Effects of addition method and fibrillation degree of cellulose nanofibrils on furnish drainability and paper properties

The objectives of this work were to investigate the effect of the degree of fibrillation and addition method of cellulose nanofibrils (CNFs) into the wet end of the papermaking process on the drainage, filler retention, strength and optical properties of handsheets. CNFs were produced with the grinding method, and the degree of fibrillation was manipulated by varying the passing number through the grinder from 15 to 50 passes. The CNFs were added to the wet end of the papermaking process in two different ways: a precipitated calcium carbonate (PCC)-CNF composite filler and a wet-end additive. The results showed that the water retention capacity of CNFs increased as the fibrillation was performed, and this resulted in an increase in drainage time. The solid content of handsheets filled with composite filler was about 1–2% higher than in the CNF-added sheets after wet pressing. Furthermore, the composite filler-filled sheets with higher ash content had similar tensile strength, compared with the CNF-added sheets. It was also found that water retention capacity of CNFs increased as the fibrillation was performed, and this resulted in an increment in drainage time and a reduction in solid content of sheets after wet pressing. The more fibrillated CNFs significantly (p < 0.05) increased the density and tensile strength of sheets, which might be due to the increased number of inter-fiber bonds. Filler retention and opacity were not significantly influenced by the fibrillation degree of CNFs.