Observations on interfibre bonding and fibre segment activation based on the strength properties of laboratory sheets

The aim of this study was to gain better understanding of bonding and activation in paper and to clarify the relationships between these two phenomena and the strength properties of paper. Bonding and activation were studied through examining the mechanical properties of paper. Both in-plane strength properties (tensile strength, tensile stiffness, elastic breaking strain and in-plane tear strength) and z-directional strength properties (Scott bond strength) were examined. The properties of fibre network were varied by beating of chemical pulp fibres and by introducing different levels of drying stress to the network at different solids contents. Different mixtures of chemical and mechanical pulps were used as well. As a generalisation, it can be said that the in-plane properties of laboratory sheets were improved by increasing drying stress. This improvement arises from increasing activation, ie. the removal of slackness and curls from the unbonded fibre segments within the network. Activation straightens those segments into load-bearing units. On the other hand, z-directional strength properties, or bond strength, in most cases decreased by increasing drying stress. The strain introduced to the network affects the bonded areas of fibres negatively, but the extent of this effect depends quite much on the fibre composition, degree of drying stress and the solids content at which strain has been introduced. It seems that the structure of the bonded area influences sheet behaviour and the development of strength properties significantly. Bonding and activation are both essential properties of fibre network in relation to its strength properties.