Imagine that you’re driving on a two-lane road. It’s dark and rainy. Maybe you’re driving faster than you should be. Perhaps some animal darts into the road. Or maybe another driver loses control of their vehicle. You swerve and slam on the brakes, but the collision has already been set into motion. Your seat belt tightens as your car crashes, and the only object between you and a serious injury or even death is a thin nylon bag full of nitrogen gas—an airbag.
The chemistry used to inflate airbags has evolved. Over the years, automakers have sought to use more efficient, less expensive chemical transformations and to reduce use of any potentially hazardous compounds. But those changes haven’t always been for the better. In the late 1990s, the automotive parts manufacturer Takata launched an airbag formulation that led to recalls that the US National Highway Traffic Safety Administration (NHTSA) describes as “the largest and most complicated automotive recalls in United States history.” Today, a combination of chemical reactions and compressed gas canisters helps save lives.
Airbags saved 50,457 lives in the US between 1987 and 2017, according to NHTSA. Bursting forth from steering wheels, dashboards, seats, and the mounting around car doors, airbags protect heads and limbs from slamming into an automobile’s hard surfaces.
Speed is of the essence for airbags to work properly. The average automobile collision takes less than 200 ms—about twice as long as it takes to blink. So safety systems need to work in even less time. From detecting a crash to deploying the airbag takes around 10–30 ms, depending on the type of airbag. The process requires some sophisticated engineering but at its heart is a chemical reaction that turns solid material into gas in a split second.
Front airbags became mandatory on all vehicles in the US in 1999, but car companies started working on airbags in the early 1970s. Inventors and engineers had been experimenting even before that. The very first automobile airbag patents date back to the early 1950s.
German inventor Walter Linderer and American engineering technician John Hetrick independently patented the idea of using compressed air to inflate a cushion in a car. But compressed air proved too slow to be practical. Two major inventions helped airbags develop into a useful safety feature: speedy detection and rapid inflation. In 1967, American Allen K. Breed patented an electromagnetic sensor that would become key to airbags’ speedy deployment. Yasuzaburou Kobori, a Japanese automobile engineer, solved the problem of creating a lot of gas very quickly in 1964. His ingenuous idea was to use a gas-generating chemical explosion to inflate airbags.
The first widespread deployment systems used sodium azide to inflate airbags. A sensor triggers a device that ignites the sodium azide, producing nitrogen gas and sodium metal. Airbag makers also added potassium nitrate and silicon dioxide to react with the resulting sodium metal. That reaction produces potassium silicate and sodium silicate, both of which stop the sodium from reacting with moisture in the air to form corrosive sodium hydroxide.
But airbag makers had phased out this chemistry by the late 1990s because the solid silicates could retain heat in an explosion, says Paul Worsey, an expert in explosives engineering at the Missouri University of Science and Technology. “You’ve got all these little hot particles” that can burn people, Worsey says.
Airbag makers were also looking for a chemical reaction that gave off more gas per gram of material so they could make the airbag system smaller and lighter, says Harold R. Blomquist, a chemist who has worked on energetic solid materials in automotive airbags and other systems at TRW Automotive and as a consultant. Some cite the toxicity of sodium azide as another reason that airbag manufacturers moved away from this chemistry, but Blomquist says there’s virtually no exposure to the compound once it is encapsulated in an airbag system. And when cars are scrapped, airbags are deployed as a matter of course, so the azide gets converted to nitrogen gas.
In the late 1990s the airbag maker Takata introduced a system that replaced sodium azide with ammonium nitrate—a fertilizer and well-known explosive. This change proved disastrous. Over time, the ammonium nitrate in Takata’s airbags broke down in a way that caused it to detonate uncontrollably when the airbag was deployed. This destroyed the container housing the ammonium nitrate, sending metal shrapnel into the vehicle. The first reported airbag rupture occurred in 2004; the defect caused 23 deaths and at least 400 injuries in the US alone, leading to recalls of 67 million airbags in tens of millions of automobiles, according to NHTSA.
But why did the ammonium nitrate break down? Blomquist, who studied Takata airbags for NHTSA’s investigation, says moisture from humid air penetrated the seals around the ammonium nitrate’s housing. Over time, moisture transformed the ammonium nitrate from a uniform solid to one riddled with channels via a process known as Ostwald ripening. The problem was exacerbated by high temperatures, so it was worse in parts of the US with warm, humid weather. When the airbag deployed, hot gas from combustion flowed through the channels quickly and burned through the material in just 3–5 ms, rather than the 30 ms it was designed to.
“So instead of pushing gas out of a hole at a nice, well-behaved rate, it comes all at once, and it blows the structure apart,” Blomquist says. He estimates that in hot, humid regions, the ammonium nitrate airbag inflators can become compromised within 6 years.
NHTSA says that because the problem was so widespread—including vehicles from 19 automakers—some airbags still haven’t been replaced. Earlier this month, automaker Stellantis (formerly Fiat Chrysler) warned owners of Dodge Magnum station wagons, Dodge Challenger coupes, and Dodge Charger and Chrysler 300 sedans from model years 2005–10 not to drive their cars at all if their airbags hadn’t yet been replaced. Instead, the automaker said, the vehicles should be towed for a repair. Drivers in the US can check to see if their automobiles are covered by the recall using the vehicle identification number at nhtsa.gov/recalls.
Today’s airbags use a different chemical to produce nitrogen gas: guanidinium nitrate, plus a copper nitrate oxidizer. When ignited, guanidinium nitrate decomposes into nitrogen gas, water, and carbon. The copper nitrate oxidizer reduces the temperature of the exhaust gas, according to Blomquist. He says this formulation has other positives: guanidinium nitrate is relatively inexpensive and, unlike ammonium nitrate, is not particularly moisture sensitive.
Chemical systems are no longer the only technology used to inflate airbags. Coming full circle, some side-curtain airbags rely on compressed helium or argon-helium mixtures. There are also hybrid systems that combine chemical propellants with compressed gas.
Hopefully, you’ll never have to experience an airbag in action. But just in case, it’s a good idea to check that your vehicle’s airbag is not one of those under recall.