3
Fiber-Forming Processes: Current and Potential Methods

INTRODUCTION

This chapter addresses the processes by which high-performance fibers are formed, with emphasis on those technologies that have been most broadly successful. The technologies include

  • Polymer fibers.

  • Pyrolytic conversion of precursor fibers.

  • Chemical conversion of precursor fibers.

  • Fibers produced by chemical vapor deposition.

  • Single crystal fibers.

Also included is the important technology associated with the coating of fibers, a technology that allows chemical and physical tailoring of the fiber surface.

The processes for converting bulk materials into fibers, while specific to the desired end product, have a series of elements in common. These include

  • Conversion of the room-temperature solid to a low-viscosity (up to 104 poise) melt or solution.

  • Passing the filtered solution through a plate of holes (spinnerette) to form fibers.

  • Solidifying the fiber over a distance of centimeters to meters under conditions of controlled temperature, stress, and mass transfer.

If the process begins with a stable melt, the process is termed "melt spinning." Solution spinning with high-volatility solvents that may be flashed off during solidification is termed "dry spinning." Conversion of solutions in low-volatility (high boiling point) solvents to fiber, requiring filament coagulation by exchanging solvent for nonsolvent, is termed ''wet spinning.'' These processes are illustrated in Figure 3.1.



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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 49 3 Fiber-Forming Processes: Current and Potential Methods INTRODUCTION This chapter addresses the processes by which high-performance fibers are formed, with emphasis on those technologies that have been most broadly successful. The technologies include • Polymer fibers. • Pyrolytic conversion of precursor fibers. • Chemical conversion of precursor fibers. • Fibers produced by chemical vapor deposition. • Single crystal fibers. Also included is the important technology associated with the coating of fibers, a technology that allows chemical and physical tailoring of the fiber surface. The processes for converting bulk materials into fibers, while specific to the desired end product, have a series of elements in common. These include • Conversion of the room-temperature solid to a low-viscosity (up to 104 poise) melt or solution. • Passing the filtered solution through a plate of holes (spinnerette) to form fibers. • Solidifying the fiber over a distance of centimeters to meters under conditions of controlled temperature, stress, and mass transfer. If the process begins with a stable melt, the process is termed "melt spinning." Solution spinning with high- volatility solvents that may be flashed off during solidification is termed "dry spinning." Conversion of solutions in low-volatility (high boiling point) solvents to fiber, requiring filament coagulation by exchanging solvent for nonsolvent, is termed ''wet spinning.'' These processes are illustrated in Figure 3.1.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 50 Figure 3.1 General processing steps for converting bulk materials to fibers.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 51 Important variations on this theme include removal of the spinnerette from the wet spinning coagulation bath (allowing separate control of spinning and coagulation temperatures) necessary for the spinning of many lyotropic polymer systems (aramid, polyphenylene polybisbenzoxazole [PBO]) (dry jet wet spinning); and the spinning of low-concentration solution to minimize chain entanglements in the resulting fiber (gel spinning). Depending on the conditions chosen, filaments may be molecularly oriented or disoriented as spun. Additional orientation may be imparted through "drawing" processes where the molecules comprising the fibers are elongated at a rate faster than their relaxation time. Heat may be applied to yarns to increase morphological stability through controlled crystallization and/or molecular relaxation processes. Finally, chemistry through the application of heat, pressure (vacuum), or reactants may be utilized as a separate step, or in conjunction with the process steps, to convert "precursor" fibers to the desired final composition. In the ensuing portions of this chapter, processing technologies that are key to the