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The Chemistry Behind Airbags. Washington
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revised October 2000): http://www.chemistry.
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Work. The Atlantic, June 27, 2015: http://www.
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Brian Rohrig is a science writer who lives in
Columbus, Ohio. His most recent ChemMatters
article, “The Cool Chemistry of Dry Ice,” appeared
in the February/March 2018 issue.
the airbag. The problem is that
the compound is easily affected
by temperature changes—which
occur often under normal driving
Theoretically, the ammonium
nitrate was supposed to be
protected from moisture. But
recent research into Takata’s
defective airbags revealed a flaw
in the metal inflator housing the
ammonium propellant. The inflators were insufficiently sealed,
allowing moisture from humid air
to seep in. Exposure to humidity
can cause ammonium nitrate to
dissolve over time.
Changes in temperature also
affect the compound, but in a
different way. Cycling between
hot and cold can cause the crystal structure
to shift between various states, which can
result in the pellets cracking into fragments.
Smaller pieces have a greater surface area and
produce gas faster than the airbag designers
intended. This creates more pressure than
the system can withstand. Instead of allowing
the gas to release into the airbag, the canister
explodes—in some cases, even when there’s
Although sodium azide is effective at inflating airbags, it has many drawbacks. For one,
it is about as toxic as cyanide compounds.
Even small amounts of sodium azide can
cause serious short- and long-term
In light of the toxicity of sodium
azide, airbag manufacturers
have been searching for safer
alternatives. They initially
made the switch to tetrazole
(CH2N4), a less toxic compound. Due to the limited
availability and high cost,
however, Takata eventually switched to ammonium nitrate (NH4NO3), a
more plentiful and cheaper
alternative. When an airbag containing ammonium
nitrate is activated, it thermally
decomposes into water vapor and
nitrous oxide (N2O), which is also
known as laughing gas.
So what went wrong?
Despite the fact that ammonium nitrate
is less toxic and cheaper than its predecessor, it became central to the Takata airbags’
Within an airbag system, the ammonium
nitrate exists in pellet form, with just the right
amount of surface area to produce a reaction
in the precise time period needed to inflate
Fixing the flaw
Newer airbag inflators that contain ammo-
nium nitrate now include a desiccant, or dry-
ing agent, that absorbs water. This addition is
designed to keep the ammonium nitrate dry,
and thus prevent it from decomposing. In July
of 2017, however, Takata filed an additional
recall for inflators containing one particular
type of desiccant containing calcium sulfate
Now, most companies that manufacture
airbag inflators have turned to stable gua-
3]NO3. It reacts
according to the following equation:
3]NO3(s) ; 3 H2O(g) +
2 N2(g)+ C(s)
Takata also announced plans to move away
from ammonium nitrate to guanidine nitrate
with the goal of preventing more airbag tragedies from occurring.
To find out if your car is affected, visit
www.safercar.gov. If your car is on
the list, call your local auto dealer. Even
though the chances of an airbag
deploying prematurely or with
excessive force is extremely rare,
it is better to be safe
Need to Be