The dielectric properties of epoxy resin mainly depend on its microstructures, especially the curing degree. In this article, the reaction process of diglycidyl ether of bisphenol A (DGEBA)/methyl hexahydrophthalic anhydride (MHHPA) system is studied by differential scanning calorimetry, and samples with different curing degrees are prepared by regulating the curing temperature and time. The evolution of microstructure during epoxy resin crosslinking is simulated by molecular dynamics simulation. It is found that the effect of the curing degree on dielectric properties can be attributed to the distribution of polar groups and the mobility of chain segments, while the contribution of free volume is insignificant. With the increase of the curing degree, the epoxy group at the chain end is transformed into an ester group with weaker polarity, as well as the epoxy molecular segments are combined more closely and the mobility of molecules decreases, resulting in an improvement in the dielectric properties of the epoxy resin. 

Curing Degree Dependence of Dielectric Properties of Bisphenol-A-Based Epoxy Resin Cured With Methyl Hexahydrophthalic Anhydride

The dielectric properties of epoxy resin mainly depend on its microstructures, especially the curing degree. In this article, the reaction process of diglycidyl ether of bisphenol A (DGEBA)/methyl hexahydrophthalic anhydride (MHHPA) system is studied by differential scanning calorimetry, and samples with different curing degrees are prepared by regulating the curing temperature and time. The evolution of microstructure during epoxy resin crosslinking is simulated by molecular dynamics simulation. It is found that the effect of the curing degree on dielectric properties can be attributed to the distribution of polar groups and the mobility of chain segments, while the contribution of free volume is insignificant. With the increase of the curing degree, the epoxy group at the chain end is transformed into an ester group with weaker polarity, as well as the epoxy molecular segments are combined more closely and the mobility of molecules decreases, resulting in an improvement in the dielectric properties of the epoxy resin.

Fluorine/silicone composite rubber is widely used as a sealing material in aerospace, missile, automotive, petroleum, and other industries, but the traditional process does not use synergistic fillers to strengthen the composite system. In this research, fumed SiO2 and black caron (N990) were used as synergistic fillers, fluorine/silicone composite rubber was prepared by mechanical mixing process, and three different fluorine rubber systems were used to find the best composite material. The mechanical properties, thermal properties, aging properties, moderate strength properties, and microstructure of the composites were evaluated. Studies have shown that mixing the two can produce a certain interface interaction and effectively improve the compatibility. The physical properties of the material tended to decrease during the increase in the added amount of silicone rubber (MVQ). The maximum tensile strength of the hybrid material can reach 15 MPa. The optimal mixing ratio is fluororubber/silicone rubber (FKM/MVQ) = 9/1. At this time, the mechanical properties of the composite material are in the best state, and SiO2 and black caron (N990) have a reinforcing effect, which can effectively improve the mechanical properties. After the composite was kept at 200 °C for 48 h, the tensile strength and elongation of the best sample A1 were 99.5 and 97.0%, respectively, showing excellent anti-aging properties. This work provides a method to fabricate high-strength fluorine/silicone composites using synergistic fillers that may be used in heat-medium-sealed environments.

https://pubs.acs.org/doi/pdf/10.1021/acsomega.2c06489

Preparation of High-Strength and Excellent Compatibility Fluorine/Silicone Rubber Composites under the Synergistic Effect of Fillers

Influence mechanism of side group types on the macroscopic electrical properties of silicone rubber (SiR) are analyzed based on the experimental comparison and the molecular simulation. The insulation properties of SiR with different side group types (vinyl, phenyl, and trifluoropropyl), as well as binary compound SiR (vinyl/phenyl and vinyl/trifluoropropyl) are compared and studied experimentally. Further, the effect of side groups on the movement of the molecular chain is analyzed by calculating the free volume, the mean square displacement (MSD), and the end distances. Then the mechanism of its influence on the macroscopic properties of SiR was studied. The experimental results indicated that the resistance and breakdown performance of the phenyl SiR surpasses both vinyl SiR and fluorosilicone rubber owing to the conjugated structure of benzene ring, which has a high electron affinity and reduces free charges and carrier migration rate. Besides, when 10wt% phenyl SiR was mixed into vinyl SiR, the resistivity increased by one order of magnitude, and the breakdown field strength increased by about 8%. In the aspect of dielectric properties, the fluorosilicone rubber is more qualified than the vinyl SiR and the phenyl SiR due to the addition of trifluoropropyl as a polar group to the non-polar dielectric SiR to boost its dielectric constant and dielectric loss. The molecular simulation results indicated that the SiR containing phenyl has a tiny free volume and weak molecular chain movement, the fluorosilicone rubber has the strongest molecular chain movement. The work has important guiding significance for improving and modifying SiR in power equipment.

