31.5 What causes an automobile airbag to inflate?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
31.5 What causes an automobile airbag to inflate?
The final cause is the production of nitrogen from 10s of grams of sodium
azide, but there are some extra chemicals involved along the way.
Sodium azide is toxic, The airbag inflators are aluminium-encased units
that contain an igniter (squib), gas generating pellets ( or wafers of
sodium azide propellant ), and filters to screen out combustion products.
The electrical signal ignites a few milligrams of initiator pyrotechnic
material. The pyrotechnic material then ignites several grams of booster
material, which ignites the tens of grams of sodium azide, and the azide
burns very rapidly to produce nitrogen gas and sodium.
The sodium azide is pelletised to control the rate of gas generation by
controlling its surface area. The free sodium would form sodium
hydroxide when it contacts the water in people's noses, mouths, and
eyes so, to prevent this, the manufacturers mix in chemicals that will
produce sodium salts ( silicates, aluminates, borates ) on combustion.
Inflator units also often have a layer of matted material of alumina and
silica called Fiberfrax in the particulate filter. The Fiberfrax mat reacts
with most of the remaining free sodium in the generated gas. A typical
reaction pathway is as follows [6];-
300C
2 NaN3 ------> 2 Na + 3 N2
10 Na + 2 KNO3 ------> K2O + 5 Na20 + N2
K2O + Na2O + SiO2 ------> alkaline silicate glass.
There are apparently also corn starch and talcum powder used as lubricants
in the bag, and if the bag explodes these are the powders that contaminate
people - not the toxic chemicals in the inflator.
One article quotes 160 grams of propellant for a drivers-side bag
( 60 litres of gas) and 450 grams for a passengers-side bags
( which are 3-5 times larger) . I suspect that may include all of the
above ingredients in the igniter, but not the bag lubricants.
The bag fills until it reaches slightly above atmospheric pressure, and
the manufacturers now control the bag inflation speed to 90-200mph, which
is less than the early models - because they were too violent and could
harm occupants. The actual sequence goes something like:-
0 - Impact
15 - 20 milliseconds - sensors signal severe frontal collision.
18 - 23 milliseconds - pyrotechnic squib fired
21 - 27 milliseconds - nylon bag inflates
45 - 50 milliseconds - the driver ( who has moved forward 5 inches)
slams into the fully inflated bag
85 -100 milliseconds - the driver "rides the bag down" as the air
cushion deflates.
Recently, there have been calls to change the crash testing procedures to
allow the test dummy to be belted in, as seat belt usage is now about 67%.
Having a belted dummy would permit the use of slower inflating airbags, as
the deaths of 30 children ( up to Dec. 1996 ) have been attributed to the
speed of inflation of the larger passenger-side bag. Early in 1997, the
US NHTSA finally permitted depowering and/or disabling of passenger-side
airbags. A major airbag supplier is Breed Automotive, Boonton Township, N.J.
More details can be found in specialist articles [7-9], and research is
continuing into alternative inflation mechanisms, such as compressed gases.
There has been extensive work over the last decade on "hybrid" airbag
systems. These two-stage systems often use cylinders of compressed gas,
which can be released at ambient temperatures for situations where low-speed
deployment is appropriate, or the gas can be rapidly heated for high-speed
deployment.
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