The chemistry behind the recent airbag recall
Curiosity Corner by Dr. Jerry D. Wilson
Emeritus Professor of Physics, Lander University
QUESTION: Why was there such a big airbag recall recently, when they have been used for years? (Asked by a curious driver.)
REPLY: The inflatable automobile airbag is quite common and is found in most cars. This safety device has saved many lives. Yet, something must have gone wrong to spur such a massive recall. The problem arose when some airbags inflated and pieces of metal flew out, causing serious injury. These metal shrapnel pieces have killed 13 people, including 10 in the U.S., and have injured more than 100. What is the cause of this dangerous defect?
A nitrogen gas is used to inflate the airbag. The chemical propellant, or source of nitrogen gas, is compressed into a pill-sized tablet and housed in a metal inflator canister. On rapid car deceleration (such as in a collision) an electric signal is sent to an ignitor that causes the propellant to explode, producing nitrogen gas that inflates the airbag. This takes place in about 30 milliseconds (0.030s) – time enough to prevent injury to a car’s passenger, who would hit the airbag about 50 milliseconds after collision. All is well with proper airbag operation, but not with pieces of metal coming from a fractured inflator canister, due to an over-reactive propellant explosion.
For some time, the propellant of choice for airbag manufacture was sodium azide (NaN3). Research for more efficient and cheaper propellants was done, and several were developed. Takata, a large Japanese airbag company with plants in the U.S. and other countries, chose an ammonium nitrate (NH4NO3) propellant. On explosion, ammonium nitrate decomposes into water and a nitrogen gas, nitrous oxide (N2O), which inflates the airbag.
In general, ammonium nitrate functioned properly as a propellant for airbag inflation, but in instances of flying metallic shrapnel, something was wrong. Because of its chemistry, ammonium nitrate can become less stable. This occurs with temperature changes – for example, the temperature change a car experiences in going from day to night. Ammonium nitrate also readily absorbs moisture from the air. These things eventually cause the propellant tablet to break down into a powder. Powder burns more quickly than a tablet, so an airbag with powdered propellant could lead to an over-explosion, rupturing the metal inflator canister and producing flying metal fragments. This occurrence is what led to the recall of millions of air bags.
C.P.S. (Curious Postscript): Ninety-eight percent of the adults in this country are decent, hardworking, honest Americans. It’s the other lousy two percent that get all the publicity. But then – we elected them. –Lily Tomlin
Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or email firstname.lastname@example.org. Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to www.curiosity-corner.net.