No Circularity without Suitable Product Design

“Without additives, there’s no circular economy” – thus Dr. Oliver Reich of BASF recently put it in a nutshell at the “Forum Plastic Recyclates” conference. Ensuring that treated plastic wastes are of an adequate and uniform quality urgently requires additives. They compensate for damage to the polymers arising during processing, service life as well as the recycling process itself. Otherwise, after a few recycling processes, or even just one, the plastics would in many cases no longer be useful for high-quality products.

However, the same can be said about other technologies, for example accurate sorting into different polymers and gentle and efficient extrusion. Without them, a circular economy would also not be conceivable. The fact that several technologies and products are essential for implementation does not diminish the contribution of each one of them. Rather, it shows how many components have to intermesh in order to manage the change to a circular economic model. “We need the entire plastics family in order to implement the necessary recycling,” explained another delegate at the conference.

Advancing the circular economy requires many different technologies. But one thing must not be forgotten: the products. Brand manufacturers, too, must invest more strongly in the recyclability of their products. Ton Emans, president of Plastic Recyclers Europe, summed it up neatly at the conference: “The biggest advances in recycling come not from innovations in recycling technologies but from innovations in product design.”

Florian Streifinger <a href="mailto:Florian.Streifinger@hanser.de">Florian.Streifinger@hanser.de</a>

16:9

Editorial Streifinger

Streifinger, Florian

No matter whether injection molding or extrusion is involved: residue plastic stuck to screws, molds,  or filters will impair system efficiency during production. Recyclates tend to build up more  quickly than virgin material on machine elements. Schwing Technologies achieves good results with  thermal cleaning by vacuum pyrolysis.

Why is it hard to remove plastics? “It sticks like chewing gum.” That is how Sales Manager Virgilio Perez Guembe at Schwing Technologies GmbH describes the challenge to removing hardened plastic residues from sometimes sensitive surfaces. In conversation with the editors of Kunststoffe, he explains why cleaning is so important – and what procedures are involved. As a specialist for the cleaning of plastics production systems, he supports his international customers with specific solutions for cleaning polymer-contaminated molds and machine elements.

Recyclates Often Exhibit Low Viscosity When Processed

“When recycled materials are processed, there is an inevitable increase in the cleaning requirements of filters, nozzles, and blow heads. Recycling alters the properties of the material, that is, its viscosity generally falls, making such plastic more likely to stick to the system that a virgin polymer material would.” says Guembe. The problem here are highly adhesive carbon residues that “stick right in the pores”. During the production process, new material that sticks to them forms additional carbon. Machine contamination by itself is a rather gradual process. The systems keep on running, but their capacity is often regulated downwards when polymer residues stick to the machine components. Only clean systems ensure long-term productivity and energy efficiency.

Plastics Removal: A Comparison  of Procedures

Generally speaking, various procedures are available for removing plastics from machine components. Schwing Technologies has reached the following assessment based on their own experience and research work in their technology center, as well as on evaluations by the Fraunhofer IPK of the cleaning of injection molds:

<ul><li>Manual cleaning: can be done in operation using scratchers, brushes, or burners, but it often results in damage to the molds or system components. The components to be cleaned may warp due to the uneven effect of heat, and damaged mold surfaces can impair product quality. Suitable, at best, for very robust system components and smooth surfaces only.</li><li>Mechanical cleaning: performed using dry ice, water, or abrasives, it is environmentally friendly and gentle on materials. Dry ice, however, means high operating expense and is thus not always economical. Other abrasive versions of mechanical cleaning often leave residue from the abrasive behind, as well as surface damage to molds and components.</li><li>Chemical cleaning: performed using detergents or chemicals and can be used against certain contaminants in reworking. Depending on the detergent, their use, however, is regarded as environmentally harmful, sometimes takes a long time, and involves high operating costs.</li><li>Thermal cleaning: an ecologically friendly, fast, and economical version of plastics removal. If thermal cleaning is done under vacuum or by circulating fluidized bed combustion, it is an especially gentle way to clean components. When certain sensitive and complex tools such as spinning nozzles are involved, follow-up treatment may be required, for example, using ultrasound.</li></ul>

The removal of plastics residues on molds, tools, and long components is performed by the Vacuclean System in an electrically heated vacuum cleaning chamber. Temperature measurements are taken directly on the component to be cleaned which is warmed up slowly and especially gently to materials, whereby a large portion of the stuck plastic melts and runs off. At approximately 450 °C, any remaining plastics decompose. Any remaining carbon is removed at the end by oxidation. Electric current and water are all that the Vacuclean System needs to run.

Schwing describes their procedure as economical – literally so, despite the high temperatures required. “Of course, energy prices are a hot issue these days. But compared to other procedures, or a system that doesn’t function productively, we are certain that pyrolysis will pay off for its user.”

Workpieces Weighing Five Tons Lie up to 40 Hours in the Vacuum Oven

How long workpieces are treated in the oven depends on their size and varies from several hours up to 40 hours for workpieces weighing five tons, for example. Pyrolysis enjoys the great advantage of being a gentle procedure. As Guembe states, “Many other procedures attack the surface of the workpiece. Many screws or injection molds are coated, since that is only way to achieved the desired flow rate of the fluid plastic during production. Thus, it is imperative not to damage the coating.” Following treatment in the pyrolysis oven, the clean components are removed. Only very fine sieves – mesh sizes of 20 µm are not uncommon in extrusion – are post-processed using ultrasound.

