User-friendly resins expanding composites' reach
Composite parts manufacturers are presented with an almost infinite number of reinforcement and resin combinations. While the huge selection can raise concerns in some quarters about lack of standardization, the upside is that today's materials can be precisely tailored to meet a customer's needs. Material suppliers continue to invest significant research and development dollars to improve their products to match the application, with the goal of making composite fabrication more efficient and user-friendly. Resin suppliers, in particular, are constantly tweaking the molecular structures and additive combinations of their thermoset products, in response to customer requests and market demands. Several of the more recent trends in resin system developments offer the potential to grab new market share for composite materials, because of the performance enhancements they bring to the table.
One new trend is the use of polyurethane resin systems for composite parts in continuous processes such as pultrusion and filament winding. Polyurethanes encompass a large family of polymers, all formed by the reaction of an organic isocyanate combined with any compound containing multiple hydroxyl groups, called a polyol (e.g., an alcohol). By virtue of their long-chain polyether chemistry, thermoset polyurethanes are tougher and resist fatigue better than styrenated polyesters, and also have greater impact strength and higher elongation-to-failure. They adhere well to fiber reinforcements, and are relatively inexpensive. And, they emit no volatile organic compounds (VOCs) during processing, which gives users of polyurethanes an advantage in environmental compliance.
Used in huge quantities for foam products in other industries, polyurethanes were not considered a viable material for composites until recently because of very fast gel times and a tendency to foam and create internal voids, which made processing a real challenge. The polyurethane components must be accurately mixed and, once combined, pot life is short and the mix proceeds rapidly to cure, typically within a few minutes. Meter/mix equipment is needed to inject the combined resin into the pultrusion die or filament winding head at the correct location to make optimum use of the resin's limited working time. Internal mold releases are an important aspect of the resin, particularly for pultrusion, to ensure that the parts pull easily at the die exit and yield a smooth surface.
Within the past several years, a number of resin producers have adapted polyurethane formulations for user-friendly composites processing that capitalizes on the proven resin benefits. Two-component (isocyanurate and polyol) systems are offered by Resin Systems Inc. (RSI, Edmonton, Alberta, Canada), Huntsman Polyurethanes (Auburn Hills, Mich.) and Bayer Polymers LLC (Pittsburgh, Pa.).
A polyurethane pioneer, RSI developed its Version family of two-component formulations several years ago. Company president Greg Pendura says the no-VOC formulation can be varied easily to tailor reactivity and pot life for the application at hand. "We can get the exact properties we want for the process," he maintains.
Version resin components are mixed at a 1:1 ratio and injected at low pressure through a resin mixing box and into a static mixing tub, through which the dry roving is passed for wetout before entering the die. A similar system has been developed for the filament winding process. The pot life of each mixed batch is approximately 20 minutes. According to Dave Slaback, director of RSI research and development, the polyurethane is compatible with sizings used on existing roving, and its high reactivity enables faster line speeds than can be maintained in standard pultrusion processes. RSI developed an internal mold release in conjunction with a supplier.
Material testing performed by the Alberta Research Council showed pultruded and filament wound parts made with Version polyurethane resin had higher in-plane and transverse properties than parts made with polyester resins, with a 62 percent higher interlaminar shear strength and 27 percent higher unnotched impact strength. Because the resin performance is so high, the company contends that product designers could decrease reinforcement loading and part wall thickness and as a result, reduce costs.
A recent RSI spin-off, RS Technologies, manufactures parts using RSI's resins. The company is filament winding thousands of power poles for several Canadian customers. Pendura says the poles are lighter in weight but tougher than traditional pultruded products and easily meet all performance requirements established by U.S. and Canadian energy regulatory agencies. RS Technologies also pultrudes hockey sticks, and light standards and wind turbine blades are two other applications in the works.
Bayer is fielding a polyurethane for pultruders called BAYDUR STR 2000 (BAYDUR STR 3000 is a higher-modulus version). Bayer's James Lambach, manager of composite urethane technology, describes how the company experimented with various component blends and tested finished pultruded parts in cooperation with the University of Mississippi, as part of product development. The 1:1 isocyanate/polyol blends were optimized for pultrusion by making subtle changes to the polyol and the catalyst to extend reactivity or pot life to about 15 minutes. Viscosity is low to ensure good fiber wetting. While the polyurethane reaction does generate significant exotherm, says Lambach, it is complete before the resin reaches its gel state. The resin requires thorough mixing with automated metering through a static mixer followed by injection into the die through an injection box to wet out the fibers. A mid-die temperature of more than 232°C/450°F allows for faster cure and faster processing speeds, as well as a better surface finish, as shown by the experimental results.
