Chemists make tough plastics recyclable
New method for producing thermoset plastics allows them to be broken down
more easily after use

Date:
July 22, 2020
Source:
Massachusetts Institute of Technology
Summary:
Chemists have developed a way to modify thermoset plastics with
a chemical linker that makes it much easier to recycle them,
but still allows them to retain their mechanical strength.



FULL STORY ========================================================================== Thermosets, which include epoxies, polyurethanes, and rubber used
for tires, are found in many products that have to be durable and heat-resistant, such as cars or electrical appliances. One drawback
to these materials is that they typically cannot be easily recycled or
broken down after use, because the chemical bonds holding them together
are stronger than those found in other materials such as thermoplastics.


==========================================================================
MIT chemists have now developed a way to modify thermoset plastics with
a chemical linker that makes the materials much easier to break down,
but still allows them to retain the mechanical strength that makes them
so useful.

In a study appearing today in Nature, the researchers showed that they
could produce a degradable version of a thermoset plastic called pDCPD,
break it down into a powder, and use the powder to create more pDCPD. They
also proposed a theoretical model suggesting that their approach could be applicable to a wide range of plastics and other polymers, such as rubber.

"This work unveils a fundamental design principle that we believe is
general to any kind of thermoset with this basic architecture," says
Jeremiah Johnson, an professor of chemistry at MIT and the senior author
of the study.

Peyton Shieh, an American Cancer Society Postdoctoral Fellow at MIT,
is the first author of the paper.

Hard to recycle Thermosets are one of the two major classes of plastics,
along with thermoplastics. Thermoplastics include polyethylene and polypropylene, which are used for plastic bags and other single-use
plastics like food wrappers.

These materials are made by heating up small pellets of plastic until
they melt, then molding them into the desired shape and letting them
cool back into a solid.



========================================================================== Thermoplastics, which make up about 75 percent of worldwide plastic
production, can be recycled by heating them again until they become
liquid, so they can be remolded into a new shape.

Thermoset plastics are made by a similar process, but once they are
cooled from a liquid into a solid, it is very difficult to return them
to a liquid state.

That's because the bonds that form between the polymer molecules are
strong chemical attachments called covalent bonds, which are very
difficult to break.

When heated, thermoset plastics will typically burn before they can be remolded, Johnson says.

"Once they are set in a given shape, they're in that shape for their
lifetime," he says. "There is often no easy way to recycle them."
The MIT team wanted to develop a way to retain the positive attributes
of thermoset plastics -- their strength and durability -- while making
them easier to break down after use.

In a paper published last year, with Shieh as the lead author, Johnson's
group reported a way to create degradable polymers for drug delivery,
by incorporating a building block, or monomer, containing a silyl
ether group.

This monomer is randomly distributed throughout the material, and when
the material is exposed to acids, bases, or ions such as fluoride,
the silyl ether bonds break.



==========================================================================
The same type of chemical reaction used to synthesize those polymers is
also used to make some thermoset plastics, including polydicyclopentadiene (pDCPD), which is used for body panels in trucks and buses.

Using the same strategy from their 2019 paper, the researchers added
silyl ether monomers to the liquid precursors that form pDCPD. They found
that if the silyl ether monomer made up between 7.5 and 10 percent of
the overall material, pDCPD would retain its mechanical strength but
could be broken down into a soluble powder upon exposure to fluoride ions.

"That was the first exciting thing we found," Johnson says. "We can make
pDCPD degradable while not hurting its useful mechanical properties."
New materials In the second phase of the study, the researchers tried to
reuse the resulting powder to form a new pDCPD material. After dissolving
the powder in the precursor solution used to make pDCPD, they were able
to make new pDCPD thermosets from the recycled powder.

"That new material has nearly indistinguishable, and in some ways
improved, mechanical properties compared to the original material,"
Johnson says.

"Showing that you can take the degradation products and remake the same thermoset again using the same process is exciting." The researchers
believe that this general approach could be applied to other types
of thermoset chemistry as well. In this study, they showed that using degradable monomers to form the individual strands of the polymers is
much more effective than using degradable bonds to "cross-link" the
strands together, which has been tried before. They believe that this
cleavable strand approach could be used to generate many other kinds of degradable materials.

If the right kinds of degradable monomers can be found for other types
of polymerization reactions, this approach could be used to make
degradable versions of other thermoset materials such as acrylics,
epoxies, silicones, or vulcanized rubber, Johnson says.

The researchers are now hoping to form a company to license and
commercialize the technology. MIT has also granted Millipore Sigma a non-exclusive license to manufacture and sell the silyl ether monomers
for research purposes.

Patrick Casey, a new product consultant at SP Insight and a mentor with
MIT's Deshpande Center for Technological Innovation, has been working
with Johnson and Shieh to evaluate the technology, including performing
some preliminary economic modeling and secondary market research.

"We have discussed this technology with some leading industry players,
who tell us it promises to be good for stakeholders throughout the value chain," Casey says. "Parts fabricators get a stream of low-cost recycled materials; equipment manufacturers, such as automotive companies, can
meet their sustainability objectives; and recyclers get a new revenue
stream from thermoset plastics. The consumers see a cost saving, and
all of us get a cleaner environment." The research was funded by the
National Science Foundation and the National Institutes of Health.


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


========================================================================== Journal Reference:
1. Shieh, P., Zhang, W., Husted, K.E.L. et al. Cleavable comonomers
enable
degradable, recyclable thermoset plastics. Nature, 2020 DOI:
10.1038/ s41586-020-2495-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200722112703.htm

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