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Nanocellulose
Chapter · December 2017
DOI: 10.1016/B978-0-08-100957-4.00011-5
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Nanocellulose: Preparation
methods and applications
11
Mohammed Nasir1, Rokiah Hashim1, Othman Sulaiman1 and Mohd Asim2
Universiti Science Malaysia, Penang, Malaysia 2Universit Putra Malaysia, Selangor,
Malaysia
1
11.1
Introduction
Cellulose is the primary constituent of the plant cell wall and can be extracted from
a variety of sources, such as wood, bast fibers, grasses, seed fibers, marine animals,
algae, fungi, invertebrates, and bacteria [1,2]. Besides cellulose, the plant cell wall
also contains hemicellulose, lignin, and small amount of extractives. Although wood
species are the main source of cellulose, nonwood plants are receiving increasing
interest due to their cheap availability and lower lignin content [3]. Consequently,
their fiber delignification and purification processes are easier and less energy consuming. For these reasons, various agricultural byproducts are under intensive study
for the manufacturing of various product ranges [1,4].
Cellulose is the most abundant and renewable natural polymer in the world. It has
been used as a manufacturing material for several commodities in the food and pharmaceutical industries, as well as in paint, textiles, etc., for years [5]. However, cellulose application in high value-added applications is still limited due to its hygroscopic
nature and lack of melting properties. In recent years a wider application of cellulose
has been proposed at the nanostructure level for developing various biocompatible
products as well as a variety of commercial cellulose derivatives [4,6]. Although it
is extracted from native cellulose it possesses remarkably high physical properties
with special surface chemistry. Nanocellulose has gained increasing interest for a
wide range of applications relevant to the fields of materials science and biomedical engineering due to its renewable nature, anisotropic shape, excellent mechanical
properties, good biocompatibility, tailorable surface chemistry, and interesting optical properties. A new scope of nanocellulose application is still under investigation
in fields such as photonics, films and foams, surface modifications, nanocomposites,
flexible optoelectronics, and medical devices like scaffolds for tissue regeneration.
The most beneficial property of nanocellulose research is the green nature of the particles, their fascinating physical and chemical properties, and the diversity of applications that can be derived from this material [7].
Various physical, mechanical, and biochemical methods have been proposed but
the commercial production of nanocellulose still involves harsh chemical treatment
[8–13]. Thus in order to use nanocellulose for universal application, it is necessary
to develop a sustainable and environmentally friendly processing technique. There
have been a number of research papers or book chapters published, dedicated to
Cellulose-Reinforced Nanofibre Composites. DOI: http://dx.doi.org/10.1016/B978-0-08-100957-4.00011-5
Copyright © 2017 Elsevier Ltd. All rights reserved.
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Cellulose-Reinforced Nanofibre Composites
production, characterization, and application of nanocellulose [8,11,14–17]. Thus
a detailed study is required to address some recent advancements such as a certain
extrusion technique, enzyme pretreatment methods, and quality assessment of cellulose nanofibril (CNF), etc. The aim of this review is to compare the various methods
of cellulose extraction and suggest a simple and environmentally friendly method for
the extraction.
11.2
Plant cell wall
Plant cell walls are complex structures comprised of diverse configurations of interlocking polysaccharides [18]. Fig. 11.1 shows the simplified structure of the cell wall
and the cellulose arrangement in the plant cell [19]. Based on its structure and composition, the cell wall is divided into three different layers: the middle lamella (ML), the
primary wall (P), and secondary wall [14]. The ML contains a high amount of lignin
and is primarily responsible for binding the neighboring cells [8]. The primary wall
is approximately 30–1000 nm thick and contains three main components—cellulose,
hemicellulose, and pectin—where cellulose microfibrils (MFs) are arranged crosswise [20]. The secondary cell wall is further divided into three layers, the outer (S1),
Figure 11.1 Simplified structure of cellulose arrangement in hierarchical order from wood to
cell wall and the structure of elementary fiber.
Source: Reproduce from Dufresne A. Nanocellulose: a new ageless bionanomaterial. Mater
Today 2013;16:220–7.
Nanocellulose: Preparation methods and applications
263
middle (S2), and inner (S3) layers, which differ in their microfibrils’ angle with
respect to the fiber axis [21]. Among all, the S2 layer is the most valuable and contains
the highest amount of cellulose.
