Carbonfiber Overview

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Carbon FibersPaul J. Walsh, Zoltek Corporation

CARBON FIBERS, after a 40 year period ofdevelopment and use in specialized applications,are now on the brink of broad commercializa-tion. Their use is growing rapidly, fueled by sig-nificant price reductions during the 1990s andincreasing availability. Changes in the perfor-mance/price ratio have resulted in the increasedpenetration of composites into applications for-

merly held by metals and has enabled their usein other applications previously not possiblewith existing materials. Additionally, marketconditions increasingly favor designs for com-mercial products that are lighter, stronger, faster,and more fuel efficientdesigns that are possi-ble with carbon fibers. No longer relegated toaerospace, carbon fiber composites are now be-ing adopted in fields such as automotive, civilinfrastructure, offshore oil, and paper produc-tion.

History

The earliest commercial use of carbon fibersis often attributed to Thomas Edisons carboni-zation of cotton and bamboo fibers for incandes-cent lamp filaments (Ref 1). However, practicalcommercial use of carbon fibers for reinforce-ment applications began in the late 1950s withthe pursuit of improved ablative materials forrockets (Ref 2). Union Carbide marketed a car-bonized rayon based fabric in the early 1960s(Ref 3). DuPonts work with black Orlon inthe late 1950s showed that acrylics could be ther-mally stabilized, while Shindo in Japan and Wattet al. in the United Kingdom demonstrated that,by using tension through the carbonization pro-cess, high mechanical properties could be real-ized (Ref 4).

Activity increased rapidly during the 1960sand 1970s to improve the performance/price ra-tio of carbon fibers. Much of this effort focusedon evaluation of various precursors, since carbonfiber can be made from almost anything thatyields a quality char upon pyrolysis. Donnet andBansal (Ref 5) present a good overview of vari-ous researchers efforts to evaluate different pre-cursors, including PAN (polyacrylonitrile),pitch, rayon, phenol, lignin, imides, amides, vi-nyl polymers, and various naturally occurringcellulosic materials.

Overall carbon fiber demand grew to approx-imately 1000 metric tons by 1980, fueled pri-marily by the aerospace industry, with the sport-ing goods industry taking some excess capacityand off-specification fiber. Polyacrylonitrile-based carbon fiber usage had exceeded all otherprecursors at that time. This was a surprise tosome, since the anticipation in the late 1970s had

been that the significantly lower raw materialprice and higher char yield of pitch would resultin the winning combination. However, higherprocessing costs are required to make a spinna-ble pitch, so better overall properties for PANfibers resulted in their dominance. Rayon wasrelegated to third place, despite having a lowerraw material cost, because inferior propertiesanda low char yield (20 to 25%) after carbonizationmade for a higher overall cost. Properties can beimproved by stress graphitization at high tem-peratures, but this increases cost further, makingthe fiber even less desirable. Rayon is still usedtoday for insulating and ablative applications butnot for structural applications.

By the mid-1990s, a new cost-effective, PAN-based carbon fiber made from a modified textileprecursor was being aggressively promoted bycompanies like Zoltek and Fortafil for commer-

cial applications. In 1995, one manufacturer nounced the goal of reaching a price level of lb ($11/kg) by the year 2000, which brought aof attention to and greatly accelerated applition development (Ref 6). An overall trendimproved performance/price ratio for both piand PAN fiber manufacturers has sustained tgrowth.

Carbon fiber demand has grown to an emated 16 106 kg (35 106 lb) per year (7). Usage in 1997 was estimated at 30% aespace, 30% sporting goods, and 30% commcial/industrial applications, with the industapplications poised for the greatest grow(Ref 8).

Manufacture of Carbon Fibers

Precursor sources used, in order of volumare PAN, pitch, and rayon. Although the specprocessing details for each precursor is differall follow a basic sequence involving spinni

stabilization, carbonization, and application ofinish or sizing to facilitate handling, as shoin Fig. 1. Discontinuous carbon fiber whiskare also now produced in a batch process fr

Fig. 1 The processing sequence for polyacrylonitrile (PAN) and mesophase-pitch-based precursor fibers showsimilarities for the two processes. Highly oriented polymer chains are obtained in PAN by hot stretching, w

high orientation in pitch is a natural consequence of the mesophase (liquid crystalline) order.

