People Report Recycling
People Report Recycling of PLA for By: Christian Hopmann Sebastian Schippers Institute of Plastics Processing (IKV) at RWTH Aachen University Aachen, Germany Fig. 3: Fractions of multiple recycled material during continuous production with constant recycling rates 21% 25% 70% 11% 2% 1% 6% Recycling of 30% waste 2% 2% 5% Recycling of 45% waste Virgin PLA 1 2 3 4 5+ 55% In cooperation with two Belgian Institutes (Celabor of Herve, and Flanders’ PlasticVision Kortrijk) and the Fraunhofer Institute for Structural Durability and System Reliability (LBF) in Darmstadt, Germany, the Institute of Plastic Processing (IKV), Aachen, analyses the recycling of PLA in flat film extrusion. The focus is on the evaluation of relevant packaging properties such as permeability and mechanical properties as well as the chemical structure (molecular weight) during various recycling routes. Preparation of internal PLA waste by means of crystallisation and drying is also included in the scope of the research. In this article the recycling with varying percentages of recycled PET, and the multiple recycling of films, is reviewed. Mechanical as well as chemical properties are evaluated. The extrusion trials are carried out on a 60 mm single screw extruder (L=38 D) and a calender stack. The extrusion line is equipped with a melt pump and a 400 mm flat film die. Additionally, a bypass-rheometer is included. Films produced from virgin PLA (Ingeo 2003D, Nature Works) are used for recycling. A shredder processes these films to flakes which are subsequently used as r-PLA (recycled PLA). The twin screw extruder ZSK26MC (Coperion) is combined with a water quenching system and a strand pelletizer unit. It is used to convert the flakes back to granules. The typical recycling route of converters includes shredding, crystallising, drying, re-granulation and reprocessing of the recycled waste to a new product. To prevent a high level of down-cycling typically the waste is not reprocessed at 100 %. Instead it is reprocessed with a defined recycled content. Depending on the packaging application, the recycling content may be up to 50 % of internal production waste. The molecular weight of different processing steps measured by GPC (Gel permeation chromatography) is shown in Fig. 1. The molecular weights shown are in a narrow range. To be able to detect any effect of a narrow variation of the molecular weight it is necessary to measure a test series with different films in the same GPC series, which is done here. In general, the molecular weight loss is low and due to deviations in the GPC measurements small effects are hardly significant. Virgin PLA loses molecular weight when processed to film. The molecular weight loss is in the range of 9 %. The molecular loss occurs due to the thermal and thermooxidative degradation which is inevitable during extrusion, especially for polycondensates like PLA. The film made of virgin PLA is milled into flakes and processed to granules by means of a twin screw extruder. Granules are advantageous since the material transport in pipes and dosing units, for example, is easier. Additionally, the process behaviour in the extruder during plastification is better. The melting process is more homogeneous than the melting of flakes since the geometry of the granules is similar to the geometry of the virgin PLA granules. During granulation in the twin screw extruder a marginal molecular weight loss is measured, which is a result of the additional processing step. During the subsequent reprocessing to new films with varying percentages of recycled PLA the molecular weight loss is low again. The difference between recycling quotas up to 45 % and film made from virgin PLA is very small. It accounts for less than 3 % during the recycling of 45 % r-PLA. The effect upon 22 bioplastics MAGAZINE [01/14] Vol. 9
Report packaging applications the film properties is negligible. Only the processing of 100 % r-PLA leads to a higher molecular weight loss in comparison with the molecular weight of the film made by virgin PLA. A loss of approx. 5 % occurs. The same effect can be seen for the mechanical properties. Fig. 2 shows the Young’s modulus in a transverse direction (TD) for film with varying recycling content. The drop in the Young´s modulus accounts for nearly 8 %. The chain scission, which is due to the down-cycling of the PLA, leads to a lower tensile strength and a lower ductility. As a result the Young´s modulus decreases. During continuous recycling a decreasing amount of material runs multiple times through the extrusion equipment. To evaluate this influence on the PLA it is recycled to 100 % for a multiple of times. This leads to a higher degradation which enables the easier detection of the effects. The fraction of multiple recycled material decreases exponentially with increasing recycling steps. Fig 3. shows, as examples, the fractions for a continuous recycling rate of 30 % and 45 %. Even for a recycling rate of 45 % the fraction of material which is recycled 3 times and more is often very low. Fig. 4 summarises the molecular weight and the viscosity (measured by the bypass rheometer) of multiple processed films. After each extrusion step the film is shredded to flakes and dried to below 250 ppm as recommended by the PLA supplier. Following this it is reprocessed to 100 %. The reprocessing in this way is repeated 6 times. A continuous decrease in both values can be seen. The increase in step 5 is the result of the blending of two different batches. R-PLA which is processed in two different trial series is mixed here. Both batches have been processed four times prior to mixing. The blending is necessary since a high amount of r-PLA is needed for the trials, which cannot be prepared in one test series. Due to start-up waste the amount of r-PLA is reduced at every step and the volume of the drying equipment is limited. The increase is visible in the molecular weight as well as in the viscosity, and other properties. The overall loss of the molecular weight over 7 extrusion steps accounts for 17 % percent. A loss of 17 % is relatively little, especially when taking into account that less than 2 % of 5 times recycled PLA will be in a product which is continuously produced with 45 % r-PLA. The Young’s Modulus in machine (MD) as well as in transverse (TD) direction and the results of the dart drop tests of PLA processed 1, 3 and 7 times are shown in Fig. 5. Weight average molecular weight [kg/mol] Weight average molecular weight [kg/mol] 220 210 200 190 180 Virgin PLA Film made of virgin PLA Granules 10% r-PLA 240 2200 230 2000 220 1800 210 1600 200 1400 190 1200 180 Virgin PLA 1 2 3 4 5 6 7 1000 Number of extrusion steps [-] Molecular weight | Average viscosity 30% r-PLA Fig. 1: Molecular weight at different process stages during recycling Young‘s modulus TD [MPa] 1850 1800 1750 1700 1650 1600 1550 1500 Film made of virgin PLA 45% r-PLA 100% r-PLA 10% r-PLA 45% r-PLA 100% r-PLA Fig. 2: Young’s modulus in transverse direction of film with varying recycling quotas Fig. 4: Molecular weight and viscosity of multipleprocessed PLA films Viscosity [Pas] bioplastics MAGAZINE [01/14] Vol. 9 23