The cell wall of wood fibers is formed of repeated crystalline structures resulting
from the aggregation of cellulose chains, also known as microfibrils [19]. The secondary wall contains most of the cellulose microfibrils, aligned parallel and packed
densely in a flat helix [16]. These microfibrils are made of elementary fibrils, which
were earlier considered as the smallest morphological units in the fiber [22]. However,
recent studies have confirmed that a single cellulose polymer chain can be extracted
during CNF production [23]. Both the cellulose, i.e., elementary fibrils, and microfibrils are categorized as CNFs. The elementary fibrils have a uniform diameter in the
range of 2–20 nm; however this is irregular in the microfibrils [21,24].
11.2.1 Cellulose
Cellulose was first discovered by Payen [25]. Since then, the physical and chemical properties of cellulose have been studied intensively. It is a well-organized fibrillar arrangement that is primarily responsible for the mechanical strength of plants. It is considered as
one of the most abundant organic compounds derived from plant biomass. Around 1010
and 1011 t of cellulose biopolymer are produced each year [26] but only about 6 × 109 t
are used by various industries such as paper, textile, material and chemical industries, etc.
[15,27]. Although cellulose is a crystalline molecule its crystallinity is imperfect; a significant portion of the cellulose structure is less ordered and can be referred to as amorphous
[28]. Thus, the cellulose chain is a two-phase model containing both crystalline (ordered)
and amorphous (less ordered) regions [29]. The degree of crystallinity of native cellulose
usually ranges from 40% to 70% depending on the origin of cellulose and the isolation
method [30]. The cellulose is present in the form of the microfibrils, which are bound
together by lignin and hemicellulose [31]. These microfibrils are made up of tiny cells
having width of 10–50 µm, depending on the source [32].
11.2.2 Cellulose chemistry
Cellulose is a natural stable polymer, containing hydrogen bond network, which does
not dissolve in common aqueous solvents and does not exhibit a melting point [33].
According to fringe-micelle theory, each cellulose molecule contains several crystalline and amorphous parts [34], thus making cellulose a semicrystalline polymer comprising ordered and disordered regions within a single microfibril. Cellulose polymers
are formed by building block unit d-glucopyranose (glucose) molecules, which are
linked together by β-1,4-glucosidic bonds [21]. The chemical formula of cellulose is
(C6H10O5)n, where n is the number of repeating sugar units or the degree of polymerization (DP) [8]. The repeating unit in cellulose consists of two glucose molecules
known as anhydrocellobiose, which is the repeating unit of cellulose polymer. The DP
of native cellulose varies from 1500 to 3500 depending on the source of cellulose and
the treatments it has received [35]. The properties of cellulose-containing materials
are highly influenced by the DP of cellulose molecules [36].
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Cellulose-Reinforced Nanofibre Composites
Figure 11.2 Four different polymorphs of cellulose.
Source: Reproduce from Lavoine N, Desloges I, Dufresne A, Bras J. Microfibrillated
cellulose—its barrier properties and applications in cellulosic materials: a review. Carbohydr
Polym 2012;90:735–64.
There are four different polymorphs of cellulose: cellulose I, II, III, and IV (Fig.
11.2) [15]. Cellulose I is native cellulose, i.e., found in nature, and it occurs in two
allomorphs, Iα and Iβ [37]. Cellulose II is also called regenerated cellulose; it is the
most stable form of crystal, and emerges from aqueous sodium hydroxide treatment
of cellulose I [13,38]. The characteristic distinction between these two forms of cellulose lies in the layout of their atoms: the chain in cellulose I is in parallel direction,
whereas it is found to be antiparallel in cellulose II [38]. Cellulose IIII and IIIII can be
obtained by ammonia treatment of cellulose I and II, respectively, whereas, cellulose
IV is derived from the modification of cellulose III [15].
11.3
Nanocellulose
In recent years, nanocellulose synthesis and application has achieved remarkable
growth as polymer reinforcement in order to create high-performance biomaterials.
Cellulose molecules with at least one dimension in nanoscale (1–100 nm) are referred
to as nanocellulose [33]. The main reason for increasing interest in nanosized material is due to the fact that highly uniform material with enhanced mechanical properties can be achieved by reducing the size of the cellulose fiber [8]. It is considered a
sustainable material due to its biodegradable nature. The characteristic properties of
nanocellulose like crystallinities, surface area, and mechanical properties vary with
the extraction methods and processing techniques [7]. For example, CNC synthesis
by sulfuric acid involves the selective hydrolysis of amorphous cellulose regions and
results in highly crystalline particles with source-dependent dimensions. Negative
charged sulfate groups are attached onto the surface of CNC particles, which restricts
the aggregation of particles in aqueous suspensions due to electrostatic repulsion.
