Application of Epoxy Impregnating Resins [PDF]

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Application of Epoxy Impregnating Resins John Phillips Managing Director Epoxylite, Division of Sterling Technology Ltd Manchester



Introduction Varnishes have been used in the electrical industry throughout the last century, whilst in the past 50 years impregnating resins have been developed and applied in the manufacture and repair of electric motors, generators and transformers in increasing volumes. Varnishes are derived from solvented coatings technology and in reality should be considered as “after treatments” to electrical insulation. Impregnating resins may be defined as liquid thermosetting plastics which undergo an irreversible chemical reaction during cure, whereby the liquid is converted to a solid. They can be designed to become an integral part of an electrical insulation system. The objective of this paper is to demonstrate how epoxy impregnating resins are selected and applied to influence and enhance the operating performance of rotating electrical machines and transformers in service.



Insulation Failure Premature failure of electrical insulation is primarily caused by excessive stress on insulation structures. There are four types of stress which may be effective either individually or as interrelated forces: Mechanical Thermal Environmental Electrical Accordingly, in order to maintain the integrity of the insulation structure and consequently extend service life, it is essential to provide optimum protection from all stresses which are encountered during normal operating conditions.



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Function of Impregnating Resin The function of an impregnating resin is fourfold, and may be described as follows: 1. ensure adequate adhesion and protect against excessive movement of the insulation structure caused by mechanical and electro-mechanical stresses. 2. reduce thermal stress by improving heat transfer and thermal endurance of the insulation structure. 3.



fill, seal, and protect the insulation structure against the effects of destructive service environments.



4. provide a composite insulation structure with optimum dielectric properties after impregnation and cure. Compatibility of an insulation system is achieved by using a correctly engineered combination of insulation materials, impregnating resin and process techniques.



Chemistry of Epoxy Impregnating Resins (a) Two Component Systems Impregnating resins comprising bisphenol A epoxy base resin cured with amine hardeners find little widespread application due to inherently short working lives, although this chemistry is often used beneficially in applications such as in rapid processing of low voltage stators and armatures. Impregnants based on bisphenol A/F or cycloaliphatic epoxy resins mixed with anhydride hardeners are commonly used to VPI process mica insulation in the manufacture of medium and high voltage motors and generators. An accelerator is either included in the resin formulation or contained in the mica insulation to enable adequate cure to be completed within an acceptable timescale at elevated temperatures. Variants of these accelerated epoxy anhydride resin impregnants containing inert fillers to modify rheology and improve thermal conductivity are also currently used in the manufacture of low voltage motors and transformers. Essentially, all anhydride cured epoxy resins are two component systems and therefore have a limited shelf life at normal ambient temperatures. They are also hygroscopic and in large bulk, require low temperature closed storage to contain the chemical reaction and maintain stability.



G - 3 of 9 (b) Single Component Systems Epoxy impregnating resins based on sophisticated Lewis Acid latent catalyst chemistry have been developed over the last two decades. Formulation of these impregnants has been refined to include the use of bisphenol F, and epoxy novolac in conjunction with bisphenol A base resin components to produce 100% solids resins and also aqueous emulsions. This chemistry offers the advantage of long term storage stability at ambient temperatures, low toxicity, low emissions, excellent cured properties and the versatility to process electrical equipment using a wide range of application techniques. Properties of epoxy impregnating resins based on this latent catalyst chemistry are compared to those of other impregnants in the following tables:



Typical Processing Properties of Impregnating Resins Polyester Resin



Polyester Resin



Polyester Resin



MF Resin



Epoxy Resin



Epoxy Emulsion



Styrene



VT



DAP



nil



nil



nil



%



25 – 30



25 – 30



20 – 25



nil



nil



nil



LD 50



5000



5000



770



> 5000



> 5000



> 5000



ºC



33



54 – 57



> 150



> 150



> 150



> 150



mPas



200



250



500



500 - 10000



300 – 5000



< 100



Cure Cycle



h/ºC



2 / 150



2 / 150



2 / 150



2 / 150



3 / 160



3 / 160



Emissions



%



10 – 20



10 – 20



5 – 10