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Airbags: Materials make a difference

Article-Airbags: Materials make a difference

Airbags: Materials make a difference

Perhaps as early as 1999, federal regulators may require automakers to introduce less complicated versions of "smart" airbags. But the National Highway Traffic Safety Administration (NHTSA) has yet to define what a smart airbag must do, adding to the complexity of design decisions. Even then, suppliers acknowledge that it won't be possible to design a fail-safe system.

Further confounding the problem is a diversity of often emotional opinions on what the law should encompass and how the final version might impact current design projects. For example, should it include a switch that would allow the car owner to deactivate the airbag system? And what about passengers who refuse to use seat belts? The latest statistics show that more than 30% of Americans decline to wear them. Must engineers be forced to factor this choice into their new airbag designs?

Historically, however, records show that current airbag designs are not all that bad. Bags inflating at speeds up to 200 mph have saved nearly 1,700 lives in higher-speed accidents. They also have been blamed for the deaths of 35 children and 20 adults in low-speed accidents. Most of the victims were not wearing seat belts.

Materials matter. Manufacturers who supply the materials that make up an airbag system's components are not taking a back seat in this design dilemma. In fact, many have introduced materials in new airbag systems that already address some of these questions. Others are looking at what materials will move to the forefront in future designs now under development by airbag makers in the U.S. and Europe.

Last year, more than 15 million airbag systems were made globally, with about 50% made in North America and 40% in Europe. By the year 2000, research indicates that more than 90 million airbag systems will be produced for driver- and passenger-side use. In fact, all 1997-model cars sold in the U.S. had to have a driver's side airbag, with passenger-side airbags mandated for all cars and light trucks by next year. Further, automotive designers are working on side-impact airbag systems that could be located in the headliner, seat, or door.

Taking this into consideration, the next major application for plastics in automobiles may involve an injection-molded nylon airbag canister housing using an innovative new airbag system design. So reports Al Winterman, director of BASF Automotive Materials.

Currently, most canister housings for passenger-side airbags are formed from steel sheet or extruded aluminum, which typically weighs about 2.3 lbs (1,040 grams). The new system uses a lightweight (729 gram) injection-molded nylon canister housing. Improved versions will be even lighter (450 gram), Winterman predicts.

The first passenger-side airbag system with a molded nylon canister housing resides in the 19951/2 Opel Vectra(R) B and Omega(R) models. Allied Signal, BASF, Lemforder Metallwaren Elastmetall, and Opel collaborated in the system's development.

For deployment of current airbags, an electronic trigger ignites a nitro cellulose-based fuel, which produces a hot gas to rapidly inflate the bag. In the new design, Arcite(R) propellant replaces the nitro cellulose-based fuel. The gas released into the airbag by this system is warm, but not hot.

The new design is said to offer several advantages. Among them:

Pressure performance and buildup are uniform and moderate. The nylon housing restricts container expansion.

  • Surface temperature of the bag increases very slightly upon deployment. Typically, temperatures do not exceed 45C above ambient.

The canister housing is molded of Ultramid(R) B3ZG8 resin, a 40% glass-fiber-reinforced, impact-modified type 6 nylon from BASF Corp. Plastics Materials (Mt. Olive, NJ). The one-piece housing, which incorporates ducts for cable and plug attachments, requires five to seven fewer parts than current designs, significantly cutting manufacturing and assembly costs.

U.S. car companies are developing passenger-side airbag systems that include the new technology and the nylon housing. However, space and crash-impact restrictions inside a vehicle's steering wheel preclude its use for that application at present. The housing now measures about 10 inches long, four inches wide, and five inches high.

Make mine magnesium. Given this push to plastics, another entry into the canister picture shouldn't be overlooked. Magnesium alloys are said to be rapidly replacing mild steel, zinc, and aluminum as the material of choice for die-cast steering components. In fact, industry sources estimate that more than 40% of cars put on the road in North America this year will use steering wheels made of die-cast magnesium alloys, according to Dow Magnesium figures. That's expected to grow to more than 60% within the next three years.