formation of important high-performance fibers critically analyzed. PROCESSES TO FORM POLYMERIC ORGANIC FIBERS Introduction The technical concepts behind the processing of ultrahigh-strength, (200-500 Ksi), ultrahigh-modulus (greater than 7 Msi) polymeric organic fibers differ from those for inorganic and metallic fibers and are related to the one-dimensional nature of polymeric molecular chains. To make a fiber that takes maximum advantage of the strength of the interatomic forces (covalent bonding) in the polymer, the molecules must be extended and oriented parallel to the axis of the fiber. The commercial processes for high-strength, high-modulus fibers are based on two physical concepts: melt or dry jet wet spinning from a nematic liquid crystalline phase in which the already rod-like molecules are uniaxially ordered, and melt or gel spinning and drawing of conventional, random-coil polymers under conditions that permit extremely high elongational forces (high draw ratios) to mechanically elongate and orient the component molecules (cf. Figure 3.1). The resulting highly oriented morphologies yield moduli that approach the theoretical. However, the relatively high strengths achieved are far (10x) from theory, offering potential for further development. These fibers are highly anisotropic, with lateral tensile properties up to two orders of magnitude lower than axial. This results in some inherent performance losses, most notably in compressive properties.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 52 Technical Description Spinning of Liquid Crystalline Solutions The first commercial high-performance organic fiber, developed by the duPont Company from poly (paraphenylene) terephthalamide, under the Kevlar trademark, was spun from a nematic (liquid crystalline) solution. In a nematic phase the individual rod-like molecules are oriented parallel to their nearest neighbors. The parallel orientation exists over a correlation volume of micrometer dimensions. The elongational force field during the spinning process orients these volumes in the direction of flow. When the initially extruded fiber is fixed by extraction of the solvent, the molecules solidify in a form that places the strong molecular chains parallel to the fiber. In addition to the aramids, rigid, para-oriented, aromatic hetrocyclic polymers (e.g., poly(benzobisoxazole) [PBO's]), are of greatest important. The cost of polymeric fibers is dependent on the cost of the starting materials and the complexity of the polymerization and conversion processes. Monomers capable of polymerization to required molecular weights must be extremely pure. The necessary steps to achieve this can add significantly to the cost and greatly complicate scale-up to commercial levels. PBO is synthesized from expensive monomers in corrosive acid solvents such as methyl sufonic or polyphosphonic acid. A related polymer with similar expensive monomers is the corresponding benzthiazole. The commercial aramid fiber from poly(paraphenylene terephthalamide) (PPD-T) is synthesized in amide solvents such as N-methylpyrollidone by the condensation of p-phenylene diamine and terephthaloyl chloride. The finished polymer is washed to remove the solvent, and the equipment required for the mixing and isolation makes this an expensive process. Both aramids and the aromatic heterocyclics are redissolved and converted to highly oriented fiber through dry jet wet spinning (cf. Figure 3.1). Aramid and PBO fibers are characterized by use temperatures up to about 250°C and by chemical inertness. The H-bonding potential to the aramids leads to moisture sensitivity with consequent property reductions under humid conditions. Spinning of Liquid Crystalline Melts The majority of polymers in this class are aromatic copolyesters that have been previously commercialized as molding resins. Their advantage is the inherently lower cost of melt spinning, which avoids dissolution and solvent recovery steps. Melt spinning has the potential for much higher spinning rates, with a consequent decrease in capital investment per output. The first commercial product to make its appearance is the fiber Vectran, by Hoechst Celanese and the Kuraray Company. This product's advantages include very high cut resistance (up to eight times that of the aramids) and excellent hydrolytic stability. Tensile properties at room temperature are similar to those of Kevlar® fibers but decrease with

