Overview

The following is an overview of the current state of composite materials. Information provided is from the Composites World's Sourcebook 2008. Additional information may be found at www.compositesworld.com.

    Composites fly high around the world

      High strength and low weight remain the winning combination that propels composite materials into new arenas, but other properties are equally important. Composite materials offer good vibrational damping and low coefficient of thermal expansion (CTE), characteristics that can be engineered for specialized applications. Composites are resistant to fatigue and provide design/fabrication flexibility that can significantly decrease the number of parts needed for specific applications — which translates into a finished product that requires less raw material, fewer joints and fasteners and shorter assembly time. Composites have proven resistance to temperature extremes, corrosion and wear, especially in industrial settings, where these properties do much to reduce product life-cycle costs. These characteristics have propelled composites into wide use. The push for fuel economy in the face of rising oil prices, for example, has made lightweighting a priority in almost every mode of mechanical transportation, from bicycles to large commercial aircraft. As the The Boeing Co.’s (Seattle, Wash.) 787 Dreamliner entered production, composites for the first time took center stage in the aerospace world. As the following will demonstrate, composites — available in increasing diverse material forms and manufacturable by an extensive array of molding and forming processes — have taken or are poised to take the spotlight in manufacturing arenas all over the globe.

    A definitive, and different, material

      Composites differ from traditional materials in that composite parts comprise two distinctly different components — fibers and a matrix material (most often a polymer resin) — that, when combined, remain discrete but function interactively to make a new material whose properties cannot be predicted by simply summing the properties of its components. In fact, one of the major advantages of the fiber/resin combination is its complementary nature. Thin glass fibers, for example, exhibit relatively high tensile strength, but are susceptible to damage. By contrast, most polymer resins are weak in tensile strength but are extremely tough and malleable. When combined, however, the fiber and resin each counteract the other’s weakness, producing a material far more useful than either of its individual components.

      The structural properties of composite material are derived primarily from the fiber reinforcement. Commercial composites for large markets, such as automotive components, boats, consumer goods and corrosion-resistant industrial parts, often are made from noncontinuous, random glass fibers or continuous but nonoriented fiber forms. Advanced composites, initially developed for the military aerospace market, offer performance superior to that of conventional structural metals, and now find applications in communications satellites, aircraft, sporting goods, transportation, heavy industry and in the energy sector, in both oil and gas exploration and wind turbine construction. High-performance composites derive their structural properties from continuous, oriented, high-strength fiber reinforcement — most commonly carbon, aramid or glass — in a binding matrix that promotes processability and enhances mechanical properties, such as stiffness and chemical resistance.

      Fiber orientation can be controlled, a factor that can improve performance in any application. In composite golf club shafts, for example, boron and carbon fibers oriented at different angles within the composite shaft enable it to take best advantage of their strength and stiffness properties and withstand torque loads and multiple flexural, compressive and tensile forces.
      A matrix can be polymeric, ceramic or metallic. The polymer matrices most widely used for composites in both commercial and high-performance aerospace applications are thermoset resins, consisting of polymer chains that are permanently cured into a crosslinked network when mixed with a catalyst, exposed to heat, or both. Curing usually occurs under elevated temperature and/or pressure conditions in an oven and/or vacuum bag or in an autoclave. Alternative curing technologies include electron beam, ultraviolet (UV) radiation, X-ray and microwave processes.

      Composites made with inherently tough thermoplastic (TP) matrices, however, are a fast-growing market sector. Their linear polymer chains are formed and can be reformed into shaped solids by melting or softening and then cooling the material. Often sold in sheet or panel form, thermoplastics can be processed by in-situ consolidation techniques, such as simple press forming (see “Fabrication methods,” p. 22) to make tough, near-net shape parts without the autoclave or vacuum-bag cure required by thermosets. TP reformability offers the potential to correct anomalies or repair in-service damage.

    Glass fibers

      In the broad composites industry, the vast majority of all fibers used are glass. Glass fibers are the oldest and, by far, the most common reinforcement used in nonaerospace applications to replace heavier metal parts. Glass weighs more than carbon but is also more impact-resistant than carbon. Depending upon the glass type, filament diameter, sizing chemistry and fiber form, a wide range of properties and performance levels can be achieved.
      Fiber properties are determined by the fiber manufacturing process and the ingredients and coatings used in the process. During glass fiber production, raw materials are transformed into delicate and highly abrasive filaments, ranging in diameter from 3.5 to 24 micrometers. Silica sand is the primary raw ingredient, typically accounting for more than 50 percent of the glass fiber weight. Metal oxides and other ingredients can be added to the silica and the processing methods can be varied to customize the fibers for particular applications.

      Glass filaments are supplied in bundles called strands. A strand is a collection of continuous glass filaments. Roving generally refers to a bundle of untwisted strands, packaged like thread on a large spool. Single-end roving consists of strands containing continuous, multiple glass filaments that run the length of the strand. Multiple-end roving contains lengthy but not entirely continuous strands, which are added or dropped in a staggered arrangement during the spooling process. Yarns are collections of strands that are twisted together.

      Electrical or E-glass, so named because its chemical composition makes it an excellent electrical insulator, is particularly well suited to applications in which radio-signal transparency is desired, such as aircraft radomes, antennae and computer circuit boards. However, it is also the most economical glass fiber for composites, offering sufficient strength in most applications at a relatively ow cost. Over time, it has become the standard form of fiberglass, accounting for more than 90 percent of all glass-fiber reinforcements. At least 50 percent of E-glass fibers are silica oxide; the remainder are composed of oxides of aluminum, boron, calcium and other compounds, including limestone, fluorspar, boric acid and clay.

      When greater strength is desired, high-strength glass, first developed for military applications in the 1960s, is an option. Variously known as S-glass in the U.S., R-glass in Europe and T-glass in Japan, its strand tensile strength is 700 ksi, with a tensile modulus of 14 Msi. S-glass has appreciably higher silica oxide, aluminum oxide and magnesium oxide content than E-glass, and is 40 to 70 percent stronger than E-glass. Both E-glass and S-glass lose up to half of their tensile strength as temperatures increase from ambient to 538°C/1000°F, although both fiber types still exhibit generally good strength in this elevated temperature range.
      While glass fibers have relatively high chemical resistance, they can be eroded by leaching action, when exposed to water. For instance, an E-glass filament 10 microns in diameter typically loses 0.7 percent of its weight when placed in hot water for 24 hours. The erosion rate, however, slows significantly as the leached glass forms a protective barrier on the outside of the filament; only 0.9 percent total weight loss occurs after seven days of exposure. To slow erosion, moisture-resistant coatings, such as silane compounds, are applied during fiber manufacturing.

      Corrosion-resistant glass, known as C-glass or E-CR glass, loses much less of its weight when exposed to an acid solution than does E-glass. C-glass and S-glass show good resistance to sulfuric acid. However, E-glass and S-glass are much more resistant to sodium carbonate solution (a base) than is C-glass. A boron-free glass fiber, with performance and price comparable to E-glass, demonstrates greater corrosion resistance in acidic environments (like E-CR glass), higher elastic modulus and better performance in high-temperature applications than does E-glass.

    High-performance fibers

      High-strength fibers used in advanced composites include not only carbon, glass and aramid, but high-modulus polyethylene (PE), boron, quartz, ceramic, newer fibers such as poly p-phenylene-2,6-benzobisoxazole (PBO), and hybrid combinations, as well. The basic fiber forms for high-performance composite applications are bundles of continuous fibers called tows. A carbon fiber tow consists of thousands of continuous, untwisted filaments, with the filament count designated by a number followed by “K,” indicating multiplication by 1,000 (e.g., 12K indicates a filament count of 12,000). Tows may be used directly, in processes such as filament winding or pultrusion, or may be converted into unidirectional tape, fabric and other reinforcement forms.

      Carbon fiber — by far the most widely used fiber in high-performance applications — is produced from a variety of precursors, including polyacrylonitrile (PAN), rayon and pitch. The precursor fibers are heated and stretched to create the high-strength fibers. The first high-performance carbon fibers on the market were made from rayon precursor. PAN- and pitch-based fiber have replaced rayon-based fiber in most applications, but the latter’s “dogbone” cross-section often make it the fiber of choice for carbon/carbon (C/C) composites. PAN-based carbon fibers are the most versatile and widely used. They offer an amazing range of properties, including excellent strength — to 1,000 ksi — and high stiffness. Pitch fibers, made from petroleum or coal tar pitches, have high to extremely high stiffness and low to negative axial CTE. Their CTE properties are especially useful in spacecraft applications that require thermal management, such as electronic instrumentation housings.

      Typical aerospace-grade tow size ranges from 1K to 12K. PAN- and pitch-based 12K carbon fibers are available with a moderate (33 to 35 Msi), intermediate (40 to 50 Msi), high (50 to 70 Msi) and ultrahigh (70 to 140 Msi) modulus. (Modulus is the mathematical value that describes the stiffness of a material by measuring its deflection or change in length under loading.) Newer heavy-tow carbon fibers, with filament counts from 48K to 320K, are available at a lower cost than aerospace-grade fibers. They typically have a 33- to 35-Msi modulus and 550-ksi tensile strength and are used when fast part build-up is required, most commonly in recreational, industrial, construction and automotive markets. Heavy-tow fibers exhibit properties that approach those of aerospace-grade fibers but can be manufactured at a lower cost because of precursor and processing differences.
      Though stronger than glass or aramid fibers, carbon fibers are not only less impact-resistant but also can experience galvanic corrosion when in contact with metal. Fabricators overcome the latter problem by using a barrier material or veil ply — often fiberglass/epoxy — during laminate layup.

      Aramid fibers, composed of aromatic polyamide, provide exceptional impact resistance and tensile strength. Standard high-performance aramid fiber has a modulus of about 20 Msi and tensile strength of approximately 500 ksi. Renowned for performance in bulletproof vests and other armor and ballistic applications, the fiber has been in increasing demand, due, in part, to growth of the personnel protection and military armor markets spurred by conflicts around the world. Aramid’s properties also make the fiber an excellent choice for helicopter rotor blades, solid rocket motors, compressed natural gas (CNG) tanks and other parts that must withstand high stress and vibration.

      Commercially available high-strength, high-modulus polyethylene (PE) fibers are known for their extremely light weight, excellent chemical and moisture resistance, outstanding impact resistance, antiballistic properties and low dielectric constant. However, PE fibers have relatively low resistance to elongation under sustained loading, and the upper limit of their use temperature range is about 98°C/210°F. PE fiber composites are used in racing boat hulls, ski poles, offshore mooring ropes and other applications that require impact and moisture resistance and light weight, but do not necessitate extreme temperature resistance. At least one aircraft manufacturer now uses high-modulus PE fibers for the bulletproof insert in aircraft cockpit doors.

    Other fiber options

      Quartz fibers, while more expensive than glass, have lower density, higher strength and higher stiffness than E-glass, and about twice the elongation-to-break, making them a good choice where durability is a priority. Quartz fibers also have a near-zero CTE; they can maintain their performance properties under continuous exposure to temperatures as high as 1050°C/1920°F and up to 1250°C/2280°F for short time periods. Quartz fibers possess significantly better electromagnetic properties than glass, a plus when fabricating parts like aircraft radomes.

      Ceramic fibers offer high to very high temperature resistance but low impact resistance and relatively poor room-temperature properties. Typically much more expensive than other fibers, ceramic, like quartz, is the fiber of choice when its advantages justify the extra cost. One application of ceramic fibers is for flame-resistant veil material in laminates for aircraft interiors, which must withstand 1093°C/2000°F for at least 15 minutes without flame penetration.

      PBO is a relatively new fiber, with modulus and tensile strength almost double that of aramid fiber and a decomposition temperature almost 100°C/212°F higher. Suitable for high-temperature applications, it is currently used in protective ballistic armor, sporting goods, insulation and tire reinforcements.

