Both the quantity of recycled plastics waste and of processed recyclates is continually increasing. However, there are still difficulties with the quality of the recycled plastics, which are impeding replacement of virgin material. In recent years, machinery manufacturers and plastics producers have introduced various further developments that improve the properties of recyclates as well as eliminate unpleasant odors and contaminants.

Plastics recycling is a laborious process. For the recycled material to have the necessary quality, it must first be sorted into the different plastics

Overcoming Difficulties in Plastics Recycling

Streifinger, Florian

Recycling

Umweltmanagement

Extruder

Kunststoffverarbeitung: Maschinen und Komponenten

Extrusionsanlagen

Trockner

Kunststoff-Aufbereitung

Waschen und Entfetten

Kunststoffverarbeitung: Verfahren

Trockner, Granulat-

Sortierer

Kunststoff-Recycling

Additive

Kunststoff Werkstoffe

Inspektion, Analyse und Sortierung von Rohmaterial

QualitÃ¤tssicherung

QualitÃ¤tssicherung Kunststoff

The Long Road to a Circular Economy

Dow, HP, Reifenhäuser, Cadel Deinking and Karlville present a feasibility study for recycling digitally printed polyethylene barrier packaging into new high-quality PE bags.

Key industry players combine forces to recycle PE-based barrier pouch into new high-quality PE pouch suitable for repeat recycling

Technik Trend

kunststoffe-international.com

Polyethylen

Thermoplaste

Kreislaufwirtschaft

Important Step towards Pouch-to-Pouch Recycling

Erema enters a new dimension with large-scale recycling plants. Up to 40,000 tonnes per year of rPET can now be produced. The first plant of this size has been shipped to Brazil.

The reactor of the Vacurema T large-scale plant almost reaches the roof of the new Erema production hall, which was commissioned in 2021.

Polyethylenterephthalat

Extrusions-Zubehör

XXL Recycling Plant for Brazil

The growing demand for plastic recycling directly reflects the machine sales for Polystar. By the end of April 2022, a total of over 110 recycling machines will have been shipped within 12 months – twice as many compared to last year.

The new factory in Sinji industrial park is the third Polystar manufacturing facility in Taiwan.

Sales Doubled for Polystar in 2021

Conventional PET recycling lines are based on single-screw extruders with maximum output rates of two and a half tons per hour. Since 2004, KraussMaffei has been researching a process to significantly increase these quantities with the help of glycol and the use of a twin-screw extruder. Given the current high demand for rPET materials, this technology is becoming increasingly important.

Glycolic Depolymerization versus Mechanical Bottle-to-Bottle Recycling

Glycol Turns the Extruder into a Turbo

The growing demand for plastic recycling directly reflects the machine sales for Polystar. By the end of  April 2022, a total of over 110 recycling machines will have been shipped within 12 months – twice as many compared to last year.

LDPE film recycling machine for postindustrial waste are on demand all over the world.

16:9

High Demand of Plastic Recycling Machines

Repeats (Recycled PE at Scale) Group, a pan-European platform for LDPE recycling, made an investment in the Spanish recycling company Anviplas.

Aleix Vintró, CEO Anviplas, and Joan Vintró at the Navacles plant. An Intarema 1716 TVEplus from Erema is in operation there. 

Anviplas Becomes Part of the Repeats Group

Static mixers are compact, but they cannot lower the melt temperature. Combining a static mixer with a powerful cooler in a single unit opens up new possibilities in process engineering. 

Combining Intensive Static Mixing with Effective Heat Exchange 

Kühlelemente, aktive

Polystyrol

Schaumstoffe und Vorprodukte

Mischer

Extrusion

Schäumen / Reaktive Verarbeitung

A Cool Mix 

Foamed polystyrene boards (XPS) have been used for generations in thermal insulation applications, e. g. for buildings. Due to its comparable mechanical and thermal properties, bio-based cellulose acetate (CA) represents a promising, sustainable alternative which is also an interesting material for technical components due to its heat resistance. However, producing such parts by foam extrusion or foam injection molding with halogen-free flame retardant systems and environmentally friendly blowing agents is a challenge. 

