Effect of Surface Modification by Oleic Acid on Physical Properties of Cellulose Nanofibers

Oleic acid was used as a hydrophobic agent to modify cellulose nanofiber (CNF) and the reaction time and fatty acid content were tested in relation to the hydrophilic properties of the products as well as the physicochemical properties of CNF. It was found that the degree of substitution (DS) increased by extending the reaction time though the fatty acid content had no effect on hydrophobicity of CNF. The success of the esterification reaction was confirmed by Fourier transform infrared spectroscopy. Higher degree of substitution led to increased contact angle of CNF surfaces with water, which indicated the increased surface hydrophobicity of modified CNF. The X-ray diffraction analyses showed a lowering trend in crystallinity index and crystallite size with increases in DS value. Surface modification changed the thermal stability of CNF by lowering the degradation temperature from 290.8°C for unmodified cellulose to 195.4°C for highly esterified cellulose. Scanning electron microscopy micrographs revealed that after esterification of CNF with oleic acid, its filamentous shape was preserved. As a result, although the surface modification of CNF by fatty acid increased its hydrophobicity and its ability to mix with non-polar polymers, but it changed CNF physicochemical characteristics and weakened its functional properties.