Modification of fluff pulp fibres: Cross-linking and alkali extraction

Licentiate thesis, 2011

Fluff pulp has been one of the key components of hygienic products, such as diapers, for many years. Fluff pulp is dry defibrated and air-laid into the absorption core of diapers. The introduction of superabsorbent polymers (SAPs) in the 1980s decreased the dependence on fluff pulp, as the SAPs became the main absorbing material. Nevertheless, in order to decrease reliance on oil-based SAPs, it is important to investigate fluff pulp once again. This thesis investigates possibilities for modifying fluff pulp for use in absorption applications. In the first part of the thesis, softwood kraft pulp fibres were modified with a diepoxide in order to introduce cross-links into the fibre wall. Results show that the wet bulk under load of the low-density fibre networks increased as a result of the modification, most likely due to higher fibre stiffness. When the fibre networks were dried after the compression test, the low-density networks with the more extensively modified fibres also regained much of their bulk. In the second part of the thesis, a softwood kraft pulp and a birch kraft pulp were alkali extracted at 3 different concentrations of NaOH (2%, 4% and 8% NaOH in suspension). The extraction was done in order to remove hemicelluloses from the fibre walls. The pulps were then dried in sheet form. Sheets were also dry defibrated and air-laid to investigate the performance of the pulps in fluff pulp applications. It could be seen that the extent of hornification of the once-dried fibres increased with increasing concentration of NaOH. Further, the tensile strength index of the sheets increased when the fibres were treated with 2% NaOH, and decreased at the two highest concentrations. The birch pulp treated with 8% NaOH had a low amount of knots after defibration. This was related to the low tensile strength index of that pulp, indicating an importance of fibre-fibre bond strength on ease of defibration. The network strength of dry defibrated and air-laid softwood pulp treated with 8% NaOH was lower than the softwood pulps treated with 0%, 2% and 4% NaOH, which was most likely an effect of the high curl index of that pulp. The network strength of the birch pulps was lower than in the softwood pulps. The network strength of the birch pulps also correlated well with the amount of knots in the pulps.