Lambach says that several types of internal mold release were tested during the trials, to find the formulation most compatible with Bayer's polyurethane. "We're using mold releases from several suppliers who have developed new products specifically for polyurethane," he says. Axel Plastics Research Laboratories (Woodside, N.Y.), for example, has just introduced a MoldWiz internal mold release (INT-1945MCH) that is specifically designed for polyurethane pultrusion.
While no customers are yet producing parts with BAYDUR STR 2000, Lambach is confident that the demonstrated superior part properties and the environmental benefits will convince pultruders to make the switch from polyester to polyurethane. "The big benefit is impact resistance and toughness," he notes, adding, "Secondary fastening is easier and the parts are much less likely to split or crack under fastener pressure." A side benefit is excellent paint adhesion to finished pultruded parts.
A radical new approach to polyurethane is a class of resins that build a polyurethane polymer into a polyester resin backbone or, conversely, graft vinyl groups into a polyurethane backbone. Crosslinking is initiated with a peroxide, while styrene provides a free radical addition reaction, so these one-component polyurethanes process like a standard polyester, but deliver the superior toughness and elongation of a polyurethane. While the presence of styrene is a potential environmental drawback, fabricators don't have to change procedures and setups like they do with typical VOC-free, two-part polyurethanes. Plexinate (Auckland, New Zealand and Beaufort, S.C.) builds vinyl into a polyurethane backbone to form a one-component, styrenated resin used in industrial products and marine applications, such as the hulls and decks of Elliott sailboats, manufactured in New Zealand. A flex additive, consisting of a polyol-modified urethane, is available to tailor resin tensile and flexural strength, says Plexinate's Tim Scott. Priced between polyester and vinyl ester, the resin can be used in pultrusion exactly like an unsaturated polyester. Reichhold Inc.'s (Research Triangle Park, N.C.) DION 31040 XTREME resin, part of the XTREME family of urethane products, is a new single- component polyurethane hybrid designed specifically for pultrusion. Nelson Douglass, Reichhold's pultrusion specialist, says customers were asking for a resin that delivered higher fatigue resistance and impact strength than could be achieved with traditional polyesters.
"Most pultruders are still using open baths, despite new environmental pressure, and don't necessarily want to invest in injection equipment," says Douglass. "The XTREME resin has the advantage of a very long catalyzed pot life, which is appropriate for an open bath setup, yet it also can be injected, if the customer prefers."
By optimizing the chemical recipe, Reichhold's research and development chemist Dr. Hildeberto Nava says his team was able to formulate a resin with a catalyzed pot life of 48 hours at room temperature -- a radical departure for polyurethane -- yet with high reactivity to enable fast pultrusion line speeds.
Nava and Douglass underscore some unanticipated benefits of the XTREME formulation, which include one-half the amount of shrinkage of a polyester and better pigmenting ability than some two-component polyurethanes. Also, surface quality is better than the team expected, with reduced fiber print-through.
Applications of XTREME include heavy-duty composite leaf springs for quieter and more fatigue-resistant theatre seats, and power poles/electrical cross-arms that outperform wooden versions. Northstar Vinyl Products LLC (Kennisaw, Ga.) has a patent pending on the use of pultruded reinforced polyurethane as sheet piling in demanding shoreline applications (photo, upper left), and is impressed with the exceptional toughness and durability of the product.
A third type of polyurethane for pultrusion is Dow Chemical's FULCRUM resin system (see CT August 2003, p. 38), now owned and marketed by FULCRUM Composites (Midland, Mich., U.S.A.). The FULCRUM technology is based on an engineering thermoplastic polyurethane that works with a temperature-activated catalyst, a diisocyanate and diol combination that disassembles the long polymer chains temporarily for lower viscosity within a tight temperature range. Upon cooling, the catalyst deactivates and the molecules reassemble to original length, with associated high strength and toughness. Several pultruders have adopted the VOC-free FULCRUM technology and are marketing parts like suspended ceiling frames.
Creative Pultrusions (Alum Bank, Pa.) — likely the most aggressive promoter of polyurethane technology based on its product conversions to date — confirms polyurethane's superior per- formance with test data. "The true measure and benefit of improved toughness is impact resistance," says Creative's Joseph Sumerak, a champion for parts made with polyurethane. "It's a difficult property to quantify, but the high-speed Instrumented Impact test [ASTM D256] really shows a difference." Static tests on a variety of pultruded profiles show that total load sustained, deflection and peak/total energy absorption are all substantially increased for a polyurethane system, compared to polyester.
