Strategies to improve the mechanical properties of high-density polylactic acid foams

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1 Original Article Strategies to improve the mechanical properties of high-density polylactic acid foams Journal of Cellular Plastics 2016, Vol. 52(1) ß The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / X cel.sagepub.com Bernd Geissler 1, Michael Feuchter 2, Stephan Laske 3, Michael Fasching 1, Clemens Holzer 3 and Günter R Langecker 3 Abstract In this study, different strategies to improve the mechanical properties of physically foamed high-density polylactic acid sheets were examined to produce polylactic acid foam sheets with tailor-made mechanical properties. The first part was the determination of the effect of different blowing agents (CO 2 and N 2 ) on the foam morphology. The second part of the study was the modification of the formulation. For this purpose, both a linear and a branching chain extender and a thermoplastic elastomer were used to improve the elongational properties (tensile modulus and strain at break) of the polylactic acid foam sheets. Additionally, the effect of the addition of cellulose fibers on the foam morphology and the mechanical properties was investigated. All experiments were carried out on a laboratory flat-film line. This extrusion line consists of a 30-mm single-screw extruder attached with a 250-mm flat sheet die. The results show a strong influence of the material formulation on the mechanical properties of the high-density foam sheets. Both the mechanical properties and foam morphology could be improved by the right material formulation. The addition of the thermoplastic elastomer leads to a better foam morphology and also to a reduced brittleness of the foam sheets. Furthermore, it could be demonstrated that cellulose fiber can be used as a nucleating agent for polylactic acid but causes a further decrease in the strain at break. 1 Polymer Competence Center, Leoben GmbH, Austria 2 Chair of Material Science and Testing of Plastics, Department of Polymer Engineering and Science, Montanuniversitaet, Leoben, Austria 3 Chair of Polymer Processing, Department of Polymer Engineering and Science, Montanuniversitaet, Austria Corresponding author: Bernd Geissler, Polymer Competence Center, Leoben GmbH, Roseggerstrasse 12, Leoben, 8700 Austria. bernd.geissler@pccl.at

2 16 Journal of Cellular Plastics 52(1) Keywords PLA, mechanical properties, flat-film extrusion, chain extender, blowing agent, thermoplastic elastomer Introduction The increasing crude oil prize and the dramatical rise of the plastic waste problem in the oceans cause an increase in the demand for new, biobased, or biodegradable solutions for the plastics packaging industry. Polylactic acid (PLA) is an interesting option to replace crude oil-based polymers in packaging applications. 1 4 Nowadays, PLA fulfills most of the requirements from the packaging industry for rigid packaging applications. 1 Nevertheless, for other applications like hot food packaging and properties like thermal stability and high brittleness of the PLA must be improved. 1 In the past, several strategies for the modification of PLA were developed, including reactive extrusion, 5 7 blending with a rubber or a thermoplastic elastomer (TPE), 8 10 and the use of fillers Typically, high-density foam sheets are manufactured with chemical-blowing agents caused by the high-acquisition costs of the equipment for physical foaming and the complexity of the physical-foaming process. 2 Though, chemical foaming of PLA is more difficult compared to other polymers as most of the standard and efficiently commercially available endothermic chemical-blowing agent masterbatches (polyolefin carrier, between 20 and 50 wt% active ingredients, which consist of sodium bicarbonate and citric acid) compose to carbon dioxide and water. 15 The formed water leads to a degradation of the PLA caused by hydrolysis and as a consequence to reduced mechanical properties. Therefore, typically, citric acid without sodium bicarbonate is used as a chemical-blowing agent for PLA but such chemical-blowing agent masterbatches are less efficient in terms of the amount of formed decomposition gases compared to the sodium bicarbonate and citric acid formulations. 15 Therefore, physical foaming is an interesting alternative to chemical foaming for high-density PLA foams. Reactive extrusion of PLA is typically used to improve the melt strength and melt elasticity for low-density foam applications 16 or the thermal stability. 17 Baltazar-y-Jimenez and Sain 18 investigated the effect of chain extenders on the mechanical properties of a PLA grade. They found out that it is possible to increase the tensile strength and modulus of injection-molded samples by using a chain extender. Similar studies were done by Pilla et al. 19 They found out that the addition of the chain extender leads to a significant decrease in mean-cell size and an increase in cell density of injection-molded PLA samples. However, they achieved dissimilar results for the specific mechanical properties compared to Baltazar-y- Jimenez and Sain 18 In their study, the tensile properties such as specific toughness, elongation at break, and specific tensile strength increased, yet the specific modulus decreased due to the addition of the chain extender.