This article is from ASM Handbook, Volume 21, Composites

Copyright ASM International, 2001

It is reproduced by permission of ASM International,

www.asminternational.org. All rights reserved.

Additional information about ASM Handbook, Volume 21

is available at www.asminternational.org


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36 / Constituent Materials

hydrocarbon gases using a vapor-liquid-solidgrowth mechanism.

PAN-based Carbon Fibers. The majority ofall carbon fibers used today are made from PANprecursor, which is a form of acrylic fiber. Pre-cursor manufacture is accomplished by spinningthe PAN polymer into filaments using variantsof standard textile fiber manufacturing pro-cesses. The PAN fibers are white in color, with

a density of approximately 1.17 g/cm3 (0.042 lb/in3) and a molecular structure comprised of ori-ented, long chain molecules. Stabilization in-volves stretching and heating the PAN fibers toapproximately 200 to 300 C (390 to 570 F) inan oxygen-containing atmosphere to further ori-ent and then crosslink the molecules, such thatthey can survive higher-temperature pyrolysiswithout decomposing. Stretching after spinningand during stabilization helps develop the highlyoriented molecular structure that allows devel-opment of a high tensile modulus and improvedtensile strength upon subsequent heat treat-ment.

Carbonization of standard and intermediate

modulus fiber typically involves pyrolyzing thefibers to temperatures ranging from 1000 to 1500C (1800 to 2700 F) in an inert atmosphere, typ-ically to a 95% carbon content. An additionalhigh heat treatment step is included just after car-bonization for some very high-modulus fibers.During carbonization, the fibers shrink in diam-eter and lose approximately 50% in weight. Re-straint on longitudinal shrinkage helps developadditional molecular orientation, further increas-ing mechanical properties.

After carbonization, the fibers may be runthrough a surface treatment step designed toclean and attach functional groups to the fibersurface, which increases bond strength with ma-

trix resins. Most manufacturers use an electro-lytic oxidation process that creates carboxyl, car-bonyl, and hydroxyl groups on the surface forenhanced bonding. A sizing or finish is then ap-plied to minimize handling damage duringspooling and enhance bonding with matrix res-ins. The fiber is then spooled.

Today, there is differentiation among manu-facturers between those who use a modified tex-tile-type PAN precursor and those who use anaerospace-type precursor. The textile-type pre-cursor is made on a very large scale in modified-acrylic textile fiber plants in tows or rovings con-sisting of200,000 filaments. The tows are thensplit down into smaller bundles (approximately48,000 filaments) after carbonization for spool-

ing. Aerospace precursor is made in smaller spe-cialty plants and processed in 3000 (3K) to 12Kfilament tows that can be assembled into 24K orlarger tows after carbonization. Manufacturingcost is lower for the textile-type precursor, dueto higher line throughputs, larger economies-of-scale, and less handling of smaller tow bundles.This type fiber is more targeted for industrial ap-plications. The aerospace-type precursor, be-cause it is processed in smaller tow sizes, is lessfuzzy and available in the small tow sizes fa-vored by the aerospace industry, for whom it was

originally developed. Physical properties can besimilar for both types.

Pitch-Based Fibers.Pitch is a complex mix-ture of aromatic hydrocarbons and can be madefrom petroleum, coal tar, asphalt, or PVC (Ref9). Starting raw material selection is importantto the final fiber properties. Pitches must be pro-cessed through a pre-treatment step to obtain thedesired viscosity and molecular weight in prep-

aration for making high-performance carbon fi-bers. The pre-processed pitch contains meso-phase, a term for a disk-like liquid crystal phase(Ref 10) that develops regions of long-term or-dered molecules favorable to manufacture ofhigh-performance fibers. Without this step, theresult is an isotropic carbon fiber with lowstrength and low modulus of less than 50 GPa(7 106 psi) (Ref 11). Process details of thefinal composition and method of spinning me-sophase pitch are generally held secret by themanufacturers.

