The anionic surface charge of cellulose fibres plays a significant role in the adsorption of strength chemicals, and this subsequently influences the strength properties of the end product – paper or board. Dissolving grade birch fibres, kraft birch fibres and carboxymethylated cellulose (CMC) modified kraft birch fibres with the same basic fibre dimension, but containing different amount of surface charges, were used in the present work. The adsorption behaviour of cationic starch, anionic polyacrylamide (APAM) and polyelectrolyte complexes (PEC) as paper dry strength chemicals were evaluated for these three differently charged fibres by polyelectrolyte titration. It was found that the saturated adsorbed mass of cationic starch was proportional to the surface charge of the cellulose fibres, and the adsorbed amount was lower than the theoretical saturated adsorbed mass when measured in 0.01M NaCl, indicating that the adsorption is a process driven by the entropic gain from the release of counter ions to the charges on the fibres and the polyelectrolytes. As a second adsorbed layer, anionic chemicals were adsorbed onto the cationic starch pre-treated fibres and there was no direct correlation between the initial surface charge and the saturation adsorption. Dissolving grade birch fibres adsorbed slightly more APAM and PEC than the other fibre types in this study, most probably due to more accessible surface area after the removal of most of the hemicellulose. Handsheets were formed with saturated adsorbed amount of the tested strength chemicals with kraft birch fibres. Tensile strength and strain at break showed significant improvement with all investigated strength agents. At the same time, the density of paper sheets was lowered following the addition of h the PEC.

Adsorption capacity and performance of paper strength agents on birch fibres with different surface charge

ACS Spring 2022

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