Vacuclean System’s range of application is wide, reaching from sieves weighing 10 g to heavy extrusion dies of up to 12 t. Even hot runner systems for 2.5 m long car bumpers are no problem. Schwing advises their customers not only as involves their version of the system, but also helps obtain official approval. “Thanks to our policy competence, we know which regulations have to be complied with. Ultimately, it is the system user’s obligation to uphold the limits, for example, for exhaust gas values.” Guembe observes.

In the Lab + Tech Center at their headquarters in Neukirchen-Vluyn, Germany, Schwing makes their entire laboratory and process pilot plant available to their customers’ development engineers and process technicians. There they can perform specific tests and test series on their own workpiece components and/or materials. Customers are invited to bring their own parts and molds contaminated with plastics along with them. The optimum cleaning procedure is established with assistance from Schwing’s process technicians.

Cleaning Also Available as a Service Including Pick-up and Delivery

Should a proprietary cleaning system not be worthwhile for economic reasons, Schwing offers needs-optimized cleaning services including associated logistical services at several worldwide locations. Each year, more than 250,000 different workpieces are cleaned according to high quality and ecological standards and reintegrated into the customer’s production processes. Their customers come from the fields of injection molding, extrusion, industrial coating, masterbatch production, and the fiber industry.

Breaker plate from an extrusion line before  and after pyrolysis treatment.

Blown film die before and after treatment. 

Srew of an injection molding machine before and after vacuum pyrolysis. 

Virgilio Perez Guembe, Sales Manager  at Schwing Technologies GmbH. 

Pyrolysis ovens are available in various versions and dimensions. This Vacuum Compact System is suitable for small format parts up to 50 kg.

A hot-runner system before and after treatment. Gentle cleaning is especially necessary for coated systems.

Name - Hauptartikel

Schröder, Susanne

Werkzeugreinigung, Kunststoffe

Kunststoffverarbeitung: Maschinen und Komponenten

Werkzeuge für die Kunststoffverarbeitung

Reinigung

Betriebsausrüstung / Betriebsmittel

Fiber Dust Formation during the Recycling of FRP

Mechanical Recycling of Fiber-Reinforced Plastics

During the shredding of fiber-reinforced plastics, fiber fragments are released that settle on the skin and clothing, which can lead to itching and may possibly also be inhaled. In a project on the subject of fiber dusts, a study is being carried out on how toxic such fiber fragments are, and which shredding technology is most suitable for preventing – or at least reducing – the release of inhalable fibers.

The demand for fiber-reinforced plastics has been increasing for many years [1], because they are highly suitable for lightweight building applications due to their weight-specific and mechanical properties. Especially organic sheet with glass fiber fabrics is being used increasingly in the automotive sector to save both resources and weight [2]. For this reason it can be assumed that, in future, large quantities of fiber-reinforced waste will be produced, which then has to be recycled. One sustainable possibility is mechanical recycling, in other words the further processing of thermoplastic fiber-reinforced waste to produce short fiber-reinforced recyclate.

A key processing step in mechanical recycling is the shredding of the waste material. Fiber-reinforced plastics can break down during shredding into inhalable fiber fragments that may be toxic [3, 4]. Fibers with WHO characteristics are considered to be particularly hazardous to health. These are fiber-shaped particles with a length of >5 µm, a diameter of &lt;3 µm and a length-to-diameter ratio of >3:1 [5].

The aim of this project is therefore to obtain know-how about the mechanisms that favor the production of recyclable ground material and, on the other hand, reduce the release of harmful fiber dust.

Previous studies on the topic of shredding technology have shown that the quality of the ground material is dependent on a number of parameters. In order to obtain a particle size distribution that is as narrow as possible and has a large particle size, it is recommended with unreinforced plastics, if possible, to initiate a cutting load into the material [6], to select a large screen mesh width [6, 7], and to prevent brittle fracture [8]. The initiation of brittle fracture – for example, through a high loading velocity, the use of more brittle materials or the deployment of the relevant shredding technology – leads to the crack front of the knife blade running ahead and thus to uncontrolled breaking-up of the material. This results in a high content of fines in the ground material [9, 10].

To what extent these relationships can be applied to the formation of airborne inhalable fiber fragments is not yet known and will therefore be studied as part of the “fiber dusts” project.

Measuring and Analyzing Airborne Fiber Dusts

For the tests on the formation of airborne fiber dust, glass fiber-reinforced organic sheet with polypropylene (Tepex dynalite 104-RG600(x)/47 %) and polyamide 6 (Tepex dynalite 102-RG600(x)/ 47 %) from Bond Laminates GmbH (Lanxess) were shredded with a single-shaft shredder WSC 250–400 (Weima Smart Cutter) from Weima Maschinenbau GmbH. During this process, the airborne dusts are analyzed. A Scanning Mobility Particle Sizer (SMPS) measures the particles in the nanometer range (9.8 – 414 nm), while an Aerodynamic Particle Sizer (APS) analyzes the particles in the micrometer range (0.5 – 20 µm). Airborne inhalable fiber dust is separated off with a VC25 sampling device on a cellulose nitrate filter and weighed. The rotor speed of the single-shaft shredder is varied from 60 rpm to 120 rpm and the screen mesh width varied from 10 mm to 20 mm. The studies with a smaller screen mesh width were only performed with the GF-PP organic sheet. The organic sheets are fed into the shredder with a size of 250 x 250 mm at a throughput of 1 kg/100 s for approx. 120 min. These parameter settings are shown in the diagrams with corresponding pictograms and arrows. The analysis of the toxicity of the fiber dusts is performed by a cytotoxicity test with a rat macrophage cell line (“lung cleaning cells”). For this, the cells were subjected to an increasing quantity of the produced glass fiber dust, and the negative effect on the energy metabolism was determined as a measure of the cytotoxicity with the fluorescence-based AlamarBlue test.