Once spun, the stabilization, carbonization,surface treatment, application of sizing, andspooling of pitch-based fibers follows a se-

quence similar to the manufacture of PAN-basedfibers, as shown in Fig. 1. Actual process param-eters, such as temperatures, ramp rates, and timeat temperature for stretch and stabilization, aredifferent for pitch than for PAN. Gas speciesevolved during pyrolysis and their onset of evo-lution are very different for PAN and pitch. Theresponse to heat treatment is also greater for me-sophase-pitch-based fibers at higher tempera-tures, a consequence of their more ordered start-ing molecular structure. For example, amesophase-pitch-derived fiber processed to thesame temperature as a PAN fiber will exhibithigher density and thermal and electrical con-ductivity, all else being equal.

Other Precursors. Rayon is processed insimilar fashion to PAN, as shown in Fig. 1; thedifference is the actual process parameters used.

Carbon fiber whiskers can be formed fromgas-phase pyrolysis via catalyzed cracking of hy-drocarbon gases like methane. One process in-volves growth of a thin carbon tube of 10 to 50nm from a submicron iron particle in a hydro-carbon-rich atmosphere, followed by a second-ary process of thickening the tube by chemicalvapor deposition of carbon on the surface (Ref12). Others have discussed similar processes,some capable of longer length fibers (Ref 13).Although only discontinuous fibers are fabri-cated, they have unique properties approachingthose of single crystal graphite in some cases.

Available Formats for Fibers.Commerciallyavailable carbon fibers are produced by a mul-titude of manufacturers with a wide range ofproperties and tow sizes. Carbon fibers are avail-able in many of the same formats as glass fiber.These formats include continuous filament-spooled fiber, milled fiber, chopped fiber, wovenfabrics, felts, veils, and chopped fiber mattes.Most fiber today is spooled, and then processedinto other formats in secondary operations. Thesize of the carbon fiber tow bundle can rangefrom 1000 filaments (1K) to more than 200K.

Generally, aerospace carbon fibers are availabin bundles of 3K, 6K, 12K, and 24K filamentwhile most commercial-grade fibers are avaiable in 48K or larger filament counts. Composifabrication equipment, such as filament windeand weaving machines, must be adapted to handle the larger cross section of commercial gradfiber.

Properties and Characteristics ofCarbon Fibers

Composites made from carbon fiber are fivtimes stronger than grade 1020 steel for structural parts, yet are still five times lighter. In comparison to 6061 aluminum, carbon fiber composites are seven times stronger and two timestiffer, yet 1.5 times lighter. Carbon fiber composites have fatigue properties superior to aknown metals, and, when coupled with thproper resins, carbon fiber composites are one othe most corrosion resistant materials availablCertain mesophase-pitch-based carbon fibe

possess thermal conductivity three times greatethan copper. The electrical conductivity of PAand pitch-based carbon fibers is used to dissipastatic electricity in a wide variety of computerelated products. They do not melt or soften witheat, allowing them to be used in such high temperature applications as rocket nozzles and aicraft brakes. In fact, their strength actually increases with temperature in non-oxidizinatmospheres. These unique properties are the result of the fiber microstructure, in both the axiaand transverse directions.

Axial Structure.Envision a single carbon fiament as a long cylinder with a diameter of approximately 7 lm. Packed within this cylinde

are tiny undulating ribbon-like crystallites whicare intertwined and oriented more or less parallto the axis of the cylinder (Ref 1416). Thlength and straightness of these crystallite ribbons determines the modulus of the fiber. Amodel of the axial structure of a PAN-based cabon fiber is shown in Fig. 2.

On a finer scale, each ribbon-like crystallite icomprised of multiple wrinkled layers. Eaclayer is made of carbon atoms arranged likchicken wire in a hexagonal structure characteistic of graphite, called a graphene plane. Stroncovalent C-C bonds within the layer plane givthe potential for high strength and stiffnesWeak van der Waals bonding between the layeplanes gives rise to poor shear resistance, bu

also allows thermal and electrical conductivityLoose electrons and thermal energy in the formof phonons take advantage of the weak bondinbetween layer planes and use the inter-planspace as a corridor to travel. The width of thribbons, the number of graphene layers compriing their thickness, and the length of the ribbonhelp determine the electrical and thermal chaacteristics of the carbon fiber, as well as contribute to fiber modulus. Typically, larger and mororiented graphene planes result in higher thermand electrical conductivity.