3 Geissler et al. 17 Ganster et al. 20 showed that it is possible to increase the elongation at break of injection molded PLA by the addition of cellulose fiber. Similar results have been published by Medina and coworkers 21 for cellulose fiber-reinforced polypropylene. Kuboki et al. 22 reported that the further addition of a compatibilizer to a HDPE/ cellulose fiber compound leads to a dramatical improvement of the notched Izod impact strength. Though, the addition of the compatibilizer results in an increase in the average cell size and widened the cell-size distribution. Anyhow, hardly, any information exists about the influence of the different modifications on the foaming behavior of PLA, especially on the manufacturing of high-density foam sheets with flat-sheet extrusion line. Therefore, this study investigates the influence of chain extenders, a TPE, and the type of blowing agent on the foaming behavior of PLA. Materials and methods Materials In this study, a PLA grade from NatureWorks Õ LLC (USA) was used as a matrix polymer. This polymer was used due to the easy processing on conventional flatfilm extrusion lines and the good mechanical properties. The physical properties of the PLA are listed in Table 1. Nitrogen (N 2, 99% pure, Linde Gas (Austria)) and carbon dioxide (CO 2, 99% pure, Linde Gas (Austria)) were applied as blowing agent. Based on previous works, a talcum grade from Mondo Minerals GmbH (Germany) was used as nucleating agent. 23 The properties are summarized in Table 2. Two types of chain extender were used to modify the PLA for the investigation of the effect of molar mass on the mechanical properties of the high-density foam sheets. Both grades were supplied by Transmare Compounding (Netherlands) as masterbatches and are based on epoxy-functionalized polymeric chain extender Joncryl Õ (BASF SE), differing in the mode of chain extension. The first of these masterbatches is a linear chain extender (MB-CL), and the second is a side chain branching chain extender (MB-CB). Table 1. Physical properties of the investigated PLA. PLA MFR (210 C/2.16 kg) 6 g/10 min Density 1.24 g/cm 3 Tensile strain at break 6 % Tensile strength at break 53 MPa

4 18 Journal of Cellular Plastics 52(1) Table 2. Properties of the nucleating agent. d 50 in mm d 98 in mm Surface coating Talcum no Table 3. Physical properties of the TPE. TPE Density 0.9 g/cm 3 Tensile strain at break 930% TPE: thermoplastic elastomer. Table 4. Characteristics of the applied cellulose fiber. Tencel 300/10 Average fiber length 300 mm Diameter fiber 10 mm Elongation at break 10.6% Young s modulus 18.4 GPa A TPE based on a styrene butadiene block copolymer was delivered by Polyone Th. Bergmann (Germany) for the improvement of the toughness of the foamed sheets. The properties are summarized in Table 3. Furthermore, the effect of a biodegradable filler on the foaming behavior of the PLA was evaluated in terms of cell morphology and the mechanical properties of the high-density foam sheets. Therefore, a cellulose fiber-grade Tencel 300/10 from Lenzing AG (Austria) was used. The characteristics of the filler are listed in Table 4. Material preparation The preparation of the compounds was carried out on a co-rotating twin-screw extruder (ZSK 25, Werner & Pfleiderer, Germany) with a screw diameter of 25 mm and a screw length of 40 D. In the first step, two masterbatches were produced with a screw speed of 150 rpm at a fixed temperature of 180 C. Masterbatch one consists of 40 wt% talcum and 60 wt% PLA and masterbatch two consists of 10 wt%

5 Geissler et al. 19 Table 5. Formulation compounds in wt%. No. PLA TPE Talcum Tencel 300/10 MB-CL MB-CB C C C C C C C C C C C C C C C PLA: polylactic acid; TPE: thermoplastic elastomer. Tencel 300/10 and 90 wt% PLA. In the second step, the masterbatches were compounded with the other ingredients (linear chain extender, branching chain extender, TPE, and additional PLA) to produce the different formulations, which are listed in Table 5. Therefore, the different ingredients of the formulation were weighed carefully and dry blended. The dry blends were used to produce the different compounds in a premix process with the twin-screw extruder at a screw speed of 150 rpm and a fixed temperature of 180 C. Prior to the processing, all components were dried at least for 4 h at 80 C to avoid any degradation caused by hydrolysis. Experimental Foaming experiments The foaming experiments were carried out on a single-screw laboratoryextruder (Dr. Collin, Germany) with a screw diameter of 30 mm and a screw length of 30 D. Additionally, a static mixer was used to increase the residence time for the formation of the single phase and the improvement of the melt homogeneity. A slit die was used for the manufacturing of the sheets. Thereby, the slit die had a width of 250 mm with the gap at the exit being set to approximately 200 mm.

