Nanoparticles are widely used as reinforcing fillers to optimise polymer properties such as tensile strength, scratch resistance and wear. An especially challenging field is the reinforcement of transparent polymers without loss of transparency as the particles must not cause any scattering of light and thus have to be very small, monodisperse and/or feature a refractive index close or equal to the polymer matrix. Nanoparticles tend to agglomerate with decreasing size due to increasing Van der Waals forces. Disintegration of the agglomerates and homogeneous dispersion of the single particles in the polymer matrix is a time and energy intensive process. Therefore the approach to minimise loss of transparency by matching of the refractive index of the nanoparticles and the polymer matrix is very promising.

In the present work, the refractive index of SiO₂/Al₂O₃ mixed oxide particles was adjusted by variation of the composition. Silica-alumina mixed oxide particles with compositions from 0 to 100 wt% alumina were produced by the flame spray synthesis. Nanoparticles were characterised by means of BET method, XRD and TEM. The refractive index was determined using immersion oils and the Becke Line method. Particles were incorporated in two-component polyurethane clear coating systems and cured at 80 °C for one hour. Transmittance of the cured films was measured by UV/vis spectroscopy.

Nanopowders with refractive indices in the range of 1.460 to 1.760 with an accuracy of 0.002 were produced at rates up to 100 g/h. Up to an alumina content of 60 wt%, corresponding to a refractive index of 1.5080, the nanopowders were amorphous. The target refractive index of the polyurethane matrix of 1.512 was met with a mixed oxide composition comprising 30 wt% alumina. Coatings were prepared and characterised with 10 and 20 wt% mixed oxide particles, respectively, and with SiO₂ (Aerosil OX50) for comparison.

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