Foamed Boards and Foamed Injection Molded Parts Made from Cellulose Acetate with a Halogen-Free Flame Retardant System and Environmentally Friendly Blowing Agents 

Flammschutzmittel

Schaumspritzgießen

Bioplastics with Halogen-Free  Flame Retardants 

Simultaneous extrusion of polyvinyl chloride profiles with a PS foam core is challenging due to the different processing conditions and has not been performed before. A trial plant for researching this process has now been set up at the SKZ.

Fig. 1. The foaming of PS was investigated in a test system developed for this purpose 

Method for the Simultaneous Extrusion of Window Profiles with a Foam Core

PVC-Profile

Halbzeuge und Zwischenprodukte

Kunststoff-Halbzeuge

Schaumstoffe

Polyvinylchlorid

Dichtungsschäumen

Rohr- und Profilextrusion

Foamed and Still Extruded  in One Step

The circular economy and more sustainable additives are the focus of AF Color’s Fakuma presence this year.

The circular economy and more sustainable additives are the focus of AF Color’s trade show presence this year

Masterbatches

Messe Fakuma

Polyamide

Biokunststoffe

Hochleistungskunststoffe

Biobased Additives and Orange for PA

Caracol and NextChem (Maire Tecnimont group) partnered to create Beluga: a joint research project to produce the first 3D printed sailing boat prototype made in a single piece with MyReplast recycled material.

Caracol and NextChem bring to life the 3D printed sailing boat Beluga

Anlagen Additive Fertigung / 3D-Druck, andere

Anlagen für die Additive Fertigung

Caracol and NextChem Bring to Life Beluga

Together with the University of Applied Sciences Magdeburg-Stendal, Germany, MultiPet GmbH is researching into injection-moldable blends that can be used to produce high-quality multi structured injection-molded parts from mixed PET.

Colorful mixture: the recycling of mixed fractions from bottles, blisters and trays remains a major challenge

Production of Injection-Moldable PET-Blends from Mixed Plastic Fractions

rPET Blends for High Standards

In the research project “UPcyclePET”, and together with Easicomp GmbH, Fraunhofer LBF developed a new material that had to prove its qualities in practical tests.

Upcycling of used PET beverage bottles into long fiber-reinforced thermoplastics

New Compound Materials for Durable Technical Applications

Verbundwerkstoffe / Werkstoffverbunde

Sustainable Components from PET Flakes

LyondellBasell has obtained the International Sustainability and Carbon Certification Plus certification for certain grades of polyethylene and polypropylene produced at four of its U.S. manufacturing sites.

Pictured is LyondellBasell’s Channelview Complex. The company has obtained the ISCC Plus certification for certain grades of polyethylene and polypropylene produced at four of its U.S. manufacturing sites

Polypropylen

LyondellBasell Gains ISCC Plus Certification

A Robust Continuous Process for the Chemical Recycling of Polystyrene

Due to their good feed characteristics and self-cleaning properties, twin-screw extruders enable even contaminated post-consumer polystyrene to be depolymerized continuously and efficiently. This opens up a promising recycling route in terms of industrial implementation via renewed polymer synthesis to produce plastic granules with a quality approaching that of virgin material.

Twin-screw extruders enable even contaminated post-consumer polystyrene to be depolymerized continuously and efficiently. This opens up a promising recycling route in terms of industrial implementation via renewed polymer synthesis to produce plastic granules with a quality approaching that of virgin material.

Polystyrene is used to produce a wide variety of food packaging and also for thermal insulation in construction. Especially in the packaging sector, it is often a short-lived disposable product that is disposed of immediately after use. The total amount of polystyrene waste in Europe (EU28+NO/CH) in 2018 was 3.28 million t [1]. This enormous amount of waste makes it necessary to develop efficient recycling methods in order to show alternatives for the utilization of plastic waste for energy. In addition to mechanical recycling, chemical recycling can also be used to return plastic waste to the material cycle [2, 3].

Besides polymethyl methacrylate (PMMA), polystyrene is one of the polymers that can be depolymerized by splitting the carbon bond, which requires a relatively high activation energy of about 240 kJ/mol [4]. The reaction is affected by exceeding the so-called ceiling temperature, at which polymerization and depolymerization occur at the same rate. A temperature above the ceiling temperature causes the reaction equilibrium to shift in favor of chain degradation. For polystyrene, this happens at a temperature of 400 °C [5]. With increasing temperature, the reaction speed rises significantly [6]. The reaction also leads to side reactions, so that dimers, trimers, α-methylstyrene and ethylbenzene are formed in addition to styrene (Fig. 1). In the absence of oxygen, the reaction starts at a random position along the molecular structure. This produces macro radicals in which the binding energy of the neighboring C-C bonds is reduced, which in turn enables a styrene molecule to be split off from the chain end [7–9].

