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Article
The matrix

The matrix material binds the fiber reinforcement, 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.

Author:
Posted on: 1/1/2014
Source: CompositesWorld

The matrix binds the fiber reinforcement, gives the composite component its shape and determines its surface quality. A composite matrix may be a polymer, ceramic, metal or carbon. Polymer matrices are the most widely used for composites in commercial and high-performance aerospace applications. Ceramic and metal matrices are typically used in high temperature environments, like engines. Carbon as a matrix is used in very high temperature applications like carbon-carbon brakes and rocket nozzles.

 

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. (Saturated polyesters are themoplastic polymers.) 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), which are processed using 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). Styrene is added in amounts up to 50 percent to reduce viscosity, making the resin easier to handle and process. 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. Dicyclopentadiene (DCPD) can be added to polyester resins to lower viscosity and provide good properties at lower styrene content. However, DCPD solidifies at room temperature, requiring, heated storage and handling facilities.

Styrene enables polyester resins to cure from a liquid to a solid by 'cross-linking' the molecular chains. However, polyester resins will gel on their own over a long period of time. Thus, small amounts of inhibitor are often added during resin manufacture to slow this action and increase shelf life. Even without inhibitors, polyester’s rate of polymerization is too slow to make composite molding practical, so catalysts and accelerators are added to speed cure. Catalysts are added to the resin prior to molding, to activate crosslinking, but do not take part in the polymerization reaction. Catalysts used with polyester include MEKP and peroxide. Thorough mixing is important and the correct ratio to resin weight affects cure rate, and potentially degree of cure. For example, 1 percent is typically considered a slow mix, 2 percent is a common supplier specification, and 3 percent is usually a fast curing mix. Adding more than 4 percent may result in a failure to cure.

An accelerator is added to the catalyzed resin to enable the reaction to proceed at workshop temperature and/or at a greater rate. Since accelerators have little influence on the resin in the absence of a catalyst they are sometimes added to the resin by the polyester manufacturer to create a 'pre-accelerated' resin. Cobalt is a common accelerator. Cure is exothermic: as the components 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 also may include inhibitors, promoters and accelerators.

A wide array of additives — sometimes known as modifiers — offer embellished processing or performance attributes, with the most common including pigments, fillers and flame or fire retardants. Although pigments are only added at about 3 percent resin weight, their use can affect curing and degrade the final laminate if they are incompatible with the resin. Fillers, such as milled fiber, chopped fiber and glass microspheres are often added in quantities up to 50 percent to help reduce cost, facilitate molding or prevent exotherm in thick laminates. Certain fillers can also contribute to improving fire-resistance in the finished composite. Halogen-free flame retardants are being developed because halogens (i.e., bromine, which, along with fluorine, chlorine, iodine and astatine, occupy Group VIIA of the periodic table) are now known to release toxic and corrosive gases when exposed to flame. Alumina trihydrate is one alternative used in MoldX products by Huber Engineered Materials (Atlanta, Ga.). They reportedly enable higher loadings without a change in viscosity for excellent mold flow and significantly reduce halogen-containing fire retardants without sacrificing flame retardant properties. R.J. Marshall Co. (Southfield, Mich.) has developed its Maxfil product line with three ATH grades offering different particle sizes as well as blends of ATH and calcium carbonate for applications where flame retardance and smoke suppression are not as critical, such as BMC/SMC sprayup of wall panels for bathrooms and pultruded pipe for oil platforms.

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). HK Research (Hickory, N.C.) has developed its REVOLUTION series, which boasts a grafted polymer matrix with a tighter network, offering a truly flexible gel coat with increased weather resistance while maintaining ease of application and pricing on par with traditional gel coats. Recent additions to this product line include the High Definition EXTREME “METAL FLEX” in-mold system which allows boatbuilders to add a ‘metallic’ coating to their fiberglass parts that compares to the popular automotive paints. The company claims it can “match any color combination imaginable,” having already done so with 2014 automotive colors for several boatbuilders.

Vinyl ester resins offer a bridge between lower-cost, rapid-curing and easily processed polyesters and higher-performance epoxy resins (described next). Their molecular structure is very similar to polyester’s, but with reactive sites only at the ends of the molecular chains and with fewer ester groups. Since ester groups are susceptible to hydrolysis, less of these increase vinyl esters’ resistance to resistance to water and chemically corrosive environments, which accounts, in part, for their higher price. Vinyl esters are favored in chemical tanks and the corrosion resistance industry and also add value in structural laminates that require a high degree of moisture resistance (such as boat hulls and decks). They are processed and cure very similarly to polyesters, with the potential to offer improved toughness, though this usually requires an elevated temperature post-cure.

