Comparative Study of Cellulose Nanofibrils Disintegrated from Different Approaches

The use of renewable materials has received a great deal of attention over the last several decades due to the environmental impact and finite resources of petroleum and other non-renewable resources. Cellulose nanofibrils are a type of nanomaterial made from natural resources like wood and non-wood plants, and they are considering promising alternative to existing materials in numerous fields (Eichhorn et al., 2010). There are essentially four types of cellulose nanomaterials, including bacterial cellulose nanofibers, electrospun cellulose nanofibers, cellulose nanocrystals, and cellulose nanofibrils (Gardner et al., 2008). Among these fibers, the latter two are extracted from natural fibers chemically and/or mechanically, and are of great potential given the available resources and viability of processing them as well as their performance. Unlike cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs) are primarily produced by subjecting pulp fibers to repeated shear and impact to produce materials with unique network structure and high aspect ratio. Since pioneers Herrick and Turbak first produced CNFs in the 1980s, many attempts have been dedicated to seek an efficient approach to disintegrate cellulose fiber to nanofibrils, involving homogenization, ultrafine grinding, refining, microfluidization, intense ultrasonication, cryocrushing in liquid nitrogen, and high-speed blending (Qing et al., 2013). However, the microfibrils in cellulose fiber are laterally combined with rigid hydrogen bonds and highly networked. Though the fibers are initially refined by removing pectin, lignin, and hemicellulose, the individualization and liberation of CNFs are still challenging, especially in terms of larger commercial scale. To loosen the rigid structure of cellulose with intention of easier defibrillation, several pretreatments including alkaline modification, enzyme hydrolysis, and chemical oxidation have been applied. For instance, Henriksson et al. (2007) proposed that endoglucanase treatment of softwood pulp fibers made it possible to facilitate production of CNFs using a Microfluidizer. Zhu and Sabo (2011) showed that extensive enzymatic treatment of cellulose fibers yielded both sugar streams and hydrolyzed cellulose fibers and that the enzymatic treatment facilitated the production of nanofibrils. Isogai and Saito (2005) reported a chemical treatment involving 2, 2, 6, 6-tetramethylpiperidinyl-1-oxyl (TEMPO) oxidization, which converts hydroxyls of cellulose to carboxyl groups, thus creating repulsive force between individual nanofibrils. The oxidization was found to promote efficient nanofibrillation of cellulose with highly uniform width of 5-10 nm. Liimatainen et al. (2012) reported another oxidization route based on a regioselective and sequential oxidation with periodate and chlorite. However, effects of pretreatments and mechanical disintegration processes on the morphology and properties of …