What's this got to do with airbags? An airbag places weight on the steering column where leverage is greatest. If not properly designed, this added weight could cause the steering wheel to vibrate under normal driving conditions. Steering wheel and column components composed of die-cast magnesium alloys lighten the load, reduce vibration, and help maintain rigidity--all factors in proper airbag deployment.

But magnesium's influence on airbags doesn't end there. Newer designs of instrument panels are sleeker, smaller, and more like fighter plane cockpits. The tighter package size and convoluted panel surface require better dimensional control of the airbag mounting locations. In turn, the fit and finish of the airbag door is governed by the position of the airbag housing.

That's why Chrysler turned to a magnesium passenger-side airbag (PAB) housing for its newest minivans. In a paper presented at this year's SAE International Congress & Exposition in Detroit, Chrysler's Fred Daris and AlliedSignal's Steven C. Bell, part of the airbag component's design team, attribute the vehicle's successful seamless passenger door and sweeping instrument panel surface to the magnesium die-cast housing .

In early PAB development, the housing generally consisted of steel. However, steel design can have as many as five stampings and several dozen rivets or welds for strength and bracket attachments; the magnesium design is one piece. The steel canister required 34 operations; the magnesium canister only seven.

Moreover, the PAB system for the minivans was more compact than its predecessor and required tighter dimensional control. A single-piece steel stamping was the design initially released for production. Deep-drawn aluminum, injection-molded plastics, HSLA steel, and magnesium were considered as alternatives.

When comparing raw material costs exclusively, the steel design won out. However, this cost didn't reflect the multitude of required secondary operations, coating, and system brackets needed to install the PAB. Although more costly than any other materials under consideration, the design team noted that new magnesium production facilities coming online by the year 2000 should stabilize the material's cost.

Comparison of Canister Materials
WEIGHT S + - +
TOTAL 0 2 -2 1

A look at the processing of magnesium vs. steel also showed the design team that most of the secondary operations and brackets could be incorporated in the canister's casting. In addition, the design eliminated the coating required for steel.

The most significant difference in the cost between the steel canister design and the magnesium, however, was that the steel design required added mounting brackets. The elimination of these alone resulted in a savings of about $1 per canister.

Where to next? The design team notes that magnesium "may not be the best material for all PAB canisters," but it adds that with careful design (for instance, care was taken to keep the hot gas from impinging directly on the housing) it is a viable solution.

No color barrier. There's also some new thermoplastic elastomer (TPE) compounds under development that enhance surface durability and make it possible to 100% precolor airbag covers. The new compounds are advanced formulations of Telcar(R) thermoplastic polyolefin (TPO) and Tekron(R) TPE products from the Plastic Division of Teknor Apex (Pawtucket, RI).

Earlier formulations of the materials had been used in airbag applications since 1994. "We are confident that these new compounds will pass the automotive industry's rigorous standardized test for scuff and mar resistance," says Charles E. Gates, automotive industry manager.

Marring and scuffing of unpainted thermoplastics have been the chief remaining issue in the automotive industry's drive to eliminate secondary finishing of interior components, such as airbag covers, Gates notes. "The punishment to auto interior surfaces is especially great on the driver side," he adds. "Currently most airbag covers are molded in black, then painted, although in some cases the substrate is molded in color like that of the paint to minimize show-through in case the paint is chipped."

In formulating the new grades, Teknor Plastics worked closely with its in-house partner, Teknor Color Co., to come up with the right ingredients, including UV resistance, physical properties, and heat aging. "We estimate that companies using these new compounds should save $1 per part by eliminating secondary finishing operations," Gates predicts. In addition, the precolored compounds eliminate volatile organic compounds (VOCs) and accompanying disposal problems.

Several OEMs in North America and Europe have the compounds under evaluation. Gates feels confident that some of the materials will make their appearance on 1999-model cars.

