Impregnation Dynamics of Liquid Resins into Rectangular Capillaries Formed by Polymer‐Coated Copper Hairpins in Electric Motors: Experiments and Theory

Herein, the impregnation dynamics of the space between rectangular, polymer‐coated copper hairpins in electric motors, using polyester‐based and epoxy‐based liquid resins, are described both experimentally and theoretically. The rectangular capillary is formed along its sides by two copper hairpins coated with a polymer, with paper along its bottom and air along its top. The following properties of the two test liquid resins are measured from 25 to 80 °C: their contact angles on the hairpin and on the paper, their dynamic viscosities, densities, and surface tension. Dynamic viscosity is modeled by the Vogel–Fulcher–Tammann model, while surface tension is modeled by the Eötvös model, used also to estimate the molar masses of the resins. Penetration times of the test liquids are measured into the capillaries for ten penetration lengths. A model is derived for the penetration length as a function of time for a liquid penetrating into horizontal, rectangular, and thin capillaries. The resulting model is similar to that of the Lucas–Washburn but the geometrical parameters differ. The temperature dependence of the penetration rate is modeled by the extended Vogel–Fulcher–Tamman model.