![the environment. To overcome this limitation with plastics, cellulose substrates are used in the
construction of various functional materials.
Cellulosic fibre products such as paper and paperboard are equally predominant with
synthetic plastic materials but as they are biodegradable, they are perfectly safe for the
environment. The hydrophilic nature of cellulose macro fibres limits the specific application in
various fields. In packaging, cellulose fibre products offer the advantages of being recyclable
and biodegradable but due to the presence of large pores and high affinity towards water and
its vapour, it has poor water vapour and oxygen barrier performance. To note, these materials
were not for fit for packaging under medium and high humidity environment. To mitigate these
limitations, the cellulose fibre products are often associated with plastics, wax and/ or
aluminium, for enhancing their barrier properties. However, these materials suffer from serious
environmental issues, as being difficult and inefficient to recycle. Recently, cellulose nanofibers
such as nanocellulose (NC) plays a predominant role in material development due to its
outstanding properties.
Cellulose is the most predominant fibrous biopolymer among natural materials. It has
outstanding properties and is readily obtainable. Cellulose from plant sources such as wood,
grass, agricultural and lignocellulosic waste has potential for replacing plastics and provides
a platform for resolving many issues such as, reduction of oil stocks, synthetic plastic pollution,
and carbon footprint. This means cellulose is a renewable and non-toxic, eco-friendly material
and biocompatible with nature. The source of cellulose via the photosynthesis process in
plants is estimated to be from 1011
–1012
tonne per annum [2]. Microbial fermentation is an
alternative way to produce bacterial cellulose.
Cellulose is used in regenerated cellulose fibres, and in cellulose ethers and esters.
These materials are used as additives, coating materials, thickeners, stabilizers, binders and
fillers in foods, pharmaceutics, sanitary, and textile etc., Due to its mechanical properties and
barrier performance, it plays a major role in developing packaging materials such as papers
and paper boards. The presence of surface hydroxyl groups in the cellulose gives a platform
of chemical modification, resulting in tailoring its properties and functionality. The reduction of
cellulose macro fibres via mechanical, chemical and enzymatic processing into cellulose
nanofibers has potential benefits and drastically changes its characteristics from macro fibres
[3]. Nanocellulose (NC) consists of entangled cellulose nanofibrils several micrometres long
containing both amorphous and crystalline regions. These regions are well connected through
intra and intermolecular bonds. The crystalline domain of the cellulose is maintained by
hydrogen bonding and van der Waals forces among the hydroxyl groups in the cellulose fibrils.
The twists and torsions are present in the amorphous structure. The reduction of cellulose](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-4-320.jpg)
![fibres into nanofibrils increases its inter-fibrillar bonds permitting a strong compact network
and drastically increases its strength, elastic modulus, viscosity, optical properties and barrier
performance. NC has high aspect ratio, forming a compact fibrous network with a considerable
density via strong interfibrillar bonds. It gives a comparable barrier performance with synthetic
polymers.
Figure 2-Molecular Structure of Cellulose, obtained from Nechyporchuk et al, 2016 [4]
Figure 1 - Hierarchical Structure of Cellulose from wood to the molecular level cellulose, obtained
from Isogai, 2011 [1]](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-5-320.jpg)
![Figure 3-Crystalline and amorphous regions of cellulose, obtained from Abdul Khalil et al,
2016 [5]
Cellulose is a linear carbohydrate polymer with repeating units of Cellobiose, a
disaccharide with the formula C12H22O11, consisting of two β-glucose molecules linked by a
β(1→4) bond. As it is a natural polysaccharide, it is susceptible to microbial and fungal
degradation. It is found as an assembly of cellulose fibrils chains forming well-organized fibre
wall in the wood. Figure 1 shows the organisation of cellulose fibres from wood to a single
fibre and Figure 2 shows the structure of cellulose and surface hydroxyl groups. Figure 3
shows cellulose containing two domains, namely crystalline and amorphous regions.
The nomenclature of cellulosic nanomaterials has not been reported uniformly in the
literature. The fibre diameter of raw cellulose was reduced via mechanical, chemical and
enzymatic processing resulting in the formation of nanocellulose. The term micro-fibrillated
cellulose (MFC) refers to the product produced via the delamination of wood fibres by
mechanical pressure with chemical or enzymatic treatment. The term MFC is consistent with
nanofibrils, micro fibrils and nano fibrillated cellulose [2] with diameter from 10 nm to 100 nm,
fibril length in the range from 500 to 10000 [6] and an aspect ratio of 100 [7]. It is composed
of formed cellulose semi-microcrystalline fibrils generally produced via high-pressure
homogenization of pulp from wood [8]. The average diameter of MFC (DIACEL –KY 100S) to
make NC films and composites is approximately 73 nm and aspect ratio of 142 ± 28 [9].
1.3.1. NANOCELLULOSE – A NOVEL FIBROUS MATERIAL
NC is a long and flexible cellulose fibril formed from an entangled fibrous network of
microfibrils and nanofibrils. NC contains both amorphous and crystalline regions. These
regions mainly contribute to functionality of materials prepared from NC. Nano-fibrils have high
aspect ratio. Figure 4 shows the cellulose nanofibrils after two pass high pressure
homogenization and the diameter of cellulose nanofibrils after this condition is approximately
20 nm. The important mechanical properties of NC are high elastic modulus (150 GPa) and
high tensile strength (10 GPa). Its properties are controlled by their cellulose fibrils’ diameter
and length. The aspect ratio of the cellulose fibrils depends on the source, processing method](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-6-320.jpg)
![and particle type. The source of NC also decides its properties such as applicability and
functionality in various fields [10].
Figure 4 - SEM Micrograph of Nanocellulose via two Pass High-Pressure Homogenization.
1.3.2. CHARACTERISTICS OF NANOCELLULOSE
The diameter of fibrils in NC ranges from 5 to 100 nm and they are typically several
micrometres in length [11]. They consist of both crystalline and amorphous regions in the fibre
chain [12]. The crystalline region contributes mostly to the functionality of materials such as
barrier and composites materials [6]. The fibril diameter distribution of NC and its aspect ratio
depends on the source, pre-treatment processing and fibrillation method etc., and indirectly
control its rheological and interfacial properties.
The diameter and aspect ratio of NC can be evaluated via an image analysis of
micrographs collected from SEM, TEM and AFM. Sedimentation of NC fibres is also performed
to evaluate their gel point. The gel point is defined as the lowest fibre concentration in the
nanocellulose suspension and a transition point from a highly dilute solution of NC fibres into
a concentrated suspension. At this point, nanofibrils in the suspension forms a compact
network of fibres due to hydrogen bonding between fibres. This network is known as the
threshold of connectivity. Beyond this concentration, the NC suspension loses its mechanical
strength due to insufficient contact of fibres with each other. This property depends on the](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-7-320.jpg)
![aspect ratio of nanocellulose fibres. This characteristic has an important effect on the drainage
time in vacuum filtration for NC film fabrication [13], [14].
The notable properties are light in weight, optical transparency, chemical functionality
and modification, dimensional stability and good barrier properties. Nanocellulose has
potential compatibility with other materials like bio- natural polymers, proteins and living cells
for biomedical applications and developing composites. The surface hydroxyl groups on NC
provide an active site for various chemical modification and functionalization. The rheological
behaviour of NC plays a major contribution in the coating process for producing barrier layers
on the paper substrates and its shows pseudoplastic behaviour under shear thinning fluids
[15].
1.3.3. SCOPE OF LITERATURE REVIEW
The predominant objective of this chapter is to critically review with the production of
films containing NC and its composites. This chapter also reviews the methods of preparation,
its advantages and disadvantages on various methods of preparation of NC film and the
reasons for the rapid processing of these materials. The potential applications for
nanocellulose film and its composites as a barrier, composites, smooth films, membranes and
other notable applications will also be discussed. Additionally, nanocellulose as sustainable
barrier materials and the need for improvements in its barrier performance, with a particular
focus on developing ultra smooth nanocellulose films and its composites, have been
addressed.
1.4. NANOCELLULOSE PRODUCTION
The large sources of cellulose are wood, seed fibres (for example, cotton, coir etc.),
bast fibres (flax, hemp, jute, ramie etc.) and grasses (bagasse, bamboo etc.)., The least
popular sources of cellulose are marine animals (tunicate), algae, fungi, invertebrates and
bacteria [16]. Wood is the predominant feedstock of cellulose and is classified into hardwood
and softwood. Apart from cellulose as a main constituent, hemicellulose, lignin and inorganic
salts are also present in the wood. The principal difference between hardwood and softwood
is the complexity in the structure with the level of heterogeneity in cellulose fibrils [17].
Hardwood fibres are more rigid than that of softwood having a high Runkel ratio, i.e. thick fibre
wall relative to internal diameter [18]. Therefore, the processing of softwood to produce NC
consumes less energy than that of hardwood as a feedstock [19]. Similarly, low energy is
required for non- woody feedstocks such as grasses and marine sources to produce NC.
These feedstocks contain low lignin with a more suitable cellulose structure for purifying and
fibrillating the cellulose [4]. Recently, agricultural and food waste have also proved to be a
potential feedstock for producing NC via a chemical-free process [20].](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-8-320.jpg)
![It is a significant challenge to fibrillate the microfibrils into nanofibrils through the
refining process. Refining processes can be classified into the mechanical, chemical and
enzymatic processes. Before refining the microfibrils, the fibres are subjected to a pre-
treatment such as de-lignification, to reduce the energy required for refining and produce high-
quality nanofibers. De-lignification is the isolation of lignin in the lignocellulosic biomass. In
this process, lignin is depolymerised by a pulping process and the solubilized lignin and
hemicellulose in the biomass is removed by washing. Further delignification can be achieved
via bleaching with chemical agents. During these pre-treatments, cellulose can undergo
mechanical and chemical changes. The pre-treatments produce high-quality cellulose fibres,
effectively removing non-cellulosic constituents. The disintegration of cellulose fibrils is
promoted through the pre-treatment, reducing the aggregation of fibres to prevent clogs in
further fibrillation process. As a result, the fibrillation into nanofibers is improved and minimizes
energy consumption [5].
1.4.1. MECHANICAL PROCESS
The most common mechanical treatment of cellulose fibres into nanofibers is primarily
disintegration. Grinding is the most accepted method for production of NC. Recently, high
intensive homogenization is reported as a novel method to produce nanofibers from cellulose
macro fibres and its particles. However, it is an energy consuming method and demands
approximately 25,000 kWh per tonne of the production of micro fibrillated cellulose. These
methods are suitable for scaling up for production of NC. In high pressure homogenization,
the critical factors namely pressure, NC solid concentration and number of passes control the
consumption of energy [5]. The various methods of mechanical disintegration of fibres are
reported in Table 1. It summarizes the concept of cellulosic fibres reduction, its isolation
methods, its advantages and disadvantages [7].](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-9-320.jpg)
![Table 1 - Mechanical Refining for Production of Nanocellulose [7]
1.4.2. CHEMICAL PROCESS
Mechanical disintegration of macro fibres is an energy consuming process, and this
could be one of the stumbling blocks for scaling up the process. Nowadays, the demand for
cellulosic materials is exponentially increasing in both research and industry. Therefore, an
energy efficient process is required to scale up production of nanocellulose [4]. Chemical or
enzymatic processes are proven low energy processes compared with mechanical
disintegration processes and have improved fibrillation efficiency [7]. Various types of
chemical processing of cellulosic nanofibers are presented, with their disadvantages and
advantages in the following Table 2.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-10-320.jpg)
![Table 2-Chemical processing for Nanocellulose [7]
Table 2 summarizes the reaction mechanism of each chemical method for cellulose fibrillation
into nanocellulose.
1.4.3. ENZYMATIC PROCESS
Enzymatic processes can be used to produce the NC from cellulosic biomass.
Enzymatic hydrolysis involves the cleavage of glycosidic bonds in the structure of the cellulose
fibres. The classical enzymes are cellobiohydrolases that attack the crystalline region of
cellulose, and endoglucanases that attack the amorphous or disordered region of cellulose.
The subsequent fibrillation efficiency of NC production depends on the contact time of the
enzymatic treatment and concentration of enzymes during the processing [7].The advantages
of enzymatic hydrolysis are good fibrillation, reduced clogging of fibrils in the mechanical
process, and less energy consumed in this processing.
1.5. APPLICATION OF NC FILMS](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-11-320.jpg)
![The small dimension of Nanocellulose (i.e. fibre diameter and length) and its large
surface area provided a great opportunity to develop it into a more functional material for
various applications [21, 22]. Nowadays, the demand for NC is increasing due to several key
technical applications and is rapidly growing in importance. In brief, NC film is a transparent,
dense packing of cellulose nanofibrils offering good barrier performance and lower surface
roughness than paper, leading to the development of many functional materials. Therefore, it
can be used in high performance packaging, and can be a good substrate for flexible and
printable electronics, low cost diagnostics and organic displays. Although the applications of
NC film are many, in this chapter we restrict discussion to a few major applications.
1.5.1. BARRIER APPLICATIONS
Barrier materials are widely utilised for preventing the physical, chemical and
microbiological deterioration of foods, nutrients, drinks, pharmaceuticals and cosmetics.
Barrier materials require low gas and water permeability to protect the contents from the
external influences and to preserve the flavour and nature of the packaged product [23].
Commonly used barrier materials are glass, metal such as aluminium and tin and synthetic
plastics made from fossil fuels. These have good barrier properties and strength, but are not
biodegradable and recyclable [24] [25]. Nowadays, the requirement of packaging materials
should have low gas and water vapour permeability.
Synthetic plastic films and sheets are predominant in the current conventional practice.
After use, these films are thrown away as waste in landfill resulting in a threat to the
environment. The recycling of these synthetic plastics is an expensive process and retaining
its barrier performance after recycling is problematic.
Cellulosic fibre products such as paper and paperboard are alternative flexible
packaging that are biodegradable in the environment. However, the hydrophilic nature of
cellulose limits the water vapour-barrier properties and oxygen barrier properties of paper.
Due to the large pore sizes and high affinity towards water and its vapour, it has poor barrier
performance [26]. These materials are not a good fit for packaging at high humidity
environment (> 65% relative humidity) such as in frozen conditions. To mitigate these
limitations, paper and paperboards can be coated with plastics, wax or extruded aluminium to
promote their barrier properties. The synthetic plastic coatings on cellulose substrates suffer
from serious environmental issues such as difficult recycling and inefficiency in conversion into
a second life barrier material.
