Carbon Fibre

1. INTRODUCTION

The development of carbon fibre has been a milestone in the history of composite materials. Although it has not achieved full commercial potential even after 48 years of its development as glass fibre has achieved, these black silky fibres have demonstrated what modern technology can achieve in terms of technological excellence. Carbon fibre is the strongest and stiffest fibre that has been produced today (excluding whiskers). 

Carbon exists in nature in several allotropic forms. These forms result due to the way carbon atoms are arranged in the material. Diamond which is the hardest of the carbon family has each carbon atom connected to four carbon atoms. Graphite is the next member in the carbon family, which has a platelet structure when each carbon atom is covalently bonded with other atoms in a plane. Carbon filament and other forms of carbon products are well below the level of graphite. When the graphite platelets are stacked in 3-dimensional form, the material is defined as graphite. Carbon fibre has two dimensional ordering of the graphite platelets.

2. BRIEF HISTORY OF DEVELOPMENT

It was Thomas Edison in 1878, more than 130 years ago, who demonstrated a process of making carbon filament from cotton fibre and bamboo for his electric filament lamps. 
Later carbon filament of low strength and modulus were made in the early 1900s for high temperature thermal insulations, heating elements and as packing glands and seats in chemical equipment.
They were made by pyrolysis of cotton, rayon and other polymers and pitches. It was during 1960, the high strength carbon fibre was made from the continuous precursor filaments. The modern carbon fibre reinforcements were thus commercially made in 1960.

3. STRENGTHENING MECHANISM 

Diamond and graphite are two strongest and stiffest materials. The theoretical density, modulus and strength of diamond are 3.51 gm/cc, 20 GPa and 90 GPa. The graphite has the respective theoretical properties with in its platelets as 2.26, 1020 GPa and 150 GPa respectively. The shear modulus and strength in the planes perpendicular to the platelets are weak. In carbon fibre, these graphite platelets are oriented in the axial direction of fibre giving high strength and modulus in the axial direction. Since defects and misalignment can still occur, the carbon fibre shows substantially less strength and modulus than the theoretical values given above. 

Fig. 1 shows the schematic picture of the layer structure of the graphite platelets in carbon fibre.


Fig 1 The graphite layer structure

4. MANUFACTURE


Almost all carbon fibres are produced from polymeric fibres referred to as precursors. Rayon, isotropic pitch, polyacrylonitrile (PAN) and liquid crystalline (mesophase) pitch are the precursor materials used. Among them, the first two precursors yield carbon fibre of lower strength and they are used in ablative layers. Reinforcement fibres are made from the PAN and mesophase pitch.



Fig 2 shows a schematic diagram of the carbon fibre making process by using both PAN and pitch based precursors.


4.1 Carbon fibre from Rayon


When a non-melting textile fibre such as cellulosic rayon is pyrolyzed up to 10000C in an inert atmosphere, hydrogen and oxygen are lost as H2O, CO2, and CO and a shrunken carbon fibre remains. Carbon fibres thus made from rayon have an axial modulus of only 35 GN m-2 because the orientation of the layer planes is completely random.


Later it was shown by Bacon and Schalamon (1969) that, if such carbon fibres are heated above 25000C, sufficient plasticity is developed that on the application of tension, the fibres can be elongated; the layer planes are pulled into an axial preferred orientation, and fibres with Young’s moduli exceeding 350 GN m-2 can be produced. This process was used commercially for a few years by the union Carbide Corporation, but has now been abandoned for the simpler processes starting from polyacrylonitrite (PAN) fibres and measophase pitch which do not use the troublesome very high temperature stretching.


4.2 Carbon fibre from PAN


PAN fibres ([CH2CH(CN)]n) are now preferred for producing most high modulus carbon fibres. They are made in large quantities for normal textile use (eg: Courtelle and Orlon) and are available in suitable small diameters. The yield of carbon is about 45% and the process requires no stretching at high temperature. PAN fibres are good precursors for carbon fibres since PAN fibres have an all carbon backbone and (of special importance) from a ladder polymer by intra-molecular rearrangement when heated to 200-3000 C (Scheme 1). A linear heterocyclic structure is formed which has an enhanced thermal stability. When the heating is carried out in an inert atmosphere, the fibres develop a deep copper colour, but heating in air gives back fibres because of an uptake of oxygen (1984). It is important that during the stabilization process the fibres are restrained from shrinking, or even extended. All synthetic textile fibres are stretched several times during manufacture to achieve an axial orientation of the linear molecules and thereby give the fibres the required properties for normal textile uses. PAN fibres are stretched about eight times at 1000C after initial coagulation into a non-oriented fibre. It follows that on heating above 1000C the fibres will shrink in length (entropic shrinkage) and loose some of their orientation. Since the ladder polymer is the template for the formation of the graphite layer planes, it is necessary to maintain the preferred orientation of the molecules.


