Fibre fractionation for high porosity sack kraft paper, TAPPI JOURNAL, June 2001, Vol. 84(6)

James Olson

Bruce Allison

Tim Friesen

Christine Peters

High performance sack kraft paper must be both strong, with high Tensile Energy Absorption (TEA), and also be porous. Refining increases pulp strength but simultaneously decreases porosity, limiting the available strength increase. Fibre fractionation can increase porosity by removing some of the fines and short fibres, enabling the pulp to be refined to a higher strength. In this study, two fractionation strategies were demonstrated to produce pulp with significantly higher TEA-porosity performance. The first strategy used a small diameter, smooth-hole screen plate to remove short fibres and fines and create a long fibre pulp that was 70 to 80 percent of the mass of the feed stream. The resulting long fibre pulp had significantly higher porosity after being laboratory refined with a PFI mill to a target TEA. The corresponding short fibre pulp was low consistency, low freeness and low porosity, making it more suitable for less demanding products that require smoothness for printing. The second strategy used a large diameter, smooth-hole screen plate, operating at a low reject ratio to fractionate the longest fibres to produce a super-porous, high-strength pulp. The short fibre pulp consisted of 60% of feed stream and only had a small drop in freeness, consistency and porosity from the feed, but had an increase in smoothness.