Fibre deformations induced by different mechanical treatments and their effect on zero-span strength

Fibre deformations have a significant effect on fibre strength and sheet properties. There is little information, however, on the kinds of deformations different types of treatments induce and how they affect the fibre strength. In the present study, first-thinning bleached pine kraft pulp was treated with three mechanical devices: a wing defibrator (high consistency treatment), an E-compactor (high consistency treatment) and a conventional Valley beater (low consistency treatment). The fibre properties were determined with a fibre analyser. The fibre cutting induced by the hydrochloric acid (HCl) treatment (‘cleavage index’) was used for the quantification of the fibre defects. The zero-span tensile strength of dry and wet paper was used to estimate the fibre strength. Each mechanical treatment induced fibre deformations in its own characteristic way. The wing defibrator induced fibre kinks and curl whereas the E-compactor, in addition to fibre cutting, favoured kinks. Low consistency Valley beating straightened the fibres and released fibre deformations. The fibre deformations, especially the number of kinks, correlated well with the wet zero-span tensile strength. The cleavage index had some correlation with the zero-span tensile strength, but the results indicated that the cleavage index may not be directly related to the mechanical defects in fibres but depend more on the chemical conditions on the fibre surface and the wall structure.