A honeycomb material created using 3-D printing technology and a special epoxy resin mimics the light weight and high strength of balsa wood. © Brett G. Compton, Lori Sanders/Harvard University
Researchers use 3-D printing to create a honeycomb of epoxy resin that mimics the low weight and high strength of balsa wood.
July 15, 2014—Researchers working at Harvard University have successfully developed a lightweight cellular composite material using 3-D printing technology and a special epoxy resin. This honeycomb structure mimics the properties of balsa wood without the natural variability and has the potential to eventually replace the wood in a variety of commercial applications.
Although many people are familiar with balsa from hobby and craft stores, engineers prize the wood because it possesses an exceptional strength-to-weight ratio. Balsa can be found in composite panels in surfboards, in high-performance cars and boats, and in the blades of wind turbines. As the sizes of such turbines continue to increase to maximize efficiency, the low weight and high strength of balsa are proving useful.
“It’s a very effective, lightweight material to use when you need to absolutely minimize weight. It has a very high specific bending stiffness and specific bending strength that rivals modern engineering materials,” says Brett Compton, Ph.D., a staff scientist at the Oak Ridge National Laboratory, in Tennessee. Compton was a postdoctoral researcher when he and Jennifer A. Lewis, Sc.D., the Hansjörg Wyss Professor of Biologically Inspired Engineering in Harvard’s School of Engineering and Applied Sciences, carried out the work. A paper summarizing their findings, “3D Printing of Lightweight Cellular Composites,” was published recently in the journal Advanced Materials.
In nature, the fast-growing balsa can be affected by variations in growing seasons and growing cycles. Most commercial balsa is grown on plantations, Ecuador providing approximately 95 percent of the world’s supply.
“The properties vary greatly according to density. The density of the wood can vary from location to location within a single tree and from tree to tree and from season to season,” Compton notes. “In our material these natural variations are eliminated, resulting in a more consistent material. We can engineer the architecture exactly how we want it to even surpass the properties of the natural material.”
Developing the correct mixture of additives was an iterative process and posed one of the greatest challenges, Compton says. The team introduced a series of special additives, including nanoclay platelets and fibers, to epoxy resin. Without the additives, the resin is a Newtonian fluid; that is, its viscosity is independent of flow rate. The clay platelets and other additives change the rheology of the resin, making it a shear-thinning, yield-stress fluid. As such, when the resin is at rest, it behaves as an elastic solid, but when it is agitated beyond a very low stress point, it flows like a fluid.
“You can extrude epoxy resin out of a very fine nozzle all you want. The hard part is getting it to stay exactly where you want it. One thing that’s nice about the clay platelets that we used is they serve a dual purpose. Not only do they give us the rheology that we want, but they also reinforce the cured material and give it higher stiffness,” Compton says.
The 3-D printing process deposits the resin through a fine-tipped syringe via air pressure. Compton says that because each new layer is applied to the previous one at the same temperature while the material is uncured, the process doesn’t have the layer-to-layer bonding problems encountered with some 3-D printing materials. The team printed a honeycomb pattern because it replicates the hierarchal structures found in natural balsa.
“If you look at a tree, you have the structure of the limbs and branches at a very large-length scale, and then you have the cellular structure of the wood itself at an intermediate-length scale, and then you have the structure of the cellulose and lignin within the cell walls at a very small-length scale. So there is architecture across many different length scales,” Compton says. “With the different size reinforcements we have in the material and the architectures we can print, we are taking advantage of the reinforcement at multiple length scales typically found in nature.”
Compton foresees the material entering the marketplace first as a core in large sandwich panels, where low weight and high stiffness are desirable. Before that happens, however, researchers will need to develop a way to increase throughput to reduce costs.
The technology, however, holds promise for doing more than producing synthetic balsa wood. Compton would like to investigate the possibility of using multifunctional additives capable of imparting electric conductivity or magnetic properties in addition to mechanical reinforcement. He explains that it might also be possible to add sensing materials that could report strains or cracks before they were observable from the exterior of the component.
“There is still a lot of work to be done just to understand how natural materials achieve the exceptional properties and precise functions they do from biological polymers and minerals available in the environment,” Compton says. “There is great potential to take the lessons learned from nature and apply them to engineering materials to create truly outstanding bioinspired composites.”