Kraft Paper & Pressboards
Kraft paper and pressboard materials are manufactured using cellulose fiber as the base material. In the case of kraft paper and pressboards, the cellulose paper is often purified for electrical application.
Kraft papers are available in a wide variety of forms and grades according to the special treatment given to the paper for a particular application. For instance, kraft paper is commonly "thermally upgraded" by applying a diamond pattern epoxy resin to either one or both sides of the kraft paper. The purpose of the diamond pattern epoxy resin treatment is to improve the temperature resistance and thermal performance of the kraft paper. Almost all oil filled transformers use thermally upgrade diamond pattern epoxy coated kraft paper for coil layer insulation.
In addition, kraft papers are commonly creped for applications involving the wrapping of wire or conductor. The creped kraft paper insulation finds application in medium and high voltage transformer applications.
Pressboard materials are also manufactured from cellulose fibers. Pressboards are manufactured by pressing layers of cellulose paper under heat and pressure much like a laminate. In fact, many thicknesses of pressboard material are actually laminated from thinner plies of material. Again, the most common application for pressboard material is oil filled transformers.
Both kraft paper and pressboards are commonly used in oil filled transformer application due to their low cost, ability to absorb the dielectric oil, and their ease of fabrication.