Carbon vol.36,No.4,pp.345-362,1998 Printed in great britain. al 0008-6223/98519.00+0.00 P:006901 THE EFFECT OF PROCESSING ON THE STRUCTURE AND PROPERTIES OF CARBON FIBERSK D. D. EDiE Center for Advanced Engineering Fibers and Films and Department of Chemical Engineering. Clemson University, Clemson, SC 29634-0909, USA (Received 25 August 1997) Abstract Even though the same three process steps (hiber formation, stabilization, and carb are used t e both polyacrylonitrile-based (PAN-based )and pitch-based carbon fibers, properties difFer significantly. This is a direct result of the precursors used to produce thes Formation, creating fibers with a high degree of molecular orientation, whereas polymers ith less ordered, fibrillar structures. Carbon fibers with high degrees of molecula light moduli and thermal conductivities. By contrast, cai bon fibers with iture ordered, fibrillar structures tend to develop higher tensile strengths. Thus it is not that PAN based carbon fibers have become the preferred reinforcement for high-strength of pitch-based carbon fibers can be improved significantly. Alternatively, linearizing the molecular Researchers now realize that understanding and controlling structure during the fiber formation step critical if the properties of carbon fibers are to be optimized. Controlling the structure during fiber ormation can also permit milder conditions to be used during subsequent process steps. As a result. recursor fiber formation olTers the best opportunity for improving properties and reducing production costs for both PAN-based and pitch-based carbon fibers, o 1998 Elsevier Science Ltd. Al Key words-A, Carbon fibers, fiber formation, structure/properties, PAN, A. mesophase. 1 INTRODUCTION the conditions used to form the precursor fiber, Post Carbon fibers are, perhaps, the most successful new treatment steps(in this case crosslinking and carbon carbon product to be commercialized in the past ization) nerely refine and perfect the as-spun struc 35 years. Their high strength and stifness, combined ture. This is not to say that fiber properties cannot with their light weight, make these fibers attractive be dramatically altered during post-treatment However, the fundamental fiber structure needed to for high-volume applications ranging from sporting develop high strength or high thermal conductivity oods to aircraft structures. Today, carbon fibers are also being developed for a new class of applications. must be created during the initial fiber formation step. This paper details the processes used to form ial carbon fibers must exhibit consistent mechanical the two dominant classes of carbon fibers, polyacry lonitrile-based(PAN-based and mesophase pitch and transport properties, and the optimum properties based(pitch-based ) Then, the relationship between may differ for each application. Because of this, the control of structure and the interaction between the structure and propertics for both classes of carbon fibers is discussed. Finally, recent breakthroughs in structure and properties has been extensively studied the control of structure for pitch-based fibers will be since high performance fibers were first commercial- ized by Union Carbide in the 1960s reviewed and the possible implications for PAN Nearly all commercial carbon fibers are produced based fiber producers discussed by first converting a carbonaceous precursor into derable progress has been made fiber form. The precursor fiber then is crosslinked in over the past 35 years. the origin and development order to render it infusible. Finally. the crosslinked f structure as well as the relationship between precursor fiber is heated at temperatures from 1200 structure and properties are still not completely o ca 3000C in an inert atmosphere to drive off understood for carbon fibers. Therefore. this review nearly all of the noncarbon elements, converting the article indicates areas where researchers are in general greement and points out phenomena that are still not created caracteristics of Based Carbon o7 at The PAn precursor fibers differ from those of mesophase Pennsylvania State Univ 3-18 July 1997. pitch precursor fibers. Because of this
Pergamon PII: MOOS-6223(97)00185-l Carbon Vol. 36, No. 4, pp. 3455362, 1998 0 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.6223/98 $19.00 + 0.00 THE EFFECT OF PROCESSING ON THE STRUCTURE AND PROPERTIES OF CARBON FIBERS* D. D. EDIE Center for Advanced Engineering Fibers and Films and Department of Chemical Engineering, Clemson University, Clemson, SC 29634-0909, USA (Received 25 August 1997) Abstract Even though the same three process steps (fiber formation, stabilization, and carbonization) are used to produce both polyacrylonitrile-based (PAN-based) and pitch-based carbon fibers, their final properties differ significantly. This is a direct result of the precursors used to produce these two types of carbon fibers (polymeric versus liquid-crystalline). Liquid-crystalline materials readily orient during fiber formation, creating fibers with a high degree of molecular orientation, whereas polymers form fibers with less ordered, fibrillar structures. Carbon fibers with high degrees of molecular orientation exhibit high moduli and thermal conductivities. By contrast, carbon fibers with more discontinuous and less ordered, fibrillar structures tend to develop higher tensile strengths. Thus, it is not surprising that PANbased carbon fibers have become the preferred reinforcement for high-strength composites. However, recent studies have proven that, by disrupting molecular orientation during fiber formation, the strengths of pitch-based carbon fibers can be improved significantly. Alternatively, linearizing the molecular orientation during fiber formation can yield pitch-based fibers with enhanced thermal conductivities. Researchers now realize that understanding and controlling structure during the fiber formation step is critical if the properties of carbon fibers are to be optimized. Controlling the structure during fiber formation can also permit milder conditions to be used during subsequent process steps, As a result, research into precursor fiber formation offers the best opportunity for improving properties and reducing production costs for both PAN-based and pitch-based carbon fibers. Q 1998 Elsevier Science Ltd. All rights rcscrved. Key Words A. Carbon fibers, fiber formation, structure/properties, PAN, A. mesophase. 1. INTRODUCTION Carbon fibers are, perhaps, the most successful new carbon product to be commercialized in the past 3.5 years. Their high strength and stiffness, combined with their light weight, make these fibers attractive for high-volume applications ranging from sporting goods to aircraft structures. Today, carbon fibers are also being developed for a new class of applications, thermal management. Like other products, commercial carbon fibers must exhibit consistent mechanical and transport properties, and the optimum properties may differ for each application. Because of this, the control of structure and the interaction between the structure and properties has been extensively studied since high performance fibers were first commercialized by Union Carbide in the 1960s. Nearly all commercial carbon fibers are produced by first converting a carbonaceous precursor into fiber form. The precursor fiber then is crosslinked in order to render it infusible. Finally, the crosslinked precursor fiber is heated at temperatures from 1200 to cu 3000°C in an inert atmosphere to drive off nearly all of the noncarbon elements, converting the precursor to a carbon fiber. Like other commercial fiber processes, the final properties are, to a great extent, determined by the material, the process, and *Based on Plenary lecture given at Curbon ‘97 at The Pennsylvania State University, 13-~18 July 1997. the conditions used to form the precursor fiber. Posttreatment steps (in this case crosslinking and carbonization) merely refine and perfect the as-spun structure. This is not to say that fiber properties cannot be dramatically altered during post-treatment. However, the fundamental fiber structure needed to develop high strength or high thermal conductivity must be created during the initial fiber formation step. This paper details the processes used to form the two dominant classes of carbon fibers, polyacrylonitrile-based (PAN-based) and mesophase pitchbased (pitch-based). Then, the relationship between structure and properties for both classes of carbon fibers is discussed. Finally, recent breakthroughs in the control of structure for pitch-based fibers will be reviewed and the possible implications for PANbased fiber producers discussed. Although considerable progress has been made over the past 35 years, the origin and development of structure as well as the relationship between structure and properties are still not completely understood for carbon fibers. Therefore, this review article indicates areas where researchers are in general agreement and points out phenomena that are still subject to debate. Unlike people, all carbon fibers are not created equal. The fundamental structural characteristics of PAN precursor fibers differ from those of mesophase pitch precursor fibers. Because of this, certain proper- 345