The studies showed that an increase in the rotor speed leads to an increase in the particle concentration and, in turn, to an increase in the inhalable dust fraction. In Figure 1 it can be seen that the nanoparticle concentration, especially at large screen mesh widths, rises with both materials through the increase in the speed. This connection can also be observed for the particle concentration in the microscale size range (Fig. 2) and for the fiber dust mass deposited on the filter (Fig. 3). The findings confirm the above-mentioned relationships that a higher rotor speed leads to a more brittle fracture behavior of the thermoplastics, producing a higher fines content of the ground material as well as increased release of inhalable fiber dust. The fact that this relationship cannot be recognized for the GF-PP in the micro and nano-scale size range when shredding at lower screen mesh widths is due to the fact that, through the increased friction between the rotor and the material, the particles become electrostatically charged. This leads to the particles sticking to the wall of the shredder or agglomerating to each other.

A reduction in the screen mesh width leads to an increase in the inhalable fiber dust mass (Fig. 3) for both speeds. On the one hand, the reduction in the screen mesh width leads to the maximum particle size being reduced, and on the other, the dwell time of the material in the grinding area is increased [6, 7]. The material must be broken down more frequently until it is small enough to leave the grinding area. Also in the nano and micro-scale ranges, a significant increase in the particle concentration can, at low speed, be seen through the reduction in the screen mesh width. This effect is not visible for the high speed because of the electrostatic charging (Figs. 1 and 2).

It was also found that the shredding of GF-PA6 organic sheet in comparison with the GF-PP sheet leads to a higher particle concentration in the micro-scale range and to a higher inhalable dust mass. This is also the reason for the greater brittle fracture behavior of the organic sheets through the use of the more brittle matrix material PA6.

The known relationships from the shredding technology for achieving a narrow particle size distribution can thus, for the most part, also be applied to the avoidance of airborne fiber dusts.

Estimating Dust Formation and Recommended Measures

Scanning electron micrographs of the fiber dusts from the cellulose nitrate filter show not only some long glass fibers but also wedge-shaped glass fiber fragments that are inhalable (&lt;5 µm) and therefore potentially harmful to health (Fig. 4). These results support the findings from the measured particle concentrations. In Europe, glass fibers may nevertheless be used if they do not have any marked biopersistence (EU Directive 97/69/EC). Fragments of glass fibers that comply in this way with the requirements for EU approval do not, despite their dimensions, exhibit any chronic toxicity because they do not persist in the respiratory tracts. This is on condition that there is no permanent exposure that constantly triggers inflammatory processes.

Tests on the cytotoxicity of the glass fiber dusts that were deposited on the cellulose nitrate filter during the shredding of the GF-PA6 organic sheets have confirmed this supposition. The determination of harmful particle effects was carried out with the help of the AlamarBlue test (Invitrogen, Life Technologies Corporation, Eugene, USA). This test measures the energy charge of the cells as a measure of the toxicity, using the fluorescence dye and redox indicator resazurin. A reduction in the fluorescence indicates cytotoxicity. Figure 5 shows the results of the test. The fluorescence is plotted against the concentration of the added particles, and remains unchanged even with very high dust concentrations. The AlamarBlue test thus shows no increase in cytotoxicity, despite the high dust concentrations. Even raising the rotor speed does not lead here to any changes in the evaluation of the toxicity.

These studies can only give a small insight into the topic. Nevertheless, the results show clearly that, in the shredding of fiber-reinforced thermoplastics, inhalable fiber fragments are formed, whose concentration can be reduced by reducing the rotor speed and by using a screen mesh width that is as large as possible. Even if these studies indicate that the glass fiber fragments do not lead to any severe toxicity in the lungs, industrial safety measures should be taken in line with TRGS 521, because, for reasons of preventive health protection, it is important to stop the release of inhalable fibers.

[1] AVK Industrievereinigung verstärkter Kunststoffe: Der Markt für Glasfaserverstärkte Kunststoffe 2020. (2020).

