High-performance, fiber-reinforced polymeric composite materials are the preferred materials for many aerospace industry, defense industry, and wind energy applications due to their high modulus-to-weight ratio and their high strength-to-weight ratio. As the applications for such materials become increasingly demanding, the materials performance must be improved. A common approach to improve the performance of such materials is to incorporate nanomaterials as an additional reinforcement to offer improved mechanical, thermal, electrical, and barrier properties plus good flame retardance if the processability challenges can be overcome while addressing the possible safety risks associated with nanomaterials. However, to date, significant improvements have been a great challenge to the research community. Now, a group of researchers from several engineering disciplines at The Ohio State University
(OSU) in Dr. L. James Lee’s laboratory has developed what might be a revolutionary strategy for combining the advantages of both long-fiber reinforcements and nanomaterial reinforcements to create a superior long-fiber-reinforced polymeric (FRP) nanocomposite.
Doctoral candidate, Dante Guerra pioneered and leads the research effort and said, “The most commonly used method to incorporate such nanomaterials is to premix them into the polymer resin and to disperse them by means of high shear mixing and the use of ultrasonic forces then conduct infusion; however this approach is limited by the amount of nanoparticles that can be successfully incorporated, typically less than 15 percent. This process leads to an increase in viscosity up to 5 orders in magnitude, significantly reducing processability. There is an even bigger challenge to incorporate nanomaterials into prepreg based composite systems due to the high resin viscosity of the partially cured resin.” He added, “Our research has focused on improving mechanical properties, such as tensile strength and modulus, with successful improvements up to 35 percent at room temperature, and greater improvements at higher temperatures.”
“We are working with companies that use nanofillers and want to use more of them, even though such materials have a long certification process to fully enter the marketplace and have to compete with composites already available and exhibiting good properties,” said Guerra. Because conventional FRP’s are high insulators and wear poorly compared to metals, nanomaterials could likely be a good substitute, especially where abrasion and wear are big issues, such as with aircraft, on the leading edges of wings as well as landing gear. “Appropriate nanomaterials could extend part lifespan from 20 to 40 years, which is why there is so much interest in overcoming the challenges.”
Another challenge, and thus another goal for the materials being developed in Dr. Lee’s laboratory, is electromagnetic interference (EMI) shielding and lightning strike mitigation. FRPs have often used metallic meshes and sputter coated or painted-on conductive coatings to achieve EMI shielding levels and lightning strike mitigation. The OSU project seeks to achieve improved performance over present technologies at a much lower weight and much lower cost by the use of nanomaterials.
Guerra and colleagues are investigating two methods for combining nanoparticles with FRPs: 1) a spray approach that incorporates nanoparticles onto the continuous fiber reinforcement preforms to achieve overall improvement in mechanical properties; and 2) a surface reinforcement approach where previously manufactured nanoparticle thin-films are incorporated on the surface of the composite material, thereby providing the desired surface properties for a given application such as electrical conductivity, EMI shielding, de-icing, and abrasion resistance. The films, now known as Buckeye NanoPaper®, are a stand-alone nanopaper produced in a continuous fashion. The nanopaper can be easily incorporated to composite materials by means of established molding techniques, and extremely tailorable -- and is thus the focus of much of the group’s research.
OSU is certainly not the only research center working on this problem. Competitors in the race for a workable, marketable product range from universities to companies that are using nanopaper manufacturing techniques that range from solvent-filtration to continuous physical vapor deposition (CVD) processing. “Our approach was to look for the middle ground – to successfully develop a scalable manufacturing process with the flexibility of producing application specific nanopapers with excellent performance, while keeping the process simple and affordable,” said Guerra. “We are dispersing different nanomaterials such as carbon nanofibers (CNFs), carbon nanotubes (CNTs), and clay nanomaterials with or without chemical surface modification in a solvent, which is then deposited onto a substrate to create a nanopaper layer. The solvent is then evaporated, leaving a stand-alone nanopaper layer. This process can be repeated multiple times to produce multi-layered nanopapers that take full advantage of electrostatic interaction between individual layers, thus optimizing the nanopapers’ properties, all at a very fast production rate.” Guerra added, “This proprietary approach can be considered a platform technology in the sense that our nanopapers are engineered specifically for each given application, optimizing performance while keeping the cost at a minimum.” In fact, tests are showing that a 300 µm thick CNF Buckeye NanoPaper incorporated on the surface of wind blade composites increased the materials’ sand erosion resistance by 85 percent.
Research to date has been funded by the Center for Multifunctional Polymer Nanomaterials and Devices
(CMPND), a Wright Center of Innovation supported by the Ohio Department of Development (ODOD) and a RCP grant from ODOD, and previously was funded by Bell Helicopter, Ashland Chemical and Owens Corning.
Guerra looks forward to future development and proof-of-concept work with the Buckeye NanoPaper. He and Dr. Lee’s team plan to conduct EMI shielding effectiveness tests, lightning strike mitigation tests, address handling and safety, scale-up the automated manufacturing process, and optimize nanopaper properties specific to application. The group has a patent pending.
The Center for Multifunctional Polymer Nanomaterials and Devices (CMPND) leads a research and commercialization partnership in polymer nanotechnology. This multi-institutional, interdisciplinary organization is centered at The Ohio State University in conjunction with research university partners, University of Akron, University of Dayton, University of Toledo, Kent State University, and Wright State University. CMPND puts Ohio at the forefront of nanotechnology research and commercialization opportunities. Other partners include three additional Ohio universities, and more than 60 large and small companies in Ohio. CMPND helps target markets that build on the research strengths of the participating universities and national labs, and develops manufacturing protocols and nanostructures for near-term industrial polymeric nanocomposites, emerging polymer photonic components and devices, and more futuristic biomedical devices and systems with nanoscale functions.