Depending on the technique and synthesis conditions of nanocellulose, which
Nanocellulose: Preparation methods and applications
265
determines its dimensions, composition and properties, it can be divided into three
main categories:
1. cellulose nanocrystals (CNCs), also known as cellulose whiskers
2. cellulose nanofibrils (CNFs), also known as nanofibrillated cellulose (NFC), microfibrillated cellulose (MFC) or cellulose nanofibers
3. bacterial cellulose (BC) and electrospun cellulose nanofibers (ECNFs)
The first two categories, i.e., CNC and CNF, are produced by disintegration of cellulose fibers into nanoscale particles (top to bottom process); however, in the case of
BC and ECNF, the low molecular weight sugars or dissolved cellulose are generated by
bacteria or electrospinning, respectively (bottom to top process) [35]. Thus, large-scale
production of BC and ECNF is difficult and commercialization remains questionable.
The commercial production of nanocellulose has already begun and is now the
focus of research is more and more on industrial applications. Earlier CNF isolation
was considered a more expensive process due to the high energy demands required in
mechanical disintegration. However, with the discovery of various pretreatment methods like 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-mediated oxidation [39,40]
or enzymatic hydrolysis [41–43], which have eased the mechanical disintegration
process, CNF has become a more attractive material for commercial applications.
Researchers are now focused on optimization of the existing techniques to develop
environmentally friendly methods, which can benefit the production process or endow
the nanocellulose with new properties.
11.3.1 Type of nanocellulose
11.3.1.1 Cellulose nanocrystals
Cellulose nanocrystals (CNCs) are commonly produced using acid hydrolysis of
cellulosic materials dispersed in water. In general, concentrated sulfuric acid is used,
which dissolves the amorphous regions of cellulose and the crystalline regions are left
alone [29]. Although this technique produces a rod-like rigid CNC with almost 90%
purity, the sulfate groups remain attached at the surface of the fibers as impurities
[16]. The length and diameter of CNCs commonly vary from a length of 200–500 nm
to a diameter of 3–35 nm.
11.3.1.2 Cellulose nanofibrils
CNFs are long entangled fibrils (µm) with a diameter in nanometer range. CNFs are
produced by high-pressure grinding of cellulosic pulp suspension and strongly entangled networks of nanofibrils are formed [44]. Unlike CNCs, which have near-perfect
crystallinity (c.90%), CNFs contain both amorphous as well as crystalline cellulose
domains within the single fibers [9]. Typically, CNFs have a diameter of 5–50 nm
and a length of a few micrometers [16]. CNF extraction from cellulosic fibers can be
obtained by three types of processes: (1) mechanical treatments (e.g., homogenization, grinding, and milling); (2) chemical treatments (e.g., TEMPO oxidation); and
(3) a combination of chemical and mechanical treatments [21].
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11.3.1.3
Cellulose-Reinforced Nanofibre Composites
Bacterial cellulose
Bacterial cellulose (BC) is also known as microbial cellulose. It is typically produced from bacteria, (e.g., Acetobacterxylinum) as a separate molecule and does
not require additional processing to remove contaminants like lignin, pectin, and
hemicellulose [45]. Furthermore in contrast to CNC and CNF biosynthesis, BC
biosynthesis involves the addition of molecules from tiny units (Å) to small units
(nm) [15]. In the biosynthesis of BC, the glucose chains are supplied inside the
bacterial body and expelled out through minor pores present on the cell wall [16].
Ribbon-shaped BC nanofibers are formed when glucose is combined with the cell
wall [45]. This ribbon-like web-shaped structure produces a 20–100 nm long unique
nanofiber system.
11.4 Preparation methods/nanocellulose synthesis
methods
11.4.1
Pretreatment methods
Pretreatment of wood cellulose fibers is a technique to reduce the energy consumption of mechanical nanofibrillation processes to improve the degree of nanofibrillation
[17]. Since energy consumption is the main drawback for the production of nanofibers
by mechanical isolation processes, the pretreatment process has become an important
step. Furthermore it improves the fibrillation process with increase in production of
nanofibers [46].