D. D. EDiE ties are easier to develop in PAN-based carbon fibers, 2.2 Production of PAN precursor fibers while other properties are easier to develop in pitch Although PAN fibers can be produced by either based carbon fibers. Therefore. to understand struc- wet or dry spinning processes, wet spinning is used ture and properties of these two classes of carbon to produce nearly all precursor fibers used in commer fibers, one must begin by detailing the materials and cial PAN-based carbon fiber processes. The solution processes used to form the precursor fibers from 10 to 30% by weight of PAN or(PAN copoly mer)dissolved in a polar solvent, such as sodium PAN-BASED CARBON FIBERS isocyanate, nitric acid or dimethylacetamide. Th Nearly all commercial fibers are produced using solution is first filtered and then extruded through a one of three techniques; melt spinning: wet spinning spinnerette into a coagulation bath [3, 4](scc Fig. 1) or dry spinning. In melt spinning the precursor is The coagulation bath can contain various solutions, merely melted and extruded through a spinneret ranging from water and sodium thiocyanate or ontaining numerous small capillaries. As the precur dimethylacetamide to ethylene glycol and dimethyl sor emerges from these capillaries, it cools and solidi cetamide or dimethylformamide. The rate of fiber fics into fibcr form. In wct spinning a concentrated formation is controlled by adjusting parameters such solution of the precursor is extruded through a as the solution concentration, the concentration of spinneret into a coagulation bath. The solvent is the coagulation bath, the bath temperature, the dray more soluble in the coagulation fluid than it is in the down rate, and the rate of extrusion [5] precursor. Therefore, as the solution emerges from Mass transfer at the fiber/liquid interface is rela the spinneret capillaries, the precursor precipitates tively slow in the wet-spinning process. The reason into fiber form. Dry spinning also involves spinning is that the solvent concentration of the coagulation concentrated solution through a spinneret bath is relatively high. Because of this the solvent However, in dry spinning the solution is extruded can diff use radially through the solidifying fiber faster nto a drying chamber. Here, the solvent evaporates than it can diffuse from the fiber surface. As and the precursor precipitates into fiber form. result, the solvent concentration is relatively uniforn Because melt spinning converts a pure recurse across the fiber's cross-section during solidification directly into fiber form and does not involve the Therefore, the fiber shrinks uniformly in the radial added expense of solvent recycling and recovery, direction, giving the circular cross-section that is is the preferred fiber formation process. However. characteristic of wetspun PAN. H if the pol either wet or dry spinning must he employed if the precursor degrades at or near its melting temperature. relatively rigid fiber skin can also form in this process before the center of the fiber has solidified, yielding 2.1 Production of PAN precursor PAN is an atactic, linear polymer containing highly As the PAN solution is forced through the spin polar nitrile pendant groups. Because of its highly neret capillaries. the shear field tends to orient the olar nature, pure PAN has a glass transition temper- solidifying polymeric structure parallel to the direc- ature of ca 120 C and tends to decompose before it tion of flow. In fact, various studies have found that melts. Therefore, PAN precursor fibers must be pro- a solvent call decrease the entanglement of polymers duced by either wet- or dry-spinning processes using during extrusion and enhance orientation. Like many a highly polar solvents. Actually, PAN homopolymer other polymers, PAN tends to precipitate into fibril rarely, if ever, used as a carbon fiber precursor. form. Various processing parameters, such as coag Commercial PAn precursor fibers normally contain lation bath temperature, solvent concentration and from G to 9% of other monomers, such as itaconic stretch. can influcncc thc fibrillar structure and its cid, acrylic acid, methacrylic acid, methyl acrylate, orientation within the as-spun PAN fiber(see Fig. 2) vinyl bromide, etc. [1, 2]. These additions lower the [5]. In other words, wet-spinning yields a precursor glass transition temperature and affect the reactivity fiber in which the PAn molecules are organized into of the polymer structure. Both of these changes can fibrils which, in turn, are generally oriented parallel dramatically influcncc subscquent process steps to the fiber axis. Electron micrographs of as-spun Storage tank Dry and heated-draw Spinnerette Coagulation bath Wash bath Wash bath Nind-up Fig. I. Schematic of wet-spinning process used to produce PAN precursor fibers [41
346 D. D. EDK ties are easier to develop in PAN-based carbon fibers, while other properties are easier to develop in pitchbased carbon fibers. Therefore, to understand structure and properties of these two classes of carbon fibers, one must begin by detailing the materials and processes used to form the precursor fibers. 2. PAN-BASED CARBON FIBERS Nearly all commercial fibers are produced using one of three techniques: melt spinning; wet spinning; or dry spinning. In melt spinning the precursor is merely melted and extruded through a spinneret containing numerous small capillaries. As the precursor emerges from these capillaries, it cools and solidifies into fiber form. In wet spinning a concentrated solution of the precursor is extruded through a spinneret into a coagulation bath. The solvent is more soluble in the coagulation fluid than it is in the precursor. Therefore, as the solution emerges from the spinneret capillaries, the precursor precipitates into fiber form. Dry spinning also involves spinning a concentrated solution through a spinneret. However, in dry spinning the solution is extruded into a drying chamber. Here, the solvent evaporates and the precursor precipitates into fiber form. Because melt spinning converts a pure precursor directly into fiber form and does not involve the added expense of solvent recycling and recovery, it is the preferred fiber formation process. However, either wet or dry spinning must be employed if the precursor degrades at or near its melting temperature. 2.1 Production of PANprecursor PAN is an atactic, linear polymer containing highly polar nitrile pendant groups. Because of its highly polar nature, pure PAN has a glass transition temperature of ra 120°C and tends to decompose before it melts. Therefore, PAN precursor fibers must be produced by either wet- or dry-spinning processes using a highly polar solvents. Actually, PAN homopolymer is rarely, if ever, used as a carbon fiber precursor. Commercial PAN precursor fibers normally contain from 6 to 9% of other monomers, such as itaconic acid, acrylic acid, methacrylic acid, methyl acrylate, vinyl bromide, etc. [1,2]. These additions lower the glass transition temperature and affect the reactivity of the polymer structure. Both of these changes can dramatically influence subsequent process steps. Storage tank A 2.2 Production of PANprecursorJibers Although PAN fibers can be produced by either wet or dry spinning processes, wet spinning is used to produce nearly all precursor fibers used in commercial PAN-based carbon fiber processes. The solution used in a wet spinning process normally consists of from 10 to 30% by weight of PAN or (PAN copolymer) dissolved in a polar solvent, such as sodium thiocyanate, nitric acid or dimethylacetamide. This solution is first filtered and then extruded through a spinnerette into a coagulation bath [3,4] (see Fig. 1). The coagulation bath can contain various solutions, ranging from water and sodium thiocyanate or dimethylacetamide to ethylene glycol and dimethylacetamide or dimethylformamide. The rate of fiber formation is controlled by adjusting