The influence of dextran molecular weight (77 000 to 2 000 000) on the stre
ngth of fibre-fibre bonds was investigated. A series of cationic dextrans w
ith 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 ads
orbed layer thickness on colloidal silica increased with increasing molecul
ar weight. At saturation coverage, the molecular weight of dextran did nor
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 acc
essible to dextran of molecular weight 2 000 000 was estimated as 35 m(2)/g
.