11.4.1.1
Enzyme hydrolysis
Enzymes with the ability of selective hydrolysis, such as laccase, can degrade or
modify the lignin and hemicellulose contents without disturbing cellulose content
[5,47]. Since cellulosic fibers contain many different organic compounds as a composite structure, a single specific enzyme cannot degrade the fiber. The following set
of enzymes are required to decay extra cellulose compound [48]:
1. Cellobiohydrolases: A & B type cellulases—attack greatly on crystalline cellulose
2. Endoglucanases: C & D type cellulases—attack disordered structure of cellulose
Pääkkö and Ankerfors [43] produced NFC from bleached softwood pulp. In this
method a mild enzymatic hydrolysis was applied followed by refining and homogenization. It was observed that the mild hydrolysis using endoglucanase increased
the aspect ratio without a harsh reaction compared to acid hydrolysis. An additional
advantage of enzyme pretreatment is that it increases the solids level, which allows
a smooth pass during HPH processing [49]. Janardhnan and Sain [50] studied the
TEM micrographs of enzyme-treated nanofiber and observed that more than 90%
of enzyme-pretreated nanofibers had a diameter less than 50 nm. Furthermore they
showed a higher aspect ratio and more distinct compared to untreated.
Nanocellulose: Preparation methods and applications
267
11.4.1.2 Alkaline–acid
Alkaline–acid pretreatment is the most common method used for lignin, hemicellulose, and pectin solubilization before mechanical isolation of NFC [10,11,51]. This
method included the following steps [52]:
1. Sodium hydroxides (NaOH). Soaking fibers in 12–17.5 wt% solution for 2 hours. This raises
the surface area of cellulosic fibers and eases the hydrolysis.
2. Hydrochloric acid (HCL). Soaking fibers in 1 M solution at 60–80°C. This solubilizes the
hemicelluloses.
3. Sodium hydroxides (NaOH). Treating with 2 wt% solution for 2 hours at 60–80°C. This
disrupts the lignin structure, and breaks the linkages between carbohydrate and lignin.
Alkaline pretreatment is an effective method that can improve cellulose yield from
43% to 84% [53]. It also helps to removed lignin and hemicelluloses partially from
soy hull fibers and wheat straw.
11.4.1.3 Ionic liquids
Ionic liquids (ILs) are organic salts having special properties such as nonflammability, thermal and chemical stability, and infinitely low vapor pressure [54,55]. It is an
increasing interest of researchers to study as solvents of cellulosic materials. Li, Wei
[56] treated sugarcane bagasse with 1-butyl-3-methylimidazolium chloride [(Bmim)
Cl] as ionic liquid and followed the high-pressure homogenization (HPH) technique
to prepare NFC. The pulp passed through homogenizer without clogging; these fibers
were then precipitated in water solution and regenerated by freeze-drying.
11.4.2 Mechanical process
Cellulosic materials are required to go through mechanical treatment for defibrillation. Pretreatment processing, either by chemicals or enzymes, is done before
mechanical fibrillation to ease the process [57]. Chemical treatments help in widening
the space between hydroxyl groups, increasing the inner surface, altering crystallinity, and breaking cellulose hydrogen bonds, thus enhancing surface areas, which
helps boost the reactivity of the fibers [58]. There are many mechanical methods for
converting cellulosic fiber to nanocellulose, such as homogenizing [59–61], microfluidization [62,63], grinding [64], cryocrushing [65], and high-intensity ultrasonication
(HIUS) [66–68].
11.4.2.1 High-pressure homogenization
High-pressure homogenization (HPH) is an efficient method for refining of cellulosic
fibers. It was introduced in 1983, to use nanofibril cellulose from wood pulp [69,70].
This procedure is very simple and does not involve addition of any organic solvents
[71]. In this process, cellulosic pulp is passed through a very small nozzle at high
pressure. There are many types of forces that can be applied on cellulose pulp such as
high velocity and pressure, as well as impact and shear forces, which influence fluid
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Cellulose-Reinforced Nanofibre Composites
to generate shear rates in the stream and decrease the size of fibers to the nanoscale
[66]. Many researchers have used HPH for many other raw materials, such as bleached
sugar beet by Leitner et al. [72], extraction of prickly pear by Habibi et al. [73], etc.
There are some drawbacks in HPH, such as fiber clogging. Cellulosic fibers must be
chopped very small and passed through HPH to avoid this problem. Mechanical pretreatments are before HPH to produce nanofibers from kenaf bast fiber [74].
11.4.2.2
Microfluidization
Microfluidizers work similar to HPH in the production of nanocellulose fiber.
Microfluidizers use an intensifier pump to enhance pressure, while the interaction
chamber is used for shear and impact forces against colliding streams to defibrillate the fibers [62]. Lee et al. [75] studied the aspect ratio of cellulose fiber and
observed that NFC fiber with higher surface area could be obtained by multiple
passes through the microfluidizer. The cellulose fibrils showed a higher amount of
hydroxyl (OH) groups as well as agglomeration behavior due to more surface area.
