Explosive that save life

Do you know something? A fast chemical reaction makes driving car feels safe. Most cars have airbags in the dashboard and steering wheel that can blow up like super-fast balloons during a collision, and protect passengers from getting hurt. Which makes the airbag blow up like a balloon? Yeah! You guess right, Chemistry! Instead of carrying compressed gas in the car to inflate the airbag, we take advantage of a very fast chemical reaction that produces the gas.

Anyway we don't usually associate automobiles with chemistry but a lot of chemistry takes place in a working car. For example the burning of fuel to run the engine, chemical reactions occur in the battery to generate electricity and most important chemical reaction is involved in the air bag. The chemical sodium azide (NaN3) is called as the heart of the air bag.

Who have idea?

A Japanese automobile engineer named as Yasuzaburou Kobori, have an ingenuous idea to solve the problem of creating a lot of gas very quickly to inflate the airbags by using a gas-generating chemical explosion in 1964.

What's in it? 

The air bag itself is made up of a thin nylon fabric, which is folded into the steering wheel or dashboard. The airbag system ignites a solid propellant, which burns extremely & rapidly to produce a large volume of gas to inflate the bag. The bag then literally bursts from its storage site at up to 322 kmph speed which is faster than the blink of an eye! After a second, the gas quickly dissipates through tiny holes in the bag, thus deflating the bag then you can move.

How is it work?

1. The airbag control unit receives the signal from the crash sensors which are typically found at the front of the car after a sudden deceleration is detected.

2. The main control unit receives data from other sensors, like wheel speed sensors, brake pressure sensors, and seat occupant status sensors.

3. After analyzing the information, which provided by the sensors the airbag control unit can activate security features like the seatbelt lock, automatic door lock, and airbag deployment.

4. When an accident is detected, the main control unit alerts the inflator.

5. The inflator starts a chemical reaction which produce an explosion of nitrogen gas to fill the airbag.

6. When the airbag is fully filled with nitrogen, it ruptures through the panel that is holding it and cushions the passenger.

7. After deployment, the airbag is deflated automatically. 

Where is chemistry?

 Many car airbag inflators contain small amounts of a toxic chemical called sodium azide (NaN3). Its breaks down very fast when heated or shocked. This reaction is called decomposition. The products of the decomposition are sodium atoms and hot blasts of the nitrogen molecules. Under normal condition, this molecule is quite stable. If heated, it will fall apart. The chemical equation 2 NaN3 --> 2 Na + 3 N2 describes exactly how it falls apart. The second product of the above reaction is N2, also known as nitrogen gas.

A relatively small amount of sodium azide (130 g) will produce 67 liters of nitrogen gas very quickly; it takes almost five party balloons’ worth of gas to fill a normal air bag! That's not the only chemistry involved. Notice that the other chemical into which sodium azide falls apart is sodium (Na).

Sodium is a very reactive metal that will react rapidly with water to form sodium hydroxide which is a strong base, it would be quite harmful if it got into your eyes, nose or mouth. So, to decrease the danger of exposure, air bag manufacturers mix the sodium azide with other chemicals such as potassium nitrate (KNO3) and silicon dioxide (SiO2) to react with the resulting sodium metal. That reaction gives potassium silicate (K2SiO3) & sodium silicate (Na2SiO3), both can able to stop the sodium from reacting with moisture in the air to form corrosive NaOH.

The powdery substance which released from the airbag after collision is regular cornstarch or talcum powder. This is used by the airbag manufacturers to keep the airbags flexible and lubricated while they are in storage.

What's going on?

Today’s airbags use a different chemical to produce nitrogen gas: guanidinium nitrate with a copper nitrate as oxidizer. When heated, guanidinium nitrate breaks down into nitrogen gas, water, and carbon. 

The copper nitrate oxidizer reduces the temperature of the released gas. This composition has other advantages like guanidinium nitrate is relatively cheap unlike ammonium nitrate and it is not moisture sensitive.

Is this safe?

The airbag is one of the most important automotive safety features. Airbags are considered "passive" restraints and act as a supplement to "active" restraints. Because no action by a vehicle occupant is required to activate or use the airbag, it is considered a "passive" device. This is in contrast to seat belts, which are considered "active" devices because the vehicle occupant must act to enable them.

Airbags will activate and inflate within a second of a severe crash only not In the event of an emergency brake. Because forces greater than 20 times the gravitational force (20Gs) or force equal to running into a brick wall at 16 to 24 km per hour can only activate them. Thus, an accelerometer is required for a car's airbag system. It takes this sensor between 12 and 20 milliseconds for the frontal airbag to rise, and another 60 to 65 milliseconds for it to fully inflate.

What are the types?

Frontal airbags: These airbags are used to damper collisions between the front dashboard, steering wheel and passenger.

Door Mounted Airbags: They are assembled in the front doors on both sides.

Roof Rail Airbags: These are located at the back of the trim that covers the roof rails.

Seat Mounted Airbags: Seat-mounted airbags are situated on the outboard side of the driver's seat, as well as the front and the back passenger side.

Knee Airbags: These airbags ensure the safety of the driver and the front-seat passengers. They are located in the lower portion of the instrument panel.

In India the cars usually feature this types of airbags.

You may be think sodium azide is also used to inflate airplane escape chutes in case of an accident. 

Comment your thoughts of other situations where fast production of a gas is needed.

References:

1) https://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/airbag1.htm

2) https://www.explainthatstuff.com/airbags.html

3) https://www.scientificamerican.com/article/how-do-air-bags-work/

4) https://en.wikipedia.org/wiki/Airbag

5) https://www.laboratory-supply.net/blog/how-does-a-vehicle-airbag-work/