Nanocellulose is a term referring to
nano-structured cellulose. This may be either cellulose nanocrystal (CNC or
NCC), cellulose nanofibers (CNF) also called microfibrillated cellulose (MFC),
or bacterial nanocellulose, which refers to nano-structured cellulose produced
by bacteria.
CNF is a material composed of nanosized
cellulose fibrils with a high aspect ratio (length to width ratio). Typical
fibril widths are 5–20 nanometers with a wide range of lengths, typically
several micrometers. It is pseudo-plastic and exhibits thixotropy, the property
of certain gels or fluids that are thick (viscous) under normal conditions, but
become less viscous when shaken or agitated. When the shearing forces are
removed the gel regains much of its original state. The fibrils are isolated
from any cellulose containing source including wood-based fibers (pulp fibers)
through high-pressure, high temperature and high velocity impact
homogenization, grinding or microfluidization (see manufacture below).
Nanocellulose can also be obtained from
native fibers by an acid hydrolysis, giving rise to highly crystalline and
rigid nanoparticles which are shorter (100s to 1000 nanometers) than the
nanofibrils obtained through homogenization, microfluiodization or grinding
routes. The resulting material is known as cellulose nanocrystal (CNC).
History and terminology
The terminology
microfibrillated/nanocellulose or (MFC) was first used by Turbak, Snyder and
Sandberg in the late 1970s at the ITT Rayonier labs in Whippany, New Jersey, USA
to describe a product prepared as a gel type material by passing wood pulp
through a Gaulin type milk homogenizer at high temperatures and high pressures
followed by ejection impact against a hard surface.
The terminology first appeared publicly in
the early 1980s when a number of patents and publications were issued to ITT
Rayonier on a new nanocellulose composition of matter. In later work
Herrick[who?] at Rayonier also published work on making a dry powder form of
the gel. Rayonier has been one of the world's premier producers of purified
pulps interested in creating new uses and new markets for pulps and not to
compete with new customers. Thus, as the patents issued, Rayonier gave free
license to whoever wanted to pursue this new use for cellulose. Rayonier, as a
company, never pursued scale-up. Rather, Turbak et al. pursued 1) finding new
uses for the MFC/nanocellulose. These included using MFC as a thickener and
binder in foods, cosmetics, paper formation, textiles, nonwovens, etc. and 2)
evaluate swelling and other techniques for lowering the energy requirements for
MFC/Nanocellulose production. After ITT closed the Rayonier Whippany Labs in
1983–84, Herric worked on making a dry powder form of MFC at the Rayonier labs in
Shelton , Washington ,
USA
In the mid 1990s the group of Taniguchi and
co-workers and later Yano and co-workers pursued the effort in Japan U.S.
patents issued to P&G, J&J, 3M ,
McNeil, etc. using U.S.
Manufacture
Nanocellulose, which is also called
cellulose nanofibers (CNF), microfibrillated cellulose (MFC) or cellulose
nanocrystal (CNC), can be prepared from any cellulose source material, but
woodpulp is normally used.
The nanocellulose fibrils may be isolated
from the wood-based fibers using mechanical methods which expose the pulp to
high shear forces, ripping the larger wood-fibres apart into nanofibers. For
this purpose, high-pressure homogenizers, ultrasonic homogenizers,[better
source needed] grinders or microfluidizers can be used. The homogenizers are
used to delaminate the cell walls of the fibers and liberate the nanosized
fibrils. This process consumes very large amounts of energy and values over 30
MWh/tonne are not uncommon.
To address this problem, sometimes
enzymatic/mechanical pre-treatments and introduction of charged groups for
example through carboxymethylation or TEMPO-mediated oxidation are used. these
pre-treatments can decrease energy consumption below 1 MWh/tonne.
Cellulose nanowhiskers are rodlike highly
crystalline particles (relative crystallinity index above 75%) with a
rectangular cross section. They are formed by the acid hydrolysis of native
cellulose fibers commonly using sulfuric or hydrochloric acid. Amorphous
sections of native cellulose are hydrolysed and after careful timing,
crystalline sections can be retrieved from the acid solution by centrifugation
and washing. Their dimensions depend on the native cellulose source material,
and hydrolysis time and temperature.
In April 2013 breakthroughs[clarification
needed] in nanocellulose production were announced at an American Chemical
Society conference.
At ICAR-Central Institute for Research on
Cotton Technology, Mumbai , India 10 kg per
day. This facility was inaugurated in 2015.
