The Relation between the Residual Stress and the Crazing in Polymer Films on Magnet Wires

Abstract

It is well known that crazing occurs when a magnet wire is immersed in an impregnating bath of varnish after the wire was coiled up. A quantitative study was carried out in this paper for clarifying the relation between the crazing and the residual stress in the polymer film on the wire. As test specimen were used magnet wires which had poor adhesion strength between the polymer film and the conductor. The crazing in the polymer film on the magnet wire occurred most severely between 3% and 5% elongation. The residual stress in the film attained a maximum value in, the same range of elongation. That is, the crazing occurred most severely in the range of elongation where the stress in the polymer film was most hardly relaxed. A morphological study of crazing was made by using two types of magnet wires coated with cross-linked and linear polymers. The crazing observed in the cross-linked polymer films, polyesterimide, was identified as a sort of micro-cracks. In the case of linear polymer films, ployamide (Nylon 66), orientation of polymer chains in crazing was observed.