General Materials
Glass Fibers,Carbon Fibers Materials,Fibers
GLASS FIBERS are among the most versatile industrial materials known today. They are readily produced from raw materials, which are available in virtually unlimited supply (Ref 1). All glass fibers described in this article are derived from compositions containing silica. They exhibit useful bulk properties such as hardness, transparency, resistance to chemical attack, stability, and inertness, as well as desirable fiber properties such as strength, flexibility, and stiffness (Ref 2). Glass fibers are used in the manufacture of structural composites, printed circuit boards and a wide range of special-purpose products (Ref 3). Fiber Forming Processes. Glass melts are made by fusing (co-melting) silica with minerals, which contain the oxides needed to form a given composition. The molten mass is rapidly cooled to prevent crystallization and formed into glass fibers by a process also known as fiberization. Nearly all continuous glass fibers are made by a direct draw process and formed by extruding molten glass through a platinum alloy bushing that may contain up to several thousand individual orifices, each being 0.793 to 3.175 mm (0.0312 to 0.125 in.) in diameter (Ref 1). While still highly viscous, the resulting fibers are rapidly drawn to a fine diameter and solidify. Typical fiber diameters range from 3 to 20 μm (118 to 787 μin.). Individual filaments are combined into multifilament strands, which are pulled by mechanical winders at velocities of up to 61 m/ s (200 ft/s) and wound onto tubes or forming packages. This is the only process that is described in detail subsequently in the present article. The marble melt process can be used to form special-purpose, for example, high-strength fibers. In this process, the raw materials are melted, and solid glass marbles, usually 2 to 3 cm (0.8 to 1.2 in.) in diameter, are formed from the melt. The marbles are remelted (at the same or at a different location) and formed into glass fibers. Glass fibers can also be down drawn from the surface of solid preforms. Although this is the only process used for manufacturing optical fibers, which are not discussed in this Volume, it is a specialty process for manufacturing structural glass fibers such as silica or quartz glass fibers. These and other specialty processes are highlighted wherever appropriate but not discussed in full. Additional details about fiber forming are provided in the section “Glass Melting and Fiber Forming” in this article. Sizes and Binders. Glass filaments are highly abrasive to each other (Ref 4). “Size” coatings or binders are therefore applied before the strand is gathered to minimize degradation of filament strength that would otherwise be caused by filament-to-filament abrasion. Binders provide lubrication, protection, and/or coupling. The size may be temporary, as in the form of a starch-oil emulsion that is subsequently removed by heating and replaced with a glass-to-resin coupling agent known as a finish. On the other hand, the size may be a compatible treatment that performs several necessary functions during the subsequent forming operation and which, during impregnation, acts as a coupling agent to the resin being reinforced.
General-purpose glass fibers (E-glass variants) are discussed in the following section of this article, which provides an indepth discussion of compositions, melt properties, fiber properties (Ref 12), methods of manufacture, and significant product types. An in-depth discussion of composite applications can be found in other articles in this Volume. Glass fibers and fabrics are used in ever increasing varieties for a wide range of applications (Ref 13). A data book is available (Ref 14) that covers all commercially available E-glass fibers, whether employed for reinforcement, filtration, insulation, or other applications. It lists all manufacturers, their sales offices, agents, subsidiaries, and affiliates, complete with addresses, and telephone and fax numbers. And it tabulates key properties and relevant supply details of all E-glass fiber grades, that are available in the market today. Special-Purpose Glass Fibers. S-glass, D- glass, A-glass, ECR-glass, ultrapure silica fibers, hollow fibers, and trilobal fibers are special-purpose glass fibers. Selected special-purpose glass fibers are discussed in the subsequent section of this article. That section reviews compositions, manufacture, properties, and applications to an extent commensurate with their commercial use (Ref 15). A companion data book (Ref 16) is available that covers all commercially available high- strength glass fibers including S-glass and, all silica or quartz glass fibers, including Astroquartz and Quartzel. It also lists a wide range of woven fabrics, that are commercially available in the market of today, ranging from S-glass/aramid, S-glass/carbon, silica/aramid, and silica/ carbon yarns to silica/boron yarns. In addition, it covers all commercially available carbon, ceramic, boron, and high-temperature polymer fibers and yarns. This data book also lists all yarn counts, fabric constructions, fabric weights, and commercial sources. ASTM Test Methods. ASTM has published standard test methods for glass density (Ref 17), alternating current loss characteristics and dielectric constant (Ref 18), direct current conductance of insulating materials (Ref 19), dielectric breakdown voltage and dielectric strength (Ref 20), softening point of glass (Ref 21), annealing point and strain point of glass by fiber elongation (Ref 22), annealing point and strain point of glass by beam bending (Ref 23), viscosity (Ref 24), liquidus temperature (Ref 25), and coefficient of linear thermal expansion of plastics (Ref 26). Some fiber properties (Ref 4), such as tensile strength, modulus, and chemical durability, are measured on the fibers directly. Other properties, such as relative permittivity, dissipation factor, dielectric strength, volume/surface resistivities, and thermal expansion, are measured on glass that has been formed into a bulk patty or block sample and annealed (heat treated) to relieve forming stresses. Properties such as density and refractive index are measured on both fibers and bulk samples, in annealed or unannealed form.