Ashland Performance Materials (Dublin, Ohio) is one resin supplier making significant effort to commercialize bio-based resins in this arena with its ENVIREZ line, which replaces glycol derived from crude oil with bio-glycol derived from corn or soybeans. Canada’s Campion Marine (Kelowna, British Columbia) is the first volume boatbuilder to convert to a bio-resin, using special laminating grade Envirez L 86300 in all of its models since 2009, after tests verified that it had strength equal to, and elongation and elasticity superior to, the petroleum-based polyesters used previously. 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. Entropy Resins Inc. (Hayward, Calif.) claims to have the industry’s first U.S. Department of Agriculture (USDA) “bio-preferred” certified epoxy, Super Sap Epoxy 100/1000, with 37 percent of its backbone derived from naturally occurring pine oil. Entropy claims it adheres well to reinforcements and has better elongation properties than petroleum-based epoxies. Pasadena, Texas-based Dixie Chemical Co. formulates bio-based unsaturated polyesters (ortho-, iso-, and terephthalic, DCPD-modified and bisphenol A fumarate) and vinyl esters using a methacrylated fatty acid (MFA) reactive diluent, which is a partial styrene replacement derived from palm kernel and coconut oil. Applied in 15 to 18 percent loadings, MFA reduces styrene emissions up to 27 percent, with good toughness and elongation, low-to-no odor and 60 percent bio-content.

For advanced composite matrices, the most common thermosets are epoxies, phenolics, cyanate esters (CEs), bismaleimides (BMIs), benzoxazines 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, diglycidyl ether of bisphenol F (lower viscosity), creosol novolacs, or phenolic novolacs. Epoxies are not cured with a catalyst, like polyester resins, but instead use a hardener (also called a curing agent). The hardener (part B) and the base resin (part A) co-react in an ‘addition reaction’ according to a fixed ratio. Thus, it is critical to use the correct mix ratio of resin to hardener in order to ensure a complete reaction, otherwise the resin will not fully cure nor attain its full properties. Meter mix dispense equipment is used widely now to automate and accurately control this mixing of resin and then deliver it into the molding process. The type of hardener used affects the final properties of the cured resin, and thus, the composite. These include: aliphatic amines, cycloaliphatic amines, polyamides, aromatic amines, anhydrides, phenols, thiols and latent hardeners (e.g., Lewis acids). Many aerospace applications use amine-cured, multifunctional epoxies that require cure at elevated temperatures and pressures. Toughened epoxy — with thermoplastics and reactive rubber compounds added to counteract brittleness due to high degree of cross-linking — have become the norm in high percentage composite airframes like the Boeing 787 and Airbus A350.

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. However, phenolics polymerize via  condensation reaction, releasing 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, thus phenolics are not typically processed using RTM.

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 have traditionally exhibited higher moisture absorption and lower toughness values than CEs or epoxies, but significant progress has been made in recent years to create tougher formulations, and BMIs are now touted as having better resistance to fluid ingression than epoxies. Increased use of BMI is being driven not just by tooling and applications where service temperatures exceed 350°F/177°C, but also by the increasing use of composites in structures that need improved hot/wet and open hole compression (OHC) performance at moderate temperatures, e.g. 80°C to 120°C (176°F to 248°F). This is the reason behind much of its use on the F-35 Joint Strike Fighter, enabling damage-tolerant structures at lower mass vs. epoxy. OEMs continue to beat the drum for resins with at least a 20 percent increase in OHC vs. the toughened epoxies being used on the 787 and A350. Previously, BMI was thought to be too expensive, but at $75/lb for carbon fiber prepreg it competes very well with intermediate-modulus (IM) carbon fiber/epoxy systems at $70/lb, and previous issues with long cycle times are also being addressed.

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 are formed by reacting phenol, formaldehyde and amine in an additive reaction with ring opening polymerization which produces a high molecular weight polymer with near-zero cure shrinkage, reactive sites that greatly facilitate hybridizing with other resins, and the ability to polymerize with itself (homopolymerize) to form polybenzoxazine networks very similar to phenolic. Discovered in the 1940s, benzoxazine development intensified during the 1980s and 90s, and were qualified into printed circuit boards (PCBs) in 2000, yet systems for structural composites weren’t commercialized until 2008. Interest in benzoxazines is increasing due to its high stiffness, excellent thermal properties, lower moisture absorption, better resistance to flammability and also to ultraviolet (UV) radiation than epoxies. Though exotic benzoxazines can have a Tg as high as 300°C to 350°C (572°F to 662°F), more common formulations range between 150°C and 250°C (302°F and 482°F). Like BMI, these higher-Tg systems can be brittle and need to be toughened to prevent microcracking. Suppliers claim the processing is basically the same as epoxy but with a lower heat of reaction. Henkel Aerospace (Bay Point, Calif.) reports that a 19mm/0.75-inch thick, 150-ply laminate can be cured with a 5°C/min (9˚F/min) ramp rate without an uncontrolled exotherm.

