Nicole Pfeiffer

• Synthetisch hergestelltes Siliziumdioxid (SiO2) im Submikro- und Nanopartikelmaßstab findet bereits in Farben und Lacken, sowie in Kunst- und Klebstoffen Einsatz, um deren Brillanz, Kratz- und Kohäsionsfestigkeit zu verbessern. Die gute Verfügbarkeit und der geringe Preis des SiO2 machen es für viele Anwendungen zu einem interessanten Füllstoff. Ziel dieser Arbeit war es, durch werkstoffwissenschaftliche, grundlagenorientierte Betrachtungen ein tieferes Verständnis für die Funktionsmechanismen von modifizierten SiO2-Partikeln in einer EP-Matrix zu erlangen und eine Verbesserung der tribologischen Eigenschaften (Reibungskoeffizient und Verschleißrate) der Komposite zu erreichen. Der erste Teil dieser Arbeit befasst sich mit der Herstellung und Charakterisierung von modifizierten Siliziumdioxid-Partikeln mittels eines modifizierten Stöberprozess. Es wurden die Katalysatoren Ammoniak, Tetramethylammoniumhydroxid (TMAH) und Tetramethylethylendiamin (TMED) getestet. Als Referenz wurden kommerziell erhältliche SiO2-Nanopartikel (Aerosil 200) sowie ein ebenfalls kommerziell erhältliches Partikelgemisch (NanoVit) als Füllstoffe getestet. Zur Ermittlung der tribologischen Eigenschaften wurde an den Kompositen eine Gleitverschleißuntersuchung (Stift-auf-Scheibe (PoD)) mit unterschiedlichen Parametervarianten (Geschwindigkeit-/Druckvariationen) durchgeführt. Es konnte gezeigt werden, dass die modifizierten SiO2-Partikel einen positiven Einfluss auf die tribologischen Eigenschaften der EP-Komposite haben. Weiter konnte bewiesen werden, dass durch die Zugabe der neu synthetisierten SiO2-Partikel auf eine Zugabe von Graphit als Schmiermittel verzichtet werden kann. Auch wurde festgestellt, dass durch das Eleminieren des Graphites sich ein sehr dünner Gleitfilm auf dem Gegenkörper ausbildete, der als ein Indiz für die Ursache der Verbesserung des Reibwertes gelten kann. Ein Prozess für dieses Kompositverhalten wurde in dieser Arbeit postuliert.
• Epoxy resins (EP), are increasingly being used as templates for the development of anti-wear coatings and composites. An improvement in the tribological properties of EP composite materials is thus of great importance. As a basic functionalization for high performance EP composites micro scale short carbon fibers (SCFs) and micro scale graphite are being utilized. In the last years the use of nanomaterials as fillers in polymer matrices has started to increase. A common way to produce nanoreinforced polymer composites is to purchase microagglomerates with a primary particle size on the nanoscale directly from the market, to transfer those particles into the respective polymer matrix and to destroy those microagglomerates by the application of various “milling techniques”. In case of thermosets e.g. epoxy resins dissolver techniques like a torus mill are applied, in case of thermoplastics a twin screw extruder will be used. At the end of the “milling process” a broad distribution of nanoparticles as well as remaining microagglomerates are present in the respective polymer matrix. Therefore an exactly controlled synthesis of defined and homogenously controlled particle sizes is not given. In order to obtain such nano- or submicroparticles respectively the so called bottom-up method has to be applied. The basis of this method are molecules e.g. tetraethylorthosilicate (TEOS) which will be condensed in alkaline medium to aggregates with a defined particle size by using stabilizing agents and temperature control. Nanoparticles such as titanium dioxide (TiO2), silicium dioxide (SiO2), zinc oxide (ZnO) or molybdeniumdisulfide (MoS2) are now an important part of industrial materials. Silicon dioxide (quartz) is caused by dehydration of silicic acid and is one of the hardest natural materials. To synthesize this material as nanoparticles, to use it as fillers in composites, offers a high potential for the creation of new nanomaterial composites. The good availability and low price of silica make it attractive for many applications. Synthetically produced SiO2 in the sub-micron and nano-scale can already be found in paints and varnishes, plastics and adhesives to use boost their brilliance, as well as to improve scratch resistance and cohesive strength. Depending on the desired particle properties and production rates, a suitable production method must be selected from the large number of possible processes to generate the perfect nanoparticles. One challenge is the transfer of these manufacturing processes from the laboratory to an industrial scale. Here the sol-gel process takes an attractive position for synthesizing innovative nanoscale SiO2 particles. The aim of this work is to stabilize the Stöber process for producing SiO2 nanoparticles, so that it allows a scale-up in the industrial scale, without significant changes in the SiO2 particles quality. The SiO2 particles are controlled during the synthesis in terms of their shape and size. For this purpose, SiO2 particles with two different stabilizers, Tetramethylammonium hydroxide (TMAH) and Tetramethylethylenediamine (TMED) are manufactured. A SiO2 particle growth law is defined by which carefully coordinated synthesis parameters (concentration, temperature and pH) homogeneous, monodispersed particles can be produced with a defined particle size between 50 nm and 700 nm (±10nm). These particles are characterized by analytical methods (IR spectroscopy, DLS analysis, SEM images, NMR spectroscopy and BET measurements). The SiO2 particles, which have been produced with the aid of the stabilizer TMAH, show a higher concentration of silanol groups in their structure. After ensuring the controlled SiO2 - manufacturing process, the particles thus produced are used as filling material for EP composites. With the modified Stöber SiO2 particles in combination with SCF and graphite, the tribological properties (coefficient of friction and wear) of the EP- composites should be improved. After the production of the SiO2 particles, these are introduced, differentiated by their different size in an EP- matrix. In addition to provide a reference the fillers SCF and graphite can be tested in an EP matrix as well. These composites are now being analyzed by the tribological pin-on-ring method in terms of wear values. In the next step EP composites are produced which include the best SiO2 particle sizes from the previous chapter together with SCF and graphite as filler materials to create better tribological properties. After this they were characterized by the pin-on-disc method with respect to their wear rate and coefficient of friction. A commercially available SiO2 nanoparticle (Aerosil 200) as filler is used in an EP composites in the next chapter as a reference. This composite is compared to two other composites containing the self-produced SiO2 particles, wherein both types differ by the choice of the stabilizer used during the synthesis. In the last section of this work a composite in which a commercially available filler mix (NanoVit) is included were tested, compared with a composite with the best previously determined fillers. It is shown that the modified SiO2 particles have a positive effect on the tribological characteristics of the EP-composites. They produce a lower coefficient of friction and improved wear performance. It is also proven that by adding the newly synthesized SiO2 particles the addition of graphite as a lubricant is no longer relevant. Furthermore, it is proven that by eliminating the graphite a very thin film of lubricant on the counter body is formed which is an indication of the cause of improving the coefficient of friction. A process for this behavior is postulated.