2005 Volume 2

Using the colloidal probe technique, the interaction between carboxymethylated cellulose films and a cellulose sphere was studied. At low pH (pH 3.5), the interaction was dominated by dispersion forces. This was due to the low dissociation of carboxyl groups within the film at pH 3.5. However, at pH greater than 5, the interaction was dominated by an electrostatic repulsion. The increase in pH had the two-fold effect of completely dissociating the charged groups as well as causing appreciable swelling of the film leading to a decrease in the van der Waals component of the interaction. From these results it can be concluded that these cellulose films are suitable for a range of surface forces measurements including electrostatic, van der Waals’, steric and adhesion forces. Furthermore, the measurement of forces using the colloidal probe technique can be extended to inorganic particles as well as for interactions between surfaces in the presence of for example wet and dry strength agents and other materials relevant to the paper-making industry.

The purpose of this study was to investigate the effect of polyelectrolytes on the adhesion forces between polyelectrolyte-coated surfaces and to relate the observed phenomena to the performance of dry strength additives in papermaking systems.

The adhesive properties (pull-off forces) between polyelectrolyte-coated surfaces in air were determined as a function of the polymer charge density and the number of contacts. We found
strong adhesion energy for model systems coated with polymers of the highest (cationic) charge density, which suggests that electrostatic surface-polyelectrolyte-surface bridges are the main cause of the adhesion. At low charge density another force contribution due to entanglement of polymer chains gives rise to a significant contribution to the adhesion forces. The disruption of the contact leads to irreversible changes in the adsorbed layers when either the number of electrostatic bridges formed is large or when entanglement effects are important for polymers with a relatively low binding strength to the surface. These results are discussed in terms of our current understanding of development of dry strength in fiber systems.

The influence of surface roughness and surface hydrophobicity on ink detachment of water-based flexographic ink was studied. It was shown that increased surface roughness and an increased surface hydrophobicity both had a negative effect on ink detachment. The increased surface roughness was suggested to increase the molecular contact area between ink and cellulose and thereby also to decrease ink detachment. Ink cellulose interaction was evaluated from interfacial energies and contact angle measurements. A new technique in which the adhesion properties between ink and the model cellulose surface were directly measured using a Micro Adhesion Measurement Apparatus (MAMA) was also used. Upon increasing the hydrophobicity of the model cellulose surface it was shown that the work of adhesion between ink and model cellulose surfaces decreased. At the same time the interfacial energy between cellulose and ink increased, as did the interfacial energy between cellulose and water resulting in a lower degree of ink detachment.