The effect of molecular weight on the performance of paper strength-enhancing polymers

The influence of dextran molecular weight (77 000 to 2 000 000) on the strength of fibre-fibre bonds was investigated. A series of cationic dextra with different molecular weights, from 77 000 to 2 000 000, was prepared and the adsorption behaviour on fibre was characterized. The maximum amount of adsorbed dextran increased with decreasing molecular weight, while the adsorbed layer thickness on colloidal silica increased with increasing molecular weight. At saturation coverage, the molecular weight of dextran did not affect the tensile strength of handsheets made from unbeaten bleached kraft pulp. The strength improvement with polymer addition did not correspond to increased optical bonded area (opacity). The external surface of fibre accessible to dextran of molecular weight 2 000 000 was estimated as 35 m²/g.