The effect of fines on fibre bonding:Cross-sectional dimensions of TMP fibres at potential bonding sites

The density and tensile strength properties of handsheets made of the long fibre fraction of a pulp can be significantly improved by the addition of chemical or mechanical fines. This raises questions about how fines affect potential bonding sites, what fibre crossing points look like in wet-pressed and dried sheets, and how they appear in cross-sectional view. An attempt has been made to answer these questions by using a confocal laser scanning microscope (CLSM) to measure changes in cross-sectional dimensions of drying fibres at potential bonding sites. Handsheets made of just the long fibre fraction of a thermomechanical pulp were bulky and had little strength, while those containing fines additions were far denser and stronger. Sheets containing kraft fines were significantly stronger than those containing TMP fines. Measurements showed that there was no significant difference in the cross-sectional dimensions of fibres in sheets containing fines. It was concluded that fines additions helped to bring fibres into closer contact, thus facilitating bonding, and that physicochemical differences between kraft and TMP fines accounted for the greater density and strength properties of sheets containing kraft fines.