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Micro Structural and Electrical properties of Liquid Silicone Rubber Used for External Insulation
This study examines the function of Al 2 O 3 nanofiller in liquid silicone rubber (LSR) for the purpose of outdoor insulation. Several characteristics, including structural (Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), physical (density), mechanical (tensile, tear, percentage elongation at break, and shore A hardness), thermal (thermal conductivity, thermo gravimetric analysis (TGA)), electrical (dielectric constant, dielectric breakdown strength, DC volume resistivity, arc resistance, and recovery of hydrophobicity), although chemical (immersion of samples for 72 h in NaCl, HCl, and H 2 SO 4 solutions and measurement of sample weight and PH of the solution before and after immersion) were investigated after the addition of Al 2 O 3 nanofiller with different compositions, such as 5, 10, and 15 weight percentages. In comparison to pure LSR, it was discovered that the inclusion of nano Al 2 O 3 particles enhanced the material’s density by 11%, thermal conductivity by 12%, dielectric constant by 24%, DC volume resistivity by 2%, arc resistance by significantly 1205%, dielectric breakdown strength by 54%, and hydrophobicity. Other improvements include a 7% rise in tensile strength, a 23% rise in tear strength, a 17% rise in hardness, and an 18% reduction in percentage elongation at break when compared to pure LSR for the composite samples. Mechanical qualities are affected by the curing process duration. More specifically, we discovered there is a proportionality relationship between the tensile strength and the elastic modulus. Findings of the chemical examination revealed that the composites have corrosion resistance in the form of negligible weight change in the sample when exposed to salt and acid solutions. The combination of these exceptional qualities makes Al 2 O 3 composites appealing within an insulating materials realm.
In recent years, some hollow core composite insulators, of which housings were made of liquid silicone rubber (LSR) of current and voltage transformers used in China Southern Power Grid have showed different levels of degradation and cracking phenomena. Based on SEM, FTIR, XPS and XRD methods, analyses have been conducted from several aspects such as organic functional groups concentration, microscopic appearance, element valence etc. It is found that organic groups (mainly Si-(CH3)2) on the side-chains of LSR are much more vulnerable than the backbone during the degradation process, thus Si-O/Si-C absorption peak ratio could be applied to characterize the degradation level of LSR. For severely aged LSR samples, the ratio is around 1.787-2.436. The valences of Si element and Si-O, Si-C functional group relative concentration of the aged LSR surface have changed and crystal structure is formed. Macro properties tests show that the surface of aged LSR is almost hydrophilic and tracking and erosion resistance is deteriorated remarkably. It is concluded that the crystallization caused by thermal/optical oxidation over crosslinking reaction is the main reason why degradation occurred.
Purpose
Diverse commercial implantable medical devices were developed for the convenience and life-quality of patients, and tether-free diagnostics and therapeutics. Those devices need implantable cable, which connects each part of their devices for transcutaneous energy or signal transfer. For prolonged implantation into human body, it should be safe, robust, and be long-term operable without failure. In this paper, we introduce an implantable PDMS-coated cable.
Methods
By using PDMS as an encapsulation material, we developed a biocompatible, flexible and durable cable. Several tests were carried out for the evaluation of cable performance. Leakage test confirmed that coating using PDMS was sufficient to prevent the invasion of body fluid to conducting wire. Tensile test, torsion-durability test and bending-durability test demonstrated its mechanical durability and robustness to motion. The resistance variation recorded in the durability test was monitored to verify the electrical stability during movement of cable. Experiments of subcutaneous tissue implantation for 8 weeks were performed to observe the degree of biocompatibility as an implantable cable.
Results & conclusions
Our cable was mechanically stable and biocompatible enough to be used for long term implantable medical devices.
Rapid crosslinking of silicone chains on the nanosilica surface during arc discharge is proposed as an efficient strategy to improve the tracking and erosion resistance of silicone rubber. Urethane-containing silane (URS) was synthesized by using (3-isocyanatopropyl) triethoxysilane, isopropyl alcohol and allyl alcohol via nucleophilic addition and trans-etherification. The suppression effect and mechanism of URS on the tracking and erosion of addition-cure liquid silicone rubber/silica (ALSR/SiO2) nanocomposite were investigated. It was found that URS could significantly enhance the tracking and erosion resistance of ALSR/SiO2. When only 2.5 phr URS was added, all the ALSR/SiO2/URS specimens passed the inclined plane test at 4.5 kV and the erosion rate decreased from 3.44% to 0.25%. This might be because URS tightly constrained silicone chains on the SiO2 surface by strong hydrogen-bonding interactions. During arc discharge, URS at the interphase enhanced the catalytic activity of platinum compound to promote the radical crosslinking of silicone chains absorbed on the SiO2 surface. Hence, a compact and intact ceramic layer was quickly formed on the ALSR surface and protected the underlying materials from further arc ablation. Our finding provides a new method and theoretical basis for the design, preparation and application of the novel high-performance polymer insulating material.