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 53 increasing temperature. Use temperatures are intermediate between Kevlar® and polyethylene, 150° to 200°C depending on composition. Gel Spinning High molecular weight linear polyethylene is gel spun commercially (see Introduction) with the resulting fiber being drawn to very high (20-100) draw ratios. The elongational forces cause the molecular chains to be extended and highly oriented in the fiber direction, resulting in high levels of tensile modulus and tensile strength. The solvent recovery aspect of the process and the controlled low-speed drawing increase the cost of the fiber to a level comparable to other high-strength fibers, despite the low polymer cost. Performance problems inherent in highly oriented polyethylene fibers include low-use temperature (<100°C) and creep. "Normal" Solution Spinning Spinning of a normal isotropic (nonliquid crystalline) solution with subsequent drawing of the fiber to induce orientation can be accomplished with certain rigid molecules. Teijin has commercialized an aramid (Technora) that is a polymer of paraphenylene diamine, 4,4"-diamino diphenylether, and terephthalic acid. The fiber, after removal of the amide (n-methylpyrollidone)/LiCl solvent, is drawn at 485°C. Technical Future A major class of polymers with demonstrated fiber potential but no commercial product yet is the polyimides. Two Japanese companies (Ube and Toray) have announced the manufacture of fibers based on polyimides with tenacities of about 20 gpd. It is unlikely that new spinning processes will be applied to high modulus fiber production. However, the postheating processes that have been shown to improve fiber properties are poorly understood. Better understanding of these processes could lead to improved fibers. The cost of the additional step will remain a factor. Research on the orientation of molecules in elongational and shear fields could lead to incremental but significant improvements in tenacity and modulus. Research on fiber physics, which could lead to commercially important discoveries, is not being done at an appropriate level by industry and only at a very few university laboratories. Rheological and morphological studies have the ultimate potential of finding processes to form high-strength fibers from commodity polymers other than polyethlyene. High-strength nylon or conventional polyester (polyethylene terephthalate) fibers made by a melt process could be the first high-strength fibers with prices compatible with commercial ground transportation applications and land-and marine-based applications. Improved low-cost routes to monomers for liquid crystalline melts, or different low-cost monomers for the same polymers, could have significant commercial impact.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 54 No process variable (other than, perhaps, molecular cross linkage) is likely to significantly improve the tendency toward compressive failure in these highly oriented materials. Process parameters are also unable to affect use temperatures, which will remain in a range well below those of ceramics. The application of blend concepts has been exploited to a relatively small extent. The field of engineering resins has demonstrated the potential of blending to result in order-of-magnitude property changes. The commercially announced fibers from liquid crystalline melts illustrate the strong Japanese effort toward development of fibers discovered in the United States. This reflects the unwillingness of American industry to commercially develop new materials with a limited current market. Although industry is doing little fundamental research on organic fibers, development in general will be handled by industry more effectively than for the other fibers discussed in this report because the organics have a broader commercial market. This indicates that limited government resources should be applied to the more specialized inorganics. FIBER FORMATION BY PYROLYTIC CONVERSION OF PRECURSOR FIBERS Introduction The use of precursors that can be pyrolyzed to form continuous inorganic filaments has provided a route to the manufacture of synthetic inorganic fibers of many different compositions. The precursor materials include polymers, concentrated salt solutions that may behave like polymeric materials, polymer-modified solutions and slurries, and sol-gel systems. Polymeric precursors are used for the fabrication of continuous nonoxide filaments such as carbon, graphite, and silicon carbide (sic). Oxide fibers are produced from all of the precursor types listed above. Since the rheological properties necessary for spinning continuous filaments at high speed are provided by the polymeric-type material present in the fiber-forming precursor composition, spinability of the final fiber composition in its fused form is not a necessary property, as it is in the forming of traditional glass filaments. Therefore, materials such as aluminum oxide (A12O3) or zirconium oxide (ZrO2) and many others may be prepared in fiber form, even though the properties of their liquid phases would not normally permit fiber spinning at any practical rate. Similarly, fibers of materials not generally considered to melt, such as carbon (c) and SiC, are also prepared by this technology. Polymeric materials may embody in their composition all of the precursor components of the final solid inorganic, such as SiC or C, or they may be present principally for their contribution to fiberizing properties. In the latter case, the polymer may be completely fugitive, while the actual fiber components exist in the precursor formulation as decomposable salt compounds or as colloids added as particles or compatible sols such as aqua sols.1 Thus, formation of the inorganic fiber precursor (or organic in the case of carbon fibers) involves spinning melted polymeric

