Fourteen million cars worldwide have now been recalled because their air bags, made by the Japanese auto supplier Takata (7312:JP), can rupture and hurl metal fragments like the shrapnel in a bomb. Six deaths and more than 30 injuries have been linked to the faulty air bags. The problem, according to a New York Times report, is that Takata began using a new propellant in its air bags: ammonium nitrate.
Ammonium nitrate—two nitrogen atoms, three oxygen, four hydrogen—is one of the world’s workhorse chemicals. The nitrogen makes it an effective fertilizer, and because of the oxygen it’s valuable in blasting agents used in mining and industrial demolition. Because it’s cheap, it’s widely used in both areas.
Compared with other blasting agents, ammonium nitrate is relatively stable. But it has been implicated in some of the biggest industrial disasters of the 20th century. An aluminum nitrate explosion at a chemical plant in Germany killed almost 600 people in 1921, another blast killed a similar number in 1947 in Texas City, Tex. And last spring a pile of ammonium nitrate at a fertilizer plant in West, Tex., exploded, killing 15 people and injuring 160.
This is the compound that Takata used as the propellant in its air bags. The problem in this case is that ammonium nitrate is sensitive to temperature swings, which can change its crystalline structure. The ammonium nitrate in an air bag that has been sitting in the hot sun all day, or out in the cold all night, is not the same compound that was tested in the lab and won’t explode in the same way when it’s set off. ”Once it’s partially degraded, it’s not what you put in there in the first place,” says Jimmie Oxley, an explosives expert and professor of chemistry at the University of Rhode Island.
In testimony this morning before a U.S. Senate panel, the Takata senior vice president in charge of global quality assurance apologized for the flaw and promised that it would meet the demand from automakers for replacement units. The company may well have been criminally reckless in going with a compound that behaved unpredictably in different temperatures.
Yet, as Oxley says, the difficulty for air-bag manufacturers is that there is no perfect propellant. Sodium azide, an early propellant, causes chemical burns, and several other compounds turn into oxygen once released, creating a fire hazard, or carbon monoxide, which is lethal in an enclosed space such as a car. ”If we’re going to have a 100 percent safe airbag,” Oxley says, “we’re going to have to put more research into it.”
