This specification covers pressboard for electrical insulating purposes as well as for dielectrical or structural purposes in transformers and other electrical apparatus. Pressboards under this specification are of three types: Type 1 consists of high-purity (Grade 1.1) calendered pressboards, while Type 2 consists of normal-purity (Grades 2.1.1, 2.2.1, 2.3.1, and 2.3.2) calendered pressboards. Precompressed pressboards (Grades 3.1.1, 3.2.1, and 3.3) fall under Type 3. Not included in this specification, however, are pressboards comprised of two or more sheets laminated together using an adhesive. Pressboards shall be manufactured from unbleached kraft pulp, cotton pulp, or a combination of both, and shall conform to the thickness, density, surface texture (smooth calendered surface for Types1 and 2, and fine-textured finish for Type 3), and color (from tan to blue-gray, depending on the pressboard's grade) requirements specified. The pressboard must also be free of dirt, metal particles, and other foreign material. Tests for apparent density, thickness, moisture and ash content, aqueous extract conductivity, chloride content, tensile strength, pH of aqueous extract, dielectric strength in air and in oil, shrinkage, and compressibility shall be performed and shall conform to the requirements specified.

1.1 This specification covers pressboard for electrical insulating purposes, manufactured from kraft, cotton, or kraft and cotton pulps. This board is intended for dielectrical or structural purposes in transformers and other electrical apparatus.

1.2 Electrical insulating boards are most commonly referred to (and will be referred to herein) as pressboard. Other terms used for pressboard include transformer board, fuller board, and presspan.

1.3 This specification covers pressboard having a nominal thickness of 0.030 to 0.315 in. (0.8 to 8.0 mm). For thinner material refer to Specification D1305.

1.4 The maximum thickness available will differ with the type and the manufacturer. The maximum sheet size will differ with the thickness, type, and manufacturer.

1.5 Pressboard shall normally be plied wet without pasting. Unless specified by the purchaser, this specification does not include pressboard comprised of two or more sheets that have been laminated together using an adhesive.

Note 1—The materials described in this specification are similar to corresponding types of pressboard described in IEC Specification 641-3, Sheet 1, Types B.0.1, B.2.1, B.2.3, B.3.1, and B.3.3.

1.6 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.