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.