What makes airbags inflate

It also spreads the impact over a larger area of the body. That way, no single area forehead, chin, neck bears the brunt of it.

There are six main parts of an airbag system: an accelerometer; a circuit; a heating element; an explosive charge; and the bag itself. The accelerometer keeps track of how quickly the speed of your vehicle is changing. When your car hits another car—or wall or telephone pole or deer—the accelerometer triggers the circuit. The circuit then sends an electrical current through the heating element , which is kind of like the ones in your toaster, except it heats up a whole lot quicker.

This ignites the charge, often solid pellets of sodium azide NaN3 , which explodes. The goal is for the bag to be deflating by time your head hits it. That way it absorbs the impact, rather than your head bouncing back off the fully inflated airbag and causing you the sort of whiplash that could break your neck.

A handful grams of sodium azide will produce 67 liters of nitrogen gas--which is enough to inflate a normal air bag. That's not the only chemistry involved. Notice that the other chemical into which sodium azide falls apart is Na, or sodium. Sodium is a very reactive metal that will react rapidly with water to form sodium hydroxide; as a result, it would be quite harmful if it got into your eyes, nose or mouth.

So to minimize the danger of exposure, air bag manufacturers mix the sodium azide with other chemicals that will react with the sodium and, in turn, make less toxic compounds. What prompts an air bag to inflate by way of this reaction?

There are sensors in the front of the automobile that detect a collision. These sensors send an electric signal to the canister that contains the sodium azide and the electric signal detonates a small amount of an igniter compound. The heat from this ignition starts the decomposition of the sodium azide and the generation of nitrogen gas to fill the air bag.

Hot blasts of nitrogen gas inflated the airbag. Because sodium azide is extremely toxic, these chemicals were widely phased out during the s in favor of more efficient, less expensive, and less toxic alternatives. The alternative propellants may incorporate, for example, a combination of nitroguanidine, phase-stabilized ammonium nitrate NH4NO3 or other nonmetallic oxidizers, and a nitrogen-rich fuel other than azide e.

As illustrated in the above figure, when a crash is sensed, the control unit sends an electrical signal to the inflator. The chemical reaction is initiated by the igniter, generating primarily nitrogen gas to fill the airbag causing it to deploy through the module cover. The original idea of using compressed air turned out to be not workable because the air cylinder itself represented a risk.

What if it was damaged in an accident and took off like a rocket? Furthermore, at the time, car manufacturers were more interested in enticing customers with huge engines and tail fins than airbags. But as the slaughter on highways continued unabated, carmakers realized that something had to be done. Air bags began to be seriously considered but how could they be inflated safely within a few milliseconds of impact without using compressed gases?

The answer would be found in a fascinating chemical called sodium azide, NaN3. When this substance is ignited by a spark it releases nitrogen gas which can instantly inflate an airbag. The problem, however, is that the reaction also forms sodium metal which reacts with moisture to generate sodium hydroxide, a highly corrosive substance.

A burst airbag could wreak havoc. Chemical ingenuity, however, came to the fore. If potassium nitrate and silicon dioxide were also included with the sodium azide, the only products that would form in addition to nitrogen would be potassium silicate and sodium silicate. Both of these are inert, harmless substances. An airbag is designed to release some of the gas just after it deploys to help cushion the impact against the body.

Hitting a fully inflated, unyielding airbag could be catastrophic. So before widely promoting this protective device, the safety of the contents had to be ascertained.