Nanocellulose has good barrier properties due to its fibrous network [10] and crystalline
region. The cellulose nanofibrils in the dense network form a compact structure reducing the
passage of water vapour and oxygen. This complex fibrous network increases the tortuosity of](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-12-320.jpg)
![the pathway for oxygen and water vapour transmission, thus lowering its permeation rate. NC
films have low oxygen permeability (OP) due to the complex entanglement of fibrils [26]. The
OP of NC film is reported to be 0.004 cm3
.μm.m-2
.day-1
.kPa-1
[27]. This OP value is quite
comparable with synthetic plastics such as polyvinylidene (PVDC) ( OP- 0.1-3 cm3
.μm.m-2
.day-
1
.kPa-1
) [28], poly(vinyl chloride) (PVC) (20-80 cm3
.μm.m-2
.day-1
.kPa-1
) [28]. The water barrier
properties of NC films is also extremely good under dry conditions although it is less effective
in high humidity [26] Therefore, NC is a potential alternative for synthetic plastics in barrier
application [29]. The source of fibres for making NC films, their chemical composition, physical
structure, and the pretreatment methods used to produce the nanocellulose have an effect on
their barrier performance [6].
1.5.2. PRINTED ELECTRONICS
Cellulose substrates such as paper are sustainable and low-cost substrates for
developing functional materials such as printed electronics, solar cells [30], Radio- Frequency
Identification Device (RFID) tags, Organic Light Emitting Diode (OLED)s, photovoltaic (PV)
cells, printed diagnostics, batteries, memory cards etc. [31]. Paper substrates is also used in
the fabrication of transistors and super capacitors. However, paper is a porous substrate,
highly susceptible to moisture absorption and has high surface roughness. Their properties
limit the utilization of paper in the construction of electronic materials. The surface roughness
of paper-based substrates varies from 2 to 10 micrometres and this range of roughness limits
the performance of solar cells [32]. Consequently, nanocellulose is nowadays used as a
potentially sustainable substrate for developing printed electronics due to its low surface
roughness [33]. NC provides smoothness on the substrates, is thermally stable and provides
good barrier against water and air for the construction of flexible electronic device.
1.5.3. OTHER APPLICATIONS
The surface and mechanical properties of nano-fibrillated cellulose can be tailored.
Films made from nanocellulose can have various outstanding mechanical, optical and
structural properties and these properties are useful in fabricating various functional materials
such as cellulose nanocomposites [34], inorganic nanocomposites [35], organic Transistors
and conducting materials [36], Immunoassays and Diagnostics materials [37]. As a result, it is
used in the field of photonics, as a surface modifier, in nanocomposites, biomedical scaffolds
and optoelectronics [10]. Recently, NC films have become one of the most promising high-
performance functional materials potentially used as filters for removal of virus [38],
adsorbents, catalysts [39], cell culture substrate, thermal insulator and drug carrier for drug
delivery system [40]. Moreover, due to the barrier performance of nanocellulose and size, it is
used to functionalize base sheets via creating barrier layers as a coating or to make](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-13-320.jpg)
![freestanding sheets/films and nanocomposites [4]. Recently the application of nanocellulose
in various fields has increased rapidly. There is a demand for a rapid manufacturing method
for NC film. In particular, the demand for barrier application NC film to replace synthetic
plastics in practice is urgent[41]. Table 3 summarizes the application of NC depending on its
quantity in various fields. Figure 5 shows the products developed with NC in High-performance
Fibre material Group, Department of Chemical Engineering, Monash University, Australia.
Figure 5- Applications of Nanocellulose into functional Materials from High-Performance Fibre
material research Group, BIOPRIA, Monash University, Australia.
(1). K.Shanmugam et al, Cellulose, 2017, 24, 7, 2669–2676. (2) Z. he et al, RSC Adv., 2016, 6, 21435-
21438 [42]. (3). U.M. Garusinghe et al, Scientific Reports, 8, 2306 (2018) (4). V. Swambabu et al,
Chemical Engineering Journal 265 (1) 2015. (5). Mendoza et al, J Colloid Interface Sci. 2018 1;509:39-
46. (6) Maliha , 2018 (7). U.M. Garusinghe et al, Colloids and Surfaces A, 540, 2018,233-241.
1.5.4. PRODUCTS OF HIGH-PERFORMANCE FIBRE MATERIAL RESEARCH
GROUP, MONASH UNIVERSITY.
1.5.4.1. PACKAGING
An homogenized NC film from MFC was prepared, via vacuum filtration, with an air
permeance less than 0.003 µm/Pa.S, confirming that it is a good impermeable film for
packaging applications [43]. The water vapour transmission rate (WVTR) and oxygen
transmission rate (OTR) of the NC film at 23°C and 50% RH were 44.7 g/m2
.day and 20.1
cc/m2
.day respectively [44]. These values confirmed that a vaccum filtered NC film is a good
barrier.
1.5.4.2. OIL AND WATER SEPERATION
Crosslinking between nanocellulose and polyamideamine-epichlorohydrin (PAE) was
used to produce an aerogel filter using a freeze drying cross linking method. The NC based](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-14-320.jpg)
![aerogel filter was capable of de-emulsification of oil water mixture producing 100% oil water
separation efficiency even after 10 cycles of usage and yielding 98.6 % oil water surface free
emulsion [45].
1.5.4.3. PHOTO CATALYST COMPOSITE
This composite was developed from nanocellulose -polyamide-amine-epichlorohydrin
(PAE)-TiO2 nanoparticles via vaccum filtration, and was designed as a photo catalyst for
treatment of waste water. Figure 6 shows that the titanium dioxide in the composite degrades
95% of methyl orange dye in water in 150 min under UV light irradiation. The NC matrix with
PAE strongly hold TiO2 nanoparticles and avoid the release of nanoparticles into the waste
water. This composite can be used as a recylable, low cost photo catalyst membrane for waste
water treatment [46].
Figure 6- The degradation of methyl orange solution byTiO2-MFC composite as photo catalyst.
The spectrum indicates two absorption maxima. The colour of dye solution reduced using
photocatalysis composite. The image obtained from Garusinghe et al 2018 [46].
1.5.4.4. MEMBRANES
Cellulose nanofibre membranes can be an alternative to synthetic plastic membranes
and they are recyclable and biodegradable. In this case, the novel composite was prepared](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-15-320.jpg)
![via vaccum filtration and the composite was produced with nanocellulose, silica nanoparticles
(NP) and polyamide-amine-epichlorohydrin (PAE).The size of silica nanoparticles were 22 nm
and they acted as a spacer for controlling the pore of the composite. The function of PAE in
the composite was to adhere with the negativly charged silica nanoparticles and promote the
wet strength of composite. This composite membrane has a water flux of 80 Lm-2
h-1
, Liter per
square meter and hour (LMH) and molecular weight cutoff, (MWCO) of 200 kDa. The quantity
of silica nanoparticle could be altered to tune the pore size. The function of composite mainly
play as membrane and can be used mainly in ultrafiltration applications [47].
1.5.4.5. GELS
Nanocellulose can be converted via TEMPO oxidation method, into hydrogels and gels
for numerous biological and medical appliocations. Nanocellulose gels can be used for
separation of protein via elctrophoresis and in diagnostics for bioseparation [48].
Nanocellulose hydrogels and gels are also suitable substrates for 3D cell culture for producing
microenvironment for cell growth, tissue engineering scaffolds and drug delivery vehicles [49].
Carboxylated nanocellulose via TEMPO oxidation can be converted into foam structures as
super adsorbent material (SAM) [50].
1.5.4.6. ANTI-MICROBIAL NANOCOMPOSITES
Anti-microbial nanocellulose composites using an anti-microbial agent phenyl bismuth
bis(diphenyl phosphinate), via vacuum filtration [51] and spraying [52] were developed. These
composites can act as anti-microbial packaging materials, biomedical bandages, and anti-
microbial coating on the surface. Recently, bismuth based anti-microbial agents have been
shown to have broad spectrum activity against pathogens including antimicrobial resistant
species. The anti-microbial potential of the 5 wt. % bismuth loaded composite was shown by
the 15 mm zone of inhibition against both gram negative microorganisms such as Escherichia
coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa), gram positive bacterium such as
Staphylococcus aureus (S. Aureus), anti-microbial resistant pathogens namely Vancomycin-
resistant Enterococcus (VRE) and Methicillin-resistant Staphylococcus aureus (MRSA). The
WVP of the same composite was evaluated to be 4.4 x 10-11
g/Pa.s.m, comparable with
synthetic plastics [52].
1.5.4.7. HIGH PERFORMANCE NANOCOMPOSITES
NC based nanocomposites for high performance barrier materials were developed via
an in-situ precipitation process. In this method, calcium carbonate nanoparticles were
precipitated via a chemical reaction between sodium carbonate and calcium chloride in the](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-16-320.jpg)
![NC film to yield a low permeability porous composite. The precipitated nanoparticles reduced
the pore volume of the NC film, resulting in the low permeance of the composite. Figure 7
shows the difference between pure NC film and in-situ precipitated composite prepared from
0.2 M solutions and the presence of CaCO3 nanoparticles interior of the composite. The WVTR
and OTR of 1 wt. % CaCO3 precipitated in NC film were 4.7 g/m2
.day and 2.7 cc/m2
.day
respectively at 23°C and 50% RH. The tensile strength and elastic modulus of this composite
were 91.96 ±14.99 MPa and 4.6 GPa respectively, confirming good strength and stiffness [44].
Figure 7 - SEM micrographs of CNF film and its composite via in-situ precipitation of
nanoparticles. (A) Pure CNC film and (B and C) Surface and cross-sectional view of composite.
The image obtained from Swambabu et al, 2018 [44].
High performance membranes were also achieved with a nanocellulose (NC)-Silicon
–Di- Oxide , SiO2 composite fabricated with a conventional vacuum filtration method. The pore
size of the composite was reported to be less than 100 nm. The porosity of the composite
could be tailored by varying SiO2 loading in the NC suspension [53]. Similarly, a NC-](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-17-320.jpg)
![Montmorillonite (MMT) composite was developed via vacuum filtration for packaging
application. The WVP of the homogenised NC – MMT composite was reported to be 6.33±1.5
g.µm/m2
.day.kPa with 23.1 wt.% MMT comparable with synthetic plastics [54].
Table 3 - Application of NC [7]
1.5.5. NANOCELLULOSE COMPOSITES AND ITS APPLICATIONS
Although NC films are a flexible, strong, biodegradable and sustainable material. It has
specific limitations in oxygen and water vapour permeability at high humidity where the pore
size of NC film increases by the swelling of the cellulose nanofibrils. Aggregation is a problem
in the preparation of NC films. Normally, cellulose molecules aggregate due to hydrogen](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-18-320.jpg)
![bonds between fibrils. To overcome this problem in NC films, NC has been combined with
other biopolymers or inorganics such as nanoclay to develop composites. One of the most
important applications of NC is to develop composites with nano-inorganics [54-56]. The
nanoparticles of inorganics interact with cellulose nanofibrils and fill the pores in their matrix,
forming the composite with drastically enhanced barrier and mechanical properties. These
composites have specific applications as barrier, fire retardant, membrane and heat insulating
materials. The incorporation of nanoclay such as Montmorillonite (MMT) into NC matrix
enhances its mechanical and barrier properties [54]. The nano-inorganics interact with
cellulose nanofibrils promoting the functionality of the composite [55] [56]. The functionality of
the composites can be enhanced by filling the pores in cellulose fibrous matrix resulting in
good barrier composites, by forming aggregates in cellulose network resulting in fire retardant
material, or by tuning the pores of cellulose matrix in membrane applications [47, 53] [57].
Table 4 summarizes the potential use of NC based composites.
1.5.5.1. BARRIER COATING
Nanocellulose–MMT suspension can be used for coating to produce a composite
barrier layer on the cellulose substrates. The composite barrier coating lowers the oxygen and
water vapour permeability of the sheet. 5 wt. % MMT and 1.4 wt. % NC coating on base sheet
with thickness of 22 ± 0.5 µm and with a coat weight of 29 ± 0.5 g/m2
has a WVTR of 8 ± 0.3
g m−2
day and OTR of 36400 ± 1100 cm3
m−2
day. With only 1.4 wt. % NC coating on a base
sheet with thickness 11 ± 0.3 µm and coat weight 9.9 ± 0.2g m-2
had a WVTR of 24 ± 0.7 g
m−2
day and OTR was above measurable range. Hence the 5 wt. % addition of MMT in NC
suspension causes a significant decrease in WVTR and brought OTR in the range [58].
1.5.5.2. MEMBRANE
NC-MMT can be used for construction of membranes. For example, MMT in
membranes can adsorb cationic dyes in the waste water and act as a cation exchanger for
wastewater treatment. Additionally, these composites can be used for the removal of heavy
metals in waste water [59], [60].](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-19-320.jpg)
![Table 4-Application of NC based Nanocomposites [61]
1.5.5.3. FLAME AND FIRE-RETARDANT
The development of non-toxic sustainable flame and fire retardants to replace
synthetic materials is a challenging task. Cellulose-based flame- and fire-retardant materials
are a green and sustainable alternative to halogen based retardant materials. Generally,
halogen based retardant materials are a toxic hazard that can cause halogen compounds to
enter the food chain. Recently, nanocellulose has been shown to be capable of meeting the
requirements of flame retardant materials. The addition of nano-clay is another approach to
tailor the properties of composites for flame retardant properties [57]. The OP of 5 wt.% MMT
composite film prepared via casting is 0.006 mL μm m−2
day−1
kPa−1
at 0 % RH and its
mechanical strength is 18 GPa of E-Modulus and 509 MPa tensile strength. Increasing the
MMT to 50 wt.% in the composite decreases its OP to 0.0008 mL μm m−2
day−1
kPa−1
. However,
the mechanical strength of the composite film decreased at 50 wt.% of MMT loading. These
composite could be used as gas barrier material [62].
The 50 wt. % MMT–50 wt. % NC composite in a continuous fibrous matrix of
nanocellulose formed via vacuum filtration has also been investigated. The strength of this
composite was 124 MPa and 8.7 GPa of elastic modulus. The OTR of the pure NC film was](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-20-320.jpg)
![0.048 cm3
mm m-2
day-1
atm-1
at 50% RH and 17.8 cm3
mm m-2
day-1
atm-1
at 95% RH. By
comparison the OTR of 50 wt. % MMT–50 wt. % NC composite was 0.045 and 3.5 cm3
mm
m-2
day-1
atm-1
at 50 % and 95% RH respectively. The OP of pure NC film was increased by
370% from 50 % RH to 95%. In the case of composite, the increase in OP was only 13 % from
50 % to 95% RH. This demonstrates that the addition of MMT into fibrous structure formed a
composite with potential gas barrier and flame retardancy properties [57]. Both 30 wt. % and
50 wt. % MMT in NC composite demonstrated a good fire retardancy material. Flammability
test and calorimetry confirmed self-extinguishing properties of the composite. This property is
important to autonomously control or prevent a fire or flame. The oriented MMT in the NC
fibrous network hinders diffusion of oxygen and ignition process [63].
