Yingyan Liu

Investigation on permeation properties of liquids into HTV silicone rubber materials

In this paper, the permeation properties of three types of liquids into HTV silicone rubber with different Alumina Tri-hydrate (ATH) contents had been investigated by weight gain experiments. The influence of differing exposure conditions on the diffusion into silicone rubber, in particular the effect of solution type, solution concentration, and test temperature were explored. Experimental results indicated that the liquids permeation into silicone rubber obeyed anomalous diffusion ways instead of the Fick diffusion model. Moreover, higher temperature would accelerate the permeation process, and silicone rubber with higher ATH content absorbed more liquids than that with lower ATH content. Furthermore, the material properties of silicone rubber before and after liquid permeation were examined using Fourier infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM), respectively. The permeation mechanisms and process were discussed in depth by combining the weight gain experiment results and the material properties analyses.

Aging and self-healing properties of superhydrophobic silicone rubber

Silicone rubber is widely used for outdoor insulation because of its hydrophobicity. Meanwhile, superhydrophobic surfaces with self-cleaning property are ideal as outdoor insulating materials. An efficient technique to fabricate superhydrophobic silicone rubber with a contact angle of 151° and a sliding angle of 5° is reported. Long-term stability of polymeric superhydrophobic insulating surfaces was examined under electrical and non-electrical stresses. Results show that silicone rubber transforms from superhydrophobic to superhydrophilic after corona aging. Superhydrophobic characteristic is fully recovered through a self-healing process. After water immersion, it results in large contract angle hysteresis, although the static contact angle on the surface is large. The increase in surface free energy caused by aging for self-healing superhydrophobic material affects the stability of superhydrophobic properties. Once the hydrophobicity decreases and Wenzel contact mode is formed, the sliding angle and contact angle hysteresis are difficult to recover because of a large energy barrier between the Wenzel-Cassie transitions.

A capacitance study of anomalous diffusion of water into HTV silicone rubber materials

Capacitance and gravimetric measurements are used to study the diffusion of water into high temperature vulcanized (HTV) silicone rubber. Silicone rubber with different Alumina Tri-hydrate (ATH) filler contents was used in this research, and deviations from the classical Fickian diffusion were observed. A model is presented for the calculation of the diffusion coefficient of water in silicone rubber by capacitance method in which the anomalous diffusion mechanism and the water concentration profile are considered. The present model is compared to other models and the difference between the diffusion coefficients obtained is analyzed.

Cleaning effect of rainfall on salt in pollution layer of silicone rubber insulators

Electric properties of insulators are significantly affected by the amount of salt in pollution layer. In this paper, cleaning effect of rainfall on salt in pollution of silicone rubber insulators was studied under artificial rainfall. Factors including duration and intensity of rainfall, hydrophobicity of pollution layer as well as discharge on surface of pollution were studied with artificially and naturally polluted insulators. The obtained results indicate that: factors mentioned above all had significant impact on cleaning effect. Hydrophobicity and surface discharge could weaken cleaning effect of rainfall. With cleaning duration increasing, residual salt would fall down to a stable percent which was related to intensity of rainfall. A modified diffusion model based on Fick diffusion law could be used to characterize the salts cleaning process of naturally polluted silicone rubber insulators under rainfall.

Influence of AC corona discharge on contamination layer of composite insulator surface

Pollution layer on insulator surface may change under corona discharge during the long-term service of insulators, which would influence the performance of contaminated insulators. In this paper, AC corona discharge experiments are studied by utilizing a 31-needle-to-plate electrode system. The Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) are used to study the characteristic variation of pollution after corona process. It was concluded that hydrophobicity, microstructure and chemical composition of pollution obviously changed after corona process.

Effect of alumina tri-hydrate surface modification on liquid permeation and electrical performance of silicone rubber

This paper investigates the liquid permeation performance of silicone rubber containing alumina tri-hydrate (ATH) that has been modified with N-[3-(trimethoxysilyl) propyl] aniline (Y-9669). From the change in weight during immersion in water, saline solution or nitric acid, and during subsequent drying in air as well as a second immersion test, it is revealed that the majority of the absorbed liquid exists in filler and interface for silicone rubber with original ATH. Surface modification of ATH filler can protect silicone rubber from liquid permeation. It is easier for liquid to permeate into silicone rubber than to escape from it due to the hydrophobicity of the silicone rubber matrix. A cycle of immersion and drying is found to have little impact on subsequent immersion in the case of water and saline solutions; however, damage inflicted on the matrix and filler by nitric acid means that the rate of permeation increases during subsequent immersion. The volume resistivity and surface resistivity of samples with unmodified ATH also decrease more appreciably than those with modified ATH after a cycle of immersion and air drying due to the amount of the residual absorbed liquid.

