Hydrocarbon Processing - April 2022 - 67

Green
Petrochemicals
H. SIEBERT and B. SIGGELKOW, Clariant,
Frankfurt, Germany; and E. DONGIOVANNI,
Clariant, Muttenz, Switzerland
Plastics circularity: How to solve the challenge
of retaining quality in recycled polymers
Businesses and governments around
the world have united behind the vision
of a circular economy for plastics. Realizing
this vision calls for industry-scale
innovation to ensure that plastics are reusable,
recyclable or compostable.
Among the many obstacles to overcome
is the need to maintain high polymer
quality over multiple recycling loops,
which is critical to the viability of plastics
recycling and to increasing the use of recyclates.
Demand from brand owners for
high-quality recycled polymers remains
high. This is in combination with their
willingness to reduce plastic waste and
use it as future raw material. Therefore,
enormous efforts are going into technology,
process and product development to
achieve these key targets.
Industry requirements. In Europe, a
pledge by the European Union (EU) in
2018 revealed that 10 MMt of high-quality
post-consumer resin (PCR) will be
needed in 2025 to meet industry requirements.
In 2018, the recycling rate globally
was at 21%, with a significant share
of recycled material being used in lower
value applications.
Due to its quantities in the market,
plastics packaging is playing a prominent
role in the discussion of how to transition
towards a circular economy. Many brand
owners have committed to ambitious
targets. For example, they are reducing
their use of virgin polymers by increasing
the share of recycled polymers and
by designing recyclable packaging. The
drivers behind these ambitions are supporting
the reduction of plastics waste in
the environment and mitigating carbon
footprints through increased circularity.
Moving packaging towards a circular
economy demands an understanding of
the value chain and the impact that each
step has on the polymer's quality. The
steps influencing the quality of the polymer
are numerous, and some are more
controlled than others.
Let us first start with controlled steps.
Those steps influencing the quality of
the recyclate start immediately after
the production of the virgin polymer. It
is widely known that polyolefin resins
must be stabilized with antioxidants and
that acidic catalyst residues must be neutralized
with an acid scavenger.1
These
acid scavengers protect the polymer during
the conversion process. After potential
further enhancement of the plastic
item by labeling, printing and filling,
the product is put into the market. It is
in these steps where polymer design and
composition have been optimized for
packaging during the last few decades.
Additive amounts have been optimized
for cost effectiveness in such single use
applications, balancing quality and processing
efficiency and reflecting the
short lifetime of the products.
Additivation
is
fundamental
to
achieving a circular economy. In a
circular economy, the requirements for
plastic packaging change. When polymers
are recovered through mechanical
recycling, additional stresses are placed
on the polymer. After being collected,
the plastics are grinded, extruded and
converted again. For polyolefin recycling,
the question that arises is: What
happens during these steps with the
polymer and the additives, especially the
antioxidants and the acid scavengers?
While it is the industry standard to
incorporate additives into virgin polymers
to optimize quality and processing
throughput, the polymer is only partially
protected by additives during recycling.
Furthermore, re-additivation is not practiced.
Often, additives are even considered
as an unwanted impurity that should
be omitted. This has a significant implication
on the product quality and processing
efficiency. Proper additivation also
within the so-called " second life " of the
polymer must be considered and adjusted
to the requirements of such recycling and
subsequent conversion processes.
In addition, there are influences on
plastic quality that go beyond the control
of processing (e.g., if the packaging is exposed
to sunlight and heat by consumers
or if the plastics are exposed to contamination
during their collection and sorting).
This can lead to polymer destruction,
crosslinking and acid formation.
The implications of these uncontrolled
influences on polymers must be considered
and counteracted with the required
additivation to maintain the processing
efficiency and product quality of the
resulting recyclate.
The standard industry procedure for
additivation is to use a primary antioxidant
(used as a heat stabilizer and typically
a phenolic antioxidant) together with a
secondary antioxidant-a processing stabilizer,
typically phosphorous-based like
a phosphite or phosphonite.
To analyze the consumption of antioxidants
during the circular lifetime of a
commercial high-density polyethylene
Hydrocarbon Processing | APRIL 2022 67
Hydrocarbon Processing - April 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - April 2022
