Effect of cooking and bleaching conditions on the properties of eucalyptus kraft fluff pulps

A series of Eucalyptus globulus kraft pulps was cooked under the same conditions (160 °C, 2 h) using different active alkali (AA) charges of 13%, 15%, 17% and 18%. All pulps were further oxygen delignified (100 °C, 1 h) and bleached using the elemental chlorine free (ECF) and the totally chlorine free (TCF) sequences. The obtained bleached pulps were dry-defibrated on a pilot scale hammermill at different intensities (rpm) and analysed for knot content, absorption capacity and air-laid fibre network strength. These fluff pulp parameters were related to the chemical composition of pulps, fibre morphology and the physical structure of the cellulose assessed by solid-state 13C nuclear magnetic resonance spectroscopy (13C NMR). Being dry-defibrated, pulps cooked at an increased AA revealed a reduction of the knots and the ECF pulps showed better defibration ability than TCF pulps. It was concluded that absorption capacity of the fluff pulps is the result of multiple occurring factors, such as the knot content, fibre´s hydrophilicity and morphology and the water retention capacity of the fibre web, whose contributions are not always unambiguous. This explains the non-evident dependence of the absorption capacity of fluff pulps on cooking and bleaching conditions. At the same time, the fibre network strength was sensitive to the conditions of pulping and bleaching and always increased with the increase in cooking AA. The network strength of TCF fluff pulps was 20–70% greater than that of ECF fluff pulps.