Influence of cellulose viscosity and residual lignin on water absorption of nanofibril films

The quality of cellulose nanofibril (CNF) films is affected by processing and raw material related factors. The causes of potential excessive cellulose degradation during defibrillation that may directly affect the quality of CNF films may be assessed by cellulose viscosity, which need further investigations. This study aims to investigate the influence of cellulose viscosity and residual lignin of four different hardwood species (Eucalyptus grandis - eucalyptus; Brosimum parinarioides - amapá; Parkia gigantocarpa - faveira; and Cordia goeldiana - freijó ) on water absorption of the subsequent CNF films. Viscosity of the unbleached and bleached pulps was measured by capillary viscosimetry after alkali and bleaching pre-treatments of the raw wood sawdust. Residual lignin was also determined for the bleached pulps. The films were obtained by casting method after pulp nanofibrillation through 10, 20, 30 and 40 passages in a mechanical defibrillator. The amount of water absorbed per film area (g/m2) after water immersion of the CNF films for 60 s was determined and normalized by the thickness. Overall, CNF films obtained from pulps with residual lignin and higher viscosity, and hence lower degradation by the alkali and bleaching pre-treatments, showed lower water absorption. The lowest water absorption of B. parinarioides CNF films may be highlighted. For pulps with lower viscosity, water absorption continuously increased with the increase of passages through the defibrillator, possibly due to combined degradation of pre-treatments and nanofibrillation. This tendency was observed from 10 to 40 passages for eucalyptus CNF films and from 10 to 30 passages for P. gigantocarpa CNF films.