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 [41346 D. D. EDK ties are easier to develop in PAN-based carbon fibers, while other properties are easier to develop in pitch￾based carbon fibers. Therefore, to understand struc￾ture 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 precur￾sor emerges from these capillaries, it cools and solidi￾fies 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 temper￾ature of ra 120°C and tends to decompose before it melts. Therefore, PAN precursor fibers must be pro￾duced 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 commer￾cial 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 copoly￾mer) 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 dimethyla￾cetamide 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 draw￾down rate, and the rate of extrusion [ 51. Mass transfer at the fiber/liquid interface is rela￾tively 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 poly￾mer 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 spin￾neret capillaries, the shear field tends to orient the solidifying polymeric structure parallel to the direc￾tion 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 coagu￾lation 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]