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 55 precursors, viscous polymeric solutions, or mixtures of polymers, salt solutions, and colloids. Regardless of the precursor used, the principal common steps for the fabrication of fibers with polymeric precursor systems are as shown in Figure 3.2. Figure 3.2 Simplified flowsheet for pecursor pyrloysis processes. An especially important feature of the precursor pyrolysis process, in addition to its versatility in the preparation of fibers over a wide range of compositions, is the low temperatures (350°C or lower) used for precursor fiber spinning. Despite the straightforward flowsheet of Figure 3.2, it is important to realize that precise control of conditions in each step is required for the fabrication of filaments with optimum properties. Since the pyrolysis step may involve the decomposition and removal of up to 50 percent or more of the polymeric precursor fiber, it becomes obvious that careful control at this point is extremely critical to ensure that fiber defects are minimized or that the fiber even survives this part of the process. Control at this point will include factors such as the atmosphere, temperature, and rate of temperature increase, it being necessary to control rates of decomposition and removal of volatile products as well as rates of reaction of residual precursor components of the final fiber composition. The specific details for the fabrication of fibers vary according to the composition of the final inorganic fiber and precursors used for their preparation. The major portion of development work on inorganic fibers via the pyrolytic conversion of precursor fibers has been done since 1960. The review by Bracke et al.2 provides a useful summary of the state of the art on inorganic fibers in patents and other publications up to 1984. More complete details on processing, current status, performance, and needs in the area of high-performance synthetic fibers are provided in the following sections on nonoxide and oxide fibers. Carbon Fibers Even though high-performance carbon fibers were first introduced in the 1960s, the physical properties of these reinforcing fibers have improved dramatically over the past decade. This can be attributed to three developments: significant improvements in both quality and performance of the precursor fiber, a substantial increase in worldwide production capacity and the process improvements resulting from this gain in production experience, and a strong push on the part of the United States and Western European aerospace industry for high- performance carbon-based reinforcing fibers.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 56 There are three types of precursors used on a commercial basis to manufacture carbon fibers: rayon, polyacrylonitrile (PAN), and pitch. Rayon, the raw material used for the first high-performance carbon fibers, was the dominant precursor material in the 1960s and early 1970s. However, because of its low carbon yield (20-30 percent), high processing cost, and limited physical properties, today rayon precursor is used for less than 1 percent of commercial carbon fibers. At the present time, North American Rayon is the U.S. supplier of rayon precursor, and Amoco Performance Products is the only U.S. supplier of rayon-based carbon fiber. Use of rayon- based fiber is primarily limited to C-C ablative shields. Since PAN and pitch are used as the precursor for over 99 percent of today's commercial carbon fibers, this discussion will concentrate on the strengths and limitations of the carbon fibers produced from these feedstocks. Noncontinuous carbon fibers have also been made on a developmental basis from a radically different process that does not use polymeric precursors. This process, often referred to as vapor grown carbon fiber, involves the combination of a hydrocarbon vapor with a catalyst source and hydrogen. A discussion of its process is included in the section on whisker-like materials. PAN-based Carbon Fibers Technical Description and Present Status. PAN-based carbon fibers are derived from polyacrylonitrile or acrylic copolymers. Normally, a solution of the polymer is either wet or dry spun into PAN precursor fibers that are ultimately converted into carbon fibers. Figure 3.3 gives a flow diagram for the process, which is well documented in the literature.1 The precursor fibers used by the major PAN-based carbon fiber suppliers differ significantly from the acrylic fibers used for textile acrylic apparel and industrial applications. PAN precursor fibers have fewer filaments per tow stage, a higher level of purity, smaller filament diameter, and higher acrylonitrile (AN) content (i.e., normally greater than 90 percent AN) than fibers used in textile applications. Also, the polymer composition and molecular weight must be modified to produce the desired carbon fiber properties. Because of these differences, the cost of acrylic precursor fibers is three to five times greater than that of acrylic fibers used in textile applications. Stabilization of PAN involves heating the fiber in air to temperatures ranging from 200° to 300°C for approximately 1 hour. The stabilization treatment is followed by carbonization in an inert atmosphere at temperatures greater than 1200°C. Orientation of the graphite-like crystal structure, and thus the fiber modulus, can be further increased by heat treatment (termed "graphitization") at temperatures up to 3000°C. The continuous carbon or graphite fiber is then surface treated and coated with a sizing agent prior to winding the continuous filaments on bobbins. The surface treatment is an oxidation of the fiber surface to promote adhesion to the matrix resin in the composite, and the size promotes handleability and wettability of the fiber with the matrix resin. Carbon fiber strength is primarily controlled by the defect level, and modulus is controlled by crystalline orientation and degree of crystallinity. Thus, by varying those process parameters that can influence orientation and

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 57 Figure 3.3 PAN Based Process