1.6. PROPERTIES OF NANOCELLULOSE FILMS
The physical properties of NC film are related to their thickness, basis weight, density
and porosity. These properties strongly rely on the properties of cellulose nano-fibrils such as
their diameter and aspect ratio and the method of fabrication [64].
1.6.1. THICKNESS AND BASIS WEIGHT
The thickness and basis weight of NC film affects numerous properties such as
strength, density and barrier performance. The method used to fabricate the NC film also
controls its characteristics and properties. In vacuum filtration, the thickness of NC films varies
from 50 to 100 µm and basis weight varies from 60 to 80 g/m2
[43]. The dewatering time in the
forming process increases when NC suspension concentration is increased for higher basis
weight NC film. Casting, a simple method, gives a non uniform NC film due to shrinkage and
wrinkles formed. Cast NC films are produced with thickness varying from 5 to 30 µm [65].
1.6.2. APPARENT DENSITY
The density of the fibrous network in NC film controls the barrier and mechanical
properties [66]. The density of the NC film is controlled by fibril interactions and film processing.
The NC film preparation method controls film density by agglomeration and non-uniformity in
the surface of NC film. The clumps of cellulose nanofibers during processing of NC film affects
its density. Partial homogenised or non-homogenised NC can produce a drop-in density of
film in vacuum filtration. The NC film produced with homogenization of fibres shows
densification with lower thickness due to the packing of nanofibrils. The mean densities of
micro fibrillated cellulose, MFC film via vacuum filtration was 820 k g/m3
[43].](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-21-320.jpg)
![1.6.3. FILM UNIFORMITY
In the processing of cellulose fibres products such as paper and paper board, the
quality of the film/sheet is evaluated by its uniformity. This property is a confirmation for
evaluating uniformity of NC film. This property affects directly surface quality, strength and
printability of the film [67]. A paper perfect formation tester is used to estimate relative
formation value (RFV) of the sheet/film. The RFV measures the uniformity of NC film. If the
RFV is ≥ 1, the film has good uniformity, if it is < 1, the film is said to have lower formation [68].
1.6.4. MECHANICAL PROPERTIES
The strength of NC film is important in maintaining functions of NC films in barrier
materials, membrane and composite. NC films should have enough support for the matrix.
The fibres in the NC film form a compact network with hydrogen bonds between neighbouring
fibres developing the strength and modulus and high density of the film. The strength of the
NC film also depends on a number of factors such as diameter and length of fibrils, source of
biomass and quality of interaction with fibres in a network. The strength of NC film depends
on the fabrication methods and cellulose nanofibrils characteristics. The mechanical
properties of nanocellulose film prepared by various methods are summarized in Table 5. This
shows that spraying is the fastest method to prepare NC films and properties achieved by
spraying are similar to those of film from conventional methods.
1.6.5. BARRIER PROPERTIES
The main motivating factors to use cellulose nanofibers is its ability to form a fibrous
compact structure that plays a role in barrier performance. When comparing with other
synthetic plastics and other natural polymers, nanocellulose has low oxygen permeability and
reasonable water vapour permeability. Figure 8 shows oxygen permeability (OP) and water
vapour permeability (WVP) of conventional synthetic films and biopolymer films, confirming
the notable barrier performance of nanocellulose comparing with other biopolymers [29].
Oxygen and water vapour permeability are the main barrier properties of NC film and Figure
8 reveals that the barrier properties of NC film is better than that of other biopolymers. Figure
9 shows the tortuous pathway of NC against oxygen and water vapour.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-22-320.jpg)
![Table 5-Properties of NC films [69]
1.6.5.1. OXYGEN PERMEABILITY (OP)
The permeation of oxygen and other gases including volatile organic compounds can
degrade the quality of foods. The atmospheric oxygen can react with the double bond of the
unsaturated fatty acids in oils and fats resulting in rancidity. Because NC films have low OP
they can provide good barriers to protect the foods. It can increase the shelf life of foods by
avoiding the formation of unwanted chemical reactions such as oxidation and assist the
retention of the flavour of food and its fragrances. The oxygen permeability of nanocellulose
film is evaluated and compared with the synthetic plastics in Table 6. The data for OP of
synthetic plastics and nanocellulose films are shown in Table 6 [29]. This table also shows the
effect of relative humidity on NC film and carboxymethylated NC film compared with synthetic
plastics. At low RH%, NC film has low OP, but it increases with increasing %RH. The
increment is quite comparable with conventional synthetic plastics.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-23-320.jpg)
![Figure 8-Overview of Barrier Materials, obtained from Aulin et al 2011 [70]
Figure 9-Simple diagram of the structure of NC film giving it good barrier performance
against oxygen and water vapour
Table 6-Oxygen Permeability of NC and Synthetic Plastics
Materials OP Conditions](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-24-320.jpg)
![(cc µm/m2
day KPa)
NC 0.6 65% RH
NC
carboxymethylated
0.0006 0% RH
NC Carboxy
methylated
0.85 50% RH
Cellophane 0.41 0% RH
Polyethene (PE) 50–200 50% RH
Polyvinylidene
chloride (PVdC)
0.1–3 -
Polyvinyl alcohol
(PVOH)
0.20 0% RH
Ethylene vinyl
alcohol (EVOH)
0.01–0.1 0% RH
NC film has more and higher nano-fibrillar entanglements and provides a more closed
surface, and a complex internal tortuous diffusion path, showing a good barrier against oxygen
up to 65% RH[72]. [26]. In this range, the NC film shows comparable barrier performance with
synthetic polymers [71]. The crystalline regions in particular, contribute to increased
impermeability of this film against oxygen [73]. Hence, the nanocellulose are suitable for dry
foods packaging.
1.6.5.2. LIMITATIONS OF NC IN OP
The OP of NC film is very low and comparable with synthetic plastics at 0 % RH.
However, if the RH% reaches the 65%, it is not considered as a good barrier material. At low
humidity, NC film is rigid and with good resistance. At higher RH, It becomes rubbery and
viscous due to the high moisture content which softens the cellulose nanofibrils and weakens
the cellulose chains. As a consequence, the interfibrillar space is increased and the local
deformation permits increased permeation of oxygen molecules across the film [26]. The OP
of NC film is 0.6 cc µm/m2
day KPa at 65 % RH, 0.85 cc µm/m2
day KPa at 50 % RH and 0.011
(cm3
µm)/(m2
day kPa) at 0% RH. The OP of micro fibrillated cellulose, MFC at various
humidities and in comparison, with synthetic plastics is shown in Table 7. Table 8 confirms
OP of cellulose nanofiber, CNF film decreases with increased thickness of the film and
increased RH [74].](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-25-320.jpg)
![Table 7-OP of MFC [75]
Material OP
(cm3
µm)/(m2
day kPa)
Relative humidity
(%)
MFC 0.011 0
MFC 3.52 - 5.03 50
PET 10 - 50 50
PLA 184 0
LDPE 1900 50
Table 8-Reported oxygen permeability of selected carboxymethylated nanofibril (CNF) films
Material OP
(cm3
.μm)/
(m2
.day.kPa)
Film
thickness
µm
Conditions
CNF
(carboxymethylated)
0.009 2.54 0% RH
CNF
(carboxymethylated)
0.0006 3.19 0% RH
CNF
(carboxymethylated)
0.85 3.19 50% RH
1.6.5.3. WATER VAPOUR BARRIER PROPERTIES:
Paper and paper board are cellulose fibre-based substrates giving mechanical support
to protect contents, but they have poor resistance against water vapour and oxygen due to
the presence of large pore size and the hydrophilic nature of the cellulose fibres.
Nanocellulose has a fibrous network with small pores and shows reasonable resistance
against water vapour. The incorporation of cellulose reinforcing materials into the cellulose
fibrous matrix has enhanced barrier performance reasonably and reduced the water vapour
permeability [26]. Table 9 shows the barrier performance of nanocellulose against water
vapour and compared with synthetic polymers.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-26-320.jpg)
![To understand the barrier mechanism of NC film, the diffusion of oxygen and water vapour
from high concentration to low concentration separated by an NC film has three following steps
namely
1. Permeating molecules adsorbed on the surface of NC films
2. Diffusion of these molecules through the interior of the film.
3. The molecule desorbed from the other side of the NC film.
The permeability through a NC film mostly depends on diffusion rate internal to the film.
Equation 1- Permeability of gas or water vapour through films
𝑃 = 𝐷 ∗ 𝑆
Where P is the permeability, D is the diffusion coefficient and S is the solubility coefficient.
Fick Laws of diffusion is used to describe the permeability coefficient P.
Equation 2-Permeability coefficient
𝑃 = 𝐷 × 𝑆 =
𝑞 × 𝑙
𝐴 × 𝑡 × 𝛥𝑃
Where q is the amount of gas or water vapour passing through the film, l is the flow path or
thickness, A is the cross-sectional area, t is time, and p is the pressure difference across the
film [29].
1.7. METHODS FOR IMPROVING THE BARRIER PERFORMANCE
Methods available for enhancing barrier performance of NC films are chemical
modification of the cellulose fibrils, use of additives, coating on the film, curable cross linking
treatment of fibres and keeping residual lignin in the NC [75]. Apart from these methods,
incorporation of montmorillonite, MMT or similar nanoparticles influence the water vapour
resistance of the NC films [76]. Vermiculite added into NC film increased its oxygen resistance
even at high humidity conditions [77]. Adding MMT into biopolymers to develop
nanocomposites, improve their strength and thermal stability. Moreover, MMT or similar clays
with high aspect ratio [75] provide good reinforcement for bio polymeric matrices [78].
The common clay used in developing composite is bentonite, which is also known as
montmorillonite (MMT). “It is an alumina-silicate layered clay hydrated. This structure consists
of an edge-shared octahedral sheet of aluminium hydroxide between two silica tetrahedral
layers” [79]. Figure 10 shows the structure of MMT. It is a potential filler at the nanoscale,
which provides reinforcement in the nanocomposite and offers a high surface area with a large
aspect ratio from 50 to 1000 [80]. It is a di-octahedral clay with 2:1 layer linkage. MMT is a
rigid and impermeable filler. The 5 wt.% MMT is a permissible concentration used as food](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-28-320.jpg)
![additives in Australia and no adverse effects on human health [81]. Nanocomposites from
nanocellulose and MMT has low oxygen and water vapour permeability [82]. Apart from the
barrier performance of nanocomposite, incorporation of clay in the biopolymer network
promotes its mechanical properties and increased the glass transition and thermal resistance.
However, adding clay in the nanocellulose reduces the transparency of NC film [75].
1.7.1. ROLE OF CLAYS INTO THE NANOCELLULOSE NETWORK
The packaging industry prefers low-cost additives and eco-friendly materials that
should be universally accepted for easy processability by industry. Layered inorganics used
as additives are two-dimensional, usually about 1 nm in thickness and several microns in
length depending on the types of silicates or clays. As already discussed the tortuosity of NC
film is increased through clay particles sitting in the diffusion path of the film, and the barrier
performance and potential of the given nanocomposite is thus improved [83].
Figure 10-Structure of MMT. The image obtained from [84]
1.7.2. STRUCTURE OF NANOCELLULOSE COMPOSITE
There are three types of structure in composites, namely phase separated
nanocomposites, intercalated nanocomposites and exfoliated nanocomposites as shown in
Figure 11. The method of fabrication of composites, the configuration of clay platelets and the
fibril size of the nanocellulose control the structure of the composite and its barrier
performance. In phase separated nanocomposites, clay platelets exist in a tactoid structure in
the composite where the polymer and clay platelets are not mingled. As a result, clay platelets
are aggregated in the composite resulting in the formation of a macroscopic composite with](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-29-320.jpg)
![poor barrier performance. In the second type of composite, the clay platelets are intercalated
with polymer fibres. The nanocellulose fibres penetrate the interlayer region of the nanoclay
platelets resulting in the formation of an ordered structure with a multilayer of alternative
NC/clay layers at nanometre scale. In the exfoliated structure, the clay platelets are well
delaminated and distributed in the interior of the nanocellulose region of the nanocomposite
and extensive nanocellulose penetration into the layer of clays is observed [83, 85]. Figure 11
shows the orientation of clay platelets in nanocomposites and how this affects barrier
mechanism demonstrated in figure 12.
Figure 11-Structure of nanocomposite. The image obtained from Alexandre, M et al , 2000
[86]
1.8. FABRICATION METHODS
NC films/composites can be used in various applications hence the demand for NC
films/composites is rapidly increasing. The market for NC is predicted to reach USD 700 million
by 2023 as NC demand increases at a compound annual growth rate of more than 33%,
especially increasing in the pulp and paper sector [41]. However, the commercialization of
these products is limited by the existing methods for preparing NC films/composites. The key
issue is processing time, and therefore, a rapid and flexible process to handle high NC
suspension concentration is required to prepare free-standing NC films with properties such
as strength, barrier properties and roughness required for specific functionalities.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-30-320.jpg)
![Methods used for the fabrication of free-standing nanocellulose films/ composites are
solvent casting, spin coating, Roll to Roll (R2R) coating, layer by layer assembly, vacuum
filtration and spraying. Vacuum filtration is now considered as a conventional laboratory
method for NC film fabrication.
1.8.1. SOLVENT CASTING
Solvent/Solution casting is a laboratory scale technique for NC film fabrication. In this
method, NC suspension is poured on a glass or plastic Petri dish. The solvent is evaporated
from the poured suspension on the substrate under standard laboratory conditions [87].
Freeze-drying of the cast suspension is another option., Normally, the time taken for forming
the films varies from a minimum of 3 days to a week, depending on the concentration of NC
suspension and the thickness of suspension [74]. The higher the water content of the NC
suspension the more time is required for evaporation. The suspension consistency normally
used in solvent casting varies from 1.13 wt. % to 2 wt. % NC content.
Although, this method is a quite simple method, controlling the uniformity of the NC
film thickness and basis weight is quite challenging. After drying, films generally tend to shrink
and wrinkle, resulting in poor uniformity of the NC film that affects its strength and other
characteristics. Good distribution of the NC in the suspension is also required to achieve
quality films and sheets, as it controls the properties of the films and sheets [65].
