Mechanical properties of paperboard with a needled middle layer

Needling of formed kraft pulp at a dry content of 20 % was previously shown to result in a redistribution of the fibres in the plane, resulting in local regions with a higher fibre structure density. The bending properties of a three-ply paperboard with a needled middle layer have here been studied, and it is shown that a needled middle layer can increase the bending resistance of a layered board by 8 % and that the peak bending angle can be increased by up to 20 %, although the bending stiffness may not be changed significantly. The needled middle layers had a higher compressive stiffness in the out-of-plane direction, but the effect of the needling on the tensile strength in the out-of-plane direction was insignificant.