Advanced materials for aircraft interiors

Applications aren't as demanding as airframe composites, but requirements are still exacting — passenger safety is key.
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While the new 787 Dreamliner and the Airbus A380 have attracted kudos for groundbreaking structural composite airframe designs, composites have long been standard issue for virtually every visible part of a commercial aircraft's cabin, primarily because of the material's light weight and high strength-to-weight ratio. Less glamorous perhaps, than airframes, aircraft interiors are subject to performance requirements no less demanding. Parts have to deliver mechanical strength, dimensional stability and low heat release in the event of a fire (see "FAR Part 25 Standards"sidebar, p. 28), while keeping aircraft weight as low as possible for maximum efficiency.

"Interiors may be less demanding structurally when compared to airframe parts, but there are a lot of specialized needs and mechanical requirements that have to be met, most notably, very stringent fire, smoke and toxicity numbers,"explains Bill Webb, marketing and business development manager for Cytec Engineered Materials (Tempe, Ariz.). "Reaching these goals is not easy,"says Webb. "And it's certainly not a commodity market."

But it is huge. Thousands of pounds of advanced composites materials, including thermoset prepregs, honeycomb core and advanced engineering thermoplastics, are used in each aircraft - according to one source, more than 11,000 lb/5,100 kg for a single Boeing 777. These materials are found most often in floor and ceiling panels, cargo liners, overhead stowage bins, panels, window surrounds, lavatory modules, galleys, food and drink trolleys and bulkheads/class dividers. Moreover, the market is not limited to new original equipment manufacturer (OEM) construction; refurbishment of older planes also is significant because interior components have a service life of between three and seven years, says Dale Brosius of Brosius Management Consulting (Brighton, Mich.) "Both are very important markets for composite material suppliers,"he notes, pegging the current total annual prepreg usage for aircraft interiors, new and aftermarket combined, at about 13 million lb (6,000 metric tonnes). Transport & VIP Interiors magazine claims that close to 2,000 supplier companies design, manufacture, assemble, repair and distribute aircraft cabin interior equipment - composite and noncomposite - for commercial and private planes.

Getting inside interiors

The materials used in aircraft are similar regardless of airline or type of aircraft, since all passenger-carrying craft must meet the flammability requirements noted in the sidebar (p. 25). Key composite parts include ventilation ducting and "linings"- thermoplastic or reinforced thermoset shapes around windows, doors and cabin sidewalls - that require contoured shapes. By far, the largest composite applications are sandwich panels made with honeycomb core and thermoset resins, used for flooring, ceilings, galley walls, lavatories and cargo hold liners. Low-density, lightweight core between thin facesheets dramatically increases a panel's stiffness with little added weight. The core functions like the connecting web of an I-beam, separating the facesheets at a uniform distance, while the skins themselves function as the beam flanges; panel bending stiffness is proportional to core thickness. Sandwich panels are cost-effective because core material is less expensive and weighs less than skin composite and can be cured or processed with the skins in a one-shot operation.

A typical interior sandwich panel is made up of Nomex honeycomb core (made from aramid fiber paper supplied by DuPont Advanced Fibers Systems, Richmond, Va.), approximately 0.5-inch/13mm thick, with a 0.12-inch/3 mm cell size. Usually, the core is faced with one or two skin plies of glass/phenolic prepreg for most ceilings and walls, and glass/epoxy or carbon/epoxy for floor panels, which require higher tensile strength.

Flat panels, usually 4 ft by 8 ft (1.2m by 2.4m), are typically processed with flat press molding, in which the lower skin prepreg, core and upper skin prepreg are stacked onto a heated steel caul plate. A second, heated caul plate in a hydraulic press is lowered onto the layup and the laminate is held under pressure for periods ranging from 30 minutes to more than 100 minutes. Usually the press is "bumped"or cracked open slightly during the process to release gas and water vapor, byproducts of phenolic's condensation reaction cure, then closed again to complete the cure cycle. High-rate lines use multi-opening presses (MOPs), with five or more mold openings; each is loaded with a layup then processed as above. Pressures range from 50 psi to 100 psi (3.4 bar to 6.8 bar). Phenolic resin cure proceeds faster with increasing temperature, so a high-temperature-capable mold, at least 160ºC/320ºF, is required.

