Composites from Spouted-Bed Granulation Process
- Hannah Sophia Rothberg, M.Sc., Institut für Feststoffverfahrenstechnik und Partikeltechnologie -
Many of natural materials show very interesting mechanical properties for technical applications. For this reason, the structure of these materials has been investigated during the last decades very well. 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.  Because of the remarkable mechanical properties of natural materials many attempts are made to reconstruct this complex hierarchical structural design. The production of a tailor-made highly-filled composite materials is the main aim of the “Sonderforschungsbereich SFB 986 M3”, which starts in the third funding period next year. In subproject A3 a 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. For the composite production, a spouted bed is used to coat fine iron oxide particles with a polymeric film. The spouted bed is a special form of the fluidized bed with the difference that the gas is supplied via a narrow gap instead of a porous distributor plate. As a result, higher shear forces and fluidization velocities are achieved and thus very fine, cohesive and as well very coarse and heavy particles can be fluidized. For this work a miniaturized spouted bed was developed to enable the usage of organic solvents for tailor-made polymers and small amounts of bed material. During the experiment, the particles are fluidized by the fluidization gas and wetted with a polymer solution by spraying liquid droplets with a two-phase nozzle (see Fig. 1). As the particles rise, the solvent evaporates and the polymer remains on the particles remaining in a core shell structure (right image in Fig. 1).
After the spouted bed process the coated particles are compacted to a composite pellet. Due to the warm compaction the polymer around the particles forms a continuous matrix where the particles are embedded (see Fig. 2).
In a last step mechanical tests are conducted where bending strength and Young’s modulus of the produced composite are measured. In a recent study the influence of the composition on the mechanical properties where studied. The study showed that maximal values for the mechanical properties of the composites were reached at 70 vol-%. iron oxide. Presumable at this composition there is just enough polymer in the composite to fill the voids between the particles. The polymer acts as a binder between the particles and enhances the load transfer when mechanical stress is applied. When there is less polymer in the composite, remaining voids are weak spots leading to weaker properties. On the other hand, the properties of the soft component will predominate when there is an excess of polymer in the composite leading to weaker strength and Youngs modulus. 
Within the new funding period functionalized particles and polymers, which are produced in other subprojects of the SFB, should be used in order to optimize the mechanical properties. Furthermore, a nano spray drying process is investigated for the production of those highly-filled materials in order to decrease the unit size in the composite and thus optimize the load transfer.
This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – project number 192346071 – SFB 986.
- Barthelat, F.; Li, C.-M.; Comi, C.; Espinosa, H.D. Mechanical properties of nacre constituents and their impact on mechanical performance. J. Mater. Res. 2006, 21(6), 1977-1986.
- Rothberg, H.S.; Pietsch, S.; Schneider, G.A.; Heinrich, S. Fabrication of Highly Filled Composites with an Innovative Miniaturized Spouted Bed. Processes 2020, 8, 521.