Effect of charge on the fibre and paper properties of bleached industrial kraft pulps

The influence of fibre charge on fibre and paper properties was investigated using five industrially produced hardwood (Betula verrucosa) and five softwood (Pinus sylvestris) ECF and TCF bleached kraft pulps. All pulps had been dried before the experiments.

The relationship between total charge and tensile strength of paper could be explained as follows: (1) The swelling of the rewetted fibres (and fines) increases with increasing charge. (2) Fibre flexibility increases with increasing swelling. (3) Increasing flexibility promotes the conformability of the fibres. This results in the formation of a denser paper sheet with higher relative bonded area (RBA) and higher tensile strength. The fibre-fibre bond strength did not vary much between these fully bleached fibres.

Both the swelling and wet fibre flexibility of hardwood increased more rapidly with charge than in the case of softwood. In both cases, swelling increased with charge even at such high ionic strengths that the effects of the osmotic pressure created when ionizable groups dissociate from (Donnan equilibrium) suppressed. It could be concluded that the swelling of rewetted fibres was governed by the following factors: (1) the swelling of fibre polysaccharides (hemicellulose), (2) the Donnan equilibrium, (3) the extent to which interfibrillar bonding increases when the fibres are dried (''hornification''), and (4) the ability of the fibre walls to resist the swelling pressure. The charge affected not only the Donnan equilibrium but also hornification, which decreased as the content of ionizable groups in the fibres increased. Swelling was greater for hardwood than for softwood fibres because the charge on hardwood fibres was higher, but also because the cell walls were thinner and contained more hemicellulose, i.e. their elastic response to swelling was weaker.

For softwood, fibre flexibility and bonding properties were influenced not only by the charge content but also by fibre coarseness. For the hardwood samples investigated, the morphological properties of the fibres did not vary greatly. It seems that in terms of pulp and paper properties, the charge on the fibre is of greater importance for bleached hardwood than for bleached softwood kraft pulp.

The amount of energy required to reach a certain bonding level by beating decreased considerably when the mechanical action was combined with a high concentration of ionizable groups. This may be due to both the swelling pressure created by ionization and to decreasing hornification. It is evident that the properties of the fibre network in the paper formed by bleached kraft pulp can be managed mechanically by controlling the amount of the charge (ionizable groups) or by a combination of these factors.