Influence of fibre length and filler particle size on pore structure and mechanical strength of filler-containing paper

Test sheets were prepared by incorporating softwood pulp with silica filler, PW-5 (diameter 4.5 mum) or PW-20 (15 mum). Length-weighted averages of fibre were 2.5 (uncut fibre) and 1.25 mm (short-cut fibre). Pore sizes less than 150 mum were measured by mercury porosimeter. Sheets of short-cut fibres and mixed with uncut fibres at ratio of 3:1 or 1:3 had larger pore volumes than others tested. When filler content increased, the total pore volume increased for PW-20 sheets, but it did not for PW-5 sheets with short-cut fibres. Tensile index and folding endurance were very much affected by fibre length. Contact number on a fibre was calculated by computer simulation, and it had a linear relation with tensile index of sheet.