Preparation And Performances Of Epoxy Insulating Resin For Vacuum Pressure Impregnating (V.P.I.) Technique

Groundwall insulation is employed to separate the copper conductors from the grounded stator core in the electric machines. The performances and service life of the groundwall insulation of the stator is critical to the service life of electric machines. At present,the vacuum pressure impregnation process (V.P.I.) is one of the most promising technologies for the groundwall insulation of electric machines and are widely adopted by many leading electrical equipment manufacturers around the world. Groundwall insulation systems for vacuum-pressure-impregnated (V.P.I.) coils of high voltage stators are usually glass-backed mica paper tape system. The coils are insulated with mica paper tape and then vacuum-pressure-impregnated with the impregnating resin. Therefore, the performance of the impregnating resin is one of the key factors accounting for the evolution of the groundwall insulation. Accordingly, the availability of an impregnating resin with improved dielectric properties as well as excellent mechanical performances and thermal stability for the V.P.I process is eagerly desired from the viewpoint of the moter/ or generator technology development.In this study, multifunctional-reactive silicon compounds with very low toxicity have been used as reactive diluents for epoxy impregnating resin based on a kind of cycloaliphatic epoxy resin and methyl-hexahydrophthalic anhydride (MHHPA) system, with the catalysis of aluminum (III) acetylacetonate (Al(acac)3). This is based on the following considerations: firstly, the organosiloxane are effective diluents since they have very low viscosities and are mutually soluble with the epoxy resin at all temperatures; secondly, the existence of the reactive groups like epoxide, amino, or vinyl groups enables the diluents to be involved in the cross-linking networks of the epoxy resin; and at last, the reactivity of Si-OR makes the structure design possible. It can be expected that polysiloxane may be produced in the cross-linking network of the cured epoxy resin via synchronous reactions, which could improve the toughness of the epoxy matrix. Besides, nano-silica particles have been employed to enhance the epoxy impregnating resin. To develop an eco-friendly epoxy impregnating resin system for groundwall insulation of large generators, the curing reaction mechanism has been fully studied, the formulation and the curing techniques have been optimized, and the influence of different constitution on the curing reaction, microstructure, and final performances have been studied and characterized.In this paper, influences of different reactive organo-siloxanes on the curing reaction and performances of epoxy V.P.I. resin have been studied and compared. The results show that: in the presence of Al(acac)3, the addition of GPMDS can obviously reduce the epoxy viscosity, improve the processability and toughness without imparment to the insulating properties and thermal stabilities. The influences of organo-siloxanes on the curing reaction and performances of epoxy V.P.I. resin are very complicated and greatly related to the reactive structures (epoxide, amino, or vinyl groups) and the silicon structures.In this paper, the interaction between the hydrolysis/condensation of (3-glycidoxypropyl) trimethoxysilane (GPTMS) and the curing reaction of epoxy resin have been studied, as well as the influences on the properties of the cured samples. The results show that the influence of GPTMS on the curing reaction of epoxy resin is very complicate and is greatly influenced by the catalysts. With the catalysis of Nd(acac)3, the incorporation of GPTMS into epoxy resin has a slight influence on the curing reaction, and considerable enhancement in toughness have been obtained without impairments to thermal stability and insulating properties. Differently, with the catalysis of Al(acac)3, the hydrolysis and condensation of GPTMS prior to the curing polymerization of epoxide and anhydride can been greatly facilitated and detected by the DSC analysis which induces phase separation and inhomogeneity in the micro-morphologies of the cured sample and accordingly decreased glass transition temperature, thermal stability, insulating and mechanical performances.DSC and Heating-FTIR have been employed to study the accelerating mechanism of Al(acac)3 on the curing reaction between the epoxy resin and MHHPA. The results show that the catalytic mechanism is that Al(acac)3 first reacts with MHHPA to form a carboxylic anion as a transition state at elevated temperatures, then the carboxylic anions attack the epoxide groups to initiate the curing reaction. Kissinger's and Friedman-Reich-Lev's methods have been employed to study the curing kinetics of the epoxy V.P.I. resin. The results show that the epoxy V.P.I. resin is a complicate reacting system, and the introduction of GPTMS or GPMDS into the system can lower down the gel temperature and the curing temperature.Nano-silica particles have been introduced into epoxy resin to achieve good toughness and mechanical strength. The results have shown that with the incorporation of GPMDS, the nano-silica particles exhibit good compatibility with the epoxy matrix and the enhancement is determined by the concentration and dispersion of nano-silica particles. Nano-silica particles without surface treatment are easy to aggregate and hard to disperse, thereby enhancement can be obtained only at low silica loadings (no more than 3 wt%) when there's no great aggregations. The incorporation of untreated nano-silica particles results in decreased volume resistivity and increased dielectric loss due to the large amount of–OH groups adhered to the particle surface. Surface treatment of nano-silica with GPMDS can improve the dispersion of the nanosilica particles, and accordingly improve the toughness, strength, the glass transition temperature, the insulating properties, and the thermal stabilities of the epoxy/nano-silica composites.