The matrix binds the fiber reinforcement and gives the composite component its shape and determines the quality of its surface. A matrix can be polymeric, ceramic or metallic. Polymer matrices are the most widely used for composites in commercial and high-performance aerospace applications.

Resin matrices: Thermosets

The polymers most widely used in composites are thermosets, a class of plastic resins that, when cured by thermal and/or chemical (catalyst or promoter) or other means, become substantially infusible and insoluble. After cure, a thermoset cannot be returned to the uncured state. While almost all thermosets in commercial use today are derived from petroleum feedstocks, R&D is growing in the field of bio-resins. Developed primarily in an effort to use renewable agricultural feedstocks, bio-resins comprise, in varying proportions, polyol (from soybeans) and ethanol (from corn).

Unsaturated polyester resins are the most widely used thermosets 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 see "Fabrication methods," under Editor's Picks," at right).

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 example, 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 next). Vinyl esters shrink less during cure and outperform polyesters in chemically corrosive environments (e.g., chemical tanks) and in structural laminates that require 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. Ashland Performance Materials, Composite Polymers (Columbus, Ohio) is one resin supplier making significant effort to commercialize bio-based resins in this arena with its ENVIREZ line, which is based in part on soybean oil. Reichhold Inc. (Research Triangle Park, N.C.) also has developed a bio-resin, POLYLITE 31325-00, a low-viscosity unsaturated polyester with 25 percent soy oil content. The material is designed for SMC/BMC applications.

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 and pressures. Toughening agents — thermoplastics and reactive rubber compounds — can be added to counteract brittleness. Considerable demand is building, particularly in the aerospace industry, for epoxies that can be cured out of the autoclave. Large carbon fiber/epoxy aircraft structures require immense, expensive autoclaves. Out-of-autoclave, aerospace-grade epoxies with modified properties can deliver nearly equivalent properties with an oven cure, say suppliers.

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 also have proven to be 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 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. Current applications range from radomes, antennae, missiles and ablatives to microelectronics and microwave products.

Among the more exotic of resins, bismaleimide 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 delamination. 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. Huntsman Advanced Materials (The Woodlands, Texas) and Henkel Corp. (Rocky Hill, Conn.) have both developed commercial benzoxazines for advan-ced 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 remelt and can be reshaped 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), polyarylsulfone (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 possess exceptional impact-resistance and vibration-damping properties. However, they can present composites manufacturers with some processing challenges because of their relatively high viscosity. Reinforced thermoplastic composites that feature these resins as matrices are also making inroads into aerospace applications. For example, Stork Fokker AESP (Hoogeveen, The Netherlands) has supplied carbon/PEI floor panels for the Gulfstream 550 executive jet, and will do the same for the forthcoming Gulfstream 650. Other applications include aircraft seatbacks, floor beams and brackets.

Resin matrices: Thermoset or thermoplastic

Polyurethane resins are available in both thermoset and thermoplastic formulations. Thermoset polyurethanes are used to pultrude tough 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 RIM, see “Fabrication methods,” under "Editor's Picks," at right. 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.

Also available in either form are polyimides (the thermoset form of which already has been described). In thermoplastic form, polyimide resin readily releases volatiles under heat and pressure, producing parts with fewer voids.

Two other resins have been added to this category in the past decade 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 thermoforming) or overmolded (via extrusion and/or injection molding) to create products such as threaded rod, without the use of 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 that such applications require.

Editor's Note: To continue reading the SOURCEBOOK "Industry Overview, Part I," click on "Fabrication methods," under "Editor's Picks," at right.