The future of composites may lie in carbon nanotubes. Nano composites have already found their way into cars and sports gear, and now specialists in this technology are looking for aerospace applications.
Carbon nanotubes are cylinders of carbon atoms arranged like a roll of chicken wire with hemispheres at both ends to complete the structure. Each hemisphere looks like a football, with atoms arranged in pentagons and hexagons. “Carbon nanotubes were first described in 1991 by Sumio Iijama, a Japanese researcher who investigated arc-discharge soots using an electron microscope,” Kai Schierholz, vice president for technology and research and development at Nanoledge, a Montpellier, France-based company specializing in nano materials R&D and production.
Nano composites use carbon nanotubes to replace (partly or totally) conventional fibers in a resin matrix. Both single- and multi-wall tubes exist, with the latter being made of several concentric cylinders.
One family of applications is in electricity, with nanotubes replacing other conductive materials in the matrix. A thin coating of conductive nano composite establishes electronic shielding and the resulting antistatic properties have already been used, for example, to prevent sparks in car fuel lines.
“For a given conductivity, a smaller load of carbon nanotubes is needed, compared to other conductive materials,” Schierholz told Aviation International News. This means that the matrix better retains its mechanical properties. Experience shows that a load of 2 to 5 percent (in weight) of carbon nanotubes is fine for those applications that need good conduction. Antistatic applications need less than 1 percent.
Another family of applications is reinforcement for structural materials. This yields better comfort and enhanced performance in some sports gear, Schierholz said. For example, in high-end bicycle frames, carbon nanotubes give better stiffness and foreign object impact resistance, and the combined flexibility and stiffness are major benefits in ski applications, he said.
The first family of applications should be of great interest to the aerospace industry. Making the matrix conductive would enable designers simply to eliminate the wire mesh that is usually added to composite subassemblies. “We are ready and the material is here. It is a nanotube-loaded thermoset. We still have to test it to convince designers it is time for change, said Schierholz.
In structural applications, nanotubes may help cut weight, with a combination of carbon fibers, carbon nanotubes and epoxy resin. “You can develop thinner parts
because nanotubes improve mechanical properties and so need less material,” Schierholz said.
However, weight saving isn’t necessarily the main advantage over conventional composites. Schierholz emphasized that the use of carbon nanotubes could allow composites to replace those metal parts that currently still outperform their composite equivalent. Weight savings would still be significant, but the nanotubes also can improve the composites’ mechanical properties such as shock resistance, flexural strength and modulus as well as compression strength. Nano composites also are less prone to delamination.
According to Andy Upinie, Cessna’s director for research and advanced technology, company experts are closely watching nanotube technology for applications in five to 10 years. And Gulfstream’s structures staff scientist Frank Simmons added that “nano composite technology has the potential to make a huge impact on the composite industry.”
Nevertheless, there has been significant progress in conventional composites. As Simmons noted, over time fiber stiffness has doubled and fiber strength has improved by more than 50 percent. Simultaneously, advances in resin chemistry are yielding lower cure temperatures and pressures.
In recent enhancements, Serge Dellus, head of Dassault’s advanced technologies development center, sees the development of injection techniques. “Resin transfer molding and resin film infusion, for example, enable more integrated parts, with more precise geometries,” he said. In addition, better resins are giving improved damage tolerance.
At the same time, composite users are trying to simplify the curing phase or even abandon the autoclave, which is expensive, as are autoclave-capable tools. It also is more expensive to operate an autoclave than an oven, and use of an autoclave can create production bottle necks.
In response to these issues the industry has been developing systems that allow curing without the use of high pressures or temperatures. According to Tim Shumate, Cytec Engineered Materials’ marketing manager, one new process needs only relatively low temperatures of 180- to 250-deg F (80- to 120-deg C), local vacuum and a post cure phase. The latter needs little equipment.
According to Shumate, the Holy Grail of composite technology could be advanced fiber placement and in situ curing, “that is, curing the material as you lay it down on the tool with the fiber placement machine.” There have been some studies where thermoset resins are cured in place using pressure and temperature applied by the fiber placement machine. Post curing would probably be necessary, which would not be much of a disadvantage. However, Shumate believes that porosity could still be an issue with such a process.
So there is probably even more potential in manufacturing thermoplastic (as opposed to thermoset) resin-based systems in situ. “I say in situ manufacturing versus curing since thermoplastic materials do not cure like thermosets,” Shumate explained.
“Thermoplastics are heated to a temperature that allows them to flow. So, if you can develop a fiber placement machine that applies temperature and pressure at the correct levels you could make a part out of thermoplastic composites and totally avoid the autoclave and oven. This would be a game changer.” R&D is already being done in this field but the results appear to be some years off.
A majority of aerospace composites are thermosets. Is this because they outperform thermoplastics? Maybe not. “Roughly speaking, the preference for thermoset is simply because they were more ready for use when composites hit the market,” Shumate said.
Nonetheless, thermoplastics do have some advantages, such as easier storage. In addition, it can be argued that thermoplastics perform better mechanically.
But thermosets (like epoxy) were adopted first and are well established. They are easier to handle when using a hand lay-up process, which was the predominant composite manufacturing process in the 1980s and early 1990s. But now that automated processes are being developed for thermoplastics, they are gaining in popularity, Shumate asserted. “Change is slow in the industry but one day thermoplastics may catch on,” he said. Dutch-based Stork Aerospace is already offering floor beams made of fiber-reinforced thermoplastic.