parameters such as the solution concentration, the concentration of the coagulation bath, the bath temperature, the drawdown rate, and the rate of extrusion [ 51. Mass transfer at the fiber/liquid interface is relatively slow in the wet-spinning process. The reason is that the solvent concentration of the coagulation bath is relatively high. Because of this the solvent can diffuse radially through the solidifying fiber faster than it can diffuse away from the fiber surface. As a result, the solvent concentration is relatively uniform across the fiber’s cross-section during solidification. Therefore, the fiber shrinks uniformly in the radial direction, giving the circular cross-section that is characteristic of wetspun PAN. However, if the polymer concentration in the spinning solution is low, a relatively rigid fiber skin can also form in this process before the center of the fiber has solidified, yielding a dogbone-shape fiber. As the PAN solution is forced through the spinneret capillaries, the shear field tends to orient the solidifying polymeric structure parallel to the direction of flow. In fact, various studies have found that a solvent can decrease the entanglement of polymers during extrusion and enhance orientation. Like many other polymers, PAN tends to precipitate into fibril form. Various processing parameters, such as coagulation bath temperature, solvent concentration and stretch, can influence the fibrillar structure and its orientation within the as-spun PAN fiber (see Fig. 2) [5]. In other words, wet-spinning yields a precursor fiber in which the PAN molecules are organized into fibrils which, in turn, are generally oriented parallel to the fiber axis. Electron micrographs of as-spun Dry and heated-draw Coagulation bath Wash bath P Fig. 1. Schematic of wet-spinning process used to produce PAN precursor fibers [4]
Efect of processing on carbon fibers (a)0°C (b)30°C (c)50C ig. 2. Effect of coagulation bath temperature on the fibril and pore structure of wet-spun PAN fiber after 6 x stretch [5] Tonsion Control PAN Supply Creel Oxidized Fiber Wind-uip Fig. 3. Schematic of commercial PAN stabilization oven [2] fibers show that these fibrils are joined together in a relaxation and chain scission during the final carbon three-dimensional network [6] ization step [7, 8]. In most commercial processes, the After being spun into fibers, the orientation within PAn precursor fiber is stabilized by ng it to the PAn is enhanced by stretching. Although the ir at temperatures ranging from 230 to 280C, maximum degree of crystallinity within the PAN fiber Tension must be applied during this step to limit is only 50%6, this step is cssential for producing a relaxation uf the polymer structure(see Fig 3).Most final carbon fiber with adequate strength and modu- would agree that both cyclization and dehydrogena lus. Like most polymeric fiber processes, stretching tion can occur during the stabilization step does not greatly increase the crystallinity or the Cyclization, in particular, is highly exothermic, but molecular order within the PAN; rather, it enhances the exotherm is reduced when PAN copolymer pre- the axial oricntation of the pan fibrils. This fibrillar ursors are employcd. Evidently, the e network appears to be the precursor of the graphene as an initiator for the stabilization reaction [91 network that develops during final heat treatment Numerous studies [10, 11] have shown that the rate of oxidative stabilization is affected by the copolyme 2.3 Stabilization of PAN precursor fibers The primary function of the stabilization step is to to composition of the Pan precursor fiber, the temper- ature and even the applied tension. Although most slink this as-spun structure, insuring that both researches agree that a ladder polymer forms during he molecular and the fibrillar orientation will not be this process, its exact structure lost during final heat treatment. To accomplish this, recent review article, Bashir [12 It that the either the inherent stiffness of the pan molecul cyclization reaction may be ste cific. In fact. a lust be increased or the molecules must be "tied study by Colman et al. [13] suggests that cyclization together in order to eliminate, or at least limit would occur preferentially in isotactic sequences
Effect of processing on carbon fibers 341 (a) 0°C (b) 30°C (c) 50°C Fig. 2. Effect of coagulation bath temperature on the fibril and pore structure of wet-spun PAN fiber after 6 x stretch [5]. Tension Coti Exhaust Gases Oxiied Fiber Wind-up Tension Contml Fig. 3. Schematic of commercial PAN stabilization oven [2]. fibers show that these fibrils are joined together in a three-dimensional network [6]. After being spun into fibers, the orientation within the PAN is enhanced by stretching. Although the maximum degree of crystallinity within the PAN fiber is only 50%, this step is essential for producing a final carbon fiber with adequate strength and modulus. Like most polymeric fiber processes, stretching does not greatly increase the crystallinity or the molecular order within the PAN; rather, it enhances the axial orientation of the PAN fibrils. This fibrillar network appears to be the precursor of the graphene network that develops during final heat treatment. 2.3 Stabilization of PANprecursorfibers The primary function of the stabilization step is to crosslink this as-spun structure, insuring that both the molecular and the fibrillar orientation will not be lost during final heat treatment. To accomplish this, either the inherent stiffness of the PAN molecules must be increased or the molecules must be “tied” together in order to eliminate, or at least limit relaxation and chain scission during the final carbonization step [7,8]. In most commercial processes, the PAN precursor fiber is stabilized by exposing it to air at temperatures ranging from 230 to 280°C. Tension must be applied during this step to limit relaxation of the polymer structure (see Fig. 3). Most would agree that both cyclization and dehydrogenation can occur during the stabilization step. Cyclization, in particular, is highly exothermic, but the exotherm is reduced when PAN copolymer precursors are employed. Evidently, the comonomer acts as an initiator for the stabilization reaction [9]. Numerous studies [lO,ll] have shown that the rate of oxidative stabilization is affected by the copolymer composition of the PAN precursor fiber, the temperature and even the applied tension. Although most researches agree that a ladder polymer forms during this process, its exact structure is still in doubt. In a recent review article, Bashir [ 121 pointed out that the cyclization reaction may be stereospecific. In fact, a study by Colman et al. [ 131 suggests that cyclization would occur preferentially in isotactic sequences
D. D. EDIE However, Chen et al. [14] found that syndiotactic minimized by applying tension during heat treatment sequences were equally capable of cyclization. The 18]. By contrast, the degree of preferred orientation problem may be that the polymer chains within the within the fiber and thus. the modulus of the pan fibril form an irregular rod-like helix due to the based carbon fibers increase continuously as heat intramolecular repulsion of the nitrile groups. These treatment temperature is increased [17]. Because of fibrils, in turn, contain both crystalline and amo this, the various grades uf PAN-based carbon fiber ta and Harrison [15] found that available from a particular manufacturer are, nor tramolecular reactions within the rod-like heli mally, the result of changes in heat treatment dominate at lower temperatures(below ca 290"C) whereas intermolecular reactions between adjacent helices occur between 300 and 380.C. Perhaps both 2.5 Structure of PAN-based carbon fibers Colman et al. and Chen et al. are correct-intramolec Pioneering studies by Diefendorf and Tokarsky lar reactions are stereospecific, but intermolecular [191, Johnson [20] and others showed that the struc reactions are not ture of PAN-based carbon fiber is fibrillar in nature Thus, one might expect a stereoregular precurso mimicking the fundamental structure of the poly lymer to offer advantages. Howcvcr, to date, no meric precursor fiber. Diefendorf and Tokarsky also research in this area has been reported showed that the amplitude of the undulation in the fibrillar structure was highest in the center and lowest 2.4 Carbonization of stabilized PAN fibers near the surface of the PaN-based carbon fibers Once stabilized. the pan fiber is carbonized at This indicated that the modulus of a PAN-based 1000 1500C in an inert atmosphere [2, 8]. During carbon fiber varies throughout its cross-section. In a this step most of the noncarbon elements within the recent study Huang and Young [21] confirmed this hydrogen cyanide, water, carbon monoxide, carbon fibers using Raman spectroscopy, Both Johnson [201 ioxide, ammonia and various other gases [6.16]. and Endo [22]employed wide-angle X-ray diffraction The evolution of these compounds decreases the mass to show that the layer planes of PAN-based carbon of the fiber by from 55 to 60 wt%. As a result the fibers have no regular three-dimensional order. Also, fiber shrinks in diameter. Therefore, in a typical Pan by subjecting longitudinal and transverse sections to cess, the precursor fiber might begin with an small-angle X-ray difiraction and transmission as-spun diameter of 35 um and then be stretched to electron microscope(TEM) analysis, Johnson [20] a diameter of 10.5 um. Finally, shrinkage during showed that needle-shaped voids exist between crys carbonization yields a carbon fiber with a diameter tallies in the outer skin of the fiber and that in this of 7 um. In other words, small carbon fiber diamete region, the layer planes are essentially parallel to the are a characteristic of the high weight loss of PAl surface. However, in the core region, Johnson found during processing. Even though the diameter of that the layer planes were folded extensively, often PAN-based carbon fibers is smaller than that of most through angles of 180. Based on these results. itch-based carbon fibers, the as-spun diameter of Johnson developed the three-dimensional schematic the precursor fiber is actually significantly greater representation of the microstructure of PAN-based Initially, increasing the final heat treatment ter carbon fiber shown in Fig. 5. Guigon et ul. [23]have ature es tensile strength. However, as Fig 4 proposed that the microtexture of PAN-based carbon shows, the tensile strength suddenly drops when heat fibers is even more complicated-crumpled sheets treatment temperatures exceed 1600C [17]. Fitzer form these fibrils claims that this decrease is associated with the release of niTrogen and tliat the reduction in strength can be · sAF230一270c40min Final Heat Treatment Temperature(C) Fig 4. Influence of final heat treatment temperature on the Fig. 5. Microstructure of PAN-based carbon fiber proposed ensile strength of PAN-based carbon fiber [17
348 D. D. EIIII: However, Chen et al. [14] found that syndiotactic sequences were equally capable of cyclization. The problem may be that the polymer chains within the fibril form an irregular rod-like helix due to the intramolecular repulsion of the nitrile groups. These fibrils, in turn, contain both crystalline and amorphous regions. Gupta and Harrison [ 151 found that intramolecular reactions within the rod-like helix dominate at lower temperatures (below ca 290°C) whereas intermolecular reactions between adjacent helices occur between 300 and 380°C. Perhaps both Colman et al. and Chen et al. are correct - intramolecular reactions are stereospecific, but intermolecular reactions are not. minimized by applying tension during heat treatment [ 181. By contrast, the degree of preferred orientation within the fiber and, thus, the modulus of the PANbased carbon fibers increase continuously as heat treatment temperature is increased [17]. Because of this, the various grades of PAN-based carbon fiber available from a particular manufacturer are, normally, the result of changes in heat treatment temperature. Thus, one might expect a stereoregular precursor polymer to offer advantages. However, to date, no research in this area has been reported. 2.4 Carbonization of stabilized PANJibers Once stabilized, the PAN fiber is carbonized at 1000~1500°C in an inert atmosphere [2,8]. During this step most of the noncarbon elements within the fiber are volatilized in the form of methane, hydrogen, hydrogen cyanide, water, carbon monoxide, carbon dioxide, ammonia and various other gases [6,16]. The evolution of these compounds decreases the mass of the fiber by from 55 to 60 wt%. As a result the fiber shrinks in diameter. Therefore, in a typical PAN process, the precursor fiber might begin with an as-spun diameter of 35 pm and then be stretched to a diameter of 10.5 ,nm. Finally, shrinkage during carbonization yields a carbon fiber with a diameter of 7 pm. In other words, small carbon fiber diameters are a characteristic of the high weight loss of PAN during processing. Even though the diameter of most PAN-based carbon fibers is smaller than that of most pitch-based carbon fibers, the as-spun diameter of the precursor fiber is actually significantly greater. Initially, increasing the final heat treatment temperature increases tensile strength. However, as Fig. 4 shows, the tensile strength suddenly drops when heat treatment temperatures exceed 1600°C [ 171. Fitzer claims that this decrease is associated with the release of nitrogen and that the reduction in strength can be 2.5 Structure of PAN-based carbonJibers Pioneering studies by Diefendorf and Tokarsky [ 191, Johnson [20] and others showed that the structure of PAN-based carbon fiber is fibrillar in nature, mimicking the fundamental structure of the polymeric precursor fiber. Diefendorf and Tokarsky also showed that the amplitude of the undulation in the fibrillar structure was highest in the center and lowest near the surface of the PAN-based carbon fibers. This indicated that the modulus of a PAN-based carbon fiber varies throughout its cross-section. In a recent study Huang and Young [21] confirmed this skin-core structural difference in PAN-based carbon fibers using Raman spectroscopy. Both Johnson [20] and Endo [22] employed wide-angle X-ray diffraction to show that the layer planes of PAN-based carbon fibers have no regular three-dimensional order. Also. by subjecting longitudinal and transverse sections to small-angle X-ray diffraction and transmission electron microscope (TEM) analysis, Johnson [ 201 showed that needle-shaped voids exist between crystallites in the outer skin of the fiber and that. in this region, the layer planes are essentially parallel to the surface. However, in the core region, Johnson found that the layer planes were folded extensively, often through angles of 180”. Based on these results, Johnson developed the three-dimensional schematic representation of the microstructure of PAN-based carbon fiber shown in Fig. 5. Guigon rt al. [23] have proposed that the microtexture of PAN-based carbon fibers is even more complicated~“crumpled sheets” form these fibrils. 40 - I ,.., I I ,, I I I * , I,, , , I,, 500 ,000 1500 2000 2500 Final Heat Treatment Temperature (“C) Fig. 4. Influence of final heat treatment temperature on the tensile strength of PAN-based carbon fiber [ 171. Fig. 5. Microstructure of PAN-based carbon fiber proposed by Johnson [20]
Effect of processing on carbon fibers Misoriented crystallite linking two Tensile stress causes basal Catastrophic failure occurs if crystallites parallel to fiber axis. plane rupture in La direction crystallite size> critical flaw size (b) Fig. 6. Reynolds and Sharp mechanism for tensile failure of carbon fibers [20] Each of these show that the PAN-based failure mode explains the difference between the flaw extensively folded and sensitivity(and, therefore, the tensile strength) of the interlinked turbostratic layers of carbon with nongraphite, fibrillar PAN-based carbon fiber and interlayer spacings considerably larger than those of graphitic, mesophase pitch-based fiber. Recent work raphite. As a result, PAN-based carbon fibers have by Dobb er al. [24] indicates that inter-crystalline a low degree of graphitization. The turbostratic layers and intra-crystalline disorder, most likely caused by within PAN-based carbon fibers appear to follow the the fibrillar structure of the PAN-based carbon fiber. original fibril structure of the PAN precursor fiber. is responsible for the superior compressive strength Although the turbostratic layers within these fibrils of this class of carbon fiber tend to be oriented parallel to the fiber axis, they are Obviously, the fundainental fibrillar structure of not highly aligned. As first proposed by Johnson PAN-based carbon fibers is created during initial [20]. it is this fibrillar structure that makes PAN fiber formation. However, little if any research into based fibers less prone to faw-induced failure. He this fiber formation process has been done since based his argument on the brittle-failure mechanism PAN's adoption as a carbon fiber precursor. Instead, proposed by Reynolds and Sharp. As