Influence of electrolytes on the swelling and strength of kraft-liner pulps

In the present paper, the effect of common process electrolytes NaCl, Na(2)SO(4) and CaCl(2) on fibre swelling and paper strength has been investigated. Results show that there is an initial increase in the water retention value (WRV) of the fibres when the salt concentration is increased at a pH of 5 in the solution surrounding the fibres. At higher salt concentrations, the WRV decreases. This behaviour is most pronounced for Na(2)SO(4) and smaller for NaCl and CaCl(2). The increase in swelling is explained as being due to an increase in pH in the fibre wall and a subsequent increased dissociation of the carboxyl groups during the initial increase in ionic strength. When the electrolyte concentration is further increased, the difference in osmotic pressure between the interior of the fibre wall and the surrounding solution decreases and eventually the pressure difference drops to zero. It is also found that there is no unique relationship between the WRV and the strength of paper formed from the fibres treated with different electrolyte concentrations.