The influence of a calendering pre-treatment on the refining of chemical softwood pulps

This study examines the potential offered by a calendering pre-treatment of chemical pulp fibres before refining. This mechanical treatment involves the application of linear loads, shear forces and heat, applied in a single pass and in multiple passes. The effects of the treatment were examined with both unrefined and refined fibres and with untreated fibres and once dried fibres. For unrefined fibres, the mechanical treatment tended to cause collapse, flattening, straightening and cle-swelling of the fibres. These effects were reflected as an improvement in tensile index compared to never dried and once dried pulps. The tear index was also improved compared to that of never dried pulp. Laboratory refining of mechanically treated fibres produced a higher freeness than refining of conventional never dried and once dried pulps at the same level of refining energy. This is because refining of mechanically treated fibres causes less external fibrillation and produces less fines and less short fibres. In dewatering, the mechanically treated fibres showed a similar behaviour to dried fibres, with lower dewatering resistance and a more open network than never dried fibres. The strength properties of handsheets showed that mechanically treated refined fibres have a higher tensile index, tensile stiffness and Scott bond than once dried pulps and a higher tear index than never dried pulps. Accordingly, the treatment results in a good compromise between good strength properties and drainability compared to the properties obtained with never dried pulp and dried pulp. The treatment was carried out using an industrial calender device which makes it suitable for industrial application on existing pulp drying machines and wet-lap machines.