Intelligent and Sustainable Compounding
Author: Christian Makert / Andreas Liebing
More and more companies are looking for new ideas as to how recycling plastics can be incorporated into end products for consumers and technical components for industry. As a result, compounding is coming more into focus – provided that process stability, high availability and durability of the machines used in the compounding process can be assured.
A glance at the shelves in the supermarket is enough. Even today you can already find a large number of packagings that advertise a high recycling plastic content, and this share is set to increase even further. It is not only the large manufacturers of goods and articles for the ultimate consumer who are changing over or re-evaluating their processes, but also many other industries.
But changing over a process to enable the use of recycled materials is anything but simple. In the meantime, it has become possible to produce customized recycling plastics even for high-grade applications by adding additives, fillers or reinforcing materials such as talcum or chalk (CaCO3), glass fibers or master batch pigments. In view of the fluctuating quality of the input material, however, the know-how about material properties, wear protection and process stability is more important than ever when it comes to the compounding of recycling plastics. Only then can the property profile of the end product be maintained without compromising quality.
In this respect, raw materials based on recycled material no longer differ much from virgin material in terms of their properties. However, this does have an impact on the pumps, filters and pelletizing systems required for the compounding process. Ultimately different compositions, residual moisture contents and viscosities are the order of the day with recyclates, and various contaminants further complicate the compounding process.
To give just one example: For cost reasons, recycled material is often mixed with fillers, especially talcum or chalk. They can often account for up to 60 percent of the material. The mineral filler has a wear-enhancing effect. Furthermore, higher contamination levels and thus larger particle sizes are involved. Extremely resilient pumps and melt filters are therefore required. This makes it all the more important to have well-matched components in the entire process.
Each component combines with the next
Maag supplies complete solutions for the compounding process. Here – from melt conveying and filtration to pelletizing or grinding and drying through to the control system – all the components combine seamlessly with one another to create and efficient production process. The know-how of the entire Maag family with the product brands Automatik, Ettlinger, Gala, Maag, Reduction, Scheer Witte and Xantec goes into the process. These individual elements are brought together into one system, resulting in a complete system downstream of the extruder. The carefully combined unit with the Xantec control concept has coordinated interfaces. It ensures optimum process monitoring and can even be extended to the extruder control system and the dispensing, i.e. to the entire recycling line.
Vast system know-how
Long service lives, especially when working with recyclates and fillers, are achieved not only by innovative materials and surface know-how, but also by the way the system is operated. To achieve fast start-up and shutdown cycles of processing and compounding extruders, and minimize the load on them, the pressure of the plastic melt is generated by a gear pump. Only with a constant pressure and volume in the process can pellets of the highest quality be continuously produced.
In the Maag melt pump, on the other hand, the material is only briefly pressurized and is subject to very little shearing. The reason for this is that the Maag gear pumps operate with very high precision. Depending on the viscosity of the product, they guarantee a tolerance range of a few bar pressure variation. This is ensured by the special gearing and the special geometry of the gear wheels. The precise and stable volume flow rate reduces the extruder’s pressure demand, delivering greater efficiency and extended service life, which are highly valued attributes in the recycling industry.
The Maag melt pump was developed one step further for this industry. The extrex6 recycling version looks just like the standard model. This recycling version, however, features special tooth clearances, larger toothing gaps, and new types of bearings protected to prevent large-sized particles penetrating into their lubrication gaps, all of which allows tolerance for particles up to 4 mm in size, depending on the pump rating. In such cases there is no need even for a protective filter upstream of the pump, as a study at a European customer demonstrated over a period of several months. The pump doubled its service life, even in post-consumer recycling – the pinnacle of plastics recycling, as it is where the highest contamination levels are found.
To remove impurities and contaminants from the recycling melt stream and so achieve the desired product quality, Maag supplies the ERF and ECO Series of continuous-running high-performance melt filters. Melt flows from the outside to the inside through a permanently rotating filter drum with a large number of conical holes. The impurities contained in the melt are held back on the filter surface where they are immediately removed by a stripper, fed to the discharge system and discharged from the process. A clean filter surface is therefore available at every rotation.
Uniform pellets as a finished product
During underwater pelletizing that combines a number of advantages and innovations in the current Pearlo model, the melt reaches the die plate via a process-controlled, hydraulically actuated start-up valve, where it is forced through die holes into the cutting chamber through which process water flows. The result is a highly uniform spherical pellet. In the downline dryer, 95 per cent of the water is already separated via a pre-dewatering system, with the remaining water then being removed by means of centrifugal separation. The residual heat of the pellets ensures thermal residual drying and thus results in very low residual moisture values. The process water is then filtered and its temperature regulated in the water treatment unit before being returned to the cutting chamber.
During strand pelletizing, the plastic melt is routed to the die head. Here polymer strands are extruded via the die plate and conveyed to the cutting rotor. The exact dwell time in the cooling water is determined according to the polymer, before cylindrical pellets of the highest quality are cut.
From floor covering to engine compartment
The systems supplied by Maag are suitable for small pilot lines right up to processes for large throughputs of 12 – 13 tonnes per hour. The latter often run around the clock and are switched off at most once a year for revision. It is therefore all the more important that the systems work very robustly and that the need for spare parts is minimized.
In the meantime, many industries are rethinking the use of recycling materials. For example, floor coverings are usually made of compressed wood fibers, but increasingly they are also being produced with recycled plastics. – By contrast with MDF floor coverings, plastic flooring can also be installed in damp rooms. Furthermore it is possible to recycle the laminate at the end of its life and return it to the material cycle.
Another example is the production of pallets. They have to be particularly impact-resistant and strong in everyday use in order to bear the heavy loads. Recycled materials now make a valuable contribution here, too. Pallets containing recycled material have the same properties as pallets produced from virgin material.
In addition to these applications, however, there are other compounding examples in which Maag components play a role. In automobile production, for example, a material mix of PE, PP and other components is used for the lining of the engine compartment. Domestic plastic waste from the yellow bag – sorted, washed and compounded – is often used in this process. The Maag systems have also been precisely adapted to this mix of materials, so that there are only minor differences to the virgin material when it comes to reuse.
One final example for which Maag supplied a line for compounding of virgin material: Unlike the recycling applications, this involves a polymer compound for the production of films. Here, the quality of the later film surface was particularly important. No specks were to be visible caused by gels or agglomerates as residues from the compounding process. The constant-pressure filtration of elastic particles from the virgin material, with enormous, uninterrupted service lives, in combination with a melt pump and underwater pelletizing, is the optimum solution for this customer.
Conclusion
The examples show that there is now nothing to prevent the use of recycled material even for high-quality products. However, compounding processes require a great deal of know-how about materials, surfaces and the interaction of the individual systems. Only with carefully matched components, such as melt conveying, filtration, pelletizing and drying, can high-quality pellets and micropellets be produced.
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