Figure 12-Mechanism of Barrier Performance in Nanocomposites. This image obtained from
Garusinghe et al, 2018 [54]](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-31-320.jpg)
![Casting is also a common laboratory scale method for producing clay–nanocellulose
composites. Using a suitable base surface, the key factor is proper dispersion of the clay in
the suspension which is required to avoid the stacking and aggregation of clay in the
composites. Improper dispersion leads to exfoliation, with resultant poor barrier performance.
Therefore, an additional step is required to improve the dispersibility of clay platelets in the
NC suspension, aiming at an intercalated composite [87, 88]. The clay dispersibility can be
promoted by intensive agitation such as by stirring and shear mixing. Recently, ultra-sonication
has been used to provide a good dispersion of nanoclay in suspension and break the nanoclay
clusters or aggregates. However, this may not be a solution for large scale production of
composites [54].
Several examples of cast films are worth noting. Cast composites from vermiculite
nanoplatelets in homogenized NC showed good oxygen and water vapour barrier properties
and achieved good tensile strength and stiffness[77]. A cast composite of cellulose foam with
homogenized NC and Tween 80- 2.5 wt.% MMT achieved a WVP of 216000 g µm m-2
day-1
KPa-1
, a strength of 13.1±1.7 MPa and a Young's modulus of 0.455 GPa [88]. The low WVP
and high strength was due to the composite containing a lower loading of MMT.
The composite from the casting of poly (vinyl alcohol) PVA- Poly acrylic acid (PAA) –
NFC – 50 wt. % Sodium MMT achieved a WVP of 105 g µm m-2
day-1
kPa-1
a strength of
190±11 MPa and a modulus of 15.9±0.3 GPa [89]. The cast composite showed better WVP
and good strength due to the presence of biodegradable polymers and high content of MMT.
The biodegradable polymer (PAA) promoted the dispersion of clay particles and crosslinking
between PVA and PAA [56]. NFC – vermiculite composite prepared via two pressure
homogenization stages followed by solution casting was reported to have a WVP of 60 g µm
m-2
day-1
kPa-1
a tensile strength of 244±24 MPa with a modulus of 17.3±0.4 GPa [77]. The
processing time for forming the composite film via casting consumed days and is considered
as a huge constraint for commercialization.
In solvent casting, NC suspension and clay platelets are mixed together for better
dispersion and allow swelling of the clay layers. Therefore, the clay platelets are retained onto
the surface of the poured suspension as a single layer and causes exfoliation of platelets in
the sheet. The exfoliated composite gives poor barrier performance against water vapour and
oxygen due to its poor maze structure in the diffusion pathway. Therefore, an additional step
is required in improving the dispersibility of clay platelets into NC suspension to get an
intercalated composite [90, 91]. The clay dispersibility could be promoted by an intensive
stirring and shear mixing. Recently, ultra-sonication has been used to provide a good
dispersion of nanoclay in suspension and break the nanoclay clusters or aggregates. However,](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-32-320.jpg)
![it may not be a solution for large scale production of composites [54]. Solution blending
followed by casting has drawbacks of being a time-consuming process, challenging to form a
composite with exfoliation of clay platelets. It requires an additional process to disperse clay
to produce the composite at laboratory scale.
1.8.2. SPIN COATING
Spin coating is a technique for producing freestanding thin nanocellulose film with
nanometric thickness. Spin-coated thin NC films have been used for investigating the
interaction of biological molecules with NC[92]. In this method, high-speed spinning is applied
to remove the excess NC suspension, thus leaving ultra-thin NC films. This is not a scalable
method and is not suitable to meet the commercial requirements of NC film as per the current
demand [87], but, it is a perfectly suitable method to make films for looking at biomolecule
interaction with cellulose fibrils at laboratory scale. Spin coating was also used to produce
cellulose nanomaterial layers as a film on the polypropylene. A 1 µm thin layer of CNC coated
on a UV treated Poly Propylene (PP) film reduced oxygen permeability by 39% compared to
uncoated PP[93]. Spin-coated CNC films promote their gas barrier functionality and
multilayered coated CNC films can be used in gas separation and as barrier material [94].
1.8.3. ROLL TO ROLL COATING
The Roll to Roll Coating (R2R) has been used to produce NC film [65] and NC barrier
layers on cellulose substrates to enhance the mechanical and barrier properties [95]. In
making films this way, NC is coated evenly on pre-treated substrates, normally plastic films.
The spreading of NC is controlled and adheres to the substrate followed by drying under a
controlled environment. The NC films are then detached from the substrate and pressed. The
NC film prepared via this method has no shrinkage. The average roughness of NC film made
this way is 400 nm on the side exposed to the plastic substrate and 800 nm on the free side.
This is a scalable method for producing translucent, smooth and densely packed NFC film.
The time required for making the film is 15 minutes [65]. The OTR of NC film was less than
0.1 cm3
/m2
/d (0% RH; 15 μm thickness) and tensile strength of 200 Mpa, while water vapour
barrier was not reported. The coating weight of NC using R2R can vary from 0.1 to 400 g/m2
.
The quality of NC film can be controlled by various parameters such as NC solid concentration,
wettability of NC suspension on the substrates and velocity of webs etc.
1.8.4. LAYER BY LAYER ASSEMBLY
This method deposits macromolecules and nanoparticles containing charged groups
in solution to form a film with variation in thickness from nanometre to micrometres. This
method can be used to produce thin functional films and for coating of nanocellulose for](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-33-320.jpg)
![specific functionality. One application used a thickness of the film of 5 µm [96] and reported a
surface roughness of 9 nm. It is not a scalable process for commercial production of NC films.
In the past three decades, Layer–By-Layer assembly carried out at mild reaction
conditions has been used as a rapid technique for surface engineering of cellulose fibre sheets.
The size and shape of the substrate do not affect this process. The film produced via this
process can be flexible in composition, with tailorable thickness and stable performance. The
thickness of the composite can be engineered via multiple coatings on the surface of base
substrates. The multilayer coating of MMT on the base substrates affects the strength of the
composites and can cause aggregation of MMT Layer-by-layer coating of MMT on
biodegradable polymer sheet enhanced its barrier performance including giving a low oxygen
permeability [97]. The surface coating of MMT on sisal fibre cellulose microcrystals via this
method was utilised to develop a flame retardant material [98].
1.8.5. VACUUM FILTRATION
Vacuum filtration is the most common method for forming NC films, as it used in
cellulosic fiber sheets such as paper and paper board. In the laboratory method, NC
suspension is poured into a column which contains a metal mesh as a filter at the bottom. A
wet film of NC fibres is formed on the mesh. The sequences involved in the preparation of NC
film via vacuum filtration are shown in Figure 13. The retention of NC on the filter depends on
the mesh size and its pore size; however, smaller mesh size generally increases the drainage
time in the filtration process. The commonly used filters in practice [99] are tetrafluoroethylene
(PTFE) membrane (pore size 0.1 μm), woven filter fabrics (pores – 55 µm, 109 µm, 150 µm)
[100] and Millipore filter membrane (Pore- 0.65 µm) [101]. The wet NC film is formed on the
mesh and subjected to air or drum drying. The time required to prepare films is varied to 10
minutes[43] to 4 hours [102]. Reasons for variation in the time for the NC film prepared via
vacuum filtration is dewatering time, the pore size of the filter medium, the pore volume of the
NC film and thickness of the NC film formed on the filter medium. Normally, filtration time also
increases with NC suspension concentration [43].
Laboratory scale production of homogenized NC film via filtration requires
approximately 10 minutes to produce a film with basis weight of 60 g/m2
. It is a conventional
method to produce NC films. However, it has a limitation in that the drainage time increases
with basis weight of the film. The increase of thickness and basis weight of the film with
drainage time is exponential. The thickness of NC film produced via the filtration varies from
50 to 100 µm. The notable issue in this method is the separation of the film from the filter after
couching. Sometimes a filter mark is imprinted on the NC film affecting the uniformity of the](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-34-320.jpg)
![film [65]. Although this method can achieve excellent barrier and mechanical properties [43],
preparation time is a bottleneck for scaling up the process for commercialization of NC film.
Figure 13 - Vacuum filtration for NC film fabrication. Reprinted adapted with permission from
Sehaqui et al 2010 [103]
1.8.6. NANOCOMPOSITE VIA FILTRATION
Vacuum filtration is a conventional cellulose fibre / cellulose based composite sheet
making process in which the suspension of cellulose nanofibrils with nano-inorganics is
collected on the filter to form the composites. The wet composite on the filter are couched
and followed by pressing and drying process. In the mixing stage, inorganics are well
distributed into the 3D matrix of fibrous network.
The MMT–NC composites have been produced via vacuum filtration and reported low
oxygen and water vapour permeability [54, 56, 82]. The WVP of MMT –NC composites via
filtration are reported to be 13.3 ± 2.0 g µm/m2
.day.KPa with 16.7 wt. % of MMT loading in
the composites. Increasing MMT in 0.2 wt. % of NC suspension increase drainage time in the
filtration process and high constraint for commercialization. A composite film from 25 wt. %
Clay loaded in Eucalyptus NC and P. radiata NC and prepared via vacuum filtration resulted
in WVTR of the composites ranging from 183 to 197 g/m day. [100]. Increasing the clay loading
from 0 wt.% to 25 wt.% reduced WVTR from 299 to 197 in P. radiata NC composites and from
285 to 183 in Eucalyptus NC composites [104]. The processing time for composites was not
reported.
1.8.7. IMPREGNATION PROCESSES
This method has been used to develop composites as samples for evaluating
mechanical, thermal and optical properties. In this method, a thin cellulose film initially
produced via membrane filtration is then laminated with nanomaterial. The composite was
promoted for its dimensional stability, flame retardancy function and its strength [105]. The
barrier performance of the composites via impregnation was not reported.
1.8.8. HOT PRESSING](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-35-320.jpg)
![Hot pressing is used to develop nanocellulose – inorganics composite for barrier
applications. Hot pressing is a final step applied after filtration. It is performed at a temperature
of 105°C and pressure of 150 bar. Time consumption of pressing step is 3 to 4 hours including
cooling and heating of the composites. The WVP of Trimethyl ammonium –modified
nanofibrillated cellulose and layered silicate ( TMA-NFC/Mica R120) was 2200 g µm m-2
day -
1
KPa-1
at a condition of 23 ° C and 50% RH. The strength of this hot-pressed composite was
104±5 MPa with Young's modulus of 10.3±1.1 GPa [104]. The WVP of a TMA-NFC/Vermiculite
composite was 1400 g µm m-2
day -1
kPa-1
, with a strength of 135±5 MPa and a modulus of
10.3±1.1 GPa [106].
1.8.9. SPRAY COATING
Spray coating is a bench scale technique for fabricating nanocellulose film. This
method is used to develop free-standing films by spraying on to a solid surface either
permeable [69, 107] or impermeable [108] . Spraying allows contour coating and the coating
is performed contactless with the surface [109]. In addition to that, the coating process is not
influenced by the surface’s topography [109]. Films produced by spraying are stronger than
cast film [110].
Figure 14 shows the sequences involved in the spraying process. In spraying process,
the mechanism consists of the formation of spray jet of the liquid and then atomization of the
sprayed jet into the mist of the liquid. The atomization involves two mechanisms namely liquid
lamella disintegration and disintegration of the liquid jet. The fine droplets formed from
atomization coalesce on the contact surface and a film is formed on the base surface. Spraying
of these biopolymers onto the solid surface causes coalescence of sprayed droplets due to
the film-forming properties of the polymers via hydrogen bonding [111]. The film that is formed
on the surface can be peeled from the substrates after drying.
Figure 14 - Mechanism of spraying](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-36-320.jpg)
![Compared with laboratory and conventional methods for NC film making, spraying is
a novel and fast method to create a coating layer either on cellulosic substrates or solid
surfaces. Spraying of nanocellulose on a paper substrate has been performed to enhance its
barrier and mechanical properties [112][107]. Similarly, spraying NC on nylon fabric was
attempted to make free-standing nanopaper as barrier materials [69]. The spray coating of
chitin and cellulose nanofibers onto poly lactic films has been performed to enhance barrier
performance and mechanical properties [113] . In this thesis, the preparation of free-standing
NC film via spraying is the focus. This method can be classified into two approaches - spraying
on a permeable substrate or spraying on an impermeable substrate.
1.8.9.1. SPRAYING OF NC ON PERMEABLE SUBSTRATES
Spraying can provide a good range of coat basis weight significantly higher than
filtration. The spraying process is independent of the NC suspension concentration provided
the suspension remains within the sprayable viscosity. Spraying of NC is a method that has
been investigated for creating barrier layers on cellulosic fibrous substrates [107] by spray
coating an homogeneous layer of NC on paper substrates [107]. Up to 2 wt. % NC was used
for spraying, thus reducing the water to be removed in subsequent drying.
1.8.9.2. SPRAYING OF NC ON IMPERMEABLE SUBSTRATES
Spraying of NC on smooth and polished solid surfaces has been investigated to
produce free-standing NC film. Recently, spraying of micro-fibrillated cellulose (MFC) on a
preheated metal surface was used to produce MFC film with 3D structures [108]. The basis
weight and thickness of the film varied from 59 to 118 g/m2
and 46 to 68 µm respectively. The
limitations in this method were cracks and wrinkles formed on the surface of the NC film. In
this investigation, the amount of water was reduced but spraying with both 4.5 wt. % and 9
wt. % NC caused the formation of a disturbed spray jet due to the gel nature of the MFC.
Consequently, the cracked film was produced [108]. The replication and evaluation of surface
roughness of spray coated NC film from the base surface was not reported. In another
investigation, ethyl cellulose was well dispersed in water with addition of plasticizer and the
liquid was sprayed on Teflon plates. The spraying produced a uniform and reproducible
cellulose film with tensile strength better than cast films [110]. The spraying of cationic chitin
fibres and anionic cellulose nanofiber on poly lactic acid (PLA) films was investigated to
promote barrier performance. The multilayer spray coating of these fibrous materials on PLA
reduced O2 permeability but had a lower maze than when spray coated with only chitin
nanofiber or cellulose nanofiber [113].
1.8.9.3. LIMITATIONS IN SPRAYING OF NC ON PERMEABLE SURFACE
Using spray coating [69], an NC suspension was sprayed on a permeable nylon fabric
substrate having opening size of 20.4 µm, 14 % open area and 470 mesh size and then](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-37-320.jpg)
![vacuum filtration was used to remove the excess water from the spray coated NC layer on the
nylon fabric. This spraying method is a time-consuming process as it took 10 to 20 minutes.
