Individual Filaments 109 crystalline structure and the 3-D nature of the structure.After graphitization,the fibers are surface treated,a sizing or finish is applied, and the fibers are then wound for shipment. 2.2.1.2.Pitch Precursor [1] Pitch is the lowest grade obtained from the distillation of petroleum products.It is also the least expensive as compared to other grades.At the beginning of the fiber-making process,pitch must be converted into a suitable fiber.In the conversion process,pitch goes through a mesophase,called liquid crystal phase,in which the polymer chains are somewhat oriented even though the material is liquid.This orientation is responsible for the ease of consolidation of the pitch-based product into a carbon/graphite form.Subsequently the process follows a similar se- quence as the PAN precursor,as shown in Figure 3.8. 2.2.2.Surface Treatment of Carbon Fibers [1] Surface treatment of the carbon fibers improves the bonding between the fibers and matrix and thus improves the interlaminar shear strength (ILSS).Surface treatment can be divided into two types:oxidative and non-oxidative. PAN PROCESS STRETCH LLALIA PAN THERMOSET PITCH PROCESS CARBONIZE GRAPHITIZE am PETROLEUM MELT THERMOSET CARBONIZE GRAPHITIZE PITCH SPIN SPOOL EPOXY SIZING SURFACE TREATMENT FIGURE 3.8 Process of making carbon fibers(Reproduced from Carbon/Graphite Fi- bers by Russell J.Diefendorf in Engineered Materials Handbook,Vol.1,Composites, 1987,with permission from ASM International)
crystalline structure and the 3-D nature of the structure. After graphitization, the fibers are surface treated, a sizing or finish is applied, and the fibers are then wound for shipment. 2.2.1.2. Pitch Precursor [1] Pitch is the lowest grade obtained from the distillation of petroleum products. It is also the least expensive as compared to other grades. At the beginning of the fiber-making process, pitch must be converted into a suitable fiber. In the conversion process, pitch goes through a mesophase, called liquid crystal phase, in which the polymer chains are somewhat oriented even though the material is liquid. This orientation is responsible for the ease of consolidation of the pitch-based product into a carbon/graphite form. Subsequently the process follows a similar sequence as the PAN precursor, as shown in Figure 3.8. 2.2.2. Surface Treatment of Carbon Fibers [1] Surface treatment of the carbon fibers improves the bonding between the fibers and matrix and thus improves the interlaminar shear strength (ILSS). Surface treatment can be divided into two types: oxidative and non-oxidative. Individual Filaments 109 FIGURE 3.8 Process of making carbon fibers (Reproduced from Carbon/Graphite Fibers by Russell J. Diefendorf in Engineered Materials Handbook, Vol. 1, Composites, 1987, with permission from ASM International)
110 REINFORCEMENTS-FIBERS Liquid phase oxidative treatment can be done by simply drawing the fibers through a bath of some convenient oxidative agent(such as nitric acid,potassium permanganate,or sodium hypochlorite),or drawing the fiber through an acidic or alkaline electrolytic bath.The electrolytic bath is preferred since the process can be done continuously.The net result of this method is to clean the carbon fiber surface and then to attach chemi- cal groups,such as hydroxides,which can bond with the matrix or a sur- face coating (sizing).Excessive oxidation,however,can result in a decrease in fiber tensile strength,presumably due to pitting of the fiber surface. Non-oxidative treatments are of three types:whiskerization,pyrolytic surface coatings,and polymer grafting.In whiskerization,single crystals of ceramic materials(SiC,TiO2,or SiN)are grown on the surface of the fibers.These whiskers are perpendicular to the fiber axis.The resulting improvement in interlaminar shear strength (ILSS)is excellent but the process is very expensive and is not commercially available at present. Pyrolytic coating consists of a vapor phase deposition of pyrolytic car- bon on the fiber surface.This method shows good increase in ILSS but is expensive and unavailable commercially.The attachment of a polymer graft involves the preliminary attachment of some group that can be used as an anchor for the polymer.These reactive groups are generally at- tached by one of the oxidative methods,so the polymer graft method is really a subgroup of the oxidative method in which a polymer is intro- duced to act as a bridge between the fiber and the size of matrix. The effectiveness of the fiber is often measured by the wettability of the fiber.The extent of wetting of the fiber is directly proportional to the number of chemical groups attached,and therefore the effectiveness of the surface treatment.This wettability is most often measured by looking at the contact angle of the fiber/water interface. 2.2.3.Surface Coatings(Sizing)of Carbon Fibers In some cases organic coatings(sizings)are also added to the fibers to further improve the fiber/matrix bonding and to protect the fibers from damage during subsequent processing.These sizings are supplied to both treated and untreated fibers by passing the fibers through a heated bath of the sizing agent.The most common sizing is an epoxy,although polyvinyl alcohol and polyimides have also been used. The value of the sizing or finish on carbon fibers is not as dramatic as it is in glass fibers in improving the mechanical properties in adverse envi- ronmental conditions.The reason for this is the inherently better mois- ture resistance of the carbon fibers compared to glass fibers
