Influence of Interface on the Properties of Silicone Nanocomposites

Abstract

The increase in voltage levels and electrical stresses of the electrical equipment have resulted in demands for electrical insulations that have high breakdown strength, low losses, high thermal conductivity, and high mechanical performance. Use of dielectric polymer nanocomposites is a promising approach as nanocomposites have superior properties over traditional materials. The improvements of electrical, mechanical and thermal properties are related to the plurality of interfaces introduced with nanoparticles as fillers. Addition of fillers to the polymer materials not only enhance their performance but also reduce the cost. In this thesis, influence of interface on the electrical properties of silicone nanocomposites has been investigated. Poor interaction between nanoparticles and base polymer, and particle agglomerations limit the performances and applications of nanocomposite materials unless the fillers are dispersed and distributed uniformly. Base polymers, nanofillers, surface treatment of the fillers, filler concentrations, and dispersion techniques are the main factors that determine dispersion status and overall properties of the composites. In this study, filler loading levels, surface treatment of nanofillers and different processing techniques are investigated. Incorporation of treated and untreated nano-alumina into RTV 615 silicone rubber with different weight percentages of nanofillers 5wt %, 7.5wt% 10wt%, and 20wt% has been investigated. Surface treatment are used to change the hydrophilic surface of nano-alumina to hydrophobic and to enhance the dispersion. Electrostatic disperser (ES) and high shear (HS) methods have been used to obtain further improvements in the properties of the composites. Dielectric spectroscopy, thermographic analysis (TGA), laser erosion tests, and mechanical performance evaluations are used to study the effect of the interface on the electrical, mechanical and thermal properties of nanocomposites. Results obtained with the above techniques, and scanning electron microscopy (SEM) images have demonstrated that treated nano-alumina prepared by high shear (HS) and electrostatic disperser (ES) have a better filler dispersion and distribution than untreated nano-alumina. Also treated nano-alumina composites showed less dielectric loss and high erosion resistance than those composites with untreated nano-alumina. Results further confirm that particle dispersion using ES mixing is better than that using HS mixing. Filler polymer interactions and dispersion can be analyzed by using dielectric spectroscopy, in specific using the frequency responses below 0.01 Hz. The low frequency spectra can reveal the interfacial polarization, hence the reflected permittivity. The present result indicate that treated nano-alumina filled silicon rubber has lower permittivity than untreated nano-alumina at very low frequency because of the restriction of polymer chain mobility. In addition, untreated fillers being hydrophilic can absorb moisture and have resulted in composites that show high permittivity values particularly at frequencies below 0.001Hz.