[2] Seidel, S.: Organoblech-Verschnitte recyceln. Kunststoffe (2021)1, S. 25–27

[3] Mattenklott, M.; van Gelder, R.: Carbonfaser und carbonfaserverstärkte Kunststoffe (CFK). Gefahrstoffe Reinhaltung der Luft (2019) 9, pp. 317–322

[4] Westphal, G. A.; Monsé, C.; Walter, D.; Brüning, T.; Bünger, J.: Gefährdungsanalyse für Carbonfaser-verstärkte Kunststoffe. IPA Journal (2019) 1, pp. 10–14

[5] BAuA Bundesanstalt für Arbeitsschutz und Arbeitsmedizin: Bekanntmachung zu Gefahrenstoffen, Hergestellte Nanomaterialien – BekGS 527 (2016), S. 754–767

[6] Löhr, K.; Melchiorre, M.; Kettemann, B.-U.: Aufbereitungstechnik: Recycling von Produktionsabfällen und Altprodukten. Carl Hanser Verlag, München 1995

[7] Shuaib, N. A.; Mativenga, P. T.: Effect of Process Parameters on Mechanical Recycling of Glass Fibre Thermoset Composites. Procedia CIRP, (2016) 48, pp. 134–139

[8] Bauer, W.: Untersuchung des Einzelzerkleinerungsvorganges in Schneidmühlen am Beispiel von Polypropylen. Fortschrittberichte VDI: Verfahrenstechnik, 694. VDI-Verlag, Düsseldorf 2001

[9] Stiess, M.: Mechanische Verfahrenstechnik. Springer Verlag, Berlin Heidelberg 1994

[10] Wolters, L.; Marvick, L.; vonRegel, K.; Lackner, V. (Hrsg.); Schäfer, B. (Hrsg.): Kunststoff-Recycling: Grundlagen Verfahren – Praxisbeispiele. Carl Hanser Verlag, München 1997

Fig. 1. Mass-based nanoparticle concentration resulting from the shredding of the GF-PP and GF-PA6 organic sheets using different shredding parameters.

Fig. 2. Mass-based particle concentration in the microscale size range, which resulted from the shredding of the organic sheets under different shredding parameters.

Fig. 3. Inhalable dust mass resulting from the shredding of the organic sheets. The dust mass has been standardized by dividing it through the mass of the input material and the quantity of the filtered air. 

Fig. 4. Scanning electron micrographs of the glass fiber dusts formed during the shredding of the GF-PA6 organic sheets 

Fig. 5. AlamarBlue Test for glass fiber sheets that were shredded at different speeds but with the same screen mesh width. No cytotoxicity is visible up to the largest quantity in the study.

Lisa Tölle, M. Sc., has been a research assistant at the Institute for Plastics Technology at the University of Paderborn (KTP), Germany, since 2019.

Dr.-Ing. Matthias Hopp has been Chief Engineer at the KTP since 2017.

Prof.-Dr. Jürgen Bünger is a specialist consultant for occupational medicine and, since 2005, has headed the Competence Center for Medicine within the Institute for Prevention and Occupational Medicine of the “Deutsche Gesetzliche Unfallversicherung” at the Ruhr University of Bochum (IPA), Germany.

PD Dr. Götz Westphal is a specialist toxicologist GT and, since 2008, has been a research assistant at the IPA since 2008.

Nina Rosenkranz has been a medical-laboratory assistant in the field of occupational medicine research and consulting at the IPA since 2007.

Dr. rer. nat. Christian Monsé has been employed as a research assistant at the IPA since 2009.

The authors would like to thank the Confederation of Industrial Research Associations “Otto von Guericke” e.V. (AiF) for its financial support for the research project, which was financed from funds of the Federal Ministry for Economic Affairs and Energy (BMWi). We would also like to thank the companies from the project-accompanying committee.

Faserverbundwerkstoffe

Andere Werkstoffe

Kunststoff-Recycling

Kunststoffverarbeitung: Verfahren

Kunststoff-Aufbereitung

Kreislaufwirtschaft

Efficient Filtration and Cake Washing for Improved Chemical Recycling of PET

Chemical recycling processes permit treatment of plastic waste – also if highly contaminated. The results are recyclates that have the same quality as virgin material. For example, newly developed processes enable used PET plastics to be returned to the material cycle. Hereby, the end products‘ quality is determined by the recycled material‘s purity. This can be significantly increased by means of efficient filtration and cake washing.

Plastic waste causes severe environmental problems. However, suitable recycling processes permit waste plastic to be used as a valuable secondary raw material – a decisive step, which not only permits the huge amounts of waste to be reduced, but also counters the raw material shortage. The need for increased waste recycling has also been recognized by politicians, and corresponding regulatory frameworks have been launched. For example, in the EU from 2025 onwards, bottles made of polyethylene terephthalate (PET) must contain at least 25 percent of recycled plastic, and by 2030, all packaging must be recyclable.

But higher recycling rates also offer considerable economic potential, provided that the plastics industry introduces the right steps. According to the consulting firm McKinsey, reutilization and recycling of plastics could lead to increased earnings of up to USD 60  billion  for industry [1].

However, mechanical recycling processes have limitations. For mechanical recycling, the plastic is melted and then converted to recyclate, whereby the material‘s quality suffers. Therefore, it is not possible e.g. to produce new bottles from 100 % recycled PET. Consequently, manufacturers are searching for alternative methods. Meanwhile, an increasing number of companies has successfully implemented chemical recycling.

Chemical Recycling: Products Achieve the Quality of Virgin Material

Chemical recycling includes all methods for the depolymerization of plastics. For this, waste plastic is first shredded mechanically, and then chemically decomposed into the basic constituents, frequently by using a catalyst or enzymes. The resulting recyclate exhibits the same properties as virgin materials produced from crude oil, and can be returned directly into the plastics manufacturing process. The decisive difference to mechanical recycling is: with the chemical method, very pure and high-quality materials from the recycling operation are available for plastic production. The quality of the end products is equal to that of virgin material.

