Insulating Varnishes and Impregnating Resins Information
Insulating varnishes and impregnating resins are thermosetting resins such as epoxies or phenolics. They are used to ensure that electrical devices, including motors, generators, transformers, sensors, and other devices that function by electromagnetic induction, have the necessary electrical insulation and structural integrity for operation. These materials crosslink when applied and are, therefore, inherently strong, environmentally stable, impervious, and durable. Transformers, for instance, are comprised of coil windings where the electric wires are protected by a thermosetting resin or an insulating varnish (also called a wire enamel or primary insulation) to prevent electrical shorting. The completed transformer, comprised of an assembly of coil windings and insulating paper, is sealed with a thermosetting resin to provide environmental protection (especially from moisture and dust) to eliminate air in the transformer that can conduct electricity and heat, and to provide strength.
This sealing process can be done by potting, casting, or encapsulation. These names are used interchangeably, which can cause confusion of the terms. The most commonly used terms are potting and encapsulation. Common industry definitions are:
Potting—The potting method uses a case (pot) to load the device and then fill with liquid potting. The case becomes part of the unit. This is the most often used method, particularly for high production conditions.
Casting—Casting is basically the same as potting but the molding case is removed after encapsulation and curing.
Encapsulation—Encapsulation also refers to a device dipped into a resin and a thick coating encases the unit. This method is often referred to as “dip coating.”
Three common insulating resins are epoxy, phenolic, and polyurethane.
Epoxy resins (EP) exhibit high strength and low shrinkage during curing. Epoxies are known for their toughness and resistance to chemical and environmental damage. Most epoxies are two-part resins cured at room temperature. Some thermally cured or thermoset one-part epoxies are also available. Depending on the formulation, epoxy resins are used as casting resins, potting agents, resin binders, or laminating resins in fiberglass or composite construction. They are also used as encapsulates, electrical conductors in microelectronic packaging, and adhesives in structural bonding applications.
Phenolic and formaldehyde resins are thermosetting molding compounds and adhesives that provide strong bonds and good resistance to high temperatures. Phenolic, or phenol formaldehyde, urea formaldehyde, furan, and melamine resins are all part of this category. Generally, the most durable phenolic resin adhesives are made from chemicals of the phenol group and formaldehyde. Phenolic resins come in liquid, powder, and film form. Special phenolic resins are available that harden at moderate temperatures when mixed with suitable accelerators. Phenol-formaldehyde, resorcinol-formaldehyde, resol, and novalac resins are types of phenolic resins. Urea resin adhesives are made from urea, formaldehyde, and catalysts, or hardeners. Urea formaldehyde resins can harden rapidly at moderate temperatures, but generally do not have the properties of phenolic resins. Melamine resins are made through a reaction of dicyandiamide with formaldehyde. Most of the resins in this group have excellent dielectric properties. Furan formaldehyde (FF) resins are made by the polymerization, or poly-condensation, of furfural, furfural alcohol, or other compounds containing a furan ring, or by the reaction of these furan compounds with other compounds (not over 50%). Fire-retardant furans are used in hand lay-up, spray-up, and filament winding operations. Furans are commonly used in foundry binders, grinding wheels, refractories, and other high-temperature applications. Furan resins and chemicals are also used in fiberglass composites, hybrid resins combined with epoxy or phenolics, and in corrosion-resistant cements.
Polyurethane (PUR) resins provide excellent flexibility, impact resistance, and durability. Polyurethanes are formed through the reaction of an isocyanate component with polyols or other active hydroxyl group compounds. PUR resins require a catalyst, heat, or air evaporation to initiate and complete curing.
The material selection process should consider that insulating varnishes and impregnating resins must be superior electrical insulators with a long life. The products must not degrade with exposure to heat, cold, water, chemicals, or other environmental factors that might reduce the insulating characteristics of the products. If these products become electrically conductive or degraded such that electricity can flow through the wire, transformer or other electrical device to the surrounding area, system failure and damage will likely occur.
Often times insulating varnishes and impregnating resins appear to be redundant, applied to already insulated components. What insulating varnishes and impregnating resins provide are several benefits that improve the performance and life span of the equipment. It provides protection from mechanical damage and dampens vibrations in the windings and other components. Insulating varnishes and impregnating resins prevent moisture and contamination from lodging between coils and components and damaging the insulation. They also increase the dielectric strength of paper insulation and help carry heat away from the windings and other components.