6 20 Journal of Cellular Plastics 52(1) Two different pumps were used for the injection of the blowing agent. Supercritical carbon dioxide was injected by a positive displacement pump (Syringe pump 260D, Teledyne Isco, USA) at a fixed amount of approximately 0.1 wt% carbon dioxide for each formulation. Nitrogen was injected by a semi industrial gas injection device (DSD 500, Linde Gas, Germany) at a fixed amount of 0.08 wt%. A roll stack with three rolls was used for the calibration and cooling of the melt film. The temperature of the rolls was set to 30 C. The haul-off speed was kept constant for all formulations. The axial-temperature profile of the extruder was adjusted to achieve a melt temperature of 190 C in front of the die. The maximum temperature reached 200 C at the location of the gas-injection point. All experiments were executed with a screw speed of 15 rpm caused by the limitations in the maximum pressure of the extruder (300 bar at the tip of the screw). All experiments were started without any injection of blowing agent to completely remove the previous material of the extruder. After reaching a steady state, which was typically reached after 15 min of flushing with the new material, samples were collected for the rheological tests. Then the volume flow rate was determined, and the blowing agent injection was started at the desired mass flow rate. Sufficient time was given to stabilize the process, and if necessary, the flow rate of the blowing agent was adjusted. Samples (3 m of the sheets) were taken, after no further change was noticeable. The whole process was repeated three times for every setting. Again, all compounds were dried for at least 3 h at 80 C. Rheological characterization The rheological properties of the chain-extended samples were measured with a rotational rheometer (Anton Paar, Physica MCR 501, Germany) with cone-plate geometry at 190 C. The cone had an angle of 5 and a diameter of 25 mm. For the characterization of the dynamic rheological properties, creep and recovery measurements were done to characterize the material s elasticity and the zero viscosity. The creep-recovery test consists of a 300-s creep phase with a constant load of 200 Pa followed by a 700-s recovery phase. For the calculation of the equilibrium compliance and the zero viscosity, the software Anton Paar Rheoplus V3.61 was used. Foam morphology characterization The foam morphology was characterized with an infinite focus microscope (Alicona IFM, Austria). Thereby, the foam sheets were investigated with a transmitted light microscopic technique to get information about cell length in machine direction and perpendicular to the machine direction. Additionally, the high-density foam sheets were fractured in a brittle manner to investigate the cross section of the sheets. The determination of the mean-cell diameter (dm in mm) was carried out with the Software ImageJ (open source software).

7 Geissler et al. 21 Mechanical characterization Room temperature uniaxial tensile properties were evaluated for the different foam sheet samples using a Universal Tensile Testing Machine Z010 (Zwick Ltd and Co. KG, Germany). The tensile tests were carried out according to ISO The tests were repeated six times for each formulation with a test velocity of 1 mm/min for all measurement. The tests were done at standardized conditions of 23 C and 50% r.h. The analysis of the measurements was carried out with the software TestXpert II (Zwick Ltd and Co. KG, Germany). Results Influence of the type of blowing agent The first part of this study was to evaluate which blowing agent leads to the better foam morphology in terms of cell size and density with the used setting (flat film die, blowing agent content, temperature, and mass flow rate). It is known from the literature that in terms of cell density, the combination of nitrogen and talcum is less sensitive to the pressure drop rate than carbon dioxide and talcum. 24 Nevertheless, the solubility of nitrogen in PLA is much lower than the solubility of carbon dioxide in PLA. 25 Therefore, the phase separation between PLA and nitrogen occurs at an earlier point inside the die compared to PLA and carbon dioxide at similar blowing agent loadings. Additionally, the point of phase separation is influenced by die geometry especially due the manifold and die island geometry. With 5 wt% nucleating agent content, nitrogen leads to smaller cell sizes compared to carbon dioxide (see Figure 1). With increasing nucleating agent content, however, nitrogen causes much bigger cell sizes and a rougher surface of the foam sheets. Two pictures of different sheet surfaces are shown in Figure 2. While picture (a) shows a smooth, closed-cell surface, picture (b) shows a collapsed foam morphology with holes in the surface of the foam sheets. This behavior is related to the increase in the degassing pressure, which is caused by the flow conditions and the nucleating agent (content, type, and particle size). The increase in the degassing pressure leads to an earlier phase separation inside the die, and as a result, the bubbles start to grow in the die. It is important to mention that the point of phase separation along the width of the die was distributed similar to the coat hanger geometry. This means that the foam sheets had a rough surface in the middle of the sheets with most of the cells being collapsed while next to the edges the surface of the sheets was smooth, and the cells were not collapsed. Due to these results, carbon dioxide was chosen as blowing agent for all other experiments, as the foam morphology of the carbon dioxide blown sheets was pretty much uniform over the whole width for both nucleating agent contents.