The report detailed that the NC films were made with 5 pass homogenized NC suspension
with fixed consistency of 2 wt. %. The surface roughness of the NC film and the effect of
suspension consistency on the properties of the NC film were not reported. The thickness and
basis weight of the NC film were varied from 10µm to 72 µm and 13.7 g/m2
to 124 g/m2
respectively by varying the speed of the conveyor in the experimental set up from 0.5 to 3
m/min. The constant MFC slurry flow from the nozzle of spray system was 0.75 kg/min. After
spraying, the time taken for filtration varied from 15 sec to 90 sec. After dewatering, the wet
MFC film was placed between new nylon fabrics and two blotting papers and two pass
compressed with a 3 kg roll, and vacuum dried at 90° C. The time taken for drying a 55 g/m2
film and another thicker film were 10 and 25 mins respectively. The nylon fabric imprints on
spray-coated nanopaper was reported, however these markings were not evaluated. The
effect of the base surface on the properties of NC film was not investigated [69]. Figure 15
shows a typical operation of the spray coating process.
Spraying process produced high thickness and basis weight of the NC film or barrier
layers on the cellulosic substrates compared to vacuum filtration. Spraying homogenized
micro-fibrillated cellulose onto a nylon fabric moving on a conveyor belt set at a speed of 0.5
m/min, yielded 124 g/m2
of the NC film. The spray coated films have a Young’s Modulus of
18 GPa and its air permeability is reported to be < 0.7 nm Pa-1
s-1
[69]. Spraying has not so
far been used to make discrete NC film.
1.8.9.4. SPRAY COATING FOR COMPOSITES LAYER DEVELOPMENT
Recently, the spraying of nanocellulose with inorganics onto permeable substrates
such as paper and cardboard has been found to be a simple approach for developing
composites barrier layers. This approach permits the coating of a larger surface area and
reduces operation time [107][114].](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-38-320.jpg)
![MFC -SiO2 composites have been developed via spray coating with operation time on
a base sheet of almost 310 cm2
of less than 30 minutes for spraying, independent of SiO2
concentration. SiO2 was added in the range 0 to 33 wt. % in the spraying suspension, tailoring
the properties of the composite. The coating layer weight on the cellulosic substrates was
controlled by varying the velocity of the conveyor in the experimental setup and the inorganics
content in the NC suspension. These composites produced a material impermeable to air,
confirming its good barrier performance [114]. Similarly, a composite made up of graphite and
MFC as an electrode were developed via spraying [115].
Spraying of cellulose nanofibrils with nanoclay (MMT) on a Kraft paper and printing
paper as base cellulosic substrates were developed and increased its barrier properties
against oxygen and water vapour and tensile strength. The operation time for producing the
composite barrier layers on the paper were 30 sec and 50 sec for clay suspension varying
from 0 wt. % to 5 wt. %. At 3 wt. % and 5 wt. % clay composite layer with basis weight of 22
and 29 g/m2
the oxygen barrier permeance was 88600±2000 and 36400±1100 cm3
m-2
day
respectively. The water vapour barrier performance of these composites was reported to be
14 and 8 gm-2
day. The time consumed for 19 and 29 µm thickness of barrier layer on paper
Figure 15-Spray coating to prepare NC film. The image obtained from Beneventi et al, 2015
[69]](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-39-320.jpg)
![was less than 30 sec in the spraying process [58]. In this process, the water in the sprayed
suspensions was removed by vacuum filtration. Peeling the layer as a film from the base
surface was a challenge. Surface treatment of the substrates such as wetting, and corona
discharge treatment was required to enhance the wettability between substrates and cellulose
nanofibrils. NC concentration was varied between 1 wt. % and 2 wt. % NC but the effect of
NC suspension concentration on the properties of composite layers was not investigated.
Some surface non-uniformity of composite layers such as agglomeration was observed.
Layered spray coating of cationic chitin fibres and anionic cellulose fibres on the PLA
films has also been investigated. This method provided alternative multi-layer coating of these
fibres on PLA film to tune its barrier properties and mechanical properties. The OP of
composite layer with PLA film is 22 cm3
µm/m2
/day/kPa at 50% RH compared to that of the
plain PLA film which was OP 70 cm3
µm/m2
/day/KPa at 50% RH. The WVTR of the composite
layer with PLA film is reported to be 60 g water/m2
/day at 50 % RH, slightly lower than the
WVTR of pure PLA film [113].
1.8.9.5. SPRAY COATING FOR DEVELOPING FREE-STANDING
NANOCOMPOSITE
Recently, spraying is a rapid process to produce NC film and its operation time less
than 30 mins [69]. Spraying is an effective method for producing composites and therefore
implemented to make NC–MMT Composite. Generally, free standing NC films have been
prepared using either vacuum filtration or casting. Both methods are time-consuming
processes and required from 10 minutes to 3 days for the film to form and drying. Wrinking of
the film during drying also places limitations on these processes. The operation time for
nanocomposite via vacuum filtration required minimum of 24 hours to 4 days. Additionally,
there are shortcomings in the separation of the film from the filter. The handling of the peeled
wet film was difficult before being dried. This is also a conventional method which has
limitations in tailoring the basis weight and thickness of the composites.
1.8.9.6. PROS AND CONS OF THE DIFFERENT METHODS
In summary, casting methods are too slow and the quality of the film, because of
wrinkling, is a major difficulty. In filtration methods, the time required is excessive, limiting the
range of suspension concentrations permissible. In addition, peeling is a challenge and filter
marking is a problem. While spraying appears a good option because it is quicker, studies on
spraying methods for fabricating NC sheet have not explored sprayable limits of NC
suspension. The effect of base surface on the surface properties of NC film has not so far
been explored, and the effect of NC suspension and spraying parameters on the uniformity of
NC films has not been investigated.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-40-320.jpg)
![1.8.9.7. IDEAL REQUIREMENT OF PREPARATION OF NC FILMS
When compared to synthetic plastics films, the manufacturing rate of NC films is very slow
[75]. Rapid manufacturing of NC films will facilitate their applications in packaging, as air and
water filters, membranes and films for tissue engineering applications. Ideally, for commercial
preparation of NC films methods must be able to
• Increase film performance in water vapour and oxygen barrier properties via
preparation method
• Tailor the thickness and basis weight of the NC film via processing method without
affecting operation time
• Engineer the strength and bulk properties of NC film by varying the NC suspension
concentration without affecting operation time in the method
• Produce a film with an outstanding uniformity
• Permit composites with the addition of nanoparticles not affecting the process
variables in the method
• Produce NC film in a single step to avoid further processing such as dewatering,
vacuum drying, couching of the wet film
• Produce a smooth nanocellulose film and engineer their roughness for developing
potential material without any additional treatment in the methods
1.9. RECYCLABILITY OF NANOCELLULOSE FILMS
Sustainable packaging requires recyclable packaging films and minimizing the carbon
footprint and environmental pollution. Safety and sustainability are the important requirement
in engineering food packaging materials [5]. The toxicity of cellulosic fibres is very low based
on low genotoxicity against hepatocytes and aquatic species. Like cellulose macro fibres, NC
is also biodegradable and does not enter or interfere into the food chain [5][116].
Nanocellulose has high surface area which contributes to high microbial degradation.
Therefore, nanocellulose can be recyclable and used in sustainable barrier materials.
Cellulose nanofibers are obtainable from virtually inexhaustible resources that are renewable
and widely available around the world. Nanocellulose satisfies the sustainable packaging
concepts defined by the Sustainable Packaging Alliance (SPA). Table 10 shows the
sustainability potential of nanocellulose in packaging applications.](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-41-320.jpg)
![Table 10 - Sustainability features of Cellulosic nanofibers [119]
Cellulose fibre substrates such as paper and paper board are recyclable, renewable,
and biodegradable, but show poor barrier performance against oxygen and water vapour due
to the presence of large pores in the fibrous network. To improve the barrier performance of
cellulose fibres substrates, lamination and extrusion with synthetic plastic have been
performed, but, recycling of these coated substrates has shown to be problematical [117]. In
laminated and composite mixtures of many packaging materials, the layers are strongly
bonded and the recycling of this material is an effort [75].
The NC coated LDPE was mechanically recycled into a second life barrier material.
The Roll to Roll coating was used to coat NC on LDPE. During recycling, the LDPE and
agglomerates from NC coatings were mixed together and formed a recycled film. The OTR of
1.4 g/m2
coat weight of pure NC on virgin LDPE film and recycled film were 37±4 cc/m2
/day
and 924±47 cc/m2
/day, respectively. The WVTR of pure NC on virgin LDPE film and recycled
film were 0.9±0.1 g/m2
/day and 2.0±0.2 g/m2
/day at a condition of 23°C and 50% RH,](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-42-320.jpg)
![respectively. The barrier properties of recycled film was decreased after recycling and the
effect of NC agglomerates was insignificant [118]. This concluded the possibility of recycling
of NC coating on the LDPE film and gave an advice on the focus of recycling of free-standing
NC film and its effect on the barrier performance.
1.10. SURFACE ENGINEERING OF NANOCELLULOSE FILMS
The roughness of nanocellulose film is one of the important criteria for developing
functional material from nanocellulose. The surface roughness of the film normally controlled
by the diameter distribution of nano fibril in the film and the processing method. The minimal
surface roughness (high level of smoothness) is required for constructing flexible electronics.
Because the conductive ink should spread on the surface of the substrate to avoid any
discontinuity in the ink layers. In addition to that, spreading and penetration of the conductive
ink on the substrates should be minimal on substrates for printed electronics [119]. Spreading
and penetration of conductive ink on the surface of NC film depends mainly on their surface
roughness.
The method of NC film preparation is another approach to tailor its surface roughness.
The smooth surface and texture of the base surface like plastic material can be replicated on
the nanocellulose film [120][121] either with solvent casting or roll to roll methods. The average
roughness of the NC film via roll to roll method is approximately 450 nm on the base surface
side and approximately 800 nm on the free side of the film [65]. The surface roughness of NC
film via vacuum filtration as a conventional method is 418 nm on free side and 331 nm on the
filter side [122]. Sometimes, the NC film via filtration contains filter markings on the surface of
the film affecting its roughness. The average roughness of plastic as the base surface is
approximately 400nm.
1.11. PERSPECTIVE AND CONCLUSION
Recently, nanocellulose films and their composites have potential applications in
various fields. Predominantly, these sheets have been implemented in barrier applications, air
filters, and anti-microbial sheets, substrates for electronics and light emitting diodes etc., and
replacing synthetic plastics in practice. The methods for production of NC film in practice has
time-consuming and limited in rate of production. The properties of NC films and their
composites mostly depends on the preparation methods.
The rate of manufacturing of NC films should be increased to be similar of plastic films.
Through a critical review of the literature, this chapter provides a great need of rapid production
of NC film and its composites. The rapid process should have a flexibility in the processing
and the ability to tailor the properties of pure NC film and its composite for specific functionality.
