Foam Foams made from
Foam Foams made from modified standard PLA Figure 1: XPS as thermal insulation material in buildings By: Svenja Göttermann, Sandra Weinmann, Christian Bonten Institut für Kunststofftechnik, University of Stuttgart, Germany Tobias Standau, Volker Altstäd Department of Polymer Engineering, University of Bayreuth, Germany Figure 2: Loosefill packaging chips for product protection Nowadays, polymer foams are widely used because the cellular structure leads to a low thermal conductivity and specific impact resistance, which are necessary for applications such as insulation or packaging (fig. 1 and fig. 2). In fact, thanks to the material reduction, weight and costs can be saved as well as CO 2 emissions lowered. Still, the disposal of polymer foams causes environmental issues in countries without a good waste system, because most foamed moldings are used in relatively short-lived applications like transport packaging, e. g. electronic devices or disposable dishes. Usually, these are made of petroleum-based extruded polystyrene foam (XPS). However, already things are changing. In attempt to green up the City of New York has banned XPS. The use of biodegradable and biobased plastics, especially in short-lived packaging, allows an alternative disposal route and can replace fossil-based raw materials. Until today, the price of mass produced PLA had already dropped to less than 2 €/kg even for small quantities. This is still more expensive than polystyrene. But in return, PLA is a promising bioplastic, because it is based on natural resources and at the same time is biodegradable. But a main drawback of conventional, cost-effective PLA is its low melt strength, which is among other properties disadvantageous in terms of foaming. There are special PLA types for foaming on the market. However, they are not easy to handle and are cost-intensive. As part of a project funded by the German Research Foundation, the Institut für Kunststofftechnik (IKT) in Stuttgart, Germany in cooperation with the department of Polymer Engineering of University Bayreuth, Germany, are working together on modified standard PLA compounds for foaming. Within the research consortium new modifiers are being investigated that induce crosslinking, chain extension or grafting to increase the molecular weight and the melt properties (shear and elongational viscosity). The material used in this study was a commercially available cost-effective PLA from NatureWorks. The chosen resin is specifically designed for the use in injection/stretch blow moulded applications. Different modifiers with different functionalities were used for these tasks. The modifiers were added on a twin-screw extruder (fig. 3) in the compounding lab of the IKT. Table 1 shows an overview of the produced materials. The selected modifier concentration is based on preliminary examinations. At the department of Polymer Engineering at University Bayreuth the batch foaming process of the modified PLA was performed on round melt-pressed samples in a heated high pressure autoclave with CO 2 as the physical blowing agent. Here, foaming of the gas-loaded specimens 38 bioplastics MAGAZINE [04/15] Vol. 10
Foam is initiated by a thermodynamic disequilibrium due to a sudden pressure drop. PLA Modifier To determine the characteristic properties of the modified PLA and the foamed samples different thermal, rheological, chemical and morphological 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. 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 or 54 % compared to the initial molecular weight. The lowest change of 2 % occurred in PLA3. 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 materials PLA2 and PLA3. As already mentioned, the modifiers lead to altered chain structures in the form of branching, crosslinking and chain extension, so the rheological properties and therefore the processing characteristics are affected. Hence the rheological behaviour under shear (fig. 4) and elongation (fig. 5) was investigated. Within the materials, two different groups are visible. All materials of the first group (PLA ex, PLA3 – PLA5) show a typical shear thinning behavior. Here, it is striking that the modified materials have a lower zero shear viscosity than the extruded, unmodified PLA, which should have a linear structure. PLA1 and PLA2 have a very different flow behaviour, which is due to their significantly greater average molecular weight and broader molecular weight distribution. PLA2 shows an s-shaped curve without forming a Newtonian plateau, which indicates the presence of a partially crosslinked structure. PLA1, however, shows a typical shear thinning behavior as well, while the Newtonian plateau is only indicated in the low frequency range. The transient elongational viscosity (η E+ ) curves indicate, that all investigated materials show strain hardening, except PLA4 which was not measurable due to its brittle nature. While PLA3 and PLA5 behaved more or less like the neat material, PLA1 as well as PLA2 showed drastically higher extensional viscosities. This is due to their significantly greater molecular weight and broader molecular weight distribution as mentioned before. Besides the differences in the rheological behavior (shear and elongation) all materials were foamable. By means of batch foaming, the influence of the modifications on foamability and foam morphology was investigated. Figure 6 shows the SEM images of the batch foamed modified PLAs compared to the unmodified PLA. Waterbath Granulator Figure 3: Principle of the PLA modification process on a twin-screw extruder Table 1: Overview of the produced modified PLA Material Modifier (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) Table 2: Characteristic properties of the modified PLAs Material M w in g/mol M n 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 4: Complex viscosity from rheological measurements in shear deformation Complex viscosity |η*| in Pas 10 6 PLA1 PLA4 PLA3 PLA2 10 5 PLA5 PLA ex 10 4 10 3 Temperature 180 °C 10 2 10 -2 10 -1 10 0 10 1 10 2 10 3 Frequency ω in rad/s bioplastics MAGAZINE [04/15] Vol. 10 39