8 22 Journal of Cellular Plastics 52(1) Figure 1. Mean cell size as a function of the blowing agent type and nucleating agent content. Figure 2. Microscope picture of the surface of sample (a) 10 wt% talcum þ 0.1 CO 2 and (b) 10 wt% talcum þ 0.08 wt% N2. Influence of different chain extenders on the mechanical properties of the high-density foam sheets The second part of this study was to investigate the effect of two different types of chain extenders on the mechanical properties and the foam morphology. These results have been already presented at the PPS In foam extrusion, chain extenders are typically used to increase the melt strength of PLA to avoid cell coalescence for low-density foams. 27 However, hardly any information about the effect of chain extender on the mechanical properties of high-density PLA foams is

9 Geissler et al. 23 Figure 3. Time-dependent creep compliance during creep and recovery of the chain extended and neat PLA. 26 Table 6. Zero viscosity and equilibrium compliance for PLA and reactive-modified compounds. Formulation Zero viscosity in Pa s Equilibrium compliance in 1/Pa C C C C C PLA: polylactic acid. available. Therefore, the two chain extenders were used in different concentrations to realize different molar masses. The characterization of the effect of the chain extenders on the PLA was done by rheological experiments, which are shown Figure 3. The creep and recovery experiments show an increase in the zero viscosity for both types of chain extenders with a higher increase for the branching one. Furthermore, the zero viscosity rises with increasing content of both chain extenders. Both chain extenders also cause an increase in the equilibrium compliance (shown in Table 6). The equilibrium compliance is a rheological parameter for the melt elasticity. The melt elasticity plays an important role in the cell nucleation and foam growth. 28 Therefore, it is expected that the compound C-5 causes the best foam morphology. However, the addition of the chain extender causes no significant positive effect on the foam morphology. To the contrary, the linear chain extender even leads to an increase in the mean-cell size, whereas the chain-branching chain extender leads to similar results as the neat PLA in terms of cell size and cell density. These results are shown in Figure 4.

10 24 Journal of Cellular Plastics 52(1) Figure 4. Mean cell size of the chain extended and neat PLA high-density foam sheets. 26 Figure 5. Density of the chain extended and neat PLA high-density foam sheets. 26 Furthermore, the modification of the material formulation has hardly any influence on the density of the foam sheets (Figure 5). All these formulations lead to an average density reduction of about 20% compared to the not-foamed material formulation. The tensile tests reveal that the linear and branching chain extender had a different effect on the mechanical properties of the PLA. The branching chain extender causes a significant increase of the Young s modulus, whereas the linear chain extender leads to a decrease of the Young s modulus. This is shown in Figure 6. However, both chain extender types cause an increase of the Young s modulus with increasing content. Figure 7 shows the influence of the chain extender on the strain at break. It is obvious that both chain extender types lead to a significant reduction of the strain at break, which means that the high-density foam sheets are even more brittle than

11 Geissler et al. 25 Figure 6. Young s modulus of the chain extended and neat PLA high-density foam sheets. 26 Figure 7. Strain at break of the chain extended and neat PLA high-density foam sheets. 26 without the rheological modification. Similar results compared to Young s modulus were found for the tensile strength (Figure 8). Again, the chain-branching chain extender leads to a slightly higher strain at break compared to the PLA. The linearchain extender causes a decrease of the strain at break. NevIt is not clear why the linear-chain extender provokes such a significant decrease of the mechanical properties, as the rheological measurements on the extruded samples show a significant increase in the zero viscosity of the PLA due to the addition of the linear-chain extender (Figure 3 and Table 4). The addition of both chain extenders leads to an increase in the surface roughness of the high-density foam sheets. It is assumed that the increase in the melt elasticity causes an earlier phase separation inside the die.