      Also a newcomer, basalt fibers are inexpensive, golden brown-colored fibers, similar to glass, and currently produced primarily in Russia and Ukraine. Basalt exhibits better chemical and alkali resistance than glass, promising an additional choice for use in reinforcing concrete in infrastructure applications. Kamenny Vek (Dubna, Russia), Technobasalt (Kyiv, Ukraine) and Hengdian Group Shanghai Russia & Gold Basalt Fibre Co. (Shanghai, China) are three of the growing number of basalt fiber and basalt fiber product distributors.

      Boron fibers are five times as strong and twice as stiff as steel. They are made by a chemical vapor deposition process in which boron vapors are deposited onto a fine tungsten or carbon filament. Boron provides strength, stiffness and light weight, and possesses excellent compressive properties and buckling resistance. Uses for boron composites range from sporting goods, such as fishing rods, golf club shafts, skis and bicycle frames, to aerospace applications as varied as aircraft empennage skins, space shuttle truss members and prefabricated aircraft repair patches.

      Fiber hybrids capitalize on the best properties of various fiber types, and may reduce raw material costs. Hybrid composites that combine carbon/aramid or carbon/glass fibers have been used successfully in ribbed aircraft engine thrust reversers, telescope mirrors, driveshafts for ground transportation and infrastructure column-wrapping systems.

      Natural fibers — abaca, coconut, flax, hemp, jute, kenaf and sisal are the most common — are derived from the bast or outer stem of certain plants. Natural fibers are enjoying increased use because of their “green” attributes (less energy to produce), light weight, recyclability, good insulation properties and carbon dioxide neutrality (when burned natural fibers give off no more carbon dioxide than consumed while growing). Natural fibers also have the lowest density of any structural fiber but possess sufficient stiffness and strength for some applications.

      The automotive industry, in particular, is using these fibers in traditionally unreinforced plastic parts and even employs them as an alternative to glass fibers. Natural fiber-reinforced thermosets and thermoplastics are most often found in door panels, package trays, seat backs and trunk liners in cars and trucks. European fabricators hold the lead in use of these materials, in part because regulations now require their automobile components to be recyclable. Natural fibers can be incorporated into molded or extruded parts and, more recently, have been used in the direct long fiber injection (D-LFT) process where kenaf, flax and natural fiber/glass hybrids are used to reinforce polypropylene.

    Critical fiber sizing

      To achieve desirable properties in composite components, adhesion between fiber and matrix must be optimized. Adhesion requires sufficient saturation with resin (wetout) at the fiber/matrix interface. To ensure good adhesion, attention must be given to fiber surface preparation, such as the use of a surface finish or coupling agent, often termed sizing. Sizing, applied to glass and carbon filaments immediately after their formation, actually serves three purposes: As it enhances the fiber/matrix bond, it also eases processing and protects the fibers from breakage during processing. Although it accounts for only 0.25 to 6.0 percent of total fiber weight, sizing is a dynamic force in fiber reinforcement performance. Sizing chemistry distinguishes each manufacturer’s product and can be optimized for manufacturing processes, such as pultrusion, filament winding and weaving. For example, developments in sizing formulations have variously resulted in more cleanly chopped glass with reduced fuzz, glass that wets out more efficiently, and glass fibers that contain no chromium compounds.

      Historically, carbon fiber was sized only for compatibility with epoxy resin. Today, fiber manufacturers are responding to demands from fabricators and OEMs to produce carbon fiber forms compatible with a broader range of resins and processes, as carbon fiber use increases in applications outside the aerospace arena.

    Fiber reinforcement forms

      Rovings, the most common form of glass, can be chopped, woven or otherwise processed to create secondary fiber forms for composite manufacturing, such as mats, woven fabrics, braids, knitted fabrics and hybrid fabrics. Rovings are supplied by weight, with a specified filament diameter. The term yield is commonly used to indicate the number of yards in each pound of glass fiber rovings.
      One interesting area is the growing use of “stretch-broken” carbon fiber tows, commercialized by Hexcel (Dublin, Calif.). The process pulls carbon tow at differential speeds, which causes random breakage of individual filaments, yet leaves the filaments aligned. The breaks make the tow more formable and give it the ability to stretch under load, with greater strength properties than chopped, random fibers.

      Mats are nonwoven fabrics made from fibers held together by a chemical binder. They come in two distinct forms: chopped and continuous strand. Chopped mats contain randomly distributed fibers cut to lengths typically ranging from 38 mm to 63.5 mm/1.5 to 2.5 inches. Continuous-strand mat is formed from swirls of continuous fiber strands. Because their fibers are randomly oriented, mats are isotropic, that is, they possess equal strength in all directions. Chopped-strand mats provide low-cost reinforcement primarily in hand layup, continuous laminating and some closed-molding applications. Inherently stronger continuous-strand mat is used primarily in compression molding, resin transfer molding and pultrusion applications, and in the fabrication of preforms and stampable thermoplastics. Certain continuous-strand mats used for pultrusion and needled mats used for sheet molding eliminate the need for creel storage and chopping.

      Woven fabrics are made on looms in a wide variety of weights, weaves and widths. Wovens are bidirectional, providing good strength in the directions of yarn or roving axial orientation (0º/90º) and facilitate fast composite fabrication. However, the tensile strength of woven fabrics is compromised to some degree because fibers are crimped as they pass over and under one another during the weaving process. Under tensile loading, these fibers tend to straighten, causing stress within the matrix system.

      Varying weaves are used for bidirectional fabrics. In a plain weave, each fill yarn (i.e., the yarn oriented at right angles to the fabric length) alternately crosses over and under each warp yarn (the lengthwise yarn). Other weaves, such as harness, satin and basket weave, allow the yarn or roving to cross over and under multiple warp fibers (e.g., over two, under two). These weaves tend to be more drapable, that is, they are more pliable and conform more easily to curved surfaces than do plain weaves.

      Woven roving is relatively thick and is used for heavy reinforcement, especially in hand layup operations and tooling applications. Due to its relatively coarse weave, woven roving wets out quickly and is relatively inexpensive. Exceptionally fine woven fiberglass fabrics can be produced for applications such as reinforced printed circuit boards.

      Hybrid fabrics can be constructed with varying fiber types, strand compositions and fabric types. For example, high-strength strands of S-glass or small-diameter filaments may be used in the warp direction, while less-costly strands compose the fill. A hybrid also can be created by stitching woven fabric and nonwoven mat together.

      Multiaxials are nonwoven fabrics, made with unidirectional fibers laid atop one another in different orientations, and held together by through-the-thickness stitching, knitting or a chemical binder. The proportion of yarn in any direction can be selected at will. In multiaxial fabrics, the fiber crimp associated with woven fabrics is avoided because the fibers lie on top of each other, rather than crossing over and under. This makes better use of the fibers’ inherent strength and creates a fabric that is more pliable than a woven fabric of similar weight. Super-heavyweight nonwovens are available (up to 200 oz/yd2) and can significantly reduce the number of plies required for a layup, making fabrication more cost-effective, especially for large industrial structures. High interest in noncrimp multiaxials has spurred considerable growth in this reinforcement category.

      Braided fabrics are generally more expensive than woven fabrics, due to their more complex manufacturing process, but are typically stronger by weight than wovens. The strength comes from intertwining three or more yarns without twisting any two yarns around each other. Braids are continuously woven on the bias and have at least one axial yarn that is not crimped in the weaving process. This arrangement of yarns allows for highly efficient load distribution throughout the braid.

      Braids are available in both flat and tubular configurations. Flat braids are used primarily for selective reinforcement, such as to strengthen specific areas in pultruded parts. Tubular braids can be placed over a mandrel to produce hollow parts, such as windsurfing masts, hockey sticks, lamp posts and utility poles. Braid is increasingly competitive with other fabrics, due to declining manufacturing costs.

      Preforms are near-net shape reinforcement forms designed for use in the manufacture of particular parts by stacking and shaping layers of chopped, unidirectional, woven, stitched and/or braided fiber into a predetermined three-dimensional form. Complex part shapes may be closely approximated by careful selection and integration of any number of reinforcement layers in varying shapes and orientations. Because of their potential for great processing efficiency and speed, a number of preforming technologies have been developed, with the aid of special binders, heating and consolidation methods and the use of automated methods for spray up, orientation and compaction of chopped fibers.

      Resin-impregnated fiber forms, commonly called prepregs, are manufactured by impregnating fibers with a controlled amount of resin (thermoset or thermoplastic), using solvent, hot-melt or powder impregnation technologies. Prepregs can be stored in a “B-stage,” or partially cured state, until they are needed for fabrication. Prepreg tape or fabric is used in hand layup, automated tape laying, fiber placement and in some filament winding operations. Unidirectional tape (all fibers parallel), is the most common prepreg form. Prepregs made with woven fibers and other flat goods offer reinforcement in two dimensions and are typically sold in full rolls, although small quantities are available from some suppliers. Those made by impregnating fiber preforms and braids provide three-dimensional reinforcement.

      Prepregs deliver a consistent fiber/resin combination and ensure complete wetout. They also eliminate the need to weigh and mix resin and catalyst for wet layup. For most thermoset prepregs, drape and tack are “processed in” for easy handling, but they must be stored below room temperature and have out-time limitations; that is, they must be used within a given time period after removal from storage to avoid premature cure reaction. Thermoplastic prepregs do not suffer from such limitations, but without special formulation, they lack the tack or drape of thermoset prepregs and, therefore, are more difficult to form.

    Resin matrices: Thermosets

      Unsaturated polyester resins are the most widely used in commercial, mass-production applications, thanks to their ease of handling, good balance of mechanical, electrical and chemical properties and relatively low cost. Typically coupled with glass fiber reinforcements, polyesters adapt well to a range of fabrication processes and are most commonly used in open-mold sprayup, compression molding, resin transfer molding (RTM) and casting. Polyesters provide the primary resin matrix used in bulk molding compounds (BMC) and sheet molding compounds (SMC), materials used in compression molding (see “High-volume molding methods,” p. 26).

      The properties of polyester formulations can be modified to meet specific performance criteria, based on the selection of glycol and acid elements and reactive monomers (most commonly, styrene). Polyester resins are often differentiated in terms of their base ingredients. Orthopolyesters, for example, build on orthophthalic acid. Isopolyester resins have isophthalic acid as their essential ingredient, and exhibit superior chemical and thermal resistance compared to orthopolyesters. Terephthalic resins incorporate terephthalic acids and have been formulated for improved toughness, compared to traditional isopolyesters.

      Specially formulated, unreinforced polyester resins, known as gel coats, improve the impact and abrasion resistance and the surface appearance of the final product. These are applied to a mold surface and gelled before layup of the composite. In the tub and shower market, for instance, gel-coated fiberglass products have been dominant, and their use continues to grow, despite strong competition from glass/acrylic units made with polymethyl methacrylate (PMMA).

      Vinyl ester resins offer a bridge between lower-cost, rapid-curing and easily processed polyesters and higher-performance epoxy resins (described below). Vinyl esters shrink less during cure and outperform polyesters in chemically corrosive environments (e.g., chemical tanks) and in structural laminates requiring a high degree of moisture resistance (such as boat hulls and decks), which accounts, in part, for their higher price.

      Cure of these thermosets is exothermic; as they crosslink, they release heat. Fabricators can control the cure profile in terms of shelf life, pot life (the time prior to cure), gel time, cure temperature and viscosity through careful formulation of the catalyst package, which may include inhibitors, promoters and accelerators.

      For advanced composite matrices, the most common thermosets are epoxies, phenolics, cyanate esters (CEs), bismaleimides (BMIs) and polyimides.

      Epoxy resins contribute strength, durability and chemical resistance to a composite. They offer high performance at elevated temperatures, with hot/wet service temperatures up to 121°C/250°F. Epoxies come in liquid, solid and semisolid forms and typically cure by reaction with amines or anhydrides. Most commercial epoxies have a chemical structure based on diglycidyl ether of bisphenol A or creosol, and/or phenolic novolacs. Many aerospace applications use amine-cured, multifunctional epoxies that require cure at elevated temperatures. Toughening agents, e.g., thermoplastics and reactive rubber compounds, can be added to counteract brittleness.

