Compostable
cutlery and other products made from the biomaterial PLA are biodegradable,
but not fully recyclable. In a recent paper, CSU researchers have
reported a breakthrough in recyclable polymers, which can be
transformed back into their original molecular states using heat. Their
breakthrough could lead to truly recyclable plastics.
The
textbooks and journals said it couldn’t be done.
But
Colorado State University chemists have done it: They’ve made a completely
recyclable, biodegradable polymer, paving a potential new road to truly
sustainable, petroleum-free plastics.
The
innovation is from the lab of Eugene Chen, professor of chemistry and recent
recipient of the Presidential Green Chemistry Challenge award. Publishing in Nature Chemistry Nov. 23, Chen and postdoctoral fellow Miao Hong describe
synthesizing a polyester that, when simply reheated for an hour, converts back
to its original molecular state, ready for reuse.
Recyclable,
in the purest sense of the word.
Their
starting feedstock was a biorenewable monomer that textbooks and journal papers
had declared non-polymerizable, or could not be bonded into large molecules –
polymers – typically required for use as a material.
Renewable
plastics
Plastics
are the most common type of manmade polymer, which is the chemical term for a
long chain of repeating small molecules, or monomers. Plastics like
polyethylene and polystyrene are king among synthetic polymers, and have come
under fire for piling up in landfills. Chen’s lab is focused on making
renewable and degradable plastics and other polymers to replace conventional
petroleum-based materials.
“More
than 200 pounds of synthetic polymers are consumed per person each year –
plastics probably the most in terms of production volume. And most of these
polymers are not biorenewable,” Chen said. “The big drive now is to produce
biorenewable and biodegradable polymers or plastics. That is, however, only one
part of the solution, as biodegradable polymers are not necessarily recyclable,
in terms of feedstock recycling.”
There
are several biodegradable plastics on the market today, chief among them a
starch-based material made from polylactic acid, or PLA. Compostable cups,
cutlery and packaging in dining halls are made from PLA. They’re biodegradable,
yes, but they’re not truly recyclable – once made, they can’t be completely
reconstituted into their original monomeric states without forming other, unwanted
byproducts.
And
what about those little numbers on the bottoms of plastic containers? Doesn’t
that mean “recyclable”? Sort of. Soda bottles, computer keyboards and millions
of other plastics can be repurposed to extend their product lifecycle. But in the
true, chemical sense of “recyclability” – biomolecules that can be synthesized
into a useful material, and then completely converted back to the same
molecules simply by heating the bulk material – is unheard of. Until now.
‘Don’t
even bother with this monomer’
The
researchers’ starting monomer is a mouthful for being such a small molecule:
Gamma-butyrolactone, or GBL. It is a colorless liquid and common chemical
reagent, derived from a top-12 biomass compound best suited to replace
petrochemicals, according to the Department of Energy.
Textbooks
and scientific literature had described these small molecules as too happy and
thermally stable in their monomeric chemical states to polymerize.
“’Don’t even bother with this monomer,’” Chen
summarized the conventional wisdom. “‘You cannot make a polymer out of it
because the measured reaction thermodynamics told you so.’ We suspected that
some of the previous reports were probably incorrect.”
Not
only did they make a polymer, Chen and Hong figured out how to get the polymers
to take different shapes, such as linear or cyclic, based on the catalysts and
conditions they selected. For their experiments, they used both metal-based and
metal-free catalysts to synthesize the polymer, called poly(GBL), which is
chemically equivalent to a commercial biomaterial called
poly(4-hydroxybutyrate), or P4HB.
 |
A
graphical illustration of the researchers’ polymer synthesis process. The
single molecules, or monomers, are cooled in order to polymerize; to cycle
back, heat is applied. Credit: Jing Tang/Chen lab
|
Precise
reaction conditions
They
employed specifically designed reaction conditions, including low temperature,
to make the polymer, and heat between 220-300 degrees Celsius to convert the
polymer back into the original monomer, demonstrating the thermal recyclability
of the polymer.
P4HB
is derived from bacteria, which is a more expensive, complex process than how
most plastics are made. By starting with the readily available GBL and ending
up with a replacement material for P4HB, Chen’s discovery has promising market
potential, and a provisional patent has been filed with the help of CSU
Ventures.
“In my 15 years at CSU, I
would probably call this my group’s most exciting piece of work,” Chen said.
“This work creates a class of truly sustainable biopolymers, as they are both
biorenewable and recyclable, based on a bioderived monomer previously declared
non-polymerizable.”
Chemist Stuns
Scientific Community With Fully Recyclable Biopolymer
Everyone
said it was impossible, but that didn’t deter Colorado State University chemist
Eugene Chen, who recently was awarded the Presidential Green Chemistry Challenge
Award. Chen did the impossible by creating a polymer material from a monomer that was thought
to be non-polymerizable by most scientists. Not only did he overturn a
long-held scientific belief, Chen’s discovery also may revolutionize the field
of bioplastics.
Chen
started his experiments with the monomer Gamma-butyrolactone. Also known as
GBL, the colorless liquid is a common reagent used as a cleaning solution, a
superglue remover, and more. The scientific community believed the monomer was
too stable to polymerize and pointed to measured reaction thermodynamics as
proof to support their assertions. Chen suspected some of these reports were
incorrect, so he decided to go with his hunch and began experimenting with GBL
polymerization.
Chen
and Hong not only worked out a method that caused GBL to form a polymer, the
pair also figured out to make the polymer into different shapes. By varying the
reaction conditions and the catalysts they used in the reaction, the
researchers found they could form either a linear or a cyclic version of the
polymer. In the end, the material they called poly(GBL) was very similar to
P4HB, a polymer derived from bacteria that is used as a biodegradable form of
plastic. Because it is more abundantly available and cheaper to produce,
however, poly(GBL) could potentially replace P4HB in the growing market for
bioplastics.
Poly(GBL)
has another property that makes it even more useful as an
environmentally-friendly source of plastic. The researchers developed a reverse
thermal reaction that caused the polymer to revert to its original monomer
form. Unlike current bioplastics such as PLA that are only partially
biodegradable, this reverse reaction makes poly(GBL) completely recyclable.
Bioplastic objects made from the poly(GBL) material can be recovered from the
waste stream and recycled using a thermal reaction that converts the plastic
polymer back to its monomer form. Once recovered, the GBL monomer is ready
again for use in future plastic products.
This breakthrough could
open the door for the production of a bioplastic material that is not only
petroleum-free, but that can also be recycled easily under heat. Knowing
its market-changing potential, Chen has filed a provisional patent for the
discovery. “In my 15 years at CSU, I would probably call this my group’s most
exciting piece of work,” Chen said in a statement. “This work creates a class
of truly sustainable biopolymers, as they are both biorenewable and recyclable,
based on a bioderived monomer previously declared non-polymerizable.” Chen and
postdoctoral fellow Miao Hong described this discovery in a recent issue of Nature Chemistry journal.
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CSU
Professor of Chemistry Eugene Chen
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Originally published (STORY 1) in SOURCE and (STORY 2) in Digital Trends
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