A study on AKD-size retention, reaction and sizing efficiency Part 2: The effects of electrolytes, retention aids, shear forces and mode of addition on AKD-sizing using anionic andcatio- nic AKD-dispersions

摘要

The effects of electrolytes and eight different cationic polymers on alkyl ketene dimer (AKD) size retention, extent of reaction and sizing efficiency were investigated using a C-14 labelled cationic AKD-dispersion. Sizing experiments were conducted in a standard British hand sheet former, using a Britt Dynamic Drainage Jar (BDDJ) to pre-shear the pulp suspension in order to simulate more practical stock preparation conditions. Some elaboration on shear conditions was also made. The C-14 labelled AKD-dispersion was stabilised using a classical cationic starch/lignosulfonic acid recipe, resulting in a weakly cationic AKD-dispersion (z-potential = +10.5 mV) at pH = 8.0. In the absence of polyelectrolytes, the cationic AKD-particles are deposited onto negatively charged fibre surfaces through a heterocoagulation mechanism. The deposition is very sensitive to electrolytes and already at a concentration of 2 x 10~(-3) M concentration of CaCl_2, the AKD retention is essentially zero. As expected more highly charged fibres were found to be less sensitive to electrolytes. The various cationic polyelectrolytes employed, were all effective retention aids for AKD-size although a cationic poly-acrylamide with a moderate charge density (molar D.S. = 10%) showed best performance. Cationic polyelectrolytes are usually not expected to interact with cationic AKD-particles, but the amphoteric surface character of the AKD-particles, due to the lignosulfonic acid and the fatty acid, readily explains the interaction. It was also found that the addition of C-PAM makes the AKD-retention less sensitive to electrolytes. In a second series of experiments an. -anionic AKD-dispersion was used. The C-14 labelled AKD-dispersion was stabilised using a classical lignosulfonic acid, resulting in a strongly negative dispersion (z-potential = -56 mV) at pH = 8.0. Among the various cationic polyelectrolytes employed for the retention of the anionic AKD-dispersion, the cationic polyacrylamides showed best performance with retention levels up to 90 %. The results show clearly that the sizing process is not very sensitive to the state of agglomeration of the AKD-particles as suggested by the Cassie equation. Irrespective of the retention aid the used Cobb_(60)-value fell on the same graph if plotted vs. the reacted amount of AKD, irrespective if anionic or cationic AKD-dispersions were used. It was only when the stock was excessively agglomerated, deviations to this behaviour were found.