Defects aside, working
airbags save lives.
Frontal airbags reduce fatalities
14 percent when no
seat belt is used, and by
percent when a seat belt
In 2015, frontal airbags
saved an estimated
lives. Between 1987—
when airbags first began to
be installed in vehicles—and
2015, an estimated 44,869
lives were saved.
SOURCE: NATIONAL HIGH WAY TRAFFIC SAFET Y ADMINIS TRATION
sensor. Newer airbags contain much more
sensitive electronic accelerometers contained
within a microchip.
As soon as a car stops suddenly, the
airbags inflate. Many people assume that a
tank containing air at high pressure releases
its contents to blow up the airbags. But this
method would be far too slow. Airbags are
actually inflated by a very rapid chemical reaction (Fig. 1). A car’s sensor sends an electrical signal to the detonator, which initiates a
chemical reaction that inflates the airbag.
To provide adequate protection for the
occupant of a vehicle involved in a crash, the
airbag must deploy in 40 milliseconds or less.
So the chemical reaction that triggers the airbag’s inflation must happen extremely fast.
Immediately after inflation, the airbag starts
to deflate, as the gas escapes through tiny
pores in the fabric. It is crucial that the head
or torso hit the airbag as it is deflating, not
inflating, as a fully inflated airbag would not
provide enough of a cushion to protect the
occupant. Contact with the airbag generally
occurs about 50 milliseconds after a collision,
allowing time for the airbag to begin to deflate. Seeking the right reaction
Older airbags contain solid sodium azide
(NaN3). Sodium azide is very stable at low
temperatures, but it decomposes at temperatures above 300 °C. When the airbag is
triggered, the detonator sends an electrical
impulse that generates enough heat to initiate
the decomposition of the NaN3. The products
of this reaction are solid sodium and nitrogen
gas. The reaction is as follows:
2 NaN3(s) ; 2 Na(s) + 3 N2(g)
Solid sodium is not a product you want
to have left over, as it reacts rapidly with
water. So the airbag also contains potassium
nitrate (KNO3), which converts the sodium
into less harmful products, including more
nitrogen gas. The additional nitrogen helps
to inflate the airbag further. However, the
other products potassium oxide (K2O) and
sodium oxide (Na2O) are corrosive. To make
the system safer, the airbag contains silicon
dioxide (SiO2), which converts the unwanted
by-products into inert silicate glass.
A typical airbag of this type contains 130
grams (g) of sodium azide, enough to generate 70 liters of nitrogen gas at standard
temperature and pressure—0 °C at 1 atmosphere—and fill the airbag.
Figure 1. In a crash, a detonator ignites the propellant (green pellets), causing it to break
down and release a burst of nitrogen gas. The reaction inflates the airbag in less time than
it takes you to blink an eye.
How airbags can keep you safe