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Carbon Fibers /

Fig. 3 The preferred orientation of the graphene planesis determined by the heat treatment temperature

and the precursor type. Source: Ref 14, 18

Fig. 2 The undulating ribbon structure of the graphenelayers for a PAN-based carbon fiber with a 400

GPa (600 106 psi) modulus. The ribbons at the surfacehave lower amplitude than in the core. There are about 20graphene layers in the ribbons in the core and about 30near the surface.

Fig. 5 A 400-GPa (60 106 psi) modulus PAN-bfiber. Source: Ref 26

Fig. 4 The modulus of a carbon fiber is determinethe preferred orientation, microstructure,

elastic constants. The relationship between moduluspreferred orientation for a pitch-based carbon fibshown.

Improving the orientation of the microstruc-ture can also increase filament tensile modulus,thermal conductivity, electrical conductivity, anddensity. This can be accomplished by plastic de-formation (for example, stretching the fiber) and/or heat treatment. Figure 3 shows x-ray diffrac-tion results relating heat treatment temperatureto the degree of preferred orientation of the mi-crostructure (Ref 17, 18). The degree of pre-

ferred orientation represents the average angle atwhich the crystallites lie relative to the fiber axis;a zero degree angle means that the crystallitesare perfectly aligned with the fiber axis. Trans-mission electron microscopy shows that the rib-bons undulate, such that their amplitude isgreater than their wavelength. Any reportedmeasurement of preferred orientation is thereforeonly an average. The data clearly shows im-proved orientation with increasing heat treat-ment temperature. Figure 3 also shows that forheat treatment temperatures above 1600 C(2900 F) the mesophase pitch-based fiber willorient more than the PAN fiber, a result of largercrystallite sizes that PAN precursors are not able

to achieve. The relationship between preferredorientation of the microstructure and modulus isillustrated in Fig. 4. Increased orientation resultsin increased fiber modulus, as expected.

Increases in fiber tensile modulus can also beobtained by stretching the fiber during stabili-zation and carbonization. In this case, mechani-cal rather than thermal energy provides the im-petus for molecular realignment.

Transverse Structure. While axial orienta-tion determines modulus, fiber strength is deter-mined by the number and size of flaws and bythe transverse and axial orientation. A variety oftransverse textures are possible (Ref 19), includ-

ing a common one described as onion skin. Inthis structure, the graphene layer planes at thefiber surface align like the layers of an onion. Inthe center core region of the fiber, the layers arerandomly oriented. Most of the microstructuralpores and flaws are found in either the transitionfrom the skin to the random core region, or inthe core region; flaws resulting from damage in-duced during precursor or carbon fiber process-

ing are observed on the surface. Some pitch fi-bers have very large graphene layers in a flatorientation reminiscent of the old Pan-Am Air-lines insignia. Some of the ultra-high modulusfibers 900 GPa (130 106 psi) have a radialstructure like the spokes of a wheel.

Unlike the axial structure, the radial structureof the carbon filament depends upon precursortype and processing (Ref 2024).

Flaw size and flaw density reduce the strengthof a carbon fiber. However, because the fiber isbundled with thousands or millions of other fi-bers in a composite, the strength is an averageeffect. Fiber manufacturers control the strengthof the overall fiber bundle through rigorous pro-

cess control.Effect of Structure on Properties. Fibers

made from PAN precursors generally exhibithigher tensile and compressive strength, higherstrain at failure, and lower modulus as comparedto mesophase-pitch-based fibers. The structureof PAN-based carbon fibers leads to a good bal-ance in properties and is responsible for theirdominance in structural applications. Relativelygood layer alignment and small crystallite stackheights minimize interlayer shear failure, whichimproves compressive strengths while maintain-ing good tensile strength (Ref 25). A model of a400 GPa (58 106 psi) PAN-based carbon fiberis shown in Fig. 5 (Ref 26).

