Airbag Recalls

In 2008, a few thousand cars were recalled in the USA to have "faulty" Takata airbags replaced.
By 2016, tens of millions of cars world-wide are being recalled to have these "faulty" airbags replaced.
What is causing these particular airbags to rupture violently, sending lethal metal fragments flying through the car's interior?

If you subscribe to AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry students and teachers, you will recall the issue in which we discussed the use of sodium azide, a compound that undergoes explosive thermal decomposition at temperatures well above ambient, to produce nitrogen gas to inflate the airbags used in cars.

What if you wanted to produce a cheaper airbag? Is there another, readily available, inexpensive, compound that can undergo explosive decomposition and produce lots of gas to fill an airbag? Yes there is, ammonium nitrate, NH₄NO₃, which is used in fertilizers, and, in explosives for mining and quarrying.

When anhydrous ammonium nitrate is heated above its melting point of 169.6°C, liquid ammonium nitrate decomposes to gaseous nitrous oxide (N₂O) and water vapor (H₂O) as shown in the balanced chemical equation below:
NH₄NO_3(l) → N₂O(g) + 2H₂O(g)
But this reaction is not explosive and therefore not suitable for use in airbags which must inflate incredibly quickly.
In order to produce an explosive decomposition of ammonium nitrate, you need to detonate it, which is the process that occurs if you use it in an airbag. The detonation of ammonium nitrate produces nitrogen gas, oxygen gas and water vapor, as shown in the balanced chemical equation below:
NH₄NO₃ → N_2(g) + ½O_2(g) + 2H₂O(g)

At first sight, this reaction looks like a much better, that is, safer choice, than the sodium azide reaction. But what happens if moisture gets into your airbag?
Ammonium nitrate readily dissolves in water, but if there is only a small amount of moisture present, the ammonium nitrate will "clump" together. At around 32°C (the temperature inside a pretty warm car) there is a phase transition in which the new phase occupies a greater volume than the previous phase which results in "cracking" of the "clumped" ammonium nitrate grains. These cracks can lead to catastrophic behaviour when the ammonium nitrate is detonated in the airbag, that is, the thermal decomposition of cracked ammonium nitrate grains is far more forceful and uncotrolled than that of anhydrous ammonium nitrate.

In order to safely use ammonium nitrate as the airbag propellant, you need to incorporate two safety features:

1.  a drying agent or desiccant to remove moisture and prevent "clumping" the ammonium nitrate
2.  a phase stabilizer to stabilize the phases to prevent "cracking"

Other airbag manufacturers do not use ammonium nitrate. In 2014, Key Safety Systems was reported to be using guanidine nitrate

and tetrazole

in the manufacture of its airbags.

Further Reading:
Definitions of a Mole
Mass-mole Calculations 
Molar Gas Volume Calculations 
Reaction Calculations: Mass and Moles

Suggested Study Questions:

1. What does the term "anhydrous" mean?
2. Why is a desiccant used in a desiccator?
3. Give the name and chemical formula for two commonly used desiccants in the school laboratory.
4. Write the molecular formula for each of the following compounds:
   • ammonium nitrate
   • nitrogen gas
   • oxygen gas
   • water vapor
   • nitrous oxide
   • guanidine nitrate
   • tetrazole
5. Calculate the molar mass of each of the following compounds:
   • ammonium nitrate
   • nitrogen gas
   • oxygen gas
   • water vapor 
   • nitrous oxide
6. Consider the chemical equation for the thermal decomposition of ammonium nitrate. If 2 moles of ammonium nitrate decomposed, how many moles of each of the following products would be produced?
   • nitrous oxide
   • water
7.  Consider the chemical equation for the thermal decomposition of ammonium nitrate. If 2.00 g of ammonium nitrate decomposed, calculate the mass of each of the following products in grams:
   • nitrous oxide
   • water
8. Consider the chemical equation for the detonation of ammonium nitrate in an airbag. If 2 moles of ammonium nitrate were detonated, how many moles of each of the following compounds would be produced?
   • nitrogen gas
   • oxygen gas
   • water vapor
9. Consider the chemical equation for the detonation of ammonium nitrate in an airbag. If 2.00 g of ammonium nitrate were detonated, what mass in grams of each of the following compounds would be produced?
   • nitrogen gas
   • oxygen gas
   • water vapor
10. Consider the chemical equation for the detonation of ammonium nitrate in an airbag. Calculate the mass of ammonium nitrate that would be required in order to fill a 70 L airbag at 25°C