Influence of electrostatic interactions on fibre/fibre joint and paper strength

A study was undertaken to explore the influence of electrostatic interactions between the fibres during sheet forming and sheet consolidation on the strength of both the fibre/fibre joints and the paper. To establish this relationship, the joint strength between individual fibres was determined and compared with the strength of sheets made from these corresponding fibres. Regenerated cellulose fibres with their charge varied by carboxymethylation (anionic fibres) and by treatment with glycidyltrimethylamoniumchloride (GTAC) (cationic fibres) were investigated. In addition to joint strength and sheet strength measurements, the fibre charge together with wet fibre flexibility and fibre swelling, were evaluated for the differently treated fibres.The joint strength between individual fibres decreased with increasing ionic strength whereas the sheet strength measurements showed an increase in strength with increasing ionic strength for the bulk charged fibres. These results were found for both anionic and cationic bulk charged fibres. The wet flexibility of the fibres increased with increasing bulk charge, but there was no change in flexibility with an increase in salt concentration. Furthermore, there was no change in flexibility by increasing the surface charge of the fibres. It is suggested that the increase in sheet strength with increasing charge of the fibre is due to increasing joint strength and that the decrease in joint strength with increasing salt concentration is due to a decrease in the surface swelling of the fibres and hence a less favourable interaction between the fibres.The increase in sheet strength for the bulk charged fibres with increasing salt concentration is probably caused by an increased probability for joint formation in the fibre/fibre contacts due to decreased electrostatic repulsion between the fibres. This is discussed in terms of a balance between electrostatic repulsion and attractive capillary forces between the fibres during forming and consolidation of the paper.