      Phenolic resins are based on a combination of an aromatic alcohol and an aldehyde, such as phenol, combined with formaldehyde. They find application in flame-resistant aircraft interior panels and in commercial markets that require low-cost, flame-resistant and low-smoke products. Excellent char yield and ablative (heat-absorbing) characteristics have made phenolics long-time favorites for ablative and rocket nozzle applications. They have also proven successful in nonaerospace applications, notably in components for offshore oil and gas platforms, and in mass transit and electronics applications. Phenolics, however, release water vapor and formaldehyde during cure, which can produce voids in the composite. As a result, their mechanical properties are somewhat lower than those of epoxies and most other high-performance resins. Molds must be designed with adequate venting or a “breathe” step to allow the water vapor to escape.

      Cyanate esters (CEs) are versatile matrices that provide excellent strength and toughness, allow very low moisture absorption and possess superior electrical properties, compared to other polymer matrices, although at a higher cost. CEs feature hot/wet service temperatures to 149°C/300°F and are usually toughened with thermoplastics or spherical rubber particles. They process similarly to epoxies, but their curing process is simpler, thanks to CE’s viscosity profile and nominal volatiles. Applications range from radomes, antennae, missiles and ablatives to microelectronics and microwave products.

      Among the more exotic of resins, bismaleimide (BMI) and polyimide (close relatives, chemically) are used in high-temperature applications on aircraft and missiles (e.g., for jet engine nacelle components). BMIs offer hot/wet service temperatures (to 232°C/450°F), while some polyimides can be used to 371°C/700°F for short periods of time. Volatiles and moisture emitted during cure make polyimides more difficult to work with than epoxies or CEs; special formulation and processing techniques have been developed to reduce or eliminate voids and delaminations. Both BMIs and polyimides exhibit higher moisture absorption and lower toughness values than CEs or epoxies, but significant progress has been made in recent years to create tougher formulations.

      Polybutadiene resins offer good electrical properties and chemical resistance and have been used successfully as alternatives to epoxy in E-glass/epoxy composites typically used to mold thin-walled, glass-reinforced radomes.

      Benzoxazines, a subclass of phenolic resins, are formed by reacting a phenol with an aldehyde and an aromatic amine. While the chemistry has been known since the 1940s, Huntsman Advanced Materials (Woodland, Texas) has more recently developed a family of benzoxalines for advanced composites and electronics applications. Another, lesser known resin class is phthalonitriles, originally developed by the U.S. Naval Research Laboratory for very high temperature applications. Commercialized by Eikos Inc. (Franklin, Mass.), phthalonitriles have service temperatures approaching 371°C/700°F, and have been selected for high-temperature engine parts as well as submarine vessels.

    Resin matrices: Thermoplastic

      In contrast to crosslinking thermosets, whose cure reaction cannot be reversed, thermoplastics harden when cooled but retain their plasticity; that is, they will soften and can be reshaped repeatedly by reheating them above their processing temperature. Less-expensive thermoplastic matrices offer lower processing temperatures, but also have limited use temperatures. They draw from the menu of both engineered and commodity plastics, such as polyethylene (PE), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polyamide (PA or nylon) and polypropylene (PP). High-volume commercial products, such as athletic footwear, orthotics and medical prostheses, benefit from the toughness and moisture resistance of these resins, as do automotive air intake manifolds and other underhood parts.

      High-performance thermoplastic resins — polyetheretherketone (PEEK), polyetherketone (PEK), polyamide-imide (PAI), polyarylsufone (PAS), polyetherimide (PEI), polyethersulfone (PES), polyphenylene sulfide (PPS) and liquid crystal polymer (LCP) — function well in high-temperature environments and, when exposed to moisture, neither absorb water nor degrade. Reinforced with high-performance fibers, these resins exhibit lengthy prepreg shelf life without refrigeration and provide exceptional impact resistance and vibrational damping, although they present some processing challenges because of their high viscosity.

    Resin matrices: Thermoset or thermoplastic

      Polyurethane resins are available in both thermoset and thermoplastic formulations. Thermoset polyurethanes are used to pultrude tough new parts, such as marine sheet piling and electrical power poles, and to enhance the rigidity of automotive bumper fascias made by reaction injection molding (RIM; for information about this and other “Resin infusion processes,” see p. 24).

      Also available in either form are polyimide resins (the thermoset form of which is described above). In thermoplastic form, polyimides readily release volatiles under heat and pressure, producing parts with fewer voids.

      Polyurea polymer formulations are available for reinforced reaction injection molding (RRIM), with the mineral wollastonite as reinforcement. They were the first polymers to withstand the high temperatures in automotive painting processes and also provide a Class A finish.

      In this category are two resins that, in thermoplastic form, can be processed, like thermosets, at lower viscosities. A class of cyclic thermoplastic polyesters developed originally at General Electric Co. and marketed by Cyclics Corp. (Schenectady, N.Y.) offers easier processing. Thermoplastic polyester is broken down into a cyclic oligomer form that, when heated to a specified temperature, drops to a water-like viscosity — a significant aid to fiber wetout. When it is catalyzed and then cooled, the oligomer returns to more conventional viscosity and forms a long-chain, high-molecular-weight thermoplastic. The material offers the properties of a thermoplastic but can be processed like a thermoset. Another example is the family of patented thermoplastic polyurethanes (TPUs) developed around 2000 by Dow Chemical Co. (Midland, Mich.) and spun off in 2004 to Midland-based Fulcrum Composites. These TPUs have made possible the commercialization of a thermoplastic pultrusion process. Although pultrusion has been dominated by low-viscosity thermosets, the Dow TPUs have the ability to partially depolymerize at their processing temperature and rapidly repolymerize as they cool. In other words, the monomer molecules in the long polymer chains partially unlink as the resin pellets are heated and melted, then relink again when cooled. This development has made possible the production of pultruded profiles that can be postformed, via thermofoming. or overmolded (via extrusion and/or injection molding) to create products such as threaded rod, without resort to machining processes that damage the pultruded fibers.

    Other matrices: Carbon, metal and ceramic

      Perhaps the most exotic matrix, in part because it is neither thermoset nor thermoplastic, is pyrolized and densified noncontinuous carbon, which forms the matrix in carbon/carbon (C/C) composites. C/Cs withstand extremely high temperatures — nearly 1650°C/3000°F on space shuttle components — and also find use in aircraft and race car braking components, missile engines and exhaust nozzles, which can experience short-term service temperatures as high as 2760°C/5000°F.

      Metals (e.g., aluminum, titanium and magnesium) and ceramics (such as silicon carbide) are used as matrices, as well, for specialized applications, such as spacecraft components, where minimal CTE and an absence of outgassing are required. They also are used in engine components, where polymer matrices cannot offer the extremely high temperature resistance such applications require.

    Nanotechnology

      In the news for many years but primarily the province of theoreticians and researchers until recently, nanotechnologies are beginning to cross the threshold into commercialization. Several forms of nanostructures may play significant roles in the composites industry.

      Nanotech can be defined as technology at less than 100 nanometers. A nanometer (nm) is one billionth of a meter, or 80,000 times smaller than the thickness of a human hair. Nanoparticles are simply particles with diameters in the 1 nm to 100 nm range. Nanofibers are electro-spun whiskers with diameters in the 10 nm to 100 nm range and length-to-diameter ratios of 1,000:1 or more. Carbon nanotubes, the most well-known nanotubes, are made up of carbon atoms arranged in a hexagonal pattern and resemble thin cylinders of chicken wire. They can be single-walled or multi-walled. Fullerenes are closed molecular cages made of carbon atoms. For example, the C60 fullerene (a/k/a “buckyball”) is composed of 60 carbon atoms in a soccer ball shape. The extremely small size of such nanostructures, coupled with their extremely high surface-to-volume ratios (as much as 1:5), can lead to unusual characteristics that can have profound effects on performance.

      A few nanoscale technologies already have been commercialized for use as additives in the resin matrices of composite laminates. There is little doubt that applications for these products in advanced composites will eventually include improved catalysts for resin systems; nanoporous particles, which will increase adhesive bond strength; and several types of nanoparticles which can be used as additives to increase mechanical properties, such as wear resistance, conductivity, flame retardance, stiffness, and strength.

      One promising development, the work of performance materials developer Nanocomp Technologies Inc. (Concord, N.H.), is a reportedly ready-to-use textile material made from long carbon nanotubes. Available in both nonwoven sheet and yarn forms, the material could provide the practical utility that nanocomposites have promised but have not yet delivered. Real and potential applications range from body armor to structural composites as well as commercial energy storage and electronics thermal management. The nonwoven textile sheet measures about 2 ft2/0.19m2 square, weighs about 0.04 oz/1g, and ranges from 10 to 15 microns thick. After post-treatment with different chemistries, the fabric can be folded and bent repeatedly without breakage. The company hopes by the end of 2008, to upgrade to continuous, nonbatch production machinery, creating a pricing structure (“hundreds of dollars per kilogram”) that would put the material in the same ballpark as aerospace-grade carbon fiber.

    Part design criteria

      Designers of composite parts can choose from a wide variety of fiber reinforcements and resin systems. Knowledge of material properties is a prerequisite to satisfactory product design, but cost is a major factor, as well. Over-designed composites cannot compete with lower-cost, established material systems. The well-designed part not only employs the right materials and processes to meet application requirements, but, many times, is commercially competitive with other materials, when installation, maintenance and lifecycle costs are factored into the equation.

      As noted above, fiber reinforcements provide mechanical properties, such as stiffness and strength, and resin matrices provide physical characteristics, including toughness and resistance to impact, weather, fire, UV light and corrosive chemicals.

      A significant design consideration is fiber-to-matrix ratio, which is a determining factor in the ultimate weight and cost of the component and governs the extent to which performance properties inherent in the fiber reinforcement can be optimized in the part. Fiber-to-resin ratio can range from 20:80 for low-cost, nonstructural components to as high as 70:30 in some high-end pultrusion applications for structural use. A 60:40 or higher ratio is common in advanced composites.

      Three additional factors must be considered when designing with fiber: fiber type, form and orientation or architecture. Orientation refers to fiber direction in relation to the longest part dimension. Typically, fiber architecture is tailored in the direction of the primary loads placed on a structure, a design principle comparable to what civil engineers use to orient steel reinforcing bars in a concrete structure. Common orientations are parallel (longitudinal or 0°), circumferential (90°) and helical (usually ±33° to ±45°). However, fiber direction can vary greatly. For example, a 54° winding angle satisfies both the circumferential (hoop) and longitudinal (axial) strength requirements of most pipes and pressure vessels, usually manufactured by the filament winding process. However, if more stress is placed on the pipe in the axial direction, as is the case with an unsupported span, a ±20°/±70° fiber orientation will provide a stiffer bending modulus for increased axial strength.

      Composites, by nature, allow designers to tailor fiber architecture to match the performance requirements for a specific part. Laminates may be designed to be isotropic or anisotropic, balanced or unbalanced, symmetrical or asymmetrical — depending on the in-use forces a component must withstand (see “Glossary of Terms,” p. 47). Varied fiber orientation allows a range of wall-thickness variations, making it possible to develop lightweight, complex shapes and to produce large parts with integral reinforcing members.

      An understanding of layered or laminated structural behavior is vital to effective composite component design. Adhesion between laminate layers (called plies) is critical; poor adhesion can result in delamination under stress, strain, impact and load conditions. Ply layup designers must consider mechanical stresses/loads, adhesion, weight, stiffness, operating temperature and toughness requirements, as well as variables such as electromagnetic transparency and radiation resistance. Additionally, composite component design must encompass surface finish, fatigue life, overall part configuration, and scrap or rework potential, to name just a few of the many applicable factors.

      The intended fabrication method also will influence design. For instance, manufacturers of filament-wound or tape-layed structures use different reinforcement forms and build-up patterns than those used either for laminate panels layed up by hand or for vacuum-bag-cured prepreg parts.Resin transfer molding (RTM) accommodates three-dimensional preforms more easily than do some other manufacturing techniques. The varying benefits and limitations of these fabrication techniques provide designers a very flexible set of options, in their efforts to achieve optimum performance and economies.