12 26 Journal of Cellular Plastics 52(1) Figure 8. Tensile strength at break of chain extended and neat PLA high-density foam sheets. 26 The increase in the surface roughness leads to the decrease in the mechanical properties of the foamed sheets. However, the branching chain extender leads to better results compared to the linear one due the additional side chains. The addition of the branching chain extender causes an increase of the stiffness of the high-density foam sheets. Therefore, additional strategies were applied to decrease the brittleness of the foamed sheets. Influence of the addition of the TPE on the mechanical and foam properties Figure 9 illustrates the effect on the strain at break of the PLA foam sheets for two different nucleating agent concentrations (5 and 10 wt% Talcum). It is obvious that the addition of the TPE leads to a significant increase in the strain at break for both nucleating agent concentrations where even low concentrations of the TPE cause a significant increase of the strain at break. It is important to note that the positive effect of the TPE is reduced by the increasing nucleating agent content. Though, the addition of the TPE results in a reduction of the foam-sheet stiffness, which is shown in Figure 10. The high value for the young s modulus of the compound C-7 is explained by the higher density (Figure 12) compared to the other formulations. The increase in the Young s modulus, with increasing TPE content, can be explained by the reduction of the mean-cell size, which is a result of the addition of the TPE (Figure 14). It is assumed that the TPE acts as an additional nucleating agent due to the additional interphases between PLA and TPE. Overall, the increase in the talcum content results in an increase of the stiffness of the sheets. This is contrary to the effect of the TPE on the Young s modulus. Similar results were found for the tensile strength at break as presented in Figure 11. However, the tensile strength at break showed the highest reduction with the addition of the TPE (Figure 14). The addition of the TPE causes a

13 Geissler et al. 27 Figure 9. Strain at break of PLA/TPE compounds and neat PLA high-density foam sheets. Figure 10. Young s modulus of PLA/TPE compounds and neat PLA high-density foam sheets. significant decrease of the tensile strength compared to PLA/talcum compounds. Again, the increase in the talcum content leads to slight increase in the tensile strength. It is important to note that the addition of the TPE results in slightly higher density reduction compared to PLA/talcum compounds, which is explained by the lower density of the TPE and the additional nucleating sites (Figure 12).

14 28 Journal of Cellular Plastics 52(1) Figure 11. Tensile strength at break of PLA/TPE compounds and neat PLA high-density foam sheets. Figure 12. Density of PLA/TPE compounds and the neat PLA high-density foam sheets.

15 Geissler et al. 29 Figure 13. Fracture work in tensile test of PLA/TPE compounds and neat PLA high-density foam sheets. Figure 13 shows the fracture work of the tensile test for the compounds with 10 wt% talcum. It is obvious that the addition of TPE leads to a less brittle behavior and therefore covers the opportunity to increase the application ranges of the investigated PLA. Furthermore, the addition of the TPE results in an improved foam morphology (Figure 14). This offers the opportunity to reduce the talcum content to further reduce the brittleness of the foam sheets. However, it is important to mention that the used TPE is not biodegradable. Influence of the addition of the cellulose fiber on the mechanical and foam properties The last point of this study was to evaluate the possibility using cellulose fiber as a nucleating agent for the manufacturing of PLA foam sheets and to investigate the effect of the cellulose fiber on the mechanical properties of the foam sheets. The cellulose fiber was dried before the manufacturing of the masterbatch for 4 h at 80 C in a vacuum drier due to the high-moisture content. Figure 15 illustrates the influence of the cellulose fiber on the foam morphology in comparison with talcum as nucleating agent. The results indicate that the cellulose fiber can be used as a nucleating agent to improve the cell morphology. Even low content of the cellulose fiber results in smaller cell sizes than

