On the effect of recycling on cross-sectional shapes and dimensions of sulphate pulp fibres

A prerequisite for the interfibre bonding needed in strong papers is that fibres should be flexible in order to conform and come in close contact during forming and consolidation of the sheet. Fibre flexibility depend on fibre wall thickness, fibre width and fibre collapsibility, as well as the elastic and plastic properties of the fibre wall. The purpose of this study was to investigate the effect of recycling and beating on cross-sectional shapes and dimensions of chemical pulp fibres. The distributions and/or average values of fibre wall thickness, fibre wall area, lumen area, fibre width, inner and outer perimeter, degree of collapse and cross-sectional shape are reported and discussed. In order to measure cross-sectional fibre dimensions before shrinkage due to drying, freeze dried fibre samples from virgin as well as unbeaten and beaten recycled chemical pulp fibres with either hydrogen or sodium as counter-ions of acidic groups were investigated using SEM and image analysis techniques. Results showed that recycled fibres had a somewhat smaller fibre wall area and fibre wall thickness. No difference was detected between unbeaten recycled fibres in sodium form and hydrogen form. The fibre wall thickness increased after beating of the recycled fibres. It was also shown that the cross-sectional shape of unbeaten recycled fibres was less circular compared with virgin fibres when compared at the same fibre wall thickness, and that thick-walled fibres were more circular than thin-walled fibres. This was most likely due to a loss of swelling in recycled fibres, as well as a flattening of these fibres during pressing and drying in the first papermaking cycle. After beating, the recycled fibres regained most of the circular cross-sectional shape typical for virgin fibres, and approximately the same shape was regained at the same swelling level.