Study of the joint strength between regenerated cellulose fibres and its influence on the sheet strength

The joint strength between single fibres was investigated directly by measurements of perpendicular cross bonded fibres and compared with the corresponding paper properties. Regenerated cellulose fibres, rayon, were used. They were carboxymethylated in order to introduce charges to the fibres. Two different treatments were used to prepare fibres with charges located on the surface of the fibres and charges located throughout the bulk of the fibres. The charge characteristics, with respect to total charge and surface charge of the fibre were determined. The swelling caused by these charges was analysed with respect to pore volume distribution (NMR measurements) and water retention capacity (WRV). Charge characterisation showed that surface charged fibres could be produced by pre-drying fibres prior to carboxymethylation. Furthermore, WRV and NMR-relaxation measurements showed that an increase in the surface charge had no significant influence on the swelling whereas an increase in bulk charges had a large effect. The strength of the joints and the paper tensile properties are most affected by the surface charges, probably due to an increase in the molecular contact area with increasing charge. The results indicated that increased surface softness increased the critical strain of paper made from these modified fibres.