EffEct of long-fibrEd rEinforcEmEnt pulp on mEchanical propErtiEs of short fibrEd-basEd papEr

The purpose of long-fibred reinforcement pulp in printing paper furnish is to improve the runnability of paper. Here the dry web runnability was evaluated primarily based on flaw-resisting ability of paper and secondarily based on in-plane tensile properties. Flaw-resisting ability was measured as tear strength and, more importantly, as fracture toughness. We modified the fibre length and bonding potential of chemical softwood pulp–mechanical pulp mixture. Fibre length was manipulated by proportion of pulps and interfibre bonding potential by beating level and cooking conditions of chemical pulp. Results clearly showed that higher average fibre length improved the flaw-resisting ability of dry paper. As expected, beating of reinforcement pulp and sulfite cooking ( vs. kraft) increased the interfibre bonding of pulp mixtures. However, contrary to our expectation, improved bonding increased the flaw-resisting ability only a little. According to our analysis, low-freeness mechanical pulp has, as such, rather good interfibre bonding. Other factor is that beating of chemical pulp improves specific elastic modulus largely due to increased fibre segment activation, and only to smaller part due to bonding. The flaw-resisting ability of reinforced paper can be best controlled by modifying the average fibre length of the furnish - controlling the bonding ability seems not as critical. On the other hand, tensile properties like elastic modulus are not dependent on fibre length, instead they depend on fibre segment activation, interfibre bonding and fibre strength.