The linear shrinkage of stabilized PAN fibres on conversion to carbon fibres is only about 13%, but the diameter shrinkage is about 45%, indicative of the formation of a much more closely packed structure.


The ladder polymer formation and oxidation reactions are both exothermic; the former producing 33 kJ mol –1, the latter about ten times this value. Hence, it is essential that the rate of heat release is controlled, otherwise the temperature rise of the fibre tows may be so great that the fibre will stick together. Thus in commercial practice continuous tows of PAN fibres are stabilized by heating in air at temperatures of 220-2500 C while moving through an oven with a very good air circulation, and subsequently pirolyzed by passing through a series of furnaces at increasing temperatures with just sufficient tension to pull the tows through. The pyrolysis reactions in and between the oxidized ladder evolve H2, NH3, HCN and N2, and at 1000o C an oriented carbon fibre, albeit containing 5% substiitutional nitrogen, results: the nitrogen is eliminated by heating at 1500 0C. Changes in Young’s modulus and tensile strength as a function of the final heat treatment temperature are shown in Fig. xx Thus a spectrum of carbon fibres of different strength and moduli is possible.


There are also a few ultra high modulus PAN based fibres which are commercially available, but these values are still considered less than the value of 1060 GN m-2 for a single graphite crystal parallel to the layer planes. This is because the orientation of the planes in the carbon fibres approaches a maximum only in the surface layers. The enhanced orientation in the surface layers is almost certainly due to a similar effect in te precursor PAN fibre; that is, there is a decreasing oxygen gradient from circumference to centre in the cross section of the stabilized fibre.


4.3 Carbon fibres from Pitch


At about the same time that high-strength carbon fibres were being developed from PAN, carbon fibres were reported made from Low-melting point isotropic pitches. Fibres were melt spun, chemically cross linked with oxygen and ozone to prevent them from remelting on furher heat treatment, and finally carbonised. The fibres were isotropic, were not graphitizable and had poor tensile strengths and Young’s moduli.


It was subsequently found by Hawthorne et al. (1970) that the crystallites in isotropic pitch-based carbon fibres could be oriented by hot-stretching them by as much as 200% at temperatures between 2200 and 2900 0C. Their tensile modulus could be increased to 620 GN m-2 but the highly oriented carbon fibres were still not graphitizable, although the precursor pitches were. This hot stretching step had the same practical difficulties that were encountered in the rayon –based process.


Shortly thereafter, Union Carbide found that it was easier to orient the carbon structure at the mesophase or liquid crystal stage, and then to thermoset, carbonise and graphitize the fibres. Since the axial preferred orientation was maintained during all parts of the process, troublesome hot stretching techniques were not required. By heat treating these fibres to graphatizing temperatures (>2800oC), graphite fibres were obtained with Young’s moduli approaching the in-plane value for perfect graphite 1000 GN m-2.


The formation of a liquid crystal (mesophase) as the first step in the ordering process during the thermal polymerization of pitch to coke and carbon was a remarkable discovery made by Brooks and Taylor (1965). It is at this stage that pitch, a complex mixture of thousands of different species of hydrocarbon and heterocyclic molecules, becomes an incipient graphitic structure with long-range order and orientation.


This transformation occurs as follows: as pitch is heated above 400°C, either at constant temperature or with gradually increasing temperature, the molecules double or triple in size by dehydrogenative condensation reactions and become large enough (average molecular weights of approximately 1000) and flat enough to form layers of associated molecules which constitute a somewhat more dense, ordered nematic liquid-crystal phase or mesophase. At first the mesophase appears as small liquid spheres which collide and coalesce as shown in the schmematic diagram in Fig.5. Eventually the entire isotropic pitch can convert to this more ordered, more viscous phase which, with continued heating, would ultimately become a semi-solid nematic glass, namely coke.


The aromatic molecules in the liquid carbonaceous mesophase can be depicted as a collection of slippery playing cards. The liquid mesophase is thus easily orientable above its melting point by shear or elongational forces, and can be extended and drawn into highly oriented filaments or fibres. The melt spinning-orienting process is illustrated in Fig.6 Finally, the fibres are oxidized (to cross-link and thus prevent them from remelting), carbonized and, if desired, graphitized. The mesophase pitch as-spun fibres possess a built-in thermodynamic driving force to convert thermally, without any tension or stretching, into a graphitic fibre possessing a high degree of axial preferred orientation and resultant high Young's modulus.


