When FACC AG (Ried, Austria) contracted to fabricate a demonstration composite replacement part for a highly stressed aluminum spoiler center fitting on the Airbus A340-600, the company wanted to use resin transfer molding (RTM) because the part's complex shape would be difficult to produce consistently and cost-effectively with hand layup. RTM, however, demanded a low-viscosity resin that could quickly and completely wet out the dry fiber preform, while the part's expected in-service loads dictated use of toughened epoxy for high compression strength. Toughened epoxy contains additives that make it too viscous and elastic to adequately permeate a preform without high injection pressures and temperatures - a key reason aerospace fabricators have been slow to adopt resin infusion methods. The PRIFORM system from Cytec Engineered Materials (Tempe, Ariz., U.S.A., select 217) met the viscosity challenge by putting the toughening agents in the preform rather than the resin. PRIFORM's one-part, low-viscosity resin is based on Cytec's CYCOM 977-2 toughened epoxy system, reformulated without toughening agents. The polymer toughening material is instead spun into continuous fibers and cowoven with glass, carbon or aramid structural fibers to form hybrid fabrics or 3-D stitched preforms. (Cytec says the spinning process causes no physical or chemical changes to the toughening agent.) The thermoplastic fibers in the fabric are designed to dissolve into the epoxy resin during cure, at temperatures above 100°C/212°F.
The matched metal RTM mold is loaded with a hybrid 6K carbon fiber/soluble fiber fabric preform, engineered and manufactured by Cytec. FACC injects low-viscosity PRIFORM 977-20 resin at an initial temperature too low to dissolve the toughening fibers (between 60°C/140°F and 90°C/194°F), ensuring that the material won't be carried away with the flow front before the resin exit port is closed. During tool heating and subsequent cure, the soluble fibers diffuse into the resin and through the preform within five minutes at a mold temperature of 140°C/284°F.
The spoiler fitting demonstrator weighs significantly less than the original, and has passed all Airbus-required static and fatigue tests. Additional Cytec coupon testing showed that RTM'd PRIFORM coupons have properties equivalent to hand layed/autoclave-cured prepreg coupons. Cytec expects PRIFORM to enable cocuring of "toughened" parts with larger structures made with compatible composites, thus increasing parts integration and reducing structural weight and cost by eliminating fasteners.