References cited in this section
2. F.T. Wallenberger, Structural Silicate and Silica Glass Fibers, in Advanced Inorganic Fibers Processes,Structures, Properties, Applications, F.T. Wallenberger, Ed., Kluwer Academic Publishers, 1999, p 129–168
4. D.M. Miller, Glass Fibers, Composites, Vol 1, Engineered Materials Handbook, ASM International, 1987, p 45–
48
5. “Standard Specification for Glass Fiber Strands”, D 578-98, Annual Book of ASTM Standards, ASTM
6. P.K. Gupta, Glass Fibers for Composite Materials, Fibre Reinforcements for Composite Materials, A.R. Bunsell,
Ed., Elsevier Publishers, 1988, p 19–72
7. J.F. Sproull, Fiber Glass Composition, U.S. Patent 4,542,106, 17 Sept 1985
8. W.L. Eastes, D.A. Hofman, and J.W. Wingert, Boron-Free Glass Fibers, U.S. Patent 5,789,329, 4 Aug 1998
9. “Advantex Glass Fiber, A New Era in Composites Technology—Systems Thinking”, Product Bulletin, Owens
Corning, 1998
10. F. Rossi and G. Williams, A New Era in Glass Fiber Composites, Proc., 28th AVK Conf. (Baden-Baden,
Germany), 1-2 Oct 1997, p 1–10
11. O.V. Mazurin, M.V. Streltsina and T.P. Shvaiko-Shvaikovskaya, Viscosity of Silica, Handbook of Glass Data,
Part A, Elsevier, 1983, p 75
12. P.F. Aubourg and W.W. Wolf, “Glass Fibers- Glass Composition Research,” presented at Glass Division Meeting
(Grossinger, NY), American Ceramic Society, Oct 1984
make a spinnable pitch, so better overall properties for PAN fibers resulted in their dominance. Rayon was
relegated to third place, despite having a lower raw material cost, because inferior properties and a low char
yield (20 to 25%) after carbonization made for a higher overall cost. Properties can be improved by stress
graphitization at high temperatures, but this increases cost further, making the fiber even less desirable. Rayon
is still used today for insulating and ablative applications but not for structural applications.
By the mid-1990s, a new cost-effective, PAN- based carbon fiber made from a modified textile precursor was
being aggressively promoted by companies like Zoltek and Fortafil for commercial applications. In 1995, one
manufacturer announced the goal of reaching a price level of $5/ lb ($11/kg) by the year 2000, which brought
alot of attention to and greatly accelerated application development (Ref 6). An overall trend of improved
performance/price ratio for both pitch and PAN fiber manufacturers has sustained this growth.
Carbon fiber demand has grown to an estimated 16 × 106 kg (35 × 106 lb) per year (Ref 7). Usage in 1997 wasestimated at 30% aerospace, 30% sporting goods, and 30% commercial/industrial applications, with the
industrial applications poised for the greatest growth (Ref 8).
