesn’t make such a great firesuit material.

The kink in the Nomex molecule makes it harder to align, which means that Nomex is not as strong as Kevlar; however, Nomex doesn’t melt. When the temperature reaches about 800 °F (370 °C), the atoms in Nomex re-arrange and form a protective carbon coating around the outside of the polymer fiber. The carbon helps in four ways:  First, the some of the heat energy is used in forming the carbon layer, which means less heat available to get to the person wearing the firesuit. Second, the melting temperature of carbon is a whopping 6381 °F (3777 °C). Third, carbon is a very good thermal insulator, which means that if it is hot on one side of the carbon, it takes a long time for the heat to get through to the other side. Finally, the Nomex fiber actually gets thicker when it forms its carbon coating. Firesuits are woven and when the fiber gets thicker, the weave gets tighter and keeps air out. Fire can’t burn without air.

The only negative about the carbon layer that forms when Nomex is heated is that when the charred fabric cools, it becomes brittle and weak, just like paper that gets burnt crumbles. If you saw the fire shooting up in the cockpit of Michael’s car (which probably was coming through the shifter boot, you probably know that holes in his firesuit was the last thing Michael Waltrip was concerned about.

Perhaps Jeff Burton's comments about fireproof nylon are a flashback to pre-Nomex times, when regular fabrics were dipped in fire-resistant chemicals. The problem with this approach is that the fire resistance lessened each time the material was washed. Since the fire resistance of Nomex is a property of the molecules from which the material is made, Nomex isn’t degraded by washing. (UV exposure does degrade firesuits, but that's another article.)

Attention Jeff Burton: Nylon melts around 400-500 °F. Double check with whoever buys your firesuits for the actual races and make sure they (or you) heard wrong. Molten nylon can stick itself to your skin and actually make burns much worse. Not the kind of thing one needs going into the Chase.

 

Kevlar^® and Nomex^® are registered trademarks of E.I. du Pont de Nemours and Company.