Surface Modification of Fumed Silica by Dry Silanization for PP-based Dielectric Nanocomposites

Novel nanocomposites for dielectric applications based on a polypropylene (PP) blend filled with nanosilica are developed in the frame of the European `GRIDABLE' project. A systematic study of the influence of surface modification of the nanosilica on the dielectric properties of the PP/silica blend was performed. The main goal of this investigation was to modify the chemical composition of the silica surface, which is expected to improve the charge trapping properties of the nanocomposites. For the modification of the silica surface, a "green" approach was utilized: a dry silanization method, which is performed without the need of a solvent. Eight different silanes were investigated in this study, which are categorized into three different groups: I) Aliphatic silanes with a different number of alkoxy groups II) Hydrocarbon silanes containing delocalized electron clouds III) Polar silanes containing hetero elements (nitrogen, sulfur or oxygen) The results of the thermogravimetric analysis (TGA) show higher weight loss of the modified silicas in comparison to the unmodified one. This indicates that the dry process is an effective method to perform silica surface modification using alkoxysilanes. The charge trapping properties were studied by Thermally Stimulated Depolarization Current (TSDC) measurements. The obtained TSDC results show that the trap density peak is not significantly shifted in temperature when the silica is modified with functional groups elementally similar to the polymer matrix. However, their incorporation influences the traps density and suppress the deeper traps occurring near the range of the melting temperature of PP. When the silica surface is modified with a precursor containing a hetero element, it has an effect on both, trap level depth as well as density. Depending on the type of the hetero element (sulfur, nitrogen, oxygen), the trap depth shifted to higher temperatures, and the trap density decreased to significantly lower levels. Nitrogen appears to have the strongest effect on the charge trap properties. All these first stage of results show that incorporation of modified nanosilica into a PP matrix seems to be a promising approach to tailor its electric properties. Further development of these composites would lead to benefits for high-voltage cable and capacitors applications.