Liquid phase oxidative treatment can be done by simply drawing the fibers through a bath of some convenient oxidative agent (such as nitric acid, potassium permanganate, or sodium hypochlorite), or drawing the fiber through an acidic or alkaline electrolytic bath. The electrolytic bath is preferred since the process can be done continuously. The net result of this method is to clean the carbon fiber surface and then to attach chemical groups, such as hydroxides, which can bond with the matrix or a surface coating (sizing). Excessive oxidation, however, can result in a decrease in fiber tensile strength, presumably due to pitting of the fiber surface. Non-oxidative treatments are of three types: whiskerization, pyrolytic surface coatings, and polymer grafting.In whiskerization, single crystals of ceramic materials (SiC, TiO2, or Si3N4) are grown on the surface of the fibers. These whiskers are perpendicular to the fiber axis. The resulting improvement in interlaminar shear strength (ILSS) is excellent but the process is very expensive and is not commercially available at present. Pyrolytic coating consists of a vapor phase deposition of pyrolytic carbon on the fiber surface. This method shows good increase in ILSS but is expensive and unavailable commercially. The attachment of a polymer graft involves the preliminary attachment of some group that can be used as an anchor for the polymer. These reactive groups are generally attached by one of the oxidative methods, so the polymer graft method is really a subgroup of the oxidative method in which a polymer is introduced to act as a bridge between the fiber and the size of matrix. The effectiveness of the fiber is often measured by the wettability of the fiber. The extent of wetting of the fiber is directly proportional to the number of chemical groups attached, and therefore the effectiveness of the surface treatment. This wettability is most often measured by looking at the contact angle of the fiber/water interface. 2.2.3. Surface Coatings (Sizing) of Carbon Fibers In some cases organic coatings (sizings) are also added to the fibers to further improve the fiber/matrix bonding and to protect the fibers from damage during subsequent processing. These sizings are supplied to both treated and untreated fibers by passing the fibers through a heated bath of the sizing agent. The most common sizing is an epoxy, although polyvinyl alcohol and polyimides have also been used. The value of the sizing or finish on carbon fibers is not as dramatic as it is in glass fibers in improving the mechanical properties in adverse environmental conditions. The reason for this is the inherently better moisture resistance of the carbon fibers compared to glass fibers. 110 REINFORCEMENTS—FIBERS
Individual Filaments 111 2.2.4.Properties of Carbon Fibers [1] Table 3.3 shows the properties of different types of fibers.The elastic modulus of carbon fibers can be in two categories:low modulus and high modulus.The modulus can be as low as 157 GPa for pitch based carbon fibers to as high as 383 GPa for PAN-based fibers.Tensile strength also varies from 1367 MPa for pitch based fibers to as high as 3280 MPa for PAN-based fibers.The elongation of carbon fibers is low (about 1%). The higher modulus fibers have lower elongation than the lower modulus fibers.Carbon fiber composites are therefore brittle due to the limited elongation of the fibers. Carbon fibers are moderately good conductors of electricity.The con- ductivity of carbon fiber is in the order of 104 S/cm.This conductivity is assumed to arise from the natural conductivity of graphite and the orien- tation of the graphene ring,which is parallel to the axis of the fiber.Pitch fibers have better electrical conductivity than PAN-based fibers.The co- efficient or thermal expansion (longitudinal)of carbon fibers is slightly negative and becomes more negative with increasing modulus.This con- traction can be combined with the positive coefficient of thermal expan- sion of the matrix to yield a near zero coefficient of thermal expansion. 2.3.Organic Fibers The most common organic fibers used for reinforcements are the aramids,with Kevlar,a Dupont fiber,currently being the major brand.A TABLE 3.3 Different Types of Carbon Fibers and Their Properties [1]. PAN-Based Fibers Low Modulus High Modulus Tensile modulus(GPa) 226 383 Tensile strength(MPa) 3280 2392 Elongation(%) 1.4 0.6 Density(g/cc) 1.8 1.9 Carbon assay (% 92-97 100 Pitch-Based Fibers Tensile modulus(GPa) 157 376 Tensile strength(MPa) 1367 1708 Elongation (% 0.9 0.4 Density (g/cc) 1.9 2.0 Carbon assay (% 97 97
2.2.4. Properties of Carbon Fibers [1] Table 3.3 shows the properties of different types of fibers. The elastic modulus of carbon fibers can be in two categories: low modulus and high modulus. The modulus can be as low as 157 GPa for pitch based carbon fibers to as high as 383 GPa for PAN-based fibers. Tensile strength also varies from 1367 MPa for pitch based fibers to as high as 3280 MPa for PAN-based fibers. The elongation of carbon fibers is low (about 1%). The higher modulus fibers have lower elongation than the lower modulus fibers. Carbon fiber composites are therefore brittle due to the limited elongation of the fibers. Carbon fibers are moderately good conductors of electricity. The conductivity of carbon fiber is in the order of 104 S/cm. This conductivity is assumed to arise from the natural conductivity of graphite and the orientation of the graphene ring, which is parallel to the axis of the fiber. Pitch fibers have better electrical conductivity than PAN-based fibers. The coefficient or thermal expansion (longitudinal) of carbon fibers is slightly negative and becomes more negative with increasing modulus. This contraction can be combined with the positive coefficient of thermal expansion of the matrix to yield a near zero coefficient of thermal expansion. 2.3. Organic Fibers The most common organic fibers used for reinforcements are the aramids, with Kevlar, a Dupont fiber, currently being the major brand. A Individual Filaments 111 TABLE 3.3 Different Types of Carbon Fibers and Their Properties [1]. PAN-Based Fibers Low Modulus High Modulus Tensile modulus (GPa) 226 383 Tensile strength (MPa) 3280 2392 Elongation (%) 1.4 0.6 Density (g/cc) 1.8 1.9 Carbon assay (%) 92–97 100 Pitch-Based Fibers Tensile modulus (GPa) 157 376 Tensile strength (MPa) 1367 1708 Elongation (%) 0.9 0.4 Density (g/cc) 1.9 2.0 Carbon assay (%) 97 97