PAN fibers used to be categorized into stdard modulus, intermediate, and high moduNew offerings by fiber producers have blurthese categories somewhat. The differences tween the three categories for fibers made fra particular precursor are due to combinationmechanical stretching, heat treatment, andprecursor spinning. Distinctions are also mbetween aerospace and commercial-grade c

bon fibers. The difference relates to the typeprecursor used; commercial grades use a locost, modified textile-type PAN. Polyacrylotrile chemistry is similar between the two, adifferences relate more to processing. Geneized properties for PAN-based fibers are psented in Table 1.

Larger crystallite size and greater orientatof mesophase-pitch-based fibers give them perior modulus, thermal conductivity, and lowthermal expansion characteristics as comparePAN-based fibers. Satellite applications mextensive use of pitch-based fibers and take vantage of all three properties. Table 2 lists geral properties of mesophase-pitch-based fibe

One of the most beneficial properties ofcarbon fibers is their superior fatigue resistain composites. Unlike glass or aramid fibers, cbon fibers do not suffer from stress rupture, ademonstrate complete elastic recovery upon loading (Ref 2729). Creep is not observedcarbon fibers at temperatures below 2200(3990 F) (Ref 30).

The carbon content of low-modulus carbonbers is less than 99%, largely because of retainitrogen (Ref 31). Increasing carbon conte


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Table 1 Properties of PAN-based carbon fibers

Commercial, Aerospace

Property standard modulus Standard modulus Intermediate modulus High modulus

Tensile modulus, GPa (106 psi) 228 (33) 220241 (3235) 290297 (4243) 345448 (5065)Tensile strength, MPa (ksi) 380 (550) 34504830 (500700) 34506200 (600900) 34505520 (60080Elongation at break, % 1.6 1.52.2 1.32.0 0.71.0Electrical resistivity,lX cm 1650 1650 1450 900Thermal conductivity, W/m K

(Btu/ft h F)20 (11.6) 20 (11.6) 20 (11.6) 5080 (2946)

Coefficient of thermal expansion,axial direction, 106 K 0.4 0.4 0.55 0.75

Density, g/cm3 (lb/in.3) 1.8 (0.065) 1.8 (0.065) 1.8 (0.065) 1.9 (0.069)Carbon content, % 95 95 95 99Filament diameter,lm 68 68 56 58Manufacturers Zoltek, Fortafil, SGL BPAmoco, Hexcel, Mitsubishi Rayon, Toho, Toray, Tenax, Soficar, Formosa

and densities are achieved through higher heattreatment temperature, which removes nitrogenand provides greater crystalline perfection.

Electrical and thermal conductivity also in-crease with increasing crystalline perfection andpurity (Ref 32). The electrical conductivity ofcarbon fibers must be taken into account when

processing, since free-floating fibers can shortout electrical equipment. Dust-proof, gasketedNEMA 12 enclosures (as specified by the Na-tional Electrical Manufacturers Association) arerecommended for electrical cabinets, as are cov-ers over electrical outlets (Ref 33).

Room temperature coefficients of thermal ex-pansion (CTE) in the axial direction are slightlynegative for low modulus carbon fibers, andgrow increasingly negative for the higher mod-ulus fibers. At temperatures above 700 C (1290F), the axial CTE of all fibers turns positive (Ref34). Composite designers are able to couple thenegative CTE of high modulus fibers with ap-propriate matrix materials to make composites

with a CTE of zero over limited temperatureranges.Interfacial Bonding.Resins and molten met-

als do not easily wet carbon fibers, due to therelatively inert, non-polar fiber surface. Glass fi-bers depend upon coupling agents to chemicallybond with resins; carbon fibers never achievestrong bonds. Instead, carbon fiber depends upona combination of mechanical and weak chemicalbonding with the matrix material. Surface treat-ments used by carbon fiber manufacturers pop-ulate the fiber surface with active chemicalgroups such as hydroxyls, carboxyls, and car-

bonyls (Ref 35). These form bridges between thefiber and resin, and depend upon the number ofbonds rather than the strength of the bonds toachieve a strong interface.