The number of passes through the homogenizer determine the size of NFC and its
surface area [75].
11.4.2.3
Grinding
Grinding is another strategy to break up cellulose into nanosize fibers. In this process,
pulp passes through a couple of stones, where one stone is fixed while the other stone
rotates. This mechanism provides shear forces to break down the hydrogen bond and
cell wall structure of fibers and convert pulp into nanoscale fibers [59]. Wang and
Drzal [76] used a commercial stone grinder to produce NFC from bleached eucalyptus pulp, which helps to study the relation between energy consumption and fibrillation time as a function of crystallinity. Friction of stones generates heat due to the
fibrillation process, which helps to evaporate water content and raise solid content,
which takes 11 hours to boost specific fibrillation energy. Number of cycles through
HPH and grinding processes affects the characterization of resultant NFC. In HPH,
pulp fiber is passed through 30 times but 14 repetitions showed effective results; in
the case of grinding, 10 cycles are recommended to create uniform size of NFC [77].
11.4.2.4
Cryocrushing
Cryocrushing is another mechanical method used to break the cellulose wall into
nanosize fibers. In this method, fibers are kept in water and cellulose absorbs water
in its cavity. Water-soaked cellulose is immersed in liquid nitrogen, which solidifies
the water content, and is subsequently crushed by mortar and pestle [78]. Applying
high-impact force on frozen cellulosic fibers leads to rupture due to applied pressure
through ice crystals resulting in conversion to nanocellulose [59]. Wang and Sain [51]
studied the HPH and cryocrushing processes to produce nanofibers from soybean
stock and observed the diameter of nanofibers in the range of 50–100 nm through
transmission electron microscopy (TEM).
Nanocellulose: Preparation methods and applications
269
11.4.2.5 High-intensity ultrasonication
The HIUS process uses hydrodynamic forces of ultrasound with oscillation power
to isolate cellulose fibrils [79]. In this process, cavities in cells of cellulose convert
into powerful mechanical oscillating power. Molecules absorb the ultrasonic energy
of high-intensity waves and consist of formation, expansion, and implosion of microscopic gas bubbles [80]. Various studies have been reported dealing with the synthesis
of nanocellulose fiber from cellulose through HIUS and oscillating power [66–68].
Wang and Cheng [81] studied the effect of temperature, concentration, power, size,
time, and distance from probe tip on degree of fibrillation of some cellulose fibers
using HIUS. It was concluded that high temperature, high power, and short fiber size
are required to make better fibrillation. It is also reported that a combination of HPH
and HIUS increased the fibrillation and provided uniformity in nanofibers compared
to HIUS alone.
11.4.2.6 Ball milling process
Ball milling is a mechanical process in which cellulose suspension is placed in a hollow cylindrical container for the production of CNF. This hollow cylindrical container
is partially filled with balls made up of ceramic, zirconia, or metal; the container
rotates and breaks cellulose cell walls through the high-energy collision between
the balls. Zhang, Tsuzuki [82] studied various factors affecting production of CNF
such as ball size, ball-to-cellulose weight ratio, grinding time, moisture content, and
carboxylic charge. Maintaining homogeneity of the produced CNF is always a major
challenge in this method.
11.4.3 Chemical hydrolysis
The NFCs produced by mechanical methods consist of alternating crystalline and
amorphous regions within the single cellulose domains [19]. In order to dissolve
the amorphous domain and permit longitudinal cutting of microfibrils, strong but
controlled acid hydrolysis treatment is performed. The obtained nanocellulose, as an
aqueous suspension exhibits crystallinity greater than 90%, are termed as CNCs. In
the acid hydrolysis process, the hydronium ion enters the amorphous regions of cellulose chains and promotes the hydrolytic cleavage of the glycosidic bonds. A mechanical treatment for nanocellulose dispersion, such as sonication, is required to prevent
agglomeration. Various strong acids have been studied successfully to degrade cellulose fiber but the most common are hydrochloric and sulfuric acids. However, for
the preparation of crystalline cellulosic nanoparticles, phosphoric acid [83], as well
as hydrobromic [84] and nitric acids [85], were recommended. The benefit of using
sulfuric acid as hydrolyzing agent is it initiates the esterification process on the cellulose surface and promotes the grafting of anionic sulfate ester groups. Furthermore,
the presence of anion groups induces the formation of a negative electrostatic layer on
the surface of the nanocrystals and helps their dispersion in water. However, it
reduces the agglomeration and reduces the thermostability of the nanoparticles
[86]. The reduced thermal stability of H2SO4-prepared CNCs can be improved by
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Cellulose-Reinforced Nanofibre Composites
neutralizing by sodium hydroxide [35]. Although these nanoparticles possess highaspect-ratio, rod-like nanocrystals, their geometrical dimensions depend on the
source of cellulose and hydrolysis methods. The length of the CNCs varies widely
in the range of a few hundred nanometers due to the diffusion control nature of the
acid hydrolysis, while the width is in the range of nanometers [7]. Acid hydrolysis
is the easiest and oldest method of CNC preparation. Recently, some other methods
have been studied that degrade the amorphous domain from cellulosic fibers, such as
enzymatic hydrolysis treatment [87], TEMPO-mediated oxidation [9], and treatment
with ILs [54].
