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The aim of the study was to evaluate the effect of refining on the ultrastructure of spruce pulp fibres. Pulps with different molar masses of cellulose (estimated as intrinsic viscosity) were studied after PFI-refining. The molar masses of the polymers were decreased by increases in alkali concentration during pulping. Fibre surface structures were examined using Field Emission Scanning Electron Microscopy (FE-SEM) and Transmission Electron Microscopy (TEM) was used to observe changes in the internal structure of the fibres. Pulps with lower (125 and 329 ml g −1 ) intrinsic viscosity showed more damaged during refining than pulps with higher (620 and 1120 ml g −1 ) intrinsic viscosity. Observations showed pulps with lower intrinsic viscosity to have large decreases in fibre length after refining. Fibres with low intrinsic viscosity (i.e., 125 ml g −1 ) had less primary wall and S1 layer remaining and the external fibrillation and damage of the S2 layer had increased. The S2 wall of fibres with high intrinsic viscosity showed characteristic delamination. Similar delamination was not visible for fibres with low intrinsic viscosity.

Effects of refining on the fibre structure of kraft pulps as revealed by FE-SEM and TEM: Influence of alkaline degradation