The new airbag propellant was supposed to be the next big thing for Takata in 1998. An engineer for the company, Paresh Khandhadia, declared it "the new technological edge" in an interview with a trade magazine then.Based on a compound called tetrazole, it was seen as a reliable and effective compound for inflating airbags. Yet despite the fanfare, by 2001 Takata had switched to an alternative formula, ammonium nitrate, and started sending the airbags to automakers, including Honda.
That compound, according to experts, is highly sensitive to temperature changes and moisture, and it breaks down over time. And when it breaks down, it can combust violently, experts say.
"It shouldn't be used in airbags," said Paul Worsey, an expert in explosives engineering at the Missouri University of Science and Technology. The compound, he said, is more suitable for large demolitions in mining and construction. "But it's cheap, unbelievably cheap," he added.
More than a decade later, that compound is at the center of a safety crisis involving Takata and its airbags. More than 14 million vehicles with the Takata-made airbags have been recalled worldwide over concern than they can explode violently when they deploy in an accident, sending metal debris flying into the cabin. At least five deaths have been linked to the defective airbags.
On Thursday, Takata's decision to change the propellant is expected to be among the lines of questioning before the Senate Commerce Committee, which is investigating Takata's defective airbags.
Alby Berman, a spokesman for Takata, said the switch to an ammoniumnitrate-based propellant was not driven by cost considerations. Instead, the company's engineers determined that the compound produced gas more efficiently with fewer emissions.
"This breakthrough allowed us to make the smallest, lightest inflaters available, as well as significantly improve manufacturing safety," Mr.Berman said.
Two former Takata engineers said they and other employees had concerns over switching to such a risky compound.
"It's a basic design flaw that predisposes this propellant to break apart, and therefore risk catastrophic failure in an inflater," said Mark Lillie, a former senior engineer with Takata at its propellant plant in Moses Lake, Wash. Mr. Lillie recently shared his concerns with Senate staff members.
"It was a question that came up: Ammonium nitrate propellant, won't that blow up?" said Michael Britton, a chemical engineer who worked with Mr. Lillie at the Moses Lake plant. "The answer was, not if it stays in the right phase."
Takata will be represented at the hearing by Hiroshi Shimizu, the company's senior vice president for global quality assurance. He will be joined by representatives for Chrysler and Honda, as well as a victim, Stephanie Erdman. David Friedman, the deputy administrator of the National Highway Traffic Safety Administration, will also appear.
Takata's struggle with propellant stretches back to 1991, when the Tokyo-based supplier first started to manufacture airbag inflaters in the United States.
Like other airbag manufacturers at the time, Takata based its airbag propellant on a toxic compound called sodium azide. But that compound is volatile and could release toxic fumes into the car, causing chemical burns or breathing problems when the airbags deployed.
Takata then turned to tetrazole, which it promoted to automakers at the time as a safer, more environmentally friendly alternative. Takata introduced the propellant, marketed as "Envirosure," to automakers in the mid-1990s for inclusion on 1998-model vehicles.
"I said, ‘Wow! This is the break!' " Mr. Khandhadia, Takata's lead propellant engineer, told the industry publication Automotive News at the time, describing the moment tests showed the new propellant worked.
But tetrazole, which is produced in limited quantities and can be expensive, started to squeeze margins at Takata, especially as the airbag market became more competitive, Mr. Lillie said.
By 1999, Takata researchers in Michigan, pressured by executives, developed a propellant based on ammonium nitrate, he said.
But the engineering team in the Moses Lake plant raised objections to basing a propellant on such a risky compound. To bolster its case, the team pointed to explosives manuals warning that the compound "tended to disintegrate on storage under widely varying temperature conditions" with "irregular ballistic" consequences, Mr. Lillie said.
Ammonium nitrate cycles through five solid states. As the vehicle goes from receiving 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, experts say. Ammonium nitrate also absorbs moisture from the atmosphere readily. Those two things together make ammonium nitrate tablets prone to damage, experts say.
A focus in the mushrooming recalls has been that the airbags are more susceptible to malfunction in high humidity areas.
"Speaking generally, ammonium nitrate can be unstable. Its crystal structure can change according to temperature," said Katsumi Kato, an assistant professor in safety engineering at Japan's Fukuoka University. "It changes the burn rate. It leads to various malfunctions."
Other airbag makers have said they stayed away from the explosive compound.
"We've made another choice for the propellant we produce," Thomas Jonsson, a spokesman for Autoliv, a Swedish-American automotive safety products manufacturer, said in an email. Key Safety Systems, another airbag maker, said it used guanidine nitrate and tetrazole - which experts said was less risky and more durable than ammonium nitrate - in its inflaters. TRW Automotive, a large supplier of safety parts based in Michigan, also uses a propellant based on guanidine nitrate, experts said, though the airbag maker did not respond to requests for confirmation.
Still, at Takata, the answer at the time was to try to stabilize the ammonium nitrate to try to mitigate those cycling effects, but there are limits to just how far ammonium nitrate can be stabilized, said Mr. Worsey, the explosives expert.
The doubts over Takata's propellant raise questions of whether the recalls should be limited to humid regions. The National Highway Traffic Safety Administration, however, said Tuesday that it would urge automakers to expand recalls of certain drivers' side airbags that had previously been limited to states and territories with high humidity. Takata has said it continues to use ammonium nitrate in its replacement airbags.
In addition to the Senate hearing, Takata is facing mounting legal challenges. Takata said this month that it had received a subpoena for documents related to the defects from a federal grand jury in the Southern District of New York. The company confirmed that it hired Andrew Levander, a well-known defense lawyer based in New York, as it prepares for a criminal investigation.