Effects of hydrophobic sizing on paper dry and wet-strength properties: A comparative study between AKD sizing of NBSK handsheets and rosin sizing of CTMP handsheets

Alkyl ketene dimer (AKD) and rosin sizing are used in papermaking to decrease paper’s tendency to absorb liquid water. Earlier information regarding the effects of internal sizing on paper dry strength is not consistent. In the present laboratory study, AKD sizing of handsheets made from Nordic bleached softwood pulp (NBSK), and rosin sizing of handsheets made from Nordic hardwood chemi-thermomechanical pulp (CTMP) were done via an internal sizing method, and by immersing handsheets in aqueous sizing agent dispersion. In the study, AKD sizing had no significant effect on the dry strength of NBSK handsheets. The result corresponds to practical experiences of papermakers. However, both AKD sizing methods resulted in a substantial and long-lasting increase of handsheet wet-strength. Unlike internal AKD sizing of NBSK handsheets, rosin internal sizing of CTMP handsheets resulted in decreased handsheet dry strength. The decrease indicates that, under the conditions present during the experiment, rosin sizing agents interfered with interfiber hydrogen bonding of CTMP fibers. Given that, in practice, no such undesired effects have been commonly linked to rosin sizing, the observed effect may be specific to sheet-making conditions. However, the effect of rosin sizing on strength properties and their variation cannot be ruled out completely.