Heat treatment of mechanically pre-stressed electrical insulation

Previously reported work has established that the resistance to electrical tree growth can be enhanced in insulating resins by the inclusion of fibers which are cast in under tension. On completion of the post cure and subsequent removal of the tension from the cast-in fibers, the composite material contains regions adjacent to the fibers which are in a remanent compressive state. These compressive regions, through which the growing electrical tree must pass, significantly inhibit tree growth and decrease growth rates by up to a factor of four. Remanent stress analysis was carried out by Raman spectroscopy which measured the tensile stress remaining in the cast-in fibers and by inference the compressive stress in the adjacent resin. The study of this phenomenon initially utilised aromatic polyamide fibers cast in a room temperature cure epoxy resin. This combination of materials was chosen for ease of manufacture and because a large pre-stressing effect was demonstrable. However, the room temperature cure resulted in a relatively low glass transition temperature and the composite could only retain its remanent stresses at temperatures below 40/spl deg/C. Post curing of this system at 100/spl deg/C raises the glass transition temperature significantly and permits the use of the pre-stressed material at temperatures in excess of 80/spl deg/C without loss of remanent stress.