Effect of pulp refining on mechanical and electrical properties of insulating presspaper

In this study, softwood pulps with Schopper Riegler (SR) freeness of 10 °SR, 17 °SR, 31 °SR, 40 °SR, 48 °SR were used to prepare presspaper samples. Morphological properties of fibers and presspaper were characterized by optical microscope (OM) and scanning electron microscope (SEM). Mechanical and electrical performances of obtained samples were evaluated. Results show that cellulose fibers become flat and flexible after refining. Presspaper made from refined pulp has fewer voids between fibers. With the increasing of refining degree, relative permittivity of presspaper first increases, and then keeps almost constant. Presspaper made from pulp with 17 °SR has a lower dielectric loss at frequency below 100 Hz. Both mechanical strength and electrical breakdown field can be reinforced by pulp refining. Tensile strength and AC breakdown field of presspaper made from pulp without refining are 51 MPa and 4 kV/mm, respectively. Whereas those of presspaper prepared from pulp with 31 °SR are 118 MPa and 11.1 kV/mm. When refining degree is higher than 31 °SR, mechanical and electrical strengths, however, are not significantly improved with further refining.