Structure and properties
Dimensions and crystallinity
The ultrastructure of nanocellulose derived
from various sources has been extensively studied. Techniques such as
transmission electron microscopy (TEM), scanning electron microscopy (SEM),
atomic force microscopy (AFM), wide angle X-ray scattering (WAXS), small
incidence angle X-ray diffraction and solid state 13C cross-polarization magic angle spinning (CP/MAS),
nuclear magnetic resonance (NMR) and spectroscopy have been used to
characterize typically dried nanocellulose morphology.
A combination of microscopic techniques
with image analysis can provide information on fibril widths, it is more
difficult to determine fibril lengths, because of entanglements and
difficulties in identifying both ends of individual nanofibrils.[page needed]
Also, nanocellulose suspensions may not be homogeneous and can consist of
various structural components, including cellulose nanofibrils and nanofibril
bundles.
In a study of enzymatically pre-treated
nanocellulose fibrils in a suspension the size and size-distribution were
established using cryo-TEM. The fibrils were found to be rather mono-dispersed
mostly with a diameter of ca. 5 nm although occasionally thicker fibril bundles
were present. By combining ultrasonication with an "oxidation
pretreatment", cellulose microfibrils with a lateral dimension below 1 nm
has been observed by AFM. The lower end of the thickness dimension is around
0.4 nm, which is related to the thickness of a cellulose monolayer sheet.
Aggregate widths can be determined by CP/MAS
NMR developed by Innventia AB , Sweden
Pulp chemistry has a significant influence
on nanocellulose microstructure. Carboxymethylation increases the numbers of
charged groups on the fibril surfaces, making the fibrils easier to liberate
and results in smaller and more uniform fibril widths (5–15 nm) compared to
enzymatically pre-treated nanocellulose, where the fibril widths were 10–30 nm.
The degree of crystallinity and crystal structure of nanocellulose.
Nanocellulose exhibits cellulose crystal I organization and the degree of
crystallinity is unchanged by the preparation of the nanocellulose. Typical
values for the degree of crystallinity were around 63%.
Viscosity
The unique rheology of nanocellulose
dispersions was recognized by the early investigators. The high viscosity at
low nanocellulose concentrations makes nanocellulose very interesting as a
non-caloric stabilizer and gellant in food applications, the major field
explored by the early investigators.
The dynamic rheological properties were
investigated in great detail and revealed that the storage and loss modulus
were independent of the angular frequency at all nanocellulose concentrations
between 0.125% to 5.9%. The storage modulus values are particularly high (104
Pa at 3% concentration) compared to results for cellulose nanowhiskers (102 Pa
at 3% concentration). There is also a particular strong concentration
dependence as the storage modulus increases 5 orders of magnitude if the
concentration is increased from 0.125% to 5.9%.
Nanocellulose gels are also highly shear
thinning (the viscosity is lost upon introduction of the shear forces). The
shear-thinning behaviour is particularly useful in a range of different coating
applications.
Mechanical properties
Crystalline cellulose has interesting
mechanical properties for use in material applications. Its tensile strength is
about 500MPa, similar to that of aluminium. Its stiffness is about 140–220 GPa,
comparable with that of Kevlar and better than that of glass fiber, both of
which are used commercially to reinforce plastics. Films made from
nanocellulose have high strength (over 200 MPa), high stiffness (around 20 GPa)
and high strain[clarification needed] (12%). Its strength/weight ratio is 8
times that of stainless steel. Fibers made from nanocellulose have high
strength (up to 1.57 GPa) and stiffness (up to 86 GPa).
Barrier properties
In semi-crystalline polymers, the crystalline
regions are considered to be gas impermeable. Due to relatively high
crystallinity, in combination with the ability of the nanofibers to form a
dense network held together by strong inter-fibrillar bonds (high cohesive
energy density), it has been suggested that nanocellulose might act as a
barrier material. Although the number of reported oxygen permeability values
are limited, reports attribute high oxygen barrier properties to nanocellulose
films. One study reported an oxygen permeability of 0.0006 (cm3 µm)/(m2 day
kPa) for a ca. 5 µm thin nanocellulose film at 23 °C and 0% RH. In a related study, a more than
700-fold decrease in oxygen permeability of a polylactide (PLA) film when a
nanocellulose layer was added to the PLA surface was reported.
