Unbleached linerboard kraft pulps with different kappa number (yield) - effects on tensile properties and compression strength with free or restrained drying

摘要

This laboratory investigation examines the effect of softwood kraft pulp yield on tensile stiffness, tensile strength, and compression strength, as developed by low-consistency refining. The yield range from 51 to 56% corresponded to a kappa number range from 53 to 81. Free drying and restrained drying were used in parallel. A reduced kappa number enabled an improved strength combination. First, a reduced kappa number resulted in higher maximum levels for tensile strength both when using free and restrained drying and for tensile stiffness when using free drying. Second, the maxima for these properties were shifted to higher sheet density, and thereby to higher compression strength and tensile stiffness (restrained drying). Compression strength increased with increasing density unaffected by the kappa number both when using free and restrained drying. Tensile stiffness increased with increasing density unaffected by the kappa number when using restrained drying. The improved strength combination is chiefly attributed to the fact that pulp with lower kappa number requires less refining to reach a given density, resulting in less fiber shortening and lower sheet shrinkage (free drying) at a given density.