Fibre strength in relation to molecular weight distribution of hardwood kraft pulp - Degradation by gamma irradiation, oxygen/alkali or alkali

In order to study the relation between fibre strength and molecular weight distribution (MWD), a birch wood kraft pulp was degraded by gamma-irradiati on, oxygen/alkali or alkali. The pulp samples were characterised with respe ct to zero-span tensile strength, shape factor and viscosity. The effect of degradation on the MWD of cellulose and hemicellulose was studied by size- exclusion chromatography of underivatized pulp samples. The results demonstrated a correlation between rewetted zero-span tensile s trength and shape factor. In the studied viscosity range the decrease in st rength and shape factor depended on type of degradation. In spite of a seve re decrease in viscosity, the initial pulp strength was essentially preserv ed during the gamma-treatment, slightly decreased during the oxygen/alkali treatment whereas the alkali treatment caused a large reduction in strength . The results imply that gamma-treatment neither degrades nor removes hemicel lulose. This suggests that the presence of hemicellulose is of importance f or maintaining a straight and strong fibre. Gamma-degradation leads to a fa irly even distribution profile of the cellulose portion, while the MWD of t he chemically degraded pulps acquires an additional shoulder on the low mol ecular weight end of the cellulose distribution, as the degradation proceed s. This change in the MWD profile of the chemically degraded pulps is propo sed to reflect heterogeneous degradation of the cellulose portion of the fi bre, which may contribute to augmenting inherently weak points in the fibre . The observed decrease in shape factor and fibre strength is proposed to b e a combination of heterogeneous degradation and removal of xylan, whereas the decrease in M-W of the cellulose in the studied range is of minor impor tance.