Inter-fiber bonding effects of beating, starch or filler

Adding filler generally impairs the mechanical properties of paper. Beating and addition of starch can be used to compensate for this. When the fiber network density (excluding fiber mass) is used as a measure of network structure, addition of filler caused exactly the opposite effect from beating for all inplane mechanical properties except elastic stiffness. Elastic stiffness decreased less with addition of filler than it did with less beating. Addition of starch improves the strength properties without changing the structure of sheets made solely from kraft pulp, while beating or addition of filler changes their structure. In mixture sheets, consisting of kraft pulp and TMP addition of starch had no effect on the in-plane properties. Out-of-plane fracture energy improved with addition of starch, irrespective of whether TMP was included in the furnish or not.