      A common type of composite structure — sandwich construction — combines a lightweight core material with laminated composite skins (facesheets), similar to the construction of corrugated cardboard. These very lightweight panels have the highest stiffness-to-weight and strength-to-weight performance of all composite structures and are extremely resistant to bending and buckling. Suitable core materials include closed-cell foams, balsa wood and celled honeycomb in a variety of forms (aluminum, paper or plastic). Some foam cores are syntactic (i.e., containing hollow microspheres) for even lighter weight. Sandwich construction is used extensively on modern aircraft and boats as well as in applications such as cargo containers and modular buildings.

    Fabrication methods

      The most basic fabrication method for thermoset composites is hand layup, which typically consists of laying dry plies or prepreg plies by hand onto a tool to form a laminate stack. Resin is applied to the dry plies after layup is complete (e.g., by means of resin infusion) or, in a variation known as wet layup, each ply is coated with resin and “debulked” or compacted after it is placed.

      Several curing methods are available. The most basic is simply to allow cure to occur at room temperature. Cure can be accelerated, however, by applying heat, typically with an oven, and pressure, by means of vacuum. For the latter, a vacuum bag, with breather assemblies, is placed over the layup and attached to the tool, then evacuated before cure. The vacuum bagging process consolidates the plies of material and significantly reduces voids due to off-gassing that occurs as the matrix progresses through its chemical curing stages.

      Many high-performance thermoset parts require both heat and high consolidation pressure to cure, conditions that require the use of an autoclave. Autoclaves, generally, are expensive to buy and operate. Manufacturers equipped with autoclaves usually cure a number of parts simultaneously. Computer systems monitor and control autoclave temperatures, pressure, vacuum and inert atmosphere, which allows unattended and/or remote supervision of the cure process and maximizes efficient use of the technique.

      When heat is required for cure, the part temperature is “ramped up” in small increments, maintained at cure level for a specified period of time, then “ramped down” to room temperature, to avoid part distortion or warp caused by uneven expansion and contraction. When this curing cycle is complete and after parts are demolded, some parts go through a secondary freestanding postcure, during which they are subjected for a specific period of time to a temperature higher than that of the initial cure, to enhance crosslink density.

      Electron-beam (E-beam) curing holds promise as an efficient curing method for thin laminates. In E-beam curing, the composite layup is exposed to a stream of electrons that provide ionizing radiation, causing polymerization and crosslinking in radiation-sensitive resins. X-ray and microwave curing technologies work in a similar manner. A fourth alternative, ultraviolet (UV) curing, involves the use of UV radiation to activate a photoinitiator added to a thermoset resin, which, when activated, sets off a crosslinking reaction. UV curing requires light-permeable resin and reinforcements.

      An emerging technology is the monitoring of the cure itself. Dielectric cure monitors measure the extent of cure by gauging the conductivity of ions — small, polarized, relatively insignificant impurities resident in resins. Ions tend to migrate toward an electrode of opposite polarity, but the speed of migration is limited by the viscosity of the resin — the higher the viscosity, the slower the speed. As crosslinking proceeds during cure, resin viscosity increases. Other methods include dipole monitoring within the resin, the monitoring of micro-voltage produced by the crosslinking, monitoring of the exothermic reaction in the polymer during cure and, potentially, the use of infrared monitoring via fiber-optic technology.

    Open molding

      Open contact molding in one-sided molds is a low-cost, common process for making fiberglass composite products. Typically used for boat hulls and decks, RV components, truck cabs and fenders, spas, bathtubs, shower stalls and other relatively large, noncomplex shapes, open molding involves either sprayup or hand layup. In an open-mold sprayup application, the mold is first treated with mold release. If a gel coat is used, it is typically sprayed into the mold after the mold release has been applied. The gel coat then is cured and the mold is ready for fabrication to begin. In the sprayup process, catalyzed resin (viscosity from 500 cps to 1,000 cps) and glass fiber are sprayed into the mold using a chopper gun, which chops continuous fiber into short lengths, then blows the short fibers directly into the sprayed resin stream so that both materials are applied simultaneously. To reduce VOC emissions, piston pump-activated, non-atomizing spray guns and fluid impingement spray heads dispense gel coats and resins in larger droplets at low pressure. Another option is roller impregnators, which pump resin into a roller similar to a paint roller.

      In the final steps of the sprayup process, workers compact the laminate by hand with rollers. Wood, foam or other core material may then be added, and a second sprayup layer imbeds the core between the laminate skins. The part is then cured, cooled and removed from the reusable mold.

      Hand layup and sprayup methods are often used in tandem to reduce labor. For example, fabric might first be placed in an area exposed to high stress; then, a spray gun might be used to apply chopped glass and resin to build up the rest of the laminate. Balsa or foam cores may be inserted between the laminate layers in either process. Typical glass fiber volume is 15 percent with sprayup and 25 percent with hand layup.

    Resin infusion processes

      Ever-increasing demand for faster production rates has pressed the industry to replace hand layup with alternative fabrication processes and has encouraged fabricators to automate those processes wherever possible.

      A common alternative is resin transfer molding (RTM), sometimes referred to as liquid molding. RTM is a fairly simple process: It begins with a two-part, matched, closed mold, made of metal or composite material. Dry reinforcement (typically a preform) is placed into the mold, and the mold is closed. Resin and catalyst are metered and mixed in dispensing equipment, then pumped into the mold under low to moderate pressure through injection ports, following predesigned paths through the preform. Extremely low-viscosity resin is used in RTM applications for thick parts, to permeate preforms quickly and evenly before cure. Both mold and resin can be heated, as necessary, for particular applications. RTM produces parts that do not need to be autoclaved. However, once cured and demolded, a part destined for a high-temperature application usually undergoes postcure. Most RTM applications use a two-part epoxy formulation. The two parts are mixed just before they are injected. Bismaleimide and polyimide resins are also available in RTM formulations. “Light RTM” is a variant of RTM that is growing in popularity. Low injection pressure, coupled with vacuum, allow the use of less-expensive, lightweight two-part molds.

      The benefits of RTM are impressive. Generally, dry preforms for RTM are less expensive than prepreg material and can be stored at room temperature. The process can produce thick, near-net shape parts, eliminating most post-fabrication work. It also yields dimensionally accurate complex parts with good surface detail and delivers a smooth finish on all exposed surfaces. It is possible to place inserts inside the preform before the mold is closed, allowing the RTM process to accommodate core materials and integrate “molded in” fittings and other hardware into the part structure during the molding process. Moreover, void content on RTM’d parts is low, measuring in the 0 to 2 percent range. Finally, RTM significantly cuts cycle times and can be adapted for use as one stage in an automated, repeatable manufacturing process for even greater efficiency, reducing cycle time from what can be several days, typical of hand layup, to just hours — or even minutes.

      In contrast to RTM, where resin and catalyst are premixed prior to injection under pressure into the mold, reaction injection molding (RIM) injects a rapid-cure resin and a catalyst into the mold in two separate streams; mixing and the resultant chemical reaction both occur in the mold instead of in a dispensing head. Automotive industry suppliers combine structural RIM (SRIM) with rapid preforming methods to fabricate structural parts that don’t require a Class A finish. Programmable robots have become a common means to spray a chopped fiberglass/binder combination onto a vacuum-equipped preform screen or mold. Robotic sprayup can be directed to control fiber orientation. A related technology, dry fiber placement, combines stitched preforms and RTM. Fiber volumes of up to 68 percent are possible, and automated controls ensure low voids and consistent preform reproduction, without the need for trimming.

      Vacuum-assisted resin transfer molding (VARTM) refers to a variety of related processes, which represent the fastest growing new molding technology. The salient difference between VARTM-type processes and standard RTM is that in VARTM, resin is drawn into a preform through use of a vacuum, rather than pumped in under pressure. VARTM does not require high heat or pressure. For that reason, VARTM operates with low-cost tooling, making it possible to inexpensively produce large, complex parts in one shot.

      In the VARTM process, fiber reinforcements are placed in a one-sided mold, and a cover (rigid or flexible) is placed over the top to form a vacuum-tight seal. The resin typically enters the structure through strategically placed ports. It is drawn by vacuum through the reinforcements by means of a series of designed-in channels that facilitate wetout of the fibers. Fiber content in the finished part can run as high as 70 percent. Current applications include marine, ground transportation and infrastructure parts.

      Resin film infusion (RFI) is a hybrid process in which a dry preform is placed in a mold on top of a layer or interleaved with layers of high-viscosity resin film. Under applied heat, vacuum and pressure, the resin is drawn into the preform, resulting in uniform resin distribution, even with high-viscosity, toughened resins, because of the short flow distance.

    High-volume molding methods

      Compression molding is a high-volume thermoset molding process that employs expensive but very durable metal dies. It is an appropriate choice when production quantities exceed 10,000 parts. As many as 200,000 parts can be turned out on a set of forged steel dies, using sheet molding compound (SMC), a composite sheet material made by sandwiching chopped fiberglass between two layers of thick resin paste. To form the sheet, the resin paste transfers from a metering device onto a moving film carrier. Chopped glass fibers drop onto the paste, and a second film carrier places another layer of resin on top of the glass. Rollers compact the sheet to saturate the glass with resin and squeeze out entrapped air. The resin paste initially is the consistency of molasses (between 20,000 and 40,000 cps); over the next three to five days, its viscosity increases and the sheet becomes leather-like (about 25 million cps), ideal for molding.

      When the SMC is ready for molding, it is cut into smaller sheets and the charge pattern (ply schedule) is assembled on a heated mold (121°C to 262°C or 250°F to 325°F). The mold is closed and clamped, and pressure is applied at 24.5 bar to 172.4 bar (500 psi to 2,500 psi). Material viscosity drops and the SMC flows to fill the mold cavity. After cure, the part is demolded manually or by integral ejector pins.

      A typical low-profile (less than 0.05 percent shrinkage) SMC formulation for a Class A finish consists, by weight, of 25 percent polyester resin, 25 percent chopped glass, 45 percent fillers and 5 percent additives. Fiberglass thermoset SMC cures in 30 to 150 seconds and overall cycle time can be as low as 60 seconds. Other grades of SMC include low-density, flexible and pigmented formulations. Low-pressure SMC formulations now on the market offer open molders a low-capital investment entry into closed mold processing with near-zero VOC emissions and the potential for very high-quality surface finish.

      Automakers are exploring carbon fiber-reinforced SMC, hoping to take advantage of carbon’s high strength and stiffness-to-weight ratio in exterior body panels and other parts. Newer, toughened SMC formulations help prevent microcracking, a phenomenon that can lead to paint “pops” during the painting process (surface craters caused by outgassing, the release of gasses trapped in the microcracks during oven cure).

      Composites manufacturers in industrial markets are formulating their own resins and compounding SMC in-house to meet needs in specific applications that require UV, impact and moisture resistance, and have surface-quality demands that drive the need for customized material development.

      Injection molding is a fast, high-volume, low-pressure, closed process using, most commonly, filled thermoplastics, such as nylon with chopped glass fiber. In the past 20 years, however, automated injection molding of BMC has taken over some markets previously held by thermoplastic and metal casting manufacturers. For example, the first-ever BMC-based electronic throttle control (ETC) valves (previously molded only from die-cast aluminum) debuted on engines in BMW’s Mini and the Peugeot 207, taking advantage of dimensional stability offered by a specially-formulated BMC supplied by TetraDUR GmbH (Hamburg, Germany), a subsidiary of Bulk Molding Compounds Inc. (BMCI, West Chicago, Ill.).

      In the injection molding process, a ram- or screw-type plunger forces a metered shot through a heated barrel and injects it (at 5,000 to 12,000 psi) into a closed, heated mold. In the mold, the liquefied BMC flows easily along runner channels and into the closed mold. After cure and ejection, parts need only minimal finishing. Injection speeds are typically one to five seconds, and as many as 2,000 small parts can be produced per hour in some multiple-cavity molds.

      Parts with thick cross-sections can be compression-molded or transfer-molded with BMC. Transfer molding is a closed-mold process wherein a measured charge of BMC is placed in a pot with runners leading to the mold cavities. A plunger forces the material into the cavities, where the product cures under heat and pressure.

