Injection moulding PLA
Injection moulding PLA in technical applications In technically demanding applications and markets, such as electronics products, transportation, or the construction sector, bioplastics have so far hardly been represented to any significant extent. There are many reasons for this, ranging from insufficient economic competitiveness to inadequate properties for technical products. However, the material properties play an important role in the market success for technical applications. For example, plastics for technical products usually have to meet a large number of requirements, such as high heat resistance, high toughness, and low flammability. Bioplastics like polylactic acid (PLA) can hardly comply with these specifications. Despite intensive research and development in recent years, PLA still does not have a marketable property profile for technical injection moulding products. In a research project funded by the German Federal Ministry of Food and Agriculture, the Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT (Oberhausen, Germany), together with Evonik Operations (Essen, Germany), the Institute for Plastics Processing at RWTH Aachen (IKV) (Aachen, Germany) and FKuR (Willich, Germany), was able to develop formulations with simultaneously improved impact strength, heat resistance, and flame retardancy. Additionally, processing parameters to reach an economic manufacturing process via injection moulding were revealed. The material development was carried out in close consultation with partners from the E&E industry. Application-related requirements were defined with regard to important material characteristics such as Glow Wire Ignition Temperature (GWIT), Charpy Impact Strength and Heat Deflection Temperature (HDT). Here, the special focus was on fulfilling the combination of properties. PLA materials with increased flame retardancy or increased impact strength are known from past research. Adjustment of heat resistance via modification of crystallinity is already part of commercially available material solutions. The challenge is that the properties affect each other. For example, a material made of PLA without functional additives or blending partners can pass the UL94 flame retardancy test with the best result V0. If functional additives to increase the impact strength are added, however, this test cannot be passed. To achieve a desired impact strength, blend partners or performance additives can change the morphology of the PLA, especially in the amorphous regions. If high heat resistance is required at the same time, crystalline phases with a high degree of order must be present; accordingly, for PLA-based materials, heat resistance and impact strength are opposed. By studying the impact of different additives and processing conditions alone and in combination with each other, the project partners have found formulations of PLAbased blend systems that meet the requirements regarding flame resistance, heat deflection temperature, and impact strength in several combinations. At the beginning of the project, the impact of the different additives was investigated separately. The resulting typical behaviour can be seen in Figure 1 (top), which shows the notched impact strength as a function of the flame-retardant content. Because the flame-retardant is a solid component, it lowers the impact resistance by providing weak points for crack propagation. On the other hand, the lower graph of Figure 1 depicts that the heat deflection temperature not only depends on the crystallinity. The HDT remains at a constant level at higher contents of the flame-retardants, although the crystallinity declines. Additionally, the two different impact modifiers used, affect this behaviour differently. One of them leads to a higher heat deflection temperature than the other, while the crystallinities are more or less the same. This led to the conclusion that detailed investigations are necessary to understand the underlying concepts of the dependency of the final material properties on the combination of the different additives. Notched Impact Strength [kJ/m²] Heat Deflection Temperature [°C] 3 2 1 0 140 120 100 80 Impact Modifier 1 Impact Modifier 2 0 1 2 Flame Retardant [AU] 60 Impact Modifier 1 Impact Modifier 2 0 0 1 2 Flame Retardant [AU] 60 50 40 30 20 10 Figure 1: Top: notched impact strength as a function of flameretardant content for an unmodified compound. Bottom: heat deflection temperature and crystallinity as a function of flameretardant content. 0 Crystallinity [%] 38 bioplastics MAGAZINE [03/22] Vol. 17
By: Alexander Piontek, Research Associate Philip Mörbitz, Group Manager Polymer Rechnology Fraunhofer UMSICHT Oberhausen,Germany In addition to the effects of the formulation, the influence of the processing conditions on the material properties was investigated in detail. The project partners found a solution to overcome the contradiction of high crystallinity combined with high impact toughness and/or high heat deflection temperature. The solution was accomplished by combining both – additives and processing conditions – in a special way. It was shown that it is possible to tune the material properties by adjusting cooling times and rates during the injection moulding process adapted to the different formulations. The final compounds contain suitable additives, which enhance crystallinity, as well as others that influence impact strength or flame resistance. It was thus possible to obtain materials that meet at least two of the requirements simultaneously. The schematic diagram in Figure 2 depicts the material properties with regard to the chosen requirements. Automotive Impact Strength Target Compound 1 Compound 2 Compound 3 PLA GWIT HDT Figure 2: Qualitative representation of the property range achieved in the project. The developed material composition has successfully proven its applicability under industrial conditions. Different components were produced via injection moulding. It was shown, that even small delicate parts for electronic devices like the exemplary switch casing shown in Figure 3 can be produced. All demonstration parts show excellent haptic and optic properties and are UV weldable. In conclusion, the consortium was able to develop material compositions for technical applications with high PLA content and desired properties for manufacturing technical products. Fig 3: Iinjection moulded component of the developed material (white part, produced at Georg Schlegel GmbH & Co. KG) Join us at the 17th European Bioplastics Conference – the leading business forum for the bioplastics industry. 6/7 December 2022 Maritim proArte Hotel Berlin, Germany REGISTER NOW! www.umsicht.fraunhofer.de/en.html | https://corporate.evonik.com/en www.ikv-aachen.de/en/ | www.fkur.com/en @EUBioplastics #eubpconf2022 www.european-bioplastics.org/events For more information email: conference@european-bioplastics.org bioplastics MAGAZINE [03/22] Vol. 17 39
