Novel nanocomposites for dielectric applications-based polypropylene/poly(ethylene-co-octene) (PP/POE) blends filled with nano silica are developed in the framework of the European ‘GRIDABLE’ project. A tailor-made low-pressure-plasma reactor was applied in this study for an organic surface modification of silica. Acetylene gas was used as the monomer for plasma polymerization in order to deposit a hydrocarbon layer onto the silica surface. The aim of this modification is to increase the compatibility between silica and the PP/POE blends matrix in order to improve the dispersion of the filler in the polymer matrix and to suppress the space charge accumulation by altering the charge trapping properties of these silica/PP/POE blends composites. The conditions for the deposition of the acetylene plasma-polymer onto the silica surface were optimized by analyzing the modification in terms of weight loss by thermogravimetry (TGA). X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray fluorescence spectroscopy (EDX) measurements confirmed the presence of hydrocarbon compounds on the silica surface after plasma modification. The acetylene plasma modified silica with the highest deposition level was selected to be incorporated into the PP/POE blends matrix. X-ray diffraction (XRD) showed that there is no new crystal phase formation in the PP/POE blends nanocomposites after addition of the acetylene plasma modified silica. Differential scanning calorimetry results (DSC) show two melting peaks and two crystallization peaks of the PP/POE blends nanocomposites corresponding to the PP and POE domains. The improved dispersion of the silica after acetylene plasma modification in the PP/POE blends matrix was shown by means of SEM–EDX mapping. Thermally stimulated depolarization current (TSDC) measurements confirm that addition of the acetylene plasma modified silica affects the charge trapping density and decreases the amount of injected charges into PP/POE blends nanocomposites. This work shows that acetylene plasma modification of the silica surface is a promising route to tune charge trapping properties of PP/POE blend-based nanocomposites.

Functionalized nanoparticles have various applications, for which grafting of a chemical moiety onto the surface to induce/improve certain properties is needed. When incorporated in polymeric matrices, for instance, the modified nanoparticles can alter the interfacial characteristics leading to improvements ofthe macroscopic properties of the nanocomposites. The extent of these improvements is highly dependent on the thickness, morphology and conformity of the grafted layer. However, the common liquid-phase modification methods provide limited control over these factors. A novel gas-phase modification process was utilized, with 3-aminopropyltriethoxysilane (APTES) as precursor, to chemically deposit amino-terminated organic layers on fumed silica nanoparticles in a fluidized bed. A self-limiting surface saturation was achieved when the reaction was done at 200 °C. With this self-limiting feature, we were able to graft multiple layers of aminopropylsiloxane (APS) onto the silica nanoparticles using water as the coreactant. The feasibility of this process was analyzed using thermogravimetric analysis (TGA), diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and elemental analysis (EA). By altering the number of APTES/water cycles, it was possible to control the thickness and conformity of the deposited aminopropylsiloxane layer. This novel approach allows to engineer the surface of nanoparticles, by introducing versatile functionalized layers in a controlled manner.

This paper focuses on novel insulation polypropylene/poly(ethylene-co-octene) (PP/POE) nanocomposites for High Voltage Direct Current (HVDC) cable application. The composites contain silica modified by a solvent-free method using silanes differing in polarity and functional moieties. Thermogravimetric Analysis and Fourier Transform Infrared Spectroscopy showed that the solvent-free method is an effective way to modify silica by silanes. Silica/PP/POE nanocomposites were prepared in a mini twin-screw compounder, and the effect of silica on crystallization, dispersibility and dielectric properties of the samples was investigated. Differential Scanning Calorimetry results showed that the unmodified and modified silicas acted as nucleation agents and increased the onset of the crystallization temperature of the polymeric matrix. Scanning Electron Microscopy images showed that the silica is mostly located in the PP phase matrix. For the PP/POE nanocomposites filled with unpolar silica, a higher trap density (measured by Thermally Stimulated Depolarization Current, TSDC) was found; this might be caused by the larger interfacial area due to a better dispersion of the unpolar silica in the polymeric matrix. Polar silicas introduce deeper traps than the unpolar ones, which is most likely due to the hetero-atom introduction. Nitrogen atoms were found to have the strongest effect on the charge trapping properties. According to these results, amine-modified silica is a promising candidate for PP/POE nanocomposites for HVDC cable applications.