Airbag: Can “explosion” save lives?

Author: Xue Bin

When we think of explosions, we often think of dangerous and even life-threatening situations. However, there are two sides to every coin. If explosive chemicals are used properly, they can also benefit mankind. For example, the working process of airbags in cars is called "life-saving explosions."

Inspiration from a car accident

The efficiency and convenience brought by cars have changed people's lifestyles. However, with the increase in car power and speed and the rise of highways, traffic accidents are becoming more and more frequent. In this regard, people have to consider how to reduce personal injuries when a car accident occurs.

In the 1950s, after experiencing a car accident, American engineer John W. Hetrick came up with the idea of ​​installing airbags on cars to reduce the damage to drivers and passengers caused by the huge impact force when the car is hit. At that time, his plan was to use compressed air as a buffer material - that is, to install a cylinder full of compressed air on the car - when a car accident occurs, the air is released into the airbag to act as a cushion.

But the drawbacks of this type of airbag are obvious: the compressed air cylinder not only takes up space and increases the weight of the car, but also has the potential for explosion. More importantly, the compressed air cannot be released quickly.

Are there airbags that don't take up space and only inflate when a severe collision occurs?

The "slightly better tempered" member of the Explosive Family

After exploration, automotive engineers turned their attention to the chemical sodium azide (NaN3). Sodium azide is an azide salt, which is composed of positively charged sodium ions and negatively charged azide ions. Among them, the azide ion is composed of three nitrogen atoms connected in a string, like a human pyramid, so it is named "azide". Azide salts have a common characteristic of "hot temper" - they are easy to explode when stimulated by heat, impact and strong light, and are converted into nitrogen and metal.

Sodium azide is a white crystalline solid, which is a "slightly better tempered" member of the azide salts and relatively stable. It can be decomposed into metallic sodium and nitrogen under controlled conditions, so it was selected as the detonating material for airbags. According to statistics, on average, each car carries 300 grams of sodium azide, which can release 155 liters of nitrogen, enough to fill the airbags.

However, although nitrogen is a harmless gas, metallic sodium is very active. Especially after contacting water, it will undergo a violent chemical reaction and produce corrosive sodium hydroxide, which may cause secondary damage to the occupants of the vehicle. To solve this problem, engineers added potassium nitrate and silicon dioxide to sodium azide. The metallic sodium produced by the explosive decomposition of sodium azide will react chemically with potassium nitrate and silicon dioxide to produce safe and harmless sodium silicate and potassium silicate, thereby improving the safety of the airbag.

In addition, the airbag system also includes collision sensors, control systems, ignition components, nylon airbags, etc. The entire airbag is usually placed in a folded state in the steering wheel lining and in the dashboard opposite the passenger seat. Some vehicles also have built-in airbags in the roof, doors and seats.

0.03 seconds to save your life

How do airbag systems containing sodium azide work?

When an accident occurs, the collision sensors located on the left and right ends of the front of the vehicle will sense the stall state of the vehicle and then send an electrical signal through the control system; after the ignition element receives the transmitted electrical signal, it starts the electric heating program, and the sodium azide in it will explode in just 0.03 seconds after being heated.

The nitrogen released after the explosion will quickly expand, fill, and eject the folded airbag, blocking the steering wheel, dashboard, driver, and front passenger. In addition, there are tiny pores on the airbag that can gradually release the nitrogen in the airbag, allowing it to fully play its cushioning role without affecting people's activities.

Studies have shown that if airbags are used correctly in the event of a car accident, the driver's mortality rate can be reduced by 29%, and the passenger's mortality rate can be reduced by 32%. It can truly be said that "the explosion saves lives"!

The automobile airbag system has gone through many iterations and finally achieved effective safety protection by cleverly utilizing violent chemical reactions. It is a microcosm of the benefits that science and technology bring to mankind. If you were an automobile engineer, what system would you like to invent to protect everyone?

(Author's unit: Shanghai Ocean University)

Editor-in-charge: Hu Huiwen