Changes in the chemical reactivity and swelling behaviour of pulps caused by refining and drying processes

Recent changes in digestion and bleaching processes have yielded modern ECF and TCF pulps which differ in their chemical composition and microfibril s tructure. When these pulps are refined and dried, they will behave differen tly in terms of reactivity and swelling. Theoretically, as the surface is e nlarged, the chemical components become more easily accessible and lignin a nd hemicelluloses are redistributed between pulp and water under the refini ng action. All this will impact chemical reactions, because the substances contained in chemical pulps and which are accessible after refining, influe nce the circuit water composition. The reasons for the different swelling b ehaviour of pulps are manifold and can hardly be covered by the WRV sum par ameter. Swelling of a pulp fibre is impacted by capillary structures formin g in the delignification process, by the content of swellable cell wall com ponents and of charged functional groups. Swelling of fibres is accompanied by enhanced fibre flexibility which essentially influences interfibre bond ing in the sheet and thus the paper strength.