Silicones coming on strong. Another trend appears to be a continuing switch away from Neoprene to silicone for coating airbags. Silicone fabric coatings have a long, successful history for use in industrial textiles, including conveyor belts, electrical and protective sleeving, and welding blankets. The material's heat resistance and long-term aging stability makes it the choice over organic rubber coatings, according to Joanne Schwark, group leader, textiles, Wacker Silicones Corp. (Adrian, MI).

These same attributes also are critical when it comes to airbag fabrics. It's for this reason that many airbag-system manufacturers are turning to silicone coatings instead of organics for their latest designs, Schwark told attendees at a recent Airbag Conference in Detroit sponsored by Technomic Publishing (Lancaster, PA).

For driver-side airbags, which must deploy rapidly, coated fabrics dominate. The coating, applied after the fabric is scoured and heat set, may be either a silicone or Neoprene elastomer. The coating must: protect the fabric from hot inflator gases; prevent burn-through (pinholes) by hot particulates produced by the inflator; control fabric permeability; and enhance the bag's smooth deployment. Passenger-side bags have joined their driver-side counterparts in adapting to the coated fabrics.

"To give equivalent heat protection, the Neoprene must be coated at a weight more than twice that of the silicone," says Schwark. This results in a heavier, stiffer, and thicker fabric. In contrast, the silicone-coated fabric is lighter, thinner, and softer. This means that the same fabric yardage can be folded into a more compact module, giving an engineer greater latitude in module design.

Another benefit of silicone coating: "It is more chemically compatible with nylon fabric (the other chief material used in airbags) than Neoprene," Schwark notes. Uncoated nylon can be attacked by moisture (hydrolysis), while the Neoprene coating generates hydrochloric acid during aging, which actively damages the fiber. The silicone coating provides a protective layer against hydrolysis and also remains chemically inert.

Which airbag material will win out?
Contenders Interesting but expensive
Nylon 6,6 Aramid
Nylon 6 Nylon 4,6
Polyester Modified polyester films

Not only does the silicone coating resist hot gas and particulates at lower coating weights than Neoprene, the lower weight makes the fabric softer and more packageable. Moreover, the silicone-coated fabric is inherently non-blocking; Neoprene-coated material must be dusted with talc to prevent self-adhesion. The talc creates dust in the manufacturing process, and its presence in a vehicle interior following airbag deployment is an issue, Schwark points out.

"The automotive industry is demanding silicone-coated fabrics for airbags," says Schwark. "The drive will continue for lighter weight, less stiff, more packageable fabrics, which allow more flexibility in design. For silicones, this means formulations must be designed that can be coated to even lower weights, without sacrificing performance."

Crash cushion. TRW's airbag system also makes use of a generous helping of silicone. The system contains several sensitive components packed into a tightly designed "inflator." A die-cut cushion of silicone in the inflator relieves mechanical stress brought on by the effects of variable coefficients of thermal expansion. It also cushions shock and vibration that result from vehicle operation.

TRW selected PORON BF-1000 silicone, supplied by Rogers Corp.'s Bisco Materials Unit (Elk Grove Village, IL), because of its low compression-force deflection and instant recovery features. This allows for limited, but stabilized movement of the inflator's components.

The compression set resistance of the material also proved vital to the application. The foam spacer, loaded into the canister in a compressed state, must retain its load-force resilience for the 10-year operating life of the system. "PORON silicone has no trouble handling temperature extremes that range from arctic cold to desert heat," says Bill Thomas, marketing manager at Bisco.

In addition, the foam is naturally flame retardant, enabling it to meet such industry standards as FAR 25.853, UL 94HF-1, and 94V-O. Volatile and toxicity emissions remain nearly untraceable in the material, providing an added safety factor.

Winning converts. In spite of the fact that plastics have won acceptance as the material of choice for airbags, they have only begun to gain followers for other airbag-system components. Take electronic devices, for example.

Although plastic housings are used for many electronic components in automotive applications, metal housings still prevail for airbag triggering units. The critical question has always been: "Can the accelerometers mounted inside the bag's triggering unit discriminate the crash pulse satisfactorily?" says Derrick Zechmair of TEMIC Telefunken Microelectronic GmbH. "And can they do so under all environmental circumstances and over the specified life span of the system?" he adds.

