The team created the material with cellulose nanocrystals mixed with a small amount of synthetic polymer. The organic crystals make up about 60-90% of the material, claimed by an MIT announcement as “the highest fraction of CNCs achieved in a composite to date”.
A single wood cell wall is constructed from fibres of cellulose, nature’s most abundant polymer and the main structural component of all plants and algae. Within each fibre are reinforcing cellulose nanocrystals, or CNCs, which are chains of organic polymers arranged in nearly perfect crystal patterns.
At the nanoscale, CNCs are stronger and stiffer than Kevlar. If the crystals could be worked into materials in significant fractions, CNCs could be a route to stronger, more sustainable, naturally-derived plastics.
The engineers hit on a recipe for the CNC-based composite that they could fabricate using both 3D printing and conventional casting. They printed and cast the composite into penny-sized pieces of film, which they used to test the material’s strength and hardness. They also machined the composite into the shape of a tooth to show that the material might one day be used to make cellulose-based dental implants – or many other plastic products – that are stronger, tougher, and more sustainable.
“By creating composites with CNCs at high loading, we can give polymer-based materials mechanical properties they never had before,” said mechanical engineer Professor A John Hart. “If we can replace some petroleum-based plastic with naturally-derived cellulose, that’s arguably better for the planet as well.”
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 mould for casting. 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.
After feeding some of the gel through a 3D printer, and moulding the rest, they 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,” said researcher Abhinav Rao. “We took the best components of wood, which is cellulose nanocrystals, and reconstructed them to achieve a new composite material.”
When the team examined the composite’s structure under a microscope, they observed that grains of cellulose settled into a brick-and-mortar pattern, similar to the architecture of nacre, the hard inner shell lining of some molluscs. In nacre, this zig-zagging microstructure stops a crack from running straight through the material. The researchers found this to also be the case with their new cellulose composite.
They tested the material’s resistance to cracks, using tools to initiate nano- and then micro-scale cracks. They found that the composite’s arrangement of cellulose grains prevented the cracks from splitting the material.
“This resistance to plastic deformation gives the composite a hardness and stiffness at the boundary between conventional plastics and metals,” the MIT announcement said.
Going forward, the team is looking for ways to minimise the shrinkage of gels as they dry. In bigger objects, shrinkage could cause buckles or cracks during drying.
“If you could avoid shrinkage, you could keep scaling up, maybe to the metre scale,” said Rao. “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 by the US National Defence Science and Engineering Graduate Fellowship.
The work was published in Cellulose.
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