Henrik Hillborg

Crosslinked polydimethylsiloxane exposed to oxygen plasma studied by neutron reflectometry and other surface specific techniques

Abstract Spin-coated specimens of crosslinked polydimethylsiloxane (PDMS) exposed to radio-frequency (RF) and microwave (MW) oxygen plasma were studied by specular neutron reflectometry, X-ray photoelectron spectroscopy (XPS), Wilhelmy balance, contact angle measurements, scanning electron microscopy and atomic force microscopy. Neutron reflectometry and XPS showed that the oxygen plasma led to the formation of a smooth (

Hydrophobicity recovery of polydimethylsiloxane after exposure to corona discharges

Abstract A high-temperature-vulcanized polydimethylsiloxane (PDMS) elastomer has been subjected to corona discharges for different periods of time in dry air. The loss and recovery of hydrophobicity of the surface have been characterized by contact angle measurements. Immediately after exposure to corona discharges, samples showed a low surface hydrophobicity and, on storage in dry air, a continuous increase in hydrophobicity finally approaching the hydrophobicity of the unexposed material. The activation energy of the hydrophobicity recovery was two to four times greater than the activation energy of the diffusivity of low molar mass PDMS in PDMS elastomers, indicating that the diffusivity properties of the oxidized surface layer were different from that of the bulk. PDMS elastomers quenched in liquid nitrogen or subjected to small mechanical deformation (

Hydrophobicity changes in silicone rubbers

Water repellency, high surface resistivity, vandalism resistance, low density and good processability have made silicone rubbers based on polydimethylsiloxane (PDMS) very attractive materials in housings for outdoor insulation. Their ability to recover hydrophobicity after oxida. tion or contamination is of paramount importance and this is the topic of this review. A critical evaluation of the chemical and physical mechanisms responsible for hydrophobicity loss and recovery is presented.

Graphene Oxide Filled Nanocomposite with Novel Electrical and Dielectric Properties

IO N The globally increasing demand of energy is a technical challenge for the electrical generation, transmission and distribution systems. This requires often contradictory features such as increasing voltage levels in combination with more compact designs in urban environments. This leads to an increased electric stress on the insulation systems. This can be addressed by using insulating materials with tunable non-linear conductivity, as well as high dielectric constant and low loss, for electric fi eld grading applications. [ 1 ] Current fi eld grading materials consist of polymer, semi-conducting ceramic particles such as SiC, ZnO etc., as well as lower amounts carbon black, embedded in a polymer matrix. [ 2 , 3 ] The nonlinear electrical conductivity is created by the percolated structure of these particles within the matrix, at a typically volume loading of 30% to 40%. As a result of the high fi ller level, current fi eld grading materials are limited in terms of poor mechanical properties, high weight and tendency to overheat at elevated electric fi elds. Graphene oxide (GO) is commonly prepared from natural fl ake graphite by the Hummers method and serves as an intermediate product to prepare graphene. [ 4 ] It has disrupted sp 2

Hydrophobic recovery of polydimethylsiloxane after exposure to partial discharges as a function of crosslink density

Polydimethylsiloxanes with different crosslink densities were exposed to corona discharges or GHz air plasma and studied by contact angle measurements, X-ray photoelectron spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. The degree of surface oxidation increased with increasing exposure time with a limiting depth of 100-150 nm. Surface oxidation was faster in more highly crosslinked polymers. Within the oxidised layer, a brittle, microporous silica-like layer with a minimum organic silicone content of 40% gradually developed with increasing exposure time. The strain at which the silica-like layer cracked decreased with increasing dose of corona or air plasma. The hydrophobic recovery following the corona/air plasma exposure occurred at a slow rate by diffusion of oligomers through the microporous but uncracked silica-like layer or at a much higher rate by transport of oligomers through cracks in the silica-like layer.