Auto SMC breakthrough
Polyester sheet molding compound (SMC) offers many advantages to automotive OEMs - among them, high strength-to-weight, greater design flexibility and less-expensive tooling when compared to steel. But one issue that has dogged molders of SMC body panels is the tendency for paint "pops" to develop as freshly painted panels cure in the automaker's high-temperature baking ovens. The problem stems from microcracks along the edges of the relatively brittle panels, which result from repeated handling and flexing during the manufacturing process. During preparation for painting, solvent migrates into the cracks, and despite the fact that the paint completely covers the crack, the solvent vaporizes and rises through the paint layer while in the curing oven, leaving a small crater on the panel surface.
Automotive component molder ThyssenKrupp Budd Co. (Troy, Mich.) devoted considerable research-and-development time to improving SMC's ultimate appearance and performance, and has come up with a revolutionary new SMC product called Tough Class A SMC. AOC Resins (Collierville, Tenn.) was a Budd development partner and worked on synthesis of the polyester resin that goes into Budd's new sheet molding compound, says AOC's Mike Dettre. AOC offers its own version of the resin, which it calls Atryl TCA, to other customers.
Budd's patent-pending polyester formulation has nearly 70 percent higher toughness than standard polyester, as quantified by measuring the resin's tensile strength, generating a stress/strain plot and measuring the area under the curve (that is, strain energy per unit volume). To test the resin's ability to stop paint pops, Budd incorporated it into SMC containing 10 percent chopped glass fibers (a lower percentage than normal SMC to better evaluate the resin strength) and molded a series of coupons or plaques. The test coupons were subjected to flexure to induce cracking, then painted and evaluated for paint pops. Budd researchers report that the test material reduced paint pops by more than 80 percent compared to standard SMC.
Budd produces its own SMC in-house, using compounding equipment supplied by Finn & Fram Inc. (San Fernando, Calif.). The SMC incorporates about 25 percent glass fibers (1-inch in length) and approximately 50 percent fillers, including multiple, low-profile additives (LPAs) for reduced shrinkage and improved surface quality. Tough Class A SMC has been introduced into hoods, decklids and fenders on several Ford models, including the Ranger truck, the Mustang Mach I, the Thunderbird and the Lincoln Navigator. Michael Siwajek, supervisor of material development in Budd's R&D group, says that Tough Class A SMC is a huge breakthrough. "We've essentially eliminated paint pops at Ford," says Siwajek. "The material is performing way beyond our wildest dreams. The biggest upside is that Ford is starting to design in Class A SMC again, because they're excited about the results."
Despite Tough Class A SMC's breakthrough performance improvement, environmental pressure may change the way body panels are painted altogether, which raises another appearance issue for SMC. DaimlerChrysler and General Motors are trying to eliminate paint solvents by going to powdercoating (electrostatic painting technology), in which an electric charge is applied to the body panel, causing the negatively charged paint particles to be attracted to it. SMC panels, however, tend to absorb moisture after molding. When powdercoated and baked at high temperature, the moisture comes up through the coating and causes a "foaming" effect. Both AOC and Budd are working on various approaches to resolve this new issue, including electrically conductive SMC and new additives that reduce moisture absorption. AOC's latest SMC material, which is still in development, showed moisture absorption of less than 0.7 percent after exposure to hot/wet conditions during recent testing.
Resins are organic materials, that is, they can burn. For that reason, ever more stringent U.S. and European fire regulations often put composites at a disadvantage for use in the interior architecture of public buildings and in passenger-carrying marine vessels, aircraft and mass transit vehicles. Historically, resin suppliers have produced compliant resins by adding a variety of fire retardants, like bromine or halogenated compounds (those that contain chlorine). These additives are very good at minimizing flame spread, but they produce smoke and lethal gases, which has led to usage bans in Europe. Further, additive and filler loadings reduce the mechanical performance of the cured laminate.
"It's a conflict," says Joe Parker, technical sales manager at Gougeon Brothers Inc. (Bay City, Mich.), maker of Pro-Set epoxy products. "The more material for fire performance you add to the formulation, the more problems you can create in terms of resin strength and workability."
To meet new fire performance guidelines, resin makers have been able to improve fire performance and reduce smoke and byproduct generation, while maintaining or increasing physical properties.
Magnolia Plastics Inc. (Chamblee, Ga.) is a specialty formulator of epoxies for a wide range of applications, from automotive to aerospace. Magnolia sells its products to makers of aircraft interiors in planes that often operate throughout Europe, so the interiors must comply with the more stringent European regulations, says company president Rick Wells. He adds that the firm was "ahead of the curve" when the European Union first proposed banning pentabromodiphenyl ether (PBDE) as a fire retardant. PBDE and other halogens were removed from Magnolia's epoxy products before the regulation took effect, and the resins were reformulated with other proprietary fillers to improve mechanical properties. More recently, the company has introduced a new fire-retardant adhesive, called MagnoBond 6151A/B, a two-part epoxy paste packaged in a dual-cartridge format. The new adhesive meets Boeing's Material Specification BMS528 for aircraft interiors, reportedly offering both excellent fire performance and good peel and shear strength.
