New Composite Lifts Off

Changing the sheet: Plastic sheet made from Ultem PEI has been used in aircraft interiors for more than a decade. Now a new lightweight, formable composite version of the material is about to open up new structural applications like this windo mask.

Plastic sheet made from polyetherimide (PEI) first found a home in aircraft interiors years ago. Most recently, Boeing picked GE Advanced Material's Ultem PEI for use in its C17 jetliner, while interiors supplier Adder has used it in a new movable cabin divider for the Airbus A320. In both cases, PEI got the nod for its aesthetics, strength, thermal performance, and compliance with the industry's stringent burn-test standards. Now, GE has come up with a new composite form of Ultem, which could further expand the use of the material in aircraft and other transportation applications.

GE created the new Ultem-based composite using its SuperLite technology. These composites consist of a thermoplastic base resin and long-glass fiber reinforcement. Unlike other reinforced plastics, though, SuperLite composites are produced in a modified papermaking process: GE first combines chopped glass fibers, resin powder, and additives in an aqueous slurry. It then transforms the slurry into a finished composite by sending it through a series of drying and lamination steps. GE can finish off the composite by applying a variety of cosmetic and functional surface layers or "skins."

The resulting composites have an airy, open celled structure that has very low densities compared to traditional thermoset composites such as FRP. Ultem-based SuperLite composites have specific gravities ranging from 0.5 to 0.9, while compression molded composites usually have gravities between 0.6 to 2.0. Accounting for such a huge density difference, Benny David, a PhD chemical engineer who now manages global transportation applications for GE, points out that the new composites contain 30 to 80 percent air by volume. In real-world designs, this density advantage means that SuperLite components would typically weigh 30 to 45percent less than comparable FRP parts, he says.

At the same time, SuperLite composites don't scrimp on the glass content that provides much of the stiffness desirable in a composite. With a glass content ranging from 40 to 55 percent, Ultem-based SuperLite composites have a flexural modulus up to 600,000 psi and a flexural strength up to 10,000 psi. "They have excellent stiffness- and strength-to-weight ratios," David concludes.

And that isn't all these composites have going for them.

Forms easily

Like earlier SuperLite composites, the new Ultem variety has a likely manufacturing advantage in that it can be thermoformed in matched metal tooling rather than just compression molded. With typical cycle times between three and five minutes for a typical aircraft interior part, thermoforming promises a significant productivity gain over the 15-60 minute cycle times needed to compression mold comparable thermoset composite parts. Thermoforming also requires just a fraction of the pressure used for compression molding—with low process pressures translating into less expensive tooling and processing equipment.

Thermoforming opens up important new design possibilities too. Rob Butterfield, director of design at Fitch, an industrial design firm affiliated with GE, explains that thermoforming offers deeper draws than compression molding—six inches for thermoforming versus about three for compression molding. "The main benefit of the deep draw is that it allows us to create 3D, curved shapes," he says.

And the thermoforming process enables a form of in-tool assembly, reducing the need for secondary operations. GE has experimented with heat-activated adhesives to join plastic subparts to the main composite component while both are inside the matched metal tooling. The company has also been able to press metal attachment points into the composite surface during the thermoforming operations. Butterfield says that the ability to create complex curves and form in attachment points could together reduce the complexity of joints, particularly where metal meets plastic. "We can do away with features like returns and flanges," Butterfield says.

Varying properties

SuperLite composites also offer the ability to "dial-in" desired final densities though tooling design and control of the thermoforming process. In essence, matched metal tools can be designed to squeeze the composites to varying degrees, allowing them to retain more or less of their loft. "The composite first expands like a spring when heated in the thermoformer. The matched metal tooling then brings it down to the target part thickness," David says.

This capability means that the same initial composite sheet can create parts with very different finished densities and mechanical properties. The same principle applies across the surface of a single part. "We can also vary the density within the same part," David says. This kind of localized density control ultimately lets engineers maximize strength and stiffness in the high-stress regions of the part. For example, compressing the corners and edges of a part would add more rigidity to those regions.

The SuperLite composites have a couple of more attractive design capabilities. One is that they fail in a ductile manner. "This is very different than most glass filled products, which fail in a catastrophic brittle mode," David says. The reason for the difference: High loads on the composite will ultimately tear apart the glass and resin matrix, but even as it tears the composite exhibits a "stick-slip" behavior that allows it to support some load.

The other property is the material's low thermal expansion even in large parts. David describes the CTE of SuperLite as falling "between steel and aluminum," though close to both from a design standpoint. For example, a 2.5 mm sample of a 2000 grams/m², 55 percent glass-filled polypropylene SuperLite has a CTE of 18.9 × 10^-6 mm/mm/^°C versus 12.1 × 10^-6 mm/mm/^°C for steel and 23.5 × 10^-6 mm/mm/^°C for aluminum.

According to David, the new Ultem composites exhibit the low smoke toxicity and heat release properties derived from their proven base resin. When it comes to burn performance under high heat loads, though, the composite outperforms the unreinforced sheet. The Ultem used in sheet form has a relatively high HDT of 220F, but some of the aerospace industry's burn tests can challenge just about any thermoplastic.

Doesn't feel the burn

For example, in large-scale burn tests of cargo panels, temperatures can hit 1,400F, seemingly too high for Ultem. But David says that GE scientists "noticed some very interesting properties" related to the new composite's thermal performance. For one, the composite has proved to be more resistant to melting and dripping that you might expect from a thermoplastic subjected to such high heat. "The glass fiber forms a skeleton that supports the resin," David explains. For another, only a relatively small amount of the resin actually participates in the burn since the outside layers of the composite form a "sealing char." David likens this char to a ceramic insulating material and says it and the glass fibers near the surface together help protect the bulk of the composite.

GE's new Ultem composite addresses applications in a variety of aircraft, mass transit, and marine components, including many whose structural loads would preclude the material in its sheet form. In aircraft interiors alone, the potential applications for Ultem composites include ceiling panels, window masks, seat backs, arm rests, bins, tray tables, partitions, doors and even cargo panels. To support these applications, GE initially will offer Ultem SuperLite in a variety of weights, ranging from 1,025 to 2,500 grams/m².