In this way, for the first time, a closed raw materials loop for plastics has been created. Therefore, chemical recycling opens up new possibilities, and must presently be seen as the only way to achieve the targeted recycling rates. The German Federal Environmental Agency agrees with this view, and has classified chemical recycling of plastics as the preferred process in accordance with the Circular Economy Act.

Apart from thermochemical decomposition in the absence of oxygen (pyrolysis), and gasification, the process known as solvolysis (dissociation using solvents) has been discussed and tested as a recycling method in recent years. Under the influence of suitable agents, mainly thermoplastic materials such as PET are liquefied and then dissociated into their monomers. Because different solvents are used, solvolysis processes can be further distinguished: the most common methods include glycolysis, methanolysis, and hydrolysis, partially with enzymatic boosting [2]. All three processes are suitable for decomposing PET (Info box). Which process is used, depends on the quality of the input material.

The processes named above produce monomers in the same quality as their petroleum-based pendants. The monomers are dissolved in the process liquids, and must be separated. As soon as depolymerization has been completed, the monomers are precipitated from the solution, and the suspension passed through a filter. Here, the solvents, catalysts, and other constituents are removed.

To a great extent, the quality of the produced PET virgin material depends on the monomer‘s purity, as contaminations clearly impair the transformation in polymers, and the reactions take a different course. The product purity required for successful recycling is decisively achieved by solids/liquids separation with efficient cake washing (Fig. 1). For this step, several international companies who employ one of the described recycling processes make use of the BHS-Sonthofen processing solutions.

Multi-Stage Counterflow Washing with Lower Consumption of Resources

For one of these customers, BHS-Sonthofen supplied two Type BF indexing belt filters (Fig. 2). The machines operate in parallel in the production line with a throughput of 2 t/h. The Type BF indexing belt filter is a continuously working horizontal vacuum filter that is used for efficient and gentle separation of sedimenting solids from suspensions.

A multi-stage counterflow washer with washing filtrates is used, which represents a highly efficient form of cake washing. Hereby, fresh, application-specific washing liquid is used in the last washing zone, which is then re-used in the previous zones. This procedure is especially suited to generate high purity – with a low consumption of washing liquid and energy.

Particularly in the context that processing efficiency will also decide how chemical plastic recycling will become established in the market in future, cake washing plays a very important role. Another advantage is that fresh liquid is used for washing the filter cloth, before it is used again for filtration and later cake washing. The solids washed off the cloth are returned to the process and recycled, which increases product yield.

BHS-Sonthofen has adapted its filtration process to the demands of chemical recycling. This permits high-purity monomers to be reclaimed efficiently, which can be used for the synthesis of new PET without additional processing steps. Hereby, the company was able to resort to the experience acquired from the use of their filters in numerous plastic applications.

Rotary Pressure Filter for  Chemical Recycling

In addition, the Type RPF rotary pressure filter (Fig. 3) is suitable for chemical recycling of PET. It is a continuously operating device for pressure filtration, cake washing, and drying of suspensions. Experience values and comprehensive trials have shown that both technologies can be used successfully for chemical recycling of PET. BHS-Sonthofen‘s many years of experience in the production of virgin PET material proved to be advantageous during development of the filter.

Chemical plastic recycling offers several benefits: it prevents the combustion of plastics (thermal utilization), and the consumption of fossil raw materials is reduced. Highly contaminated plastic waste can also be recycled, and waste flows of materials that previously were out of the question can now be recycled. However, ecological and economic challenges must still be overcome. Regarding energy and consumption of resources, the processes must be designed so that they exhibit a positive ecological balance, and are also economically viable. Therefore – and apart from the quality of the reclaimed monomer – processing efficiency plays a major role when it comes to the application of the described processes on an industrial scale. For this, smart processing solutions are required, which ensure product quality and efficiency in equal measure.

High Monomer Yield with Suitable Solids/Liquids Separation

The use of efficient machines permits a high yield of monomers to be achieved, for a reduced consumption of energy and resources. A customized solution for solids/liquids separation for the respective customer application is a decisive contribution for introducing chemical plastics recycling on a large scale. If attempts are successful to establish technically functional and commercially viable solutions in the market, the industry will have made a large step towards the sustainable production of plastics.

[1] McKinsey (2018): How plastics waste recycling could transform the chemical industry.

[2] Die nächste Generation des Recyclings – neues Leben für Kunststoffmüll: Angew. Chem. Int. Ed. (2020), 59, S. 15402–15423.

Fig. 2. The BHS indexing belt filter is made of stainless steel and supports multiple counterflow washing.   

Fig. 3. Pressure filtration  can also be used in chemical recycling of PET, e.g. with the rotary pressure filter RPF. 

The RPF rotary pressure filter is a continuously operating device for pressure filtration, cake washing, and drying of suspensions. Cake washing plays an important role for improving the processing efficiency of chemical recycling.

Fig. 1. During chemical recycling of PET, solids/liquids separation is a particularly important processing step. Corresponding systems from BHS-Sonthofen are already being used by several companies.

KU 5-22_BHS - Hauptartikel

For glycolysis, various glycols are used to dissociate the polymers. The glycol diffuses into the polymer‘s structure. Hereby, a mixture of bis(hydroxyethyl)terephthalate (BHET) and short-chain oligomers is created, which is cleaned and can then be reused for the production of PET.