discussed as the above revicw bove, the crystallites within PAN-based carbon based carbon fiber research has concentrated on fibers are not perfectly aligned, and misoriented abilization and carbonization, by contrast, research itively common (see Fig. 6(a) in pitch-based carbon fibers has concentrated on When a stress is applied parallel to the fiber axis, the perfecting precursor chemistry and the development ystallites align until their movement is restricted by of structure during fiber formation. As will be seen, a disclination in the structure( Fig. 6(b)). If the stress pitch researchers appear to have chosen the more is sufficient, the misoriented crystallite will rupture critical area for control and optimization of fiber and relieve the stress within the fiber(Fig. 6(c)). properties When the size of the ruptured crystallite(perpendicu lar to the fiber axis)is larger than the critical faw size,a catastrophic failure occurs, and the fiber 3. PITCH-BASED CARBON FIBERS breaks. Even if the rupture crystallite is smaller than Its highly condensed aromatic structure the critical flaw size, catastrophic failure can occur if phase pitch (the precursor for pitch-ba he crystallites surrounding the disclination are con- fibers) relatively good thermal stability ous enough to allow a crack to propagate inte this, mesophase pitch precursor fibers are me/t sn,or boring crystallites. According to Johnson, this As previously mentioned, melt spinning is the pre-
Effect of processing on carbon fibers 349 Misoriented crystallite linking two crystallites parallel to fiber axis. (4 ! I Tensile stress causes basal plane rupture in La direction. (b) Catastrophic failure occurs if crystallite size 5 critical flaw size. cc> Fig. 6. Reynolds and Sharp mechanism for tensile failure of carbon fibers [20]. Each of these studies show that the PAN-based carbon fibers contain extensively folded and interlinked turbostratic layers of carbon with interlayer spacings considerably larger than those of graphite. As a result, PAN-based carbon fibers have a low degree of graphitization. The turbostratic layers within PAN-based carbon fibers appear to follow the original fibril structure of the PAN precursor fiber. Although the turbostratic layers within these fibrils tend to be oriented parallel to the fiber axis, they are not highly aligned. As first proposed by Johnson [20], it is this fibrillar structure that makes PANbased fibers less prone to flaw-induced failure. He based his argument on the brittle-failure mechanism proposed by Reynolds and Sharp. As discussed above, the crystallites within PAN-based carbon fibers are not perfectly aligned, and misoriented crystallites are relatively common (see Fig. 6(a)). When a stress is applied parallel to the fiber axis, the crystallites align until their movement is restricted by a disclination in the structure (Fig. 6(b)). If the stress is sufficient, the misoriented crystallite will rupture and relieve the stress within the fiber (Fig. 6(c)). When the size of the ruptured crystallite (perpendicular to the fiber axis) is larger than the critical flaw size, a catastrophic failure occurs, and the fiber breaks. Even if the rupture crystallite is smaller than the critical flaw size, catastrophic failure can occur if the crystallites surrounding the disclination are continuous enough to allow a crack to propagate into neighboring crystallites. According to Johnson, this failure mode explains the difference between the flaw sensitivity (and, therefore, the tensile strength) of the nongraphite, fibrillar PAN-based carbon fiber and graphitic, mesophase pitch-based fiber. Recent work by Dobb ef al. [24] indicates that inter-crystalline and intra-crystalline disorder, most likely caused by the fibrillar structure of the PAN-based carbon fiber, is responsible for the superior compressive strength of this class of carbon fiber. Obviously, the fundamental fibrillar structure of PAN-based carbon fibers is created during initial fiber formation. However, little if any research into this fiber formation process has been done since PAN’s adoption as a carbon fiber precursor. Instead, as the above review indicates, nearly all recent PANbased carbon fiber research has concentrated on stabilization and carbonization. By contrast, research in pitch-based carbon fibers has concentrated on perfecting precursor chemistry and the development of structure during fiber formation. As will be seen, pitch researchers appear to have chosen the more critical area for control and optimization of fiber properties. 3. PITCH-BASED CARBON FIBERS Its highly condensed aromatic structure gives mesophase pitch (the precursor for pitch-based carbon fibers) relatively good thermal stability. Because of this, mesophase pitch precursor fibers are melt spun. As previously mentioned, melt spinning is the pre-
D. D. EDIE Coal-tar-derived mesophase Petroleum-derived mesophase Fig. 7. Typical polynuclear aromatic hydrocarbons in mesophases produced from coal-tar and petroleum [321 ferred fiber formation process less complicated spinning proce as sel avoids Chwastiak [ 32] found that if an inert gas spurge was that this used to agitate the pitch during thermal polymeriza tion, a spinnable 100% mesophase product could be potential low cost of the precursor, would make produced. A single-phase precursor is preferable for pitch-based fibers a low-cost alternative to PAN melt-spinning processes because it avoids the stability based carbon fibers. While this may come to pass problem associated with two-phase extrusion eventually, the economics of pitch fiber processing Nevertheless, based on a recent TEM study by Fitz are not quite this simple. Also, as researchers have Gerald et al. [33 it would appear that some commer discovered, the fundamental structure of pitch-based cial pitch-based fibers are still produced from a mixed carhon fibers is very different from that of PAN mesophase/isotropic pitch precursor based carbon fibers and each structure offers certain Diefendorf and Riggs developed an alternative advantages. Like PAN-based fibers, the structure of technique, solvent extraction that produced a spinn pitch-based fibers is largely developed during fiber feed [34. In their process a portion of highly aro- matic pitch was extracted using a solvent mixture 3.1 Production of mesophase pitch such as benzene and toluene. The extraction step Like PAN-based carbon fibers, the peculiarities of removes the smaller disordering molecules and con- iLch-based fibers are the direct result of the precursor cenTrales the higher molecular weight mat and the process used to convert it to fiber form. In higher molecular fraction can be converted to 100% this case the precursor is mesophase pitch, a liquid mesophase by heating it to between 230 and 400C rystalline material consisting of large polynuclear for only 10 minutes. In either the thermal polymeriza romatic hydrocarbons, The properties of meso- hase. its formation, an the subject of numerous articles[25-28 The first commercial mesophase precursors were produced by Union Carbide using a thermal polyme Pitch ization process. Evidently, the original process pro Nitrogen, zone 1: zone2:zone3Ⅲ可 duced a mixture of isotropic and mesophase pitcl [29, 30]. These early patents claim that small amounts a0onldnooddala f isotropic pitch are needed to reduce the viscosit of the polymerized mesophase and, therefore, the Fllter Extruder spinning temperature. These mixed precursors were prepared by thermally polymerizing a highly aromatic isotropic pitch feed (originating from either petro- leum or coal tar) at temperatures of 400-410'C for as long as 40 hours [29, 30. Coal tar pitch produ Quench Air mesop ith higher aromaticity whereas petroleum pitch yields a mesophase product with a more open structure and a higher content of ( see Fig.7)19.31 by Lewis[30], agitation during heat treatment pre duces a lower molecular weight mesophase and cre- Fig 8 Schematic of melt-spinning prooess used lo produce pitch, making the material easier to spin. Later mesophase pitch precursor fibers [39]