The influence of nanocellulose film density
and porosity on film oxygen permeability has recently been explored. Some
authors have reported significant porosity in nanocellulose films, which seems
to be in contradiction with high oxygen barrier properties, whereas Aulin et
al. measured a nanocellulose film density close to density of crystalline
cellulose (cellulose Iß crystal structure, 1.63 g /cm3) indicating a very dense film with a
porosity close to zero.
Changing the surface functionality of the
cellulose nanoparticle can also affect the permeability of nanocellulose films.
Films constituted of negatively charged cellulose nanowhiskers could
effectively reduce permeation of negatively charged ions, while leaving neutral
ions virtually unaffected. Positively charged ions were found to accumulate in
the membrane.
Multi-Parametric Surface Plasmon Resonance
is one of the methods to study barrier properties of natural, modified or
coated nanocellulose. The different antifouling, moisture, solvent,
antimicrobial barrier formulation quality can be measured on the nanoscale. The
adsorption kinetics as well as the degree of swelling can be measured in
real-time and label-free.
Foams
Nanocellulose can also be used to make
aerogels/foams, either homogeneously or in composite formulations.
Nanocellulose-based foams are being studied for packaging applications in order
to replace polystyrene-based foams. Svagan et al. showed that nanocellulose has
the ability to reinforce starch foams by using a freeze-drying technique. The
advantage of using nanocellulose instead of wood-based pulp fibers is that the
nanofibrills can reinforce the thin cells in the starch foam. Moreover, it is
possible to prepare pure nanocellulose aerogels applying various freeze-drying
and super critical CO
2 drying techniques. Aerogels and foams can
be used as porous templates. Tough ultra-high porosity foams prepared from
cellulose I nanofibrill suspensions were studied by Sehaqui et al. a wide range
of mechanical properties including compression was obtained by controlling
density and nanofibrill interaction in the foams. Cellulose nanowhiskers could
also be made to gel in water under low power sonication giving rise to aerogels
with the highest reported surface area (>600m2 /g) and lowest shrinkage during drying (6.5%) of
cellulose aerogels. In another study by Aulin et al., the formation of
structured porous aerogels of nanocellulose by freeze-drying was demonstrated.
The density and surface texture of the aerogels was tuned by selecting the
concentration of the nanocellulose dispersions before freeze-drying. Chemical
vapour deposition of a fluorinated silane was used to uniformly coat the aerogel
to tune their wetting properties towards non-polar liquids/oils. The authors
demonstrated that it is possible to switch the wettability behaviour of the
cellulose surfaces between super-wetting and super-repellent, using different
scales of roughness and porosity created by the freeze-drying technique and
change of concentration of the nanocellulose dispersion. Structured porous
cellulose foams can however also be obtained by utilizing the freeze-drying
technique on cellulose generated by Gluconobacter strains of bacteria, which
bio-synthesize open porous networks of cellulose fibers with relatively large
amounts of nanofibrills dispersed inside. Olsson et al. demonstrated that these
networks can be further impregnated with metalhydroxide/oxide precursors, which
can readily be transformed into grafted magnetic nanoparticles along the
cellulose nanofibers. The magnetic cellulose foam may allow for a number of
novel applications of nanocellulose and the first remotely actuated magnetic
super sponges absorbing 1 gram of water within a 60 mg cellulose aerogel foam
were reported. Notably, these highly porous foams (>98% air) can be
compressed into strong magnetic nanopapers, which may find use as functional
membranes in various applications.
Surface modification
The surface modification of nanocellulose
is currently receiving a large amount of attention. Nanocellulose displays a
high concentration of hydroxyl groups at the surface which can be reacted.
However, hydrogen bonding strongly affects the reactivity of the surface
hydroxyl groups. In addition, impurities at the surface of nanocellulose such
as glucosidic and lignin fragments need to be removed before surface
modification to obtain acceptable reproducibility between different batches.
Safety aspects
Health, safety and environmental aspects of
nanocellulose have been recently evaluated. Processing of nanocellulose does
not cause significant exposure to fine particles during friction grinding or
spray drying. No evidence of inflammatory effects or cytotoxicity on mouse or
human macrophages can be observed after exposure to nanocellulose. The results
of toxicity studies suggest that nanocellulose is not cytotoxic and does not
cause any effects on inflammatory system in macrophages. In addition, nanocellulose
is not acutely toxic to Vibrio fischeri in environmentally relevant
concentrations.
