Most car recalls are pretty tame. But the recent Takata airbag recall is not one of them. It’s not your ordinary situation where something might malfunction in a mild way – say, a wire shorting out which might lead to a fire, which in turn might injure someone who handled the situation wrong. With these airbags, the risk is that when set off, they might explode and send shrapnel into you.

A recent article about this stated that Takata hasn’t yet found the root cause of the failures; it went on to say that, according to Takata, “the ammonium nitrate used in the airbags was safe and stable”.

Wait, what? Ammonium nitrate, in airbags? Did I read that wrong? Did the news site get it wrong? No; checking, plenty of other sources confirm.

Well, they may not know what the root cause of their airbag malfunctions is, but I do: they used ammonium nitrate. Ammonium nitrate is hygroscopic. It’s “stable” in the sense that it won’t decompose under normal circumstances; but it absorbs water from the atmosphere and, above a certain humidity, turns itself into a little puddle. Then as it dries out, it recrystallizes into a different form. Presumably they tried to seal water out of the airbags; but seals often fail over time, or are flawed from the start; and lots of materials (like plastic films) which seem impervious to water actually let it slowly diffuse through. Once moisture has gotten in, this process (absorbing water and turning into a liquid, then drying out and recrystallizing) would repeat itself every day, due to the humidity dropping during the day then rising at night. (With a constant amount of moisture present, humidity falls when the temperature rises.)

In the airbags, they must have mixed the ammonium nitrate with something else, as pure ammonium nitrate is not a suitable propellant. And the exact nature of the mixture must have been important. The more finely divided the material, the faster the boom. The changing shape of the ammonium nitrate must have played havoc with whatever they’d done to control the speed of the boom.

I don’t know how they could ever have thought this would work. If they didn’t get problems with exploding too fast, they’d have gotten problems with exploding too slow. (And I wouldn’t be surprised if the latter also sometimes turned out to be a problem with these airbags.) But ammonium nitrate is among the cheapest of materials, which is presumably why they tried to make it work.

Still, you just can’t field millions of devices, and expect to keep moisture out of all of them. On a laboratory scale, that might work; out in the world, subjected to all sorts of abuse and to the forces of aging, it never will.

(Most of this was originally posted as a comment to the article linked above.)

Addendum (Jan 17, 2015): On further reflection, there is one technology for sealing out water that has been widely and successfully fielded, namely enclosure in a glass capsule. Vacuum tubes generally work fine even after sitting in storage for decades; and they were produced by the billions and sent all over the globe. Their technology includes, of course, electrical connections through the glass, as would be necessary here. Having airbag propellant sealed inside glass might be a bit tricky, as the flying glass shards would have to be filtered out before they reached the bag itself; but that could be done. My guess is that the expense of such an assembly would eliminate ammonium nitrate’s cost advantage, though.