350 D. Eon Coal-tar-derived mesophase Petroleum-derived mesophase Fig. 7. Typical polynuclear aromatic hydrocarbons in mesophases produced from coal-tar and petroleum [32] ferred fiber formation process because it avoids solvent-related issues. Initially, it was felt that this less complicated spinning process, combined with the potential low cost of the precursor, would make pitch-based fibers a low-cost alternative to PANbased carbon fibers. While this may come to pass eventually, the economics of pitch fiber processing are not quite this simple. Also, as researchers have discovered, the fundamental structure of pitch-based carbon fibers is very different from that of PANbased carbon fibers, and each structure offers certain advantages. Like PAN-based fibers, the structure of pitch-based fibers is largely developed during fiber formation. 3.1 Production of’mesophase pitch Like PAN-based carbon fibers, the peculiarities of pitch-based fibers are the direct result of the precursor and the process used to convert it to fiber form. In this case, the precursor is mesophase pitch, a liquid crystalline material consisting of large polynuclear aromatic hydrocarbons. The properties of mesophase, its formation, and mode of growth have been the subject of numerous articles [25-281. The first commercial mesophase precursors were produced by Union Carbide using a thermal polymerization process. Evidently, the original process produced a mixture of isotropic and mesophase pitch [29,30]. These early patents claim that small amounts of isotropic pitch are needed to reduce the viscosity of the polymerized mesophase and, therefore, the spinning temperature. These mixed precursors were prepared by thermally polymerizing a highly aromatic isotropic pitch feed (originating from either petroleum or coal tar) at temperatures of 400-410°C for as long as 40 hours [29,30]. Coal tar pitch produces a mesophase product with higher aromaticity. whereas petroleum pitch yields a mesophase product with a more open structure and a higher content of aliphatic side chains (see Fig. 7) [9.31]. As revealed by Lewis [30], agitation during heat treatment produces a lower molecular weight mesophase and creates an emulsion of the mesophase and isotropic pitch, making the material easier to spin. Later Chwastiak [32] found that if an inert gas spurge was used to agitate the pitch during thermal polymerization, a spinnable 100% mesophase product could be produced. A single-phase precursor is preferable for melt-spinning processes because it avoids the stability problem associated with two-phase extrusion, Nevertheless, based on a recent TEM study by Fitz Gerald et (I/. [33], it would appear that some commercial pitch-based fibers are still produced from a mixed mcsophase/isotropic pitch precursor. Diefendorf and Riggs developed an alternative technique, solvent extraction. that produced a spinnable 100% mesophase precursor from an isotropic feed [34]. In their process a portion of highly aromatic pitch was extracted using a solvent mixture such as benzene and toluene. The extraction step removes the smaller disordering molecules and concentrates the higher molecular weight material. The higher molecular fraction can be converted to 100% mesophase by heating it to between 230 and 400°C for only 10 minutes. In either the thermal polymerizaQuench Air Variable Speed Winder - Fig. 8. Schematic of melt-spinning process used to product mesophase pitch precursor fibers [39]
Effect of processing on carbon fibers 351 Fig. 9. Predicted influence of major process variables during mesophase melt spinning [41 tion or the solvent extraction process, a free radica but uses large amounts of solvents, Supercritical mechanism is believed to be responsible for polymer- extraction and catalytic polymerization produce rela ization of the carbonaceous material. Although tively uniform products with w molecular solvent extraction does reduce the molecular weight weight distributions. However, supercritical extrac distribution somewhat prior to heat treatment, this tion has yet to be proven on a commercial scale, and reaction mechanism still tends to create a product catalysT with a relatively broad molecular weight distribution. to increasing environmental regulations Recently, Thies and coworkers [35] developed a The mesophase products produced by these four variation of this solvent-extraction process that uses processes ditter considerably, but they also exhibit a supercritical fluid instead of a conventional liquid many similarities. For instance, each process yields a solvent. In this process, an aromatic isotropic feed product with a different molecular weight distribution tch is initially dissolved in an aromatic solvent, and a different concentration of aliphatic side chains such as toluene at supercritical conditions, The n the individual mesophase molecules. Consequently resulting homogencous solution is then fractionated their viscous characteristics differ and, as will be nventional manner, using changes in either explained later, their rate of stabilization diffe temperature or pressure to produce pitch fractions well. However all of these mesophase products ce f relatively narrow molecular weight distribution. tain a range of molecular weights, with an average Recent tests have demonstrated that the process can from 800 to 1200. Because of this these mesophases desired molecular weight and softening point(3b a reach a viscosity of 200 Pas-i ur lower well below their degradation temperature. Also, although some- Mochida [37] also has developed a process which what irregular, the individual mesophase molecules oduces a spinnable 100% mesophase precursor with are, in general, disc-like in shape. Recent work by a relatively narrow molecular weight distribution. Korai and Mochida [38] indicates that mesophase This proccss, reccntly commercialized by Mitsubishi molccules can form a substructure and that the Gas Chemical Company, uses a strong Lewis acid chemical nature of the molecule influences the size catalyst(HF-BF3) to catalyze a pure chemical feed, of this substructure methyl-naphthalene, to a While coalesced mesophase can exhibit compli- 100% mesophase product. As Mochida has shown, cated extinction patterns caused by disclinations, no the use of HF BF, greatly reduces the molecular grain boundaries appear to be present [ 39]. In other weight distribution of the mesophase product com- words, one might expect bulk mesophase to behave pared to that produced by thermal polymerization. as an ideal liquid crystalline fuid-a single-domain Each of these processes are being explored, and liquid crystal. As will be seen, this appears to be to varying degrees, used on a commercial scale. While both true and false ach process and its phase product offer certain advantages, they also suffer from some disadvan- 3.2 Production of mesop se pitch precursor tages. Thermal polymerization avoids the use of fibers solvents, but produces a product with a broad molec- As previously mentioned, the mesophase pitches ular weight distribution form carbon fibers soften and How well below phase mixture). Solvent extraction produces a pro- their degradation temperature. Therefore, they can duct with a narrower molecular weight distribution be melt spun into fiber form. The schematic for a
Effect of processing on carbon fibers 351 Quench Cross Air winder Spinning Temperature Velocity Velocity Temperature Fig. 9. Predicted influence of major process variables during mesophase melt spinning [41]. tion or the solvent extraction process, a free radical mechanism is believed to be responsible for polymerization of the carbonaceous material. Although solvent extraction does reduce the molecular weight distribution somewhat prior to heat treatment, this reaction mechanism still tends to create a product with a relatively broad molecular weight distribution. Recently, Thies and coworkers [35] developed a variation of this solvent-extraction process that uses a supercritical fluid instead of a conventional liquid solvent. In this process, an aromatic isotropic feed pitch is initially dissolved in an aromatic solvent, such as toluene, at supercritical conditions. The resulting homogeneous solution is then fractionated in a conventional manner, using changes in either temperature or pressure, to produce pitch fractions of relatively narrow molecular weight distribution. Recent tests have demonstrated that the process can be used to produce 100% mesophase fractions of a desired molecular weight and softening point [ 361. Mochida [37] also has developed a process which produces a spinnable 100% mesophase precursor with a relatively narrow molecular weight distribution. This process, recently commercialized by Mitsubishi Gas Chemical Company, uses a strong Lewis acid catalyst (HF-BF,) to catalyze a pure chemical feed, such as naphthalene or methyl-naphthalene, to a 100% mesophase product. As Mochida has shown, the use of HF-BF, greatly reduces the molecular weight distribution of the mesophase product compared to that produced by thermal polymerization. Each of these processes are being explored, and, to varying