Now, researchers have developed a potentially more
sustainable way to make sharp knives: using hardened wood. The method, presented
October 20th, 2021, in the journal Matter, makes wood 23 times harder, and a
knife made from the material is nearly three times sharper than a
stainless-steel dinner table knife.
“The knife cuts through a medium-well done steak easily,
with similar performance to a dinner table knife,” says Teng Li, the senior
author of the study and a materials scientist at the University of Maryland.
Afterward, the hardened wood knife can be washed and reused, making it a
promising alternative to steel, ceramic, and disposable plastic knives.
Li and his team also demonstrated that their material can be
used to produce wooden nails as sharp as conventional steel nails. Unlike steel
nails, the wooden nails the team developed are resistant to rusting. The
researchers showed that these wooden nails could be used to hammer together
three boards without any damage to the nail. In addition to knives and nails,
Li hopes that, in the future, the material can also be used to make hardwood
flooring that is more resistant to scratching and wear.
While Li’s method to produce hardened wood is new, wood
processing in general has been around for centuries. However, when wood is
prepared for furniture or building materials, it is only processed with steam
and compression, and the material rebounds somewhat after shaping. “When you
look around at the hard materials you use in your daily life, you see many of
them are man-made materials because natural materials won’t necessarily satisfy
what we need,” says Li.
“Cellulose, the main component of wood, has a higher ratio
of strength to density than most engineered materials, like ceramics, metals,
and polymers, but our existing usage of wood barely touches its full potential,”
he says. Even though it’s often used in building, wood’s strength falls short
of that of cellulose. This is because wood is made up of only 40%–50%
cellulose, with the rest consisting of hemicellulose and lignin, which acts as
a binder.
Li and his team sought to process wood in such a way to
remove the weaker components while not destroying the cellulose skeleton. “It’s
a two-step process,” says Li. “In the first step, we partially delignify wood.
Typically, wood is very rigid, but after removal of the lignin, it becomes
soft, flexible, and somewhat squishy. In the second step, we do a hot press by
applying pressure and heat to the chemically processed wood to densify and
remove the water.”
After the material is processed and carved into the desired
shape, it is coated in mineral oil to extend its lifetime. Cellulose tends to
absorb water, so this coating preserves the knife’s sharpness during use and
when it is washed in the sink or dishwasher.
Using high-resolution microscopy, Li and his team examined
the microstructure of the hardened wood to determine the origin of its
strength. “The strength of a piece of material is very sensitive to the size
and density of defects, like voids, channels, or pits,” says Li. “The two-step
process we are using to process the natural wood significantly reduces or
removes the defects in natural wood, so those channels to transport water or
other nutrients in the tree are almost gone.”
This wood-hardening process has the potential to be more
energy-efficient and have a lower environmental impact than for the manufacture
of other man-made materials, although more in-depth analysis is necessary to
say for sure. The first step requires boiling the wood at 100° Celsius in a
bath of chemicals, which could potentially be reused from batch to batch. For
comparison, the process used to make ceramics requires heating materials up to
a few thousand degrees Celsius.
“In our kitchen, we have many wood pieces that
we use for a very long time, like a cutting board, chopsticks, or a rolling
pin,” says Li. “These knives, too, can be used many times if you resurface
them, sharpen them, and perform the same regular upkeep.” Courtesy: www.scitechdaily.com
