Many of natural structural materials possess very interesting mechanical properties for technical applications. For this reason the structure of these materials has been investigated during the last decades very well. It turned out the natural materials exhibit a hierarchical structure and have a high filling degree of minerals (ceramic). The other components of the natural materials are bio-polymers. Despite a very high filling degree the natural materials possess unexpected high fracture toughness. A typical example of such a damage tolerant natural material is nacre. Nacre consists of about 95 vol.% calcium carbonate and about 5 vol.% of polymeric material.Due of good mechanical properties of natural materials many attempts are made to reconstruct this complex hierarchical structural design.
Main aim of the project is the fabrication of tailor-made high-filled and hierarchical composite materials for different applications. For this aim novel processing routes based on fluidization of particles are developed and optimized and the mechanical properties of produced highly filled ceramic-polymer and metal-polymer composites are investigated. Furthermore the influence of the shape of reinforcement particles is investigated.
1) Fluidized Bed Spray Granulation
One suitable process route for the production of composites with a high-volume fraction of the reinforcedparticulate phase involves the fluidized bed spray granulation process, followed by subsequent warm compation. Fine particles are coated and granulated with polymer to replicate the structural characteristics found in natural composites. For this project, a miniaturized fluidized bed (see Fig. 1) designed specifically for processing fine particles and small quantities of valuable materials was developed. The granules are compacted using a hot press at a temperature above the polymer's glass transition temperature. This results in the formation of a continuous polymer matrix phase around the particles within the composite pellet. Composites with combinations such as Al2O3 and PVB (poly vinyl butyral), as well as Fe3O4 and SBS (poly(styrene-butadiene-styrene)), were manufactured and subjected to characterization. Mechanical properties are evaluated through three-point bending and nanoindentatoin tests.
2) Spray Drying Process
To replicate the structure and mechanical traits of natural composites, the size of the constituent building blocks plays a crucial role. Smaller units faciliate effective load distribution when subjected to mechanical stress. However, the fluidized bed process faces limitations with small particles, as cohesion forces leasing to aggregates formation and preventing the fluidization of individual particles. Consequently, spray drying emerges as an alternative method fot the production fo composite materials featuring finely sized particles. In this project, suspensions containing submicron Al2O3 or Fe3O4 particles in diverse polymer solutions are dried in a nano spray dryer by the company Büchi. Utilizing a piezo-electic membran nozzle with a mesh size of 7 µm enable the generaton of exeptionally fine droplets, producing coated particles with sized under 20 µm (see Fig. 2). The same process employed for producing composite pellets from fluidized bed approach is applied. Highly-filled magnetite-PVB composites featuring 50 to 62 vol.% magnetite content were successfully manufactured. These composites exhibited robust mechanical characteristics compared with those of the individual polymers, boasting a maximum Young's modulus of 9.2 GPa and a bending strength of 91.2 MPa (see Fig. 3). Comparable outcomes were achieved with Al2O3 particles. Throughout these studies, employing commercial particles and polymers unveiled the interface between both phases as the weakest point in the composite structure. Consequently, the current focus lies in optimizing mechanical properties by enhancing the interaction between the rigid and pliable components within the composite. This is achieved through the creating of silane-functionalized alumina particles, subsequently coated with PVA (poly vinyl alcohol). PVA is adept at forming covalent bonds with the ligands on the particle's functionalized surface, resulting in a more robust interface and stronger composites. The process route is illustrated in Fig. 4.
- Rothberg, H.S., Pietsch-Braune, S., Spahr, L., Kanina, Y., Heinrich, S.: Production of magnetite-polyvinyl butyral composites using a Nano Spray Dryer, Powder Technology, 2021, 394 (394-402), DOI: 10.1016/j.powtec.2021.08.063
- Rothberg, H.S., Pietsch, S., Schneider, G.A., Heinrich, S.: Fabrication of Highly Filled Composites with an Innovative Miniaturized Spouted Bed, Processes, 2020, 8(5), 521, DOI: 10.3390/pr8050521
- Eichner E., Fischer, P.-K., Heinrich, S., Schneider, G.A.: Production of composites with high relative permittivity using the spouted bed technique, Particuology, 42 (2019), 184-189.
- Eichner E., Heinrich, S., Schneider, G.A.: Influence of particle shape and size on mechanical properties in copper-polymer composites, Powder Technol. 339 (2018), 39-45.
- Eichner E., Salikov V., Bassen P., Heinrich S., Schneider G.A.: Using dilute spouting for fabrication of highly filled metal-polymer composite materials, Powder Technol. 316 (2017) 426–433.
- Georgopanos P., Eichner E., Filiz V., Handge U.A., Schneider G.A., Heinrich S., Abetz V.: Improvement of mechanical properties by a polydopamine interface in highly filled hierarchical composites of titanium dioxide particles and poly(vinyl butyral), Compos. Sci. Technol. 146 (2017) 73-82.
- Wolff M.F.H., Salikov V., Antonyuk S., Heinrich S., Schneider G.A. (2014): Novel, highly-filled ceramic–polymer composites synthesized by a spouted bed spray granulation process, Composit. Sci.&Techn. 90
- Wolff M.F.H., Salikov V., Antonyuk S., Heinrich S., Schneider G.A. (2013): Three-dimensional discrete element modeling of micromechanical bending tests of ceramic–polymer composite materials, Powder Techn. 248.
• Institute of Advanced Ceramics, Hamburg University of Technology (Prof. G.A. Schneider)
• Institute of Physical Chemistry, University of Hamburg (AG Prof. V. Abetz)