Techology Profile: Carbon Fibre – Space age material brought down to earth
Like so many other technologies, carbon-fibre has filtered into the consumer mainstream from military and aerospace applications.
During the 1950s and ‘60s, rayon-based fibres were used for aircraft parts, for rocket nozzles and missile nose tips and for spacecraft heat shields. Since then, continued research and development has led to the widespread use of carbon fibrereinforced plastics (CFRPs) in commercial planes and helicopters, lithium batteries, sporting goods, structural reinforcement in construction materials, prosthetic limbs, bicycle frames, wind turbines, motorcycles, racing automobiles and high-performance consumer vehicles.
While there is certainly agreement that using CFRPs in manufacturing has advantages, the cost to produce them is still an obstacle for use in mass production — in particular, for automobiles. There is a major incentive, however, that will likely pressure industry to develop advanced CFRP manufacturing techniques resulting in lower costs.
The Corporate Average Fuel Economy (CAFE) standards set by the U.S. National Highway Traffic Safety Administration (NHTSA) and the U.S. Environmental Protection Agency (EPA) is aimed at dramatically increase fuel economy over the next nine years. The current CAFÉ standard is 8.5 litres per 100 kilometre (33 miles per gallon). By 2025, fuel consumption must be 4.2 L/100 km (67 mpg). These same standards also apply to Canadian vehicles.
Essentially, CAFE is forcing automakers to find ways to improve gas mileage. A huge part of that is to decrease the weight of vehicles without sacrificing performance or safety. According to the U.S. Department of Energy, automobile components made with advanced composite materials could reduce the weight of passenger cars by half and improve fuel efficiency by nearly 35 per cent. Not surprisingly, automakers are counting on the use of CFRPs because of the material’s almost-too-goodto- be-true properties.
It’s a dream material, says Dale Brosius, chief commercialization officer for the Institute for Advanced Composite Manufacturing Innovation (IACMI) in Knoxville, Tex. “Carbon-fibre composites have several times the strength of steel and a long fatigue life despite being much lighter than steel,” says Brosius, who has more than three decades of experience in the chemicals, plastics and composite materials field.
“In fact, the material is 30 to 40 per cent lighter than aluminum. That’s why, in its early days, the material was used in satellites and eventually aircraft. The trade-off is it costs more to make than steel.” CFRP is a composite material made of a polymer matrix — essentially, a resin, reinforced with carbon fibres. The fibres are long, thin strands composed mostly of carbon. According to Zoltek Corp. of Bridgeton (St. Louis), Missouri, a major CFRP manufacturer, the main reason for CFRP’s tensile properties is “the carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fibre.
The crystal alignment makes the fibre incredibly strong for its size. Several thousand carbon fibres are twisted together to form a yarn, which may be used by itself or woven into a fabric. The yarn or fabric is then combined with epoxy and wound or moulded into shape to form various composite materials. The raw material used to make carbon fibre is called the precursor. About 90 per cent of the carbon fibres produced are made from polyacrylonitrile (PAN). The remaining 10 per cent are made from rayon or petroleum pitch.
The exact composition of each precursor varies from one company to another. During the manufacturing process, a variety of gases and liquids are used. Some of these materials are designed to react with the fibre to achieve a specific effect. Others are designed not to react or to prevent certain reactions with the fibre. As with the precursors, the exact compositions of many of these process materials are considered trade secrets.
The process for making carbon fibres is part chemical and part mechanical. The precursor is drawn into long strands or fibres and then heated to high temperatures without allowing it to come in contact with oxygen. Without oxygen, the fibre cannot burn. Instead, the high temperature causes the atoms in the fibre to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization, and leaves a fibre composed of long, tightly interlocked chains of carbon atoms with only a few non-carbon atoms remaining. The fibres are coated and then wound onto cylinders called bobbins. The bobbins are loaded into a spinning machine and the fibres are twisted into yarns of various sizes.”
The fibres are then incorporated into the epoxy or resin for use in manufacturing parts in a variety of ways depending on what the end use will be. For instance, for a single layer of carbon fabric backed with fibreglass, a chopper gun is used to spray resin onto the shell formed by the carbon fibre. Parts can also be formed by placing layers of carbon fibre cloth into a mold and filling it with the epoxy. Edison’s bright idea The American Chemical Society (ACS) points out that Thomas Edison produced what may have been the first commercial carbon fibre when he used cotton threads or bamboo slivers as filaments for the incandescent light bulbs he invented.
