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PLA Recycling PLA

PLA Recycling PLA recycling with degassing The Institute of Plastic Processing (IKV) evaluates the recycling behaviour of PLA. Recycling helps cut raw material consumption and lowers material costs. Additionally, it improves the ecological balance. The different industrially practiced recycling strategies are analysed. A review is given about the processing by means of melt degassing. With the biggest production capacities of all bioplastics Polylactide (PLA) is a promising bio-plastic. However, although many raw material suppliers are starting production lines, the amount of commercially available PLA is still limited [1]. The end-of-life scenario for PLA has rarely been analysed, yet. Mechanical recycling is a reasonable option, but not well known in industry. The aim of this research project is the analysis of the material and process behaviour during mechanical recycling. This knowledge helps converters to improve their production. Production costs and raw material input are reduced. Four research institutes are analysing the recycling of internal PLA waste. Within the project the Flanders’ PlasticVision (Kortrijk, Belgium) analyses injection moulding while the Institute of Plastics Processing (IKV) focusses on the extrusion process. The chemical analysis of the recycled material and the development of biological chain extenders are done by the Fraunhofer Institute for Structural Durability and System Reliability (LBF), in Darmstadt, Germany. Celabor (Herve, Belgium) characterizes the physical properties of the recycled products and does a Life Cycle Analysis of the different recycling options. In the flat film extrusion process production waste arises mainly from the side cuts. In the thermoforming process punch scrap is produced. Both accounts for almost 40 % of the used raw material. The thermoplastic waste can be melted and reprocessed into a new product. But like every thermoplastic material PLA is exposed to degradation. The hydrolytical degradation is crucial for the processing of PLA. To achieve a sufficient quality certain production steps have to be followed during recycling, e.g. to avoid hydrolytical degradation PLA has to be dried [2]. The material handling of PLA is important. Figure 1 shows the moisture absorption of r-PLA under real storage conditions. After a very strong increase in the beginning the moisture reaches the saturation level at given humidity and temperature. If moisture is present during the plasticization, hydrolysis leads to a very fast degradation. This results in a decrease of the average chain length, which can be described by the molecular weight. A low molecular weight induces insufficient product properties, e.g. bad mechanical properties and low chemical resistance. Furthermore, the extrusion process is affected. A low molecular weight is followed by a low viscosity and an unstable process. In extreme cases, the process collapses. To prevent hydrolysis an expensive, time and energy consuming pre-drying step has to be conducted. An alternative is the processing by means of a degassing extruder. The degassing allows processing of moist material. By applying a vacuum the moisture is removed during the process and pre-drying is not necessary anymore. Previous analyses have shown that nearly 30 % energy can be saved by using a degassing extruder [3]. Extrusion experiments are done with the IKV equipment on a 60 mm single screw degassing extruder (L=38 D) and a calandar stack. The degassing zone can be closed. In that case the extruder operates as normal extruder. The extrusion line is equipped with a melt pump and a 900 mm flat film die. Additionally, a bypass-rheometer is implemented. Films produced from virgin PLA are used for recycling. A shred mill processes these films to flakes which are subsequently used as r-PLA. Figure 2 shows the melt viscosity depending on the shear rate measured with the bypass-rheometer. It correlates directly with the molecular weight and therefore with the quality of the film. Virgin PLA is processed with a moisture content of less than 250 ppm and without melt degassing. The artificially moistened r-PLA is processed with the specified moisture content. As shown in Figure 2 the melt viscosity drops with increasing PLA moisture. The higher the moisture the stronger is the hydrolysis. At 3200 ppm the viscosity drop is very high. As a result the melt stability is very low and the production of film is not possible anymore. The very strong viscosity drop at low shear rates is a result of the longer dwell times at low shear rates in the rheometer. This shows the very fast reactivity of the hydrolysis. By degassing the polymer melt its moisture is removed during processing. The hydrolytical degradation is lessened. At 750 ppm the viscosity is comparable to the viscosity of virgin PLA. The quality loss is marginal. The degassing of the r-PLA with moisture of 3200 ppm leads not to a sufficient viscosity. One reason is that the capacity of the degassing system is limited. Another reason is that the polymer has to be plasticized before the degassing takes place. Between plasticization and actual degassing, hydrolysis has already started to degrade the polymer chains. At high moisture rates the degradation is too heavy. Hence, the degassing effect is limited. 46 bioplastics MAGAZINE [03/13] Vol. 8

PLA Recycling by Ch. Hopmann S. Schippers Institute of Plastics Processing (IKV) at RWTH Aachen University Aachen, Germany The results of the viscosity are confirmed by measurements of the molecular weight of the produced film in Figure 3. The higher the moisture content, the lower is the molecular weight. This results from the hydrolysis. By using degassing the molecular weight loss can be reduced. The molecular weight of the 750 ppm r-PLA is nearly as high as the molecular weight of the film made from virgin PLA. Overall, the molecular weight is more stable compared to the viscosity. The trends are the same but the effect on the molecular weight is minor. Apart from the r-PLA with very high moisture content (3200 ppm) the achieved viscosity and the molecular weight are comparable to the values of virgin PLA. The molecular weight loss is little. Conclusion The material handling of PLA is important for the production process and for the later product quality. PLA shows a strong hygroscopic behaviour. To avoid the expensive pre-drying step the production using melt degassing is recommended. Hydrolysis of the PLA can be reduced as long as the moisture content does not exceed 2000 ppm. At a moisture content higher than 3200 ppm the process and product quality is affected even though degassing is used. In that case a pre-drying step has to be conducted or a degassing system with a higher capacity has to be implemented. Acknowledgment The research project 44EN of the Forschungsvereinigung Kunststoffverarbeitung has been sponsored as part of the Collective Research Networking (Cornet) by the German German Federal Ministry of Economics and Technology (BMWi) due to an enactment of the German Parliament through the AiF. We would like to extend our thanks to all organizations mentioned. [1] Auras, R.; Lim, L.T.; Selke, S.E.M.; Tsuji, H.: Poly Lactic Acid - Synthesis, Structures, Properties, Processing, and Application. Hoboken, New Jersey, USA: John Wiley & Sons Inc., 2010 [2] Brandrup, J.: Recycling and Recovery of Plastics. München, Wien: Carl Hanser Publishing, 1996 [3] Schmitz, T.: Verarbeitung von PET auf einem Einschneckenextruder mit Trichter-und Schmelzeentgasung. RWTH Aachen, Dissertation, 2005 PLA moisture [ppm] 4400 55 4000 50 3600 45 3200 40 2800 35 2400 30 2000 25 1600 20 1200 15 800 10 400 5 0 0 0 7 14 21 28 time [days] PLA moisture [ppm] enviromental humidity [%] temperature [°C] Figure 1. Moisture absorption under storage conditions 1000 viscosity [Pas] virgin PLA moisture degassing 750 ppm no 750 ppm yes 3200 ppm no 3200 ppm yes 100 1 10 100 1000 shear rate [s -1 ] Figure 2. Melt viscosity with and without degassing weight average molecular weight [kg/mol] 250 200 150 100 50 0 Moisture [ppm] virgin 750 750 3200 3200 degassing no yes no yes Figure 3. Molecular weight with and without degassing enviromental humidity / temperature bioplastics MAGAZINE [03/13] Vol. 8 47

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