Monday, October 27, 2014

Any good theories about the airbag recall?

So I have a theory on the airbag recall which came from this NPR story:
SHEPARDSON: Right. The issue is the propellant in the airbags, the material that actually explodes the airbag into your face, you know, milliseconds after sensors detect a crash is about to happen or has happened, in some cases is damaged and as a result - and what investigators believe is that's mostly linked to high humidity areas where - so in other words, after being exposed to humidity, they're more likely to have defects and as a result rather than simply expand, send this shrapnel into passengers in the vehicles. 
GREENE: So this is not part of the actual bag itself? This is like, a piece of the thing that makes the bag explode actually exploding itself and spraying shrapnel at people. 
SHEPARDSON: That's right. And also metal parts of the airbag around it. So no it's - you can imagine, it's a very violent incident and it's resulted in serious injuries, as well. I mean, people having - losing eyes or serious lacerations, other cuts. So it's not anything to not take seriously, for sure.
So sodium azide is the propellant for air bags, right? So I have a theory that the humidity problems is letting water into the azide compartment and making hydrazoic acid... which is quite explosive/shock sensitive. Readers, what do you think?

(Of course, if the airbags turn out to use another compound (like the nitroguanidines talked about in the Wikipedia entry), then my theory is falsified.) 

10 comments:

  1. The authors of this paper - http://pubs.acs.org/doi/pdf/10.1021/ol5027975 - report a gram-scale synthesis of triazoled by heating sodium azide and tosic acid in DMF. If a little moisture and some sodium azide is enough to cause an explosion via hydrazoic acid, I would hate to be working in proximity to these guys.

    Interestingly, they note in the paper "Due to the explosive and toxic nature of hydrazoic acids, Brønsted acids should not be mixed with NaN3", then proceed to do just that. Brave, or just crazy? I can't decide.

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  2. sodium azide (with a sensitizer additive) was used in older air bags, I think it is now being phased out in favor of aminotetrazole + oxidizer combo, because of concerns about azide toxicity

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  3. also, I don't think hydrazoic acid release is the culprit - my guess would be more likely a corrosive action of the propellant on the canister material, which would weaken it over time especially if some moisture seeps in

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  4. Now I don't know what the canister material is made of, I assume some sort of metallic compound. It could be that moisture and the NaN3 is leaching out some heavier metals and forming a heavy metal azide which is probably shock sensitive. This used to happen in the lead piping of older labs, especially those who used NaN3 as a bactericide and poured the waste down the sink. over the years lead azide built up confusing the plumber who had to fix the leaks!

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  5. It looks like they were unique in using ammonium nitrate in their airbags. Bad seals in high humidity climates allowed water to get in and produced more gas than intended.

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    1. Huh, that's a very interesting bit of information! Thanks.

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    2. Here's a link to Ruth's post explaining the issue: http://www.thechemistryofcars.com/?p=188

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    3. Wow! Yet another thing missing from my safety check list.... Always learning....

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  6. I saw the innards of an air bag made in 2003. The propellant was housed in a half-filled heavy-walled steel tube. Both ends were closed and small vent holes were drilled toward the end of the empty half. Presumably the restricted vents slowed down the gas expansion to prevent ripping the bag.

    I can imagine that a bad design or manufacturing problem could result in rupturing of the steel tube sending shrapnel through the bag canvas.

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  7. Old post, but there's a NY Times piece today that says the airbag problems may be because they cheaped out and starting using ammonium nitrate, instead of tetrazole or guanidinium nitrate. (My phone isn't letting me paste the link)

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