Other easily orientable pitches and other processes for making meso phase pitch have been described. The most comprehensive review of carbon fibres derived from mesophase pitches has been published by Rand.


The structural characteristics of mesophase pitch-based fibres are unique compared to other carbon fibres derived from Rayon, PAN or isotropic pitch. They are more graphitizable and possess larger crystallites and oriented domains than do fibres from other precursors. The filaments have a fibrillar-like longitudinal appearance and the large elongated domains are evident under polarized light. The transverse structure can be either circumferential, radial or random in character. Figure 7 shows SEM photomicrographs of the radial and circumferential (onion skin) types of carbon fibre.


The thermal conductivities of ultrahigh-modulus mesophase pitch-based fibres are remarkably high and in some cases surpass that of copper at room temperature. An interesting comparison (Fig.8) of the relationship between thermal conductivity and electrical resistivity for carbon fibres and metals has been devised by Schulz. Note that the highest modulus P-130X fibre has almost three times the room-temperature thermal conductivity of copper. It will be of interest to see how closely the thermal conductivity of mesophase pitch-based fibres can be made to approach that of highly oriented pyrolytic graphite (HOPG), which has approximately six times the thermal conductivity of copper at room temperature.


4.4 Carbonization


The operation is identical for both PAN and pitch based fibres. However, the stabilization of anisotropic pitches involves simple cross-linking of plate-like molecules whereas the stabilization of PAN involves many different chemical reactions. Stabilization of both PAN and pitch is an exothermic process, so great care must be taken to control the rate of reaction and to avoid thermal runaway which melts the fibre and is a fire hazard.


Commercial stabilization is carried out by heating the PAN fibre in air between 200 and 260°C for a period of time that varies between thirty minutes and several hours. During stabilization, several interpendent chemical reactions occur. The reaction that dominates is primarily determined by the chemical composition of the initial precursor, the spinning history, the final composition of the as-spun fibre and the stabilization heating schedule.


A PAN polymer mainly consists of acrylonitrile entities -CH2CH(CN)-which are able to cyclize with the help of an initiator into a presumably linear 'ladder polymer'. In general, the pendant nitrile groups of PAN first become crosslinked to form a ladder polymer. Initiation of this process is catalyzed in some cases by the presence of a small amount of reactive copolymer such as itaconic acid. Oxygen is then incorporated into the ladder polymer under a number of possible schemes. Cyclization and stabilization induce tremendous shrinkage into the polymer. Longitudinal shrinkage is resisted mechanically, however, the diameter of the fibre is allowed to decrease.


A balance should be kept during stabilization as to hydrogenation degree. A large hydrogen content can result in a small N/C ratio which increases the temperature at which the local molecular ordering occurs. Conversely, increasing the available oxygen decreases the size of and the temperature at which the units of local molecular order (LMOs) are formed. In addition, since the viscosity increases as cross-linking (stabilization) proceeds, the mobility and growth rate of the LMOs decrease; hence their final size remains small. The smaller the size of the LMOs, the less graphitizable is the carbon and the lower the properties of the fibre. In order to ensure appropriate N/C ratios at reasonable temperatures, stabilization should result in only a moderate degree of cross-linking. In addition, slow heating rates during precarbonization should permit hydrogen and delay nitrogen emissions; both of these effects lower the temperature at which extensive formation and growth of LMOs occur.


A commercially acceptable rate of stabilization requires the use of as high a temperature as possible. However, since the reactions that occur during stabilization are exothermic, it is most important to limit the oxidation rate and to prevent uncontrollable temperature increases. These conflicting requirements have resulted in the development of alternative methods of stabilization.


4.4. 1 Chemical Changes During Carbonization: The carbonization of stabilized PAN and pitch involves controlled heating to a temperature of about 1500°C. The majority of gases emitted from either the PAN or the pitch are emitted before a temperature of 1000°C is reached and the emission is primarily from unstabilized regions. Indeed, the quantity of gases emitted from an unstabilized central core of either PAN or pitch can be so large that the fibre can disintegrate. Great care should therefore be taken to determine the optimum heating rate for stabilized or under-stabilized fibres. In some cases, hold times should be incorporated into the heating cycle. Both materials emit a variety of gas molecules containing oxygen, hydrogen and carbon. However, a major difference between PAN and pitch involves nitrogen containing compounds which are only emitted from PAN. The temperature and rate of emittance are important control parameters since they affect the strength of the resultant carbon fibres.


