Tikalon Blog by Dev Gualtieri

Carbon Fibers

April 3, 2012

As we all know, through the many news stories of actual and alleged violations, that performance-enhancing drugs are prohibited in sports . Someone needs to draw the line somewhere, or else all sporting competitions will be battled in the laboratory , rather than on the playing field .

Golf (as regulated by The Royal and Ancient Golf Club of St Andrews and the United States Golf Association ) has decided to preserve the purity of its sport by having a specification for its balls that purposely limits performance. Golf balls must have a minimum diameter of 43 mm (1.68 inch ), a maximum weight of 45.9 grams (1.620 ounce ), and they must have a speed less than 270 km/h (250 feet per second ) for a particular test condition. No such prohibition on equipment technology exists in cycling .

Figure one of US Patent No. 701,736, "Golf-Ball," by Eleazer Kempshall, June 3, 1902.

(Via Google Patents).

Lance Armstrong became a major proponent of using molded carbon fiber as a cycle frame material to replace the typical magnesium alloy , Now, all high performance cycles are made from carbon fiber. Sometimes a materials advance in a sport can give a competitor an advantage before the common use of that material. There's an interesting paper, published in JOM (formerly, the Journal of Metals) in 1997, entitled, "Is the Use of Advanced Materials in Sports Equipment Unethical?"[1] Although it's easy to define steroid use, how would you define an "advanced material?"

The allure of carbon fiber composites is that they have a high ratio of elastic modulus to density , as can be seen in the following table.[1] Ceramics have a better ratio, but their lack of fracture toughness precludes their use in most sporting applications.

Material	Modulus E (Mpa)	Density ρ (g/cc)	E/ρ
Metal	40,000 - 210,000	~2-8	24,000 - 30,000
Glass	73,000	~2.5	~30,000
Ceramic	400,000 - 700,000	~3.5	100,000 - 230,000
Carbon Fiber Composite	200,000	2.0	100,000

Bicycles are not the only pieces of sporting equipment that are made using carbon fiber composites. There are also fishing rods , the shafts of golf clubs , tennis racquets , hockey sticks , skis and snowboards . Carbon fiber composites account for more than half of composites used in sporting goods.[3] An estimated 6,710 metric tons (14.8 million pounds ) of carbon fiber, or 24% of global carbon fiber, were used for such equipment in 2006.[3]

The usual method of carbon fiber manufacture should be of no surprise to a materials scientist . You start with polymer fiber , such as rayon or polyacrylonitrile , which is heated in air to carbonize the fiber. A subsequent higher temperature anneal (~ 2000 °C) in an oxygen -free atmosphere densifies the resultant carbon . The processed fibers are about 95% carbon. Since the original polymer fiber was produced by drawing, the graphitic crystallites are oriented generally in the axial direction.

A lot of research has been devoted to such a useful material. Oak Ridge National Laboratory worked on carbon fiber produced from a polyethylene precursor as early as 1971. At that time, Charles R. Schmitt patented a process for turning polyethylene-impregnated rayon into carbon fiber.[4] Recently, ORNL scientists have been doing some novel synthesis of polyethylene-derived carbon fiber, and their research is reported in an early edition of Advanced Materials .[5-6]

A team led by Amit K. Naskar of the Polymer Matrix Composites Group of ORNL's Materials Science and Technology Division has been producing carbon fibers with non-circular cross-section derived from polyethylene fibers (see figure). This increases the area/volume ratio, which should allow for better bonding to the composite matrix . Other scientists involved in this research are Marcus Hunt, Tomonori Saito and Rebecca Brown of ORNL; and Amar Kumbhar of the University of North Carolina at Chapel Hill .[ORNL]

Carbon fibers with a variety of cross-sections, as produced by Oak Ridge National Laboratory. (Oak Ridge National Laboratory Image/Amit K. Naskar).[6]

The ORNL team is using an additional step in carbonizing the fibers, first treating the fibers to incorporate sulfur , a technique known as sulfonation. This makes the fibers more resistant to heat, so they can be carbonized at a higher temperature. A similar approach was patented by Japanese scientists in the late 1970s.[7] To quote from their patent, their invention was

"A process for the production of carbon fiber, which comprises sulfonating polyethylene fiber with chlorosulfonic acid, sulfuric acid, fuming sulfuric acid or a mixture of two or more kinds thereof and carbonizing the resulting precursor fiber by heating at a temperature of 600° to 3,000°C, optionally while giving a tension to the fiber."

The ORNL process, as described by Naskar, is as follows:

"We dip the fiber bundle into an acid containing a chemical bath where it reacts and forms a black fiber that no longer will melt... It is this sulfonation reaction that transforms the plastic fiber into an infusible form. At this stage, the plastic molecules bond, and with further heating cannot melt or flow. At very high temperatures, this fiber retains mostly carbon and all other elements volatilize off in different gas or compound forms."[ORNL]

The ORNL process allows the porosity to be tuned, so the material could be useful for filtration , catalysis and electrochemical energy harvesting .[ORNL] Funding for the ORNL work was provided by United States Department of Energy , Office of Energy Efficiency and Renewable Energy .[ORNL]

Carbon Fibers

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