Bio-Sourced Materials Could Make Composites Greener

Wood, a natural composite material, made a crucial contribution to aviation’s beginnings and is still in use in light aviation. However, it was supplanted in commercial aircraft production by metal for several reasons, such as the stiffness of aluminum relative to its weight. But now that the industry has turned its attention to becoming sustainable, wood may help aviation again.

Composite materials, especially carbon-fiber-reinforced plastics, are gradually becoming widespread in airframe construction, thanks to their light weight. That quality translates into lower fuel consumption and a path toward sustainability. Fuel savings are the preponderant outcome of using composites over an aircraft’s life cycle, allowing the high energy consumption of composite production to be largely recouped over time.

. Research into a more sustainable production process is underway

. Carbon fiber could be created from wood rather than oil

As the aerospace manufacturing sector seeks to reduce its environmental footprint, changes to the process of making fossil-sourced composites would be welcome. This is where wood may serve an important role: Carbon fiber could be created from cellulose or lignin—the main two ingredients in wood—thanks to a more efficient technique.

When using composites, the fossil origin of the raw materials cannot be ignored. When the world stops relying on oil, it will be left under the ground, and a different substance will be required for composite materials.

And those are just two of several environmental issues researchers are considering involving composites production. At stake, too, is the toxicity of some ingredients, such as the Bisphenol A contained in epoxy resins.

Attempts to replace fossil-sourced carbon fiber are underway. Lufthansa Technik is pitching AeroFLAX as the first renewable, eco-efficient and aerospace-grade preimpregnated fabric. Fibers come from flax, and the resin uses agricultural waste, such as from corn harvests, as feedstock. AeroFLAX is at the research and technology stage. It is suitable for cabin interior components, but not airframe parts that sustain strong loads.

Led by the IRT Jules Verne research and technology center in Nantes, France, the Suspens program is studying the environmental effect of manufacturing light composite structures. It started in January and is planned to last 3.5 years. The programs stands to benefit from €4.9 million ($5.3 million) of funding as part of the framework of a European Commission support scheme.

Along with the leisure boat and automotive industries, aerospace is expected to benefit from the work, thanks to IRT Jules Verne’s historically strong orientation toward aviation. IRT Jules Verne’s board is chaired by the manager of Airbus’ Nantes factory.

At the heart of the problem is an energy-intensive carbonization process. The usual, fossil-sourced precursor of carbon fiber is called polyacrylonitrile (PAN). It needs to be carbonized at temperatures of 1,500-2,000C (2,730-3,630F) to morph into carbon fiber, says Mehdi Marin, head of the Suspens program at IRT Jules Verne.

One idea in the Suspens program is to use lignin—wood’s matrix, as opposed to the cellulose fiber reinforcement—as the precursor, thus replacing PAN. From a solution of lignin, a thread would be created and then carbonized at temperatures several hundreds of degrees Celsius lower than PAN, Marin says.

The priority in Suspens is to ensure that the resulting carbon fiber performs as well as conventional fiber manufactured from PAN, he emphasizes.

Paradoxically, cellulose—wood fiber—is not as good a carbon fiber precursor as lignin. As part of Suspens, cellulose will be used directly as a reinforcement (as opposed to a precursor), Marin says. It might find an application for cabin interior panels.

Meanwhile, 95% of bio-sourced resins could be synthesized from flax, rape, castor oil or algae, Marin says. Suspens is focusing on thermoset composites rather than thermoplastics. For aerospace, engineers are looking at creating epoxy resins.

Although the Suspens program is not conducting a complete study of the supply chain for bio-sourced matter, that aspect is being considered for the ingredients in resins, Marin says.

At the part-manufacturing stage, Suspens engineers are studying changes to the resin’s chemical composition and behavior to shorten the curing time. Curing currently involves several hours at temperatures of 100-180C, Marin says.

As part of the program, a small-scale, 60-cm-long (24-in.) winglet will be designed and built. The idea is to validate Suspens’ advancements on a hollow-construction component with a complex shape and a strong need for stiffness. It also could be used as a foil demonstrator for the leisure boat industry.

One criterion an aerospace-grade composite material must meet will be under particular observation: a glass transition temperature high enough to avoid the deterioration of mechanical performance. Certification requirements call for the airframe to withstand 80C (such as at an extremely hot airport), and a typical 30C margin to glass transition must be ensured, Marin says.