      Filament winding is a continuous fabrication method that can be highly automated and repeatable with relatively low material costs. A long, cylindrical tool called a mandrel is suspended horizontally between end supports, while the “head” — the fiber application instrument — moves back and forth along the length of a rotating mandrel, placing fiber onto the tool in a predetermined configuration. Computer-controlled filament-winding machines are available, equipped with from 2 to 12 axes of motion.

      In most thermoset applications, the filament winding apparatus passes the fiber material through a resin “bath,” just before the material touches the mandrel. This is called “wet winding.” Towpreg, that is, continuous fiber pre-impregnated with resin, also be wound, eliminating the need for an onsite resin bath. In a slightly different process, fiber is wound without resin (“dry winding”). The dry shape is then removed and used as a preform in another molding process, such as RTM.

      Following oven or autoclave curing, the mandrel may remain in place and become part of the wound component or it may be removed. One-piece cylindrical or tapered mandrels, usually of simple shape, are pulled out of the part with mandrel extraction equipment. Some mandrels, particularly in more complex parts, are made of soluble material and may be dissolved and washed out of the part. Others are collapsible or built from several parts that allow disassembly and removal in smaller pieces. Filament-winding manufacturers often “tweak” or slightly modify off-the-shelf resin to meet specific application requirements. Some manufacturers develop their own resin formulations.

      In thermoplastics winding, material is in prepregged form, so a resin bath is not needed. Material is heated as it is wound onto the mandrel — a process known as curing “on the fly” or “in situ consolidation.” The prepreg is heated, layed down, compacted, consolidated and cooled in a single, continuous operation. Thermoplastic prepregs eliminate autoclave curing (cutting costs and size limitations), reduce raw material costs and the resulting parts can be reprocessed to correct flaws.

      Filament winding yields parts with exceptional circumferential or “hoop” strength. The highest-volume single application of filament winding is golf club shafts. Fishing rods, pipe, pressure vessels and other cylindrical parts comprise most remaining business.

      Pultrusion, like RTM, has been used for decades with glass fiber and polyester resins, but in the last ten years the process also has found applications in the advanced composites industry. In this relatively simple, low-cost, continuous process, the reinforcing fiber (usually roving, tow or continuous mat) is typically pulled through a heated resin bath, then formed into specific shapes as it passes through one or more forming guides or bushings. The material then moves through a heated die, where it takes its net shape and cures. Further downstream, after cooling, the resulting profile is cut to desired length. Pultrusion yields smooth finished parts that typically do not require any post-processing.

      A wide range of continuous, consistent, solid and hollow profiles are pultruded, and the process can be custom-tailored to fit specific applications.

      Tube rolling is a long-standing composites manufacturing process for producing finite-length tubes and rods. It is particularly applicable to small-diameter cylindrical or tapered tubes in lengths up to 20 ft/6.2m. Tubing diameters up to 6 inches can be rolled efficiently. Typically, a tacky prepreg fabric or unidirectional tape is used, depending on the part. The material is precut in patterns that have been designed to achieve the requisite ply schedule and fiber architecture for the application. The pattern pieces are laid out on a flat surface and a mandrel is rolled over each one under applied pressure, which compacts and debulks the material. When rolling a tapered mandrel — e.g., for a fishing rod — only the first row of longitudinal fibers falls on the true 0° axis. To impart bending strength to the tube, the fibers must be continuously reoriented by repositioning the pattern pieces at regular intervals.

      The fiber placement process automatically places multiple individual preimpregnated tows onto a mandrel at high speed, using a numerically controlled placement head to dispense, clamp, cut and restart each tow during placement. Minimum cut length (the shortest tow length a machine can lay down) is the essential ply-shape determinant. The fiber placement heads can be attached to a 5-axis gantry or retrofitted to a filament winder or delivered as a turnkey custom system. Machines are available with dual mandrel stations to increase productivity. Advantages of fiber placement include processing speed, reduced material scrap and labor costs, parts consolidation, and improved part-to-part uniformity. The process is often used to produce large thermoset parts with complex shapes.

      Tape laying is an even speedier automated process in which prepregged tape, rather than single tows, is laid down continuously to form parts. It is often used for parts with highly complex contours or angles. Tape layup is versatile, allowing breaks in the process and easy direction changes, and can be adapted for both thermoset and thermoplastic materials. Capital expenditures for computer-driven, automated equipment can be significant, however.

      Suitable for both simple and complex parts, thermoset tape laying is the current method of choice for wingskin panels on the F-22 Raptor fighter jet and the new Boeing 787 Dreamliner.
      Centrifugal casting of pipe from 1 inch/25 mm to 14 inches/356 mm in diameter is an alternative to filament winding for high-performance, corrosion-resistant service. In cast pipe, 0°/90° woven fiberglass provides both longitudinal and hoop strength throughout the pipe wall and brings greater strength at equal wall thickness compared to multiaxial fiberglass wound pipe. In the casting process, epoxy or vinyl ester resin is injected into a 150G centrifugally spinning mold, permeating the woven fabric wrapped around the mold’s interior surface. The centrifugal force pushes the resin through the layers of fabric, creating a smooth finish on the outside of the pipe, and excess resin pumped into the mold creates a resin-rich, corrosion- and abrasion-resistant interior liner.

      Fiber-reinforced thermoplastic shapes now can be produced by extrusion, as well. Breakthrough material and process technology has been developed with long-fiber glass-reinforced thermoplastic (ABS, PVC or polypropylene) composites to provide profiles that offer a tough, low-cost alternative to wood, metal and injection-molded plastic parts used in office furniture, appliances, semitrailers and sporting goods. A huge emerging market is extruded thermoplastic/wood flour (or other additives, such as bast fibers or fly ash) composites, used to simulate wood decking, siding, window and door frames and fencing (see “Construction,” p. 36).

    Tooling considerations

      The molds used for forming composites, also known as tools, can be made from virtually any material. For parts cured at ambient or low temperature, or for prototyping, where tight control of dimensional accuracy isn’t required, materials such as fiberglass, high-density foams, machinable epoxy “boards” or even clay or wood/plaster models are often suitable. Tooling costs and complexity increase as the part performance requirements and the number of parts to be produced go up. High-rate production tools are generally made of robust metals that can stand up to repeated cycles and maintain good finish and dimensional accuracy.

      The molds in which high-performance composite parts are formed can be made from carbon fiber/epoxy, monolithic graphite, castable graphite, ceramics or metals. Each material offers unique capabilities and drawbacks. Sometimes called “hard” tooling, ceramic and metal tooling is relatively heavy and able to withstand many thousands of production cycles. Composite tools, sometimes called “soft” tooling, are more vulnerable to wear and typically find service in low-volume production.

      Steel and aluminum are less expensive and more readily available than high-performance metal alloys, but during autoclave cure, the CTE mismatch is often too extreme for compatibility with most advanced composite parts. Higher-priced metal alloys, such as Invar, offer closer CTE matches.

      Composite tools made from traditional tooling prepregs offer several advantages, among them a CTE close to the part CTE, helping the part maintains dimensional integrity during cure. Plus, in relatively short-run applications, several suitable tools can be made with composite materials for less than the cost of a single hard tool. Like those made on hard tooling, parts made on composite tools can be cured in an autoclave or oven, or by integral heating, in which individual heating elements are placed inside the tool.

      One such product in this category is HexTOOL from Hexcel (Dublin, Calif.), a machinable carbon fiber/bismaleimide (BMI) composite tooling material. It’s comprised of prepreg strips randomly distributed onto a release paper to form a larger mat. After layup and cure, it can be machined like metal, has a coefficient of thermal expansion to match carbon/epoxy parts and can survive 500 autoclave cycles, all with a build and cost time comparable to existing alternatives.

      Commercialized tooling design software is reducing the time it takes to model and manufacture a tool — including back-up structure — in some cases, by 80 percent. New inspection systems give tooling suppliers and fabricators a way to verify a tool’s dimensional accuracy prior to and during production. In recent years, a variety of low-cost modeling materials that maintain dimensional stability at higher temperatures have made inroads into traditional toolmaking.

      No matter what the tooling material, the importance of mold release agents cannot be over-emphasized. Releases create a barrier between the mold and part, preventing part/mold adhesion and facilitating part removal. For open molding, most releases today are waxes or are based on polymer chemistry. Of these, most are polymers in solvent-based carrier solutions, such as an aliphatic hydrocarbon blend. Some manufacturers prefer naptha-based releases, which have longer shelf life and faster evaporation rates and are considered to be less damaging to composite tool surfaces. Increasingly strict environmental regulations encouraged development of water-based release agents, which do not produce VOCs and clean up more easily, with less skin irritation.

      Semipermanent polymer mold release systems enable multiple parts to be molded and released with a single application, in contrast to traditional paste waxes that need to be reapplied for each part. Semipermanents, which are preferred for better control over VOC emissions, have been formulated specifically to meet the needs of resin transfer molding (RTM) and other closed mold processes.

      Internal release agents, added to the resin or gel coat and used instead of or in addition to external agents on the mold surface, further reduce emissions, and have negligible effects on a part’s physical properties and surface finish. Internal release formulations are required for pultrusion processing, because the part is pulled continuously through the die, allowing no opportunity for intermittent application of external releases to the die surface.

    Safety/environmental concerns

      Fabricators and OEMs must address health, safety and environmental concerns when producing and handling composite materials. Their methods for maintaining a safe workplace include periodic training, adherence to detailed handling procedures, maintenance of current toxicity information, use of protective equipment (gloves, aprons, dust-control systems and respirators) and development of company monitoring policies. Both suppliers and OEMs are working to reduce emissions of highly volatile organic compounds by reformulating resins and prepregs and switching to water-dispersible cleaning agents.

      The U.S. Environmental Protection Agency has continued to strengthen its requirements to meet the mandates of the Clean Air Act Amendments, passed by Congress in 1990. Specifically, the agency’s goal is to reduce the emission of hazardous air pollutants or HAPs, a list of approximately 180 volatile chemicals considered to pose a health risk. Some of the compounds used in resins and released during cure contain HAPs. In early 2003, the EPA enacted regulatory requirements specifically for the reinforced plastic composites industry, requiring emission controls using maximum achievable control technology, or MACT. The regulations took effect in early 2006.

    Repair considerations

      As more composite materials find a place on aircraft, boats, bridges and hundreds of other applications where part replacement is difficult and expensive, OEM engineers are considering the repairability of structural and secondary composite components during the initial design phase.
      According to a recent report by Aerostrategy Management Consulting (Ann Arbor, Mich. and Amersham, Buckinghamshire, U.K.) spending by air transport maintenance and repair organizations (MROs) on air transport maintenance materials was $16 billion (USD) and likely to increase to $20 billion by 2011. Today, composite repair materials account for a fraction of that: $15 million to $25 million annually. With the recent roll out of Boeing’s 787, MROs soon will have to repair planes that are 50 percent composites by weight. One answer may be automated repairs.

      Publicly unveiled in 2005, the Inspection and Repair Preparation Cell (IRPC) concept is the result of an initiative begun in 1999 to improve aircraft repair practices and reduce repair time through automation. The ongoing work of a consortium of companies championed by American GFM (AGFM, Chesapeake, Va., a member of the GFM machine tool organization headquartered in Steyr, Austria), the IRPC is designed to integrate into a single, automated work cell a variety of tasks regularly performed manually by MROs. An IRPC “cell,” as currently conceived, will include a 3-D digitizing station, nondestructive testing (NDT) capability, radio frequency identification (RFID) tagging, automated machining equipment (to remove damage and cut out repair plies and core plugs) and advanced in-cell collision avoidance technologies. (For more on the IRPC concept, visit www.compositesworld.com/hpc/issues/2007/March/111325.)