He and fellow engineers at TEMIC believe so. In fact, they feel that plastic housings offer a number of advantages over metal housings, including:

Reducing airbag ECU weight from 35 to 85 grams per device.

  • Allowing for very complex mechanical designs to make more efficient use of the small space in a vehicle.

  • Eliminating the need for separate labels, with the text written onto the housing directly via laser.

  • Employing integrated connectors to reduce mechanical stress on the PCB, as well as providing a low-cost approach for weatherproof sealing of the connector/housing interface.

  • Implementing a plastic/plastic interface as a more cost-competitive measure than sealing a metal/metal interface.

  • Using plastic snap-in assemblies that would eliminate screws needed for a metal structure.

  • And, since the coefficient of expansion of a plastic housing resembles a PCB, reducing mechanical stress due to temperature changes through the use of plastics.

Based on research conducted by the three TEMIC engineers, "newer-generation airbag triggering units are ready for innovative plastic design."

More changes coming. With the advent of smart-airbag systems other material innovations are in the works. Although designs under development by the various manufacturers of these systems might vary, a typical system includes these components:

Proximity sensors and a ranging algorithm to determine occupant presence, position, and size.

  • A crash severity detector.

  • A seat-belt-use sensor and weight sensor to adjust airbag deployment.

  • A variable-level (dual bag) inflator to adjust the airbag filling rate to better control airbag/occupant interaction.

Richard Otero, Jr., business manager, air bags, DuPont Nylon (Wilmington, DE) reviewed some of the material demands that address the new systems at the May 1997 Air Bag Technology Conference. Nylon 6,6, nylon 6, and polyester will still dominate as the major materials for airbags over the next several years, he believes. However, he feels that polyester, a favorite material for seat belts, will not be the material of choice for newer systems because of its price and stiffness. "Manufacturers want a tough, flexible material for airbags," Otero says, "not one that's stiff."

Otera also expects nylon 6,6 to win more converts because of its higher-heat resistance, which could be required depending on the type of inflator systems used. The introduction of side-impact airbags in the headliner, seat, or door further complicates material choices. There's even work underway to develop "rollover" tubes. For some of these systems, Otera contemplates that a material like DuPont's Kevlar(R) aramid fiber coated with Teflon(R) might work best.

The main question to be answered, says Otera, is how much will the car buyer be willing to pay for these added safety features. "Adding $2,000 to the price of a car might not set well with the buying public," he surmises. And, he adds, the buyer should be aware that these systems are not designed for 70 mph crashes.

Perhaps Larry E. Coben, who heads the legal firm of Coben & Associates (Scottsdale, AZ), has the best solution. In a recent review of airbag injuries and fatalities, he warned: "Until certain design changes can be filtered into the restraint systems in vehicles, the only answer to this risk-benefit (system) is one of instruction and warnings. Motorists need to be informed of the grave risk that they can be exposed to by a safety device that must still be viewed as a life saver."

Are airbags needless killers?

Airbag systems were first installed in the U.S. in thousands of cars during 1974 through 1976, mainly in large-size vehicles made by GM and Ford. At the time, the intent was to provide frontal crash protection for motorists who, on the whole, did not wear seat belts. Consequently, the bags were quite large in circumference--occupying fully the entire front of the vehicle from door to door.

The few accident studies of accidents that involved the early airbags concluded that these systems were working to minimize injury potential. Those studies were consistent with laboratory testing, which demonstrated that these passive-restraint systems provided protection in barrier impacts through the 30- to 40-mph crash speed range.

These systems used compressed gas (sodium azide) in a chemical can that rapidly inflated the bag by ignition and conversion to harmless nitrogen gas when sensors detected a pre-planned "severe" frontal crash. More recent analyses of these older systems determined that they were found to be somewhat effective in preventing driver injuries, but not nearly as effective in the case of passengers.

