Chemistry is not usually associated with cars, but there’s one very important piece of safety equipment which relies on a cleverly devised chemical reaction. And it’s one that most drivers would rather not experience first-hand – air bags. These safety devices are not inflated with compressed air, as this would be too slow to be effective. Instead, the gas produced is the product of a somewhat violent chemical reaction, based on sodium azide (NaN3).
The chemistry behind airbags
Under room temperature, sodium azide is a stable compound. However, when heated by an impulse it disintegrates to produce sodium (Na) and nitrogen (N2). It only takes about 50-100 grams sodium azide to produce enough nitrogen gas to fill a normal air bag for the driver (it takes a little more for the passenger).
This, however, is not the only chemistry involved. Unfortunately, sodium will quickly react with water to form sodium hydroxide (NaOH), which can be toxic if inhaled. To minimise this danger, manufacturers add other chemicals, including potassium nitrate (KNO3) and silicon dioxide (SiO2), which can react with sodium, to eventually produce a safe and harmless compound, silicate glass (Na2O3Si).
(1) 2 NaN3 → 3 N2 + 2 Na
(2) 10 Na + 2 KNO3 → N2 + K2O + 5 Na2O
(3) K2O + 2 SiO2 + Na2O → K2O3Si + Na2O3Si (silicate glass)
Most car-manufactures prefer KNO3 and not NaNO3, as it’s less hygroscopic. To ensure the air bag is fully operational even in the presence of moisture, it’s essential to choose non-hygroscopic components, otherwise the reaction may fail.
How is the air bag prompted to inflate?
The most common way is with sensors located in the front of the car. These sensors detect a collision and send a signal to a container with sodium azide, to ignite the reaction. The propellants used to generate the necessary high temperature vary widely according to each manufacturer. Examples include nitroguanidine, ammonium nitrate (NH4NO3) and a nitrogen-rich component other than an azide, such as triazoles and tetrazoles. Alternatives include nitrocellulose or high-oxygen nitrogen-free components (e.g. tricarboxylic acid) mixed with inorganic oxidisers (e.g. chlorate or perchlorate).
The heat produced initiates the decomposition of sodium azide and the production of nitrogen to inflate the air bag. Incredibly, the reactions only take about 30-40 milliseconds to occur and inflate the air bag. This means less than one second after the impact, the driver has a fully inflated air bag to absorb the tendency to move forward following a frontal collision.