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Airbags

Airbags

When purchasing a vehicle, what is the single most important feature to you? Is it the speed? The mileage? The sound system? Well, hopefully it’s the safety features. I don’t know about you, but I wouldn’t dare enter a vehicle prepared with poor quality safety features. Aside from having properly functioning brakes, it would be in my best interest to have a vehicle equipped with practical airbags. So, what is it about airbags that makes them important? Airbags save lives. The sole purpose of an airbag is to act as a protective device. When an automobile accident occurs, an airbag works as a shock absorber.1 Airbags are generally located at 2 positions in a vehicle: inside the center of the steering wheel and above the glove compartment.1 Have you ever wondered what goes on inside an airbag system and how it works? Well, let’s take a closer look and see!

Timing is an extremely vital aspect for airbags as they must be able to inflate within milliseconds upon severe impact or collision.1 The airbag must also be able to deflate once it has had physical contact with the individual(s) in the vehicle, in order to support their heads with air.2 There are 4 parts to every airbag system: the gas generator, the polyamide bag, the electronic sensors, and the microprocessor.1 The sensors can be found at the very front of the vehicle or near the frontal foot area. The microprocessor contains a set of crash pattern data within it.3 The microprocessor is continuously analyzing brake patterns, shocks, acceleration and speed impulses all while simultaneously comparing those values to that of the crash pattern data.1,2 When an accident occurs, the microprocessor is able to calculate how critical the collision is and deploys the airbag but only if the speed of the vehicle is greater than 20 mph at time of impact.3

Within the gas generator exists a combination of sodium azide (NaN3), silicon dioxide (SiO2), and potassium nitrate (KNO3).1,2 The reaction of these compounds is initiated through the conduction of an electrical signal, which causes them to undergo deflagration.1 Deflagration is a type of gradual detonation that releases a predetermined amount of nitrogen gas that inflates the bag. What is the role of KNO3 and SiO2? Their sole responsibility is to remove the highly reactive and potentially explosive sodium metal by transforming it into harmless material first.3

As shown in reaction (1), the products (sodium and dinitrogen) are generated. The sodium metal is used in reaction (2) to form potassium oxide (K2O), sodium oxide (Na2O) and nitrogen (N2).1  In the third reaction, potassium oxide and sodium oxide react with silicon dioxide (SiO2) to form this alkaline silicate glass.3 This compound is presumed to be safe and fireproof. Sodium oxide and potassium oxide are classified as Group 1A metal oxides (meaning that they are metals found in Group 1 of the Periodic Table and contain an oxygen). Group 1A metal oxides are known to be highly reactive.3 To say that something is highly reactive means that the metal element will combine with a very large number of other elements and do so very vigorously with the addition of a significant amount of heat.1,2 This would be unsafe to leave them in this state because sodium azide is notorious for causing explosions.

When production of N2 comes to a halt, the gas molecules are able to leave the airbag through vents.1 When this happens, the pressure in the bag drops and the bag shrinks mildly enough to work as a cushion.3 Two seconds after the collision or impact has occurred, the pressure within the airbag will have reached atmospheric pressure (1 atm).

In order for an airbag to function properly, it must be filled with a certain amount of nitrogen gas.3 How do we go about calculating the amount of gas needed? For this, we will need to refer to the Ideal Gas Law, PV=nRT .3

                                                P = Pressure (measured in atm)

                                                V = Volume (measured in liters)

                                                n = Number of moles of gas

                                                R = Universal constant (0.08205 L◦atm/mol◦K)

                                                T = Temperature (measured in Kelvin)

Dinitrogen has low reactivity with other gasses, making it an inert gas. Because of this, it can be classified as an ideal gas at the pressure and temperature required to inflate the airbag. The Ideal Gas Law will provide a good approximation of the relationship between the volume, pressure and moles of N2 contained in the bag.3 In order for the airbag to inflate within milliseconds, it needs to be filled a certain amount of pressure. First, the acceleration of the airbag and force exerted on the airbag to create this acceleration must be calculated.3 The amount of pressure can be acquired only after finding the acceleration and force measurements. Once the amount of pressure has been generated, the Ideal Gas Law will calculate the amount of N2 required to fill the airbag.3 The sodium azide (NaN3) in the gas generator will be the chemical used to create this amount of N2. Airbag volume is generally 60-70 L and is normally equipped with about 200 grams of sodium azide. 3, 4

References

  1. A. "The Chemistry Behind the Airbag: High Tech in First-Year Chemistry," (1996) J. Chem.  Ed., 73 (4), p. 347-348.  
  2. Bell, W.L. "Chemistry of Air Bags," (1990) J. Chem. Ed. 67 (1), p. 61. 
  3. Casiday, Rachel, and Regina Frey. "Gas Laws Save Lives: The Chemistry Behind Airbags." http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html (accessed Sept. 22, 2016).
  4. Stiles, Lori. "Sodium Azide in Car Airbags Poses Growing Environmental Hazard, UA Scientist Says." https://uanews.arizona.edu/story/sodium-azide-in-car-airbags-poses-growing-environmental-hazard-ua-scientist-says (accessed Nov. 2, 2016).