16 30 Journal of Cellular Plastics 52(1) Figure 14. Mean cell size of PLA/TPE compounds and neat PLA high-density foam sheets. Figure 15. Mean cell size of cellulose fiber compounds and neat PLA high-density foam sheets. achieved with 5 wt% talcum. With increasing cellulose fiber content, the nucleating performance is rising. However, the cell morphology is slightly poorer with the highest cellulose fiber content than with 10 wt% talcum. The good nucleating performance for high-density foam applications is in good agreement with Geissler et al. 29

17 Geissler et al. 31 Figure 16. Tensile strength at break of cellulose fiber compounds and neat PLA high-density foam sheets. Figure 17. Young s modulus of cellulose fiber compounds and neat PLA high-density foam sheets. Figure 16 shows the influence of the cellulose fiber on the tensile strength at break. It is obvious that the cellulose fiber causes a decrease in the tensile strength compared to the PLA/talcum compounds. Though, the reduction is much lower than the one observed with TPE. It is assumed that the cellulose fiber and the residual moisture did not induce a hydrolytical degradation.

18 32 Journal of Cellular Plastics 52(1) Figure 18. Strain at break of PLA/TPE compounds and neat PLA high-density foam sheets. Similar results were found for the Young s modulus and are shown in Figure 17. The higher Young s modulus of compound C-15 is explained by the slightly smaller cell sizes and the higher fiber content. Figure 18 illustrates the strain at break of the cellulose fiber compounds. The addition of the cellulose fiber leads to a decrease of the strain at break. The reduction of the strain at break is hereby no function of the cellulose fiber content. Therefore, a bad dispersion of the cellulose fiber is expected. However, it was not possible to clarify this with microscope picture. For that reason, additional work has to be done to understand the behavior of the cellulose fiber and if possible to improve the mechanical properties of cellulose fiber-filled PLA foam sheets. Summary and conclusions In this study, the manufacturing of PLA high-density foam sheets on a laboratory flat-film extrusion line was investigated. It was shown that nitrogen leads to smaller cell sizes compared to supercritical carbon dioxide for lower nucleating agent contents. However, if the nucleating agent and the amount of the filler were increased, the quality of the foam sheets got worse with nitrogen. This is explained by the higher degassing pressure of nitrogen compared to carbon dioxide. The higher degassing pressure causes an earlier phase separation inside the die. As a consequence, all other experiments were done with supercritical carbon dioxide. The addition of the two chain extenders led to completely different mechanical properties of the foam sheets. Though, both types of chain extenders led to a

19 Geissler et al. 33 significant increase in the zero viscosity but had no significant improving effect on the foam morphology. To the contrary, the linear chain extender led to an increase in the cell size. Similar to the foam morphology, the branching chain extender led to a slight increase in the stiffness of the PLA foam sheets, whereas the linear chain extender caused a decrease. Both chain extenders resulted in a decrease of the strain at break. Furthermore, it could be demonstrated that a TPE can be used to decrease the brittleness of the foam sheets. However, the addition of the TPE resulted in a reduction of the tensile strength at break. Additionally, the TPE acted as a nucleating agent, which could be used for the reduction of the talcum content. On the other hand, it is important to note that a high-talcum content is necessary to achieve good quality foam sheets with the used setting (die, blowing agent content, and processing conditions). Though, high-talcum content led to higher brittleness of the sheets. Finally, it was shown that cellulose fiber can be used as a nucleating agent for PLA, but additional work needs to be done to improve the mechanical properties of the resulting foam sheets and to reduce the effort for the drying before the manufacturing of the cellulose fiber masterbatches to avoid hydrolysis. The cellulose fiber showed a good nucleating behavior and is biodegradable. Therefore, foamed PLA cellulose fiber compounds are an interesting option for totally compostable, light, and green packaging materials. Acknowledgements The research work of this paper was performed at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within the framework of the COMET-program of the Federal Ministry for Transport, Innovation and Technology and the Federal Ministry of Economy, Family and Youth with contributions by the Department of Polymer Engineering and Science, Chair of Polymer Processing, Montanuniversitaet Leoben. The author thanks Anna Uray and Astrid Rauschenbach for their help. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The PCCL is funded by the Austrian Government and the State Governments of Styria and Upper Austria. References 1. Maazouz A, Mallet B and Lamnawar K. Compounding and processing of biodegradable materials based on PLA for packaging applications: in greening the 21st century materials world. Front Sci Eng 2011; 1 44.

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