The modern world is filled with plastic in part because plastic is, well, plastic: flexible and moldable, polymers can be formed into a seemingly infinite variety of shapes. Aluminum, too, has its merits, and has maintained a place in daily life. But plastic’s origins in fossil fuel extraction and burning, and its tendency to persist long after its useful life – along with the environmental and human costs of aluminum mining – have prompted interest in alternative materials.
Wood is one alternative, but no matter how intricately carved or expertly carpentered, wood can’t be as plastic as plastic or as extrudable as aluminum and still maintain its mechanical strength. Scientists have invented ways to break down wood into cellulose nanofibers, a process that makes it moldable, but is expensive and energy intensive. Turning wood into a liquid slurry also eliminates its star quality: the rigidity inherent in wood’s porous arrangement of parallel fibers and vessels.
Researchers from the University of Maryland and Yale University, with funding from the U.S. Department of Energy, have developed a new process that renders hardwood foldable and moldable, while also making it stronger. They described their method in the October 20, 2021 issue of the journal Science.
First, they used an alcohol-based solution, similar to what is used to make wood pulp, to remove a little more than half the lignin and hemicellulose (cellular components that influence strength) from the cell walls of a thin sheet of basswood. As the remaining cells absorb water, the wood expands and softens. The wood is then dried, shrinking the fibers and vessels, and then quickly re-hydrated. As the vessels re-open, they create space within the wood that allows it to undergo accordion-like compression and tension without cracking.
“This rapid water-shock process forms a distinct partially open, wrinkled cell wall structure that provides space for compression as well as the ability to support high strain, allowing the material to be easily folded and molded,” wrote Shaoliang Xiao, University of Maryland researcher and lead author. Xiao and his colleagues tested the molded wood against a similar piece of aluminum alloy. The wood stayed intact after being folded and unfolded along the grain 100 times. The aluminum broke after just three folds.
A final drying to remove remaining water leaves a final, fixed, wooden shape. The researchers showed examples of zigzag, corrugated, rolled, twisted, honeycomb, and star-shaped structures. The process preserves wood’s longitudinal rigidity while enhancing interactions among the wood fibers, increasing the mechanical strength – as much as six times stronger than the original wood material, according to Yale University.