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 58 crystallinity (e.g., precursor purity, stretch ratios, overall process tension levels, carbonization furnace temperature,) in the manufacture of both the precursor and the carbon fiber, the resulting carbon fibers can have a broad range of properties. Generally, carbon fibers are classified into three major groupings (see Table 3.1): standard or aerospace grade, intermediate modulus (I.M.), and high modulus (H.M.). To date, most sales and aerospace qualifications have been with the standard-grade carbon fiber, and this is likely to continue as long as the present applications continue in production. The newer aerospace (and some recreation) applications, however, are primarily involved with the I.M. and H.M. Fibers. TABLE 3.1 Carbon Fiber Classifications Grade Product Modulus % Strain to Failure Development Activity Standard 32-35 1.6 Lower Cost 32-35 2.0 Increased Strain to Failure Intermediate (I.M.) 40-50 2.0 Improved Composite Balance of Properties (Compression, Toughness) Increased Strain to Failure High Modulus (H.M.) 55-85 1.0 Increased Strain to Failure Carbon fibers possess the highest specific modulus (modulus/density) of all commercially available reinforcing fibers (several times that of conventional metals). However, since commercial PAN-based carbon fibers were first introduced 20 years ago, their physical properties have improved dramatically. In fact, over the past 7 to 8 years improvements in precursor as well as carbon fiber technology have increased the tensile strength of this class of fibers from approximately 3.45 GPa (500 Ksi) to 6.9 GPa (1000 Ksi). Today, specific strengths (tensile strength/density) achievable with PAN-based carbon fibers are among the highest of all commercially available reinforcing fibers. They also have a very good balance of properties, particularly compressive strength. Approaches toward improved compression performance, however, are an area where extensive research is being directed since polymeric-matrix composites (PMCs) made with the new higher-strength, higher

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 59 modulus fibers show corresponding improvements in tensile and flex properties but little improvement in compression. A wide variety of fibers are now commercially available that range in tensile modulus from 207 GPa (30 Msi) to 586 GPa (85 Msi) with elongation from less than 1 percent to over 2 percent. Finally, a substantial amount of progress has been made in the ability to convert carbon fibers into intermediate product forms. The material can be braided, woven, knitted, and converted into unidirectional tapes that ultimately can be processed into composites. These materials, however, are inherently more difficult to handle than conventional textile fibers and conventional processing speeds are not practical with them. Very specialized techniques are therefore continually being developed to improve processing efficiencies. Future Potential. Even though PAN-based carbon-fibers have been commercially available for over 20 years, the manufacturing technology, including both precursor and carbon fiber formation, is still very low on the technology growth curve, and there is great potential for improvements in product and process areas. Technological developments for PAN-based carbon fibers will center on the development and implementation of domestic precursor; lower-cost manufacturing technology (e.g., reduced capital as well as operating costs); new high-strength, intermediate, and high-modulus fibers; fibers with an improved overall balance of mechanical properties (e.g., compression); improved composite performance through optimization of the fiber and matrix interface; tailored physical properties (i.e., independent control of electrical and mechanical properties); and improved thermal oxidative stability. Precursor Technology. Currently, all commercial production of PAN-based precursor fibers is based on solution spinning of PAN polymer. Typically, a dilute solution of acrylic polymer is extruded into a coagulation bath (wet spinning) or a hot-gas environment (dry spinning). The use of large amounts of solvents is a fundamental factor in the production process, which results in environmental as well as product design limitations. As indicated previously, all of the major U.S. carbon fiber producers have installed or will be installing domestic precursor capacity principally to meet the Department of Defense's precursor directive. Most of these companies are simply installing the standard wet or dry spinning technology that is the domestic equivalent of the technology used overseas to produce their precursor. However, one company, BASF Structural Materials, Inc., has developed and is now commercializing a unique new technology that utilizes melt-assisted extrusion as the basis for the spinning process.3 This process eliminates the need for conventional solvent-recovery systems, reducing the required wastewater treatment. In short, this new process can easily produce the current standard high-quality precursor fiber, but it also has the versatility to modify fiber composition and even fiber cross-sectional shape, offering the potential to create a new family of carbon fibers with unique properties. Carbon Fiber Technology. The development of processes capable of yielding higher-strength, higher- modulus, PAN-based carbon fibers is a critical research goal. Most of this research is centered around routes to eliminate defects from the fiber (the strength is flaw controlled), to obtain

