New plant-based composite is as tough as bone and hard as aluminum

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The strongest part of a tree is not its trunk or sprawling roots, but its microscopic cell walls.

A single wooden cell wall is constructed from cellulose fibers, nature’s most abundant polymer and the main structural component of all plants and algae. Within each fiber are reinforcing cellulose nanocrystals, or CNCs, which are chains of organic polymers arranged in near-perfect crystalline patterns. At the nanoscale, CNCs are stronger and stiffer than Kevlar. If crystals could be made into materials in large fractions, CNCs could be a pathway to stronger and more durable naturally-occurring plastics.

Now a team from MIT has designed a composite composed mainly of cellulose nanocrystals mixed with a little synthetic polymer. Organic crystals occupy approximately 60-90% of the material, the highest fraction of CNC achieved in a composite to date.

The researchers found that the cellulose-based composite is stronger and tougher than some types of bone, and harder than typical aluminum alloys. The material has a brick-and-mortar microstructure that resembles mother-of-pearl, the hard inner wall of some molluscs.

The team found a recipe for the CNC-based composite that they could make using both 3D printing and conventional molding. They printed and cast the composite into penny-sized pieces of film that they used to test the material’s strength and hardness. They also machined the tooth-shaped composite to show that the material could one day be used to make cellulose-based dental implants – and for that matter, all plastic products – which are stronger, tougher and more durable.

“By creating composites with high-load CNCs, we can give polymer-based materials mechanical properties they never had before,” says A. John Hart, professor of mechanical engineering. “If we can replace some petroleum-based plastics with naturally occurring cellulose, that’s probably better for the planet as well.”

Hart and his team, including Abhinav Rao PhD ’18, Thibaut Divoux and Crystal Owens SM ’17, published their results today in the journal Cellulose.

Gel bonds

Each year, more than 10 billion tons of cellulose are synthesized from the bark, wood or leaves of plants. Most of this cellulose is used to make paper and textiles, while some is ground into powder for use in food thickeners and cosmetics.

In recent years, scientists have explored the uses of cellulose nanocrystals, which can be extracted from cellulose fibers by acid hydrolysis. The exceptionally strong crystals could be used as natural reinforcements in polymer-based materials. But the researchers were only able to incorporate small fractions of CNC, because the crystals tended to clump together and bind only weakly to polymer molecules.

Hart and his colleagues sought to develop a composite with a high CNC fraction, which they could shape into strong, durable shapes. They started by mixing a synthetic polymer solution 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 ultrasound probe to break up clumps of cellulose in the gel, making it more likely that the dispersed cellulose would form strong bonds with the polymer molecules.

They fed part of the gel into a 3D printer and poured the rest into a casting mold. They then let the printed samples dry. In the process, the material shrunk, leaving behind a strong composite composed primarily of cellulose nanocrystals.

“We basically deconstructed the wood and rebuilt it,” says Rao. “We took the best components of wood, namely cellulose nanocrystals, and rebuilt them to obtain a new composite material.”

stubborn cracks

Interestingly, when the team examined the structure of the composite under a microscope, they observed that the cellulose grains settled in a brick-and-mortar pattern, similar to the architecture of mother-of-pearl. In mother-of-pearl, this zigzag microstructure prevents a crack from going straight through the material. The researchers found that this was also the case with their new cellulose composite.

They tested the material’s resistance to cracking, using tools to initiate cracks first at the nanoscale and then at the microscopic scale. They found that at multiple scales, the arrangement of the cellulose grains in the composite prevented cracks from splitting the material. This resistance to plastic deformation gives the composite a hardness and rigidity on the boundary between conventional plastics and metals.

Going forward, the team is looking for ways to minimize shrinkage of the gels as they dry. While shrinkage isn’t much of an issue when printing small objects, anything larger can warp or crack when the composite dries.

“If you could avoid the shrinkage, you could keep increasing, perhaps on the meter scale,” Rao says. “Then, if we were to dream big, we could replace a significant fraction of plastics with cellulose composites.”

This research was supported, in part, by the Proctor and Gamble Corporation and the National Defense Science and Engineering Graduate Fellowship.

– This press release was originally posted on the Massachusetts Institute of Technology website

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