The chemical recycling of polystyrene can be carried out by several discontinuous processes, for example in a batch reactor. For industrial implementation of the chemical recycling of polystyrene, the achievable throughputs are too low and the sensitivity to contamination of these processes is too high. Therefore, a reactor technology has to be developed which allows high throughputs and at the same time can continuously process highly contaminated post-consumer polystyrene with high process stability [10, 11]. The feasibility of continuous depolymerization of polystyrene in a twin-screw extruder has already been demonstrated [12].

Continuous Depolymerization of Polystyrene in a Twin-Screw Extruder

The continuous depolymerization of polystyrene takes place in a co-rotating, tightly intermeshing twin-screw extruder (DSE). The polystyrene (type: Styrolution PS 156F, manufacturer: Ineos Styrolution Group GmbH, Frankfurt/Main, Germany) is fed into the DSE (type: ZSK 26 Mc, manufacturer: Coperion GmbH, Stuttgart, Germany) by means of a gravimetric screw feeder (manufacturer: Brabender Technologie, Duisburg, Germany). In the melting zone, the plastic is plasticized by the introduction of mechanical energy and by the housing temperature.

The residence time depends on the parameters of screw length, mass throughput, screw speed and screw configuration as well as on the input material itself and can therefore be varied over a wide range. Using a vacuum system (manufacturer: Pfeiffer Vacuum, Asslar, and Busch Vacuum Solutions SE, Maulburg, Germany), the gaseous reaction products are removed from the extruder and liquefied in a condenser. Impurities and incompletely degraded polystyrene are discharged from the DSE by the self-cleaning of the screws.

The aim of the depolymerization of polystyrene in the DSE is to process plastic waste with high contamination. After separation and cleaning of the waste according to type, impurities are still present in the plastic stream to be recycled. In addition to foreign matter, these are also fillers and additives that were added to the plastic for customized properties in its original use. Polystyrene is often mixed with stabilizers, antioxidants, flame retardants and colorants [13]. In the packaging industry, fillers are frequently used to color the packaging. Titanium dioxide is often used for white coloration due to its low cost and food compatibility. Optically black polystyrene is usually produced by adding carbon black.

In order to analyze the influence of these fillers on depolymerization, one plastic compound is filled with 5 wt. % titanium dioxide and one with 5 wt. % carbon black. These two plastic compounds will be used to clarify whether the fillers have a negative influence on the continuous depolymerization of polystyrene and whether an industrial conversion is basically feasible.

Influence of Screw Speed and  Material Composition

The depolymerization process was performed at a housing temperature of 450 °C and a vacuum pressure of 400 mbar. The mass throughput was 10 kg/h, the screw speed was varied (400, 800 and 1200 rpm). This process parameter selection is based on preliminary work [12]. Over a period of 5 min, both the discharged residue and the condensate were weighed. Additional condensate and residue samples were used for material analysis.

The thermally or mechanically induced degradation of polystyrene was estimated by studying the melt flow rate (MFR) and differential scanning calorimetry (DSC). A higher melt flow rate due to depolymerization and the resultant reduced viscosity allows conclusions about degradation. The reason for the change in viscosity is the advancing chain degradation, which can also be observed by a reduced glass transition temperature in DSC. The heating rate in the DSC measurements was 20 K/min. A triple determination was performed, whereby the test chamber was flushed with nitrogen. The glass transition temperature is determined during the second heating cycle, thus ensuring a comparable thermal history. Furthermore, the condensate yield as well as the chemical composition of the condensate were determined by gas chromatography by Neue Materialien Bayreuth GmbH, Bayreuth, Germany, and evaluated at IKV.

