Porosity of wood pulp fibers in the wet and highly open dry state

The porosity of kraft and dissolving pulp samples with different drying histories were studied in the wet (Water Retention Value, Solute Exclusion) and dry state (N₂ sorption, Scanning Electron Microscopy) and the results were compared. An efficient drying protocol was created to minimize porosity loss upon water removal for dry analyses. The samples were first solvent exchanged by dialysis to dry acetone and then critical point dried (CPD) from CO₂. It was found out that even after reaching relatively high specific surface areas of 200–300 m²/g for CPD pulp, the samples had shrunk and most of the micropores were not detectable until rewetting. However, the dry state mesoporosity correlated well with the wet state values. SEM was used to examine the pore structure and fibril arrangement of the CPD fiber sample. The kraft pulps were found to have a larger specific pore volume and surface area than the dissolving pulps as measured by N₂ sorption. However, SEM analysis of the samples showed the dissolving pulps had a more homogenous and more open surface porosity than the kraft samples. The kraft pulp showed less hornification after drying and rewetting than the dissolving pulp. Heating of kraft pulp to 100 °C during drying increased the irreversible pore closure. Pores were often arranged in chain-like formations and signs of surface fibril aggregation were detected.