Toyota last week announced the recall of units of its Corolla Altis sedan in India to rectify a problem with the passenger side airbag supplied by Japanese auto parts firm Takata Corp. The recall, expected to cover 23,157 Altises built between January 2010 and December 2012, is part of the ongoing recall of 2.9 million vehicles globally for defective Takata airbags.
The recall concerns airbag inflators which, if exposed to heat for long, could explode or deploy inadvertently. At least 16 deaths have been linked to exploding Takata inflators, mainly in the US, prompting the auto industry’s biggest-ever global recall.
This January, Honda recalled 41,580 units of previous generation Accord, Civic, City and Jazz models in India to fix airbag problems.
On February 27, Takata — which was founded in the 1930s as a textile maker that produced parachutes for the Japanese Army during World War II, and in the 1960s diversified into high-end auto components such as airbags — had agreed to pay $ 1 billion in penalties for concealing a defect blamed for the 16 deaths, of which 11 were in the US, and over 180 injuries worldwide. Takata admitted to concealing evidence that millions of its airbag inflators could explode with too much force, hurling lethal shrapnel into car occupants.
HOW AIRBAGS WORK
Airbags soften the impact of collisions by keeping occupants from coming into contact with the steering wheel, dashboard, front glass and other parts of the automobile. To do this, airbags need to inflate quickly. They are inflated with gas created by igniting propellants. The propellant — such as those used in space rockets or in detonators used for mining blasts — is compressed into candy-size pellets and placed in a metal tube called an inflator. When a crash occurs, the tablets are ignited and convert from solid to gas, which shoots out of the inflator and into the airbag in milliseconds.
The crucial thing about an airbag is that it has to be small enough to fit into the steering wheel and other tight spaces, and has to deploy with just the right amount of force. That’s where propellants come in. Each of the world’s five main airbag manufacturers has developed its own trademark chemical compound to act as proprietary propellant.
Takata’s original propellant was based on sodium azide, which was used for launching submarine torpedoes and missiles. Sodium azide had its drawbacks, including the fact that it was prone to exploding when exposed to air or light, and was toxic if inhaled. Takata’s second-generation propellant, introduced in the late nineties, was based on tetrazole.
Ammonium nitrate — among the world’s most widely used commercial chemical explosives that is nearly as powerful as dynamite but costs about a tenth that of tetrazole — was the company’s third generation propellant. While it delivered as an explosive, the problem with ammonium nitrate was that changes in temperature could induce changes in its stability.
Ammonium nitrate is a strong oxidant that reacts with combustible and reducing materials. It has a critical relative humidity of 59.4%, which is the value of relative humidity of the surrounding air, above which the material absorbs moisture, and below which it does not. Beyond the critical relative humidity level, the propellant tablet could break down into powder form. Since the compound as powder combusts faster than as a denser pellet, airbags in which the propellant had disintegrated due to temperature or humidity changes tended to have far greater combustion qualities, and were prone to deploying faster and with greater punch.
This is at the heart of the problem with Takata airbags using ammonium nitrate-based propellant. If the propellant ignites with explosive force, the airbag’s inflator — the metal cartridge in which the propellant pellets are loaded — could rupture, and a spray of shards can rocket out into the passenger cabin.