Joining Bioplastics in
Joining Bioplastics in technical applications Thermal direct joining of PLA with aluminium How can bioplastics be efficiently combined with metals? And how do such multi-material joints behave in comparison to established petrochemical polymers? These were questions that researchers at the University of Applied Sciences Dresden, Germany (HTW) and the Fraunhofer Institute for Material and Beam Technology (IWS), Dresden, Germany sought to find an answer to. The idea was to establish whether the application fields of PLA could be broadened, as PLA is currently mainly used for packaging or as a construction material for household appliances, electronic articles and decorative elements in the automotive industry. For this purpose of this study, PLA and other thermoplastics were joined to aluminium using various processes. Hot and solid - direct thermal joining In industrial applications, materials must not only be resilient but they must also be able to be joined together quickly. While the advantages of direct thermal joining are well known, the process is still relatively uncommon for bioplastics. The principle of thermal metal-plastic joining is to convert the polymer-based joining partner into a plastic or liquid state through the localized introduction of heat at the joining surface in order to wet the metal surface. Ultrasound, induction or laser radiation can serve as the heat source. When a joining pressure is applied, the polymer molecules interlock with the microscopic structure of the metal surface and cool down. The better the surface is structured, the better the adhesion. The use of adhesion promoters is also possible. As with adhesive bonding, this is also the key to achieving the desired load-bearing capacity: insufficiently pre-treated surfaces result in significantly lower strength. The heat input is usually indirectly applied through heating of the metal, although heating the plastic directly is also possible. The resulting joints are permanent and cannot be separated without damaging at least one of the joined parts. Advantages over other joining methods, such as adhesive bonding or assembly with fasteners (drilling, deburring, threading, setting), are the extremely short process times, as both the heating and cooling rates are extremely fast. The joined components achieve their final strength immediately after cooling and can be further processed directly. No additional materials such as adhesives, solders and fluxes or connecting elements like screws, rivets or bolts are required. The process is not suitable for contaminated surfaces, as is the case with adhesive bonding, as the joining surface is liquefied and contaminants are partly absorbed in the liquid phase. Unlike with the use of an adhesive, curing times are unnecessary. Like bonds, thermally joined joints can compensate for geometrical tolerances due to the local deformation. On the test bench The materials tested included the bioplastic PLA, PET and PP, with the latter two serving as a reference for typical petrochemical plastics. All the materials tested are available as commercially used films. Material PLA Bioplastic PET Reference plastic PP Reference plastic AW 6082 T6 Aluminium alloy Thickness 0.5 mm 0.5 mm 0.5 mm 1.5 mm Melting temperature ca. 155°C ca. 250°C ca. 165°C ca. 660°C Strength 61 N/mm² 57 N/mm² 22 N/mm² ca. 300 N/mm² The metal part used was a 1.5 mm thick aluminium sheet made of EN AW6082 T6, an alloy widely applied in the automotive and mechanical engineering industry. Induction joining, laser joining, ultrasonic joining and, as a reference, the current market standard, adhesive bonding were used for the respective pairs. The surface of the aluminium sheet was given a regular linear structure using a laser-assisted ablation process, similar to an engraving process. The test specimen geometry was based on DIN EN 1465, which was developed for the overlapping lap shear testing of adhesives. All test specimens had the same overlapping surface and were tested with the same test parameters until destruction in order to establish direct comparability. All adhesively bonded joints showed continuous adhesive failure of the plastic, with PP achieving the highest values for this test configuration and being significantly ahead of the biobased PLA. The ultrasonically bonded samples showed predominantly adhesive failure. The two thermally joined samples - using induction and laser as the heat source - behaved similarly, with PLA and PET achieving the highest strengths and showing mixed adhesive and cohesive failure. PP showed a slightly lower strength compared to adhesive bonding, and suffered adhesive failure. Knowledge for practice - summary The tests showed that PLA can indeed be joined to aluminium using direct thermal joining with induction or laser heating. With appropriate pre-treatment of the surface, high joining strengths could be achieved, in some cases far exceeding the adhesive joints. The biological origin of PLA does not constitute a disadvantage to using these joining technologies. The results are comparable to the behaviour of PET under similar conditions. Thermal joining processes are therefore a very good alternative to established joining processes and may help to introduce more eco-friendly materials into the production chain. The extremely short process times allow for fast handling and thus high productivity. By dispensing with additives and chemicals, a high level of sustainability and environmental compatibility is also achieved. This work is co-financed by the European Social Fund (ESF) and tax funds on the basis of the budget adopted by the Saxon State Parliament. 42 bioplastics MAGAZINE [02/20] Vol. 15
Joining By: Philipp Zink Research associate Fraunhofer Institute for Material and Beam Technology IWS Dresden, Germany COMPEO Leading compounding technology for heat- and shear-sensitive plastics A PLA-textile-laminate thermally directly joined with aluminium by ultrasound. Joining takes place without any additional materials. Ultrasonic joining makes it possible to not only create joints of the same type. 25 — Comparison of the joining process tensile shear strength (N/mm 2 ) 20 — 15 — 10 — 5 — 0 — — Adhesive bonding — Ultrasonic joining Induction joining PLA PET PP Laser joining Although known to be difficult to bond, PET and PLA were successfully joined to aluminium using direct thermal joining methods and showed an increase in strength up to a factor of 4 compared to the reference (adhesive bonding). — — — Uniquely efficient. Incredibly versatile. Amazingly flexible. With its new COMPEO Kneader series, BUSS continues to offer continuous compounding solutions that set the standard for heat- and shear-sensitive applications, in all industries, including for biopolymers. • Moderate, uniform shear rates • Extremely low temperature profile • Efficient injection of liquid components • Precise temperature control • High filler loadings www.busscorp.com https://www.htw-dresden.de/ | www.iws.fraunhofer.de/en.html bioplastics MAGAZINE [02/20] Vol. 15 43
