Nano bits of eggshells
let two biodegradable plastics work better together.
An old proverb says
you’ve got to crack a few eggs to make an omelet. Most people then toss those
eggshells out. Indeed, a lot of eggshells end up in landfills, observes
Boniface Tiimob. He’s a materials scientist and graduate student at Tuskegee
University in Alabama. But now, he says, there’s a promising new use for that
waste. Those shells can be recycled into packaging for more eggs or other
goods.
Vijaya Rangari is a
materials scientist, also at Tuskegee. He and Tiimob got their idea when the
National Science Foundation challenged researchers to find ways to put wastes
to good use. The team even decided to make its new packaging biodegradable. In other
words, they designed it to break down into safe materials over a fairly short
period. Then nature can recycle those breakdown products into new resources.
For their design, the
team members drew on their chemistry knowledge and did some homework. For
instance, they studied some earlier work on biodegradable polymers. Polymers
are made from long chains of repeating groups of atoms. Two biodegradable ones
are PLA and PBAT. PLA, or polylactic acid, comes from cornstarch. On its own,
it’s strong but very brittle. “It doesn’t have flexibility at all,” Tiimob
says. PBAT — short for poly (butylene adipate-co-terephthalate) — comes from
petroleum. It’s very flexible but not very strong.
One earlier study
combined calcium carbonate with PLA. This allowed the plastic to flex a bit
more before it broke. Another study showed that PLA was also more flexible with
some PBAT added to it. But the two plastics generally don’t mix well, Rangari
notes.
He and Tiimob believed
they could build upon those studies. They reasoned that calcium carbonate could
help the polymers blend, at the same time adding flexibility. Eggshells, as
they are mostly calcium carbonate, could be a good source, Rangari and Tiimob
figured. So they mixed bits of eggshell in with the plastics. Tiny bits.
The researchers chose
nano-scale particles because they would have lots of surface area.
Rangari’s group had already figured out how to make the nanoparticles. First,
they ground clean eggshells into a fine powder. That process was a lot like
grinding wheat into flour. Next, they blasted the fine powder withultrasonic waves.
These are energy waves that have a frequency higher than audible sound. The
force broke the powder down even further into teeny, tiny bits. Each was shaped
like a stacked deck of cards and was 10 nanometers across. (A human hair is
more than 350,000 times as thick.)
The team blended the
eggshell nanoparticles into various mixtures of PLA and PBAT. The best performance
came from a mix that was about 0.5 percent eggshell. The rest of it was seven
parts of PBAT to every three parts of PLA. The result was about seven times
more flexible than other biodegradable polymer blends, Rangari said. The
plastic was also strong enough for most packaging uses.
The researchers
described their work on March 16 in San Diego at the spring national meeting of
the American Chemical Society.
