IN A NUTSHELL: Glass and carbon fiber reinforced epoxy composites have found preferred use in a wide range of high performance engineering applications in which low weight combined with high strength are key attributes, such as wind turbines, aircraft structural members and surfaces and marine applications. Injection Resin Transfer Molding (RTM), Vacuum Assist Resin Transfer Molding (VARTM), and Seemans Composites Resin Infusion Molding Process (SCRIMP) resin infusion methods currently provide fabricators with a cost efficient means of building large composite structures. These manufacturing methods require resins with relatively low viscosity, which permits the polymer to flow into the dry laminate stack and properly saturate the fibers and core materials. Sufficient toughness to generate good laminate mechanical properties and thermal properties high enough to create a durable functional structure in expected use conditions are also key requirements.

Although wet hand-lay-up, filament winding, and prepreg technologies are used to fabricate wind turbine blades, resin infusion technology is now used in the manufacture of the majority of wind turbine blades, especially longer turbine blades in the 40 to 61 m length range. Wind turbine blades are exposed to the possibility of tensile/compressive and fatigue failures in use. The key to durable blades is to avoid dry spots and void formation in the final assembly because voids are the principle origin of buckling failure in wind turbines. High-quality, easily processed epoxy formulations, such as we are developing, will provide excellent fiber wetting and bonding, significantly less shrinkage on curing and, thereby, improved composite performance.

A resin with relatively low viscosity will have a distinct advantage in vacuum-assisted processing. Low resin viscosity is a necessary attribute for easy flow in the VARTM process. Resin injection is accomplished by drawing a vacuum and having the resin injected by atmospheric pressure. This is the step in which easy resin flow and good fiber wetting without toughening particle filtration are critical for the successful fabrication of a composite part. The composite is then cured and demolded, after which the finishing of the part is completed.

When impact modifiers are formulated with the epoxy resins before processing, marked increased viscosity usually results, thus making infusion processing more difficult. Clearly, an epoxy matrix resin with superior flow and wetting attributes along with good toughness and fatigue resistance will aid in the fabrication of high quality wind turbine blades. A key to the development of the new epoxy system is enhancing its toughness with second phase nanosiloxane hyperbranched polymer particles (NSHP) for which the polymer is initially soluble in the resin and forms nano-scale size range particles after infusion and cure. Resin infusion and final composite particles will be enhanced because the filtering out of the elastomeric particles by the fiber will be avoided.

We are fine tuning the NSHP in regards to molecular weight and final crosslink density and are evaluating epoxies with the NSHP additives in glass fiber reinforced composite panels. The resulting resin viscosity is targeted to be limited to about half of that of the epoxy alone to enhance the production rate for VARTM type processing.