Elimination of Crazing of Enameled Wires

Abstract

This paper describes a study of the elimination of crazing by even treatment in polymer coatings on wire. The crazing became invisible under an optical microscope after appropriate heat treatments, leaving some traces observable under an electron microscope. The thermal expansion of the polymer coatings and the enhancement of micro-Brownian motion of polymer segments by heat were found to be main factors affecting the elimination of crazing. The electrical and mechanical properties of enameled wires after the elimination of crazing were measured and compared with those of the original wires. Judging from the breakdown voltage of the treated wires, we found that the perfect elimination of crazing was not realized. However, examination of the stress-strain curve of the polymer coating removed from the wire showed that the crazing was almost perfectly eliminated by the heat treatment. The apparent activation energy for the elimination of crazing was evaluated for a variety of polymer coatings, and it was found that the increased energy was generally accompanied by increased glass transition temperature.
The occurrence of crazing in the polymer coatings on wire can be prevented by relaxing the residual stresses by appropriate heat treatment. The time of treatment required for the prevention of crazing was measured at various temperatures, and the apparent activation energy for the process was evaluated. This energy was considerably lower, for all the tested polymers, than the activation energy for the elimination of crazing.