Preparation of porous sheets with high mechanical strength by the addition of cellulose nanofibrils

Porous sheets with high mechanical strength were prepared with natural pulp fibers (hardwood and softwood bleached kraft pulp) and polyethylene terephthalate (PET) by adding cellulose nanofibrils (CNF) through a wet-laid forming method. The structure and mechanical properties of the porous sheets were investigated in this study depending on the type of mixed fibers, the CNF ratio, and drying methods (cylinder-drying and freeze-drying). The apparent density and porosity of porous sheets changed according to the morphology of the mixed fibers. PET fibers resulted in a sheet with a bulkier structure, whereas natural pulp fibers produced relatively dense sheets. Sheets with high porosity could be prepared by freeze-drying due to the inhibition of hydrogen bonds and low shrinkage during drying. Pore characteristics were evaluated in terms of air permeability and average pore diameter. The air permeability of porous sheets decreased as the amount of CNF increased. Mercury porosimetry showed that the average pore diameter in sheets ranged from 0.6 to 36.7 A mu m depending on the type and mixing ratio of fibers and drying conditions. Sheet tensile properties were improved by the addition of CNF and changed depending on mixing combinations and drying methods. The applicability of freeze-dried porous sheets as filter media was examined by evaluating the filtration efficiency and pressure drop. Porous sheets with a HwBKP/CNF combination showed a filtration efficiency of 99.944 %, which approached the efficiency of a high efficiency particulate air filter. However, further study is required to reduce the pressure drop during filtration for better use as an air filter.