A stabilized polyacrylonitrile fibre which contains about 11 wt.% oxygen can be thermally degraded by heating at a slow heating rate in an inert atmosphere such as nitrogen or a reactive environment where nitrogen gas is bu~bled through acid (US Patent 3 972 984, 1976) or water (US Patent 3 677 70S, 1976; US 3 656 903, 1972; US Patent 4 039 341, 1976). As the temperature increases, many complex reactions take place resulting in the evolution of volatile products. For example, when the fibre is initially heated, cyclization occurs with the release of large amounts of HCN and NH3. Up to 450°C. HCN, acrylonitrile, propionitrile, NH3 and H2O are emitted. Subsequently, at around 500°C and 700°C copious quantities of HCN and water vapour are emitted, respectively. All of these emissions are believed to come from reactions involving crosslinking of individual molecules. Evolution of nitrogen starts near 700°C; so fibres produced after being heated to 1000°C retain only about 5.8 wt.% nitrogen and have lost about 50 wt. % of the original PAN precursor fibre


Results obtained from experiments involving slow pyrolysis at 4°C/min indicate that optimum mechanical and physical properties are unobtainable unless high nitrogen contents are retained within the precursor until the later stages of carbonization. Therefore, a large nitrogen content (large N/C ratio) should be present when local molecular ordering (LMO) begins and the carbon skeleton is being formed. Since the N/C atomic ratio depends inversely upon the H.C ratio, LMO should occur at large N/C and small H/C ratios. Essentially, small amounts of aromatic hydrogen and a relatively large amount of nitrogen present during the LMO stage allow the carbon skeleton to remain flexible enough at high temperatures that molecular rearrangement is easy. Within this overall fibrous texture, the nanotextural features of the carbonized fibres are the consequence of the variations in cyclization, stabilization, carbaonization and graphitization conditions. If the original precursor is CH- and NH-rich but oxygen-poor, the corresponding carbonized fibres will have low porosity, high compactibility and stacking order and a relatively high strength. Likewise, if two nitrogen atoms are present in two aromatic rings contained within adjacent sheets, they are able to promote bonding of contacting BSUs together with a N2 release (Watt, 1972). The ultimate strength value of the fibres increases as the compactibility and the availability of 'efficient' nitrogen (i.e. the nitrogen remaining at the moment of LMO occurrence) increases (Oberlin and Guigon, 1988; Guigon, 1985). Bright and Singer (1979) agreed with others when they found that the tensile strength of most heat treated fibres tends to decrease with higher temperature exposures and release of nitrogen


Up to 1000°C, carbonization leads to an effluent loss and increasing aromatization. As a result, the solid residue transforms from a viscoelastic into a brittle solid material. The stabilized PAN normally carbonizes into a statistically isotropic but nanoporous carbon material \NI1ich, because of the small dimensions of the initial LMOs, is inherently non-graphitizable (Joseph and Oberlin, 1983a). Continuing pyrolysis up to 1500°C eliminates most of the residual nitrogen and completes theconversion of the PAN molecules into sheets of carbon that are appreciably anisotropic (Mair and Mansfield, 1987). Continued heating eliminates the remaining nitrogen and, since the material is then only made of pure carbon, further modifications are only structural.


Graphitization is the name of the process that involves heating the carbonized fibre to approximately 2500°C in times as short as a minute (US Patent 4005 183, 1977). Graphitized pitch fibres exhibit a larger, more graphitic and better oriented crystal structure than PAN-based carbon fibres which are inherently non-graphitizable. Parallel to the fibre axes, pitch fibres have higher stiffness and thermal conductivity values and a reduced thermal expansion coefficient. These changes due to graphitization do not produce any significant increase in relative strength values. As a result of the extreme temperatures required to process them, graphitized pitch- based carbon fibres are more expensive and are fabricated for specialized applications


4.4.2 Microstructural Changes During Carbonization: The initial heating of stabilized PAN fibres causes growth of graphite-like ribbons by a dehydrogenation mechanism. Denitrogenation, which occurs as the temperature is increased, is responsible for the growth in area and the transformation of these ribbons into thin sheet-like structures  and, at higher temperatures, for the bonding of adjacent sheets. These sheets contain numerous vacancy imperfections and are folded to enclose pencil-shaped voids oriented in the general direction of the fibre axis. The lengths of each block or sheet are relatively short, with each succeeding block disoriented with respect to its neighbour. A schematic illustration of the microstructure as it exists within the actual fibre is shown in Fig.4. This structure is typically exhibited by high strength PAN-based carbon fibres


A schematic of this microstructure is shown in Fig.4 The longer, better oriented and more graphitic microstructures exhibit both higher values of modulus and thermal and electrical conductivities; unfortunately, the misalignment of the larger microstructural units causes large stress concentrations and, hence, weaker fibres.





