If you want to produce plastics in a more environmentally friendly way and improve their recyclability, you have to try out new ingredients and keep separating and remixing the components of the "plastic". A twin-screw extruder costing 800,000 euros has recently been installed at Bielefeld University of Applied Sciences to help with this. Among other things, the researchers want to use it to test algae-based dyes and break down stubborn plastic compounds in order to be able to recycle the raw materials.



Bielefeld (hsbi). If Johannes Brikmann wasn't standing in the experimental hall at HSBI, you might think he was working in the canteen kitchen of the neighboring cafeteria: "We can chop, divide, mix and portion with the equipment," says the 37-year-old engineer about his workplace. Even though this often involves recipes or ingredients, Brikmann and AG Bielefelder Kunststofftechnik are not concerned with preparing food, but with the plastics of the future. Their workplace is located in the large experimental hall in the basement of the main HSBI building and goes by the rather cute name of twin-screw extruder.



The aim of the elaborate system is to realistically simulate industrial processes

The various components of the extruder measure around five meters long and three meters wide, allowing the team led by Johannes Brikmann and his laboratory manager, Professor Dr. Bruno Hüsgen, to realistically simulate plastics production processes. "With the extruder, we can both develop new materials and modify existing ones in a targeted manner, for example by adding new substances or breaking down others," says Hüsgen, explaining the main areas of application of the system, which is also known as a compounder. He stands in front of one of the platform supports in the hall and points to several low trolleys on which pumps and hoppers are mounted. From here, hoses lead into a box-shaped, elongated housing, at the end of which a stately electric motor is mounted. Under this casing is the element that gives the system its name: two spiral-shaped torsion bars - called "screws" - that rotate in a cylinder encased in heating bands.



From granulate to extrudate: the extrusion process

At the beginning of the extrusion process, plastic granulate, which forms the basis of every plastic product, is filled into this barrel via a hopper above the system. The rotation of the screws transports the granules along the barrel to a nozzle at the end of the line. This creates friction. At the same time, heating belts are switched on, causing up to 450̊ C of heat to act on the material filled in. Under the influence of these two factors, the granulates can be put into all conceivable aggregate states. They are often melted. Other raw materials retain their consistency, are remixed or given individual properties through additives. At the end of the process, the mass, now called "extrudate", is pressed through a nozzle, as in the molding of cookies, and is thus given its final shape.



Additives give the plastic its shape and color

But before that, the roller carriages with their pump containers come into play: with their help, certain additives can be mixed into the plastic during the process. These "additives" give the plastic individual properties. They guarantee, for example, that the end product does not deform, lose its color or become fireproof even under stress. "The special thing about this compounder is that we can not only mix in all the additives used in industry, but can also try out new plant-based additives that require gentle temperature control," says Bruno Hüsgen, pointing out another characteristic of the extruder - it can be used to develop sustainable plastics.



Biopolymers and recycling - how can plastics become sustainable?

A current example of the working group's research into sustainable plastics is a new blend of the two naturally occurring raw materials polylactide (PLA for short) and polyhydroxybutyric acid (PHB). Brikmann believes that the compound could be used in sustainable packaging in the future, which could be completely biodegradable under the right conditions. When developing the new material mix, the team made the weak points of the two polymers their own: "PHB is very brittle, while PLA only has a lower temperature resistance. The mixture cancels out the disadvantages of both types. In combined form, the disadvantages become the positive properties of a new raw material that does not require the use of crude oil," says Brikmann, who is currently working on his doctorate, illustrating the development process. The polymer, which is both tough and heat-resistant, could also be used in technical components with a short half-life or as packaging for medical products, which generally have a short lifespan.



Attempts to recycle plastics that were previously almost impossible to recycle

Another approach of Hüsgen and Brikmann's team is to extend plastic life cycles through recycling. To this end, they have chosen the group of elastomers as a rather "difficult candidate". This basic material for tires, seals and rubber bands has the advantage of being extremely stretchable and elastic due to its long molecular chains. However, when a plastic product made with elastomers reaches the end of its life, it is precisely the cross-linked, long molecular structure that becomes a problem. This is because the chains of an elastomer cannot be restored by the usual recycling process of shredding, melting and hardening. This is precisely why the main option for elastomer products at the end of their useful life has so far been downcycling: a worn-out car tire, for example, can at best be turned into flooring or infill for artificial turf pitches. In most cases, however, such waste products are only thermally recycled, i.e. they are sent to a waste incineration plant or used as fuel in a cement plant. For Bruno Hüsgen and his team, this is an unacceptable situation: "We want to use the shear forces acting on the material in the compounder to separate the sulphur bonds in the elastomer," says Hüsgen, explaining the tests currently underway. "If we succeed, material recycling would be possible and the material could therefore be reused."



All components must be considered for true sustainability

To be truly sustainable, however, such a cycle must not only focus on the main material. All other components of an end product must also be able to do without the use of new petroleum-based plastic. This also includes, for example, the coloring of the resulting plastics. This is because paints and varnishes are also often based on petroleum products. This is the mission for Tessa Strümpfler and Dr. Thomas Zimmermann. The two are part of the team of Prof. Dr. Anant Patel, process engineer and HSBI Vice President for Research and Development, and are actually involved in researching new crop protection agents. Their work often revolves around the different properties of algae. Today, however, they are particularly interested in twin-screw extruder technology, as it could also serve the group well in their current research projects. In their research on blue-green algae, the working group developed a process with which they can extract a natural and easily degradable dye from algae. As this could be the missing building block for closing a sustainable plastics cycle , an innovative collaboration has developed between the two working groups: The Patel working group provides its knowledge of plant-based dyes, while the Hüsgen working group contributes its expertise on the process technology of the extruder for the experiments of the biotechnology group.



Working with the extruder promotes unexpected synergies

"Simply replacing the plastic and continuing to use dyes from the petrochemical industry is not a completely green solution. This also requires bio-based innovations on the additives side," says Thomas Zimmermann, explaining how the ball started rolling for the collaboration between the two research groups. "Conversely, the extruder enables our research group to combine previously separate processes in the formulation of our active ingredients into a single process," says Tessa Strümpfler, describing the aim of the collaboration. By "formulation" , the biotechnologist means the form in which an active ingredient arrives at its place of use. In their field of research, Strümpfler and Zimmermann currently often work with capsules, which are first formulated in a two-stage process and then coated. This is often cost-intensive. Due to its higher productivity and scalability, the extruder could already make the formulation considerably cheaper. In the best case scenario, the final coating could become superfluous in the future.



Previously separate steps could be combined by the extruder

"We may be working with different ingredients and requirements, but we both need a pretty expensive stove to transfer our products into an industrial process," concludes Johannes Brikmann, finding himself a kitchen metaphor for the cooperation between the two research groups. In future, the work with the extruder should also open up new solutions for further questions from both working groups or for thematically similar projects such as InCamS@BI. As the group leaves the experimental hall, Brikmann turns back to the extruder as if to say goodbye : "In the end, it takes a creative chef who uses the right technology and a good recipe to create a product that impresses," he reveals with a wink. Just like next door in the cafeteria kitchen.