A new composite material engineered from cellulose nanocrystals by a team of MIT researchers could pave the way for sustainable plastics. The cellulose-based composite is stronger and tougher than some types of bone, and harder than typical aluminium alloys.
Cellulose, considered nature’s most abundant polymer, is also the main structural component of all plants and algae. A single wood cell wall is constructed from fibres of cellulose, and within each fibre are reinforcing cellulose nanocrystals, or CNCs, which at the nanoscale, are stronger and stiffer than Kevlar.
The composite created by the MIT researchers is made mostly from cellulose nanocrystals mixed with a bit of synthetic polymer. The researchers found that the cellulose-based composite is stronger and tougher, and has a brick-and-mortar microstructure that resembles nacre, the hard inner shell lining of some molluscs.
The study, which was published in the journal Cellulose, was led by A. John Hart, professor of mechanical engineering at MIT with the research team also including Abhinav Rao PhD ’18, Thibaut Divoux, and Crystal Owens SM ’17.
Looking to develop a composite with high CNC content that could be shaped into strong, durable forms, they mixed a solution of synthetic polymer with commercially available CNC powder in a ratio that would create a gel with a consistency that could either be fed through the nozzle of a 3D printer or poured into a mould for casting. An ultrasonic probe was used to break up any clumps of cellulose in the gel.
The next step was to fabricate the composite using both 3D printing and conventional casting methods. While a part of the gel was fed through a 3D printer to be cast and printed into penny-sized pieces of film to test the material’s strength and hardness, the rest was poured into a mould to be cast. Once dry, the material shrank, leaving behind a solid composite composed mainly of cellulose nanocrystals. The composite was machined into the shape of a tooth to show the material’s potential use in making cellulose-based dental implants.
“By creating composites with CNCs at high loading, we can give polymer-based materials mechanical properties they never had before,” Professor Hart said. “If we can replace some petroleum-based plastic with naturally-derived cellulose, that’s arguably better for the planet as well.”
“We basically deconstructed wood, and reconstructed it,” Rao explained. “We took the best components of wood, which is cellulose nanocrystals, and reconstructed them to achieve a new composite material.”
When examining the cellulose composite’s structure under a microscope, the researchers observed that grains of cellulose settled into a brick-and-mortar pattern, similar to the architecture of nacre. In nacre, this zig-zagging microstructure stops a crack from running straight through the material; the new composite also exhibited similar behaviour.
When testing the composite’s resistance to cracks, they found that the material’s arrangement of cellulose grains prevented the cracks from splitting the material.
The next step for the research team is to minimise the shrinkage of gels as they dry, to be able to scale the casting and fabrication to larger objects.
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