"Crush core"is a faster, more economical process conducted at higher temperature and pressure, in which a cored laminate is placed in a large press and crushed down to a predetermined thickness. The process, practiced mainly by Boeing and Airbus and their largest Tier I interiors suppliers, is most commonly used to produce curved side panels and to achieve shapes like the tapered depressions around aircraft windows. Pressures up to 300 psi/20.7 bar cause the honeycomb cell walls to fold over and flatten, creating more bonding surface area for the thin prepreg skins. The method creates panels of consistent thickness, ensuring good fit and finish during installation. Material processing times are on the order of 8 to 12 minutes, resulting in very efficient production, says Cytec Engineered Materials' senior principal research chemist Billy Harmon. Large presses are needed to generate the pressure required, together with heated metal molds, and high production volumes are needed to justify the capital costs.

More complex shapes, such as luggage bins, can be made from flat pressed panels with simple "cut and fold"methods. A strip of the upper skin is removed to expose the core; the tighter the fold or radius angle, the wider the strip removed. Adhesive is applied to the exposed core, which is not cut, and the panel is simply folded to the desired position and clamped until the adhesive cures. "The method is very economical, because it can be done without complicated jigs or fixtures,"says Hexcel's (Dublin, Calif.) European head of marketing Neil Howard. Fastener locations require extra material, such as adhesive potting to hold the bolt or fastener; a ferrule or tube bonded to both skins in which the fastener is held; or a threaded insert held in place with extra resin or foam. Panel edges require some sort of finishing, such as edge potting, bonded-on injection-molded profiles or a press-fit plastic cover, all of which prevent moisture ingress into the core and present a decorative finish.

Because sandwich panel facesheets are very thin, core print-through is common. Further, pinholes can form as phenolic resins outgas during reaction and cure. Therefore, panels visible to passengers typically are surfaced with some type of decorative material to cover surface flaws. Floor panels can be painted or covered with flame-retardant carpet.

Decorative surfaces include plastic films or laminates, cloth "tapestries,"plastic laminates or paint. Painting requires filling, sanding and primer application. A decorative film or laminate eliminates some of the surface-prep and finishing steps but adds cost and weight to the part. Decorative plastic laminates differ from films in that they combine several film layers, which perform different functions. A typical decorative laminate features an optically clear outer layer of polyvinyl fluoride (PVF) wear film that covers and protects one or more layers of printed and/or textured films. Films and laminates are attached to panels with a layer of film adhesive (epoxy, phenolic or polyurethane). DuPont Engineering Polymers (Wilmington, Del.) supplies Tedlar, a well-known PVF product.

Some manufacturers are opting for thermoplastic rather than thermoset materials. In contrast to thermosets, thermoplastics process in a matter of minutes because the heated resin only has to be shaped and cooled - no chemical cure reaction occurs. While high-temperature, high-performance thermoplastic resins like polyetherketoneketone (PEKK) cost significantly more than phenolic resin, the material has better flexibility and impact resistance and processes faster, which makes it better for some applications, says Cytec's thermoplastics business manager Steve Cease, adding that it easily meets the cabin fire/smoke/toxicity (FST) requirements. The small window shade panels that slide down to cover airline windows, for example, are typically thermoformed PEKK plastic sheet. Other manufacturers are investigating a switch to thermoformed thermoplastic parts for load-bearing components that require good impact strength.

Major material suppliers

Because of the size of the interiors market, a large number of suppliers offer resins, woven or unidirectional reinforcements, honeycomb core materials, adhesives and potting compounds, films and a wealth of related products. The principal phenolic resin suppliers include Hexion Specialty Chemicals (Columbus, Ohio), which consolidated former Borden, Bakelite and Shell brands into one entity; Georgia Pacific Resins Inc. (Atlanta, Ga.); Durez Corp. (Addison, Texas), a subsidiary of Sumitomo Bakelite; and Arizona Chemical (Jacksonville, Fla.). Thermoplastic resin and sheet suppliers include Cytec; GE Plastics (Pittsfield, Mass.); Ten Cate Advanced Composites (Nijverdal, The Netherlands) and its U.S. subsidiary Bryte Technologies Inc. (Morgan Hill, Calif.); Solvay Advanced Polymers, maker of the Radell brand (Alpharetta, Ga.); and Kleerdex Co. LLC (Aiken, S.C.). Major core suppliers are Hexcel, M.C. Gill (El Monte, Calif.), Aerocell (Marysville, Wash., now owned by interiors manufacturer C&D Zodiac Inc.), Plascore Inc. (Zeeland, Mich.), Euro-Composites Corp. (Elkwood, Va.), Advanced Honeycomb Technologies Inc. (San Marcos, Calif.) and Schütz (Selters, Germany). All core manufacturers purchase aramid fiber papers from DuPont Advanced Fiber Systems to produce the core.

