The use of polyelectrolyte multilayers of cationic starch and CMC to enhance strength properties of papers formed from mixtures of unbleached chemical pulp and CTMP-II : Part II: Influnence of addition strategy, fibre treatment and fibre type

Polyelectrolyte multilayers (PEM) consisting of cationic starch and anionic carboxymethylcellulose (CMC) have been applied to different pulp fibres in order to enhance the out-of-plane sheet strength properties of a typical packaging board furnish. An unbleached softwood chemical pulp was treated with multilayers consisting of two layers of cationic starch and one layer of CMC, and then mixed with different mechanical and chemimechanical pulps. Hand sheets were prepared with the aid of the Rapid Köthen sheet former from stocks consisting of 20% treated chemical pulp and 80% mechanical or chemimechanical pulp, which was either PGW from spruce, HT-CTMP from spruce or birch, or a standard spruce CTMP. Multilayer treatment significantly improved Scott Bond values and in some cases improved the tensile index, with the achieved effects being significantly larger than the effects of applying starch alone. Positive effects were obtained by treating only 20% of the furnish, showing a very high efficiency of the adsorbed multilayers. Compared to earlier work, one important finding was that the PEM treatment should preferably be applied only to the chemical pulp and not on the entire stock. It was possible to increase the out-of-plane strength properties, measured as Scott Bond values, with just a very small increase in density of the sheets. Multilayer treatment of the chemical pulp improved the joint strength between the fibres while maintaining the high bulk of the sheets prepared from the stiff mechanical and chemimechanical fibres