Fiber Fracture Episode 6 doc

FRACTURE OF CARBON FIBERS 161 8 7 6 m5 a c3 5 04 2 5 c 3 2 1 0 Isotropic Pitch ia. 200 400 600 800 1000 Modulus, GPa Fig. 4. Strength and stiffness of carbon fibers. From Lavin (2001b). 0 200 400 600 800 1000 Modulus, GPa Fig. 5. Compressive strength of carbon fibers. From Lavin (2001b). 162 J.G. Lavin 1000 5oa Y s loa ._ 2; 2 50 0 3 U c 0 0 - 2 % la 5 1 0 200 400 600 800 1000 Modulus, GPa Fig. 6. Thermal conductivity of carbon fibers. From Lavin (2001b). 0 200 400 600 800 1000 Modulus, GPa Fig. 7. Electrical resistivity of carbon fibers. From Lavin (2001b). FRACTURE OF CARBON FIBERS 163 PAN-BASED CARBON FIBERS Polyacrylonitrile (PAN) fibers are made by a variety of methods. The polymer is made by free-radical polymerization either in solution or in a solvent-water suspension. The polymer is then dried and re-dissolved in another solvent for spinning, either by wet-spinning or dry-spinning. In the wet-spinning process the spin dope is forced through a spinneret into a coagulating liquid and stretched, while in the dry-spinning process the dope is spun into a hot gas chamber, and stretched. For high-strength carbon fibers, it is important to avoid the formation of voids within the fiber at this step. Dry-spun fibers are characterized by a 'dog-bone' cross-section, formed because the perimeter of the fiber is quenched before much of the solvent is removed. The preferred process for high-strength fiber today is wet-spinning. Processes for melt-spinning PAN plasticized with water or polyethylene glycol have been developed, but are not practiced commercially. A significant improvement in carbon fiber strength was obtained by Moreton and Watt (1974) who spun the PAN precursor under clean room conditions. The strength of fibers spun in this way and subsequently heat treated was found to improve by >SO% over conventionally spun fibers. The mechanism is presumed to be removal of small impurities which can act as crack initiators. This technology is believed to be critical for production of high strength fibers such as Toray's T800 and T1000. Polyacrylonitrile 7^r('(* Cyclize CN HHH OPO Oxidize HHH Stretch Fig. 8. PAN-based carbon fiber chemistry: cyclization and oxidation. 164 J.G. Lavin Initially, commercial PAN-based carbon fibers were made from the polymers devel- oped for textile applications. However, these fibers were neither very stiff nor strong. Development efforts over the 1960s and 1970s focused on increasing molecular weight, introducing co-monomers to assist processing, and eliminating impurities which limited mechanical strength. The chemistry of conversion of PAN to carbon is quite complex, and the interested reader is referred to an excellent treatment in Peebles (1994). The critical steps are outlined below. The first critical step in making carbon fiber from PAN fiber is causing the pendant nitrile groups to cyclize, as illustrated in Fig. 8. This process is thermally activated and is highly exothermic. The activation temperature is influenced by the type and amount of co-monomer used. It is also important to keep the fiber under tension in this process, and indeed, during the whole conversion process. The next step is to make the fiber infusible: this is accomplished by adding oxygen atoms to the polymer, again by heating in air. The reaction is diffusion limited, requiring exposure times of tens of minutes. When about 8% oxygen by weight has been added, the fiber can be heated above 600°C without melting. When the fiber is heated above this temperature, the processes of decyanization and dehydrogenation take place, and above 1000°C large aromatic sheets start to form, as illustrated in Fig. 9. Carbonize H H H H H H 00-600 C Dehydrogenation 600-1 300 C Denitrogenation + Stretch Fig. 9. PAN-based carbon fiber chemistry: carbonization. FRACTURE OF CARBON FIBERS 165 fibre axis Fig. 10. Model of microtexture of PAN-based carbon fiber. (Copyright 1984, reproduced with permission from Elsevier Science.) The weight loss experienced in the production of carbon fibers from PAN precursor is approximately 50%. Guigon et al. (1984) showed that this leads to a structure containing many longitudinal voids, as shown in Fig. 10, and a density of -1.8 g/cm3, compared with 2.28 g/cm3 for pure graphite, and 2.1 for pitch-based carbon fibers. Boyes and Lavin (1998) showed evidence