The influence of the fillers on the melt mass flow rate and glass transition temperature is shown in Figure 2. The MFR values obtained are between 34.3 and 79.65 g/10 min. In the case of unfilled polystyrene, the MFR in the delivery state is 0.09 g/10 min. In contrast, a clear influence of the screw speed on the MFR can be seen at a carbon black content of 5 wt. %. The MFR value increases from 34.3 g/10 min at a screw speed of 400 rpm to 61.1 g/10 min at 1200 rpm. As a result of the increased surface renewal at increasing screw speed, the polystyrene is degraded more strongly and a higher MFR value is achieved. Titanium dioxide as a filler does not lead to a clear effect. At a low screw speed (400 rpm) a high MFR value of 75 g/10 min is achieved. In contrast to carbon black, a high screw speed leads to a reduced MFR value of 61.85 g/10 min.

These trends are also confirmed by observing the glass transition temperature. The glass transition temperature drops from 80.0 °C at 400 rpm to 71.5 °C at 1200 rpm with a carbon black content of 5 wt. %. and indicates an increasing molecular degradation with increasing speed. Titanium dioxide achieves a higher molecular degradation at 400 and 800 rpm compared to carbon black and unfilled PS (higher MFR value; lower glass transition temperature). In contrast, a high screw speed of 1200 rpm leads to a higher glass transition temperature for titanium dioxide. One possible reason is that the reaction products of depolymerization cannot be removed fast enough via the vacuum system and the reaction is inhibited.

Figure 3 shows the dependence of the condensate yield for the test materials as a function of the screw speed. The yield is generally in the range of 47.4 to 66 wt. %. The maximum yield of 66 wt. % is achieved by polystyrene with a carbon black content of 5 wt. % at a screw speed of 800 rpm. PS with 5 wt. % carbon black achieves the highest condensate yield for each screw speed. The increase in speed also tends to lead to higher yields. One explanation for this is that the increased speed increases surface renewal and thus the reaction products can be removed more quickly. In addition, a higher speed also reduces the dwell time of the material in the twin-screw extruder, so that an optimum is achieved for the material. For the carbon black-filled material, the optimum is 800 rpm, while for titanium-filled and unfilled PS it is 1200 rpm. In general, however, the tests show that the condensate yield for carbon black and titanium dioxide is at the same level or higher than for unfilled polystyrene and therefore has no negative influence on the process in this respect.

Figure 4 shows the condensate composition for the test materials determined by gas chromatography measurements at a screw speed of 1200 rpm. The styrene content in the condensate is highest for titanium dioxide at 72.5 wt. %. The compound with 5 wt. %. carbon black achieves the lowest styrene content of 69 wt. %. Unfilled PS showed a styrene content of 70.4 wt. %. Based on the tests, there seems to be a small influence of the fillers on the side reactions, so that the sum of the corresponding products has a higher proportion for carbon black and a lower proportion for titanium dioxide. However, the styrene fractions are very similar, so that no significant negative influence due to the presence of fillers was found.

Overall, for an input mass flow of 10 kg/h and the respective optimum screw speeds, the results of condensate yield and composition result in a calculated styrene yield of 4.1 kg/h for titanium dioxide, 4.3 kg/h for carbon black and 4.3 kg/h for pure polystyrene. Therefore, contamination with the fillers carbon black and titanium dioxide in the test chamber under consideration does not have a negative effect on depolymerization.

The addition of carbon black and titanium dioxide influences the reaction equilibrium of the depolymerization and the associated styrene content in the condensate. Especially when processing plastic waste, the results of depolymerization cannot be estimated due to different fillers. Compared to pure polystyrene, plastic compounds filled with carbon black achieve a higher condensate yield of up to 7 wt. %. The condensate composition is only slightly influenced by the addition of carbon black or titanium dioxide. Nevertheless, the depolymerization of polystyrene with carbon black and titanium dioxide achieves a comparable styrene yield in condensate of 4.1 to 4.3 kg/h. The depolymerization of polystyrene in a twin-screw extruder thus represents a robust process for the chemical recycling of polystyrene.

