contaminated or difficult to separate. Coated paper
cartons and boxes, where polymers are tightly adhered, present insurmountable challenges in polymer
recycling. Processing the polymers for energy content
is one option for keeping these materials out of the
environment. Additional processing can turn the
polymers into liquid fuels or the polymers can be
used directly in energy production. Successful pilot
efforts proved that community recovery is possible
and a growing number of municipalities are pursuing
options for energy production.
Chemical recycling is in its infancy when compared to
mechanical recycling. Depolymerizing plastics back to
monomers and using those monomers to make virgin
material is more energy intensive than mechanical
recycling, but allows polymer producers to regain
control over the end product properties, producing the highest
value products. Technology can be polymer dependent or
completely agnostic to the feed composition.
Loop Industries is representative of a chemical route specific
to a single polymer. Their process depolymerizes polyethylene
terephthalate (PET) into dimethyl terephthalate and ethylene
glycol in a catalytic process, and, after purification, produces
PET again. The PET produced is suitable for any and all uses
of virgin PET, including food and water contact. This process
is not commercial on a wide scale and there are competing
processes. Others are proposing new polymer systems that are
more easily depolymerized, but introducing a new-to-the-world polymer is difficult.
The polyolefin resins are not well suited for depolymerization. There are no reports of high yield back to the monomers.
The current options for chemical recycling involve gasification
of the polymers, as pure plastic or as a plastic-containing
municipal solid waste stream, to form synthesis gas or pyrolysis
to make an oil suitable for steam cracking after additional processing. Synthesis gas can be converted to methanol,
ethanol, or naphtha for subsequent processing to ethylene and/
or propylene. Olefins are produced, albeit through a complex
pathway, that are identical to the olefin feedstocks used in
the production of the virgin resin. Chemical recycling is in its
infancy and is less economical than production from virgin
feedstock, due largely to high capital. Technology improvements are clearly needed and likely attainable.
Some slip of polymers to the environment is likely
inevitable. Reducing the impact of polymers that escape
into the environment means shortening the lifetime in the
environment. Polyethylene, polypropylene and PET are widely
used because they are robust, resilient and largely immune
from biodegradation. If released, they can persist for years.
Development of new materials or modification of existing
polymers to speed degradation reduces impact.
Many bidegradable polymers are known, yet none have
managed to compete with polyethylene, polypropylene and
PET. Research efforts to develop better biodegradable and en-
vironmentally degradable polymers continues. The robust and
mature recycling infrastructure hinders new polymer introduc-
tion. Inroads are being made were the waste can be controlled,
such as in stadiums specifying only compostable containers
and food service items. New polymers must be compatible
with the existing recycling infrastructure for use more broadly.
One of the more tantalizing, though largely unrealized,
options is modification of conventional resins to degrade in the
environment. Methods employing biological fillers, like starch,
are problematic. These lead to visual reduction of litter when
the filler degrades leaving only small particles of plastic. Recent
focus on persistent microplastic particles calls into question the
wisdom of such approaches. Additives that improve environ-
mental oxidation of polyethylene are not widely used.
Concern over contamination and degradation of the recycle
stream remains an issue. Newer research is changing perceptions of what is possible. Enzymatic degradation of polyethylene is now proven, providing new avenues of exploration.
Additives that render polyethylene biodegradable are now
on the market. New discoveries hold the promise of making
conventional polymers decompose in the environment.
Reducing the amount of plastic in the environment is an
active research area. Improving plastics so that less is used,
developing methods to recycle more and harnessing nature to
reduce the impact of what escapes to the environment all are
rich areas of study. Societal pressure and willingness to support
new options will ultimately determine what is succeeds. What
is clear is that the R&D community is providing new options
for a cleaner, more circular future.