Cutting costs of carbon composites

  • 27-Feb-2013 08:39 EST
BMW_i3_Concept_Coupe27.jpg

The BMW i3 urban EV will be the first mass-produced carbon-fiber car.

When the BMW i3 city car rolls out of the company’s Leipzig plant later this year, it will represent the first carbon-fiber car that will be manufactured in any quantity—about 40,000 vehicles a year at full output. The lightweight but sturdy nonmetallic structure of the new commuter car, the result of BMW’s joint venture with SGL Technologies in Wiesbaden, will mark a milestone in the development of carbon-fiber-reinforced plastic (CFRP) materials, which have traditionally been too costly for use in automotive mass production.

CFRPs are engineered materials that are fabricated by embedding webs of carbon fiber inside molded polymer resins. The fibers bolster the physical properties of the plastic matrix component in the same way that a skeleton of steel rebar strengthens a poured-concrete structure.

Although the i3 electric vehicle (EV) won’t exactly come cheap—estimates run from $40,000 to $50,000—BMW reportedly claims that forthcoming improvements in the production process during the next three to five years should cut carbon composite costs enough to match those of aluminum chassis, which still command a premium over standard steel car frames.

High-priced, high-performance option

CFRP structures weigh half that of steel counterparts and a third less than aluminum ones. Add the inherent corrosion resistance of composites and the ability of purpose-designed, molded components to cut parts counts by a factor of 10, and the appeal to automakers is clear. But despite the benefits of using CFRPs, composites cost significantly more than metals, even allowing for their lighter weight. The high prices have so far limited their use to high-performance vehicles such as jet fighters, spacecraft, racecars, racing yachts, exotic sports cars, and notably, the latest Airbus and Boeing airliners.

Whereas steel goes for between $0.80 and $1/kg, and aluminum costs between $2.40 and $2.60/kg, polyester and epoxy resins range from $5 to $15/kg and the reinforcing fiber costs an additional $2 to $30/kg, depending on quality. To enable cars to clear the U.S. government’s fast-approaching 54.5-mpg average fuel-economy bar, automakers and their suppliers are striving to come up with ways to produce affordable carbon-fiber cars on the mass-scale.

But adapting structural composites to low-cost mass production has always been a technical and manufacturing challenge, said Ross Kozarsky, Senior Analyst at Lux Research, an independent research and consulting firm that focuses on emerging technologies.

Kozarsky follows composite materials and led a study team that last year assessed CFRP manufacturing costs and identified potential innovations in each step of the complex process.

“Our methodology is to follow, through visits and interviews, the entire value chain from the tow, yarn, and grade level onwards, examining the supplier structure and the general market costs,” he explained. The Lux team then developed a cost model that combines material, capital expenditure, infrastructure, labor, and utility consideration and the chances for cost reductions.

Overall market growth

Although the sporting goods, military, and aerospace industries have traditionally developed and first applied composite materials, the pre-eminence of those segments in terms of sales is ending, Kozarsky said. The wind-turbine business will contend with aerospace for the top market as larger, more-efficient offshore wind-power installations are built.

“It’s more economical to use bigger turbine blades, which can only be made using carbon-fiber materials,” he noted.

The Lux report predicted that the global market for CFRPs will more than double from $14.6 billion in 2012 to $36 billion in 2020, as innovative new production technologies lower carbon-fiber costs—the major cost-driver. During the same period, demand for carbon fiber is expected to rise fourfold from the current 27,000 million ton (24,500 million t) to 110,000 million ton (99,800 million t).

Major suppliers of carbon fiber include Toray, Zoltek, Toho, Mitsubishi, Hexcel, Formosa Plastics, SGL Carbon, Cytec, AKSA, Hyosung, SABIC, and more than a dozen smaller Chinese companies.

“A lot of people are talking about automotive uses now, which is totally at the other end of the spectrum from aerospace applications, since it has a much higher volume and a lot more cost-sensitivity,” Kozarsky said. After a slow start, the auto industry will enjoy the second-largest average industry segment improvement throughout the decade, growing at a 17% clip, according to the Lux forecast.

The Lux analysis indicates that CFRP technology remains expensive mainly because of high material costs—particularly the carbon-fiber reinforcements—as well as slow manufacturing throughput, he reported.

“The industry has reached an interesting precipice,” he said, wherein industrial ingenuity will vie with the traditional technical challenges to try to meet the new demand while lowering costs and speeding production cycle times.

The PAN fiber process

The best-performing carbon fibers—the higher grades used in defense and aerospace applications—start out as what is called PAN (polyacrylonitrile) precursors. Because of the difficulty of the manufacturing process, PAN fibers cost about $21.5/kg, according to Kozarsky, who explained that makers subject the PAN to a series of thermal treatments in which the material is polymerized and carbonized as it is stretched. The resulting “conversion” leaves the filaments oriented along the length of the fiber to give it the optimal strength and toughness. Various post-processing stages and the surface-acting additives help ensure durability and “handleability.”

“The biggest potential for cost-saving is finding an alternative for making PAN, which constitutes 50% of the cost of the fiber,” he stressed.

Kozarsky singled out an industrial/government R&D collaboration at the new Carbon Fiber Technology Facility at Oak Ridge National Laboratory (ORNL), which has been funded with $35 million in U.S. Department of Energy money as one of the more promising efforts to lower fiber costs. Part of the project is to identify cheaper precursor materials that can be processed into good-quality fibers (see “Oak Ridge collaborates for cheaper carbon fiber,” http://www.sae.org/mags/aei/11104). The plan is to test many types of potential low-cost fiber precursors such as the cheaper polymers, inexpensive textiles, some made from low-quality plant fibers or renewable natural fibers such as wood lignin, and melt-span PAN.

Near term the Lux team expects the work that ORNL is performing with Portuguese acrylic-fiber maker FISIP (majority owned by SGL) on textile-grade PAN to achieve costs at the pilot-line scale of $19.3/kg in 2013. Although significant, it would be only a modest reduction when compared to the 50% needed for penetration in high-volume auto applications.

One of the major limitations of PAN, he said, is that “at best 2 kg of PAN yields 1 kg of carbon fiber, which gives you a conversion efficiency of only 50%.” Dow Chemical is investigating using polyolefins—polyethylene, polypropylene—as the feedstock because they could offer potential conversion efficiencies of 70% to 75%. If mechanical performance targets can be met, pilot-line costs of $13.8/kg could be achieved by 2017, stated the report.

The Oak Ridge group, Kozarsky said, is also working on novel microwave-assisted plasma carbonization techniques that can produce useful, uniform fiber properties. And ORNL’s nonthermal plasma oxidation process has been shown to have the potential to stabilize and cross-link the precursor materials rapidly and efficiently.

Polyolefin-precursor carbon fiber, combined with these kinds of alternative thermal-treatment mechanisms, should reduce costs to sub $11/kg at pilot-line scale in 2017, he noted. Kozarsky added that “there’s lots of interest in improving the resin matrix as well,” with research focusing on using thermoplastics instead of the existing thermosets and producing higher-toughness, faster-processing polymers.

The Lux analyst also cited other industrial partnerships, collaborations, and investments that could yield progress on CFRPs. They include General Motors’ link-up with Teijin (Toho Tenax) to develop a high-throughput “part-per-minute” production process for carbon-fiber car parts; Ford’s partnership with Dow Chemical; Toray’s R&D efforts with Toyota, Daimler, and Honda; Volkswagen’s and BMW’s investments in SGL; as well as joint research by Zoltec and Weyerhauser to develop lignin-based fiber precursors.

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