      Other composite components that require repair include heavy truck front ends and other automotive and marine products. Whether damage is minor or extensive, training and specific repair materials are available. A number of materials suppliers provide kits containing low- or room-temperature curing adhesives and potting compounds formulated especially for onsite repair, along with low-quantity dry fiber or prepreg and vacuum-bagging materials. At least a dozen private training companies nationally offer from one- to ten-day courses in composite repair.
      Composites are increasingly being used to repair structures with other materials, such as bridge beams, bridge decks, parking garages and pipelines, including underground systems. A number of specialty contractors, like Structural Preservation Systems (Hanover, Md.) and Fyfe Co. (San Diego, Calif.) use composite fabrics and precured strips to add load capacity and strength to floors and beams. Composites also can be used to repair product pipelines, such as natural gas and petroleum pipes at oil refineries and offshore platforms.

    Future composites engineers

      To support the composites industry, an increasing number of colleges and universities offer composite materials and design courses within mechanical, chemical and civil engineering degree programs. Educators have networked progressively with suppliers and OEMs, cosponsoring projects that give students hands-on experience in materials evaluation and procurement, part design and fabrication. A notable recent effort is The Initiative, a “transferable model” for composites training developed by an unusual alliance in Maine that includes government entities, educational institutions, private trainers, and associations, such as the Maine Boat Building Assn. (MBBA), Maine Composites Alliance (MCA) and the ACMA. This consortium has acquired facilities, developed formal curricula and will issue Certifications for Composites Training. The group’s ultimate goal is to create a two-year Science Degree in Composites. More importantly, proponents say that, with the federal funding behind it, the concept is a repeatable blueprint that could be used in any U.S. state or region. (To read more, visit www.compositesworld.com/ct/issues/2007/October/112059.)

    A multitude of markets

      The overall outlook for the composites industry remains healthy, driven by developments in the commercial aircraft market — the advent of Boeing’s 787 and Airbus Industrie’s (Toulouse, France) A380, the future arrivals of the Airbus A400M military transport and its mid-sized A350 XWB, and a host of general aviation aircraft, all with unprecedented levels of composite components. The cautious optimism of 2007 has segued to confidence, especially among suppliers and users of carbon fiber. But in the short-term, lulls in the U.S. boating industry and the overpriced housing market in the last half of 2007, the result of slowing overall economic growth, tempered the mood in some quarters, but were considered by some to be temporary setbacks. Also, resin prices continued to uptick in 2007 as manufacturers sought relief from continued high prices for crude oil.

      According to figures compiled by the Freedonia Group Inc. (Cleveland, Ohio), demand for reinforced plastics will increase to more than 4.2 billion lb by the year 2009, a market value of $6.7 billion (USD). During that period, manufacturers of composites are expected to consume 2.7 billion lb of resin and 1.5 billion lb of reinforcements.

      Boatbuilding:

        Despite the 2007 lull, composites continue to offset wood and aluminum construction in boat hulls, decks and superstructures. Market research firm Freedonia Group (Cleveland, Ohio) estimates that global demand for recreational boating products will grow 7 percent annually to $33 billion by 2010. New boat sales, which hit bottom in 2003, rebounded in 2004 and since have grown at roughly prerecession rates, according to the National Marine Manufacturer’s Assn. Although sales for outboard and inboard motorboats were down in 2006, new boat unit sales overall were up 6 percent. North America is still the largest recreational boating market, but Europe is the fastest growing. U.S. demand for recreational boating products should increase 4.8 percent annually through 2009 to $16.7 billion. Open molding continues to give place to closed molding. Although the market is dominated by glass fiber-reinforced polyesters and vinyl esters, boatbuilders are employing carbon fiber reinforcement not molding. Although the market is dominated by glass fiber-reinforced polyesters and vinyl esters, boatbuilders are employing carbon fiber reinforcement not only in sailing yacht rigging systems (masts, shrouds, stays and spreaders) where it has become the standard, but in the upper deck structures of megapoweryachts, primarily in Europe, to decrease topside weight and increase boat stability. Observers, however, see breakout growth potential for carbon, for the same reasons, in the much larger 20-ft to 40-ft cruiser boat segment. Additionally, some believe carbon has great potential in military boats (see photo and caption on p. 26). Estimates of current carbon fiber use range from 450,000 lb to 590,000 lb (227 to 267 metric tonnes) per year.

      Automotive and transportation:

        Rising fuel prices have provided greater motivation for automotive OEMs to lightweight their vehicles to improve fuel economy. Composites continue to be attractive replacements for steel in automotive body panels, structural components and under-the-hood parts. But composites for horizontal body panels (previously considered impractical) made the biggest splash in 2007. Fuel-cell powered concept cars — GM’s Volt, Hundai’s QarmaQ and Ford’s Focus FCX premiered with composites technology that may be a harbinger of a near-future automotive lightweighting frontier. The Focus FCX features a lightweight carbon composite hood, while on the QarmaQ, the lightweight hood (similar to that on the Volt) was hot-tool compression molded from high-performance thermoplastic composite (HPPC), which is a product of GE Plastics and partner Azdel Inc. (Forest, Va.). Developed specifically to make reinforced thermoplastics practical for horizontal body panels, HPPC consists of a sandwich of Azdel Superlite glass mat thermoplastic between two-ply skins of continuous unidirectional glass fiber wet out with thermoplastic resin, For the QarmaQ hood, the Superlite core resin is Valox iQ, GE’s new “green” polyester made with feedstock components extracted from reclaimed polyethylene terephthalate (PET) soda bottles. The skins are wet out with Xenoy polycarbonate/polybutylene terephthalate resin blend. The latter has sufficient service temperature to withstand online painting at 210°/410°F.

      Corrosion-resistant applications:

        The direct cost of metallic corrosion in the U.S. alone is estimated at $300 billion per year by CC Technologies Laboratories Inc. (Dublin, Ohio) with support from NACE International (the National Association of Corrosion Engineers). Every sector of the economy has significant corrosion costs, including water and sewer piping systems, highways and bridges, electrical utilities and industrial plants. Corrosion-resistant composite materials are ideally suited to replace metal structures. Demand for fiber-reinforced polymer (FRP) composites to replace expensive stainless steel and high-nickel alloy scrubbers and chimneys (called stacks) that remove sulpher dioxide SO2 from flue gas emissions in coal-burning plants is exceeding supply as coal-burning power plants in the U.S. push toward compliance with the U.S. Environmental Protection Agency (EPA) Clean Air Interstate Rule (CAIR). CAIR calls for a reduction in emissions of 70 percent by 2015. Innovative new technologies for stack construction include computer-controlled, stationary vertical winding equipment that can field-wind preprogrammed sections of a stack around a rotating mold. Winding systems are available for tanks up to 124-ft/38m diameter, according to Robert Brady, of engineering consultancy The Brady Group (Portland, Ore.). As each section is wound, it can be jacked up above the mold to permit the next section to be wound using the same mold. This “jack-and-wind” process can be repeated until the stack reaches its design height — in some cases, more than 500 ft/152m.Cured-in-place pipe (CIPP) rehabilitation technology is a burgeoning market that eliminates the disruptive digging that is otherwise required to repair underground water/wastewater piping. Currently, several CIPP processes use nonwoven glass/polyester mat that, when wet out with resin, bond with the inside surface of existing pipe, forming a tough, seamless, corrosion-resistant liner. UV-curable CIPP technology, developed in Europe and now gaining attention in the U.S., eliminates the need for hot water or steam cure and the equipment necessary to heat it, and ends concern about the presence of styrenated water in the flow media following cure. International Pipe Lining Technologies (IPL, San Diego, Calif.) and Reline America (Saltville, Va.) offer UV-curable liners in diameters up to 48 inches/1.2m. The lining systems rely on stitch-bonded, light transparent fiberglass fabric wetout with resin formulated for ultraviolet (UV) cure. Cure is effected by a UV light “train” — a collection of UV lamps on a wheeled device (see photo, p. 28) — which is pulled through the pre-impregnated CIPP at a speed calculated to provide UV energy at an intensity and exposure duration sufficient to completely through-cure the laminate.

      Construction:

        Composite materials continue to play an increasingly significant role in construction, primarily in residential housing applications. However, the U.S. housing market, which was strong in 2005 and 2006, suffered from overpricing in 2007. Despite that, composites are having a strong showing in the still strong remodeling market, on the strength of wood plastic composites (WPCs). According to The Freedonia Group LLC (Cleveland, Ohio), demand for wood-filled thermoplastic composite lumber is unprecedented, especially in deck board and railing, molding and trim, fencing and door and window components. According to Freedonia’s Composite & Plastic Lumber report (issued in February 2006), WPC lumber production is forecast to expand nearly 14 percent per annum through 2009 to 2.7 billion lb per year. WPC deck board, alone, accounts for about 50 percent of overall WPC volume and has staked claim to a good piece of the $4.6 billion U.S. decking market, increasing from 4 percent in 1996 to 14 percent in 2006. That percentage is expected to grow to 23 percent by 2011 and 32 percent by 2016. Despite the weaker new housing market in the U.S. going into 2008, the decking market is relatively stable because more than 85 percent of demand is generated through inherently less cyclical remodeling and repair activity. According to strategic consulting firm Principia Partners (Exton, Pa.), approximately 88 percent of WPC decking is from R&R. WPC sales are expected to hold: U.S. demand for decking is projected to advance 2.2 percent per year through 2011 reports Freedonia in its latest Wood & Competitive Decking report (July 2007).

      Civil infrastructure:

        More than 250,000 deficient or obsolete structures, such as bridges and parking garages, need repair, retrofit or replacement in the U.S. alone. Glass, glass/aramid hybrids and carbon fibers, used with epoxy resin, continue to find application as cost-effective column-wrapping and jacketing systems for seismic and structural upgrading (see “Repair considerations,” p. 33).Fiberglass composites are finding niche applications in areas such as stay-in-place concrete forms, reinforcing rebar, bridge decks, wind fairings and enclosures, as well as entire bridges. Exhibiting corrosion resistance, light weight (approximately one-fifth the weight of steel), high strength and ease of installation, composite materials are gradually being accepted as alternatives to traditional materials to reduce dead load and extend structure life. Carbon fibers are finding a niche here as well, particularly in precast concrete products such as those provided by TechFab LLC (Anderson, S.C.) for architectural cladding, insulating sandwich panels, hardwall panels and double tees.Governments and engineering associations worldwide are cooperating to standardize workable international design parameters, and the composites industry is forging critical alliances with the civil engineering community and associations. A notable example is the American Concrete Institute (ACI) and CERF (Civil Engineering Research Foundation) which published its Guide Test Methods for Fiber-Reinforced Polymers for Reinforcing or Strengthening Concrete Structures (ACI 440.3R-04) in 2006 and is working on a state-of-the-art document that addresses the durability of fiber-reinforced polymers when they are used in conjunction with concrete.Bridge decks, a continuing focus of composites development, have been dominated by glass-fiber-reinforced pultruded components. However, other composite designs made some headlines in 2007.A carbon fiber bridge, manufactured by FiberCore Europe (Rotterdam, The Netherlands) and installed mid-year in the Dutch town of Dronton, featured an unusual carbon fiber-reinforced sandwich structure. This bridge, 24.5m long, 5m wide and weighing but 12 metric tonnes, features a deck with a honeycomb core laid in a sinusoidal pattern, with facesheets featuring 3 tons of Panex 35 fiber (Zoltek Inc., St. Louis, Mo.) impregnated with vinyl ester resin from DSM Resins (Heerlen, The Netherlands). An adaptable tooling system allows FiberCore to accommodate a variety of lengths, widths and other dimensions. Fibercore says the cost of the bridge is competitive with concrete and weighs nine times less.Meanwhile, Composite Advantage (Dayton, Ohio) fabricated 14,000 ft2 (1,300m2) of composite decking for the Anacostia River Walk pedestrian bridge, which will span an active railroad corridor and connect bicycle and pedestrian trails near Washington, D.C.In contrast to composite bridge designs that must compete with less costly concrete or steel proposals, the Anacostia bridge is a cost-competitive hybrid design that incorporates an integrally molded sandwich deck supported by steel girders. The deck’s sandwich construction, features a z-directional composite-reinforced foam core made by WebCore Technologies (Miamisburg, Ohio) molded on simple, reconfigurable tooling.