The barrier and mechanical performance of NC films from spraying as new and proposed](https://image.slidesharecdn.com/kirubanandanfinalversionchapter-201222113642/85/NANOCELLULOSE-AND-ITS-COMPOSITE-FILMS-APPLICATIONS-PROPERTIES-PREPARATION-METHODS-AND-LIMITATIONS-43-320.jpg)
NANOCELLULOSE AND ITS COMPOSITE FILMS – APPLICATIONS, PROPERTIES, PREPARATION METHODS AND LIMITATIONS
- 1. NANOCELLULOSE AND ITS COMPOSITE FILMS – APPLICATIONS, PROPERTIES, PREPARATION METHODS AND LIMITATIONS CONTENTS 1.1. ABSTRACT ................................................................................................................ 3 1.2. KEYWORDS .............................................................................................................. 3 1.3. INTRODUCTION........................................................................................................ 3 1.3.1. NANOCELLULOSE – A NOVEL FIBROUS MATERIAL.......................................... 6 1.3.2. CHARACTERISTICS OF NANOCELLULOSE ........................................................ 7 1.3.3. SCOPE OF LITERATURE REVIEW ....................................................................... 8 1.4. NANOCELLULOSE PRODUCTION ........................................................................... 8 1.4.1. MECHANICAL PROCESS...................................................................................... 9 1.4.2. CHEMICAL PROCESS......................................................................................... 10 1.4.3. ENZYMATIC PROCESS....................................................................................... 11 1.5. APPLICATION OF NC FILMS .................................................................................. 11 1.5.1. BARRIER APPLICATIONS................................................................................ 12 1.5.2. PRINTED ELECTRONICS ................................................................................ 13 1.5.3. OTHER APPLICATIONS................................................................................... 13 1.5.4. PRODUCTS OF HIGH-PERFORMANCE FIBRE MATERIAL RESEARCH GROUP, MONASH UNIVERSITY. .................................................................................. 14 1.5.4.1. PACKAGING.............................................................................................. 14 1.5.4.2. OIL AND WATER SEPERATION ............................................................... 14 1.5.4.3. PHOTO CATALYST COMPOSITE............................................................. 15 1.5.4.4. MEMBRANES............................................................................................ 15 1.5.4.5. GELS ......................................................................................................... 16 1.5.4.6. ANTI-MICROBIAL NANOCOMPOSITES ................................................... 16 1.5.4.7. HIGH PERFORMANCE NANOCOMPOSITES........................................... 16 1.5.5. NANOCELLULOSE COMPOSITES AND ITS APPLICATIONS......................... 18 1.5.5.1. BARRIER COATING.................................................................................. 19 1.5.5.2. MEMBRANE .............................................................................................. 19 1.5.5.3. FLAME AND FIRE-RETARDANT............................................................... 20 1.6. PROPERTIES OF NANOCELLULOSE FILMS......................................................... 21 1.6.1. THICKNESS AND BASIS WEIGHT................................................................... 21 1.6.2. APPARENT DENSITY....................................................................................... 21 1.6.3. FILM UNIFORMITY........................................................................................... 22 1.6.4. MECHANICAL PROPERTIES........................................................................... 22 1.6.5. BARRIER PROPERTIES .................................................................................. 22 1.6.5.1. OXYGEN PERMEABILITY (OP)................................................................. 23
- 2. 1.6.5.2. LIMITATIONS OF NC IN OP ...................................................................... 25 1.6.5.3. WATER VAPOUR BARRIER PROPERTIES:............................................. 26 1.6.5.4. MECHANISM OF BARRIER PERFORMANCE .......................................... 27 1.7. METHODS FOR IMPROVING THE BARRIER PERFORMANCE............................. 28 1.7.1. ROLE OF CLAYS INTO THE NANOCELLULOSE NETWORK ......................... 29 1.7.2. STRUCTURE OF NANOCELLULOSE COMPOSITE ........................................ 29 1.8. FABRICATION METHODS....................................................................................... 30 1.8.1. SOLVENT CASTING......................................................................................... 31 1.8.2. SPIN COATING................................................................................................. 33 1.8.3. ROLL TO ROLL COATING ............................................................................... 33 1.8.4. LAYER BY LAYER ASSEMBLY ........................................................................ 33 1.8.5. VACUUM FILTRATION..................................................................................... 34 1.8.6. NANOCOMPOSITE VIA FILTRATION .............................................................. 35 1.8.7. IMPREGNATION PROCESSES........................................................................ 35 1.8.8. HOT PRESSING ............................................................................................... 35 1.8.9. SPRAY COATING............................................................................................. 36 1.8.9.1. SPRAYING OF NC ON PERMEABLE SUBSTRATES ............................... 37 1.8.9.2. SPRAYING OF NC ON IMPERMEABLE SUBSTRATES ........................... 37 1.8.9.3. LIMITATIONS IN SPRAYING OF NC ON PERMEABLE SURFACE .......... 37 1.8.9.4. SPRAY COATING FOR COMPOSITES LAYER DEVELOPMENT............. 38 1.8.9.5. SPRAY COATING FOR DEVELOPING FREE-STANDING NANOCOMPOSITE..................................................................................................... 40 1.8.9.6. PROS AND CONS OF THE DIFFERENT METHODS................................ 40 1.8.9.7. IDEAL REQUIREMENT OF PREPARATION OF NC FILMS ...................... 41 1.9. RECYCLABILITY OF NANOCELLULOSE FILMS .................................................... 41 1.10. SURFACE ENGINEERING OF NANOCELLULOSE FILMS.................................. 43 1.11. PERSPECTIVE AND CONCLUSION.................................................................... 43 1.12. REFERENCES ..................................................................................................... 44
- 3. NANOCELLULOSE AND ITS COMPOSITE FILMS – APPLICATIONS, PROPERTIES, PREPARATION METHODS AND LIMITATIONS Kirubanandan Shanmugam, Christine Browne, Robert Johnston, Gil Garnier and Warren Batchelor, Department of Chemical Engineering, Bioresource Processing Research Institute of Australia (BioPRIA), Monash University, Clayton, Vic 3800, Australia. *Corresponding Author, E-mail: Warren.Batchelor@monash.edu skirubanandan80@gmail.com 1.1. ABSTRACT Nanocellulose (NC) is a biodegradable nanomaterial produced from lignocellulosic biomass. The mechanical, chemical or enzymatic processes are normally used for defibrillation of wood fibres into nanofibrils. It is a cellulose nanofibril with unique characteristics leading to potential applications in functional materials. These nanomaterials can replace synthetic plastics and be used as biomaterials for developing recyclable nanocomposites. The fibrils at nanosize provide high surface area and strength and are biodegradable. NC films and their composites have been used in developing functional materials such as barrier materials, substrates for printing circuits in flexible electronics, barrier coatings on the paper and paper board etc., In addition, there is a potential demand for NC films and composites to replace or eliminate conventional plastics. There are many methods available to prepare free-standing NC films with composites and these are critically reviewed and addressed in this chapter. Conventional methods are seen to be time-consuming, and unable to produce outstanding quality films, which set limitations on commercialization. Therefore, a rapid process to produce the NC films is required to meet the potentially large requirement for packaging and other functional material. 1.2. KEYWORDS Nanocellulose; Nanocomposites; Applications; Oxygen Permeability and Water Permeability. 1.3. INTRODUCTION Synthetic plastics are extensively used in developing functional materials such as barrier layers on cellulosic substrates, packaging films and substrates for printed electronics. However, they have poor recyclability, biodegradability and their wastes are highly harmful to
- 4. the environment. To overcome this limitation with plastics, cellulose substrates are used in the construction of various functional materials. Cellulosic fibre products such as paper and paperboard are equally predominant with synthetic plastic materials but as they are biodegradable, they are perfectly safe for the environment. The hydrophilic nature of cellulose macro fibres limits the specific application in various fields. In packaging, cellulose fibre products offer the advantages of being recyclable and biodegradable but due to the presence of large pores and high affinity towards water and its vapour, it has poor water vapour and oxygen barrier performance. To note, these materials were not for fit for packaging under medium and high humidity environment. To mitigate these limitations, the cellulose fibre products are often associated with plastics, wax and/ or aluminium, for enhancing their barrier properties. However, these materials suffer from serious environmental issues, as being difficult and inefficient to recycle. Recently, cellulose nanofibers such as nanocellulose (NC) plays a predominant role in material development due to its outstanding properties. Cellulose is the most predominant fibrous biopolymer among natural materials. It has outstanding properties and is readily obtainable. Cellulose from plant sources such as wood, grass, agricultural and lignocellulosic waste has potential for replacing plastics and provides a platform for resolving many issues such as, reduction of oil stocks, synthetic plastic pollution, and carbon footprint. This means cellulose is a renewable and non-toxic, eco-friendly material and biocompatible with nature. The source of cellulose via the photosynthesis process in plants is estimated to be from 1011 –1012 tonne per annum [2]. Microbial fermentation is an alternative way to produce bacterial cellulose. Cellulose is used in regenerated cellulose fibres, and in cellulose ethers and esters. These materials are used as additives, coating materials, thickeners, stabilizers, binders and fillers in foods, pharmaceutics, sanitary, and textile etc., Due to its mechanical properties and barrier performance, it plays a major role in developing packaging materials such as papers and paper boards. The presence of surface hydroxyl groups in the cellulose gives a platform of chemical modification, resulting in tailoring its properties and functionality. The reduction of cellulose macro fibres via mechanical, chemical and enzymatic processing into cellulose nanofibers has potential benefits and drastically changes its characteristics from macro fibres [3]. Nanocellulose (NC) consists of entangled cellulose nanofibrils several micrometres long containing both amorphous and crystalline regions. These regions are well connected through intra and intermolecular bonds. The crystalline domain of the cellulose is maintained by hydrogen bonding and van der Waals forces among the hydroxyl groups in the cellulose fibrils. The twists and torsions are present in the amorphous structure. The reduction of cellulose
- 5. fibres into nanofibrils increases its inter-fibrillar bonds permitting a strong compact network and drastically increases its strength, elastic modulus, viscosity, optical properties and barrier performance. NC has high aspect ratio, forming a compact fibrous network with a considerable density via strong interfibrillar bonds. It gives a comparable barrier performance with synthetic polymers. Figure 2-Molecular Structure of Cellulose, obtained from Nechyporchuk et al, 2016 [4] Figure 1 - Hierarchical Structure of Cellulose from wood to the molecular level cellulose, obtained from Isogai, 2011 [1]
- 6. Figure 3-Crystalline and amorphous regions of cellulose, obtained from Abdul Khalil et al, 2016 [5] Cellulose is a linear carbohydrate polymer with repeating units of Cellobiose, a disaccharide with the formula C12H22O11, consisting of two β-glucose molecules linked by a β(1→4) bond. As it is a natural polysaccharide, it is susceptible to microbial and fungal degradation. It is found as an assembly of cellulose fibrils chains forming well-organized fibre wall in the wood. Figure 1 shows the organisation of cellulose fibres from wood to a single fibre and Figure 2 shows the structure of cellulose and surface hydroxyl groups. Figure 3 shows cellulose containing two domains, namely crystalline and amorphous regions. The nomenclature of cellulosic nanomaterials has not been reported uniformly in the literature. The fibre diameter of raw cellulose was reduced via mechanical, chemical and enzymatic processing resulting in the formation of nanocellulose. The term micro-fibrillated cellulose (MFC) refers to the product produced via the delamination of wood fibres by mechanical pressure with chemical or enzymatic treatment. The term MFC is consistent with nanofibrils, micro fibrils and nano fibrillated cellulose [2] with diameter from 10 nm to 100 nm, fibril length in the range from 500 to 10000 [6] and an aspect ratio of 100 [7]. It is composed of formed cellulose semi-microcrystalline fibrils generally produced via high-pressure homogenization of pulp from wood [8]. The average diameter of MFC (DIACEL –KY 100S) to make NC films and composites is approximately 73 nm and aspect ratio of 142 ± 28 [9]. 1.3.1. NANOCELLULOSE – A NOVEL FIBROUS MATERIAL NC is a long and flexible cellulose fibril formed from an entangled fibrous network of microfibrils and nanofibrils. NC contains both amorphous and crystalline regions. These regions mainly contribute to functionality of materials prepared from NC. Nano-fibrils have high aspect ratio. Figure 4 shows the cellulose nanofibrils after two pass high pressure homogenization and the diameter of cellulose nanofibrils after this condition is approximately 20 nm. The important mechanical properties of NC are high elastic modulus (150 GPa) and high tensile strength (10 GPa). Its properties are controlled by their cellulose fibrils’ diameter and length. The aspect ratio of the cellulose fibrils depends on the source, processing method
- 7. and particle type. The source of NC also decides its properties such as applicability and functionality in various fields [10]. Figure 4 - SEM Micrograph of Nanocellulose via two Pass High-Pressure Homogenization. 1.3.2. CHARACTERISTICS OF NANOCELLULOSE The diameter of fibrils in NC ranges from 5 to 100 nm and they are typically several micrometres in length [11]. They consist of both crystalline and amorphous regions in the fibre chain [12]. The crystalline region contributes mostly to the functionality of materials such as barrier and composites materials [6]. The fibril diameter distribution of NC and its aspect ratio depends on the source, pre-treatment processing and fibrillation method etc., and indirectly control its rheological and interfacial properties. The diameter and aspect ratio of NC can be evaluated via an image analysis of micrographs collected from SEM, TEM and AFM. Sedimentation of NC fibres is also performed to evaluate their gel point. The gel point is defined as the lowest fibre concentration in the nanocellulose suspension and a transition point from a highly dilute solution of NC fibres into a concentrated suspension. At this point, nanofibrils in the suspension forms a compact network of fibres due to hydrogen bonding between fibres. This network is known as the threshold of connectivity. Beyond this concentration, the NC suspension loses its mechanical strength due to insufficient contact of fibres with each other. This property depends on the
- 8. aspect ratio of nanocellulose fibres. This characteristic has an important effect on the drainage time in vacuum filtration for NC film fabrication [13], [14]. The notable properties are light in weight, optical transparency, chemical functionality and modification, dimensional stability and good barrier properties. Nanocellulose has potential compatibility with other materials like bio- natural polymers, proteins and living cells for biomedical applications and developing composites. The surface hydroxyl groups on NC provide an active site for various chemical modification and functionalization. The rheological behaviour of NC plays a major contribution in the coating process for producing barrier layers on the paper substrates and its shows pseudoplastic behaviour under shear thinning fluids [15]. 1.3.3. SCOPE OF LITERATURE REVIEW The predominant objective of this chapter is to critically review with the production of films containing NC and its composites. This chapter also reviews the methods of preparation, its advantages and disadvantages on various methods of preparation of NC film and the reasons for the rapid processing of these materials. The potential applications for nanocellulose film and its composites as a barrier, composites, smooth films, membranes and other notable applications will also be discussed. Additionally, nanocellulose as sustainable barrier materials and the need for improvements in its barrier performance, with a particular focus on developing ultra smooth nanocellulose films and its composites, have been addressed. 1.4. NANOCELLULOSE PRODUCTION The large sources of cellulose are wood, seed fibres (for example, cotton, coir etc.), bast fibres (flax, hemp, jute, ramie etc.) and grasses (bagasse, bamboo etc.)., The least popular sources of cellulose are marine animals (tunicate), algae, fungi, invertebrates and bacteria [16]. Wood is the predominant feedstock of cellulose and is classified into hardwood and softwood. Apart from cellulose as a main constituent, hemicellulose, lignin and inorganic salts are also present in the wood. The principal difference between hardwood and softwood is the complexity in the structure with the level of heterogeneity in cellulose fibrils [17]. Hardwood fibres are more rigid than that of softwood having a high Runkel ratio, i.e. thick fibre wall relative to internal diameter [18]. Therefore, the processing of softwood to produce NC consumes less energy than that of hardwood as a feedstock [19]. Similarly, low energy is required for non- woody feedstocks such as grasses and marine sources to produce NC. These feedstocks contain low lignin with a more suitable cellulose structure for purifying and fibrillating the cellulose [4]. Recently, agricultural and food waste have also proved to be a potential feedstock for producing NC via a chemical-free process [20].
- 9. It is a significant challenge to fibrillate the microfibrils into nanofibrils through the refining process. Refining processes can be classified into the mechanical, chemical and enzymatic processes. Before refining the microfibrils, the fibres are subjected to a pre- treatment such as de-lignification, to reduce the energy required for refining and produce high- quality nanofibers. De-lignification is the isolation of lignin in the lignocellulosic biomass. In this process, lignin is depolymerised by a pulping process and the solubilized lignin and hemicellulose in the biomass is removed by washing. Further delignification can be achieved via bleaching with chemical agents. During these pre-treatments, cellulose can undergo mechanical and chemical changes. The pre-treatments produce high-quality cellulose fibres, effectively removing non-cellulosic constituents. The disintegration of cellulose fibrils is promoted through the pre-treatment, reducing the aggregation of fibres to prevent clogs in further fibrillation process. As a result, the fibrillation into nanofibers is improved and minimizes energy consumption [5]. 1.4.1. MECHANICAL PROCESS The most common mechanical treatment of cellulose fibres into nanofibers is primarily disintegration. Grinding is the most accepted method for production of NC. Recently, high intensive homogenization is reported as a novel method to produce nanofibers from cellulose macro fibres and its particles. However, it is an energy consuming method and demands approximately 25,000 kWh per tonne of the production of micro fibrillated cellulose. These methods are suitable for scaling up for production of NC. In high pressure homogenization, the critical factors namely pressure, NC solid concentration and number of passes control the consumption of energy [5]. The various methods of mechanical disintegration of fibres are reported in Table 1. It summarizes the concept of cellulosic fibres reduction, its isolation methods, its advantages and disadvantages [7].
- 10. Table 1 - Mechanical Refining for Production of Nanocellulose [7] 1.4.2. CHEMICAL PROCESS Mechanical disintegration of macro fibres is an energy consuming process, and this could be one of the stumbling blocks for scaling up the process. Nowadays, the demand for cellulosic materials is exponentially increasing in both research and industry. Therefore, an energy efficient process is required to scale up production of nanocellulose [4]. Chemical or enzymatic processes are proven low energy processes compared with mechanical disintegration processes and have improved fibrillation efficiency [7]. Various types of chemical processing of cellulosic nanofibers are presented, with their disadvantages and advantages in the following Table 2.