degrees, used on a commercial scale. While each process and its mesophase product offer certain advantages, they also suffer from some disadvantages. Thermal polymerization avoids the use of solvents, but produces a product with a broad molecular weight distribution (and perhaps even a twophase mixture). Solvent extraction produces a product with a narrower molecular weight distribution, but uses large amounts of solvents. Supercritical extraction and catalytic polymerization produce relatively uniform products with narrow molecular weight distributions. However, supercritical extraction has yet to be proven on a commercial scale, and processes using HF-BF, catalysis are being subjected to increasing environmental regulations. The mesophase products produced by these four processes differ considerably, but they also exhibit many similarities. For instance, each process yields a product with a different molecular weight distribution and a different concentration of aliphatic side chains on the individual mesophase molecules. Consequently their viscous characteristics differ and, as will be explained later, their rate of stabilization differs as well. However, all of these mesophase products contain a range of molecular weights, with an average from 800 to 1200. Because of this these mesophases reach a viscosity of 200 Pa s-l or lower well below their degradation temperature. Also, although somewhat irregular, the individual mesophase molecules are, in general, disc-like in shape. Recent work by Korai and Mochida [38] indicates that mesophase molecules can form a substructure and that the chemical nature of the molecule influences the size of this substructure. While coalesced mesophase can exhibit complicated extinction patterns caused by disclinations, no grain boundaries appear to be present [ 391. In other words, one might expect bulk mesophase to behave as an ideal liquid crystalline fluid-a single-domain liquid crystal. As will be seen, this appears to be both true and false. 3.2 Production of mesophase pitch precursor jibers As previously mentioned, the mesophase pitches used to form carbon fibers soften and flow well below their degradation temperature. Therefore, they can be melt spun into fiber form. The schematic for a
D. D. EDIE the manifold it enters a metering pump. The purpose of this positive-displacement pump is to minimize any pressure fluctuations created by the rotating extruder screw. The metering pump forces the molten precursor into the spin pack. Normally, the spin pack houses a filter, which is capable of Radia Random small solid particles from the molten precursor. As the precursor exits the pack, it is forced through a plate containing numerous small holes (i.e. the spin- neret). Finally, as the molten precursor exits these ing atmosphere and drawn down by the windup device, forming solid fibers. At first glance. this would appear to be a relatively simple process. In fact, the Flat-layer Radial-folded melt spinning is simple. However, melt-spinning mesophase is far from simple Fig 10. Transverse textures of mesophase-pitch-based By applying heat, mass and momentum balane carbon fibres [56] Edie and Dunham [41] showed that the melt-spinning process is extremely sensitiv typical melt-spinning process that might be used to changes in process conditions. Although oduce mesoph did not account for the liquid crystalline behavior of shown in Fig 8. The precursor (in this case, meso- the mesophase precursor, it nevertheless demon- hase pitch) is loaded into the feed hopper of the strated that, at typical process conditions, the tensile xtruder as solid chips. The extruder's rotating screw stress on mesophase fibers is ca 20% of that required to the melting secti to break fiber (see Fig 9), and extruder where the chips are heated, forming a vis- measurements confirm these predictions In compari cous melt. Then the molten precursor is conveyed son, during melt spinning the tensile stress developed into the pumping section of the extruder. In this within a nylon fiber is 1% of the breaking strength section of the extruder the channel narrows, increas- of the filament. This stability problem is the direct the fluid pressure, The molten w at result of two peculiarities of mesophase: its highly a relatively high pressure, exits the extruder and flows temperature-dependent viscosity. and the brittle through the transfer manifold As the precursor exits nature of as-spun mesophase fibers PAN-based fibers Current PAN-based fibers (Prior to 1990) Akzo Nobel, Mitsubish Current mesophase-pitch-based fibe Nippon Steel)(Amoco, Mitsubishi) Isotropic-pitch-based fibers Mesophase-pitch-based fibers (Textron) Prior to 1990) Fiber modulus, GPa Fig. 11. Mechanical properties of commercial PAN-based and mesophase pitch-based carbon fibers as of 1989 compared to the properties of current commercial PAN-based and pitch-based fibers
352 D. D. EDK Radial Onion-skin Random Flat-layer Radial-folded Line-origin the manifold it enters a metering pump. The purpose of this positive-displacement pump is to minimize any pressure fluctuations created by the rotating extruder screw. The metering pump forces the molten precursor into the spin pack. Normally, the spin pack houses a filter, which is capable of removing any small solid particles from the molten precursor. As the precursor exits the pack, it is forced through a plate containing numerous small holes (i.e. the spinneret). Finally, as the molten precursor exits these holes, it is simultaneously quenched by the surrounding atmosphere and drawn down by the windup device, forming solid fibers. At first glance, this would appear to be a relatively simple process. In fact, the melt spinning is simple. However, melt-spinning mesophase is far from simple. nature of as-spun mesophase fibers. By applying heat, mass and momentum balances, Edie and Dunham [41] showed that the mesophase melt-spinning process is extremely sensitive to small changes in process conditions. Although their model did not account for the liquid crystalline behavior of the mesophase precursor, it nevertheless demonstrated that, at typical process conditions, the tensile stress on mesophase fibers is ca 20% of that required to break the fiber (see Fig. 9), and experimental measurements confirm these predictions. In comparison, during melt spinning the tensile stress developed within a nylon fiber is < 1% of the breaking strength of the filament. This stability problem is the direct result of two peculiarities of mesophase: its highly temperature-dependent viscosity, and the brittle Fig. 10. Transverse textures of mcsophasc-pitch-based carbon fibres [56]. typical melt-spinning process that might be used to produce mesophase pitch precursor fibers [40] is shown in Fig. 8. The precursor (in this case, mesophase pitch) is loaded into the feed hopper of the extruder as solid chips. The extruder’s rotating screw conveys the chips into the melting section of the extruder where the chips are heated, forming a viscous melt. Then the molten precursor is conveyed into the pumping section of the extruder. In this section of the extruder the channel narrows, increasing the fluid pressure. The molten precursor, now at a relatively high pressure, exits the extruder and flows through the transfer manifold. As the precursor exits 7 6 5 4 3 2 1 0 C I I I I I I I I I PAN-based fibers Current PAN-based fibers (Prior (Amoco, Hercules, Akzo Nobel, Mitsubishi) Isotropic-pitch-based fibers Mesophase-pitch-based fibers l v (Textron) (Prior to 1YYO) I I I I I , I I I ) 100 200 300 400 500 600 700 800 900 1 Fiber Modulus, GPa 10 Fig. 11. Mechanical properties of commercial PAN-based and mesophase pitch-based carbon fibers as of 1989 compared to the properties of current commercial PAN-based and pitch-based fibers