1. N.N.: Plastics—The Facts 2019. An analysis of European plastics production, demand and waste data. PlasticsEurope, 2019

2. Rudolph, N.; Kiesel, R.; Aumnate, C.: Einführung Kunststoffrecycling: Ökonomische, ökologische und technische Aspekte der Kunststoffabfallverwertung. Carl Hanser Verlag, München 2019

3. Brandrup, J; Bittner, M.; Michaeli, W.; Menges, G.: Die Wiederverwertung von Kunststoffen. Carl Hanser Verlage, München 1995

4. Peterson, J. D.; Vyazovkin, S.; Wight, C.A.: Kinetics of the thermal and thermo‐oxidative degradation of polystyrene, polyethylene and Poly (propylene). Macromolecular Chemistry and Physics 202 (2001) 6, pp. 775–784

5. Fink, J. K.: Reactive polymers fundamental and applications 2. New York: William Andrew, 2013

6. Song, H.S.; Lee, J.S.; Hyun, J.C.: A kinetic model for polystyrene (PS) pyrolysis reaction. Korean Journal of Chemical Engineering 19 (2002) 6, pp. 949–953

7. Guyot, A.: Recent developments in the thermal degradation of polystyrene. Polymer degradation and stability 15 (1986) 3, pp. 219–235

8. Ohtani, H.; Yuyama, T.; Tsuge, S.: Study on thermal degradation of polystyrenes by pyrolysis-gas chromatography and pyrolysis-field ionization mass spectrometry. European Polymer Journal 26 (1990) 8, pp. 893–899

9. Zhou, J.; Qiao, Y.; Wang, W.: Formation of styrene monomer, dimer and trimer in the primary volatiles produced from polystyrene pyrolysis in a wire-mesh reactor. Fuel 182 (2016) 8, pp. 333–339

10. Onwudili, J. A.; Insura, N.; Williams, P. T.: Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time. Journal of Analytical and Applied Pyrolysis 86 (2009) 2, pp. 293–303

11. Kaminsky, W.; Predel, M.; Sadiki, A.: Feedstock recycling of polymers by pyrolysis in a fluidised bed. Polymer Degradation and Stability 85 (2004) 3, pp. 1045–1050

12. Schäfer, P.; Hopmann, Ch.; Facklam, M.; Hollerbach, L.; Kolb, T.; Schedl, A.; Schmidt, H. W.; Nosić, F.; Wilhelmus, B.: Continuous Chemical Recycling of Polystyrene with a Twin–Screw Extruder. In Advances in Polymer Processing (2020), pp. 37–49

13. Maier, R.D.; Schiller, M.: Handbuch Kunststoff Additive. München, Carl Hanser Verlag, Wien 2016

Institut für Kunststoffverarbeitung IKV in Industrie und Handwerk an der RWTH Aachen

https://www.ikv-aachen.de

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Continuous process in a twin-screw extruder: For industrial implementation of chemical recycling of polystyrene, a reactor technology is required that allows high throughput rates and also processes heavily contaminated post-consumer material continuously with high process stability

Fig. 1. Depolymerization mechanism of polystyrene to styrene (in brackets: boiling temperatures)

Fig. 2. Characterization of polymer degradation based on melt mass flow rate and glass transition temperature as a function of material composition and screw speed

Fig. 3. Condensate yield as a function of material composition and screw speed

Fig. 4. Composition of the condensates depending on the input materials

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Univ.-Prof. Dr.-Ing. Christian Hopmann holds the chair for plastics processing at RWTH Aachen University and is Head of the Institute for Plastics Processing (IKV) in Industry and Craft at RWTH Aachen University, Germany.

Philipp Schäfer, M.Sc., has been working as a research assistant in the field of compounding and reactive extrusion at IKV since 2017; <a href="mailto:Philipp.Schaefer@ikv.rwth-aachen.de">Philipp.Schaefer@ikv.rwth-aachen.de</a>

Nils Dauber, M.Sc., wrote his master thesis on the chemical recycling of polystyrene at IKV and has been working as a research assistant in the field of forming simulation at IKV since 2020.

Dr.-Ing. Martin Facklam has been Head of the Extrusion and Rubber Technology Department at IKV since 2019.

The project “Recycling of Polystyrene by means of Raw Material Recovery” (ResolVe), on which this report is based, was funded by the Federal Ministry of Education and Research under the funding code 033R194C. Our thanks go to the BMBF. Our thanks also go to our project partners: Institute for Processing and Recycling at RWTH Aachen University, Neue Materialien Bayreuth GmbH, Ineos Styrolution Group GmbH, Ineos Manufacturing Deutschland GmbH.