The corona-resistant composite material in this proposal would form a coating or film over the electrical and electronic components and related equipment to be protected, having a thickness no greater than 50 microns (about 2 mils). It can withstand at least 20,000 hours of corona discharges, remaining flexible (70% or more elongation upon breaking), and functional in a wide temperature range, from -100°C to 250°C. High energy impacts of coronal discharges are characterized by accelerated electrons, charged molecules, atoms, and radicals which cause high velocity physical impacts on material, acting like lightning bolts, causing damage which appears on a small scale, creating “pinholes” through films or coatings of polymer materials. Also, these events can emit UV radiation which can deteriorate polymers.
Both polymer matrix and reinforcing fibers in the proposed composite have high dielectric strengths, a measure of the electrical strength of a material as an insulator. Dielectric strength is the maximum voltage required to produce a dielectric “breakdown” through the material and is expressed as Volts per unit thickness of the material (V/mm or V/mil, for example). The higher the dielectric strength of a material is, the better its quality as an insulator. The proposed composite is resistant to high energy particle and electromagnetic (UV) radiation, the physical impact of such particles, and the related aggressive chemical interactions that further deteriorate some materials. The proposed protective composite material will be in contact with coronal discharge plasma over extended periods of time, and will be subjected to aggressive “attack” on chemical bonds and high energy radiation which could otherwise cause “scission” reactions in some materials, that could degrade the linkages or chemical bonds in the “backbone” of some polymers.
Because of its radiation and chemical resistance, wide functional temperature range, excellent mechanical properties and high dielectric strength, polyimide is the preferred polymer matrix for a fiber-reinforced composite. Dielectric strength is 291 kV/mm. Corona Resistance is 100,000 hr at 20 kV/mm at 50 Hz. Closely related polyamide-imide or polyetherimide, may be used, depending on applications, and materials to be coated. The best fiber, because of its radiation, chemical, temperature, and mechanical properties, high dielectric strength and volume resistivity (200 kV/mm; 4 x 1012 Ω/m), and inexpensive cost, is basalt fiber, incorporated up to 70% by weight of the proposed composite.
Polyimide (PI) is a class of aromatic, linear polymers having excellent mechanical and electrical (insulating/dielectric) properties, and high temperature resistance. While polyimide is probably the best choice among polymers for long term corona resistance, overall performance and durability can be improved, by using polyimide as the matrix of a polymer-fiber composite, using a basalt fiber up to 70% by weight of the composite composition. The major disadvantage of PI is that it is generally expensive, compared to some other polymers. This can be significantly mitigated and overcome altogether, however, by incorporating relatively inexpensive basalt fiber to form polyimide composite materials, at up to 70% by weight of the composite, significantly offsetting the expense of using polyimide in corona resistant material applications.