Researchers have discovered a way to chemically recycle PVC into usable material, finding a way to use the phthalates in the plasticizers -- one of PVC's most noxious components -- as the mediator for the chemical reaction.
From: University of Michigan
November 30, 2022 -- PVC,
or polyvinyl chloride, is one of the most produced plastics in the United
States and the third highest by volume in the world.
PVC makes up a vast
amount of plastics we use on a daily basis. Much of the plastic used in
hospital equipment -- tubing, blood bags, masks and more -- is PVC, as is most
of the piping used in modern plumbing. Window frames, housing trim, siding and
flooring are made of, or include, PVC. It coats electrical wiring and comprises
materials such as shower curtains, tents, tarps and clothing.
It also has a zero
percent recycling rate in the United States.
Now, University of
Michigan researchers, led by study first author Danielle Fagnani and principal
investigator Anne McNeil, have discovered a way to chemically recycle PVC into
usable material. The most fortuitous part of the study? The researchers found a
way to use the phthalates in the plasticizers -- one of PVC's most noxious
components -- as the mediator for the chemical reaction. Their results are
published in the journal Nature Chemistry.
"PVC is the kind
of plastic that no one wants to deal with because it has its own unique set of
problems," said Fagnani, who completed the work as a postdoctoral
researcher in the U-M Department of Chemistry. "PVC usually contains a lot
of plasticizers, which contaminate everything in the recycling stream and are
usually very toxic. It also releases hydrochloric acid really rapidly with some
heat."
Plastic is typically
recycled by melting it down and reforming it into the lower quality materials
in a process called mechanical recycling. But when heat is applied to PVC, one
of its primary components, called plasticizers, leach out of the material very
easily, McNeil says.
They then can slip into
other plastics in the recycling stream. Additionally, hydrochloric acid
releases easily out of PVC with heat. It could corrode the recycling equipment
and cause chemical burns to skin and eyes -- not ideal for workers in a
recycling plant.
What's more, phthalates
-- a common plasticizer -- are highly toxic endocrine disruptors, which means
they can interfere with the thyroid hormone, growth hormones and hormones
involved with reproduction in mammals, including humans.
So, to find a way to
recycle PVC that does not require heat, Fagnani began exploring
electrochemistry. Along the way, she and the team discovered that the
plasticizer that presents one of the major recycling difficulties could be used
in the method to break down PVC. In fact, the plasticizer improves the
efficiency of the method, and the electrochemical method resolves the issue
with hydrochloric acid.
"What we found is
that it still releases hydrochloric acid, but at a much slower, more controlled
rate," Fagnani said.
PVC is a polymer with a
hydrocarbon backbone, Fagnani says, composed of single carbon-carbon bonds.
Attached to every other carbon group is a chlorine group. Under heat
activation, hydrochloric acid rapidly pops off, resulting in a carbon-carbon
double bond along the polymer's backbone.
But the research team
instead uses electrochemistry to introduce an electron into the system, which
causes the system to have a negative charge. This breaks the carbon-chloride
bond and results in a negatively charged chloride ion. Because the researchers are
using electrochemistry, they can meter the rate at which electrons are
introduced into the system -- which controls how quickly hydrochloric acid is
produced.
The acid can then be
used by industries as a reagent for other chemical reactions. The chloride ions
can also be used to chlorinate small molecules called arenes. These arenes can
be used in pharmaceutical and agricultural components. There is material left
from the polymer, for which McNeil says the group is still looking for a use.
Fagnani says the study shows how scientists might think about chemically
recycling other difficult materials.
"Let's be
strategic with the additives that are in plastics formulations. Let's think
about the during-use and end-of-use from the perspective of the additives,"
said Fagnani, who is now a research scientist at Ashland, a company focused on
making biodegradable specialty additives to consumer goods such as laundry
detergents, sunscreens and shampoos. "Current group members are trying to
improve the efficiency of this process even more."
The focus of McNeil's
lab has been to develop ways to chemically recycle different kinds of plastics.
Breaking plastics into their constituent parts could produce non-degraded
materials that industry can incorporate back into production.
"It's a failure of
humanity to have created these amazing materials which have improved our lives
in many ways, but at the same time to be so shortsighted that we didn't think
about what to do with the waste," McNeil said. "In the United States,
we're still stuck at a 9% recycling rate, and it's only a few types of
plastics. And even for the plastics we do recycle, it leads to lower and lower
quality polymers. Our beverage bottles never become beverage bottles again.
They become a textile or a park bench, which then ends up in a landfill."
https://www.sciencedaily.com/releases/2022/11/221130114529.htm
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