Assessment of the material degradation in laboratory similar to field conditions is still a challenge. In the present work an attempt is made to study the effect of environment conditions in service life of polymeric insulators on tracking and erosion resistance. Evaluation of tracking and erosion resistance of Silicone Rubber insulating samples of high temperature vulcanized (HTV) rubber and liquid silicone rubber (LSR) are carried out under AC and positive polarity DC voltage. The samples are treated under normal contaminants as per IEC 60587 and acidic contaminant solution simulating acid rain. The conditions of pollution/ contamination severity level are taken into consideration by maintaining the conductivity approximately 2.5 mS/cm for both solution to observe the effect of acidity alone on the samples, the acidity level is varied to understand the material behavior under acidic case with AC and DC stress application. A new experimental arrangement is set up based on IEC 60587 and ASTM D-2303 standards. The constant voltage method is employed to find the relative tracking performance of silicone rubber samples for different chemical composition. Physico-chemical analysis involving SEM, EDAX and FTIR is carried out on treated samples to understand the surface morphology and chemical changes. A comparative analysis is carried out for the acid rain solution and the standard solution of NH
4
Cl. Further leakage current performance and analysis is carried under both AC and DC stress.
Silica is the most widely-used filler to reinforce liquid silicone rubber (LSR), but the high viscosity of LSR/silica suspension significantly limits its processing flexibility. To balance the processibility and reinforcing efficiency of LSR/silica systems, two kinds of enols (propenol and 1-undecylenyl alcohol) and a saturated alcohol (1-undecylic alcohol) were employed to modify the silica surface. Various rheological tests were carried out to investigate the processibility as well as filler networking and crosslinking processes of the modified systems. Tensile tests were also adopted to verify the reinforcing effect. It was found that surface modification of silica by 1-undecylenyl alcohol could significantly reduce the viscosity of its suspension with LSR. Meanwhile, the mechanical strength of LSR could be largely enhanced by six times with 10 wt % modified silica. This work will merit design and production of LSR materials with balanced processibility and mechanical performance. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45544.
Ureido-modified MQ silicone resin (DIPUPES-MQ) was prepared by the hydrolytic condensation of tetraethoxysilane (TEOS), (γ-diisopropylureidopropyl) triethoxysilane (DIPUPES), hexamethyldisiloxane and divinyltetramethyldisiloxane. Subsequently, DIPUPES-MQ was employed to improve the resistance to tracking and erosion of addition-cure liquid silicone rubber (ALSR), and it showed that DIPUPES-MQ could significantly enhance the tracking and erosion resistance as well as the mechanical properties of the ALSR. When the content of DIPUPES-MQ was 3.75 phr, all the test samples passed the inclined plane tracking and erosion (IP) test at 4.5 kV. The thermogravimetry (TG), thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR) and FTIR results showed that the striking enhancement in the tracking and erosion resistance of the ALSR was probably ascribed to the decomposition of the ureido groups. In the pyrolysis process, the ureido groups of DIPUPES-MQ could generate isocyanic acid to hinder the “unzipping” depolymerization through reacting with Si-OH groups, so that the amount of cyclic dimethylsiloxanes inducing intense and continuous dry-band arcs was decreased.
Graphene oxide (GO) was heat treated at different temperatures between 120 and 220 °C and the structural changes were assessed by thermogravimetry, infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy (SEM). The resulting reduced graphene oxide (rGO) fillers showed markedly lower oxygen contents (primarily by reduction of epoxide and hydroxyl groups) than GO. SEM of silicone rubber composites containing 3 wt.% rGO or GO filler showed that the nanoparticles were uniformly distributed in the polymer. The rGO-filled composites exhibited electric field-dependent resistivity; the resistivity decreased from 1014 to 1011 ohm m as the electric field was increased from 0.2 to 6 kV (mm)-1. The composites exhibited an increased resistivity after being exposed to a combined thermal cycling and electrical field. An increase in the resistivity of samples aged at 120 °C for more than 17 h was observed; the resistivity-electric field behavior and the dielectric constant of the aged composite resembled that of GO-filled composite. The composites exhibited dielectric constant values between 4.0 and 5.2 and a low tan delta (≤ 0.015) at frequencies between 10-2 and 104 Hz. The results suggest that the resistivity of the composites can be tuned by adjusting the degree of reduction of GO. The low rGO-filler content that was required to achieve this adequate property profile is attractive, which makes these composites potentially useful as electric field-grading material in HVDC cable accessories. However, this requires that the long-term stability problem can be sensible addressed.