Diagnostic Methods for Outdoor Polymeric Insulators

Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Aging of silicone rubber under ac or dc voltages in a coastal environment

Material samples of silicone rubber with known differences in their composition, i.e. different filler content and extra silicone oil added, have been aged at the Anneberg field station on the west coast of Sweden. ac or dc voltage supplied to cylindrical samples at stress levels of 50 or 100 V/mm. The work includes laboratory examination of material changes together with on-site, visual observations and leakage current measurements. Material samplings for the laboratory tests were made after 18 months of electrical aging, which went on for one more year in order to gather further information on the long-term electrical performance of the material. The dominant aging factor was the level of the applied stress, independent of ac or dc voltage. The dc stressed samples showed a higher leakage current and exhibited larger surface degradation compared with samples exposed to ac voltage. The material parameter, an addition of extra silicone oil, initially led to an increase in adhesion of pollutants, whereas the overall performance was improved by the higher suppression of the leakage current related to oligomer diffusion. Samples with lower levels of alumina trihydrate (ATH) exhibited a delayed onset of degradation, but once damaged they degraded more rapidly than the specimens with a higher ATH content. Infrared spectroscopy showed that the ATH was completely consumed at the eroded surface regions. The aging of the surfaces was further assessed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The low molar mass siloxanes present in the pollution layer were extracted and analyzed by size exclusion chromatography and gas chromatography-mass spectroscopy. The results indicated that the main degradation factor was thermal depolymerization activated by electrical discharges. Oxidative crosslinking of the silicone rubber, usually attributed to surface close corona discharges, appeared to have played a minor role.

Effect of high aspect ratio filler on dielectric properties of polymer composites: a study on barium titanate fibers and graphene platelets

High aspect ratio fillers are predicted to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures. Using high aspect ratio fillers is a promising route for creating high dielectric constant, low loss materials at a low filler volume fraction, for use as capacitor and electric field grading materials. In this work, two high aspect ratio fillers were mixed into a polymer matrix, and the dielectric properties of composites were studied. Barium titanate fibers were synthesized by electrospinning a sol-gel, followed by a heat treatment to obtain a perovskite crystal structure. The heat treatment conditions were found to be crucial for obtaining tetragonal barium titanate fibers with high dielectric constant. Graphene platelets were prepared by a thermal shock method, which was found to result in a larger dielectric constant. A combination of barium titanate and graphene platelets yielded the highest dielectric constant when used in a polydimethyl siloxane matrix. The increase in dielectric loss over the pure matrix was small when the volume fraction was below the percolation threshold of graphene platelets. Electric flux density-electric field (D-E) measurements showed a linear dielectric constant in barium titanate filled composites and higher loss when graphene was added. The ac breakdown strength was reduced compared to the neat polymer and was affected by filler aspect ratio. The mechanisms that lead to the observed phenomena are discussed.

Dielectric breakdown strength of epoxy bimodal-polymer-brush-grafted core functionalized silica nanocomposites

The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy.

Tracking and erosion resistance of high temperature vulcanizing ATH-free silicone rubber

Traditionally aluminum trihydrate (ATH) is added to high-temperature vulcanized (HTV) silicone rubber for outdoor insulation applications to improve flame retardancy and the electrical tracking and erosion performance. In this work, the flammability of ATH-free HTV silicone rubber formulations were evaluated by the UL94 vertical burning test. The most promising candidates, as well as reference formulations filled with silica or silica/ATH only, were then evaluated by the inclined plane tracking and erosion test at 4.5 kV. The best result was obtained by silicone rubber containing 100 phr silica and 15 phr melamine cyanurate. It was suggested that this was a result of the arc quenching ability of the melamine hence extinguishing the electrical arc. If the filler level was too high, resulting in poor dispersion, the resistance to tracking and erosion was significantly reduced. Thus formulating a HTV silicone rubber with good tracking and erosion resistance is a balance between having a high enough filler content, while still maintaining a good dispersion of the particles in the polymer matrix, as well as good bonding between the particles and the matrix.