The future of organically inspired structures and recyclable composites

In the evolution of aerospace materials, additive manufacturing using continuous fiber is the next generation. It allows for the creation of strong, lightweight, energy-efficient, complex designs that resemble natural structures such as tree branches or the human circulatory system rather than traditional designs. Researchers at the University of Illinois Urbana-Champaign are changing the designs and refining the manufacturing processes while also working to make the structures recyclable.

“Continuous fiber additive manufacturing brings together with the strength and stiffness of composite material with the ability to make new and complex designs,” said Jeff Baur, a professor in the Department of Aerospace Engineering at the University of Illinois Urbana-Champaign.

Baur said traditional aircraft frames are made from ribs and spars that are individually easy to produce and then assembled into a grid-like structure. With continuous fiber additive manufacturing, the structure can be much more complicated in design. By letting the computer determine the optimum location to put material and the direction of the fiber, the structure can be optimized for the specific loading conditions. This often results in organically shaped designs that are 20 to 40 percent lighter and have the ability to carry more weight with less structural material.

“We’ve been building truss-like structures for a long time,” Baur said. “With these new manufacturing processes, we can create branched structures that appear more organic. For example, the hierarchical branched root and limb structure of a tree mechanically stabilizes the tree against high winds. Such a structural design would typically be too time-consuming and expensive to manufacture by traditional means. However, with continuous fiber additive manufacturing, there is little processing penalty for hierarchically branched structures,” he said.

While design complexity can increase the structural efficiency of a structure it can also make it more difficult to recycle structural materials at the end of a structure’s useful life.  

Baur is a researcher in the Center for Regenerative Energy-Efficient Manufacturing of Thermoset Polymeric Materials. Illinois is the lead institution in this Department of Energy Basic Energy Sciences Energy Frontier Research Center along with partners: Sandia National Laboratories, Massachusetts Institute of Technology, Harvard University, Stanford University, and the University of Utah. Other U of I researchers in REMAT are Director Nancy Sottos, Deputy Director Jeffrey Moore, Jeff Baur, Randy Ewoldt, Philippe Geubelle, and Sameh Tawfick.

The center is addressing fundamental scientific challenges for energy efficient manufacturing and realistic end-of-life recycling strategies for thermoset polymers and their composites. The technical challenges to recycle composites and reuse materials are significant.

“We’ve made thermoset materials robust,” Baur said. “They're durable in extreme conditions. We’ve made them so durable, that when we're done with them, they  typically can only be buried or burned.”

He said some suggest down cycle—grind it up and use it in another application.

“The thermosetting composites we use are strong and durable because they are highly cross-linked like a molecular net. In the REMAT center, we use a process called frontal polymerization to form the molecular net in a way that is energy efficient and requires very little specialized equipment. We’ve also been designing links in the molecular net that cleave when they are exposed to certain conditions that they would not experience during use.

"The vision is that we will have a robust material with a useful life span. And when it’s done, we expose it to those special conditions at which those cleavable links break apart into long chains called oligomers. The oligomers can be reactivated for repeated use as a cross-linked resin or as a starting material for another formulation."

Baur said, “Half of the researchers in the center are focusing on new molecules that can both frontally polymerize and cleave under select conditions, while he and other researchers are involved with assessing the processing and properties of the materials. For composites, breaking down the polymer matrix would allow us to recover the carbon fiber, which is the component that is the most expensive and energy intensive to produce. We can then reuse it instead of making new fiber.”

Baur said using materials for aerospace and aircraft applications may be a long way off because qualifying a new material and certifying an aerospace structural design made with new material is a lengthy and costly process.

“We likely can’t start with aerospace structures because those are the hardest,” Baur said. “But we can start with other structures that don’t have the extremes in environmental conditions or the same level of testing rigor and performance as aerospace structures. Everybody is thinking about potential applications, but the center is focused on fundamental chemistry and understanding the connection between the molecular structure and the material’s ability to be energy efficiently processed, the resulting mechanical properties, and the potential to be reused multiple times.”