April 2022

Green Petrochemicals

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.

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 (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.

FIG. 1. HDPE blow molding grade—not enough stabilizer for circular recycling.

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) 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.

FIG. 2. HDPE blow molding grade, with the addition of antioxidants (AO 1010, PS 168 and a proprietary processing stabilizera) at each recycling step.

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 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 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.
  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.

The procedure to measure corrosion is shown in FIG. 3. Two soft iron plates are put separately into an aluminum plate filled with a PP compound and placed in a ventilated oven at 230°C for 4 hr. After taking the soft iron plates out of the oven and removing the molten PP compound from their surfaces, the soft iron plates are hung in a tightly-closed glass bottle saturated with water vapor at room temperature for 7 d. Weights and photographs of the soft iron plates before and after testing are recorded.

FIG. 3. Corrosion test.

FIG. 4. shows the appearance of metal plates that are non-treated, treated with recycled PP from the market, and the same recycled PP compounded with 500 ppm proprietary acid scavenger and stabilizerb. It becomes obvious that an acid scavenger is needed to neutralize acidic residues from recycled PP; 500 ppm of the proprietary acid scavenger and stabilizerb reduces corrosion significantly, but it does not eliminate it fully. Depending on the quality of the mold and the intensity of exposure during injection molding, it remains to be clarified whether 500 ppm of the proprietary acid scavenger and stabilizerb is sufficient in such a case to neutralize the acidic content.

FIG. 4. Appearance of soft iron plates after exposure to saturated water vapor conditions at room temperature for 7 d.

The weight increase of the iron plates varies from 0.0002 without polymer exposure to polymer melt, to 0.0016 with recycled PP, to 0.0004 with recycled PP and 500 ppm of the proprietary acid scavenger and stabilizerb. This reflects an improvement of 75% by using this high-performance acid scavenger. Therefore, it is recommended to include additional additivation with acid scavengers at the stage of recyclate extrusion at the recycler to increase the quality of the recyclates and reduce the negative impacts on processing efficiency.

Collaboration is a key enabler to identifying circular solutions

Understanding the root causes for polymer quality loss along the lifecycle of the product calls for an understanding of the requirements and opportunities within several technological steps in the lifecycle. Working in and analyzing the conditions within value chain collaborations is one way forward to achieve this understanding.

Industry consortia—e.g., the EU-founded Circular Plastics Alliance (CPA), with more than 290 signatories from public and private sectors along the plastics value chain—are well placed to combine expertise and propose new ideas and solutions to overcome the obstacles to implementation. For example, the CPA recently announced a design work plan supporting the development of a guideline on design for plastics recycling.

Companies around the globe are also launching activities to foster value chain collaboration. Within the chemical industry, for example, the authors’ company launched its circular plastics program EcoCircle in 2019. This program facilitates discussions with customers and players across the value chain, focusing expertise and combining product solutions with established system competences. By developing products collaboratively with customers and proving the solutions against circularity criteria along the value chain, real product solutions can be created that support the circularity of the plastics value chain. Similar programs have since been set up by other chemical companies.

Whereas this article concentrates on the impact that the implementation of correct additivation within the mechanical recycling step can have on polymer quality, it is well acknowledged that there are industry developments covering other recycling technologies such as, for example, chemical recycling. While immediate solutions are needed to make these technologies viable, the need for joint development, collaboration and an understanding of value chains remains the same, independent of the recycling technologies approached.

Product design, product composition and additivation, processing parameters and recycling technologies all must be aligned. This can be best achieved when collaborating across the value chain.

Takeaway

This article demonstrates how additivation in the recycling process can play a crucial part in providing the required recyclate quality for achieving a circular economy. It also emphasizes that industry collaboration is a key enabler to developing new solutions and transitioning towards a circular economy.

It should be noted that from a statistical point of view, a long polymer chain is attacked with a higher probability than a short polymer chain and that it is almost impossible to reconnect polyolefin chains in a linear way once they are broken—long chain polyolefins are connected to high processability and melt stability and have good ESCR. Accordingly, the avoidance of any chain breakage while using antioxidants should be a priority within polyolefin recycling to help bring circular plastic to fruition.

NOTE

   a  Clariant’s Hostanox® P-EPQ® powder

   b  Clariant’s Hycite® 713

LITERATURE CITED

  1. Zweifel, H., R. Maier and M. Schiller, Plastics Additives Handbook, 6th Ed. Hanser Publications, Liberty Township, Ohio, June 2009.
  2.  Hanel, H. and A. Roth, “Methodology for improving the quality of LDPE recyclate from the Newcycling® process with the help of tailor-made stabilizer systems,” Forum Plastics Recyclates, March 2021.

The Authors

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