Effects of fibre deformations on strength and runnability of wet paper

Fibre kinks and curl, i.e. the shape of fibres are known to have a great effect on strength properties of dry and wet paper. In this study, the effect of fibre shape factor on water removal, wet web tensile and especially relaxation properties, i.e. the tension holding capacity of wet web was studied. Bleached softwood kraft pulp (CSF 500 ml) was thickened to dryness level 25% and mechanically treated in a kitchen mixer in order to change the shape factor of fibres. Relaxation properties of paper were measured by IMPACT test rig which is especially designed for testing of runnability potential of furnishes. The tension holding capacity of wet paper and the wet web runnability in paper machine dryer section are known to correlate well. The relaxation tests were carried out on 2% straining level followed by 0.475 seconds relaxation. Four trial points were used: Hot disintegrated pulp, pulp with no mechanical treatment and pulps mechanically treated for 15 and 45 minutes. The results indicated that the decrease of the shape factor of fibres reduces the stregth and relaxation properties of dry and wet paper. The tension after relaxation was dropped dramatically after mechanical treatment due to the lowered shape factor of fibres. One unit change in shape factor decreased the residual tension and strength of wet paper by 10%. For wet samples the effect of shape factor increased by increasing dryness. Mechanical treatment of fibres led to a lowered WRV and density and at the same time to a higher shrinkage of hand sheets. This indicates that mechanical treatment lowered the axial stiffness of fibres. The tensile strength of dry paper was affected strongly by the shape factor, but also by the drying method. Compared to free shrinkage samples, the restricted shrinkage samples enhanced dry paper tensile strength on average by 30% at a constant shape factor levels. The results indicate that in order to achieve good runnability on the paper machine, the shape factor of fibres should be controlled.