Reconstructing wood: MIT develops plant composite tougher than bone for plastic replacement

15 Feb 2022 --- Researchers at the Massachusetts Institute of Technology (MIT), US, have engineered a composite made 60%-90% from cellulose nanocrystals (CNCs) mixed with synthetic polymer. The scientists say this composite is stronger and tougher than some types of bone, harder than aluminum alloys and has the potential to replace “a significant fraction of plastics” in packaging.

“By creating composites with CNCs at high loading, we can give polymer-based materials mechanical properties they never had before,” says study author John Hart, professor of mechanical engineering. “If we can replace some petroleum-based plastic with naturally-derived cellulose, that’s arguably better for the planet as well.”

The institute’s research has shown that the strongest part of a tree lies not in its trunk or sprawling roots but in the walls of its microscopic cells. Furthermore, the organic crystals present in the newly developed material form the highest fraction of CNCs achieved in a composite to date.

Naturally derived plastics
The researchers explain a single wood cell wall is constructed from fibers of cellulose, nature’s most abundant polymer, and the main structural component of all plants and algae. Within each fiber are reinforcing CNCs, which are chains of organic polymers arranged in nearly perfect crystal patterns.

A tooth printed by the MIT team (Image credit: A. John Hart, Abhinav Rao; edited by MIT News).CNCs are stronger and stiffer than Kevlar at the nanoscale. The study authors say if the crystals could be worked into materials in significant fractions, CNCs could be a route to stronger, more environmentally sustainable, naturally derived plastics.

The researchers found the cellulose-based composite is stronger and tougher than some types of bone and harder than typical aluminum alloys. Furthermore, the material has a brick-and-mortar microstructure that resembles nacre – the hard inner shell lining of some mollusks.

Polymer-based potential
According to the researchers, more than 10 billion tons of cellulose are synthesized from bark, wood, or plant leaves each year. Most of this cellulose is used to manufacture paper and textiles, while a portion is processed into powder for use in food thickeners and cosmetics.

In recent years, scientists have explored uses for cellulose nanocrystals, which can be extracted from cellulose fibers via acid hydrolysis. The “exceptionally” strong crystals could be used as natural reinforcements in polymer-based materials.

However, researchers have only been able to incorporate low fractions of CNCs, as the crystals have tended to clump and only weakly bond with polymer molecules.

The researchers found the cellulose-based composite is stronger than some types of bone, and harder than aluminum alloys.Increasing CNC levels
Hart and his colleagues looked to develop a composite with a high fraction of CNCs that they could shape into strong, durable forms. They started by mixing a solution of synthetic polymer with commercially available CNC powder.

The team determined the ratio of CNC and polymer that would turn the solution into a gel, with a consistency that could either be fed through the nozzle of a 3D printer or poured into a mold to be cast. They used an ultrasonic probe to break up any clumps of cellulose in the gel, making it more likely for the dispersed cellulose to form strong bonds with polymer molecules.

The scientists then fed some of the gel through a 3D printer and poured the rest into a mold to be cast and let the printed samples dry. In the process, the material shrank, leaving behind a solid composite composed mainly of cellulose nanocrystals.

“We basically deconstructed wood, and reconstructed it,” Rao says. “We took the best components of wood, which are cellulose nanocrystals, and reconstructed them to achieve a new composite material.”

In related developments, Toraphene has developed a graphene-enhanced bioplastic that is “significantly stronger” than any other plastic alternative.

Edited

By Natalie Schwertheim

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