Tag Archives: airbag

Why does an airbag inflate?

For years, seat belts have represented the unique real safety device in our cars. Despite its not being immediately accepted, eventually the seat belt demonstrated all its efficiency.

The same happened to the airbag, whose concept has been around for many years. Its invention goes back to 1952, by John W. Hetrick who submitted the patent the following year. The first use is remembered at the end of the sixties, together with the big improvements to the other components necessary to its proper functioning. After all the prototypes introduced and tested by practically all the biggest car companies, the first vehicle carrying this kind of innovation was the Oldsmobile Toronado in 1973, followed by other models produced by Buick and Cadillac. In Europe it was Mercedes Benz the first one to offer the accessory on its top cars in 1980. After a first period of indifference and skepticism the airbag briefly took over at industrial level, turning into one of the strongest safety devices on a vehicle. The National Highway Traffic Safety Administration estimates that the combination of an airbag plus a lap/shoulder belt reduces the risk of serious head injury by 85 percent compared with a 60 percentage reduction for belts alone.

But how does an airbag work? Why does it inflate? Well you can believe it or not but chemistry saves our lives!

airTiming is absolutely crucial for the airbag to save a life. An airbag must be able to inflate and work properly in a few milliseconds after the first collision. Meanwhile, it has to be projected in order to be restrained from deploying when the accident is negligible. Therefore, the primary component of the system is a well calibrated sensor able to reveal front strokes and to generate an immediate inflation of the device. One of the simplest mechanism studied for the collision sensor is a steel little sphere which is free to slide inside a smooth pipe. The ball is controlled by a magnet or by a rigid spring, which reduces the movements of the sphere when encountering bumps or potholes. Nevertheless, when the car decelerates rapidly, as for example during a crash, the sphere moves fast to the front triggering an electric circuit.

When the sensor switches on the circuit, a tiny mass of sodium azide (NaN3) starts burning in a very fast reaction, developing nitrogen (N2). This gas fills a nylon or polyamide envelope, which inflates completely after only 40 milliseconds. Ideally the body of the driver should not hit the airbag during the phase of inflation but just after, when it is beginning to lose pression. Otherwise the surface of the envelope would be too hard and may hurt the driver.

imagesInside the airbag there is a gas generator containing a mixture of NaN3, KNO3, and SiO2. When the car undergoes a collision, a series of three chemical reactions occur inside the gas generator. These reactions produce gas (N2) to fill the airbag and convert NaN3, a highly toxic substance, to harmless sodium and potassium silicate, a major ingredient of glass. Sodium azide (NaN3) can decompose at 300oC to produce sodium metal (Na) and nitrogen gas (N2). The signal from the deceleration sensor ignites the gas-generator mixture by an electrical impulse, creating the high-temperature condition necessary for NaN3 to decompose. The nitrogen gas that is generated then fills the airbag. The purpose of the KNO3 and SiO2 is to remove the sodium metal (which is highly reactive and potentially explosive) by converting it to a harmless material.

air-bag-deployed

First, the sodium reacts with potassium nitrate (KNO3) to produce potassium oxide (K2O), sodium oxide (Na2O), and additional N2 gas. The N2 generated in this second reaction also fills the airbag, and the metal oxides react with silicon dioxide (SiO2) in a final reaction to produce silicate, which is harmless and stable. (First-period metal oxides, such as Na2O and K2O, are highly reactive, so it would be unsafe to allow them to be the end product of the airbag detonation.)

Well… as I said chemistry saves lives!

That’s it! Cool, isn’t it?

(Some information summarized from “Gas Laws Saves Lives: The Chemistry Behind Airbags”)

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by Francesco Pochetti