With cost and performance between epoxy and BMI, how benzoxazine is compared to both is based on the very different approaches taken by its two leading suppliers. Henkel — supplying structural prepregs, infusion resins and film adhesives, while partnering with Airtech (Huntington Beach, Cal.) to offer tooling prepreg — sees benzoxazine as a way to cut the cost of composite structures throughout the supply chain due to its room temperature stability (no frozen storage) and processing advantages, satisfying not just complex structural demands, but also supply, handling, surface finish and health and safety requirements. Meanwhile, Huntsman Advanced Materials (Basel, Switzerland and The Woodlands, Texas) is selling benzoxazine building block components to prepreggers, resin formulators and adhesives manufacturers. It views benzoxazine’s unique and almost endless ability to hybridize with other resins — e.g. epoxy, phenolic, BMI, thiol and more — as the path to achieving truly tailored polymers to meet the needs of individual composite applications and even further optimized structural designs in the future. Offering the flammability, smoke and toxicity (FST) performance of phenolic without its voids and processing difficulties, benzoxazine seems poised to push large, integrated structures into aircraft and other transportation interiors. But some users caution that it has not yet attained the maturity of BMI, still requiring further development in several areas, including OOA processing.

Benzoxazine has been adopted by a major airframe manufacturer for bonding large BMI tooling assemblies via Airtech’s Beta 8610 film adhesive, and also qualified for the Airbus A380 auxiliary power unit (APU) housing, replacing the original BMI with a Henkel benzoxazine prepreg using Toho Tenax (Tokyo, Japan) carbon fiber. Airtech Beta Prepreg autoclave-cure tooling system using Henkel benzoxazine resin has been used at GKN Aerospace Munich for production of 100 carbon fiber T-stringer support tools used in manufacturing the A350 inboard and outboard wing flaps.

Another, but less well-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. Huntsman Polyurethanes (Auburn Hills, Mich.) kicked off the “snap-cure” revolution with its VITROX product. VITROX can remain at a consistently low and stable viscosity until the “kick off” temperature is reached, which activates a snap cure. This trigger temperature is determined by the resin’s catalyst blend and specific formulation, and can be precisely tailored to a customer’s application, with pot life tailorable from less than five minutes to up to several hours compared to traditional PUR pot life of 20 to 25 minutes. Designed for infusion, RTM, VARTM and filament winding applications, and targeted at the automotive industry, VITROX offers a Tg of more than 200°C/392°F and reportedly has good mechanicals and inherent FST performance.

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.

 

Out of autoclave processing

There has been considerable demand in the aerospace industry for matrix resins that can deliver autoclave-equivalent laminates (including less than 1 percent void content) without the need to be cured in autoclaves, which are not only large and expensive to buy, but also not cheap to operate. Recent industry surveys indicate that for parts 8 m2 to 130 m2 (86 ft2 to 1,400 ft2), ovens can cost 1/7 to 1/10 that of a comparatively-sized autoclave, and the cost of dry fiber and liquid resin can be as much as 70 percent less than the same materials converted to prepreg.

TenCate offers six different out of autoclave (OOA) cure epoxy prepregs with Tg ranging from 257°F/125°C to 397°F/203°C, including TC250 toughened epoxy with properties in the NCAMP database, TC275 products designed for general aviation use and TC350 toughened high temperature system for use in military and commercial aerospace structures. The latter two materials are listed as capable of very low (<0.05%) void content parts. Hexcel’s HexPly M56 is a 180˚C/350˚F curing epoxy product line for aircraft structures, while Cytec OOA  products include: Cycom 5320-1 developed for aircraft primary structure which cures at 180˚C/350˚F, Cycom 5215 with a wet Tg of more than 300°F/150°C after a 350°F/177°C freestanding postcure, MTM44-1 toughened epoxy with a max. wet Tg of 150°C/302°F and adopted by GE Aviation to manufacture the outer and mid-section fixed trailing edge panels for the Airbus A350 XWB wing, MTM46 with a max. wet Tg of 130°C/266°F, MTM27 with a 15-min cure cycle at 150°C/302°F followed by 110°C/230°F postcure, MTM45 with a max. wet Tg 160°C/320°F and used throughout the structure on the Advanced Composite Cargo Aircraft (ACCA), MTM57-2 reduced tack vacuum bag only ZPREG,  and a wide variety of LTM low temperature tooling materials.

BMIs have also moved out of autoclave with systems commercially available from Cytec, TenCate, Renegade Materials Corp. (Springsboro, Ohio) and Stratton Composite Solutions (Marietta, Ga.). Hexcel also has an OOA BMI in development.

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, for example, on NASA's 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.


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