Reactivity with Other Compounds. As aninorganic material, carbon fibers are not affectedby moisture, atmosphere, solvents, bases, and

weak acids at room temperature (Ref 36). How-ever, oxidation becomes a problem at elevatedtemperatures. For low-modulus PAN-based fi-bers and high-modulus PAN- or pitch-based fi-bers, the threshold for oxidation for extended op-erating times is 350 C (660 F) or 450 C (840F), respectively (Ref 37). Impurities tend to cat-alyze oxidation at these low temperatures andsomewhat improved oxidation resistance can beexpected with higher-purity fibers (Ref 38).

Typical Applications ofCarbon Fibers

Carbon fiber usage is growing in a variety ofapplications, including aerospace, sportinggoods, and a variety of commercial/industrialap-plications. Growth is fastest in the commercial/industrial applications. In many instances, car-bon composites have displaced metal parts,despite being more expensive on a direct-re-placement purchased cost basis. Where success-ful, carbon composites have lowered total sys-tem costs through reduced maintenance, fasterprocessing speeds, and improved reliability.Many new uses under development are enabling,meaning applications that were not practicalwith

metal or other materials are now possible witcarbon composites.

Aerospace. Perhaps nowhere is the need tsave weight greater than in the aerospace indutry. Early growth of the carbon fiber industry wadriven almost exclusively by the desire fohigher performance aircraft made possible wit

carbon fiber composites. Today, carbon fiber used on aircraft for primary and secondary strutures. Use is growing, having already establishea strong track record in primary structures omilitary aircraft. All of these applications uscarbon fiber for its high specific strength anspecific stiffness. Fiber formats used include prpreg for layup processes and fabrics for resitransfer molding and similar processes.

Satellites incorporate very high modulupitch-based carbon fibers, partly for the higstiffness-to-weight ratios and partly for thenegative axial coefficient of thermal expansion

Sporting Goods. Golf club shafts are preently the largest sporting goods application fo

carbon fibers. Lighter weight and higher stiffneshafts, made possible with carbon fiber, alloclub manufacturers to place more weight in thclubhead, which increases club head speed foimproved distance. Most golf shaft manufactuing today is done with unidirectional prepreggesheets of carbon fiber in a roll wrapping operation. Some shafts are filament wound.

Carbon fiber fishing rods are favored by fisherman for their lightweight and sensitive touchThe rods are manufactured via a roll-wrappinprocess similar to golf shafts, using unidiretional prepreg. Most racquets for tennis, raquetball, and squash are made from prepreggecarbon fiber that is sheeted, wrapped around bladder, and cured. Carbon composite arrows ar

fabricated by either of two processes: pultrusioor roll wrapping. Skis and bicycle componentend to use fabrics made from carbon fiber.

Commercial and Industrial ApplicationLarge volumes of milled and chopped carbon fiber are used to impart static dissipating propeties to trays for processing semiconductors, anfor computer printer and copier machine partParts are injection molded from thermoplasticthat have been pre-blended with carbon fiber ia compounding extruder. The carbon fiber used for its electrical conductivity and ability t

Table 2 Properties of mesophase pitch-based carbon fibers

Property Low modulus High modulus Ultra-high modulus

Tensile modulus, GPa (106 psi) 170241 (2535) 380620 (5590) 690965 (100140)Te nsile s tren gth, M Pa (ksi) 1 380 3 10 0 (20 0 450 ) 1 900 27 50 (275 4 00 ) 2 410 (35 0)Elongation at break, % 0.9 0.5 0.40.27Electrical resistivity,lX cm 1300 900 220130Thermal conductivity, W/m K

(Btu/ft h F). . . . . . 4001100 (230635)

Coefficient of thermal expansion inaxial direction, 106 K

. . . 0.9 1.6

Density, g/cm3 (lb/in.3) 1.9 (0.069) 2.0 (0.072) 2.2 (0.079)Carbon content, % 97 99 99Filament diameter,lm 11 11 10Manufacturers BPAmoco, Mitsubishi Kasei BPAmoco


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Carbon Fibers /

provide lightweight reinforcement to thermo-plastics.

