New plant-derived composite is tough as bone and hard as aluminum
By Jennifer
Chu, MIT News Office
February 10, 2022 -- The strongest part
of a tree lies not in its trunk or its sprawling roots, but in the walls of its
microscopic cells.
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
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.
Now, an 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 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
molluscs.
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 today 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.
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.”
This research was supported, in part, by
the Proctor and Gamble Corporation, and by the National Defense Science and
Engineering Graduate Fellowship.
https://news.mit.edu/2022/plant-derived-composite-0210
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