In a dramatic expansion of what is already the largest automotive recall in history, Takata Corp.—which recently filed for bankruptcy protection—has just added 2.7 million vehicles to the total.
NHTSA has determined the root cause of the problem is airbags that use ammonium nitrate-based propellent without a reliable chemical drying agent, i.e., without an effective desiccant. Environmental moisture, high temperatures, and age contribute to the process that can disastrously and explosively overinflate the airbags’ inflator mechanism.
Vehicles made by Ford, Nissan and Mazda are using a type of airbag inflator including a desiccant, or drying agent, that previously was thought to be safe. Commonly encountered desiccants are solids that absorb water. They are commonly used in packaged foods to retain crispness and are in no way explosive in their own right.
However the desiccant’s role in the Takata airbag inflation mechanism is to prevent the propellant, largely ammonium nitrate, from becoming hydrated and therefore unstable. In chemical terms instability can affect the burn rate of the compounds and behave unpredicably leading to malfunction.
Ammonium nitrate is an inexpensive propellant that cycles through five solid states depending on temperature. Generally this occurs in a predicable way and as the vehicle goes from the heat of sunshine to the cold overnight, the temperature swing is large enough for the ammonium nitrate to change from one phase to another. Ammonium nitrate also absorbs moisture from the atmosphere readily, hence the need for a desiccant to keep the compounds dry. The desiccant is there to stabilize this process and, if it fails, may make ammonium nitrate dangerous, engineers say.
Exposure to moisture and temperature fluctuations can degrade the propellant ammonium nitrate, the volatile compound Takata’s inflators use to deploy airbags. The company has used a variety of chemical agents to keep the propellant dry in its devices over the years, with some combinations showing a greater propensity to fail than others, federal regulators said.
The real problem is that the inflators have an unacceptable failure rate due to occasional fragmenation of the metal inflator housing blasting shrapnel into the passenger compartment. Due to aging and humidity, the propellant that inflates the air bag oxidizes fuel granules and wafers explosively causing the inflator’s casing to rupture, and blasting the shattered fragments with sometimes lethal velocity.
Ford, Mazda and Nissan installed these inflators in vehicles manufactured for the United States market from 2005 through 2012, according to Takata. All are on the driver’s side of the vehicles. The affected vehicles are from the 2005 through 2012 model years.