Small-scale Extruders Empower Plastics Recycling Research

Recycling modern plastics is much more complicated that looking at the number inside a “chasing arrows” symbol and tossing the piece into the designated waste bin. Once plastics reach the recycling facility, they need to be treated in the correct way to optimize their recycling potential. The plastic materials used in modern-day packaging, equipment, and in many other technical applications are often complex mixtures of dozens of chemicals. This makes efforts to recycle such plastics and keep them out of a landfill a true challenge. Scientists researching recycling methods continually seek to develop efficient closed recycling loops from waste collection. The ultimate goal is to figure out how to separate out individual waste components, and then determine how to efficiently process them into recycled plastics equivalent to virgin plastic materials.

3 Types of Recycling Processes

Within the recycling sector, there are three main types of processes: chemical, mechanical, and solvent-based or physical. Each has their own advantages and disadvantages for any given situation. In the case of polymeric materials like plastics, these methods can produce very different outcomes. Chemical recycling, based on depolymerization reactions, results in a mix of different smaller building blocks or monomers. Physical recycling, makes use of solvents to dissolve plastics apart, but the polymer structures are not influenced. Mechanical recycling makes use of techniques like grinding, magnetic separation, sorting, vacuuming away volatile compounds (VOCs), and extrusion with melt filtration steps. During these mechanical steps, the polymer structure is not affected, at least not intentionally.

During the research stage of the mechanical recycling process, the aim is to ascertain which of various mechanical processing steps could be implemented that deliver the highest purity plastic recyclates at the end. A small laboratory-scale extruder can be a major asset in this testing and recycling research to figure out the best parameters within the recycling routes. Because research paths often lead to discoveries that may not be commercially viable, small scale experiments are necessary to test new protocols in cost-effective ways that do not generate substantial amounts of unusable waste.

Reducing Scrap Waste

Figuring out how to turn waste into useful material is in some cases a governmental directive, and in other cases a self-imposed goal. Organizations like the European Union (EU) have proposed directives that will require, for example, the automotive industry to use increased proportions of recycled materials, including a certain share of that comes from vehicles at the end of their lives.

Other industries may simply try to be environmentally conscious and reduce their waste. The textile industry produces items like, say, sport bags that use varying amounts of mixed materials. Since some cut material is always left over from the production process, there is waste that must be dealt with.

Reducing the amount of scrap waste has long been a goal of any responsible business, while making goods that are durable, repairable, and recyclable is a stated goal of the EU’s Strategy for Sustainable and Circular Textiles. Incorporating sufficient capacities for recycling and minimal incineration and landfilling is another objective of the strategy. Much of the nearly 5 million metric tons of clothing discarded in Europe each year is either sent to landfill or incinerated. Currently, only 1% of such material is recycled into new clothing. (source: European Commission, directorate of Energy, Climate change, and Environment) Lab-scale experimentation on the small scraps left over from production might help develop recycling methods that could increase the amount of textiles that get recycled.

The Science of Plastic Recycling

The complex process of plastic recycling involves lots of experimentation and many variables: What is the chemical composition of the material or mix of materials? How should a material be cut, or should it be ground up instead? How small should such particles be for effective recycling? Does there need to be any kind of pre-treatment to make the material processable by the extruder? In what order should the pre-treatment steps be carried out to optimize processing? Later in the process, will the plastic recyclates be put through injection molding? In the case of textiles, is fiber-to-fiber recycling possible? All these questions need to be answered to develop economical, effective recycling processes.

Because it is impractical to test all these possible variables on an industrial scale, lab-scale extruders are often used for preliminary work trials. An industrial-scale extruder would require much more starting material—a minimum of approximately 100 kilograms. Besides the fact that running unproven experiments on such large volumes would be wasteful, in some cases such a substantial amount of material is not even available. A test batch of some new material to be recycled might contain only 5 kg or 10 kg or maybe even only a few grams.

Using Extruders in Recycling

Fortunately, trials performed on small-scale extruders are representative of processing at larger scales. All the scenarios and tests performed in the lab can be carried out on manageable levels, including the measurement of material characteristics through rheological testing, determining the composition of off-gases, and more. These testing methods can be performed in-line, which makes testing easier and fast enough to have in-time results for process adaptation. The results are also likely to be more regular compared to off-line laboratory testing, which only uses a small (potentially non-representative) sample of the overall batch and often takes much longer. After a process has been experimentally determined based on a small amount of material, the method can be transferred to larger extruders with relative ease thanks to the scalability of the extrusion process.

The manageable size of a lab extruder provides other advantages. The small footprint of a lab-scale extruder means it could fit more easily into a tight workspace; some models could conceivably fit on a lab bench, making them easy and convenient to operate. Also, it is possible to pre-adjust the main processing parameters using lab extruders with a very small amount of material. As noted earlier, there are many variables when recycling plastic waste from products which were not designed for recycling and have never been recycled before—e.g., plastics waste from end-of-life vehicles, electrical scrap, rotor blades, etc. What additives to use on these materials, what size particles work best, or the like needs to be determined. Small scale extruders are highly useful for these initial attempts to discern the optimal processing parameters.

The electronics industry provides an example of just how useful lab scale extruders can be. Imagine a situation where a truckload of electronics is scheduled to be recycled. Such electronics contain a variety of metals and plastics, in parts large and small, all mixed together. How can you apply mechanical methods, and avoid using hazardous chemicals or toxic solvents, to separate those components and obtain useful recycled materials? Where do you even start?

Working on a laboratory scale, it is possible to start with as little as a few hundred grams of material. A 500-gram electronic component, for example, might be half metal and half plastic. But that plastic portion might contain a dozen or more different types of plastic. This means each amount of polymer you would have at the end is very, very small, possibly just a few dozen grams. Processing this on a semi-industrial extruder or larger would not be feasible.

In this case the process would begin by putting the sample through relatively straightforward grinding and metal separation processes. This first step extracts the metallic portions and results in a mixture of varying amounts of polymers. Then spectroscopic methods like Raman or NIR can be used to differentiate and separate the small particles of plastics based on their chemical composition, or perhaps other mechanical methods like a sink-float test might be employed to separate plastic types based on density. When this separation step is complete, the lab-scale extruders come into play, because some polymers would be present only in small amounts. There might be some form of pre-treatment, but then these small amounts of plastics would be fed into the extruder, where they undergo mechanical processing (potentially with some type of additive mixed in to alter strength or other physical properties) that transforms the material into a consistent, homogenous product.

A small injection molding machine that can be incorporated with the lab-scale extruders adds another benefit. As the polymer exits the extruder, it can be fed into an injection molder to produce specimens for subsequent testing. The samples can then be tested for purity, processability, and use performance.

The current and future value of smaller lab-scale extruders for recycling is very promising, as many applications are still at the beginning of the transition to a circular economy. The combination of technologies like extruders, injection molds, and other small-scale analytical instruments makes it possible, on a manageable scale, to develop new recycled products from what had been a messy mixture of polymers.

Additional Resources

• Application note: Preparation and analysis of PET with additives using a micro compounder and rheometer // URL: LR74-e_Preparation_and_analysis_of_PET.pdf (thermofisher.com)
• Collection of notes on small-scale recycling: Polymer sustainability applications compendium CO64729-Recycling-EN.pdf (thermofisher.com)
• European Commission publication: EU strategy for sustainable and circular textiles // URL: https://environment.ec.europa.eu/strategy/textiles-strategy_en