In the recent decades, the phenomena of space charge accumulation in the high voltage direct current (HVDC) insulation have been attracted more attention. In this paper, low density polyethylene (LDPE) nanocomposites filled with alumina nanoparticles (nano-Al2O3) were prepared employing melting blend method. Morphologies of nanoparticles and LDPE/Al2O3 nanocomposites were performed by scanning electron microscopy (SEM). Electrical properties of the LDPE nanocomposites were also investigated. Results shown that the nano-Al2O3 particles modified with vinyl silane coupling can effectively enhance the breakdown strength of LDPE nanocomposites. With the nano-Al2O3 particles loading, the volume resistivity of the LDPE nanocomposites was increased, while dielectric permittivity of the nanocomposites was decreased. Space charge of the LDPE nanocomposites was measured by pulsed electro-acoustic (PEA) method. The charge profiles indicated that space charge suppression of the LDPE nanocomposites was better than that of pure LDPE. The excellent insulation properties of the LDPE nanocomposites were attributed to the better interfacial adhesion between the surface-treated nano-Al2O3 particles and the LDPE matrix.
Silicone rubber filled with thermally conductive alumina is fabricated as a class of thermal interface materials in this work. The thermal conductivity of the prepared alumina/silicone rubber composite is measured as a function of alumina loading. The effects of alumina filler with different phases and morphologies on the thermal conductivity of the composite are investigated by comparative method. When the filler loading is low, the composite filled with porous irregular-shaped α-alumina exhibits a higher thermal conductivity than that filled with γ-Al2O3 and spherical α-Al2O3. In order to achieve a high loading, spherical α-Al2O3 has the most pronounced effect due to its intrinsic high thermal conductivity and unique morphology for homogeneous dispersion in the polymer matrix, which is superior to irregular-shaped α- and γ-Al2O3. Our results demonstrate that the composite filled with spherical alumina by the mass concentration of 82 % has six times thermal conductivity higher than pure silicone rubber. Thermogravimetric analysis studies exhibit that the thermal stability of the composite distinctly increases with filler loadings. The obtained data were compared with theoretical equations in the literatures that are used to predict the properties of two-phase mixtures.
In the current study, the effects of loading level of micro or nano size BN particles on the thermal, mechanical, and morphological properties of silicone rubber are investigated. Three micron size and two nano size BNs with different particle sizes and shapes are used. All five types of BNs are found to be well-dispersed in silicone rubber matrix despite some local agglomerates. In general, the addition of BN particles in silicone matrix decreases the tensile strength and strain at break, coefficient of thermal expansion (CTE) values, on the other hand increases modulus, hardness and thermal conductivity. Nano size fillers have more pronounced effect on tensile properties of composites in comparison to micron size BNs at any given loading level. The aspect ratio of the filler is found to be very effective in achieving high thermal conductivity in composite systems. Dielectric constants of composites vary between dielectric constant of silicone and BN.
Liquid silicone rubber (LSR) has increasingly been used as one kind of inner insulating material. Studies of space charge and its effect during pre-stressing and breakdown (BD) process in LSR were carried out. DC electrical breakdown field (BDF) of LSR specimens before and after DC electrical pre-stressing was measured. The space charge characteristics of LSR specimens under different DC electrical pre-stressing fields and that during the BD processes were measured by a pulsed electro-acoustic (PEA) method. It was found that the amount of homo space charges near electrodes increases obviously as a rising electric field is applied. Whether the polarity of BDF is the same as that of the pre-stressing field or not, the DC BDF of pre-stressed specimens is higher than that of specimens without pre-stressing. However, the pre-stressing does not show monotone and saturation effects, as generally occurs in the literature. In hetero BD experiments for example, with an increasing pre-stressing field, the DC BDF first increases up to 14.5% at the pre-stressing field of 25 kV/mm and then decreases. The influence of pre-stressing on the BD processes in LSR are discussed. The relationship between space charge and the DC BDF was related to the microstructure of nano-silica and LSR molecules.
Graphene oxide filled polymer nanocomposites are found to possess excellent nonlinear electrical conductivity, low conductivity at low field and a good combination of increased dielectric constant and low loss factor. Those outstanding electrical properties are achieved at filler loading of 3% and controllable by adjusting the oxidation state of graphene oxide.