Cytec Engineered Materials is a major interiors material supplier to OEMs and interiors manufacturers. The company formulates its own specific phenolic resin blends for a wide variety of applications. And, Cytec also produces thermoplastic interior products, including PEKK resin thermoformable sheet, trademarked Declar, for injection molded and press-formed parts.

Webb, a long-time industry veteran, emphasizes the technical expertise required on the part of all material suppliers to balance the competing goals of excellent adhesion versus excellent FST properties: "It's elusive - better adhesion of the prepreg to the core requires a different set of additives than those needed for good fire properties. We've worked hard to address this in our product development."

He says that new products are always in the pipeline, driven not so much by FST issues but by customer demands for lighter weight and faster processing. The company's newest product is CYCOM 6825-1, a unique phenolic resin system that can be used for prepregs with various fabric styles, ranging from eight-harness satin weave to plain weave to unidirectional. According to Cytec's Harmon, it's a low-moisture-release phenolic that provides a superior surface finish with very low 20/20 FST numbers. "The prepreg is self-adhesive, meaning that you get good adhesion to core in a rapid cycle crush core process, without any adhesive that would add weight to the part."

Cease points out that the pigmented Declar thermoplastic sheet can be produced in custom thicknesses in 4-ft by 8-ft (1.2m by 2.4m) size, with or without decorative film caps, and is qualified for large aircraft. A fiber/fabric-reinforced version of Declar is under development, he notes. "A great thing about thermoplastic parts is that not only do they have OSU FST numbers less than 50/50, but they also can be thermoformed at relatively low temperatures and any trim scraps can be reused."

Gurit (formerly Stesalit, Kassel, Germany), another major interior materials supplier, particularly to Airbus programs, is well known for its thermoset phenolic and epoxy prepregs. Many Gurit products have been qualified on the Airbus A380 and older generations of aircraft. Dr. Maarten de Zwart, business development manager at Gurit, explains that cabin flooring panels are produced to meet varying strength requirements: panels under passenger seats take the lightest loads, those in the aisle(s) must support foot traffic and food and beverage trolley loads while those adjacent to aircraft doors must be the strongest. Until recently, panel construction was achieved by cocuring epoxy prepregs with a specially formulated phenolic top layer prepreg to meet aerospace FST requirements. Recent A380 developments resulted in the selection of Gurit's new EP 137 UD carbon/epoxy unidirectional prepreg for floor panels that need higher strength and stiffness but lower weight. Dr. de Zwart notes that the product doesn't require a phenolic ply since the epoxy formulation has low FST properties.

Gurit also provides modified cyanate ester resin prepregs for ventilation and environmental control system (ECS) ducting. Performance requirements for ducts are even more stringent than those for exposed interior components because they also have to fulfill stringent FST values. In the past, Airbus applied phenolic/glass prepregs cured in an expensive autoclave process. By replacing them with Gurit's nonvolatile emitting PN900 low-FST cyanate ester resin system matrix, cost-saving production processes, like vacuum-bag curing, can be used. Also, the fact that free phenol and formaldehyde are no longer emitted during the manufacturing steps makes PN900 an advantageous choice, not only for ECS ducting, but also, in future, for other aircraft interior parts.

Hexcel manufactures interiors materials in both Europe and the U.S. for its OEM and aftermarket customers. A vertically integrated supplier and the originator of the honeycomb-cored sandwich panel concept, the company makes its own core, epoxy resins and prepregs (phenolic resin is purchased from outside suppliers and formulated in-house).

Hexcel offers a broad range of interiors products, says Neil Howard. One of the newest is HexPly M41, a halogen-free, fast-cure phenolic system for wall, ceiling and floor panels, offered in a variety of fabric forms and three different tack levels. Reflecting the fact that phenolic cures faster at higher temperatures, HexPly M41 cycles in four minutes at 160ºC/320ºF or 8 minutes at 140ºC/280ºF using crush core processing, compared to a 30-minute cycle with flat panel press methods. M41 exhibits good surface finish and the lower process temperature means that Tedlar films can be cocured with parts, explains Howard. Plus, FST numbers are in the low 20/20 range, he notes.

Also vertically integrated, M.C. Gill, one of the largest suppliers of prepreg and semifinished honeycomb-cored sandwich panels, produces its own prepregs, honeycomb core and adhesives. Its proprietary sandwich panel designs support both OEM and aftermarket customers and are available as either semi-finished or build-to-print panels. M.C. Gill produces custom panel sizes to order, in standard "stock"finishes or decorated with Tedlar films, says Irv Freund, VP of sales and marketing.

