Improving the papermaking properties of kraft pulp by controlling hornification and internal fibrillation

Abstract

The objective of this thesis was to improve press dewatering and paper properties of kraft pulps by controlling hornification and internal fibrillation. Hornification was induced by drying and by pressing pulps, and internal fibrillation was developed in refining.

Hornification and internal fibrillation are related to the change in fibre pore structure. In this thesis, a thermoporosimetry technique with cyclohexane as an absorbate was tested and found suitable for detecting the change in pore size and pore volume during drying and refining. The results show that for never-dried fibres, refining mainly expands the large pores in the cell wall, whereas it has only a slight effect on the small pores. Drying closes most of the large pores and a substantial amount of the small pores. For dried fibres, refining not only expands the large pores but also reopens the small pores to a certain extent. Even though the pore volume of previously dried pulps can be recovered by refining (i.e., the pulp can be reswollen), some small pores, which are closed in drying, are not reopened by normal levels of refining. In other words, refining does not completely reverse hornification.

Drying of pulps greatly reduces pulp swelling, enhancing dewatering but impairing tensile strength. Dried pulps offer a far better combination of dewatering and tensile strength than never-dried pulps. One possible reason is that some small hard-to-dewater pores in the fibre wall are irreversibly closed by drying, which enables better dewatering. However, pulp drying is energy-consuming. Pressing pulps may provide an economical way to improve dewatering, while maintaining paper strength properties. Pressing hornifies pulps, which promotes dewatering but impairs tensile strength to a certain extent. On the other hand, pressing causes fibres to flatten, with the flattened fibres providing more surface contact for bonding, thus increasing density and tensile strength. Never-dried pulps which were pressed before refining were found to give both improved dewatering and better tensile strength.

The refining results support the earlier view that internal fibrillation is largely produced by a cyclic compressive action. It is suggested that fibres need to be turned over in refining and compressed from different directions in order to disrupt their internal structure and cause internal fibrillation. Compression also facilitates fibre straightening, but does not promote external fibrillation and fines generation. At the same swelling level, more straightened pulps give higher tensile strength, and pulps with less fines and external fibrillation enable better dewatering. Hence, to achieve an optimum combination of dewatering and tensile strength, chemical pulp refining should aim at increasing internal fibrillation, straightening fibres, and keeping the amount of fines and external fibrils at a low level.