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 92 surface. This method usually requires a pretreatment of the fiber to activate the surface before entering the plating solution. The major disadvantage of this process is the time required for the reactions to occur, which limits the throughput of the process. Chemical Vapor Deposition Chemical vapor deposition (CVD), previously discussed as a method of fiber synthesis, is also used for coating. It can be broadly defined as a materials synthesis method in which the constituents of a gaseous phase react with one another and with the substrate surface to produce a solid film having the desired composition on the substrate. It is most favorable that the reactions be heterogeneous, that is, occur at the substrate surface. This usually results in high-quality films that adhere well to the substrate. If the reactions are homogeneous and take place in the gas phase, the resulting films exhibit poor properties and adhesion as a result of the formation of solid particles in the gas that eventually contact the substrate surface producing a film. The heterogeneous reaction can be viewed as a sequence of events, any one of which can be rate limiting. The growth of a film via a heterogeneous process can be viewed as a sequence of the following processes:57 • Transport of the reactants to the substrate surface via diffusion. • Adsorption of the reactants on the substrate surface. • Surface processes—reaction, surface diffusion, diffusion in the substrate. • Desorption of volatile reaction products. • Transport of the products away from the substrate surface. A schematic view of these processes is shown in Figure 3.20. The different regimes in which each of these processes may become rate limiting will be controlled by the process variables—the partial pressures of the reactants, the substrate temperature, and the flow rate through the reactor. A diagram of a typical CVD reactor for continuously coating fibers is shown in Figure 3.21. The great flexibility and inherent simplicity make the CVD process one of the most widely used methods for applying coatings to fibers. The process is naturally suited to applying coatings in a continuous manner, and since the deposition is carried out in a regime in which the deposition is gas-phase diffusion limited, all of the fibers in a tow become uniformly coated. In addition, there are usually several sets of reactions that can be utilized for producing a particular coating composition. For example, -SiC coatings have been produced by CVD from many different gaseous mixtures such as SiH4, C3H8 and H2, or (CH3)3 SiCl and various hydrocarbons58 as well as others. Table 3.5 gives a brief listing of materials that have been deposited using CVD processes. For a discussion of many more compounds and metals that have also been deposited, the reader is directed to more extensive reviews.59 For each given set of reactants, the most crucial parameter is the substrate temperature because it determines the rate and the overall thermodynamic outcome of the reaction.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 93 Figure 3.20. Schematic Diagram of the CVD Process. Permission to print by the Electrochemical Society, Inc. Coating thicknesses from a few nanometers to a few hundred micrometers can be deposited; however, because deposition rates are slow, process throughput falls off for thick coatings. Typical deposition rates can range from ˜μm/hr to ˜100 μm/hr. In many instances of materials having slow deposition rates, it is necessary to have multiple deposition stations along the fiber line. Metallorganic Deposition Metallorganic deposition is an entirely nonvacuum technique for the preparation of thin films. In this process a liquid precursor that contains a metallorganic species dissolved in an organic solvent is deposited onto the substrate surface. The substrate is then subjected to drying and heating steps that result in removal of the solvent and decomposition of the organometallic compound to produce the desired coating. The process is shown schematically in Figure 3.22. Typical precursors are metal alkoxides, metal deconates, neodeconates, and preceramic oligomers or polymers. In the case of the metal alkoxides, after the metallorganic is deposited, it is subjected to a hydrolysis reaction to convert the coating to the oxide. The remaining heat treatments then serve to remove any remaining organic residues. This process usually produces coatings that are 50 to 200 nm thick after the heat treatment stage. The process is capable of applying graded compositions by using multiple dipping stations, each having a slightly different composition.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 94 Figure 3.21. Schematic of a continuous CVD fiber-coating line. TABLE 3.5 Materials Produced by CVD Processes CVD Material Reference A1203 60 B 61 B4C 62 HfC 63 SiC 64, 65 Si3N4 66 TiB2 67 TiC 68 ZrC 69

