BMI Suppliers Target New Grades At Broader Market Penetration

By Patrick A. Toensmeier

A milestone was reached in military aviation last December with the maiden flight of the F-35 Lightning II Joint Strike Fighter (JSF), one of the most advanced aircraft ever commissioned by the U.S. Air Force. The warplane incorporates defining technologies in numerous areas that will be critical to its success in multi-role missions. Notable among these is composites. The F-35 makes extensive use of bismaleimide (BMI) and carbon fibers in wings and other structures to meet the extreme thermal and mechanical demands of flight and to improve the aircraft's radar-evading or stealth properties.

The specification of BMI by F-35 builder Lockheed Martin highlights the capabilities of a resin that to date has been a niche material in the gallery of high-performance thermosets and thermoplastics. Grades of BMI routinely withstand continuous-use temperatures of up to 500-600°F., yield compression strength of around 35,000 p.s.i. and are generally impervious to most corrosive environments. The thermoset is, however, expensive (military versions range up to $120/lb. for resin and $65/lb. for prepregs), difficult to work with and often limited in processability, characteristics that have largely confined its use to aerospace applications where performance trumps concerns over cost and manufacturability.

This could be changing. A number of BMI suppliers are developing grades they claim will be far less expensive than those qualified for military use and easier to process, while offering no tradeoff in properties or performance. These grades are beginning to enter the market and are targeted at industrial applications and cost-sensitive aviation markets like commercial jet-engine and landing-gear components, where they will compete with metal. There's also a chance that some new BMI resins will find use in larger consumer-oriented markets like automotive, where interest has been expressed in applying their high-heat properties and chemical resistance to lightweight engine-block components.

Some suppliers maintain they can provide new grades of BMI resin for around $40/lb., near the price of high-heat epoxies, which they put at about $30/lb. Producers claim that in some industrial applications the higher price for these new grades would be justified by the greater benefits achieved over epoxies in thermal and mechanical properties and in processability.

"One of our strategies is to offer BMI on the order of about $40/lb.," says Linas Repecka, owner of Raptor Resins Inc., Celina, Tenn. Fibraplex plans to commercialize a grade called BMI-2, which is targeted at industrial markets like down-hole oil-well applications. "We've gotten the price down significantly and there are no properties or performance tradeoffs."

Lower-cost BMI resins and prepregs are essential to suppliers' efforts to increase demand for the material beyond military and aerospace markets. Most of the BMI produced in the U.S. goes to these areas. The F-35 JSF and another BMI-rich fighter plane, the F-22 Raptor, are major consumers. Numbers are difficult to come by, especially since BMI is also used as a curing agent in rubber, but some experts put annual production of the resin at about 1 million lb. This contributes to high cost, as do the lengthy certification processes necessary for military and aerospace use. Another factor affecting demand and cost is the International Traffic in Arms Regulations (ITAR) act, which regulates the export of materials like BMI when used with high-modulis fibers, a composition that is critical to U.S. defense applications.

The outlook for total composites use in aerospace is strong. Consultant AeroStrategy LLC of Ann Arbor, Mich., forecasts a doubling of market value to $14 billion within a decade. While BMI will likely pick up a share of this, the ability to successfully market lower-cost grades to other end-uses would not only yield new business opportunities for suppliers but increase the performance options available to product engineers.

The strategy has pitfalls, notably in cost and the challenges to many engineers and product designers in working with an unfamiliar thermoset. "The potential for BMI in other markets is there," says Bob Lacovara, senior technical director for the American Composites Manufacturers Assn., Arlington, Va. "We can take composites to some pretty extreme physical properties. But the cost starts to increase significantly. When you fall into that mode, you may be able to fabricate in metallics less expensively. So you would really need composites for a specific reason."

Suppliers are banking on there being enough non-military applications to justify a closer look by engineers at new BMI grades.

One feature of the BMI-2 grade from Raptor Resins, is its low-temperature cure. Most BMI resins are cured at 440-480°F., Repecka says. The BMI-2 version cures at 325°F., which reduces process time. Despite the lower cure, the resin has a glass-transition (Tg) temperature of 700°F., as measured by dynamic mechanical analysis. The Tg of most BMI grades is 520-530°F., Repecka claims. "The Tg temperature of BMI-2 is not only much higher than that of other BMI resins, but greater than most polyimides that are formulated for use at higher temperatures than BMI."