Effect of processing on carbon fibers 丁↓↓;↓↓↓↓↓↓ Fig. 12. Predicted transverse molecular orientation for mesophase pitch flowing through a circular capillary and transverse texture of carbonized mesophase fiber extruded from a circular capillar [571 Because its viscosity is highly temperature-depen- so the stabilization process involves simultaneous dent, mesophase pitch fibers draw down and cool diffusion and reaction. However, unlike PAN precur very quickly during fiber formaLion. In fact, at Typical sor fibers, the as-spun structure of inesopliase precur melt-spinning conditions, mesophase fibers are sor fibers is already highly oriented, so tension does already 100.c below their glass transition temper- not need to be applied during stabilization. Most ature by the time they are 2 cm from the spinneret. mesophase precursor fibers can be stabilized by As a result, they can break easily during spinning 230 to 280"C. Often, the tcmpcrature begins ncar the exposing them to air at temperatures ranging from and are extremely difficult to handle before they are carbonized. Although the rheology of mesophase softening temperature of the mesophase and makes control of the melt-spinning more difficult, its increased in a series of steps during the stabilization iquid crystalline nature gives this precursor advan- process, Numerous studies [43-45] have shown that tages compared to polymeric precursors such as he rate of oxidative stabilization is affected by the PAN. As Yoon et al.[42] showed, unlike polymeric temperature, the concentration of oxygen, and the fibers, the molecular orientation within a mesophase chemical structure of the mesophase molecules. Most precursor fiber can be improved by increasing spin researchers agree uring ling temperature oxidative stabilization, the mesophase fiber gains weight; ketones, aldehydes and carboxylic acids are 3.3 Stabilization of mesophase pitch precursor formed; and water is given off [43, 44]. At higher temperatures, the fiber begins to lose weight as CO2 Because mesophase pitch is a thermoplastic mate- is evolved. However, the exact nature of the reactions rial, the as-spun structure must be thermoset to that occur during the stabilization step is still the prevent relaxation during final heat treatment. Like subject of active research. As in the PAN carbon he PaN carbon fiber process, oxidative stabilization fiber process, the objective is to uniformly crosslink is normally employed to crosslink the as-spun fibers the precursor fiber as fast as possible with a minimum
Effect of processing on carbon fibers 353 (4 vr = “8 = 0 n,=cosw(r) ne = sin o(r) n, = 0 Fig. 1 2. Predicted transverse molecular orientation for mesophase pitch flowing through a circular capillary a texture of carbonized mesophase fiber extruded from a circular capillar [57]. .nd transl Because its viscosity is highly temperature-dependent, mesophase pitch fibers draw down and cool very quickly during fiber formation. In fact, at typical melt-spinning conditions, mesophase fibers are already 100°C below their glass transition temperature by the time they are 2 cm from the spinneret. As a result, they can break easily during spinning and are extremely difficult to handle before they are carbonized. Although the rheology of mesophase makes control of the melt-spinning more difficult, its liquid crystalline nature gives this precursor advantages compared to polymeric precursors such as PAN. As Yoon et al. [42] showed, unlike polymeric fibers, the molecular orientation within a mesophase precursor fiber can be improved by increasing spinning temperature. 3.3 Stabilization of mesophasepitch precursor fibers Because mesophase pitch is a thermoplastic material, the as-spun structure must be thermoset to prevent relaxation during final heat treatment. Like the PAN carbon fiber process, oxidative stabilization is normally employed to crosslink the as-spun fibers, so the stabilization process involves simultaneous diffusion and reaction. However, unlike PAN precursor fibers, the as-spun structure of mesophase precursor fibers is already highly oriented, so tension does not need to be applied during stabilization. Most mesophase precursor fibers can be stabilized by exposing them to air at temperatures ranging from 230 to 280°C. Often, the temperature begins near the softening temperature of the mesophase and is increased in a series of steps during the stabilization process. Numerous studies [43-451 have shown that the rate of oxidative stabilization is affected by the temperature, the concentration of oxygen, and the chemical structure of the mesophase molecules. Most researchers agree that, during the initial stages of oxidative stabilization, the mesophase fiber gains weight; ketones, aldehydes and carboxylic acids are formed; and water is given off [43,44]. At higher temperatures, the fiber begins to lose weight as CO, is evolved. However, the exact nature of the reactions that occur during the stabilization step is still the subject of active research. As in the PAN carbon fiber process, the objective is to uniformly crosslink the precursor fiber as fast as possible with a minimum
354 D. D. EDIE Table 1. Current mechanical and electrical properties of commercial pitch-based carbon Density Average ter (g cm) strength(GPa) modulus(GPa) resistivity (uQ m) 2.0 93 931 2 XN-50A 3.83 7.0 XN-7C 3.0 XN-85-A 363 3.0 363 6.0 2.09 7.0 785 5.0 Mitsubishi 2.35 K37 2.l Carboflext 1.57 0.55 60.0 Coal ta pitch pr addition of noncarbon elements. Currently stabilize- carbonization. In other word ough the diam- on is the slowest step in the pitch-based carbon eters of most commercial pitcl b-based iber process, taking from 30 minutes to >2 hours are larger than those of most PAN-based carbon for most mesophase precursors. Therefore, the search continues for new stabilization techniques and new hase structures that can be more readily 3. 4 Carbonization of stabilized mesophase pitch based fibers nce ed at 1500-3000C in an inert atmosphere [46, 47].During 50 this step most of the noncarbon elements within the fiber are volatilized in the form of methane, hydrogen, water, carbon monoxide, carbon dioxide and various other gases. Because the mesophase precursor fiber 90% carbon and it gains only 6-8 wt% of oxygen during stabilization, the yield of the pitch-based carbon fiber process ranges from 70 to 80--consider ably higher than that of the PAN-based process Dimensionless e,T/T。 However, the lower mass loss also means that, typi- cally, a 12-um-diameter precursor fiber must be spun Fig. 13. Temperature varia if a 10 um final diameter is desired By con KL, and bend, K, [60]. The temperature is scaled by the a fully-drawn an precursor fiber with a diameter of transitional temperature Te at which K,=K3, and the scaling nstant for the frank elastic constants is the cross-ot 15 um would yield the same size carbon fiber after
354 D. D. EDW Table 1. Current mechanical and electrical properties of commercial pitch-based carbon fibers Density Average tensile (g cm? strength (GPa) Average tensile Average electrical modulus (CPa) resistivity(@ mm’) Amoco P-25 P-55s P-l% P-loos P-loos P-120 P-120s K-800x K-l 100 Nippon XN-5OA XN-70A XN-8-A XN-85-A YS-SOA* Y S-7OA* YS-50* YS-60* YS-70* YS-80* Mitsubishi Kasei Co. K133 K135 K137 K139 K321 Textron 1.90 1.38 159 13.0 2.00 I .90 379 8.5 2.00 2.10 517 7.0 2.16 2.41 758 2.5 2.16 2.07 758 2.5 2.17 2.41 827 2.2 2.18 2.41 827 2.0 2.18 2.93 931 1.6 2.20 3.10 931 I .2 2.14 3.83 520 7.0 2.16 3.63 720 4.0 2.17 3.63 785 3.0 2.17 3.63 830 3.0 2.09 3.83 520 7.0 2.14 3.63 720 6.0 2.09 3.73 490 7.0 2.12 3.53 590 7.0 2.14 3.53 690 6.0 2.15 3.53 785 5.0 2.08 2.35 2.10 2.55 2.11 2.65 2.12 2.75 I .90 1.96 Carboflex+ 1.57 0.55 *Coal tar mesophase pitch precursor. +Isotropic pitch precursor. addition of noncarbon elements. Currently, stabilization is the slowest step in the pitch-based carbon fiber process, taking from 30 minutes to >2 hours for most mesophase precursors. Therefore, the search continues for new stabilization techniques and new mesophase structures that can be more readily crosslinked. 3.4 Carbonization of stabilized mesophase pitchbased$bers Once stabilized, the pitch fiber is carbonized at 1500-3000°C in an inert atmosphere [46,47]. During this step most of the noncarbon elements within the fiber are volatilized in the form of methane, hydrogen, water, carbon monoxide, carbon dioxide and various other gases. Because the mesophase precursor fiber is 90% carbon and it gains only 6-8 wt% of oxygen during stabilization, the yield of the pitch-based carbon fiber process ranges from 70 to 80-considerably higher than that of the PAN-based process. However, the lower mass loss also means that, typically, a 12-pm-diameter precursor fiber must be spun if a 10 pm final diameter is desired. By comparison, a fully-drawn AN precursor fiber with a diameter of 15 pm would yield the same size carbon fiber after 440 540 640 740 180 35 60.0 carbonization. In other words, even though the diameters of most commercial pitch-based carbon fibers are larger than those of most PAN-based carbon 1.44 0.95 0.97 0.99 1 01 1.03 1.05 Dimensionless Temperature, TTT, Fig. 13. Temperature variation in elastic constants for splay, K,, and bend, K, [60]. The temperature is scaled by the transitional temperature T, at which K, = K3, and the scaling constant for the Frank elastic constants is K, the cross-over value of K, and K3
