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04 | 2010

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Basics Fig. 3: Turnkey

Basics Fig. 3: Turnkey plant for biodegradable polymer compounding based on a ZSK MEGAcompounder PLUS delivered by Coperion. Photo: Cabopol, S. A., Porto de Mós, Portugal References This article was prepared using the following literature and contributions from industry experts: [1] http://en.wikipedia.org/wiki/ Plastic_compounding [2] Kuehnen, U., Process Engineer for Extrusion of Biodegradable Plastics, Coperion, Germany. [3] Bonten C., FKuR Kunststoff, Germany [4] Darnedde, L., Development Department, KraussMaffei Berstorff, Germany [5] Martin, C. American Leistritz, USA [6] Steinbrecher, F.; Reifenhäuser, Germany [7] Coperion brochure 7430_CC_CE_092008_engl.pdf Compounding however, is not a single simple process. It is rather a large field of processes, according to the range of polymers, formulations, and applications. Steps that are usually carried out in compounding processes for bioplastics are: Plastification of starch, melting of the polymer, mixing with additives, fine dispersion and homogenous distribution of different phases, venting and degassing, pressure build-up, extruding through dies (with or without screen changer, start-up valve, melt pump), cooling of the melt and finally pelletizing [2]. In any case, there is a lot of know-how and experience needed to process bioplastics in order to enhance the properties and not destroy the biopolymers or additives. Most bioplastics for example require a smooth temperature profile, therefore a special arrangement of mixing elements is needed. This, in most cases, is the proprietary kow-how of the compounders [3]. The same applies to the temperature control. Additives themselves also need to resist temperatures, and need to be perfectly dispersed. If this cannot be realized, hardly any effect will be observed [3]. However, there are significant differences in the necessary process parameters (including extruder and screw set-up) for different bioplastics (like PLA, PHA, TPS, PBS etc…). Every bioplastic has a different window for processing, for instance the different melting points or stability have to be considered. But it is possible to handle all of this with a flexible modular co-rotating twin screw extruder [4]. At the end of a compounding process, the ‘compound‘ - still in a molten stage - has to be cooled and pelletized. For this final process step the melt is extruded through small orifices, cut, and thereby converted into pellet form. The product can be cut and transported either in air or in water. Some biodegradable materials, especially materials rich in starch and polyvinyl alcohol, are sensitive to water, soaking up, getting sticky, being dissolved, therefore any contact has to be avoided. Others are strand pelletized at low throughputs, or cut in an underwater pelletizer, in which the pellet cooling is also achieved under water at larger throughputs.[2] The pellets are then filled in sacks, big-bags or octabins for transporting to their final conversion into bioplastics products. As an alternative it is possible to eliminate the pelletizing step and process the molten compound by direct-extrusion into final products such as film or profiles [6]. Fig. 3 shows as an example a complete compounding line. The upper level of the steel framework is used for raw materials storage, the mezzanine level contains the dosing equipment and the twin screw extruder with its downstream equipment is on the ground level. This line is installed at Cabopol, S.A., Portode-Mós, Portugal [2]. 36 bioplastics MAGAZINE [04/10] Vol. 5

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