Composites take flight
Many vacations begin with an airplane flight. Chemistry is essential in lifting every jet off the ground. Most airplanes, though, are made of the same materials that were in vogue at the dawn of the jet age.
But novel new materials, such as lightweight polymer composites, will soon rule the sky. The Boeing 787 Dreamliner, for example, consists of nearly 50% composites by mass, which helps it reduce carbon dioxide emissions and save fuel.
Ultrastrong composites, like those in the Dreamliner, combine a polymer matrix derived from thermosetting resins and reinforcements made of materials such as carbon fibers, glass fibers, or aramid fibers (such as Kevlar).
Polymer composites used in airplanes are made of a polymeric material reinforced by carbon or glass fibers. The fibers provide strength and stiffness, while the polymer serves as the “glue” holding the fibers together.
Despite their advantages, composites are limited by operational temperatures, explains Boris Bulgakov of Lomonosov Moscow State University. “For most common materials in aerospace based on epoxy resins, operational temperatures cannot be higher than 120 °C; the usual range is −50 to 50 °C,“ he says. Some models of aircraft and parts, such as low-pressure compressor blades, can reach up to 350 °C.
Bulgakov and his team are developing alternative polymer matrices composed of bis-phthalonitriles, organic compounds with high aromatic content. Thermosetting plastics, like phthalonitrile resins, can be molded into various shapes then hardened with heat—but only once. Reheating the resins won't make them pliable again. That means phthalonitrile resins can be exposed to high operating temperatures without damage.
Although these thermosetting plastics have been around since the 1980s, they haven't taken off because the monomers' high melting points limited their use. Attempts to decrease the resins' melting points resulted in a decrease in thermoset properties, Bulgakov explains.
Now, his team has produced low-melting-point phthalonitrile resins with strong thermoset qualities. “Our resins can withstand temperatures up to 450 °C,” he says. “Also, the matrices possess the highest flame-retardant properties among the known plastics.”
Another perplexing issue for composites is bonding. Composites are built from multiple layers of material. And while they often have higher rigidity and greater strength, they are also subject to catastrophic failure.
A team from the Department of Aeronautics & Astronautics at Massachusetts Institute of Technology, working with Swedish aerospace company Saab, has devised a surprising solution: stitching composites with carbon nanotubes. It achieved this ultrastrong stitching by vertically aligning billions of carbon nanotubes per square centimeter of composite and bonding them with polymers.
The idea of boarding a plane that's “sewn together” may be unnerving for some travelers. But the team found that their carbon nanotube stitches were 30% stronger than traditional bonding methods. “Carbon nanotubes have an extremely high surface area—1,000 times more than carbon fiber,” explains team member Roberto Guzman, now a researcher at FIDAMC, the Foundation for the Research, Development & Application of Composite Materials in Madrid. “The greater the surface area, the higher the energy required to break the material.“
These new resins and bonding strategies may lead to composites that can be used in more and more extreme aerospace environments. That should lead to lighter-weight airplanes with lower emissions and fuel consumption. Travelers may not notice that from the (relative) comfort of seat 22C, but they may see it in lower ticket prices, while the more eco-minded passengers will also be cognizant of the smaller carbon footprint they're leaving.