These two natural filaments consisted mostly of cellulose. When Edison baked them at high temperatures, they became carbonized. However, it was Roger Bacon working at Union Carbide’s Parma Technical Centre near Cleveland in the late 1950s who is generally credited with developing the modern versions of carbon fibres. According to the ACS’s history of the material, Bacon used equipment that wasn’t much different than Edison’s lamps, only operating at much higher pressures.
Small amounts of vaporized carbon would travel across the arc and then deposit as liquid. As Bacon decreased the pressure in the arc, he noticed the carbon would go straight from the vapour to solid phase, forming a stalagmite-like deposit on the lower electrode.
“I would examine these deposits, and when I broke one open to look at the structure, I found all these whiskers,” he says. “They were imbedded like straws in brick. They were up to an inch long, and they had amazing properties. They were only a 10th of the diameter of a human hair, but you could bend them and kink them and they weren’t brittle. They were long filaments of perfect graphite.”
Bacon continued to experiment, and soon produced high-strength, high-tensile fibres. But he considered them as experiments and not really commercially viable. “I estimated the cost of what it took to make them, and it was $10 million per pound,” he says. To tap their full potential, manufacturers needed a cheap, efficient way to produce the fibres. That work has continued through until today, although that price per pound is only a fraction of what it was during Bacon’s lab experiments.
The current cost of carbon fibre is between $8 to $12 per pound, says Michel Dumoulin general manager of the National Research Council of Canada’s Automotive and Surface Transportation portfolio. Dumoulin, who has extensive international experience as a research scientist in the polymer materials industry, says because more companies are producing larger quantities to economies of scale, the production costs have decreased by more than half in the last decade.
But, “you still have to compare that to the cost of steel which is about a dollar per pound. So there’s still a way to go to replace steel as the standard.” Dumoulin adds production cost is only one factor to consider. He feels the key to widespread use will be part integration. “Take an automotive assembly part, for instance — the front-end module which holds the headlights, radiator and other parts,” he says.
“Years ago, the main structure that holds everything in place was made up of 20 to 30 or even 40 parts, usually riveted and welded together and screwed into place onto the vehicle. Now, using composite technology, the module can be designed as a single moulded part. It’s ready to use with the holes in the right place and no, or very little, secondary work required. “Additionally there is no welding or riveting required.
In this way, the cost of the composite part can be justified because it could reduce the overall production cost of the structure by reducing the number of operations required. The technology then reaches a level where it becomes economical.” One of the recent key milestones in the use of carbon-fibre manufacturing has been the design and development of Boeing’s 787 Dreamliner. “The fact that the Dreamliner’s fuselage is completely manufactured with carbon fibre material sends a strong message to industry,” says Dumoulin.
“We have seen the proportion of carbon materials integrated into aircraft gradually creep up, but that proportion took a quantum leap when Boeing designed the fuselage of the 787.” While the use of carbon fibre in the Dreamliner has been a breakthrough in a sense, it is still not incorporating the material into mass production.
Obstacles still remain, says Brosius. “It’s a question of scale and we’re working toward solving that obstacle,” he says. “But there are other critical considerations, as well. For instance, in automotive manufacturing, you can stamp a piece of steel in 10 seconds. When using resins to make that part, it’s not quite the same. When you combine substances, they have to react and that takes time. That means more machinery is required to produce an equivalent number of parts.
“So what is needed is for the cycle times to be reduced so that not as much machinery is required. To achieve true mass manufacturing using CFRPs, we need to reduce the cycle times to under three minutes for large parts like floor pans from today’s state-of-the-art levels of eight to twelve minutes.” In addition to improving the performance of the parts manufacturing stages, there’s the alltoo critical issue of building the carbon-fibre manufacturing facilities themselves to meet the demand for the material.
There are only about six or seven large producers and four or five moderately sized producers globally, although those number could be boosted substantially with several facilities planned in China. “But solving these obstacles is a progressive process, “says Brosius.
“Keep in mind that steel manufacturing has had centuries of development; aluminum manufacturing has been around for at least a 100 years; carbon manufacturing is really only a few decades old. So, I think we’re there on the maturity curve.”
About the author: Ernest Granson is a Calgary-based writer and editor, and a contributor to PROCESSWest.