For methanolysis, the polyester structure is dissociated by methanol, mostly by means of catalysts. Hereby, monomers are recovered for renewed PET synthesis – in this case dimethyl terephthalate and ethylene glycol. Because special safety measures are required for methanolysis, the investment costs are higher than for glycolysis.

The energy input for hydrolysis is comparatively low. Hereby, PET is dissociated into terephthalic acid and ethylene glycol by adding e.g. sulfuric acid and sodium hydroxide [3]. If supported with special enzymes, hydrolysis is particularly time & energy saving, and conserves resources.

Dipl.-Ing. Christian Gassen is a process engineer with BHS-Sonthofen Process Technology. He has been working for the company since 2011;

<a href="mailto:process-technology@bhs-sonthofen.com">process-technology@bhs-sonthofen.com</a>

Recycling

Umweltmanagement

Filteranlagen

Filter

Maschinenelemente

Konstruktionselemente-Fluidtechnik

Conference Minutes of “Forum Plastic Recyclates”

A growing number of companies and industrial sectors would like to use recyclates for their products. However, the application is frequently still confronted with considerable difficulties. And the extension of recycling to other fields and sectors also poses new questions. An overview of the current status and associated technical solutions was provided in conference “Forum Plastic Recyclates” at the end of March.

Recycling of plastic waste is now seen by politics as well as the industry as an important issue for environmental protection and more sustainability. Clear signals for this are regulations – e.g. of the EU Commission to increase the recycling of packaging and boost the use of recyclates – and the importance of circular economy as highlighted by industrial associations such as Plastics Europe Deutschland, an association of plastics producers. Consequently, the will is there, but implementation still proves to be difficult. There is a lack of necessary infrastructure for collecting plastic waste, adequate processing capacities, recycling-compatible product design, and frequently, the quality of recyclate has room for improvement. An interesting overview of the main difficulties and possible solutions for the problems was provided during the “Forum Plastic Recyclates” hosted by the Fraunhofer Institute for Structural Durability and System Reliability (LBF) on March 24, 2022.

A good summary of the present situation was given by the forum‘s chairman Professor Dr. Rudolf Pfaendner from the Fraunhofer LBF in his opening lecture. He pointed out the growing interest of consumers and therefore also of manufacturers for an increased use of recycled materials. That is why recyclates are meanwhile being used in an increasing number of products. Simultaneously, however, there is still a need for an adequate recycling infrastructure. According to Dr. Pfaendner, the growing interest is not limited to Europe, but is a global phenomenon. Presently, two thirds of the patents in the recycling business come from China.

Ton Emans, president of the European association “Plastics Recyclers Europe“, took a clear stand for recycling-compatible design. In his view, significant advances in recycling will not be the result of innovations in recycling technologies, but mainly due to corresponding product design. He therefore also sees a commitment for producers of plastic products to focus more intensely on the recyclability of their products. “Companies must concentrate far more on design for recycling, also when selecting the polymers used”, he said. Furthermore, to increase the recycling rates, better harmonization and standardization of waste collection is necessary in Europe.

Previously, low quality was a frequent argument against a more widespread use of recyclates. But quality can be increased significantly by means of additives. Which improvements are achievable in polyolefins such as polypropylene (PP) and polyethylene (PE) was explained by Dr. Oliver Reich from BASF. The chemical company investigated how the molecular weight of the polymers changes after several recycling cycles, and which effect this has on the recyclate‘s properties and their processability.

Accordingly, investigations showed that PP as well as PE require stabilization by means of additives. With unstabilized PP, for example, the melt volume flow rate already increases significantly after just a few recycling cycles, whereby the material exits from the necessary processing window. However, if it is stabilized with additives, up to three times as many recycling cycles are possible before it cannot be processed further. But according to Dr. Reich, stabilization is not a cure-all. Primarily, the long-term temperature stability of the additives must be observed. Furnace aging tests by BASF had shown that also sufficiently stabilized materials degrade over time, thereby decreasing the effect of the additives. This point is particularly important if polymers are to pass through several recycling cycles. Nonetheless, as Dr. Reich pointed out, the investigations revealed a decisive point: “Without additives, a circular economy cannot be achieved.”

The significance of additives for improving recyclate quality was also underlined by Dr. Matthias Bruch, innovation and product manager for plastic additives at Brüggemann GmbH. In his view, there are mainly three points that create large problems with recycled plastics: the different additives and fillers contained, contaminations such as traces of metals, sand, and organic compounds like inks and adhesives, plus damage during processing caused by oxidation or degradation of the polymer chains. As a remedy for the latter, Brüggemann has developed a repair technology for recyclates.

The remedy involves additives that compensate the damage inflicted on polymers during processing. Currently, the company offers these additives for PE and PP. According to Dr. Bruch, analyses show that e.g. melt volume flow rate of correspondingly additivated plastics only increased slightly even after several extrusion cycles. Moreover, their tensile strength and their elastic modulus remain constant for a far longer period. Also the effects of heat aging arise much later. And compared with previous stabilizers, the new additives partially increase the performance of recyclates more than twofold, he added.

