Magnet wire is composed of a conducting core and thin layer of electrical insulation. Copper (Cu) is typically used as conducting core, and various polymers such as polyamideimide, polyimide, and polyesterimide are used for electrical insulation. The role of the magnet wire is related to the interchange between electrical and mechanical energy for energy transformation application such as transformers, motors, generators etc. Currently, the electric vehicles (EV) industry is growing rapidly and demands on related components are therefore increasing. Compared to the combustion engine, EV needs more electrical power with higher voltages or higher currents, which can increase probability of electrical discharge. The degradation of the insulation layer can occur by polymer bond breakage due to electrical stresses under partial discharge. To keep high performance under higher voltage, insulating polymers should have high heat and chemical resistance as well as low water absorption. In addition, the adhesion between the insulating polymer and Cu should be sufficient to prevent crack issues. As an incumbent solution, polyamideimide (PAI) and polyimide (PI) is being used for this application in a varnish form. However, PAI absorbs moisture in humid environments or when immersed in water (up to 5% weight), which can cause reliability issues [1]. PI shows very poor adhesion strength with the conducting core due to its stiffness and low surface energy [2, 3], which can cause reliability problems for magnet wires [4]. Therefore, there are strong demands to solve such issues like improvements in dielectric strength, water resistance, and high heat resistance. In this report, we will share the results of excellent flexibility, adhesion, and electrical performance of polyetherimide (PEI, ULTEM™ resin) based varnish as a high voltage insulation material.