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 95 Figure 3.22. Metallorganic deposition process. The process has been used most frequently to apply oxide coatings such as SiO2, TiO2, and mixtures of various oxides. Metal deconates undergo a similar process, but the hydrolysis step is not necessary. Preceramic polymers such as polycarbosilane, which results in SiC, are applied in the same manner as the alkoxides, but as in the case of the metal deconates, the hydrolysis step is not needed. The major disadvantage of the preceramic polymer coating method is the lack of a wide variety of coating materials. Metallorganic deposition shares many common features with electrodeposition in that the process is continuous and all operations are carried out at atmospheric pressure. The major disadvantages of this coating method are that the coating thickness for a single pass is often too thin and thicker coatings put on using multiple passes often develop cracks at the interfaces between layers. Vacuum Deposition This category of fiber coatings includes sputtering, physical vapor deposition, e-beam evaporation, plasma- assisted CVD, and ion-plating techniques. With the exception of the plasma and ion plating, these processes are line-of-sight deposition techniques, making deposition of uniform film on a multifilament tow very difficult to achieve due to shadowing effects by other filaments in the tow. The film deposition rates are usually slower than with CVD methods, and hence the throughput of these processes is limited. Vacuum deposition techniques for continuous filaments are at an early state of understanding, and much more work in this area needs to be done.

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 96 Future Fiber-Coating Technology As the demand for structural materials that can operate at higher temperatures increases, it is imperative that a thorough understanding of the role of fiber coatings be developed. Given that there is a limited set of existing fibers that can be utilized at elevated temperatures, and that these fibers are usually unstable either relative to the matrix material or the environment during fabrication of the composite or in-service use, it becomes necessary to consider the use of coatings as a means of moderating the interaction of the matrix with the fiber. In the introduction to this chapter it was pointed out the fiber-coating matrix is a complex thermochemical- thermomechanical system and that a substantial amount of analysis is required to determine the optimum composition that a coating or series of coatings should have in order to keep the coating from interacting too strongly with either the fiber or the matrix. Currently, the best models available are thermodynamic chemical equilibrium calculations used to predict whether two compositions might react with one another under a given set of conditions to form a more stable series of products. These calculations usually do not take into consideration the kinetics of the reactions. It also needs to be pointed out that in many instances the coatings applied by the methods discussed are not ideal in that they may not be stoichiometric, or crystalline, and that if they are crystalline may contain defects or impurities that will play an important role in how the coating reacts with the fiber and surrounding matrix. When coatings are applied at temperatures well below that of the melting point of the coating material, as is the case for most of the coating methods discussed here, the coating can be in a metastable state. On subsequent exposure to high temperatures, it will evolve toward the thermodynamic equilibrium state, possibly producing precipitates and/or reacting with the fiber or matrix. In general, there is a need for better understanding of the coating process, so that coatings result that will have the desired stoichiometries and structure. This requires development of more useful in situ diagnostic probes for monitoring the coating process to ensure uniformity of coating properties. It is highly likely that ternary and quaternary coating systems will need to be developed, and this will require a closer relationship between synthetic chemistry for the design of organometallic molecules and the film growth community for developing new methods for coating fibers with compositionally complex systems. There is also a need for techniques to evaluate the effect of the coating on the interfacial bonding properties in the composite that relate to the structure and chemistry of the interface. Considerations Involved in Scale-up of Fiber Processing The processing of bulk materials into fibers is a broad-based technology that is relatively well understood in principle. However, scale-up of fiber processing is more difficult than that of other materials because of the simultaneous heat, momentum, and mass transfers, combined with chemical reactions and unique, complex, flow- induced orientation effects. Fiber

