New plant-derived composite is tough as bone and hard as aluminum
The material could pave the way for sustainable plastics.

Date:
February 11, 2022
Source:
Massachusetts Institute of Technology
Summary:
A new woody composite is tough as bone and hard as aluminum,
and might pave way for naturally derived plastics.



FULL STORY ==========================================================================
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 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 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.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Abhinav Rao, Thibaut Divoux, Crystal E. Owens, A. John
Hart. Printable,
castable, nanocrystalline cellulose-epoxy composites exhibiting
hierarchical nacre-like toughening. Cellulose, 2022; DOI:
10.1007/s10570- 021-04384-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220211080620.htm

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