Such findings resulted in the federal government in delaying the starting date of mandatory passive restraint criteria. This caused automobile manufacturers not to pursue the mass production of vehicles with airbags until the late 1980s. In 1980-1981, Daimler Benz introduced in Europe a test fleet with full frontal protection. Then, in the 1982 model year, the same company introduced a driver-side airbag, along with a manual lap and shoulder belt, also in Europe, followed by its introduction in the U.S. in 1984, first as an option, then as standard equipment.

Chrysler became the first U.S. manufacturer in this same time period (1980) to introduce airbags in its vehicles. Shortly thereafter, the remaining manufacturers installed airbags for both driver and passenger positions--mainly due to the mandatory passive restraint performance criteria adopted by NHTSA. By the year 2000, more than 50 million vehicles on the roads will have airbags.

In over a dozen recently published papers, details of 120 frontal accidents involving airbag deployment were analyzed. Most of the reported injuries and deaths involved drivers rather than front-seat passengers (16 driver and four passenger fatalities). Excluded were the more than 20 fatalities to children seated in the front passenger position.

Larry E. Coben of Coben & Associates, a law firm located in Scottsdale, AZ, reviewed these accident findings in a paper at this year's SAE International Congress & Exposition. In the 20 fatalities, Coben notes, a majority of these motorists were female, under 50, and unbelted. However, no injury pattern apparently existed between the belted and unbelted fatalities. There was, however, a proportionately equal number of deaths due to brain injury and spinal cord trauma. What was somewhat unexpected is that the majority of these deaths--whether the motorist was belted or not--occurred in accident sequences characterized as low-velocity collisions.

As a result, NHTSA informed Congress that airbags can have adverse effects for small-statured people, older people, and out-of-position occupants. Questions raised by the analysis of these cases, says Coben, are:

What can be done to minimize the risk of injury or death when an occupant is unrestrained?

  • What can be done to improve the performance of seat belt restraint systems to maximize the distance available to the airbag to inflate--without impacting the occupant?

  • What can be done to lessen the impact forces of an inflating airbag?

He recommends the following as possible solutions:

Design airbags to deploy faster when the seat belt is not worn, allowing for complete inflation before the occupant moves substantially forward in the seat.

  • To minimize seat-belt excursion to the "out of position" stage, the belt should be designed to lock up when deceleration of the vehicle first begins--through braking--or as a result of occupant movement within the belt--by using web sensitive retractors tuned to lock at 0.35g.

Coben's final recommendation, however, might be the most effective. "Educate the public to the need to remain as far away from the airbag container to reduce deployment injuries and deaths," he suggests. "Both motor vehicle manufacturers and the government have a responsibility to alert consumers to these inherent risks in the current airbag modules." Many have already taken this step.

Airbag 'rules' of the road

Late last December, the National Highway Traffic Safety Administration issued a final regulation and two proposed regulations "in an effort to preserve the benefits of airbags, while minimizing their danger to children and at-risk adults."

The two proposed regulations include plans for depowering airbags, and procedures for deactivation of bags under certain conditions. The final rule continues the automakers' option of installing cut-off switches in vehicles without a rear seat for children.

In brief, the NHTSA regulatory actions include:

Extending the existing policy of permitting manufacturers to install manual cut-off switches in vehicles without a back seat, or with a back seat that is too small to install a child safety seat.

  • A proposed rule making that would authorize manufacturers to depower airbags temporarily an average of 20-35%, thereby reducing the deployment force. It would remain in effect until a "smart airbag" technology is phased into new vehicles.

  • A notice of a proposed rule making that would permit dealers and repair shops to deactivate the airbags of any owner who requests it (as opposed to the current policy of case-by-case approval by NHTSA).

The action is only part of a program that includes a warning label proposal to be issued by car makers to car owners for placement in their vehicles (most automakers have already complied with this), and, perhaps more involved from a design standpoint, a forthcoming proposal that will mandate smart airbags starting in the 1999 model year.

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