But additives are not only helpful for recycling – sometimes they also prove to be a hindrance, as explained by Dr. Andreas Mäurer from the Fraunhofer Institute for Process Engineering and Packaging (IVV), and Jörg Hamann from the plant builder EPC Engineering and Technology. This is e.g. the case with plastic products that are in use for a very long time, such as components in the automotive field and the building industry. In the past, for example, insulating panels made of expanded polystyrene (EPS) were frequently treated with hexabromocyclododecane (HBCD) to ensure adequate flame proofing. However, use of this additive has been mostly banned since 2013. Therefore, plastics containing it cannot simply be returned to the recycling loop.

One possibility for removing HBCD from insulating panels involves solvent-based recycling processes. Hereby, a solvent is used to extract the required polymer from the plastic waste, subsequently cleaned and then precipitated. The plastics recovered in this way can then be reused for making new products. As the polymer is not depolymerized by the process, solvent-based processes are classified as mechanical recycling. A corresponding process has been developed by the Fraunhofer IVV and the CreaCycle company.

Additives can solve quality problems with recycled plastics. With the IrgaCycle product range, BASF has introduced versions, with which e.g. processability and long-term thermal stability can be improved.

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Recycled Hospital Waste as Raw Material for 3D-Printed Furniture

Robotics meets additive manufacturing: when Kuka robots produce furniture, they do not take work away from carpenters by any means. The chair and bench modules made of recycled hospital waste, named Twine – designed by HagenHinderdael in London – are a novelty in the world of furniture, engineering, and recycling.

The HagenHinderdael design studio was founded by Sofia Hagen and Lisa Hinderdael in 2020. “We combine product design with art to create sculptural products and immersive installations, working at the touch point between sustainable design and innovative technology,” is how the two architects and designers describe their creative approach. Hinderdael is from Belgium, Hagen from Austria – and both have enjoyed success in international settings.

Twine – urban furniture for public spaces – was designed for the third edition of the “Guiltless Plastic” campaign. It is curated by Milan’s gallery owner Rossana Orlandi, regularly prompting artists and designers to exhibit their projects to address plastic waste. “So we invented Twine. Our furniture had to be playful, comfortable and durable, and we wanted to use the filaments from Reflow for production using 3D printing,” is how Sofia Hagen describes the original project idea. Reflow, an award-winning social enterprise in Amsterdam dedicated to the fight against environmental pollution with plastic waste, obtains its materials for additive manufacturing from recycled waste plastic. The raw material for the functional, modular furniture elements is a rPET-G filament (recycled polyethylene terephthalatemodified with glycol) made of discarded, thermoformed medical bowls.

Delivering Durability and Aesthetics – without Lifting a Finger

The Twine modules are 1.06 m long, 50 cm high and 30 or 45 cm wide. The design alone was crowned as the finalist for the RO Plastic Prize 2021. Winning this prize for objects made of recycled plastic makes headlines across Europe. Therefore, a Twine seat was sent to the exhibition in Milan as part of the competition. HagenHinderdael engaged Ai Build in London, which has already done a lot of work with Reflow filaments, to produce this and another piece of furniture.

Ai Build is a SaaS (Software as a Service) business founded in 2015, “currently employing 20 people and five Kuka robots” as Chief Operating Officer Michail Desyllas describes it. The team mainly works for tier one companies in the aviation, automobile, marine, and energy industries. But more and more architects and designers are interested in the offers as well. Desyllas reports: “The geometry of the components and the desired aesthetic finishing were the greatest challenges in this special project. We were not allowed to change anything.”

Ai Build decided that two robots, a KR30 L16 of the KR Iontec series and a KR90 R2700 of the KR Quantec series, would do the job “because these robots have the range required to make such parts. They also have our software fully integrated, allowing us to monitor the quality and report defects in real time,” says Michail Desyllas.

Less than two weeks elapsed from the order to the finished furniture. Since Twine is intended to flow like waves, Ai Build constructed a special mold, a sort of curved double bed, on which the hot material from the 3D printer was allowed to cool and solidify. The narrower elements were made by the KR30 L16, the wider ones by the KR90 R2700. Each robot was equipped with a filament extruder system (model: Ai Maker).

This system can transport up to four filament flows in different colors, supporting a rich play of colors and transparency gradients within the elements. Desyllas grants that the application of the rPET-G filament was tricky: “Each layer has to be applied within a certain window. Not directly after the previous layer, to prevent sagging, but not too late either, otherwise the layers may not adhere to each other.” According to the data sheet provided by the manufacturer (Reflow), the filament is processed at a nozzle temperature of 210 °C and a print bed temperature of 75 °C – with a printing width of 6 mm, a layer thickness of 3 mm (outer surface) or 1.5 mm (ribbing) and a printing speed of 18 mm/s (outer surface) or 25 mm/s (ribbing).

Aesthetic, robust, and comfortable seating furniture was designed thanks to good preparation and the accurateness of the robots. “Every Twine element passed the quality check executed by our software and the always vigilant eye of the camera system on our robots,” Desyllas says. The AiSync software supports dynamic, multi-axis motion control of the print head with fully automated trajectory generation, along with process control and workflow monitoring for industrial 3D large format printing based on FFF (Fused Filament Fabrication).

