materials Review From Cellulose to Cellulose Nanofibrils—A Comprehensive Review of the Preparation and Modification of Cellulose Nanofibrils 1, 1, 1 1 1,2, 1,2 1 Tan Yi y, Hanyu Zhao y, Qi Mo , Donglei Pan , Yang Liu *, Lijie Huang , Hao Xu , Bao Hu 1 and Hainong Song 3 1 College of Light Industry and Food Engineering, Guangxi University, Junwu Rd, Xixiangtang District, Nanning 530004, China; [email protected] (T.Y.); [email protected] (H.Z.); [email protected] (Q.M.); [email protected] (D.P.); [email protected] (L.H.); [email protected] (H.X.); [email protected] (B.H.) 2 Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Junwu Rd, Xixiangtang District, Nanning 530004, China 3 Guangxi Bossco Environmental Protection Technology Co., Ltd., 12 Kexing Road, High-tech Zone, Nanning 530012, China; [email protected] * Correspondence: [email protected]; Tel.: +86-155-7832-3385 Co-first author. y Received: 24 September 2020; Accepted: 30 October 2020; Published: 10 November 2020 Abstract: 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. Keywords: cellulose nanofibrils; preparation process; surface modification; chemical modification 1. Introduction As the most abundant natural polymer on earth, cellulose occupies 40–50% of the earth’s total biomass reserves. The cellulose solely produced through photosynthesis is as high as 1011–1012 tons per year [1,2] and has always been regarded as an inexhaustible green resource [3]. In the natural environment, cellulose is mainly stored in plants and microorganisms [3]. Its molecular structure is shown in Figure1. The molecular backbone of cellulose is a linear rigid chain linked by β-D-glucopyranose (AGU) through 1,4-glycosidic bonds in a chair-shaped conformation [4]. The presence of tri-and hydroxyl groups, resulting from the strong hydrogen bonds composed of Materials 2020, 13, 5062; doi:10.3390/ma13225062 www.mdpi.com/journal/materials Materials 2020, 13, 5062 2 of 32 Materials 2020, 13, x 2 of 34 groups,polyhydroxyl indicate groups,that the indicatecellulose that has thea high cellulose cohesive has microfiber a high cohesive network microfiber structure network and the structure hydrogen and bondingthe hydrogen effect makes bonding cellulose effect makesdifficult cellulose to dissolve diffi incult common to dissolve solvents in common [1,5,6]. solvents [1,5,6]. Figure 1. Cellulose molecular structure. Figure 1. Cellulose molecular structure. In 1982, Turbak et al. used a high-pressure homogenizer to extract nanocellulose, namely cellulose nanofibrilsIn 1982, (CNFs),Turbak fromet al. eucalyptus used a high pulp-pressure [3,7]. With homogenizer advancing to research, extract CNFsnanocellulose, have exhibited namely high cellulosetransparency, nanofibrils excellent (CNFs), mechanical from eucalyptus properties andpulp good [3,7] biocompatibility.. With advancing They research, have beenCNFs gradually have exhibitedapplied high to antibacterial transparency, packaging, excellent corrosionmechanical inhibitor properties carriers, and good gels, biocompatibility. transparent conductive They have films, been3D printing,gradually composite applied to materials antibacterial and drugpackaging, delivery corrosion [8–10]. At inhibitor the same carriers, time, compared gels, transparent with other conductivenanocellulose films, fibers, 3D printing, CNFs have composite a simple materials preparation and process.drug delivery CNFs [8 were–10] developed. At the same earlier time, and comparedare currently with theother main nanocellulose target for the fibers, industrial CNFs production have a simple of nanocellulose. preparation Thisprocess. review CNFs focuses were on developednanocellulose earlier fibrils and [are1,11 currently,12]. the main target for the industrial production of nanocellulose. This reviewAlthough focuses CNFs on nanoc demonstrateellulose fibrils excellent [1,11,12] functionality,. they have not yet been industrialized. TheAlthough industrial CNFs production demonstrate of CNFs excellent has beenfunctionality, mainly hinderedthey have by not two yet factors.been industrialized. Firstly, CNFs The are industrialprimarily production manufactured of CNFs by high-pressure has been mainly homogenizers, hindered by high-energy two factors. ball First millsly (mechanical, CNFs are primarily chemistry), manufacturedmicrofluidizers, by ultra-lowhigh-pressure temperature homog crushingenizers, andhigh other-energy methods. ball However,mills (mechanical because of chemistry), the relatively microfluidizers,high lengths and ultra diameters-low temperature of CNFs, their crushing specific and surface other area methods. is large, However, the surface because hydroxyl of groups the relativelyeasily form high hydrogenlengths and bonds diameters and of the CNFs, fibers their easily specific form surface hard agglomerations area is large, theand surface are hyd diffiroxylcult to grodisperse.ups easily Simply form hydrogen relying onbonds mechanical and the fibers force toeasily shear form and hard separate agglomerations the fibers and generates are difficult a lot of to energydisperse. consumption. Simply relying In addition, on mechanical the instantaneous force to shear high and temperature separate the generated fibers generates during mechanical a lot of energygrinding consumption. affects the In CNFs’ addition, crystal the structure,instantaneous destroys high theirtemp networkerature generated structure during and changes mechanical the gel grinding affects the CNFs’ crystal structure, destroys their network structure and changes the gel behavior of the nanofibrils. Secondly, the fibrillation of cellulose must be completed in liquid and, after behaviordrying, of irreversible the nanofibrils. hydrogen Second bondingly, the occurs fibrillation between of thecellulose CNFsc must fibers, bethat completed is, keratinization in liquid [and,13,14 ]. after drying, irreversible hydrogen bonding occurs between the CNFsc fibers, that is, keratinization Therefore, CNFs products mostly exist as water dispersions with low solid contents and their storage [13,14]space. Therefore, is large and CNF theirs products transportation mostly cost exist is high.as water To makedispersions the preparation with low ofsolid CNFs contents easier, and reduce their the storage space is large and their transportation cost is high. To make the preparation of CNFs easier, energy consumption required to prepare CNFs and obtain CNFs with a certain functional structure, reducea “two-step the energy method” consumption can be used required for the to prepare preparation CNFs of and nanofibrils. obtain CNFs In this with technique, a certain thefunctional cellulose structure, a “two-step method” can be used for the preparation of nanofibrils. In this technique, the is pretreated first and then the microfibril structure of cellulose is peeled off by a gentle mechanical cellulose is pretreated first and then the microfibril structure of cellulose is peeled off by a gentle method to obtain the nanofibrils [11]. Among them, the pretreatment of cellulose involves three main mechanical method to obtain the nanofibrils [11]. Among them, the pretreatment of cellulose involves techniques. (i) The use of cellulase hydrolysis to cut and peel off the fibers and make subsequent three main techniques. (i) The use of cellulase hydrolysis to cut and peel off the fibers and make mechanical separation easier [15–18]. (ii) Chemical modification of cellulose, such as using coupling subsequent mechanical separation easier [15–18]. (ii) Chemical modification of cellulose, such as agents (lauric acid, malic acid, etc.) to replace the hydroxyl groups on the cellulose surface or by using coupling agents (lauric acid, malic acid, etc.) to replace the hydroxyl groups on the cellulose reacting with hydroxyl groups to reduce the hydroxyl content on the cellulose surface, thereby reducing surface or by reacting with hydroxyl groups to reduce the hydroxyl content on the cellulose surface, the inter-cellulose and fiber. The hydrogen bond content and internal strength of the element make it thereby