Panels are produced in multi-opening presses in various combinations of fiber reinforcement, resins systems and honeycomb configurations to meet the exacting specifications of the end-user. All resins, including phenolic, epoxy, polyester and acrylics, are formulated in-house to achieve optimum translation of fiber properties, cure cycles and FST. The launch of next-generation aircraft designs has necessitated new product development, notes Freund, with an emphasis on weight savings. One new product is a lightweight floor panel, trademarked GillFloor 4809, which features unidirectional carbon fiber-reinforced epoxy facings and a Kevlar honeycomb core. M.C. Gill's marketing manager Candi Burdick explains that Kevlar N636 core is a high-performance honeycomb that can offer weight savings over Nomex in certain applications. This core, coupled with M.C. Gill's ability to tailor the areal weight of the unidirectional carbon fiber skins and adhesive, says Burdick, allows the product to achieve required performance targets with maximum weight savings. One company using the GillFloor 4809 panel is Goodrich (Uniontown, Ohio). The product is employed in some of its heated floor panel products. Goodrich integrates an electro-thermal heating element, temperature sensors and power switches, along with a titanium surface for lightweight strength. The heated panels are used around the main cabin doors to provide additional heat and to minimize water and snow tracked into the passenger cabin.

Lewcott Corp. (Millbury, Mass.) is a well-known interiors supplier with a twist: the company manufactures its own phenolic base resins, unlike competitors that purchase and formulate, says the company's director of product development Carl Varnerin. "We're the only prepregger in the U.S. manufacturing our own phenolic resin. It offers the advantage of being able to tightly control the resin's performance."

While the company has supplied its "workhorse"LC194 phenolic for years, its new LC195 is a "snap-cure"phenolic system, designed for rapid crush core processing. Varnerin notes that the company targets the aftermarket exclusively and sells to many major interiors manufacturers.

J.D. Lincoln (Costa Mesa, Calif.) supplies literally hundreds of epoxy and phenolic resin system prepregs as well as film adhesives and potting compounds for core edge closeouts. The company recently acquired an additional 6,000 ft2 (557m2) of building space and has installed a new vertical solution coater and tape coater for increased production. Ten Cate Advanced Composites Group supplies thermoplastic laminates for both interiors and air exterior parts, says Nick Tiffin, the company's global business development manager for aerospace interiors. Its CETEX laminates, made from GE Plastics' Ultem polyetherimide resin with excellent FST properties, go into overhead bins, galleys, storage units and floor panels, and its Bryte Technologies division supplies thermoset prepregs for interiors as well. See "Engineered to Innovate,"HPC September 2006, p. 28 for more on Ten Cate's aircraft interiors applications. Isovolta Group (Wiener Neudorf, Austria) provides a range of products, including decorative laminates, prepregs, honeycomb panels and thermoplastic sheet. Nelcote (formerly FiberCote Industries Inc., Waterbury, Conn.) produces phenolic and epoxy prepregs for interior applications; its F529 glass fabric/phenolic product is designed for press molding of interior panels. YLA Inc. (Benicia, Calif.) supplies bismaleimide and cyanate ester resin systems and prepregs for duct applications. Its RS7 toughened epoxy system is currently being qualified for acoustic damping applications for aircraft floors. An interesting material in development is an epoxy-based bulk molding compound (BMC) for seat structures that can replace aluminum, says Sam Sher, YLA's head of marketing.

More to come

Space limits prevent an in-depth look at the myriad parts made from the materials listed above. One example is the new, ultralightweight baggage stow bin molded for the Airbus A380 by AIM Group PLC (Southhampton, Hampshire, U.K.) at its U.S. facility, AIM Aviation (Renton, Wash.). The application makes use of a unique tubular-cored thermoplastic panel from Ten Cate. But one thing is abundantly clear: as airlines move to lighter and more fuel-efficient aircraft, getting weight out of interiors is moving up the priority list for manufacturers. One area under scrutiny is the group of interior parts still made from aluminum, such as seat rails/pedestals and the brackets in overhead stow bins. At least one company is investigating low-cost compression molding of reinforced composite brackets for bins, making them more compatible with composite-bodied aircraft, such as the 787, because there would be no need for isolation plies to address the potential for galvanic corrosion.

With their unique combination of low flammability for passenger safety, low weight, high strength, durable cosmetics and efficient processing, composite materials have a promising future in passenger aircraft interiors. Watch for coverage of these expanding applications in future issues.