PROCESSING | REACTIVE COMPOUNDING
conscientious pre-drying and subsequent stabilisa- tion. However, there may be an advantage in temporarily lowering the molecular weight of the material during the recycling process. One example is large scale process filtration in PET recycling. This is carried out through the addition of ethylene glycol, which results in a reversible chain breakage and reduced molecular weight during filtration. This reaction – glycolysis of PET with the aid of ethylene glycol – has been investigated within the framework of a European-funded joint project. It has been found that the molecular weight of the starting material can be reduced to about 1% of the initial value in an economically viable, repro- ducible, continuous reactive compounding process through the use of a small percentage of ethylene glycol. The researchers believe that similar achieve- ments could be demonstrated with other polyester- based secondary raw materials, such as polyure- thanes and polylactides. Further reactive compounding research at
Fraunhofer ICT has involved a comparison of different extruder types, identifying the key process characteristics in a co-rotating twin-screw extruder compared with those of a multi-shaft extruder. Results have shown that the smallest molecular masses in absolute numbers could be generated using the twin-screw extruder. However, the multi-shaft extruder provided broader scope to influence the reaction by varying the process parameters. Figure 1 shows the molecular weights obtained after glycolysis in a twin-screw extruder (TSE) and in the multi-shaft extruder (MSE) at various machine-side process settings. While the molecular weights obtained are almost constant after glycolysis in the twin-screw extruder at settings 1 and 2, the molecular weight of the multi-shaft extruder results in different molecular weights. The same result can be seen to be achieved with process settings 3 and 4. Bergman says he sees a trend in this sector
towards the combination of processes to save energy. This has been done in a project focused on
Figure 2: System layout for a single-stage reactive
compounding process for production of a thermoplastic starch blend
Source: Fraunhofer ICT 34 COMPOUNDING WORLD | September 2017
www.compoundingworld.com
Figure 1: PET molecular weights after glycolysis in twin screw (TSE) and multi-screw (MSE) extruders at four different process settings Source: Fraunhofer ICT
production of thermoplastic starch. Starch is the most frequently found macromolecule in nature and can be obtained from waste streams. “As a result, starch is not only an interesting starting material in economic terms, it also has great potential in the sense of sustainable development and the utilisation of renewable raw materials,” he says. “However, as starch is not thermoplastic in its
natural state, it is not processable in pure form with classical compounding methods. With the incorpo- ration of low molecular weight plasticisers into the material, this thermoplastic property can be produced, which is one of the strategies used to produce ‘thermoplastic starch’,” Bergman says. “Unfortunately, the material properties of starch and thermoplastic starch are not suitable for most applications, which means for practical use the starch materials have to be blended with other polymers. In addition to the material characteristics of the blend partner, the compatibility of the polymers used is decisive for the resulting material properties. The question of compatibilisation is of particular impor- tance for starch-based blends,” he says. Fraunhofer ICT says that its strategy is aimed not
at modifying the starch, but at the blend partner. First, researchers produced an optimised, thermo-
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