References cited in this section
1. T. Edison, U.S. Patent 223,898, 1880
2. R. Bacon and M.M. Tang, Carbonization of Cellulose Fibers I, Carbon, Vol 2, 1964, p 211 3. R. Bacon and C.T. Moses, High-Performance Polymers—Their Origin and Development, R.B. Seymourand G.S. Kirshenbaum, Ed., Elseveir, 1986, p 341
4. W. Schimpf, Advanced Fiber Technologies, personal communication, 2000
5. J.B. Donnet and R.C. Bansal, Carbon Fibers, 2nd ed., Marcel Dekker, 19906. “Zoltek Corporation 1994 Annual Report,” St. Louis, 1995
7. Zoltek Corporation, unpublished data, 2000
8. K. Shariq, E. Anderson, and M. Yamaki, “Carbon Fibers,” Chemical Economics Handbook Market
Research Report, SRI International, Menlo Park, CA, July 1999
Manufacture of Carbon Fibers Precursor sources used, in order of volume, are PAN, pitch, and rayon. Although the specific processing details for each precursor is different, all follow a basic sequence involving spinning, stabilization, carbonization, and application of a finish or sizing to facilitate handling, as shown in Fig. 1. Discontinuous carbon fiber whiskers are also now produced in a batch process from hydrocarbon gases using a vapor-liquid-solid growth mechanism.
PAN-based Carbon Fibers. The majority of all carbon fibers used today are made from PAN precursor, which is a form of acrylic fiber. Precursor manufacture is accomplished by spinning the PAN polymer into filaments using variants of standard textile fiber manufacturing processes. The PAN fibers are white in color, with a density of approximately 1.17 g/cm3 (0.042 lb/ in3) and a molecular structure comprised of oriented, long chain molecules. Stabilization involves stretching and heating the PAN fibers to approximately 200 to 300 °C (390 to 570 °F) in an oxygen-containing atmosphere to further orient and then crosslink the molecules, such that they can survive higher-temperature pyrolysis without decomposing. Stretching after spinning and during stabilization helps develop the highly oriented molecular structure that allows development of a high tensile modulus and improved tensile strength upon subsequent heat treatment. Carbonization of standard and intermediate modulus fiber typically involves pyrolyzing the fibers to temperatures ranging from 1000 to 1500 °C (1800 to 2700 °F) in an inert atmosphere, typically to a 95% carbon content. An additional high heat treatment step is included just after carbonization for some very high-modulus fibers. During carbonization, the fibers shrink in diameter and lose approximately 50% in weight. Restraint on longitudinal shrinkage helps develop additional molecular orientation, further increasing mechanical properties. After carbonization, the fibers may be run through a surface treatment step designed to clean and attach functional groups to the fiber surface, which increases bond strength with matrix resins. Most manufacturers use an electrolytic oxidation process that creates carboxyl, carbonyl, and hydroxyl groups on the surface for enhanced bonding. A sizing or finish is then applied to minimize handling damage during spooling and enhance bonding with matrix resins. The fiber is then spooled. Today, there is differentiation among manufacturers between those who use a modified textile-type PAN precursor and those who use an aerospace-type precursor. The textile-type precursor is made on a very large scale in modified- acrylic textile fiber plants in tows or rovings consisting of >200,000 filaments. The tows are then split down into smaller bundles (approximately 48,000 filaments) after carbonization for spooling. Aerospace precursor is made in smaller specialty plants and processed in 3000 (3K) to 12K filament tows that can be assembled into 24K or larger tows after carbonization. Manufacturing cost is lower for the textile-type precursor, due to higher line throughputs, larger economies-of- scale, and less handling of smaller tow bundles. This type fiber is more targeted for industrial applications. The aerospace-type precursor, because it is processed in smaller tow sizes, is less fuzzy and available in the small tow sizes favored by the aerospace industry, for whom it was originally developed. Physical properties can be similar for both types.