The compression strength of BMI-2, moreover, is 43,000 p.s.i., much higher than that of high-end epoxies and many other grades of BMI. The material also cures void-free without an autoclave, which means parts engineered for use at 500-600°F. can be fabricated with a vacuum bag and an oven. "This is an innovation," Repecka says. "BMIs have typically not been processed in autoclaves due to the need for high-temperature cures and because of off-gassing." He credits the high performance and economics of the material to formulation technology.

BMI-2 was developed as a repair kit for the Air Force, which specified the low cure temperature. Repecka says it works well in structural applications. The material is processable by resin transfer molding and resin infusion molding with a vacuum bag. Viscosity is adjustable for use in compression molding compounds, sheet molding compounds and most other processes including injection molding.

Another company planning to compete with high-end epoxies and some phenolics is YLA Advanced Composite Materials, Benicia, Calif. The company, a subsidiary of Sweden's Perstorp AB, is using a combination of mechanical blending and molecular tailoring to produce BMI that also costs about $40/lb. and, like grades from Fibraplex and other suppliers, have no tradeoffs in performance. Sam Sher, director of new business development and marketing, says the materials, tentatively called Xponent RS-8HT, will debut in the second quarter. Though test data are being compiled, he says the material has improved hot-wet properties and greater impact resistance than the existing RS-8 line, which is used in missiles and aircraft.

YLA is also looking at down-hole oil-well applications for the grade and at aerospace tooling for BMI parts. One advantage of using the RS-8HT grade in tooling is it will have the same coefficient of thermal expansion as the component being fabricated, thus improving dimensional stability and related specifications. The material will be suitable for filament and tow winding, prepregs and compression molding. The company can tailor molding compounds with a variety of fiber lengths to enhance the strength of individual applications.

Hexcel Corp., Dublin, Calif., cites improved processability from its new BMI resins. The supplier's M65 grade is formulated to increase productivity of filament winding and unidirectional tape-laying, says Karen Olesen, manager of new product integration and market development for matrix materials. By improving such characteristics as tack, Hexcel produces BMI that fully impregnates fibers and tapes, preventing fuzzing and stringers that jam machines and increase downtime. "Some of our customers are doubling production rates," she says. The resin, which is slightly more expensive than high-end epoxies, can be used in any critical loadbearing component. It is also available in a prepreg called HexPly M65.

Olesen says Hexcel will soon announce a derivative of M65 with all of the process benefits and a high compression-after-impact strength rating of 45,000 p.s.i. The resin, now in the initial stages of sampling, has heat resistance of 400°F. (short-term to 450°F.), and is initially targeted at landing gear and composite fan blades with high tip speeds that resist bird impacts.

One area of interest in BMI formulations is nanotechnology. NanoSperse LLC, Akron, Ohio, utilizes dispersion technology for nanoparticles that was developed by the Air Force and the Univ. of Dayton Research Institute. Arthur Fritts, founder and president of NanoSperse, is guarded about details of his company's work, but says that developmental projects are underway for customers in three key areas of composities -- electrical, thermal and mechanical properties. One goal is to develop nanodispersions to the point where they can be routinely used to enhance the performance of matrix materials like BMI.

The high performance required of BMI is evident in components specified for the F-35. These include wing seals fabricated by EDO Corp., Salt Lake City, Utah. Wing seals are the interphase between movable and fixed surfaces, and are manufactured to extremely tight tolerances. EDO is supplying Lockheed Martin with 68 wing seals per aircraft, ranging in size from 2 to 15 sq. ft.

The wing seals are fabricated by hand lay-up with Cycom 5240-4 prepregs from Cytec Engineered Materials, Tempe, Ariz. (Cytec officials declined to be interviewed for this article, but confirmed the company's BMI is in use on the F-35.) Mike Thersen, project manager at EDO, says the prepregs are up to 70% carbon fiber.

EDO lays up the prepregs in tools where they are shaped and their asymmetrical configurations measured by laser-tracking sensors. Thersen says there are no flat surfaces on the wing seals, so it's important to make certain the proper contours and thicknesses are done during lay-up. A laser-ply template projects a beam onto the tool and transmits information to the operator about how many and what types of plies are needed for a component. "This replaces eyebrow templates, which were cumbersome and labor-intensive," he says. Once the lay-up is complete, components are moved to an autoclave for curing.

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