for the polymeric nature of the fiber in the fracture surface shown in Fig. 1 1. The fibrils are evident on the wall of the fiber. An enlargement of the fracture surface in Fig. 12 shows fibrils at the nanometer scale. The results of a remarkable experiment by Kwizera et al. (1982) are shown in Fig. 13. A Celion GY-70 fiber was fractured in vacuum, and exploded into microfibrils roughly 100 nm in diameter, further confirming the fibrillar nature of the PAN-based fiber. I 99 I FRACTURE OF CARBON FIBERS 167 Fig. 13. PAN-based carbon fiber fractured in vacuum. The fibrils are approximately 100 nm in diameter. (Copyright 1982, reproduced with permission from Elsevier Science.) A typical commercial pitch is Ashland Aerocarb 70, which has a softening temperature of 208°C and a viscosity of 1 Pas at 278°C. Additional treatments to selectively reduce low molecular weight components are described by Sawran et al. (1985). General purpose fibers are prepared by two different spinning methods, centrifugal spinning and melt blowing, both of which are high-productivity processes. A more detailed discussion of these processes will be found in Lavin (2001b). High-Performance Pitch-Based Carbon Fibers High-performance fibers are made from mesophase pitch, which is a discotic liquid crystalline material. While mesophase pitches can be made from many starting materials, there are only a few which are of commercial interest. These are dealt with in the sections which follow. These fibers are typically melt spun, and spinning technology is the same for all pitch types. There are three common elements in pitch preparation: first, a highly aromatic feed- stock; second, a process for polymerizing the molecules; third, a process for separating out the unreacted feed molecules. The feedstock is typically a decant oil from cat cracker bottoms. When polymerized, the pitch molecule will have characteristics similar to the molecule shown in Fig. 14. When they get sufficiently large, the pitch molecules ag- gregate to form spheres, as shown in Fig. 15. The spheres are named for their discov- erers, Brooks and Taylor (1965). The spheres in turn coagulate to form larger spheres and then, as polymerization continues, there is a phase inversion and a continuous ne- matic liquid crystalline phase, typically called mesophase (Greek for changing phase), is formed. Pitches are characterized by their fractional solubility in increasingly powerful sol- 168 J.G. Lavin CH3 Fig. 14. Typical pitch molecule. "Pole" Trace of lamellae direction Edges of disk of sphere "Pole" Fig. 15. Brooks and Taylor mesophase sphere. vents; for example toluene, pyridine, quinoline. The highest molecular weight fractions are not soluble in any known solvent. It is believed that the smaller molecules in the pitch are solvents for the larger ones, and allow the pitch to flow at elevated temperatures. Petroleum-based pitches are typically made from the same slurry or decant oils used to make isotropic pitches. The earliest processes for making mesophase pitches are sim- ilar to that described by McHenry (1977). They used a long heat soak (typically about 30 h at 400°C) under an inert atmosphere, while a gas sparge was used to take away volatile compounds. Such pitches might typically have a molecular weight of about 1000 Dalton, and melt at about 300°C. They would also be characterized by high quinoline insolubles. Coal tar pitches are a by-product of coke ovens associated with steel-making operations. They differ from petroleum pitches in their rheological properties; for a given molecular weight the flow viscosity is much higher. Coal tar pitches also have fewer aliphatic groups on the molecules, which makes for longer stabilization cycles. A breakthrough in preparation of coal tar pitches came when the Japanese Agency of Industrial Science and Technology (1983) developed a process for hydrogenating them, significantly reducing viscosity and reducing quinoline insolubles to zero. The physical properties of fibers from coal tar pitches are generally competitive with fibers from petroleum pitches, except that, so far, they have not been capable of making the highest modulus products (800 GPa and higher). FRACTURE OF CARBON FIBERS 169 Pitch processes have been under continual development for the last two decades, and are now at the stage where high molecular