C&EN on airbag propellants

Via Chemical and Engineering News' Beth Halford, this fascinating look at airbag chemistry: 

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.

I gotta say, I did not expect to read about Ostwald ripening in this article: 

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.

Cool article - read the whole thing!