Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: paul cotter on 14/03/2024 12:02:33
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The withdrawn Takata airbags are stated to use ammonium nitrate as a much cheaper substitute for the previous sodium azide/potassium nitrate/silica combination. There has to be more to it than just ammonium nitrate. Although ammonium nitrate is used as a common blasting agent it is always used with some form of sensitizer as the pure substance is rather inert: a #8 detonator(the most powerful commonly used) will not initiate it and a blowlamp aimed at a loose pile of the pure substance will not lead to ignition--it just melts and sputters. Molten ammonium nitrate, especially if contaminated by a variety of organic and inorganic substances, is a different scenario--this can spontaneously detonate and this has been the cause of many catastrophic accidents.
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It's pretty handy mixed with diesel fuel to make a plastic low explosive occasionally used in mining. Maybe something like that was the basis of the airbag mixture?
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Indeed, Alan, that mix is often given the acronym ANFO, ammonium nitrate fuel oil. There has to be some reducing agent added. The other factor using ammonium nitrate as an explosive is that it's minimum diameter necessary for a detonation front to propagate is very large making small charges useless. In my misspent youth I had a passion for pyrotechnics and explosives-- I tried every possible method to initiate small charges(50-100grams) of ammonium nitrate mixed with various sensitizers but invariably failed. Given my experiences with ammonium nitrate I find it hard to believe the stories about the Takata air bags, nonetheless I have to accept what has been determined.
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The problem was that the airbags used phase stabilized Ammonia Nitrate, and that they did not include a desiccant. Over time, humidity and high temps degraded the Ammonia Nitrate. This led to it burning too quickly. This, in turn, led to pressures inside the inflators beyond that which they could withstand, causing them to explode, spewing shrapnel.
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Thanks, Janus. I had already seen that description of the problem. I would read that as "allegedly" phase stabilised ammonium nitrate as a lot of effort was put into the use of ammonium nitrate in rocket propellants without satisfactory results before the adoption of ammonium perchlorate. Ammonium nitrate exhibits many phases and changes of phase leads to swelling and contraction leading to void formation and irregular performance. What I really wanted to know is what was added to make a combustible product as on it's own it will do either nothing or detonate(only realistic with large quantities), neither of which are desirable for an air bag.