- 11. Table 2-Chemical processing for Nanocellulose [7] Table 2 summarizes the reaction mechanism of each chemical method for cellulose fibrillation into nanocellulose. 1.4.3. ENZYMATIC PROCESS Enzymatic processes can be used to produce the NC from cellulosic biomass. Enzymatic hydrolysis involves the cleavage of glycosidic bonds in the structure of the cellulose fibres. The classical enzymes are cellobiohydrolases that attack the crystalline region of cellulose, and endoglucanases that attack the amorphous or disordered region of cellulose. The subsequent fibrillation efficiency of NC production depends on the contact time of the enzymatic treatment and concentration of enzymes during the processing [7].The advantages of enzymatic hydrolysis are good fibrillation, reduced clogging of fibrils in the mechanical process, and less energy consumed in this processing. 1.5. APPLICATION OF NC FILMS
- 12. The small dimension of Nanocellulose (i.e. fibre diameter and length) and its large surface area provided a great opportunity to develop it into a more functional material for various applications [21, 22]. Nowadays, the demand for NC is increasing due to several key technical applications and is rapidly growing in importance. In brief, NC film is a transparent, dense packing of cellulose nanofibrils offering good barrier performance and lower surface roughness than paper, leading to the development of many functional materials. Therefore, it can be used in high performance packaging, and can be a good substrate for flexible and printable electronics, low cost diagnostics and organic displays. Although the applications of NC film are many, in this chapter we restrict discussion to a few major applications. 1.5.1. BARRIER APPLICATIONS Barrier materials are widely utilised for preventing the physical, chemical and microbiological deterioration of foods, nutrients, drinks, pharmaceuticals and cosmetics. Barrier materials require low gas and water permeability to protect the contents from the external influences and to preserve the flavour and nature of the packaged product [23]. Commonly used barrier materials are glass, metal such as aluminium and tin and synthetic plastics made from fossil fuels. These have good barrier properties and strength, but are not biodegradable and recyclable [24] [25]. Nowadays, the requirement of packaging materials should have low gas and water vapour permeability. Synthetic plastic films and sheets are predominant in the current conventional practice. After use, these films are thrown away as waste in landfill resulting in a threat to the environment. The recycling of these synthetic plastics is an expensive process and retaining its barrier performance after recycling is problematic. Cellulosic fibre products such as paper and paperboard are alternative flexible packaging that are biodegradable in the environment. However, the hydrophilic nature of cellulose limits the water vapour-barrier properties and oxygen barrier properties of paper. Due to the large pore sizes and high affinity towards water and its vapour, it has poor barrier performance [26]. These materials are not a good fit for packaging at high humidity environment (> 65% relative humidity) such as in frozen conditions. To mitigate these limitations, paper and paperboards can be coated with plastics, wax or extruded aluminium to promote their barrier properties. The synthetic plastic coatings on cellulose substrates suffer from serious environmental issues such as difficult recycling and inefficiency in conversion into a second life barrier material. Nanocellulose has good barrier properties due to its fibrous network [10] and crystalline region. The cellulose nanofibrils in the dense network form a compact structure reducing the passage of water vapour and oxygen. This complex fibrous network increases the tortuosity of
- 13. the pathway for oxygen and water vapour transmission, thus lowering its permeation rate. NC films have low oxygen permeability (OP) due to the complex entanglement of fibrils [26]. The OP of NC film is reported to be 0.004 cm3 .μm.m-2 .day-1 .kPa-1 [27]. This OP value is quite comparable with synthetic plastics such as polyvinylidene (PVDC) ( OP- 0.1-3 cm3 .μm.m-2 .day- 1 .kPa-1 ) [28], poly(vinyl chloride) (PVC) (20-80 cm3 .μm.m-2 .day-1 .kPa-1 ) [28]. The water barrier properties of NC films is also extremely good under dry conditions although it is less effective in high humidity [26] Therefore, NC is a potential alternative for synthetic plastics in barrier application [29]. The source of fibres for making NC films, their chemical composition, physical structure, and the pretreatment methods used to produce the nanocellulose have an effect on their barrier performance [6]. 1.5.2. PRINTED ELECTRONICS Cellulose substrates such as paper are sustainable and low-cost substrates for developing functional materials such as printed electronics, solar cells [30], Radio- Frequency Identification Device (RFID) tags, Organic Light Emitting Diode (OLED)s, photovoltaic (PV) cells, printed diagnostics, batteries, memory cards etc. [31]. Paper substrates is also used in the fabrication of transistors and super capacitors. However, paper is a porous substrate, highly susceptible to moisture absorption and has high surface roughness. Their properties limit the utilization of paper in the construction of electronic materials. The surface roughness of paper-based substrates varies from 2 to 10 micrometres and this range of roughness limits the performance of solar cells [32]. Consequently, nanocellulose is nowadays used as a potentially sustainable substrate for developing printed electronics due to its low surface roughness [33]. NC provides smoothness on the substrates, is thermally stable and provides good barrier against water and air for the construction of flexible electronic device. 1.5.3. OTHER APPLICATIONS The surface and mechanical properties of nano-fibrillated cellulose can be tailored. Films made from nanocellulose can have various outstanding mechanical, optical and structural properties and these properties are useful in fabricating various functional materials such as cellulose nanocomposites [34], inorganic nanocomposites [35], organic Transistors and conducting materials [36], Immunoassays and Diagnostics materials [37]. As a result, it is used in the field of photonics, as a surface modifier, in nanocomposites, biomedical scaffolds and optoelectronics [10]. Recently, NC films have become one of the most promising high- performance functional materials potentially used as filters for removal of virus [38], adsorbents, catalysts [39], cell culture substrate, thermal insulator and drug carrier for drug delivery system [40]. Moreover, due to the barrier performance of nanocellulose and size, it is used to functionalize base sheets via creating barrier layers as a coating or to make
- 14. freestanding sheets/films and nanocomposites [4]. Recently the application of nanocellulose in various fields has increased rapidly. There is a demand for a rapid manufacturing method for NC film. In particular, the demand for barrier application NC film to replace synthetic plastics in practice is urgent[41]. Table 3 summarizes the application of NC depending on its quantity in various fields. Figure 5 shows the products developed with NC in High-performance Fibre material Group, Department of Chemical Engineering, Monash University, Australia. Figure 5- Applications of Nanocellulose into functional Materials from High-Performance Fibre material research Group, BIOPRIA, Monash University, Australia. (1). K.Shanmugam et al, Cellulose, 2017, 24, 7, 2669–2676. (2) Z. he et al, RSC Adv., 2016, 6, 21435- 21438 [42]. (3). U.M. Garusinghe et al, Scientific Reports, 8, 2306 (2018) (4). V. Swambabu et al, Chemical Engineering Journal 265 (1) 2015. (5). Mendoza et al, J Colloid Interface Sci. 2018 1;509:39- 46. (6) Maliha , 2018 (7). U.M. Garusinghe et al, Colloids and Surfaces A, 540, 2018,233-241. 1.5.4. PRODUCTS OF HIGH-PERFORMANCE FIBRE MATERIAL RESEARCH GROUP, MONASH UNIVERSITY. 1.5.4.1. PACKAGING An homogenized NC film from MFC was prepared, via vacuum filtration, with an air permeance less than 0.003 µm/Pa.S, confirming that it is a good impermeable film for packaging applications [43]. The water vapour transmission rate (WVTR) and oxygen transmission rate (OTR) of the NC film at 23°C and 50% RH were 44.7 g/m2 .day and 20.1 cc/m2 .day respectively [44]. These values confirmed that a vaccum filtered NC film is a good barrier. 1.5.4.2. OIL AND WATER SEPERATION Crosslinking between nanocellulose and polyamideamine-epichlorohydrin (PAE) was used to produce an aerogel filter using a freeze drying cross linking method. The NC based
- 15. aerogel filter was capable of de-emulsification of oil water mixture producing 100% oil water separation efficiency even after 10 cycles of usage and yielding 98.6 % oil water surface free emulsion [45]. 1.5.4.3. PHOTO CATALYST COMPOSITE This composite was developed from nanocellulose -polyamide-amine-epichlorohydrin (PAE)-TiO2 nanoparticles via vaccum filtration, and was designed as a photo catalyst for treatment of waste water. Figure 6 shows that the titanium dioxide in the composite degrades 95% of methyl orange dye in water in 150 min under UV light irradiation. The NC matrix with PAE strongly hold TiO2 nanoparticles and avoid the release of nanoparticles into the waste water. This composite can be used as a recylable, low cost photo catalyst membrane for waste water treatment [46]. Figure 6- The degradation of methyl orange solution byTiO2-MFC composite as photo catalyst. The spectrum indicates two absorption maxima. The colour of dye solution reduced using photocatalysis composite. The image obtained from Garusinghe et al 2018 [46]. 1.5.4.4. MEMBRANES Cellulose nanofibre membranes can be an alternative to synthetic plastic membranes and they are recyclable and biodegradable. In this case, the novel composite was prepared
- 16. via vaccum filtration and the composite was produced with nanocellulose, silica nanoparticles (NP) and polyamide-amine-epichlorohydrin (PAE).The size of silica nanoparticles were 22 nm and they acted as a spacer for controlling the pore of the composite. The function of PAE in the composite was to adhere with the negativly charged silica nanoparticles and promote the wet strength of composite. This composite membrane has a water flux of 80 Lm-2 h-1 , Liter per square meter and hour (LMH) and molecular weight cutoff, (MWCO) of 200 kDa. The quantity of silica nanoparticle could be altered to tune the pore size. The function of composite mainly play as membrane and can be used mainly in ultrafiltration applications [47]. 1.5.4.5. GELS Nanocellulose can be converted via TEMPO oxidation method, into hydrogels and gels for numerous biological and medical appliocations. Nanocellulose gels can be used for separation of protein via elctrophoresis and in diagnostics for bioseparation [48]. Nanocellulose hydrogels and gels are also suitable substrates for 3D cell culture for producing microenvironment for cell growth, tissue engineering scaffolds and drug delivery vehicles [49]. Carboxylated nanocellulose via TEMPO oxidation can be converted into foam structures as super adsorbent material (SAM) [50]. 1.5.4.6. ANTI-MICROBIAL NANOCOMPOSITES Anti-microbial nanocellulose composites using an anti-microbial agent phenyl bismuth bis(diphenyl phosphinate), via vacuum filtration [51] and spraying [52] were developed. These composites can act as anti-microbial packaging materials, biomedical bandages, and anti- microbial coating on the surface. Recently, bismuth based anti-microbial agents have been shown to have broad spectrum activity against pathogens including antimicrobial resistant species. The anti-microbial potential of the 5 wt. % bismuth loaded composite was shown by the 15 mm zone of inhibition against both gram negative microorganisms such as Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa), gram positive bacterium such as Staphylococcus aureus (S. Aureus), anti-microbial resistant pathogens namely Vancomycin- resistant Enterococcus (VRE) and Methicillin-resistant Staphylococcus aureus (MRSA). The WVP of the same composite was evaluated to be 4.4 x 10-11 g/Pa.s.m, comparable with synthetic plastics [52]. 1.5.4.7. HIGH PERFORMANCE NANOCOMPOSITES NC based nanocomposites for high performance barrier materials were developed via an in-situ precipitation process. In this method, calcium carbonate nanoparticles were precipitated via a chemical reaction between sodium carbonate and calcium chloride in the
- 17. NC film to yield a low permeability porous composite. The precipitated nanoparticles reduced the pore volume of the NC film, resulting in the low permeance of the composite. Figure 7 shows the difference between pure NC film and in-situ precipitated composite prepared from 0.2 M solutions and the presence of CaCO3 nanoparticles interior of the composite. The WVTR and OTR of 1 wt. % CaCO3 precipitated in NC film were 4.7 g/m2 .day and 2.7 cc/m2 .day respectively at 23°C and 50% RH. The tensile strength and elastic modulus of this composite were 91.96 ±14.99 MPa and 4.6 GPa respectively, confirming good strength and stiffness [44]. Figure 7 - SEM micrographs of CNF film and its composite via in-situ precipitation of nanoparticles. (A) Pure CNC film and (B and C) Surface and cross-sectional view of composite. The image obtained from Swambabu et al, 2018 [44]. High performance membranes were also achieved with a nanocellulose (NC)-Silicon –Di- Oxide , SiO2 composite fabricated with a conventional vacuum filtration method. The pore size of the composite was reported to be less than 100 nm. The porosity of the composite could be tailored by varying SiO2 loading in the NC suspension [53]. Similarly, a NC-
- 18. Montmorillonite (MMT) composite was developed via vacuum filtration for packaging application. The WVP of the homogenised NC – MMT composite was reported to be 6.33±1.5 g.µm/m2 .day.kPa with 23.1 wt.% MMT comparable with synthetic plastics [54]. Table 3 - Application of NC [7] 1.5.5. NANOCELLULOSE COMPOSITES AND ITS APPLICATIONS Although NC films are a flexible, strong, biodegradable and sustainable material. It has specific limitations in oxygen and water vapour permeability at high humidity where the pore size of NC film increases by the swelling of the cellulose nanofibrils. Aggregation is a problem in the preparation of NC films. Normally, cellulose molecules aggregate due to hydrogen
- 19. bonds between fibrils. To overcome this problem in NC films, NC has been combined with other biopolymers or inorganics such as nanoclay to develop composites. One of the most important applications of NC is to develop composites with nano-inorganics [54-56]. The nanoparticles of inorganics interact with cellulose nanofibrils and fill the pores in their matrix, forming the composite with drastically enhanced barrier and mechanical properties. These composites have specific applications as barrier, fire retardant, membrane and heat insulating materials. The incorporation of nanoclay such as Montmorillonite (MMT) into NC matrix enhances its mechanical and barrier properties [54]. The nano-inorganics interact with cellulose nanofibrils promoting the functionality of the composite [55] [56]. The functionality of the composites can be enhanced by filling the pores in cellulose fibrous matrix resulting in good barrier composites, by forming aggregates in cellulose network resulting in fire retardant material, or by tuning the pores of cellulose matrix in membrane applications [47, 53] [57]. Table 4 summarizes the potential use of NC based composites. 1.5.5.1. BARRIER COATING Nanocellulose–MMT suspension can be used for coating to produce a composite barrier layer on the cellulose substrates. The composite barrier coating lowers the oxygen and water vapour permeability of the sheet. 5 wt. % MMT and 1.4 wt. % NC coating on base sheet with thickness of 22 ± 0.5 µm and with a coat weight of 29 ± 0.5 g/m2 has a WVTR of 8 ± 0.3 g m−2 day and OTR of 36400 ± 1100 cm3 m−2 day. With only 1.4 wt. % NC coating on a base sheet with thickness 11 ± 0.3 µm and coat weight 9.9 ± 0.2g m-2 had a WVTR of 24 ± 0.7 g m−2 day and OTR was above measurable range. Hence the 5 wt. % addition of MMT in NC suspension causes a significant decrease in WVTR and brought OTR in the range [58]. 1.5.5.2. MEMBRANE NC-MMT can be used for construction of membranes. For example, MMT in membranes can adsorb cationic dyes in the waste water and act as a cation exchanger for wastewater treatment. Additionally, these composites can be used for the removal of heavy metals in waste water [59], [60].
