Advances in high-field insulating polymeric materials over the past 50 years

The development of electrical insulation is closely associated with the development of electrical apparatus. Up to about 1925 only naturally occurring products such as asphalt, rubber, mica, cotton thread or fabric were generally used. Mineral oil-impregnated paper was used in transformers, capacitors, and underground cables at voltages up to 69 kV [1]. Then the first synthetic insulating materials, namely phenolic (Bakelite®) and alkyd (saturated polyester) resins, were used in insulating varnishes. Polyvinylchloride was introduced in 1936 to replace natural rubber in underground cable and house wiring, but in spite of tremendous advances in compounding polyvinylchloride to improve its properties, the electrical losses could not be reduced sufficiently. It was restricted to LV applications up to a few kilovolts. A variety of synthetic elastomers was used to replace natural rubber for HV applications, e.g., butyl rubber in the 1940s and ethylene-propylene rubber later on. Epoxy and polyester resins became available around 1945 and were first used for molding and encapsulation of electronic components, then mostly in laminated form as electrical insulation for LV applications [2]. Increased knowledge of the chemistry of insulating materials, and improved processing, then led to the availability of a large number of plastic materials such as polyethylene terephthalate, polycarbonate, polystyrene, low- and high-density polyethylene, polypropylene, polytetrafluoroethylene and fluorinated ethylene-propylene copolymer, suitable in thin film form for capacitor applications. These thin films could work at high electrical stresses or, put more practically, at minimal dielectric thickness appropriate to the capacitor working voltage. Thin films (<;10 μm) of these materials have intrinsic electric strengths in the range 500 to 1,100 V/μm at room temperature, probably 2 to 10 times higher than the average breakdown strength of plastic-film capacitors at the time [3]. Polyethylene-insulated of electrical energy, a statement that is still valid today and certainly will remain valid in the future. The objective of this paper is to give an overview of the research carried out over the last 50 years that has contributed to the development of high-field polymer dielectric materials in a broad sense. During this time thermoplastic polymers have been used in a large variety of electrical systems, e.g., cables, capacitors, transformers, bushings, electrical machines, and accessories operating in a large variety of environments (space, nuclear, thermal, etc.). Progress has been highly nonlinear in time, entangled with progress in material sciences, our understanding of the electrical properties of insulating polymers, and advances in material processing. We emphasize what are, in our view, the main features.