Gougeon has developed two fire-retardant epoxy laminating systems that have meet the fire specifications of IMO (International Maritime Organization) Circular 1006 and meet the approval of Lloyd's Register of North America for lifeboats as well. The resins are "type" certified, meaning they can be used with any reinforcement type. According to Parker, the resin has very low viscosity for good fiber wetout, in contrast to traditional highly filled resins, yet has passed all flame spread tests "with flying colors." The resins can be cured at room temperature, but to qualify for the Lloyd's certification, parts must be postcured at 60°C/140°F for eight hours. Available with different hardeners, the resins' working time can be adjusted. One of the resins is suitable for infusion processes, thanks to low viscosity, while the other is better for contact or vacuum bag laminating, says Parker.
Phenolic resin has been the industry's fire-resistance standout because its phenol/formaldehyde chemistry has a very high crosslink density, making it very resistant to high temperatures. Phenolic produces very little smoke or toxins when it is exposed to flame. Traditionally, phenolics were difficult to process and required high heat and pressure for adequate cure, says Aram Mekjian of Mektech Composites (Hillsdale, N.J.), who distributes Borden Cellobond phenolic resins. Today's lower-viscosity phenolics use an active catalyst and process more like polyester, which enables high-speed fabrication methods like pultrusion. Because they perform well with few additives, phenolic parts are significantly lighter in weight than polyesters with high filler loadings -- an advantage in weight-sensitive environments, such as offshore oil platforms and submarine interiors. "Cellobond phenolic meets the very stringent MIL standard 2031 for submarine interiors, with only a small amount of filler," says Mekjian.
Charles Dore, a consultant to and technical director of fabricator Cinnabar Florida Inc. (Orlando, Fla.), has taken phenolic one step further by developing a new in-house, ultra-fire retardant formulation that reportedly outperforms all others produced to date. Cinnabar, a producer of special-effects and recreational parts, such as theme park rides, movie and commercial sets, wanted a high-performance phenolic-based material for laminating large parts to be used in confined areas without restriction, and decided to develop its own. During initial tests conducted by Dr. Nicholas Dembsey at Worchester Polytechnic Institute (Worchester, Mass.) the proprietary resin combined with E-glass did not ignite after 15 minutes of flame exposure (75 kW/m2), under ASTM E1354 cone calorimeter test conditions, reports Dore. He plans to obtain an International Code Council (ICC) listing for the material so that architects and engineers can specify it and to ensure that code officials and fire engineers know that it meets established criteria. Dore is looking for resin suppliers or compounders interested in obtaining a license for the technology. "This material really opens up a market for both interior and exterior architectural applications," Dore maintains.
To meet the requirements of British Standard (BS) 6853 and an anticipated European standard, prEN 45545 for mass transit, additive producer BYK-Chemie GmbH (Wesel, Germany), has teamed with Menzolit-Fibron (Kraichtal, Germany) to optimize the fire-retardant properties of SMC. Formulated with a combination of additives, Menzolit's SMC 2400 has shown in tests that it meets the most stringent hazard level of the new regulation, says BYK-Chemie's Michael Sommer. "This new SMC formulation can meet stringent fire and smoke requirements and help designers fulfill the needs of modern train design," he says. Valspar Composites (Elkhard, Ind.), well known for its zero-VOC resins and gel coats, is one of several polyester suppliers that has removed halogens from its gel coat products to meet changing fire regulations. The company's 5785 series of gel coats exhibits low toxicity and very low smoke production in laboratory testing, says Valspar's technical director Ehtisham Ashai. "These gel coats have opened new markets for existing customers, like amusement park rides, panels for architecture and rail interiors. They contain aliphatic monomers for better elongation and performance both during application and service," he says. The material meets the requirements of Bombardier specification TS-3050 Interior Panel LRS, among others.
Creative Pultrusion's Sumerak and his group are investigating the fire retardance of polyurethanes, an area where the resins historically have not performed well. "In our tests, hydrogen cyanide generation, anticipated to be a problem, was well below all regulatory levels in the pultruded profiles tested," says Sumerak. "Hydrogen chloride and hydrogen bromide are a nonissue, since polyurethane isn't halogenated. While carbon monoxide generation levels pass most requirements, we'll continue to research compositions that will provide a comfortable margin for all FST [flame/smoke/toxicity] situations," he concludes.
More on the horizon
With so many new resins available, we don't have room to mention them all. We'll continue our coverage of the burgeoning number of new resins, for applications like closed molding, as well as innovative new thermoplastic formulations for a growing number of markets.
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