Un nouveau composite ligneux, mis au point par une équipe du MIT, est aussi résistant que les os et aussi dur que l’aluminium, et pourrait ouvrir la voie à des plastiques d’origine naturelle. Cette image montre une dent imprimée par l’équipe reposant sur un fond de cellules de bois. Crédit : Image reproduite avec l’aimable autorisation des chercheurs et éditée par MIT News.
Ce matériau pourrait ouvrir la voie à des plastiques durables.
La partie la plus solide d’un arbre ne réside pas dans son tronc ou ses racines tentaculaires, mais dans les parois de ses cellules microscopiques.
La paroi d’une cellule de bois est constituée de fibres de cellulose – le polymère le plus abondant dans la nature et le principal composant structurel de toutes les plantes et algues. À l’intérieur de chaque fibre se trouvent des nanocristaux de cellulose de renforcement, ou CNC, qui sont des chaînes de polymères organiques disposées en motifs cristallins presque parfaits. À l’échelle nanométrique, les CNC sont plus forts et plus rigides que le Kevlar. Si les cristaux pouvaient être transformés en fractions significatives de matériaux, les CNC pourraient constituer une voie vers des plastiques plus solides, plus durables et d’origine naturelle.
Aujourd’hui, un MIT team has engineered a composite made mostly from cellulose nanocrystals mixed with a bit of synthetic polymer. The organic crystals take up about 60 to 90 percent of the material — the highest fraction of CNCs achieved in a composite to date.
The team hit on a recipe for the CNC-based composite that they could fabricate using both 3-D-printing and conventional casting. Credit: Figure courtesy of the researchers
The researchers found 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 nacre, the hard inner shell lining of some mollusks.
The team 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 that 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 — and for that matter, any 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,” says A. 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.”
Hart and his team, including Abhinav Rao PhD ’18, Thibaut Divoux, and Crystal Owens SM ’17, have published their results on February 10, 2022, in the journal Cellulose.
Gel bonds
Each year, more than 10 billion tons of cellulose is synthesized from the bark, wood, or leaves of plants. Most of this cellulose is used to manufacture paper and textiles, while a portion of it 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. But 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 team sculpted the composite into the shape of a tooth to show that the material might one day be used to make wood-derived dental implants — and for that matter, any plastic product — that are stronger, tougher, and more sustainable. Credit: Figure courtesy of the researchers
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 3-D 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.
They fed some of the gel through a 3-D printer and poured the rest into a mold to be cast. They then 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 is cellulose nanocrystals, and reconstructed them to achieve a new composite material.”
Tough cracks
Interestingly, 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. 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 first nano- and then micro-scale cracks. They found that, across multiple scales, 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.
Going forward, the team is looking for ways to minimize the shrinkage of gels as they dry. While shrinkage isn’t much of a problem when printing small objects, anything bigger could buckle or crack as the composite dries.
“If you could avoid shrinkage, you could keep scaling up, maybe to the meter scale,” Rao says. “Then, if we were to dream big, we could replace a significant fraction of plastics, with cellulose composites.”
Reference: “Printable, castable, nanocrystalline cellulose-epoxy composites exhibiting hierarchical nacre-like toughening” by Abhinav Rao, Thibaut Divoux, Crystal E. Owens and A. John Hart, 10 February 2022, Cellulose.
DOI: 10.1007/s10570-021-04384-7
This research was supported, in part, by the Proctor and Gamble Corporation, and by the National Defense Science and Engineering Graduate Fellowship.
