This review summarizes the preparation methods of cellulose nanofibrils (CNFs) and the progress in the research pertaining to their surface modification. Moreover, the preparation and surface modification of nanocellulose were comprehensively introduced based on the existing literature. The review focuses on the mechanical treatment of cellulose, the surface modification of fibrillated fibers during pretreatment, the surface modification of nanocellulose and the modification of CNFs and their functional application. In the past five years, research on cellulose nanofibrils has progressed with developments in nanomaterials research technology. The number of papers on nanocellulose alone has increased by six times. However, owing to its high energy consumption, high cost and challenging industrial production, the applications of nanocellulose remain limited. In addition, although nanofibrils exhibit strong biocompatibility and barrier and mechanical properties, their high hydrophilicity limits their practical application. Current research on cellulose nanofibrils has mainly focused on the industrial production of CNFs, their pretreatment and functional modification and their compatibility with other biomass materials. In the future, with the rapid development of modern science and technology, the demand for biodegradable biomass materials will continue to increase. Furthermore, research on bio-based nanomaterials is expected to advance in the direction of functionalization and popularization.

Because of its non-toxic, pollution-free, and low-cost advantages, environmentally-friendly packaging is receiving widespread attention. However, using simple technology to prepare environmentally-friendly packaging with excellent comprehensive performance is a difficult problem faced by the world. This paper reports a very simple and environmentally-friendly method. The hydroxyl groups of cellulose nanofibrils (CNFs) were modified by introducing malic acid and the silane coupling agent KH-550, and the modified CNF were added to cassava starch as a reinforcing agent to prepare film with excellent mechanical, hydrophobic, and barrier properties. In addition, due to the addition of malic acid and a silane coupling agent, the dispersibility and thermal stability of the modified CNFs became significantly better. By adjusting the order of adding the modifiers, the hydrophobicity of the CNFs and thermal stability were increased by 53.5% and 36.9% ± 2.7%, respectively. At the same time, the addition of modified CNFs increased the tensile strength, hydrophobicity, and water vapor transmission coefficient of the starch-based composite films by 1034%, 129.4%, and 35.95%, respectively. This material can be widely used in the packaging of food, cosmetics, pharmaceuticals, and medical consumables.Nanomaterials 2020, 10, 755 2 of 19 starch films, researchers have often added different types of enhancers to starch film to improve its strength [11][12][13][14]. Cellulose nanofibril (CNF) has become an ideal starch film enhancer due to its low cost, low density, renewability, recyclability, high surface area, chemical reactivity, strength, modulus, elasticity, transparency, tensile rigidity, light weight, low thermal expansion, and biodegradability (due to its nano-size characteristics) [15][16][17].Cellulose nanofibril comes from various sources of natural fibers, such as cotton, wood, corn cobs, sisal, wheat straw, flax, bamboo, rice husks, pea husks, coconut shells, bagasse, and cassava residues. However, CNF is hydrophilic and absorbs moisture when exposed [18]. Therefore, the surface hydrophobicity of CNF can be changed using various chemical modification techniques, thereby improving the compatibility and dispersibility of CNF in specific solvents [19]. Through phosphorylation, carboxymethylation, oxidation and sulfonation reactions, ionic charge can be introduced to the surface of cellulose [20-23]; esterification, silylation, amidation, urethanation, and etherification can make the cellulose surface hydrophobic [24][25][26]. In summary, no matter what surface chemistry is ultimately required, the modification technology depends almost entirely on the reaction of the hydroxyl groups on the surface of the CNF. The challenge for these chemical modification technologies is to change only the surface of the CNF, maintaining its original morphology and the complex structure of its internal hydroxyl groups.Wei et al. extracted CNF from oil palm waste by acid hydrolysis using natural lime juice as a cross-linking agent. The struc...

Composite films of polybutylene adipate terephthalate (PBAT) were prepared by adding thermoplastic starch (TPS) (TPS/PBAT) and nano-zinc oxide (nano-ZnO) (TPS/PBAT/nano-ZnO). The changes of surface morphology, thermal properties, crystal types and functional groups of starch during plasticization were analyzed by scanning electron microscopy, synchronous thermal analysis, X-ray diffraction, infrared spectrometry, mechanical property tests, and contact Angle and transmittance tests. The relationship between the addition of TPS and the tensile strength, transmittance, contact angle, water absorption, and water vapor barrier of the composite film, and the influence of nano-ZnO on the mechanical properties and contact angle of the 10% TPS/PBAT composite film. Experimental results show that, after plasticizing, the crystalline form of starch changed from A-type to V-type, the functional group changed and the lipophilicity increased; the increase of TPS content, the light transmittance and mechanical properties of the composite membrane decreased, while the water vapor transmittance and water absorption increased. The mechanical properties of the composite can be significantly improved by adding nano-ZnO at a lower concentration (optimum content is 1 wt%).

This study investigated the effectiveness of ester-modified cellulose nanocrystals derived from cassava residues as a reinforcement to starch films.

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