Silica formation in a rubber matrix is studied. The hypothesis that the formation proceeds via inverse micelles where the hexylamine present, being a catalyst, also behaves as a surfactant, is made highly plausible. As well-known, the conversion of TEOS into a solid silica particles proceeds via hydrolysis and condensation reactions. These transformations of TEOS comprise consequential substitution of the hydroxyl (-OH) groups with siloxane ones (-OSi), labeled as Q1: one -OSi group; Q2: two -OSi groups; Q3: three -OSi groups; and Q4: four -OSi groups. Here the kinetics of the sol–gel reaction in the rubber matrix was studied as a function of the reaction temperature (40–120 °C) by 1H high-resolution magic-angle spinning (HR-MAS) NMR spectroscopy. Real-time small-angle X-ray scattering (SAXS) measurements of the sol–gel reaction in NR and EPDM rubber matrices were performed in a time scale of 1 s to 60 min with images being acquired every 15 s. The sol–gel reaction in NR and EPDM rubber at 100 °C shows an initial fast increase in particle size that decreases gradually for longer reaction times, eventually reaching a plateau where the particle size does not change anymore. In the NR matrix the initial fast increase in particle size lasts approximately 25 min after which the plateau begins, while for the EPDM the initial phase is approximately 50 min. Moreover quantitative 29Si MAS NMR measurements used for mapping the silicon atoms showed no significant difference in the structure of the silica particles formed after 15 and 60 min reaction time at 100 °C, indicating that a good quality silica with a ratio Q4:Q3 ¿ 2 can be realized in a relatively short reaction time, thereby offering options for future industrial applications such as reactive extrusion processes.
Miloskovska, E., Hansen, M. R., Friedrich, C., Hristova - Bogaerds, D. G., Duin, van, M., & With, de, G. (2014). In situ silica nanoparticle formation in a rubber matrix monitored via real-time SAX and solid-state NMR spectroscopy. Macromolecules, 474(15), 5174-5185. https://doi.org/10.1021/ma500594a