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]
References:
- Eleazer Kempshall, "Golf-Ball," US Patent No. 701,736, June 3, 1902
- F.H. Froes, "Is the Use of Advanced Materials in Sports Equipment Unethical?" JOM, vol. 49, no. 2 (February, 1997), pp. 15-19.
- Market Outlook: Carbon fiber in sporting goods, Composites World, January 1, 2008.
- Charles R. Schmitt, "Method For Producing Fibrous Carbon Structures," US Patent No. 3,607,672, September 21, 1971.
- Marcus A. Hunt, Tomonori Saito, Rebecca H. Brown, mar S. Kumbhar and Amit K. Naskar, "Patterned Functional Carbon Fibers from Polyethylene," Advanced Materials, Advanced Online Publication, March 27, 2012, doi: 10.1002/adma.201104551.
- Ron Walli, "ORNL process converts polyethylene into carbon fiber," Oak Ridge National Laboratory Press Release, March 29, 2012.
- Shozo Horikiri, Jiro Iseki, Masao Minobe, "Process for production of carbon fiber," US Patent No. 4,070,446, January 24, 1978.
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