weight, uniform pitches can be produced in continuous processes. A detailed review of this subject will be found in Lavin (2001a). A Paradox The requirements for a strong polymer fiber are well known. They start with ex- tremely pure ingredients which are polymerized to very high molecular weights. Once spun, the crystallites are oriented parallel to the fiber axis by stretching. In the case of pitch-based carbon fibers, the situation is very different. The ingredients come from a waste stream of unknown and variable composition. Since the molecular weight of a pitch is positively correlated with its melting point, molecular weight must be kept down, so that fiber can be spun below about 300°C. Above this temperature, seals are unreliable, and equipment becomes very expensive. Finally, the as-spun pitch-based carbon fiber is too weak to stretch. These failings are compensated by the wonderful self-organizing properties of aromatic carbon; particularly its ability to orient crystallites along the fiber axis by heat treatment in the relaxed state. Fiber Formation Melt spinning of mesophase pitches, as described by Edie and Dunham (1989), is the preferred method of obtaining high-performance fibers. The controlled drawing process provides the most uniform continuous filament products, while the wound product form necessitates uniform treatment of bundles of fibers in downstream processing. However, processing rates are generally low and greatly depend upon the quality of the pitch feedstock. Pitch rheology and the arrangement of the discotic liquid crystal was found to determine mesophase pitch structure and resultant product responses in a study by Pennock et al. (1993). This structure can be defined on a macroscopic scale by scanning electron microscopy (SEM), whereas microscopic structure on the atomic scale requires use of other techniques. such as transmission electron microscopy (TEM). Bourratt et al. (1990) effectively used these techniques to determine the structure of pitch fibers. Ross and Jennings (1993) and Fathollahi and White (1994) showed that the orientation of discs relative to one another and the fiber axis is an important element to control in the filament formation step. By utilizing filament formation geometry to establish preferred flow profiles and spin conditions that complement them, structure can be manipulated and controlled. Exam- ple geometries, when coupled with appropriate feedstocks and operating conditions, conducive to structure control and resultant product responses, are shown in Fig. 16. Fiber cross-sectional structure, as defined by SEM, are schematically represented while product categorizations of physical and thermal properties are noted. The typical fiber structures illustrated here have been labelled by several researchers as ‘pacman’ radial, wavy radial and severe ‘pacman’. Other structures such as random, onion-skin and ‘Pan Am’ have also been produced and categorized. An illustration of the most common types is shown in Fig. 17. The fibers with ‘pacman’ cross-sections have longitudinal splits which may adversely affect physical properties. Downstream processing, within 170 J.G. Lavin Standard J. Variable Strength Disruption Gradual Orientation High High Thermal Strength Conductivity Fig. 16. Influence of spinneret design on fiber morphology. limits, appears to have minimal influence in changing the general 'structure' established in the filament formation step. Subsequent heat treatment densifies the initial structure, i.e., increases the packing to increase tensile and thermal properties and modulus. The use of non-round pitch carbon fiber cross-section provides an alternate approach to modify 'structure' with potential enhancement of fiber adhesion to matrices, improved surface characteristics or improved conductivity. This forced filament geometry is routinely practiced with several polymeric systems in melt spinning to control product response. Ribbon and C-shaped carbon fibers have been provided to accomplish this, as Radial Onion-sk in Random " Pac -man " Flat -layer Radial -folded Line-or igin "Dog- bone" Fig. 17. Summary of possible carbon fiber morphologies. [...]