Fig 4 Schematic illustration of carbon fibre microstructure


Two possible orientation of layer plane in the cross section of a carbon fibre: Deducted from optical microscopy.



Increasing the heat treatment temperature results in a reduction of the interlayer spacing, a decrease in void space, a growth in thickness and area of the graphitic crystallites and an increase in the preferred orientation of the microstructure. All of these changes increase the elastic modulus and the electrical and thermal conductance. A corresponding reduction of the tensile strength also occurs by mechanisms that depend on local defects as discussed in the previous section.


PAN-based fibres have the lower maximum values of the elastic modulus and the electrical and thermal conductivities.





Mechanical Properties and Applications


In this section, the unique characteristics of carbon fibres are discussed with emphasis on which structures and precursors are most suitable for particular applications. The microstructural changes discussed above have been deduced using X-ray diffraction techniques. In addition, the mean length of the graphite sheets oriented in the fibre direction La and their thickness Lc may be computed using such techniques. It has been found that both of these parameters increase with increasing temperature. The orientation of these layers also becomes increasingly aligned with the fibre axis. The net effect is to increase the tensile modulus .


At present, the high-strength standard and intermediate modulus PAN-based continuous fibres listed in Table 1 have the best balance of tensile and compressive properties for structural applications. Whether the strength-limiting concentration of flaws due to pores and other intrinsic structural defects can be reduced even further remains to be seen. PAN-based carbon fibres are available with various surface treatments and sizings to improve their compatibility with many different thermosetting and thermoplastic resins in composites. Such composites, particularly with epoxy resins, have found applications in a variety of strong, lightweight aircraft and space structures, and in sports equipment as well.


Mesophase pitch-based fibres have advantages in those applications which require combinations of extremely, low thermal expansion, and superior oxidation resistance at high temperatures. These attributes are a result of the higher density and ease of graphitizability of these fibres. Mesophase pitch-based fibres have been found to be particularly suitable for use in carbon-carbon composites in which high modulus and high thermal conductivity are important.




6. PROPERTIES AND CHARACTERISTICS


Carbon fibres have specific gravity in the 1.76 to 1.9 in the case of PAN bases fibre and 1.9 to 2. 5 in the case of pitch based fibres. Table 1 and 2 gives the preparation of PAN and pitch based fibres.


Some of the other features of carbon  fibres can be given as follows.

  • Carbon fibres do not suffer from stress corrosion or stress rupture at room temperature as glass and organic fibres do. 
  • At high temperature strength and modulus of carbon fibres are excellent. 
  • Carbons fibre are not affected by moisture, atmosphere solvents, bases and weak acids at room temperature
  • Carbon oxidise at higher temp in atmosphere. PAN based fibres oxidises above 350 oC and pitch based fibre above 4500 C 
  • Carbon fibre reacts with molten aluminium and titanium forming Aluminium carbide and titanium carbide. To prevent this, a barrier coating of copper is applied on the fibre surface
  • Nickel does not react with carbon but a small amount of solution /dissolution can occur at the interface which degrades the fibre
  • Reaction occurs with oxides like Zirconia (ZO2), thorium (ThO2) and hxx at higher temperatures(above 12000C)
  • Coefficient of thermal expansion is slightly negative and become more negative as the modulus increases. This negative CTE is used along with the positive value of resin to make composites of zero CTE. Carbon metal composites require high modulus fibres with modulus greater than 650 GPA. Only xxx pitch fibres can be used for this. PAN and pitch based carbon fibres of lower modulus can be used for making of zero CTE composites using resin matrices. 
  • Carbon fibres are thermally and electrically conductive. Thermal and electrical conductivity depend on preferred orientation, crystallite size and purity . The thermal conductivity of high modulus fibres exceeds that of copper. Electrical and conductivity can however be les and can be 1/50 of that of copper.
  • The fibres are anisotropic in properties, the transverse modulus is much less than longitudinal modulus and is positive in transverse direction.