      Oil and gas:

        As the price of oil on the world market continues to climb, and as yet untapped land and shallow offshore oil reserves become a rarity, oil exploration companies are striking out into deepwater, tapping reserves beneath the ocean floor a mile or more below the water’s surface. As a result, demand for strong yet lightweight materials able to stand up to the harsh subsea environment has spiked, with a corresponding peak of interest in composites. As the mid-year point in 2007 approached, that interest translated into action on many fronts, pushing several key projects beyond R&D and proposals and test installations into production for “live” projects.

        Although production of composite risers — a hoped-for mega market for carbon fiber composites in the offshore oil arena — has yet to be realized, carbon is making headway in a key deepsea well technology, the umbilical. An umbilicals is a bundled collection of steel and/or thermoplastic tubing and electric cabling used to transmit chemicals, hydraulic fluids, electric power and two-way communication and control between topside production vessels and subsea production equipment. Umbilicals typically range up to 10 inches (254 mm) in diameter, with internal tubes ranging from 0.5 inch to 1 inch (12.7 mm to 25.4 mm) in diameter. A dynamic umbilical is the portion of this key linking technology that is freely suspended from the semisubmersible platform to the sea floor, where it transitions to a static section that terminates at the remote subsea wellhead. Unlike the static component, a dynamic umbilical must withstand not only the stress of its own weight, but also must manage the uneven stress loads applied when it assumes its curved shape or catenary as it descends to its seabed connection. Aker Kvaerner Subsea (AKS, Lysaker, Norway) has introduced a dynamic umbilical that features an outer casing reinforced along its length with multiple carbon-fiber rods pultruded by Vello Nordic AS (Skodje, Norway). The rods feature longitudinal (0°) reinforcement with Panex 35 commercial-grade 48K carbon fiber tow provided by Zoltek Inc. (St. Louis, Mo.) wet out with vinyl ester resin supplied by Reichhold (Research Triangle Park, N.C.)

        An initial order, placed by Kerr-McGee (Oklahoma City, Okla.) for its Merganser field off the southern U.S. coast in the Gulf of Mexico, has been augmented by several follow-on orders. Anadarko Petroleum Corp. (APC, Houston, Tex.) acquired Kerr-McGee in August of 2006 and then placed an order for three more carbon rod umbilicals plus 180 km/112 miles of AKS’s conventional steel umbilicals for static placement, making it the largest umbilical order ever placed.

      Sports and recreation:

        Composites are found in products used for seven of the 10 most popular outdoor sports and recreational activities. Glass-reinforced composites (alone or in hybrids with other fibers) continue to replace wood and metal in fishing rods, tennis racquets, spars/shafts for kayak paddles, windsurfing masts, hockey sticks, kites and bicycle handlebars, as well as in niche applications such as fairings for recumbent bikes. Sporting goods consume at least 11 million lb of carbon fiber annually, worldwide, according to one carbon fiber producer.

      Aerospace:

        Developments in the aerospace world, the point of origin for advanced composites and where they are still find most use, repeatedly made headlines. Following a 2006 filled with setbacks, aircraft giant Airbus bounced back in 2007. After years of design and development work, and more than 18 months behind schedule, the first A380 superjumbo jet was delivered on Oct. 15 by Airbus to Singapore Airlines. With the help of lightweight composites (about 16 percent by weight), the A380 is reported to have a fuel consumption of less than 3 liters per passenger per 100 kilometers. The aircraft has a range of more than 8,000 nautical miles (15,000 km) and seat-mile costs 20 percent lower than the former largest aircraft, the Boeing 747. To date, total orders and commitments for the A380 are 189 from 16 customers (Airbus must build and sell more than 460 planes to break even). Subsequent aircraft for delivery to Singapore Airlines, Emirates Airlines and Qantas also are said to be on track. However, Airbus acknowledged that there would be unspecified but significant penalties for the delay, and Credit Suisse is forecasting Airbus cost overruns of about €500 million, or $710.3 million (USD).

        Relaunched as the medium-capacity, long-range A350 XWB (Xtra Wide Body), the Airbus answer to Boeing’s mid-sized 787 is now slated to enter service in 2013 at a cost of $15 billion for its development, nearly three times the original estimate. The XWB replaces two previous designs for the A350, which was first introduced in 2005 but came under criticism from Airbus customers, who thought the plane fell short of expectations. Although the plane is running well behind the 787 in terms of entry into service, airline confidence in the product appears to be high: Airbus had picked up a total, in late Octber 2007, of more than 225 firm orders since the launch. Airbus claims this latest version of the A350 will have the widest fuselage in its class (its cross-section has been increased to 232 inches/5.9m) and predicts that it will have the lowest operating and seat-mile costs of any aircraft in its category as well.The Wall Street Journal (WSJ) reported on Sept. 15 that the A350 XWB will feature an airframe of advanced composite materials instead of metal. The report notes that Airbus had been taking the view that attaching skin panels made of carbon fiber composites to an aluminum-alloy skeleton was superior to Boeing’s method of making both the frame and skin of the 787 Dreamliner from composites. Airbus, according to the WSJ, began to rethink its position after encountering resistance from customers who questioned whether the A350 would be more difficult to maintain than the 787. Airbus intends to complete its designs of the A350 late next year and expects to deliver the first A350 in 2013.

        Without doubt, however, it was the July 2007 roll out of Boeing’s 787 Dreamliner, that drew the most attention. The televised event succeeded, like no other new product launch in the composites industry’s history, making composites the subject of wide public interest. The plane also is the fastest selling plane of its type in history, having attracted 710 firm orders by November of 2007. However, by Oct. 10, Boeing officials had announced that both the first flight and first delivery of its composites-intensive 787 Dreamliner would be delayed from late 2007 until 2008. The plane has been rescheduled to take to the skies for the first time in March this year, with delivery of the first plane to Japan’s All Nippon Airways set for November or December.

        The announcement came in the wake of problems that had escalated since late summer. After the rollout of the first, but only partially completed 787, Boeing faced a shortage of fasteners and was coping with complicated rework on the plane, the result of difficulties with its far-flung supply chain. Given the large number of suppliers delivering subassemblies from all over the world, and the plane’s unprecedented use of carbon fiber composites, Boeing reported that managing the workflow complexity proved more daunting than expected.

        As Boeing and Airbus struggled with delivery schedules and jockeyed for supremacy in the air, China offically entered the fray, announcing plans to launch its own large commercial jet by 2020. Government officials in Beijing said they have accelerated development of a locally designed and built passenger craft to compete for the billions it spends on foreign-made planes. Plans for the large aircraft will be completed by 2010. It’s builder, China Aviation Industry Corp., already is manufacturing the ARJ21, a midsized jet that is expected to begin flight testing in 2008. Reports say the consortium has 70 advance orders for this plane from domestic airlines.

        The Chinese planes are not expected to compete in the international market immediately but should see good domestic support thanks to the government’s influence over the airline industry.
        The firms facing the most eventual risk in the wake of this development are Boeing and Airbus, but the giants assume there is enough potential business to bear increased competition. Since 2000, air passenger ridership in China has risen by 105 percent to 138 million trips per year. The combined fleet of the country’s airlines rose to 863 planes from 527 over the same period. Boeing already has 60 orders from China-based carriers for 787 Dreamliners; Airbus has 100 orders from China, including five for the A380.

        For its part, Airbus predicts that mainland China will need more than 3,000 passenger aircraft and freighters from 2006 to 2025, including 2,050 single-aisle aircraft, nearly 600 small, twin-aisle aircraft, more than 200 intermediate twin-aisle aircraft and 180 very large aircraft. Airbus says China’s passenger fleet will triple in the next 20 years from 760 at the end of 2005 to 2,700 in 2025.

        On the military front, Lockheed Martin (Bethesda, Md.) and partners Northrop Grumman, BAE SYSTEMS, GKN Aerospace Services and a host of subcontractors delivered, in 2007, the first planes under its $200 billion Joint Strike Fighter (JSF) contract. The U.S. Navy’s F-35C Lightning II, the aircraft carrier variant of the JSF, has completed its Air System Critical Design Review (CDR), a significant development milestone that verifies the design maturity of the aircraft. Completion of the CDR is a prerequisite for the F-35C to move into Low Rate Initial Production. The F-35C will replace the U.S. Navy’s F/A-18 Hornet and complement the newer F/A-18E/F Super Hornet. This F-35C will be the Navy’s first stealth aircraft and is specially outfitted for the catapult launches and arrested recoveries necessary for deployment to and from large aircraft carriers.

        The MV-22 Osprey, the composites-intensive tiltrotor helicopter developed by Boeing and Bell Helicopter (Ft. Worth, Texas) for the U.S. Marines, has accomplished two major steps required for initial operational capability (IOC) and was deployed to the Al Asad Air Base in Iraq in September 2007. The news is a significant milestone in the aircraft’s development history, which has been marred by serious cost overruns and delays, including a grounding and redesign following two fatal crashes during testing in 2000.

        The U.S. Marines expect the Osprey to fly twice as fast and three times farther (900-mile/1,448-km range) than the Vietnam-era CH-46 Sea Knight helicopter it will replace. The Osprey’s intensive use of composites is also is expected to help it absorb rounds fired by enemy weapons.

        Late in 2007, Airbus announced a delay in delivery of its composites-intensive A400M military transport aircraft. The first flight of the four-engine, high-wing turboprop, scheduled for January 2008, now is likely to take place instead in July, with certification to follow in 2009 and first delivery to the French air force in 2010. The massive military transport plane will replace C-130s and C-160s in Europe. Its composite structures include 18.3m/6-ft composite wing spars designed and tape layed by GKN Aerospace (Cowes, Isle of Wight, U.K.)

        In the regional/business jet market, the Japanese aerospace industry signaled its intention to become a contender with the 90-seat Mitsubishi Regional Jet (MRJ). Mitsubishi Heavy Industries (MHI, Shinagawa, Japan), revealed that the MRJ has been in development since 2003. Although there is no specific build program at this time, the MRJ, if built, would enter into service in 2012. Composites are targeted to the fuselage, wing and empennage, building on the technology development and experience gained by MHI from its development and production of the first composite wing boxes for the Boeing 787. Composites and a newly developed engine are projected to offer a 20 percent savings in fuel consumption compared to current regional jets. Meanwhile, Canadian aircraft manufacturer Bombardier Inc. (Montreal, Quebec) fielded its CRJ700 and CRJ900 NextGen aircraft with new advanced composite components, including flaps, vanes and ailerons fabricated in a resin transfer molding (RTM) process. The company claims that use of RTM has enabled significant parts consolidation — the total number of parts required for wing component assemblies has fallen by almost 80 percent for the aileron and 95 percent for the flap and vane.

        In the general aviation market, the flurry of Very Light Jet (VLJ) programs announced in recent years are moving toward fruition, with several planes, notably the Adam Aircraft (Englewood, Co.) A700, on the path for 2008 FAA certification. VLJs, weighing less than 10,000 lb and designed for a single pilot, will cater to business travelers and may spark a strong air taxi market that could cut into commercial carrier business — these lightweight planes have a range of 1,000 miles (1600 km), require runways as short as 3000 ft (914.5m), can land at regional airports and in some cases can be cost-competitive with commercial carriers. VLJ manufacturers are heavy users of a variety of composites in wing, fuselage and engine applications. In addition to Adam aircraft, notable VLJ manufacturers include Cessna (Wichita, Kan.), Eclipse Aviation (Albuquerque, N.M.), Embraer (Sao Jose dos Campos, Brazil), Adam Aircraft (Englewood, Colo.), Excel-Jet (Monument, Colo.), Aviation Technology Group (Englewood, Colo.), Diamond Aircraft (London, Ontario, Canada) and Epic Aircraft (Bend, Ore.).

      UAVs and UCAVs:

        The booming unmanned aerial vehicle (UAV) market continues to expand with hundreds of designs competing for military and civilian contract dollars worldwide. While UAV wingspans range from commercial airliner size down to palm-sized micro flyers, small long-endurance “tactical” UAVs, those that support intelligence, surveillance and reconnaissance (ISR), are becoming key components of military and homeland security missions.

