Structure-property relationships of rubber/silica nanocomposites via sol-gel reaction

Abstract

Up to date, conventional mixing of rubber with silica is time and energy consuming process where silica aggregates are dispersed throughout the rubber matrix. A way to avoid silica aggregation and to improve dispersion is to grow silica particles directly into the rubber matrix using the sol-gel reaction. The idea triggered a PhD project where the main goal is to gain clear understanding about the structure, the formation mechanism and the corresponding properties of in-situ prepared rubber/silica nanocomposites. The gained knowledge will be then used to obtain in-situ nanocomposites via reactive extrusion process. In Chapter 1 of the thesis the current state of art in the field of nanocomposites is given, with emphasis on the preparation and properties of nanocomposites produced via sol-gel reaction. Outlined are also the objectives of the study-to provide deep understanding of the mechanism and the kinetics of the sol-gel reaction in rubber matrix and to correlate the structure of the in-situ synthesized silica to the properties of the nanocomposites. Rubber/silica nanocomposites were prepared via sol-gel reaction, using TEOS as precursor and hexylamine as catalyst. The effect of different reaction parameters - amount of the precursor, reaction time, temperature and type of the rubber on the morphology of the prepared in-situ nanocomposites (loading, size and dispersion of silica) is shown in Chapter 2. The TEM images of the nanocomposites indicated excellent dispersion of the silica particles. The amount of the bound rubber was evaluated and it was correlated to the rubber-silica interaction. The kinetics and the mechanism of the sol-gel reaction in a rubber matrix were studied in detail by performing time resolved solid-state NMR and SAXS experiments. The results presented in Chapter 3 indicate that the sol-gel process in rubber matrix adopts the emulsification process behavior, where hexylamine, used as catalyst behaves as surfactant forming inverse micelles with enclosed water in TEOS-swollen rubber matrix. The growth of the silica particles with time was probed via SAXS and a comparison is made between the growth rate of different type of rubbers and at different temperatures. The structural investigation of the in-situ synthesized silica particles, presented in Chapter 4, was performed using solid state NMR and MS-TGA-IR. For the first time we show that via the sol-gel process so called ‘hairy’ silica particles are formed, with hexylamine and remnant ethoxy groups residing predominantly on the silica surface. This hairy silica surface resulted in more hydrophobic nature of the silica particles, thus improved rubber-silica interactions. In Chapter 5 the properties of the in-situ nanocomposites produced in both-static and dynamic conditions (batch mixer) are discussed and compared to those of the conventionally prepared rubber/silica nanocomposites. The RPA measurements of the in-situ nanocomposites indicated strong reinforcement effect, at much lower silica loadings in comparison to the conventional rubber/silica nanocomposites. We explain this strong reinforcement in the in-situ prepared rubber/silica nanocomposites with the improved rubber-silica interactions (caused by the specific surface topology of the in-situ silica) and by the presence of trapped entanglements and bound rubber. The in-situ rubber/silica nanocomposites were successfully produced also via reactive extrusion, which was one of the targets of the thesis. As shown in Chapter 5, the obtained in this way nanocomposite had maximum loading of 3% silica, possessed uniform dispersion of the silica particles and very good properties. In Chapter 6 the effect of different silica precursors (tetramethyl orthosilicate (TMOS), (tetraethylorthosilicate) TEOS and tetrabutylorthosilicate (TBOS)) on the morphology of the in-situ nanocomposites was studied in relation to the mechanical properties as determined by DMTA and tensile testing. Chapter 7 contains the technological assessment of this study. The importance and the possibilities for industrial application of the sol-gel process to obtain rubber-silica nanocomposites with excellent properties are discussed.