Sofia Hagen and Lisa Hinderdael also liked the result. They sent Twine to Milan, where it captured many hearts although failing to win a prize. At the end of September, the furniture became part of the “Planted” show in the course of the London Design Festival. In the future, the Twine seating elements will be customized and produced by the Italian Kuka partner Caracol AM.

Designers Lisa Hinderdael (left) and Sofia Hagen present a better side of plastic waste. The KR Quantec used to print the larger Twine elements is in the background.

The AiSync software of the Kuka system partner Ai Build supports dynamic, multi-axis motion control for industrial 3D large format printing. 

The seating modules are produced in two widths from recycled PET-G filaments using additive manufacturing.

The project was realized in cooperation between the HagenHinderdael creative studio, the sustainable materials supplier Reflow and Ai Build, a company that develops software and artificial intelligence, among other things for applications with Kuka robots: <a href="https://reflowfilament.com">https://reflowfilament.com</a>

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New Applications for the Automated Production of Fiber Composite Components

Particularly two industries have shown considerable interest in FPP technology: aerospace industry in automating the lay-up processes of multi-material lightweight components with sandwich cores, as well as manufacturers of CFRP pressure vessels for hydrogen-powered vehicles (FCEVs), who can thus achieve savings in the double-digit percentage range. In both areas, FPP is the first technology to automate critical fiber lay-up processes in order to industrialize the production.

The robotic manufacturing technology Fiber Patch Placement (FPP) expands the possibilities for the automated production of geometrically complex fiber composite components. In the FPP lay-up process, defined fiber patches are cut from a fiber tape and placed by two robots and a fexible patch gripper directly on a 3D mold (Title figure).

The size of the patches and the gripper is adjusted to the dimensions and complexity of the component. Since the process is implemented as a series of individually controlled patch placements, FPP enables a superior level of process control for the entire laminate lay-up cycle. A dedicated CAD-CAM software simplifies the creation of patch laminates.

As a core element of the process, the software includes a FE-interface and, equipped with a digital twin of the production system, performs a fully automated robot offline programming. In-process quality control systems detect potential defects before the fiber lay-up is performed (Fig. 1). Furthermore, there is the possibility of increasing the overall productivity by up to factor 5 to 10, depending on the application. This usually translates into savings of 20 to 60 % in cost and process time.

The characteristics of FPP technology are particularly beneficial in lay-up automation for complex, multi-material aerostructures and in reinforcing the laminate of filament-wound composite tanks, which are described in the following sections.

Lay-up Automation for Complex, Multi-Material Aerostructures

The high mass-specific stiffness renders composite sandwich structures suitable for the demanding weight-restricted applications on aircrafts. But in the production of these components, geometric complexity and the multi-material mix of adhesive film, glass and carbon layers (among others) pose a particular challenge. Production usually requires several process steps and involves time-consuming manual lay-up work and intermediate debulking steps.

FPP technology, with its ability to process a broad variety of fiber materials, is able to produce such components fully automated with one production system. Fiber material can be placed directly on curved or chamfered honeycomb cores (Fig. 2). This replaces manual lay-up steps and usually increases lay-up precision and repeatability.

The deposition pressure of the placement robot can be adjusted and controlled. Thus, intermediate debulking steps can significantly be reduced or even eliminated, resulting in valuable savings in process time. Test samples with 24 plies have shown porosity values of less than 1 %.

The real-time process control ensures constant high part quality: two visual inspection units analyze

<ul><li>the quality of each patch before placement, and</li><li>the relative position of the patch on the placement gripper.</li></ul>

Thereby the system guarantees that only 100  % in-spec raw material is placed precisely on its intended position.

The flexible process also allows the combination of sandwich-core structures and monolithic sections in one integrated production process (Fig. 3). The combined laminate approach is fully supported by the FPP-specific Artist Studio software. This offers flexibility for laminate adjustments in the development process whenever needed – with significantly reduced effort and time for design iterations.

H2 Composite Tanks with  FPP Dome Reinforcements

Composite pressure vessels are the key storage system for hydrogen-powered mobility. The main cost driver of the cylindrical tanks with operating pressures of up to 700 bars is the carbon fiber material with over 50 % of total tank cost. To improve efficiency of composite pressure vessels, the US Department of Energy examined the positive effect of dome reinforcements already in 2013, but until today, there was no industrial process available to produce them.

Cevotec adapted its FPP technology to create an industrial solution that places the tank reinforcements fully automated (Title figure, Fig. 4). Afterwards, the vessel including reinforcement is transferred to the established filament winding process (FW). The weight and manufacturing costs of a tank are thereby reduced by 15 % while achieving equivalent mechanical properties.

The robotic FPP system places the reinforcements patches directly on the dome areas of the liner. The patches substitute the high-angle helical layers (HAHL) traditionally applied by the FW process. However, unlike the HAHL, the patches do not span across the cylindrical portion of the vessel, where they would contribute only marginally to the mechanical performance. This translates into a considerable saving of material, depending on the aspect ratio of a vessel. The reinforcements also facilitate a smoother stiffness transition between the cylinder and dome sections, leading to a further optimized laminate design.

The patches are applied in an independent process, parallel to the FW. This parallelization leads to an overall cycle time reduction of about 15 %, resulting in more tank capacity on existing FW lines. Overall, the combined benefits for composite vessels in series production pay off fast for any manufacturer: the necessary investment for FPP dome reinforcements amortizes within one to two years.