Foam force F in N Table
Foam force F in N Table 1: Overview of the produced modified PLA PLA Material Modifier (mass concentration in %) PLA ex - PLA1 organic peroxide (0.2) PLA2 multifunctional epoxide (1.0) PLA3 styrene maleic anhydride (0.7) PLA4 isocyanurate + diisocyanate (0.2/0.2) PLA5 bisoxazoline + diisocyanate (0.2/0.2) modifier waterbath granulator Figure 1: Principle of the PLA modification process on a twin screw extruder Table 2: Characteristic properties of the modified PLAs Material Mw in g/mol Mn in g/mol PD α in % PLA 130400 98340 1.326 26 PLA1 237600 144700 1.642 19 PLA2 200600 116200 1.727 7 PLA3 133500 99860 1,337 3 PLA4 166900 118400 1.470 25 PLA5 149600 101600 1.472 20 Figure 2: Draw force F measured by Rheotens as a function of draw velocity 0,12 T = 175 °C T* = 180 °C 0,10 v = 0,5 mm/s d = 2 mm l = 40 mm 0,08 PLA PLA 2* 0,06 PLA 1 PLA 4 PLA 5 PLA 3 0,04 0,02 0,00 0 200 400 600 800 draw velocity v in mm/s Bio-foams made of modified polylactide Polymeric foams are widely used in applications such as cushioning, packaging, and insulation, due to their unique energy absorption behaviour and good thermal or acoustic insulation properties. They keep their bending stiffness, though a part of the material is exchanged by gas and so reduces the part’s mass at the same time. Currently, extruded PS (XPS) is one of the most important representatives of foams worldwide. In some countries, there have already been attempts to ban food packaging made of petroleum-based XPS foams. As a result, alternatives to foams made of PS are coming into focus. One possible alternative is polylactide (PLA). PLA is a biobased polyester that is biodegradable under certain conditions, approved for use in the food industry, available at competitive prices, and has mechanical properties similar to those of PS. However, PLA also has a number of disadvantages, which pose a particular challenge for foaming. In addition to its low melt strength and viscosity, its crystallization behaviour is also responsible for the fact that unmodified PLA is difficult to foam. As part of a project funded by the German Research Foundation, the Institut für Kunststofftechnik (IKT) in Stuttgart in cooperation with the Department of Polymer Engineering of the University Bayreuth were working together on modified standard PLA compounds for foaming. Within the research consortium, new modifiers were being investigated. The PLA selected for modification was a commercial and inexpensive grade, named 7001D, supplied by NatureWorks LLC, Minnetonka, USA. Reactive compounding was performed on a twin-screw extruder (Fig. 1). Different modifiers with different functionalities were used for modification. At the department of Polymer Engineering at the University of Bayreuth, the foam extrusion process of the modified PLA was performed on a tandem line (COLLIN Lab & Pilot Solutions GmbH) with CO 2 as physical blowing agent. To determine the characteristic properties of the modified PLA and the foamed samples, different thermal, rheological, chemical, and optical tests were performed. Results In table 2, the results of the molecular weight determination (GPC MALLS) and the calculated crystallinity α from thermal characterization by differential scanning calorimetry (DSC) are shown. 32 bioplastics MAGAZINE [01/21] Vol. 16
By: Julia Dreier Svenja Murillo Castellón Christian Bonten Institut für Kunststofftechnik, University of Stuttgart, Germany Figure 3: SEM images of modified PLA samples Foam The molecular weight of the materials and the polydispersity index of the materials were increased due to the modifications. So, the reaction between the linear PLA chains and the modifiers took place, which result in an altered chain structure. In PLA1 and PLA2 the highest molecular weights are observed, with an increase of 82 and 54 % respectively compared to the initial molecular weight. The modified samples show lower crystallinity, except for PLA4. The decrease can be attributed to the fact that benzene rings are present in the modifiers. Due to the steric hindrance defects are induced in the crystal lamellae, so that the crystallization is inhibited. This effect is most pronounced for the material PLA2 and PLA3. As already mentioned, the modifiers lead to altered chain structures in form of branching, crosslinking, and chain extension, so the rheological properties and therefore the processing characteristics are affected. A high melt strength is essential for the formation of a stable foam structure. The melt strength values of the modified polylactides PLA3 and PLA5 are slightly lower than that of the unmodified starting material. By contrast, the melt strength of PLA is much lower (0.01 N compared to 0.03 N of the unmodified PLA), but has higher melt extensibility. The PLA1 and PLA2 occupy again a special position. They both have much higher melt strength, but a lesser melt extensibility compared to the other modified materials and the neat PLA. The increased melt strength of PLA1 and PLA2 can be attributed to their significantly higher molecular weight and the greater polydispersity through the modification. The influence of the modifications on foamability and foam morphology was investigated. Figure 3 shows the SEM images of the foamed modified PLA compared to the neat PLA. In general, foams with fine, uniform, and closed cell structure in a density range of 32 to 49 kg/m³ were achieved. Compared to the neat PLA foam, a further reduction of the density was possible except for PLA2. The best foam qualities and properties are achieved with organic peroxide (PLA1). Conclusion and Future Works The aim of generating bio-foams with standard and costeffective PLA was achieved by using appropriate modifiers. It was shown that the molecular weight and the melt strength could be increased by the use of the modifiers and the foam morphology could be improved. The future work will focus on bead foaming of PLA and the investigation of the weldability of the beads. www.ikt.uni-stuttgart.de | www.polymer-engineering.de bioplastics MAGAZINE [01/21] Vol. 16 33
