EFFECT OF CHARGE ON THE FIBER AND PAPER PROPERTIES OF BLEACHED INDUSTRIAL KRAFT PULPS

The influence of fibre charge on fibre and paper properties was invest igated using five industrially produced hardwood (Betula verrucosa) an d five softwood (Pinus sylvestris) ECF and TCF bleached kraft pulps. A ll pulps had been dried before the experiments. The relationship betwe en total charge and tensile strength of paper could be explained as fo llows: (1) The swelling of the rewetted fibres (and fines) increases w ith increasing charge. (2) Fibre flexibility increases with increasing swelling. (3) Increasing flexibility promotes the conformability of t he fibres. This results in the formation of a denser paper sheet with higher relative bonded area (RBA) and higher tensile strength. The fib re-fibre bond strength did not vary much between these fully bleached fibres. Both the swelling and wet fibre flexibility of hardwood increa sed more rapidly with charge than in the case of softwood. In both cas es, swelling increased with charge even at such high ionic strengths t hat 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 fa ctors: (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, whic h decreased as the content of ionizable groups in the fibres increased . Swelling was greater for hardwood than for softwood fibres because t he charge on hardwood fibres was higher, but also because the cell wal ls were thinner and contained more hemicellulose, i.e. their elastic r esponse to swelling was weaker. For softwood, fibre flexibility and bo nding properties were influenced not only by the charge content but al so by fibre coarseness. For the hardwood samples investigated, the mor phological properties of the fibres did not vary greatly. It seems tha t in terms of pulp and paper properties, the charge on the fibre is of greater importance for bleached hardwood than for bleached softwood k raft pulp. The amount of energy required to reach a certain bonding le vel by beating decreased considerably when the mechanical action was c ombined 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 mechanicall y by controlling the amount of the charge (ionizable groups) or by a c ombination of these factors.