Filament spinning of unbleached birch kraft pulps: Effect of pulping intensity on the processability and the fiber properties

Man-made lignocellulosic fibres were successfully prepared from unbleached birch kraft pulps by using the Ioncell-F technology. Pulps with different lignin content were produced by tailored kraft pulping with varying intensity. The degree of polymerization of the pulps was adjusted by acid-catalyzed hydrolysis and electron beam treatment. All substrates were completely soluble in 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH] OAc) and the respective solutions were spinnable to yield fibres with good to excellent mechanical properties despite the use of only mildly refined wood pulp. The tensile properties decreased gradually as the lignin concentration in the fibres increased. Changes in the chemical composition also affected the structure and morphology of the fibres. Both the molecular orientation and the crystallinity decreased while the presence of lignin enhanced the water accessibility. The effects of the crystallite size and lignin content on monolayer water adsorption are discussed.