The potential of lignin to increase the hydrophobicity of micro/nanofibrillated cellulose (MNFC)

The demand for novel cellulosic material, such as micro/nanofibrillated cellulose (MNFC) is expected to face increasing growth due to its unique properties and evolving, high value applications in packaging, biomedical and nanocomposite materials. However, the hydrophilicity of MNFC has limited some of its applications, particularly in the packaging sector. Common modifications to improve hydrophobicity of MNFC often involves costly and environmentally challenging chemicals. Lignin is naturally hydrophobic, environmentally benign and as described herein, could be an effective agent to improve the hydrophobicity of MNFC. Softwood Kraft lignin (SWKL) was first considered due to its commercial availability. When SWKL was dissolved in alkaline solution and acid precipitated onto MNFC, substantial amounts of lignin were deposited on the surface, resulting in a two-fold increase in initial water contact angle as compared to the control. However, no significant improvement on MNFC hydrophobicity was observed as the contact angle was unstable over time. It was apparent that the contact angle measurements were strongly influenced by the roughness of the paper, the porous nature of cellulose and the extent of lignin homogeneity on cellulose surface. To enhance the efficacy of the approach, hot pressing lignin-containing papers near lignin’s glass transition temperature was assessed. It was hoped that this would help redistribute the lignin, resulting in better lignin homogeneity on the fiber’s surface. Initial results seemed promising as the contact angle was stable over a period of two minutes after hot pressing. Other attempt to incorporate lignin, such as spray coating with hot pressing, was evaluated to enhance homogenous lignin coverage on the paper. Contact angles as high as 85° and 95° were achieved, for SWKL and organosolv lignin respectively. Lignin coverage as low as 1% was able to impart hydrophobicity using the spray coating method. Although the water vapor transmission rate (WVTR) could be substantially reduced by hot pressing, the incorporation of lignin onto the paper did not reduce the WVTR significantly. This work showed that hot pressing lignin-containing papers resulted in improved hydrophobicity, likely due to the redistribution of lignin on fiber surfaces and the formation of a denser fiber network.Forestry, Faculty ofGraduat