        UAVs are currently the fastest-growing segment of the aerospace sector, with a worldwide value of more than $2 billion (USD). More than 40 countries have gone on record as producing at least one UAV airframe, and more than 500 systems exist.

        Composites are the material of choice for UAV airframes, which can range from a few inches in length to the size of a commercial airliner. High strength-to-weight and limited radar signature and signal transparency are the main drivers. Since pilot or passenger risk isn’t an issue, UAV designers have a wider range of possibilities open to them to meet specific mission objectives. However, standards for unmanned aerial vehicles (UAVs) have been developed by NATO’s Conference of National Armaments Directors (CNAD). Currently, special fixed-wing UAVs with a take-off weight between 150 kg to 20,000 kg do not operate under a common set of aviation standards. The NATO Working Group has, over the past 18 months, in cooperation with UAV specialists from Canada, France, Germany, Italy, The Netherlands, Spain, Sweden, the United Kingdom and the United States, put forward a set of codes dubbed USAR (UAV Systems Airworthiness Requirements). The USAR code, which represents the first international initiative toward common UAV airworthiness rules, was based on a French proposal to take a current standard for manned aircraft and adapt it to suit the characteristics of UAVs. The initiative is being formally considered by all the member countries for approval as a NATO Standardized Agreement (STANAG).

        One of the more noteworthy of many pieces of 2007 UAV news concerned the ScanEagle, a fully autonomous, composite unmanned aerial vehicle (UAV) developed by Boeing and Insitu Inc. (Bingen, Wash.). The craft proved its worth, surpassing 1,000 flight hours in southern Iraq, completing 172 sorties in less than five months. ScanEagle provided live imagery to Australian soldiers operating from Camp Terendak, Ali Air Base in the Dhi Qar province. ScanEagle, which is 4 ft/1.2m long with a 10-ft/3m wingspan, carries either an electro-optical or an infrared camera. For a vehicle of its size, its combination of endurance and payload is reportedly unmatched. The system can provide more than 15 consecutive hours of “on-station” coverage, and can be launched and recovered from land or sea, providing greater flexibility than other systems in its class.

        This and many other programs represent explosive growth in UAV development: As existing programs mature, it is likely that an additional $10 billion will be spent by 2010.

      Wind and power:

        Wind power is the world’s fastest growing energy source and the giant rotor blades on the turbines are the composites industry’s fastest growing fiber-reinforced polymer (FRP) application. The European Union still leads the way. In 2006, the latest year for which statistics existed at press time, European wind power capacity, according to the European Wind Energy Assn. (EWEA), increased by 7,588 MW in the EU, a 23 percent improvement over 2005. EWEA anticipates similar increases for 2007. The cumulative wind power capacity operating in the EU increased by 19 percent to greater than 48,000 MW, supplying about 3.3 percent of total EU electricity consumption.

        Although the U.S. is running a distant second, the American Wind Energy Assn. (AWEA, Washington, D.C.) announced in early November a substantial increase in the projected installation of new wind energy facilities in 2007. The previous projection, 3,000 megawatts (MW) of new wind-generated electric power capacity in 2007, has been increased to 4,000 MW. AWEA reports that new windfarms already have added more than 2,300 MW of generating capacity to the electrical grid in 2007, with more than 5,000 MW in various stages of construction. (One MW of electricity, on average, serves 250 to 300 households.)

        Although this is good news, AWEA warns that the lack of a long-term, national policy to promote renewable energy development could jeopardize further growth. The federal production tax credit (PTC) for renewable energy will expire in December 2008, and AWEA warned in November 2007, that there was no national renewable electricity standard (RES) or other long-term policy set to take its place. The U.S. House of Representatives in August had passed a new RES, but that bill had not, at press time, yet cleared the U.S. Senate.

        AWEA contends that the continuity in the PTC since 2005 has spurred both record-breaking new generating capacity (2,431 MW added in 2005, 2,454 MW in 2006 and 4,000 MW expected in 2007) and a wave of investment in manufacturing facilities and services across the country.

        In state-by-state reporting, AWEA noted that Texas, again, added the largest amount of new wind power capacity (600 MW). Colorado installed 264 MW and now ranks sixth in U.S. wind power generation. Washington state, with 140 MW of new wind capacity, has pulled ahead of Minnesota into fourth place. Missouri saw the completion of its first utility-scale wind farm, a 56.7-MW project that generates power for electric cooperatives, while utility-scale projects also went online in Illinois, Pennsylvania and Iowa.

        The sleeping giant in the wind power landscape, however, could be China. Currently the sixth-largest wind energy market in the world, with close to 2,620 MW of installed capacity, China ranks fifth in the amount of wind power generating capacity installed in 2006, adding more than 1,370 MW of new capacity. Although that represents half of the capacity installed in the U.S. during the same period, China is second only to the U.S. as a consumer of electricity and has the fastest growing energy demand in the world. That leaves room for huge growth in wind energy. Determined to reduce its dependence on coal and imported oil, China enacted the Renewable Energy Law in 2006, which mandates that at least 5 percent of electricity must be generated from renewable sources by 2010 and 10 percent by 2020. To meet that goal, China must have 30,000 MW (or 30 gigawatts) of wind power capacity by 2020, which translates into roughly 2,100 MW installed per year for the next 13 years.

      Utility Infrastructure:

        As natural insulators with high dielectric strength, fiberglass composites revolutionized the handling of electricity when they first replaced wood and metal in 1959. Today, utilities in the U.S. and elsewhere are working with composite suppliers to take advantage of fiberglass for both power transmission towers and distribution poles, cables, cross-arms — traditionally the province of wood and steel — and the aluminum conductor cables they support. Pultruded and filament wound composite utility poles and cross-arms have begun to overcome buyer resistance as electric power companies employ them primarily as replacements for aging wood poles in remote and/or extremely humid locations. One good example is RS Technologies’ (Calgary, Alberta, Canada) glass/polyurethane power poles, which have been specified by several utility companies for installation. Composite-reinforced aluminum conductor cables (CRAC) replace traditional steel strength members in cables with a pultruded continuous-fiber core, which is expected to reduce weight and increase power-transmission efficiency by an estimated 200 percent. If successful in upcoming tests and demonstration projects, CRAC technologies may find application in infrastructure modernization projects estimated by one CRAC developer to be well in excess of $10 billion in China alone. Meanwhile, to maintain the electrical infrastructure in North America at current levels will require an investment of $56 billion over the next decade — twice the amount presently earmarked for that purpose by utility companies. Yet CRAC cable developers claim that power needs will actually increase, by as much as 19 percent, in that time frame, making CRAC cabling an attractive alternative for upgrading power lines, without erecting new towers or obtaining additional rights-of-way.

      Fuel Cells:

        Reinforced thermosets and thermoplastics are likely candidates for the eventual materials of choice used to make the bi-polar plates, end plates, fuel tanks and other components in fuel cell systems. Fuel cell technologies of several types offer a “clean” (near-zero VOC) means to convert hydrogen to electrical power in automotive and stationary power systems. Due to their conductivity, corrosion resistance, dimensional stability and flame retardancy, vinyl-ester-based bulk molding compounds with carbon fiber reinforcement have already been selected in a least one commercially available stationary unit.

    Fiber demand and supply

      Industry statistics for carbon fiber supply and demand are in flux, as all fiber manufacturers have announced major expansion programs and new suppliers are entering the industry. Estimates of annual worldwide demand for continuous, PAN-based carbon fiber exceeded 27,000 metric tonnes (60 million lb) in 2006 and demand is expected to grow at least 15 percent per year. Current industry capacity is estimated at around 45,500 metric tonnes (100 million lb) for conventional tows and large tows combined. When all of the announced capacity expansions go on line, total nameplate capacity should be 76,000 metric tons (168 million lb) by 2010.

      Historically, carbon fiber markets have gone through boom/bust cycles, making it difficult for fiber manufacturers to predict capacity needs. Aerospace and high-volume sporting goods markets have consumed the majority of prepregs made with standard/intermediate tensile modulus, and small-tow (1K to 12K) carbon fiber. From 2001 through 2003, carbon fiber demand dipped, as economies in the U.S. and Western Europe slowed significantly, but recovery and growth was the norm in 2004 through 2006.

      Demand is now up and continuing to rise, making the slump a memory. The aerospace market has rebounded strongly, thanks to programs such as Boeing’s 787 Dreamliner, and as segments such as industrial applications for fiber multiply, the market for carbon fiber is strong. This time, it appears that new capacity additions will keep pace.

      Forecasting 15 percent growth in demand per year, Toray Industries (Tokyo, Japan) has expanded all of its facilities, including those in Japan, the U.S. and at its French subsidiary, SOFICAR. The company is a major supplier of carbon fiber prepreg materials to Boeing for the 787, and it announced yet another expansion of its fiber capacity in late November 2007 in response to talks with Boeing, to ensure supply for the aircraft as production ramps up. The new announcement will bring its total worldwide nameplate capacity to 24,000 metric tonnes (52.8 million lb) by 2010.

      Toho Carbon Fibers Inc. (Menlo Park, Calif.) announced further expansions in October 2007, including a new fiber line at Wuppertal, Germany, slated for startup in mid-2009. Cytec Industries Inc. (West Paterson, N.J.) announced in 2007 that it will double its existing capacity by early 2010, once its new plant in South Carolina comes on stream. Cytec has confirmed that the new line will consist of small tow, from 3K to 24K. Hexcel is proceeding with construction of its new fiber plant in Madrid, Spain as well as another new line at its Salt Lake City, Utah facility. A new precursor line also will be constructed. The new capacity will bring Hexcel’s total capacity to 7,300 metric tonnes (16 million lb) by the end of 2009. Mitsubishi Rayon (Tokyo, Japan), which recently built a new 5 million lb line in Japan, has again announced expansion, this time a 2,700 metric ton (6 million lb) line at its Otake production center, scheduled for startup in late 2009. Taiwan-based Formosa Plastics also has expanded, with production expected to reach about 5,000 metric tonnes (11 million lb) sometime in 2008. Zoltek recently acquired the assets of a Mexico-based acrylic fiber manufacturer, Cydsa, which it plans to retool and modify to produce precursor. Zoltek also plans to produce 2,250 metric tonnes (5,000,000 lb) of large tow fiber at the facility within the next few years. Meanwhile, SGL Carbon (Wiesbaden, Germany) is building a third major production line at its Inverness, Scotland facility, which should come on line by the end of 2008. The company says it plans to add more fiber lines in Germany, tripling its current capacity by 2012.

      Joining these established producers are several new players: Dalian Xingke Carbon Fiber and Yingyou Group Corp. are two Chinese companies reportedly producing carbon fiber, with a combined output of about 820 metric tonnes (1.8 million lb). Fiberglass producer Fiberex (Leduc, Canada) may begin producing carbon fiber in Canada within the next few years, and other fiber startups are reportedly underway in Saudi Arabia and India.

      Demand for other advanced fibers is increasing as well. Continued security concerns worldwide have stimulated growth in the market for armor products, prompting increased production of aramid and polyethylene fibers. DuPont Advanced Fibers Systems (Richmond, Va.) has significantly increased production of its DuPont Kevlar aramid fiber by more than 25 percent, with a $500 million capacity expansion announced in September 2007 that is scheduled to come online by 2010 — the largest expansion in Kevlar history says the company. Teijin Aramid (Arnhem, The Netherlands), formerly Teijin Twaron, now produces four brands of aramid fiber, including Twaron, Technora, Sulfron and Teijinconex. With the new name, the company also plans to extend its product range. It is in the midst of a $222 million expansion to boost its Twaron capacity by 15 to 20 percent from 23,000 tonnes (50.7 million lb) per year in Emmen and Delfzijl, in The Netherlands. Start-up is expected by the second half of 2008.

      Despite the tendency for “promising” market applications in the composites industry to remain out on the horizon — never quite within reach — the future, overall, bodes well. Continued automation, streamlining of composite manufacturing methods and new material forms should make composites more user-friendly — not to mention more cost-effective — encouraging their use in greater quantities in existing markets and making them even more attractive to new industrial and consumer-driven markets.