Influence of Molecular Chain Side Group on the Electrical Properties of Silicone Rubber and Mechanism Analysis

Polymeric insulators have lightweight, excellent hydrophobicity and convenient transportation and installation. They are widely used in the external insulation for distribution and transmission lines. However, due to the long-term effects of pollution, ultraviolet radiation, discharge, temperature, humidity, altitude and other natural and complex environmental and service factors, the silicone rubber and other materials of polymeric insulators gradually age and lose their hydrophobicity and electrical insulation characteristics. The operability is significantly reduced, which seriously affects the safety and reliability of the power system. Hence, there is a need for assessing and evaluating the long-term aging and degradation of polymeric insulators under various operating conditions and environments. In this review, the various aging and characterization techniques of the polymeric insulators and their aging performance under the action of multiple factors are discussed. To enhance the performance of polymeric insulators, nano-coating, surface treatment and other techniques are also indicated. In addition, future potential fields that should be explored from a high-voltage electrical insulation perspective are also presented.

https://www.mdpi.com/1996-1073/15/23/8809/pdf?version=1669129858

Aging of Polymeric Insulators under Various Conditions and Environments: Another Look

Silicone rubber is a promising insulating material that has been performing well for different insulating and dielectric applications. However, in outdoor applications, environmental stresses cause structural and surface degradations that diminish its insulating properties. This effect of degradation can be reduced with the addition of a suitable filler to the polymer chains. For the investigation of structural changes and hydrophobicity four different systems were fabricated, including neat silicone rubber, a micro composite (with 15% micro-silica filler), and nanocomposites (with 2.5% and 5% nanosilica filler) by subjecting them to various hydrothermal conditions. In general, remarkable results were obtained by the addition of fillers. However, nanocomposites showed the best resistance against the applied stresses. In comparison to neat silicone rubber, the stability of the structure and hydrophobic behavior was better for micro-silica, which was further enhanced in the case of nanocomposites. The inclusion of 5% nanosilica showed the best results before and after applying aging conditions.

Investigation of Hydrothermally Stressed Silicone Rubber/Silica Micro and Nanocomposite for the Coating High Voltage Insulation Applications

The scope of silicone rubber (SiR) is confined due to the deprivation of its dielectric propertiesupon exposure to various ambient stresses. The aim of this research is to develop silicone rubber-based composites by employing inorganic oxide fillers for improved dielectric and high voltage insulation properties for widening its scope in the field of electrical appliances. This study reports the preparation of different composites of silicone rubber with varying concentrations of micro and nano-silica fillers. The dielectric propertytrends of these as-prepared neat and impregnated samples were examined via an indigenously developed weathering chamber capable of applying multiple stresses of acid rain, heat, humidity, UVA radiation, and salt fog. Dielectric constant values were measured before and after applying stresses. Upon applying stresses, a periodic decline in dielectric constant was observed. Improved dielectric properties were obtained by adding micro and nano-silica as fillers. A nano silica-incorporated silicone rubber product exhibited good potential for dual applications as dielectric and high voltage insulation.

Multi-Stressed Nano and Micro-Silica/Silicone Rubber Composites with Improved Dielectric and High-Voltage Insulation Properties

Investigation of epoxy composites for outdoor insulation under accelerated ultraviolet exposure

Investigation of dielectric properties and methylene intactness under multiple environmental stresses for high voltage epoxy composites

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Investigation of Hydrothermally Stressed Silicone Rubber/Silica Micro and Nanocomposite for the Coating High Voltage Insulation Applications

Multi-Stressed Nano and Micro-Silica/Silicone Rubber Composites with Improved Dielectric and High-Voltage Insulation Properties

Investigation of epoxy composites for outdoor insulation under accelerated ultraviolet exposure

Investigation of dielectric properties and methylene intactness under multiple environmental stresses for high voltage epoxy composites