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 97 process development may conveniently be divided into three levels of detail and complexity, namely: feasibility, screening, and end-use evaluation. Demonstration of the feasibility of converting a given bulk material to fiber involves converting a minimal amount of the bulk material to a short length of fiber and a gross evaluation of the mechanical, thermal, and chemical properties of the filament so produced. To do this with almost any material is relatively simple and may be accomplished with as little as a few grams. Typical methods might include ''hand pulling'' of a fiber from a melt or solution or the use of a single-filament microspinning machine. Fibers so produced lack physical and structural uniformity and give only the broadest insight into the spinnability and property set of the fiber obtained. Screening involves tens to hundreds of grams of the fiber-forming precursor material (per experiment), which is spun into either a single filament or yarn with a small number of filaments using small-scale spinning equipment. This allows broad control of the key spinning variables such as temperature, stress, and mass transfer —parameters that control the structure and therefore the properties of the fiber produced. By allowing the systematic study of process-structure-property relationships, screening permits the initial evaluation of the fiber's probable utility. In screening experiments the purity and homogeneity of the precursor material start to play a major role and allow an initial characterization of the range of properties available from the material in fiber form. For example, polyethylene that is conventionally melt spun and drawn yields a modulus of around 100 g/d and a strength of perhaps 8 g/d. The same polyethylene gel spun and drawn to much higher draw ratios might yield a modulus of 2500 g/d and a strength of 30 g/d. The mere pulling of a polyethylene melt into a long thin length of material would not have given any indication that property variation over a range of several orders of magnitude was possible. The importance of purity of the material can be illustrated by the following example. Assume a melt- spinnable material being extruded into a 20 dpf fiber at the relatively slow spinning speed of 500 m/min. If the material contains one impurity particle per 100 g capable of upsetting the spinline, a single filament spinning might run for 25 min. yielding 45,000 m of yarn for evaluation and the impression of stable spinning. If the more realistic case of 10-filament spinning is attempted, the spinline might be expected to breakdown after only about 5 min. If a 100-filament process simulation is attempted, less than 1 min of stable spinning is possible. Similarly, a parameter control upset (i.e., an anomaly in temperature or quench rate) will interupt the spinning process and/or profoundly influence the fibers' properties. The difficulties associated with control are exacerbated as the fiber forming process becomes more complicated, and the small-scale simulation of highly complex processes, such as the production of ceramic fiber from polymeric precursors, may be essentially impossible to evaluate on a very small (100-1000 g) scale. True end-use evaluation of process development requires large amounts (tens to hundreds of kilograms) and highly controlled, relatively large scale equipment. The equipment must be capable of 24-hour operation because only fiber data generated under steady-state conditions are meaningful. Often,

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FIBER-FORMING PROCESSES: CURRENT AND POTENTIAL METHODS 98 major property and processability improvements are noted in large-scale fiber evaluation because of the superior parameter control possible in large batch operation. In the case of a large-scale operation, feed material purity and homogeneity requirements are obviously high for the program to be initiated, much less successfully completed. Experiments beyond the screening stage are, therefore, nontrivial and involve the commitment of major capital resources, expertise, and man-hours. OBSERVATIONS AND CONCLUSIONS ABOUT FIBER FABRICATION AND PROCESSING • Technologies for the fabrication of a wide range of high-performance fibers are known and have been used to prepare fibers of many types with great diversity in composition and properties. • The high cost of manufacturing remains a common problem from the standpoint of economics and utilization of many high-performance synthetic fibers. • Pyrolysis and chemical conversion of precursor fibers, CVD, and single-crystal growth processes are promising routes to the fabrication of continuous high-performance synthetic inorganic fibers. • The control of microstructure during fabrication and applications is a vital consideration for all classes of high-performance synthetic fibers for structural uses. • New processes and increased understanding of the processes used now will be important in the development of reinforcement fibers for future composite applications with more demanding requirements. • Continued efforts on fiber coating processes and fundamental studies on interfacial effects between fibers and matrices are needed. REFERENCES 1. Sowman, H. G., "A New Era in Ceramic Fibers via Sol-Gel Technology", Am. Ceram. Soc. Bulletin, Vol 67 [12], pp 1911-1916, 1988. 2. Bracke, P., H. Schurmans and J. Verhoest, Inorganic Fibres and Composite Materials, Pergamon Press, 1984. 3. Watt, W., "Chemistry and Physics of the Conversion of Polyacrylonitrile Fibers into High Modulus Carbon Fibers," Handbook of Composites, Vol. 1 Strong Fibers, 327-387, Eds. W. Watt and B. V. Perov, Elsevier Science Publishers B.V., North-Holland, 1985. 4. Daumit, Gene P., Y. S. Ko, C. R. Slater, J. G. Venner, D. W. Wilson, C., C. Young, and H. Zabaleta, "MSP-A Domestic Precursor For Current And Future Generation Carbon Fibers," 20th International SAMPE Technical Conference, 20, 414, 1988.

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