11.5 Application
Nanocellulose is believed to be a replacement for synthetic materials in more environmentally friendly materials, and is an addition to completely new types of biomaterials, i.e., cellulose nanocomposites. Nowadays, cellulose nanocomposites are being
used in medical, automotive, electronics, packaging, construction, and wastewater
treatment applications.
11.5.1
Nanocellulose in the paper industry
About 100 million tons per annum of commercially harvested cellulose is used for
production of paper and paperboard [11]. The papermaking process involves steps
including preparing the paper components, wet refining, forming of wet sheet, pressing, drying, calendering, and finishing. Refining of cellulose fibers in water medium
is a mandatory step in papermaking in order to obtain strong paper. Recent developments by Ioelvich and Leykin [88] have shown the likelihood to increase the strength
of paper with additive of nanocellulose particles to paper compositions. Such sheets
exhibit admirable mechanical properties. These properties, according to Henriksson
et al. [89], are at least 2–5 times higher than those of common papers formed from
conventional refining processes.
11.5.2
Nanocellulose in the composite industry
In recent years, there has been a remarkable growth in interest in the use of nanocellulose as polymer reinforcement in order to create high-performance biomaterials.
The core reason for the appeal of nanosized cellulose is that material with higher
uniformity and fewer defects with enhanced mechanical properties can be achieved
by reducing the size of the cellulose fiber [90]. It can be used as a reinforcing
filler to prepare composites with solutions of water-soluble polymers to modify the
viscosity and increase mechanical properties of dry composites. Of utmost importance has been the addition of nanocellulose to biodegradable polymers, which
permits both the improvement of mechanical properties and speeds up the rate of
biodegradation [91].
Nanocellulose: Preparation methods and applications
271
11.5.3 Nanocellulose in the biomedical industry
Nanocellulose is a natural biodegradable material, highly suitable for the biomedical industry. Pure nanocellulose is nontoxic for people and it is biocompatible. For
that reason, it can be utilized for health care applications such as personal hygiene
products, cosmetics, and biomedicines. One of the most modest applications of nanocellulose is in the stabilization of medical suspensions against phase separation and
sedimentation of heavy ingredients. Chemically modified cellulose can be a promising carrier for immobilization of enzymes and other drugs [19]. Due to its nanosize,
such a carrier-drug complex can penetrate through skin pores and treat skin diseases.
Likewise, it can be used as a gentle but active peeling agent in cosmetics.
11.6
Conclusion
Nanocellulose prepared from wood cellulose has unique and promising properties,
such as high crystallinity, aspect ratio, Young’s moduli, and tensile strengths, which
originate from the properties of natural wood cellulose microfibrils. A change in
processing technique changes the properties of the resulting nanocellulose, which is
reflected in the final product. Besides the conventional mechanical methods traditionally used to prepare CNFs, such as grinding or homogenization, other promising methods are discussed, which can be instrumental to further research and industrialization
of these materials. Pretreatment techniques are an important step for economically
efficient production of CNF, since they can strongly reduce the energy consumption
required during the mechanical disintegration process. The search for new, efficient,
and environmentally friendly pretreatments remains an important objective. Details
and comparative study of three different types of nanocellulose (CNC, CNF, and
BC) will determine their respective applications. Despite all of these difficulties in
nanocellulose production, it is readily available on the market, allowing use of all its
outstanding properties. Many established as well as proposed methods have emerged
in terms of large-scale production of nanocellulose. Nanocellulose-based materials
are carbon-neutral, nontoxic, sustainable, and recyclable.
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