Silica surface-modification for tailoring the charge trapping properties of PP/POE based dielectric nanocomposites for HVDC cable application

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.publishedVersionPeer reviewe

PP/PP-HI/silica nanocomposites for HVDC cable insulation: Are silica clusters beneficial for space charge accumulation?

New potential High Voltage Direct Current (HVDC) cable insulation materials based on nanocomposites are developed in this study. The nanocomposites are produced by blending of polypropylene (PP), propylene-ethylene copolymer (PP–HI) and a modified fumed silica (A-silica) in a concentration of 1 and 2 wt %. The A-silica is successfully modified with (3-aminopropyl)triethoxysilane (APTES) via a solvent-free method, as proven by infrared spectroscopy, thermogravimetry and transmission electron microscope mapping. A-silica in the polymer matrix acts as a nucleating agent resulting in an increase of the crystallization temperature of the polymers and a smaller crystal size. Moreover, the silica addition modified the crystals morphology of the unfilled PP/PP-HI blend. The composite containing A-silica with 2 wt% contains bigger-size silica clusters than the composite filled with 1 wt%. The composite with the higher A-silica concentration shows lower space charge accumulation and a lower charge current value. Besides, much deeper traps and lower trap density are observed in the composite with 2 wt% A-silica addition compared to the one with a lower concentration. Surprisingly, the presence of silica clusters with dimensions of more than 200 nm exhibit a positive effect on reducing the space charge accumulation. However, the real cause of this improvement might be due to change of the electron distribution stemming from the amine-amine hydrogen bond formation, or the change of the chain mobility due to the presence of occluded polymer macromolecules constrained inside the high structure silica clusters. Both phenomena may lead to a higher energetic barrier of charge de-trapping, thus increasing the depth of the charge traps

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A systematic study of the influence of surface modification of nanosilica with coupling agents having different polarities on the dielectric properties of a PP/POE/silica blend was performed. The main goal of this investigation was to tailor the chemical composition of the silica surface, in order to modify 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. The results of the thermogravimetric analysis (TGA) indicate 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 significant differences in charge trapping properties of PP/POE composites filled with differently modified silicas. Polar functional groups attached to the surface of the silica appear to have a strong effect on the charge trapping properties: The trap depth distribution becomes deeper and the trap density decreases to significantly lower levels. All results show that incorporation of surface modified nanosilica into a PP/POE matrix is a promising approach to tailor its dielectric properties. Further development of these composites may lead to benefits for application in high-voltage cable and capacitor applications. Keywords: silica surface modification, dry silanization, charge trapping, PP, POE, TSDC, high voltage cable

New Structure Proposal for Silane Modified Silica

The use of the silica / silane system inside a tire tread compound enables a significant improvement in the rolling resistance and the wet traction of the tire. In order to improve the performance of such a tread compound further, a deep understanding of the coupling mechanism of the silica to the polymer is essential. In this paper a new proposal for the picture of a modified silica surface is presented to understand this coupling of a silica via a silane towards the polymer in a tire tread compound in a better way

Neuer Strukturvorschlag für Silan-modifizierte Kieselsäure

The use of the silica / silane system inside a tire tread compound enables a significant improvement in the rolling resistance and the wet traction of the tire. In order to improve the performance of such a tread compound further, a deep understanding of the coupling mechanism of the silica to the polymer is essential. In this paper a new proposal for the coupling reaction between silica and silane as well as for the chemical covering of the silica surface is presented

Solution Modified Fumed Silica and Its Effect on Charge Trapping Behavior of PP/POE/Silica Nanodielectrics

Various dielectric nanocomposite materials are studied in the frame of the European Commission funded project GRIDABLE. This project has the aim to develop DC cable extruded insulation and medium and low voltage DC capacitor films exhibiting enhanced performance with respect to presently used materials. The nanocomposites intended for cable applications are polypropylene (PP)/polyolefin elastomer (POE) blends filled with surface modified nano-silica particles. The surface modification is carried out via the state-of-the-art solution method using a polar silane as the modifying agent. Thermally Stimulated Depolarization Current (TSDC) measurements were carried out in order to study the charge trapping behavior of the nanocomposite samples. TSDC results indicate that the addition of the treated nano-silica, for most cases, reduces the density of the deep traps significantly. The effect of the addition of silica nanoparticles - both modified and unmodified - on the crystallinity of the samples was studied using X-ray Diffraction (XRD). This is important as the charge trapping properties of the nanodielectrics can be affected by the degree of crystallinity. While more detailed studies are necessary, these results imply that the depth and the density of the deep trap states is profoundly influenced by the level of the silica modification i.e. the amount of the grafted silane on the silica surface.publishedVersionPeer reviewe

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

A systematic study of the influence of surface modification of nanosilica with coupling agents having different polarities on the dielectric properties of a PP/POE/silica blend was performed. The main goal of this investigation was to tailor the chemical composition of the silica surface, in order to modify 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. The results of the thermogravimetric analysis (TGA) indicate 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 significant differences in charge trapping properties of PP/POE composites filled with differently modified silicas. Polar functional groups attached to the surface of the silica appear to have a strong effect on the charge trapping properties: The trap depth distribution becomes deeper and the trap density decreases to significantly lower levels. All results show that incorporation of surface modified nanosilica into a PP/POE matrix is a promising approach to tailor its dielectric properties. Further development of these composites may lead to benefits for application in high-voltage cable and capacitor applications. Keywords: silica surface modification, dry silanization, charge trapping, PP, POE, TSDC, high voltage cable.publishedVersionPeer reviewe