Article originally from American Chemistry magazine.

Chemistry is an integral part of transportation safety. In fact, statistics show advancements in transportation technology made possible through chemistry-based initiatives have been essential in saving thousands of lives. One such initiative in recent decades has been the air bag, which the National Highway Traffic Safety Administration (NHTSA) cites for reducing the odds of being killed in a direct frontal car crash by approximately 30 percent. This life-saving device would not be possible without the nitrogen or argon gas that deploys in the event of an accident, along with the high-strength nylon fabric of the bag itself.

Form and function

While the first commercial air bags for automobiles appeared in the 1980s, they only became required elements on both the driver and passenger sides of new cars in model year 1998 and light trucks, minivans and sport utility vehicles (SUVs) in 1999. Most bags are mounted in the steering wheel and dashboard, sometimes hidden behind a seamless polyurethane panel. There are also door-mounted side air bags and seat-mounted air bags for passengers in back.

Air bags are supplementary restraint systems, intended to work alongside seat belts to help slow the forward motion of the driver and passengers when the vehicle stops suddenly due to a collision. The difficult task of an air bag is to cause little or no damage within tight constraints of space (i.e. between the person and the object in front of them) and time (i.e. the fraction of a second it takes for the person to move through that space). The parts of the assembly necessary for this feat include the crash sensor system, the inflation system and the nylon bag itself.

Sensor system

When the vehicle’s collision force reaches a certain level, caused by crashing at speeds of at least 8 to 14 miles per hour, the crash sensors activate the air bag’s inflation system. An accelerometer, built into a microchip, provides deceleration data to the sensor. A mass shift closes an electrical contact, which in turn flips a mechanical switch. A diagnostic module receives the electrical signal and, after testing to ensure there is a crash, allows the signal to trigger the inflation of the air bag. The diagnostic module also operates at other times, monitoring the air bag system’s readiness. When the driver turns on the vehicle’s ignition, for instance, the diagnostic module is activated and, if it detects a problem, it turns on a warning light to alert the driver.

Inflation system

In some inflation systems, sodium azide and potassium nitrate react to create nitrogen gas. Other systems create argon gas. The bag is then inflated with hot blasts of the gas. The system works somewhat like a solid rocket booster, as a solid propellant is ignited and its burning creates a large volume of gas very quickly. This process causes the air bag to burst out of its storage location at a speed between 100 and 200 miles per hour.

Most inflation systems are designed to deploy in a vehicle fire if the temperature rises to 300 to 400 F (150 to 205 C). This is a safety feature, as such high temperatures could otherwise cause the inflation system to explode within the air bag system. New systems are being developed to specify certain levels of inflation. Multi-stage inflators could deploy less forcefully in moderate collisions than in severe crashes. Also, occupant-sensing devices could tailor the deployment to a child or adult and his/her position.

Nylon bag

Air bags require engineered materials that can withstand high mechanical stresses and temperatures. During deployment, the internal pressure within the bag can rise as high as 100 kPa, while the temperature can reach 5,000 F (2,760 C). Some inflators also expel hot particulates that increase thermal loading to the system and could potentially cause burn-through.

Air bag nylon fibers are designed with energy absorption and thermal resistance properties to keep both pressure and heat within the bag. Nylon is also used for the casing for the air bag. When a driver-side air bag is fully inflated, it has approximately the diameter of a large beach ball. A passenger-side air bag, however, can inflate to twice or three times that size, since it needs to cover a greater distance (i.e. from the dashboard to the passenger).

Air bag manufacturers use either talcum powder or regular cornstarch to keep the bags lubricated and pliable during storage and deployment. This is why a powdery substance is released into the car’s interior upon air bag inflation. Small amounts of sodium hydroxide may be present, turning into sodium bicarbonate upon exposure to air. Potassium chloride is sometimes also present. Immediately after inflation, tiny ventilation holes in the bag allow the gas to dissipate rapidly. The bag deflates, allowing the driver or passenger to move once more.

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