Applications
The properties of nanocellulose (e.g.
mechanical properties, film-forming properties, viscosity etc.) makes it an
interesting material for many applications and the potential for a
multibillion-dollar industry.
Paper and paperboard
There is potential of nanocellulose
applications in the area of paper and paperboard manufacture. Nanocelluloses
are expected to enhance the fiber-fiber bond strength and, hence, have a strong
reinforcement effect on paper materials. Nanocellulose may be useful as a
barrier in grease-proof type of papers and as a wet-end additive to enhance
retention, dry and wet strength in commodity type of paper and board products.
It has been shown that applying CNF as a coating material on the surface of
paper and paperboard improves the barrier properties, especially air
resistance. It also enhances the structure properties of paperboards (smoother
surface).
Nanocellulose can be used to prepare
flexible and optically transparent paper. Such paper is an attractive substrate
for electronic devices because it is recyclable, compatible with biological
objects, and easily degrades when disposed of.
Like resin-free lignocellulose fiberboard
which are produced using wet process, high tough cellulose nanofiber board with
thickness of 3 mm was also
introduced by Yousefi et al., 2018.
Composite
As described above the properties of the
nanocellulose makes an interesting material for reinforcing plastics.
Nanocellulose has been reported to improve the mechanical properties of, for
example, thermosetting resins, starch-based matrixes, soy protein, rubber
latex, poly(lactide). The composite applications may be for use as coatings and
films, paints, foams, packaging.
Food
Nanocellulose can be used as a low calorie
replacement for today’s carbohydrate additives used as thickeners, flavour
carriers and suspension stabilizers in a wide variety of food products and is
useful for producing fillings, crushes, chips, wafers, soups, gravies, puddings
etc. The food applications were early recognised as a highly interesting
application field for nanocellulose due to the rheological behaviour of the
nanocellulose gel.
Hygiene and absorbent products
Applications in this field include: Super
water absorbent material (e.g. for incontinence pads material), nanocellulose
used together with super absorbent polymers, nanocellulose in tissue, non-woven
products or absorbent structures and as antimicrobial films.
Emulsion and dispersion
Nanocellulose has numerous applications as
a food additive, and in the general area of emulsion and dispersion
applications in other fields. Oil in water applications were early recognized.
Early investigators had explored the area of non-settling suspensions for
pumping sand, coal as well as paints and drilling muds.
Oil recovery
Hydrocarbon fracturing of oil-bearing
formations is a potentially interesting and large-scale application.
Nanocellulose has been suggested for use in oil recovery applications as a
fracturing fluid. Drilling muds based on nanocellulose have also been
suggested.
Medical, cosmetic and pharmaceutical
The use of nanocellulose in cosmetics and
pharmaceuticals was also early recognized. A wide range of high-end
applications have been suggested:
Freeze-dried nanocellulose aerogels used in
sanitary napkins, tampons, diapers or as wound dressing
The use of nanocellulose as a composite
coating agent in cosmetics e.g. for hair, eyelashes, eyebrows or nails
A dry solid nanocellulose composition in
the form of tablets for treating intestinal disorders
Nanocellulose films for screening of
biological compounds and nucleic acids encoding a biological compound
Filter medium partly based on nanocellulose
for leukocyte free blood transfusion
A buccodental formulation, comprising
nanocellulose and a polyhydroxylated organic compound
Powdered nanocellulose has also been
suggested as an excipient in pharmaceutical compositions
Nanocellulose in compositions of a
photoreactive noxious substance purging agent
Elastic cryo-structured gels for potential
biomedical and biotechnological application.
Matrix for 3D cell culture
Other applications
As a highly scattering material for
ultra-white coatings.
Activate the dissolution of cellulose in
different solvents
Regenerated cellulose products, such as
fibers films, cellulose derivatives
Tobacco filter additive
Organometallic modified nanocellulose in
battery separators
Reinforcement of conductive materials
Loud-speaker membranes
High-flux membranes
Computer components
Capacitors
Lightweight body armour and ballistic glass
Corrosion inhibitors
Commercial Production
Although wood-driven nanocellulose was
first produced in 1983 by Herrick and Turbak, its commercial production
postponed till 2010, mainly due to the high production energy consumption and
high production cost. Inventia Co. in Sweden Canada ), Nippon
(Japan ), Nano Novin Polymer
Co. (Iran ), Maine University
(USA), VTT (Finland ),
Melodea (Israel
Source from Wikipedia
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