Effect of Softwood Kraft Refining on a Mechanical-Chemical Mixture Sheet, 1999 Papermakers Conference Proceedings

Eero Hiltunen, Jaakko E. Laine, Hannu Paulapuro, Heikki Kettunen

The main objective of this study was to maximize the fracture toughness of a mechanical-chemical mixture sheet by softwood kraft refining. The second aim was to study the microscopic fracture behavior of paper during kraft refining.

According to the results, the kraft and mixture sheets are quite different. The fracture toughness of mixture sheets decreases during softwood kraft refining, while that of pure kraft sheets increases. The damage analysis suggests an explanation. First, the pull-out width of both mixture and pure kraft sheets decreases during refining. This indicates that the proportion of broken fibers increases. Second, in mixture sheets the fracture work per damage width stays almost constant during kraft refining, while in pure kraft sheets it is about three times larger at a high refining degree than for unrefined kraft. The study also showed that the fracture work of mixture sheets of TMP and unrefined kraft is larger than in pure TMP or unrefined kraft. However, at a higher refining degree, the fracture toughness seems to follow roughly the linear rule of mixtures.