Carbon fiber drive shafts and couplings havereplaced steel shafts for cooling towers andmany other torque-transmitting applications.Properties that favor carbon composites includecorrosion resistance, light weight, and high stiff-ness, which reduces vibration. Filament windingprocesses are used to fabricate the shafts and in-

corporate fiber into various angles tailored forthe torque and vibrational characteristics re-quired of the application.

The desire for faster processing in the paper-making and film casting industry has encouragedgrowth of carbon composite rollers, which spinfaster and have less deflection than steel rollers.Roller diameters up to one meter are now rou-tinely fabricated with carbon fibers on automatedfilament winding machines.

Speed and precision are also drivers for weav-ing machine components, such as rapiers, thatare made from pultruded carbon compositeshapes.

Injection of liquids into oil wells to stimulate

production is currently being performed withspoolable carbon composite pipe. The pipe ismade in a continuous process whereby a ther-moplastic liner is pulled through multiple sets ofrotating creels containing carbon fiber spools.The fiber is wetted with resin and wrapped ontothe liner, which serves as the mandrel. The tubeis cured in-line, and spooled at the other end.Although spoolable carbon composite pipe ismore expensive than spoolable steel pipe, its us-age is increasing because of superior fatigue per-formance, which results in improved reliability.Stress corrosion during unspooling and respool-ing is responsible for premature failure of steelspoolable pipe.

Depletion of shallow water oil fields and themove into deeper water has raised oil companyinterest in carbon composites. Development pro-grams are underway to moor large oil platformsto the ocean bottom via tethers constructed frompultruded carbon fiber rods. Carbon fiber tethersare the leading contender for use in water depthsbeyond 1500 m (5000 ft); this shift is based onits lightweight and high stiffness, which mini-mize the natural frequency of the platform dueto wave motion. Steel tethers used in shallowerdepths are impractical in deeper water, becausethey cannot support their own weight hangingfrom the platform.

Pultrusion companies that are eyeing thetether application are also pursuing carbon com-

posite tendons for pre- and poststressing of pre-cast concrete. Unlike glass fiber, carbon com-posite rods are inert to alkaline attack andcorrosion and satisfy the increased useful life re-quirement imposed by many municipalities forinfrastructure projects. Another pultrusion appli-cation, carbon composite rebar, is being devel-oped for use along the waterfront to combat thehigh costs of corrosion induced structural dam-age.

Seismic retrofitting of bridge columns andwalls has been extensively performed in Japan

with sheets of carbon fiber wallpaper or fabricsthat are saturated with resin and applied to theconcrete structure. The high stiffness of the car-bon minimizes movement of the concrete and theinertness to corrosion insures long term protec-tion.

The electrical properties of carbon fiber andthe ability to configure the material into a semi-permeable membrane with defined mass trans-

port properties make carbon the material ofchoice as the electrode in polymer electrolytefuel cells to power next generation engines.

Anticipated Developments inCarbon Fibers

Much of the effort expended for carbon fiberstoday is directed at cost reduction. It appears thatthe prospects for cost reduction have stimulatedinterest in many new applications. Certainly, asprices come down, opportunities for new appli-cations grow.

Future funding and technology development

for carbon fibers will most likely be directed to-wards application development, a trend experi-enced in the glass fiber industry. Areas of op-portunity include lowering cost and improvingspeed of manufacturing processes.

Education and familiarity with composite ma-terials are increasing, but are still well below thatof metals. We can expect the demand for carbonfibers to grow in large steps as more engineerslearn how to design with carbon fibers.

Property standardization is another expectedtrend. Glass fiber is easy for a designer to designwith, since E-glass and S-glass are standards bywhich many suppliers produce. Carbon fibersuppliers have many grades to choose from, with

little commonality among producers. As largeapplications for carbon fiber develop, customerswill demand standardization among carbon fiberproducers.

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