Machine Design

Keeping cool with carbon

For automotive applications where nonconductivity is important, partially carbonized fibers fit the bill.

Dan Layden
SGL Technic Ltd.
Gardena, Calif.

The original PAN fiber is oxidized at 190 to 280°C for 5 hr to partially carbonize it. The result is approximately 60% carbon content.

Firewall insulation of VW Gold IV with covering felt in oxidized PAN fiber. Good heat resistance, light weight, and electrical nonconductivity make this material a good candidate for the application.

Mercedes-Benz turbodiesel truck engine compartment showing the hood liner using nonwoven OPF. Today's high-performance engines generate more heat than current insulation can handle.

Most of us have long been aware of the sexy applications of carbon-fiber composite materials. Composites make race-car bodies, tennis rackets, aircraft parts, and masts for yachts light and superstrong. We've also known that regular carbon fiber is electrically and thermally conductive, which makes it perfect for some automotive applications and perfectly wrong for others. Not as many of us know about a whole other family of related materials based on partially carbonized fiber. These materials are becoming increasingly important to the automotive industry for applications where nonconductivity is crucial. The material has already caught on in Europe, and many of the world's largest vehicle makers are either currently using or actively studying it.

Is it carbon fiber?
"Partially carbonized" fiber sounds like a misuse of terms. Isn't a fiber either carbon or not carbon? It may help to understand that all carbon fiber begins not as carbon but as a regular textile fiber, usually white acrylic (polyacrylonitrile, but called "PAN" in the industry). This is a form of the same acrylic fiber used in making baby blankets.

To make what we know as carbon fiber, the PAN fiber is heated in multiple steps under carefully controlled conditions to 2,000°C or higher until it carbonizes almost completely, much as wood does in turning to charcoal. The high-temperature part of the heating takes place in a nitrogen atmosphere.

The fiber produced is 85 to 98% carbon. It is strong, lightweight, and black. Typically, carbon fibers are mixed with resins to produce composites, just as glass fibers are used in autobody panels. The composites show up everywhere that strength and light weight are critical.

Partially carbonized fibers are produced by stopping the multistep heating process before the fibers go into high-temperature ovens. PAN fibers are usually heated to perhaps 280°C in conventional (oxygen atmosphere) ovens.

The resulting chemical reaction yields a black, infusible fiber with a well-oriented polymer structure, and a carbon content of about 60%. The resulting fibers have properties different from those of fully processed carbon fiber. Because its heating takes place in a regular oxygen atmosphere, the partially carbonized fiber is usually referred to as oxidized PAN fiber, or simply OPF.

Super properties
Oxidized PAN fiber is nonflammable, safe to handle, and won't melt, soften, or drip at high temperatures. Unlike carbon fiber, OPF is not electrically conductive. This makes it appropriate for a somewhat different set of automotive applications. OPF is also much less expensive to produce than fully processed carbon fiber, and this broadens its potential uses.

Because it's a fiber, OPF lends itself to textile technology. It can be woven into fabrics or chopped and formed as felt — referred to as a nonwoven application. OPF can also be chopped, mixed with resin, and injection molded. The automotive industry can use the fiber in all three formats: woven, nonwoven, and molded. Because OPF is a good heat insulator, felt OPFs serve as coverings for firewalls and hood liners. They are especially good in turbo-diesel trucks where underhood temperatures soar.

Modern engines in smaller cars also generate high heat and can benefit from OPF. Volkswagen is currently using a firewall insulation consisting of 50% Panox and 50% polyester on its A and B platform cars, of which as many as 1.8 million are manufactured per year.

Because OPFs are not electrically conductive, the insulating felt can work in close proximity to computers and other underhood electronics, or as protection for them. The felt is also used to wrap around the catalytic converter and exhaust systems. Other applications include trunk liners and fire protection around fuel tanks. And it could be argued that occupants of future vehicles would be well protected with OPF under floor mats, above headliners, and within autobody panels.

In general, Europeans lead both the U.S. and Asia in using OPF. Volkswagen and DaimlerChrysler were among the first to try oxidized PAN fiber material. VW has used it in insulated mats for over a decade.

Racing suits and aircraft seats
OPFs can be woven into fabrics in protective suits for race-car drivers, firefighters, and others. OPF fibers are frequently mixed with other fibers or materials to get additional properties. For example, a mixture with Kevlar produces a hybrid that is more abrasion-resistant than OPF and more fire retardant than Kevlar.

As another example, seats in British Airways aircraft are fire-blocked with a fabric which is 70% Panox OPF. This suggests auto upholstery fabric as a potential use, as well as curtains in RVs, buses, and long-haul trucks.

OPF composites easily form into complex, smooth shapes and can serve in anything from heater/air conditioner fan enclosures to interior trim to dashboards. It's likely that they'll see wide use under the hood and on the chassis. Even in these applications they can cut the cost of assembly as a molded plastic part would. Aircraft brakes use carbon materials which are produced by processing OPF further. OPF materials are already available as a high coefficient of friction, fade-free alternative to asbestos for pads and linings in trucks and passenger cars. Clutch faces present a similar opportunity.

Carbon is an excellent filter, a property that may bear on uses with air bags. The reaction that inflates air bags in a crash creates noxious fumes. OPF can work in concert with other materials within the bags to prevent fumes from affecting the vehicle occupants.

We may well see OPF filters in the air-conditioning systems of future cars, and perhaps in engine air cleaners.

Why bother to change?
There are a dozen reasons why OPF will provide a great alternative to other materials. These reasons include government regulations, economics, product longevity, health, and the environment.

Consider that local and state governments, and especially the federal government, continue to try to legislate technology. Legislating pollution standards and gasoline economy are favorite war horses. For example, Californians are concerned about available vehicles as ever-higher corporate average fuel economy (CAFE) standards are enforced.

Both issues are impacted by vehicle weight, and both carbon fiber and OPF can help in weight reduction. For example, a carbon composite weighs approximately one-sixth as much as the metal part it replaces. A lighter vehicle means smaller, less-polluting engines and better fuel economy.

Economic arguments win with or without government interference. Parts are less expensive if they can be molded rather than assembled. OPF gives opportunities to mold more parts than before.

OPF may promote longevity in that it doesn't corrode as does iron or even fiberglass. And it doesn't have the problem of some insulating materials (asbestos isn't the only one) that can be health hazards during their use and environmental hazards afterwards. Carbon is not a threat to health or environment.

Everything from engineering principles to government legislation argues for using more oxidized PAN fibers in vehicles. European automotive engineers have shown themselves to be believers and are a few years ahead of the Americans and Asians in employing the material. It's time for American engineers to give OPF a second look.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.