Fired Up: The Science of Flames

The Scariest Part of Racing?

During the XFINITY series race at Richmond, a malfunctioning fuel can spilled a huge amount of gasoline in the pit stall. A spark ignited the fuel, engulfing gasman Josh Wittman and rear tire changer Anthony O’Brien. A crew member for a team pitting nearby (Clifford Turner, working on Eric McClure’s car) was also injured. Although all the men were conscious and moving around immediately after the incident, all three were taken to the hospital. O’Brien wasn’t released from the hospital until Monday following the Friday incident happened.

If you were to poll racecar drivers about safety, I bet the majority of them would say the scariest situation isn’t a crash. As Elliott Sadler said: Two fears you have as a race car driver: one is being on fire and two is being T-boned in the driver door — everything else you sort of accept.

That quote was from before the Gen-5 car brought additional reinforcement to the drivers side door in the form of additional tubing and IMPAXX energy-absorbing foam. But what can you do to minimize the risk due to fire?

Fire

You need three things for fire:

Without any one of these three, you don’t get fire. Which is a good thing because we pretty much walk around surrounded by oxygen and fuel all the time. Pretty much any clothing, regardless of whether it’s made of natural or artificial fibers, is fuel. The air is about 21% oxygen, with 78% nitrogen and 1% preservatives and fillers. No, actually the 1% are other gasses, like hydrogen, krypton, neon, etc. and they’re present in such tiny quantities that we don’t care. At all.

Back in The Day…

Way back in the day, drivers and crew wore street clothes and hoped they wouldn’t catch on fire. Then fire-retardant chemicals became available and people would dip their clothing in the chemicals to make it fire-resistant. The problem is that you do tend to want to wash your clothes after driving in a hot car for a couple hours and the chemicals would wash off.

And believe me, after three or four hours in a hot car, you want to wash whatever’s been in there with you.

Polymers = Repeating Molecules

Then we learned how to design polymers. 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, below is a schematic of an ethylene molecule and the polymer polyethylene, which is nothing more than a bunch of ethylene molecules hooked up together.

You can make the polymer long or short by varying how many times you repeat.

Kevlar and Nomex: First Cousin Polymers

Kevlar was discovered by DuPont chemist Stephanie Kwolek, who passed away last June at the age of 90. How Kevlar came to be is an interesting story. A looming expected gasoline shortage led DuPont in the early 1960s to look for strong, yet lightweight fibers for tire manufacturing. One of Kwolek’s attempts at making a liquid that would be spun into a fiber ended up looking rather yuck. It was cloudly and thin, totally unlike what she expected, yet she insisted that it be made into a fiber for testing anyway.

Her invention was Kevlar, a polymer that is five times stronger than steel by weight. Below is a Kevlar molecule. Grey circles are carbon, Blue are nitrogen, red are oxygen and white are hydrogen. There are actually a bunch more hydrogen atoms in the single molecule that I don’t show because it just makes the picture messy and confusing .

You’ll notice something very interesting about the Kevlar polymer — it’s very straight. That linearity is a big contributor to its strength. Kevlar chains link with each other in a very orderly way and make a fiber that can be used in bulletproof vests, as well as serving as a reinforcement for tires and carbon fiber pieces.

But Kevlar isn’t a miracle material. It has its limitations. When you heat it up to 900 degrees F, Kevlar literally falls apart. The atoms start letting go of each other.

But check this out.

Compare the molecules left to right. Exact same atoms, just arranged differently. Kevlar is this nice straight molecule, but Nomex is… well… Nomex is a little kinky.

That difference in conformation — straight vs. kinked — makes all the difference. Nomex is nowhere near as strong as Kevlar; however, when you heat Nomex, it doesn’t melt and it doesn’t burn.

It chars. While that might seem like a bad thing, it’s actually good.

When the Nomex fiber chars, it forms a layer of carbon on the outside. That makes the fiber thicker, which does two things: First, the thicker fabric gives you a little more protection from heat transfer, but second, the thickening of the fibers closes the air gaps and prevents oxygen from getting through to the skin and feeding the fire.

The video below is one of DuPont’s promoting Nomex. Reminder. Don’t try this at home.

CarbonX

Nomex used to have a monopoly on the market, but recently there’s been a new material making waves. 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. One issue is that oxidized PAN is pretty much available in your choice of black or black, so the fibers have to be blended with other fire-resistant fibers to get colors.

One of the things CarbonX has is a 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 is oxygen. Remember oxygen makes up about 21% of normal air, so to some extent, that’s a moot point because anything with a LOI over 21% is going to work about the same as far as motorsports goes. You can hold it at 2600 F for two minutes and it won’t ignite or burn.

Prices have come way down on CarbonX since I first investigated them. You can get a CarbonX sport bra for about $80 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 they both will protect you in a fire. Drivers worry about the weight of the suit, mobility and breathability. The people I know who have tried both feel like CarbonX suits are heavier, but more breathable and less scratchy.

Can We Fireproof Racing?

People are very careful with their terminology when talking about fire safety. Nomex is not fireproof. Nomex firesuits are fire-resistant. Firesuits are made in layers, with the air between the layers also providing insulation against heat. That works the same way the air gaps between double-pane windows works.

SFI, a non-profit foundation that writes specifications and tests motorsports safety equipment, rates firesuits in terms of how long you can be exposed to fire before you’d get a second degree burn. For example, a 3.2A/1 rated firesuit gives you three seconds of protection, while a 3.2A/5 rated suit gives you 10 seconds of protection. (For the curious 3.2A is the SFI specification that deals with fire resistant uniforms).

NASCAR mandates that drivers wear a 3.2A/5 rated firesuit, as well as cover the remaining parts of the body with accessories that meet SFI specifications, including shoes and gloves. Crew members who go over the wall are required to have 3.2A/1-rated suits, although the NASCAR rulebook recommends going to the 3.2A/5. The exception is that anyone handling gas must have the the 3.2A/5 suit and must wear a fire-resistant apron.

Fire resistant underwear and socks aren’t mandated, but they are recommended. The danger here is that if you close enough to a fire, synthetic fibers like nylon and rayon melt. Then they stick to the skin and are very difficult (and painful) to remove. So for the weekend racers, if you can’t afford a full set of Nomex undies, at least make sure everything else you’re wearing is 100% cotton. And ladies — no metal hooks, clasps or underwires. Metal heats up faster than fabrics and you’ll get burned in particularly bad places.

As you might expect, the higher the level of protection, the more expensive the suits are — although you’d like to think that a couple hundred dollars per suit difference is worth a few days in the hospital — or worse. Used to be the pit crews didn’t wear firesuits or helmets. If a fire similar to the one in Richmond happened then, it would likely be fatal.

Let’s also note that the fire wouldn’t have happened if there hadn’t been a malfunction in the fuel can that allowed a couple gallons of gasoline to flood the pit lane and probably get on the gas man as well. I also want to note that the stuff they use in fire extinguishers is a pretty nasty brew of chemicals that aren’t exactly good for people to breathe — but they’re a lot better than burning to death.

Safety is about protecting people on all fronts. Even though the gas can failed, the safety equipment stepped up to the worst-case scenario. Thank heavens everyone is safe.

Originally published at The Building Speed Blog.