Press dewatering and nip rewetting of paper containing nano- and microfibril cellulose

The effect of cellulose nano- and microfibrils on press dewatering and nip rewetting was studied in order to help understand how to use these challenging materials in papermaking. Rewetting was measured by using a displacement sensor setup on a press simulator. The results show that both fibril types have a detrimental effect on dewatering at higher grammages where pressing is flow-controlled. However, at lower grammages where dewatering is pressure-controlled, the effect was lower or even negligible. The dominant mechanism by which fibrils impact dewatering is by plugging interfiber pores and reducing sheet permeability. Rewetting was shown to be mainly affected by sheet grammage and fibril dosage level. At the lowest grammages, the minimum moisture ratio during press impulse was shown to be well below the water retention value of the furnishes. This indicates that all the interfiber water and much of the cell-wall water was removed in pressing, before being reintroduced in rewetting.