Hydrocarbon Processing - April 2022 - 68

Green Petrochemicals
expected that mechanical properties, such
as impact strength and environmental
stress cracking resistance (ESCR), worsen
in cases where no antioxidants are added
and are better preserved in cases where antioxidants
are added.
Another observation with recycled
FIG. 1. HDPE blow molding grade-not enough stabilizer for circular recycling.
FIG. 2. HDPE blow molding grade, with the addition of antioxidants (AO 1010, PS 168 and a
proprietary processing stabilizera
) at each recycling step.
polyolefins is an increased acidity of the
recyclate. Acidity is a well-known phenomenon
for virgin polymers caused by
residues from the catalyst. To neutralize
the acidic residues from the catalyst, an
acid scavenger is added. Here, migrating
acid scavengers such as zinc stearate for
low-linear-density PE (LLDPE) or calcium
stearate for HDPE and polypropylene
(PP) is common. For example, in cases
where migration is an issue for metallized
film or low water carryover for raffia, the
more efficient hydrotalcites are used; zinc
oxide is also sometimes used for LLDPE.
Since the acidity of the catalyst after the
polymerization reaction must be neutralized
only once, one could presume that
acid scavengers are not needed for polyolefin
recycling. Nevertheless, there are several
potential root causes for increasing acidity
levels during recycling. These include:
1. Impurities from the packed
content from foreign particles
and materials in PCR. These
impurities can be minimized with
enhanced sorting and washing.
FIG. 3. Corrosion test.
(HDPE) bottle grade, five recycling steps
were simulated by the authors' company.
Hanel and Roth2
propose to use oxidative
induction time (OIT) as the indicative
parameter for the quality of a recycling
resin. As shown in FIG. 1, the OIT of
the HDPE resin decreases significantly
from 36 min to 8 min. In parallel, the ratio
of the melt flowrate (MFR) under two
different loads 21.6/2.16, which is a measure
for the polydispersity, increases significantly
from 81 to 432. Such broadening
of the molecular weight distribution
in HDPE is caused by chain scission and
crosslinking of the resin. The polymer
structure changes significantly.
This observation suggests that the addition
of antioxidants is required during
recycling. FIG. 2 shows the same resin after
adding antioxidants at every recycling step.
A combination of PS 168, a phosphite stabilizer,
a proprietary processing stabilizera
,
a high-performance phosphonite stabilizer
and AO 1010 (as phenolic antioxidant)
68 APRIL 2022 | HydrocarbonProcessing.com
were chosen for the simulation. Ratio and
dosage were selected in a way to replenish
the consumed antioxidants. Consequently,
the concentration of the oxidized processing
stabilizer PS 168 increases during
recycling; PS 168 stays stable.
What is the impact on the polymer
structure? The ratio of MFR 21.6/2.16 increases
by a significantly smaller amount
(i.e., from 81 to 192). This suggests an improved
preservation of the original molecular
weight distribution. It must be proven
that this enables an easier incorporation
of the recyclate into packaging production
with similar processing efficiency and
packaging product properties-an important
step for achieving industry targets for
recycled content in packaging.
In parallel, OIT even increases from 28
min to 37 min, suggesting a good heat stability
of the resin even after five recycling
steps. These results should be further
confirmed by gel permeation chromatography
and rheological measurements. It is
2. Oxidation of polyolefin, formation
of aldehyde, ketone and carboxylic
acid. Here, it must be expected
that the tertiary carbon atom of PP
is more susceptible to oxidation
than the methylene group of PE.
The authors have observed more
corrosion issues with recycled PP
than recycled PE.
3. Oxidation of the processing
stabilizer to P=O and finally
phosphor and phosphoric acid,
although the concentration is
rather low.
One way to determine the acidity of a
resin is a corrosion test. Corrosion can be a
severe issue for resin converters like injection
molders. Rough surface molds are difficult
to clean once corroded because they
cannot simply be polished like a smooth
mold. Corrosion on molds can lead to a
coloring of the plastic item and a lowering
of processing efficiency. Therefore, identifying
solutions to reduce the acidity of recyclates
is important to enable a scale-up
of recyclate usage in the industry.
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