CHARACTERIZATION OF THE INTERFACIAL BOND IN PAPER-POLYPROPYLENE-PAPERLAMINATES

The interfacial strength of two paper-polypropylene-paper laminates us ed for electrical power transmission cable insulation has been measure d using a peel test after a variety of conditionings that simulate the service thermal stress conditions. Immersion in dielectric oil at 23- degrees-C causes little decrease in peel strength, but heating in the oil at 90-degrees-C causes a rapid initial loss followed by a slow los s of strength. Cyclic and static heating show that cyclic fatigue is n ot a factor as the initial loss of strength occurs in a time comparabl e to the first two test cycles and further heating causes much less lo ss of strength. Tensile and compressive stresses resulting from cable winding have no effect on the loss of bond strength during heating in oil. Vacuum drying of the laminate and oil (as in cable manufacture) b efore heating results in less decrease in peel strength than heating w ithout prior drying. Optical and electron microscopy show minimal wett ing of the paper fibrils by the polypropylene with only a few having b een either normal to or at a steep angle to the polypropylene surface. That is, the fibrils appear to be parallel to the interface and only partially immersed in the polypropylene. Micrographs show little, if a ny, differences between the interfaces on the two sides as well as lit tle paper on the polypropylene after delamination and vice versa. It i s suggested that the bonding mechanism is a combination of weak mechan ical interlocking and secondary bonds. It is also suggested that swell ing of the polypropylene relative to the paper by the dielectric oil a t 90-degrees-C weakens the interfacial bond.