Pitch-Based Fibers. Pitch is a complex mixture of aromatic hydrocarbons and can be made from petroleum, coal tar, asphalt, or PVC (Ref 9). Starting raw material selection is important to the final fiber properties. Pitches must be processed through a pre-treatment step to obtain the desired viscosity and molecular weight in preparation for making high-performance carbon fibers. The pre-processed pitch contains “mesophase”, a term for a disk-like liquid crystal phase (Ref 10) that develops regions of long-term ordered molecules favorable to manufacture of high-performance fibers. Without this step, the result is an isotropic carbon fiber with low strength and low modulus of less than 50 GPa (7 × 106 psi) (Ref 11). Process details of the final composition and method of spinning mesophase pitch are generally held secret by the manufacturers. Once spun, the stabilization, carbonization, surface treatment, application of sizing, and spooling of pitch-based fibers follows a sequence similar to the manufacture of PAN-based fibers, as shown in Fig. 1. Actual process parameters, such as temperatures, ramp rates, and time at temperature for stretch and stabilization, are different for pitch than for PAN. Gas species evolved during pyrolysis and their onset of evolution are very different for PAN and pitch. The response to heat treatment is also greater for mesophase-pitch-based fibers at higher temperatures, a consequence of their more ordered starting molecular structure. For example, a mesophase-pitch-derived fiber processed to the same temperature as a PAN fiber will exhibit higher density and thermal and electrical conductivity, all else being equal. Other Precursors. Rayon is processed in similar fashion to PAN, as shown in Fig. 1; the difference is the actual process parameters used. Carbon fiber “whiskers” can be formed from gas-phase pyrolysis via catalyzed cracking of hydrocarbon gases like methane. One process involves growth of a thin carbon tube of 10 to 50 nm from a submicron iron particle in a hydrocarbon-rich atmosphere, followed by a secondary process of thickening the tube by chemical vapor deposition of carbon on the surface (Ref 12). Others have discussed similar processes, some capable of longer length fibers (Ref 13). Although only discontinuous fibers are fabricated, they have unique properties approaching those of single crystal graphite in some cases. Available Formats for Fibers. Commercially available carbon fibers are produced by a multitude of manufacturers with a wide range of properties and tow sizes. Carbon fibers are available in many of the same formats as glass fiber. These formats include continuous filament- spooled fiber, milled fiber, chopped fiber, woven fabrics, felts, veils, and chopped fiber mattes. Most fiber today is spooled, and then processed into other formats in secondary operations. The size of the carbon fiber tow bundle can range from 1000 filaments (1K) to more than 200K. Generally, aerospace carbon fibers are available in bundles of 3K, 6K, 12K, and 24K filaments, while most commercial-grade fibers are available in 48K or larger filament counts. Composite fabrication equipment, such as filament winders and weaving machines, must be adapted to handle the larger cross section of commercial grade fiber..
References cited in this section
9. J.B. Donnet and R.C. Bansal, Carbon Fibers, 2nd ed., Marcel Dekker, 1990, p 55
10. J.D. Brooks and G.H. Taylor, The Formation of Graphitizing Carbons from the Liquid Phase, Carbon, Vol 3, 1965, p 185–193
11. R.P. Krock, D. Carolos, and D.C. Boyer, Versatility of Short Pitch-Based Carbon Fibers in Cost Efficient Composites, 42nd Conf. of Composites Institute, SPI, Feb 1987
12. A. Oberlin, M. Endo, and T. Koyama, Filamentous Growth of Carbon Through Benzene Decomposition, J. Cryst. Growth, Vol 32, 1976, p 335
13. G.G. Tibbetts, Carbon Fiber Filaments and Composites, J.L. Figueiredo et al. Ed., Kluwer Academic Publishers, 1990, p 73–94
On a finer scale, each ribbon-like crystallite is comprised of multiple wrinkled layers. Each layer is made of carbon atoms arranged like chicken wire in a hexagonal structure characteristic of graphite, called a graphene plane. Strong covalent CC bonds within the layer plane give the potential for high strength and stiffness. Weak van der Waals bonding between the layer planes gives rise to poor shear resistance, but also allows thermal and electrical conductivity. Loose electrons and thermal energy in the form of phonons take advantage of the weak bonding between layer planes and use the interplane space as a corridor to travel. The width of the ribbons, the number of graphene layers comprising their thickness, and the length of the ribbons help determine the electrical and thermal characteristics of the carbon fiber, as well as contribute to fiber modulus. Typically, larger and more oriented graphene planes result in higher thermal and electrical conductivity. Improving the orientation of the microstructure can also increase filament tensile modulus, thermal conductivity, electrical conductivity, and density. This can be accomplished by plastic deformation (for example, stretching the fiber) and/ or heat treatment. Figure 3 shows x-ray diffraction results relating heat treatment temperature to the degree of preferred orientation of the microstructure (Ref 17, 18). The degree of preferred orientation represents the average angle at which the crystallites lie relative to the fiber axis; a zero degree angle means that the crystallites are perfectly aligned with the fiber axis. Transmission electron microscopy shows that the ribbons undulate, such that their amplitude is greater than their wavelength. Any reported measurement of preferred orientation is therefore only an average. The data clearly shows improved orientation with increasing heat treatment temperature. Figure 3 also shows that for heat treatment temperatures above 1600 °C (2900 °F) the mesophase pitch-based fiber will orient more than the PAN fiber, a result of larger crystallite sizes that PAN precursors are not able to achieve. The relationship between preferred orientation of the microstructure and modulus is illustrated in Fig. 4. Increased orientation results in increased fiber modulus, as expected.
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