New and novel resin formulations facilitate composites market expansion
Novel niche formulas enable composites breakthroughs for new market applications, traditional material replacement.
By Sara Black, Technical Editor | June 2004
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.
Tough polyurethanes
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.
Source: Plexinate
New, user-friendly one-component polyurethanes can be used in a variety of applications.
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 last 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., U.S.A.) and Bayer Polymers LLC (Pittsburgh, Pa., U.S.A.).
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., U.S.A.), 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.