... nature of the surface is shown in Fig 21 Fig 19 Pitch-based carbon fiber fracture surface: enlarged view Fig 20 Pitch-based carbon fiber fracture surface: view of large crystallites FRACTURE OF CARBON FIBERS 173 Fig 21 High-resolution scanning electron micrograph of pitch-based carbon fiber surface VAPOR-GROWN CARBON FIBERS Pure carbon fibers may be grown by a catalytic process from carbon-containing... to composite mechanical properties J Mare,: Sci., 28: 569 -61 0 Edie, D.D and Dunham, M.G (1989) Melt spinning pitch-based carbon fibers Carbon, 27: 64 7 -65 5 Endo, M (1988) Grow carbon fibers in the vapor phase Chemtech, 18: 568 -5 76 Fain, C.C., Edie, D.D., DuBose, W.A and Schikner, R.C (1988) Microstructure formation during the extrusion of pitch fibers Carbon '88, The Institute of Physics, Newcastle... leading to crystallite and ultimately fiber fracture in (c) The PAN-based fiber fracture surface shown in Fig 12 gives evidence of the tremendous amount of new surface which is created, a measure of the high strength of the fiber A similar mechanism is believed to be responsible for failure of mesophase pitch-based carbon fiber However, the highly turbostratic nature of the fiber structure will inhibit crack... Fig 18 Pitch-based carbon fiber fracture surface 172 J.G Lavin An SEM image of the fracture surface of a pitch-based carbon fiber is shown in Fig 18 It will be noted that there are many zig-zag features, which allow the fiber to sustain a 40% reduction in surface area during heat treatment without introducing damaging hoop stresses The large, flat crystals which make up the fiber are evident in Figs... large flat planes which are present in the Fig 19 fracture surface, also causing generation of large amounts of new surface Compressive Failure Extensive compressive failure studies have been conducted on both individual fibers and composites Arguably, the individual fiber studies are not meaningful, since carbon 3 3 FRACTURE OF CARBON FIBERS 177 fibers are seldom subjected to large compressive forces.. .FRACTURE OF CARBON FIBERS 171 described by Fain et al (1988) and Robinson and Edie (19 96) However, the stiffness aspects of modified fiber cross-section could be adversely affected while thermal and adhesion responses may be improved Packing densities of individual fibers in fiber assemblages may also be changed Processing continuity and... (19 76) Filamentous growth of carbon through benzene decomposition J Crystal Growth, 32: 335-349 FRACTURE 0F CARBON FIBERS 179 Peebles L.H (1994) Carbon Fibers: Formution, Structure and Properties CRC Press, Boca Raton FL Pennock, G.M., Taylor, G.H and FitzGerald, J.D (1993) Microstructure in a series of mesophase pitchbased carbon fibers from DuPont: zones, folds and disclinations Curbon, 3 I: 591 -61 0... Sharp, J.V (1974) Crystal shear limit to carbon fiber strength Carbon, 12: 103-1 I O Robinson, K.E and Edie, D.D (19 96) Microstructure and texture of pitch-based ribbon fibers for thermal management Curbon, 34: 13- 36 Rodriguez, N.M.(1993) A review of catalytically grown carbon fibers J Mater: Res., 8: 3233-3250 Ross R.A and Jennings, U.D (1YY3) Pitch carbon fiber spinning process US Patent Office, Pat... Atlas o Fiber Fracfure und Damage to Textiles, 2nd ed., p 65 , f J.W.S Hearle, B Lomas and W.D Cooke (Eds.) Woodhead Publishing, Cambridge Brooks, J.D and Taylor, G.H (1 965 ) The formation of graphitizing carbons from the liquid phase Carbon, 3: 185-193 Dobb, M.G., Johnson, D.J and Park, C.R (1990) Compressional behavior of carbon fibers J Mare,: Sci., 25: 829-834 Drzal, M.T and Madhukar, M (1993) Fiber- matrix... capabilities of the fiber are restricted FAILURE MECHANISMS Tensile Failure The most revealing experiments were conducted by Bennett et al (1983), who fractured PAN-based carbon fibers in glycol, a medium which absorbed the explosive energy generated at fiber failure This allowed meaningful examination of the broken ends by SEM and TEM They observed large misoriented crystals in the internal flaws 175 FRACTURE . FRACTURE OF CARBON FIBERS 161 8 7 6 m5 a c3 5 04 2 5 c 3 2 1 0 Isotropic Pitch ia. 200 400 60 0 800 1000 Modulus, GPa Fig. 4. Strength and stiffness of carbon fibers 7. Electrical resistivity of carbon fibers. From Lavin (2001b). FRACTURE OF CARBON FIBERS 163 PAN-BASED CARBON FIBERS Polyacrylonitrile (PAN) fibers are made by a variety of methods H H H H H 00 -60 0 C Dehydrogenation 60 0-1 300 C Denitrogenation + Stretch Fig. 9. PAN-based carbon fiber chemistry: carbonization. FRACTURE OF CARBON FIBERS 165 fibre axis Fig.