Why choose the Corona-resistant enamels?
Dielectric materials used as insulators for electrical conductors may fail as a result of corona occurring when the conductors and dielectrics are subjected to voltages above the corona starting voltage. This type of failure may occur for example in certain electric motor applications. Corona induced failure is particularly likely when the insulator material is a solid organic polymer. Improved dielectric materials having resistance to corona discharge-induced deterioration would therefore be highly desirable. For some applications, mica-based insulation systems have been used as a solution to the problem, whereby corona resistance is offered by the mica. Because of the poor physical properties inherent in mica, however, this solution has been less than ideal because of the relatively large amount of space that the mica based compositions require.

How to use the Corona-resistant enamels for enameled wire?
A corona-resistant wire enamel composition which comprises a polyimide, polyamide, polyester, polyamideimide, polyesterimide or polyetherimide resin and approximately 1% to approximately 35% by weight of submicron-sized particles of alumina. The aluminum in the alumina is atomically bound only with oxygen.

It is preferred to employ fumed alumina. The alumina is dispersed in the wire enamel composition with high shear mixing, preferably, in a concentration ranging from about 1 to 20 parts by weight per hundred parts of the resin. The alumina particles are preferably less than about 0.1 micron in size. Also, a method of providing corona-resistant insulation for an electrical conductor employs the above-mentioned composition. The method comprises applying the composition to the conductor, for example wire, by using multi-pass coating and wiping dies and curing between about 330° C. and 370° C., at varying speeds.
It was noted that if dispersion was not accomplished with high shear mixing, it was impossible to obtain the smooth continuous coating that is required to produce any insulating film in the minimal thickness required in producing commercial electrically insulated wire.

Accordingly, in its broad aspects the present invention comprises a corona-resistant wire enamel composition which comprises a polyimide, polyamide, polyester, polyamideimide, polyesterimide or polyetherimide resin and approximately about 1% to about 35% by weight of submicron-sized particles of alumina, dispersed therein by high shear mixing, and to the method of preparing such composition by high shear mixing of the alumina particles in the aforesaid resins. The improvements provided by the subject invention are not only observed in the high temperature resistant resins such as polyimides, but also provide dramatically improved corona resistance for resins generally recognized as low-temperature capability materials, such as polyamides (Nylon) and polyesters.

Corona-resistant wire enamel compositions are applied to coat conductors or conductor wires by using multi-pass coating and wiping dyes and curing between about 330° C. and 370° C. at varying speeds to obtain a smooth continuous coating.

A corona-resistant two-stage wire enamel system is provided which comprises a first layer of a polyimide, polyamide, polyester, polyamideimide, polyesterimide or polyetherimide resin and a second layer coated over the first layer of a polyimide, polyamide, polyester, polyamideimide, polyesterimide or polyetherimide resin, wherein the resins of the first and second layers differ and wherein at least one of the first or second layers includes from about 1% to about 35% by weight of submicron-sized particles of alumina, dispersed therein by high shear mixing, and to conductors insulated therewith.

The corona-resistant wire enamel compositions and the corona-resistant wire enamel systems provide superior electrical insulating systems. 

Refer to US4493873A

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