Preparation and basic properties of superhydrophobic silicone rubber with micro-nano hierarchical structures formed by picosecond laser-ablated template

Hydrophobicity is the most important property of silicone rubber for outdoor isolation. Controlling wettability, along with interface and adhesion studies, has been widely investigated for technological applications, such as self-cleaning surfaces, by modifying the geometric structure and chemical composition of surfaces. Superhydrophobic surface with self-cleaning and anti-icing properties is ideal for outdoor insulation material. This paper reports on an effective strategy to modify the surface structure of silicone rubber. Microcraters with nanotexture can be fabricated on a mold steel using high-power picosecond laser incorporated with a high-speed scanning mirror. This template is used in direct replication process in which the surface morphology of silicone rubber can be changed during vulcanization. Two accessible methods are used to implement the modification process. Single-point pulse method is used to prepare regular papilla and abundant nanotexture on open architecture-type superhydrophobic surface, with contact angle reaching 151.5°±1.7°. Line scanning method is used to prepare connected papillae on enclosed-type surface, forming the secondary structure. In this case, nanotexture is richer and has a smaller scale (100 nm) compared with open architecture-type surface. Moreover, the contact angle of the enclosed-type superhydrophobic surface is 150.3°±0.9°. The contact model of water on superhydrophobic surfaces follows the Wenzel and Cassie coexisting model as shown by the air pockets, which are found experimentally and by calculation. Sliding angle of superhydrophobic surfaces reaches 5°, which indicates that this insulating surface possesses good self-cleaning properties. Repeated and large-area fabrication of superhydrophobic silicone rubber can be achieved by the proposed methods. These methods can be performed for large-scale application.

A modified 5000 h test procedure for silicone rubber insulator based on contamination and hydrophobicity change simulation

A large number of silicone rubber (SR) insulators have been used in inland areas. Long-term performance evaluation, pre-ranking and selection of these insulators are highly significant. IEC and other institutes have proposed several 5000 h tests with multiple stresses to evaluate long-term performances of composite insulators. However, these procedures have their own characteristics and limitations. None of the existing tests could provide a satisfactory simulation of the condition with severe contamination and high non-soluble deposit density (NSDD) in inland areas as well as the hydrophobicity change process of SR insulators. Based on modifications of the IEC test procedure, a novel 5000 h test procedure is proposed. There are two main modifications. The first one aims to replace the salt fog with a mixed contamination fog including both soluble and non-soluble pollutants. The second one aims to provide sufficient time for SR to recover and to transfer its hydrophobicity. A set of test apparatus based on this novel 5000h test procedure was designed and constructed. Two tests were conducted on 16 SR insulators with different material compositions and shed designs, including an AC and a negative DC test. Contamination accumulation on the surfaces of the specimens during both tests were measured. The NSDD and equivalent salt deposit density (ESDD) were close to those measured in the field in contaminated inland areas. Hydrophobicity reduction and recovery were well reproduced. Discharge activities were observed and leakage current was measured, which were consistent with the hydrophobicity statuses of the specimen. Tracking and erosion were found for some specimens, and the damage pattern was the same with that found in the field. The poor material properties and inappropriate shed designs could be identified successfully through the test. It was concluded that the modified 5000 h procedure could provide a satisfactory simulation of the desired contamination condition and hydrophobicity change process, and the insulators with different design defects are successfully screened out.

Effects of corona discharge on the properties of fiber reinforced plastics used in composite insulator

Fiber reinforced plastics (FRP) have been widely used as composite insulator inner rod material due to its excellent mechanical properties. In practice, composite insulator suffers from many kinds of environmental, electrical and mechanical stresses, such as discharge, pollution and liquids ingress. Under normal circumstances, the FRP rod is covered with the silicone rubber sheath, so there is no discharge happened on the surface of FRP rod. However, when the silicone rubber sheath is broken or the adhesion of the interface between the silicone rubber sheath and FRP rod is failed, some discharge will happen on the surface of FRP rod under operating voltage, which would lead to degradation and aging phenomena of FRP rod. In this research, the corona discharge is employed to simulate the discharge phenomena on the surface of FRP rod and investigate the influence of discharge on the properties of FRP rod.

Investigations on the basic electrical properties of Polyurethane foam material

The Polyurethane (PU) foam materials have increasingly been used as a new kind of solid insulation material, which are always inserted into post hollow composite insulators as their inner insulation section. PU foam with PU as a base material which is homogenously filled with pores, PU foam material is a possible substitute material of the air, SF6 and N2. In order to achieve a better electrical performance, the electrical properties of PU foam should firstly been studied and understood in depth. In this paper, the electrical strength, volume resistivity, permittivity and partial discharge property of PU foam have been investigated. It is the porous structure characteristic with low density that distinguishes itself from other dielectric materials. Because of the pores distributed in the PU material, the partial discharge (PD) characteristics have been put more emphasis. Four kinds of PU foam materials with different pore size and pore geometry and one kind of PU elastomer have been used in our research. The micro-pore structure of PU foam was characterized by scanning electron microscopy (SEM) method. For PD property, we mainly focus on the PD inception voltage, PD extinction voltage, discharge magnitude with applied voltage as well as its relationship with physical structure of pores. Experimental results indicated that the electrical properties of PU materials have a closely relationship with pore structure, especially for the electrical strength and PD characteristics. The electrical properties with stress on the PD behavior of PU foam were discussed in depth by combing the electrical test result and the physical property of PU foam.