- 20. Table 4-Application of NC based Nanocomposites [61] 1.5.5.3. FLAME AND FIRE-RETARDANT The development of non-toxic sustainable flame and fire retardants to replace synthetic materials is a challenging task. Cellulose-based flame- and fire-retardant materials are a green and sustainable alternative to halogen based retardant materials. Generally, halogen based retardant materials are a toxic hazard that can cause halogen compounds to enter the food chain. Recently, nanocellulose has been shown to be capable of meeting the requirements of flame retardant materials. The addition of nano-clay is another approach to tailor the properties of composites for flame retardant properties [57]. The OP of 5 wt.% MMT composite film prepared via casting is 0.006 mL μm m−2 day−1 kPa−1 at 0 % RH and its mechanical strength is 18 GPa of E-Modulus and 509 MPa tensile strength. Increasing the MMT to 50 wt.% in the composite decreases its OP to 0.0008 mL μm m−2 day−1 kPa−1 . However, the mechanical strength of the composite film decreased at 50 wt.% of MMT loading. These composite could be used as gas barrier material [62]. The 50 wt. % MMT–50 wt. % NC composite in a continuous fibrous matrix of nanocellulose formed via vacuum filtration has also been investigated. The strength of this composite was 124 MPa and 8.7 GPa of elastic modulus. The OTR of the pure NC film was
- 21. 0.048 cm3 mm m-2 day-1 atm-1 at 50% RH and 17.8 cm3 mm m-2 day-1 atm-1 at 95% RH. By comparison the OTR of 50 wt. % MMT–50 wt. % NC composite was 0.045 and 3.5 cm3 mm m-2 day-1 atm-1 at 50 % and 95% RH respectively. The OP of pure NC film was increased by 370% from 50 % RH to 95%. In the case of composite, the increase in OP was only 13 % from 50 % to 95% RH. This demonstrates that the addition of MMT into fibrous structure formed a composite with potential gas barrier and flame retardancy properties [57]. Both 30 wt. % and 50 wt. % MMT in NC composite demonstrated a good fire retardancy material. Flammability test and calorimetry confirmed self-extinguishing properties of the composite. This property is important to autonomously control or prevent a fire or flame. The oriented MMT in the NC fibrous network hinders diffusion of oxygen and ignition process [63]. 1.6. PROPERTIES OF NANOCELLULOSE FILMS The physical properties of NC film are related to their thickness, basis weight, density and porosity. These properties strongly rely on the properties of cellulose nano-fibrils such as their diameter and aspect ratio and the method of fabrication [64]. 1.6.1. THICKNESS AND BASIS WEIGHT The thickness and basis weight of NC film affects numerous properties such as strength, density and barrier performance. The method used to fabricate the NC film also controls its characteristics and properties. In vacuum filtration, the thickness of NC films varies from 50 to 100 µm and basis weight varies from 60 to 80 g/m2 [43]. The dewatering time in the forming process increases when NC suspension concentration is increased for higher basis weight NC film. Casting, a simple method, gives a non uniform NC film due to shrinkage and wrinkles formed. Cast NC films are produced with thickness varying from 5 to 30 µm [65]. 1.6.2. APPARENT DENSITY The density of the fibrous network in NC film controls the barrier and mechanical properties [66]. The density of the NC film is controlled by fibril interactions and film processing. The NC film preparation method controls film density by agglomeration and non-uniformity in the surface of NC film. The clumps of cellulose nanofibers during processing of NC film affects its density. Partial homogenised or non-homogenised NC can produce a drop-in density of film in vacuum filtration. The NC film produced with homogenization of fibres shows densification with lower thickness due to the packing of nanofibrils. The mean densities of micro fibrillated cellulose, MFC film via vacuum filtration was 820 k g/m3 [43].
- 22. 1.6.3. FILM UNIFORMITY In the processing of cellulose fibres products such as paper and paper board, the quality of the film/sheet is evaluated by its uniformity. This property is a confirmation for evaluating uniformity of NC film. This property affects directly surface quality, strength and printability of the film [67]. A paper perfect formation tester is used to estimate relative formation value (RFV) of the sheet/film. The RFV measures the uniformity of NC film. If the RFV is ≥ 1, the film has good uniformity, if it is < 1, the film is said to have lower formation [68]. 1.6.4. MECHANICAL PROPERTIES The strength of NC film is important in maintaining functions of NC films in barrier materials, membrane and composite. NC films should have enough support for the matrix. The fibres in the NC film form a compact network with hydrogen bonds between neighbouring fibres developing the strength and modulus and high density of the film. The strength of the NC film also depends on a number of factors such as diameter and length of fibrils, source of biomass and quality of interaction with fibres in a network. The strength of NC film depends on the fabrication methods and cellulose nanofibrils characteristics. The mechanical properties of nanocellulose film prepared by various methods are summarized in Table 5. This shows that spraying is the fastest method to prepare NC films and properties achieved by spraying are similar to those of film from conventional methods. 1.6.5. BARRIER PROPERTIES The main motivating factors to use cellulose nanofibers is its ability to form a fibrous compact structure that plays a role in barrier performance. When comparing with other synthetic plastics and other natural polymers, nanocellulose has low oxygen permeability and reasonable water vapour permeability. Figure 8 shows oxygen permeability (OP) and water vapour permeability (WVP) of conventional synthetic films and biopolymer films, confirming the notable barrier performance of nanocellulose comparing with other biopolymers [29]. Oxygen and water vapour permeability are the main barrier properties of NC film and Figure 8 reveals that the barrier properties of NC film is better than that of other biopolymers. Figure 9 shows the tortuous pathway of NC against oxygen and water vapour.
- 23. Table 5-Properties of NC films [69] 1.6.5.1. OXYGEN PERMEABILITY (OP) The permeation of oxygen and other gases including volatile organic compounds can degrade the quality of foods. The atmospheric oxygen can react with the double bond of the unsaturated fatty acids in oils and fats resulting in rancidity. Because NC films have low OP they can provide good barriers to protect the foods. It can increase the shelf life of foods by avoiding the formation of unwanted chemical reactions such as oxidation and assist the retention of the flavour of food and its fragrances. The oxygen permeability of nanocellulose film is evaluated and compared with the synthetic plastics in Table 6. The data for OP of synthetic plastics and nanocellulose films are shown in Table 6 [29]. This table also shows the effect of relative humidity on NC film and carboxymethylated NC film compared with synthetic plastics. At low RH%, NC film has low OP, but it increases with increasing %RH. The increment is quite comparable with conventional synthetic plastics.
- 24. Figure 8-Overview of Barrier Materials, obtained from Aulin et al 2011 [70] Figure 9-Simple diagram of the structure of NC film giving it good barrier performance against oxygen and water vapour Table 6-Oxygen Permeability of NC and Synthetic Plastics Materials OP Conditions
- 25. (cc µm/m2 day KPa) NC 0.6 65% RH NC carboxymethylated 0.0006 0% RH NC Carboxy methylated 0.85 50% RH Cellophane 0.41 0% RH Polyethene (PE) 50–200 50% RH Polyvinylidene chloride (PVdC) 0.1–3 - Polyvinyl alcohol (PVOH) 0.20 0% RH Ethylene vinyl alcohol (EVOH) 0.01–0.1 0% RH NC film has more and higher nano-fibrillar entanglements and provides a more closed surface, and a complex internal tortuous diffusion path, showing a good barrier against oxygen up to 65% RH[72]. [26]. In this range, the NC film shows comparable barrier performance with synthetic polymers [71]. The crystalline regions in particular, contribute to increased impermeability of this film against oxygen [73]. Hence, the nanocellulose are suitable for dry foods packaging. 1.6.5.2. LIMITATIONS OF NC IN OP The OP of NC film is very low and comparable with synthetic plastics at 0 % RH. However, if the RH% reaches the 65%, it is not considered as a good barrier material. At low humidity, NC film is rigid and with good resistance. At higher RH, It becomes rubbery and viscous due to the high moisture content which softens the cellulose nanofibrils and weakens the cellulose chains. As a consequence, the interfibrillar space is increased and the local deformation permits increased permeation of oxygen molecules across the film [26]. The OP of NC film is 0.6 cc µm/m2 day KPa at 65 % RH, 0.85 cc µm/m2 day KPa at 50 % RH and 0.011 (cm3 µm)/(m2 day kPa) at 0% RH. The OP of micro fibrillated cellulose, MFC at various humidities and in comparison, with synthetic plastics is shown in Table 7. Table 8 confirms OP of cellulose nanofiber, CNF film decreases with increased thickness of the film and increased RH [74].
- 26. Table 7-OP of MFC [75] Material OP (cm3 µm)/(m2 day kPa) Relative humidity (%) MFC 0.011 0 MFC 3.52 - 5.03 50 PET 10 - 50 50 PLA 184 0 LDPE 1900 50 Table 8-Reported oxygen permeability of selected carboxymethylated nanofibril (CNF) films Material OP (cm3 .μm)/ (m2 .day.kPa) Film thickness µm Conditions CNF (carboxymethylated) 0.009 2.54 0% RH CNF (carboxymethylated) 0.0006 3.19 0% RH CNF (carboxymethylated) 0.85 3.19 50% RH 1.6.5.3. WATER VAPOUR BARRIER PROPERTIES: Paper and paper board are cellulose fibre-based substrates giving mechanical support to protect contents, but they have poor resistance against water vapour and oxygen due to the presence of large pore size and the hydrophilic nature of the cellulose fibres. Nanocellulose has a fibrous network with small pores and shows reasonable resistance against water vapour. The incorporation of cellulose reinforcing materials into the cellulose fibrous matrix has enhanced barrier performance reasonably and reduced the water vapour permeability [26]. Table 9 shows the barrier performance of nanocellulose against water vapour and compared with synthetic polymers.
- 27. Table 9 – Water vapour barrier properties of Nanocellulose and Synthetic polymers Barrier Polymers Water Transmission Rate (g/m2 day) Average Thickness of the film (µm) Water Vapour Permeability (g/m.s.Pa) NC Film - Vacuum Filtration (100 g/m2 ) 52.9±1.2 119.4 4.97 x 10-11 Cellulose nano fibrils 234 42 8.12 x 10-11 Acetylated CNF (Acetylation Time- 30 mins) 167 46 6.35 x 10-11 Polyvinylidene chloride (PVC) 3.07 12.7 1.27x 10-13 Polyethylene (PE) 16.8 18.3 1.00 x 10-12 Plasticized (PVC) 118.56 12.7 4.90 x 10-12 Aluminium Foil 2.376 18.3 1.42 x 10-13 LDPE 18 25 8.75 x 10-13 HDPE 9 25 4.37 x 10-13 Oriented Nylon 6 260 15 7.59 x 10-12 Oriented Polystyrene 170 25 8.27 x 10-12 EVA 70 25 3.41 x 10-12 EVOH 22 -124 25 1.07 x 10-12 - 6.032 x10-12 PA 155 25 7.54 x 10-12 PET 16-23 25 7.78 x10-13 - 1.12 x 10-12 PC 139.5 25 6.78 x10-12 PS 109 to 155 25 5.30 x10-12 - 7.54 x10-12 PP 6 25 2.94 x 10-13 1.6.5.4. MECHANISM OF BARRIER PERFORMANCE
- 28. To understand the barrier mechanism of NC film, the diffusion of oxygen and water vapour from high concentration to low concentration separated by an NC film has three following steps namely 1. Permeating molecules adsorbed on the surface of NC films 2. Diffusion of these molecules through the interior of the film. 3. The molecule desorbed from the other side of the NC film. The permeability through a NC film mostly depends on diffusion rate internal to the film. Equation 1- Permeability of gas or water vapour through films 𝑃 = 𝐷 ∗ 𝑆 Where P is the permeability, D is the diffusion coefficient and S is the solubility coefficient. Fick Laws of diffusion is used to describe the permeability coefficient P. Equation 2-Permeability coefficient 𝑃 = 𝐷 × 𝑆 = 𝑞 × 𝑙 𝐴 × 𝑡 × 𝛥𝑃 Where q is the amount of gas or water vapour passing through the film, l is the flow path or thickness, A is the cross-sectional area, t is time, and p is the pressure difference across the film [29]. 1.7. METHODS FOR IMPROVING THE BARRIER PERFORMANCE Methods available for enhancing barrier performance of NC films are chemical modification of the cellulose fibrils, use of additives, coating on the film, curable cross linking treatment of fibres and keeping residual lignin in the NC [75]. Apart from these methods, incorporation of montmorillonite, MMT or similar nanoparticles influence the water vapour resistance of the NC films [76]. Vermiculite added into NC film increased its oxygen resistance even at high humidity conditions [77]. Adding MMT into biopolymers to develop nanocomposites, improve their strength and thermal stability. Moreover, MMT or similar clays with high aspect ratio [75] provide good reinforcement for bio polymeric matrices [78]. The common clay used in developing composite is bentonite, which is also known as montmorillonite (MMT). “It is an alumina-silicate layered clay hydrated. This structure consists of an edge-shared octahedral sheet of aluminium hydroxide between two silica tetrahedral layers” [79]. Figure 10 shows the structure of MMT. It is a potential filler at the nanoscale, which provides reinforcement in the nanocomposite and offers a high surface area with a large aspect ratio from 50 to 1000 [80]. It is a di-octahedral clay with 2:1 layer linkage. MMT is a rigid and impermeable filler. The 5 wt.% MMT is a permissible concentration used as food
- 29. additives in Australia and no adverse effects on human health [81]. Nanocomposites from nanocellulose and MMT has low oxygen and water vapour permeability [82]. Apart from the barrier performance of nanocomposite, incorporation of clay in the biopolymer network promotes its mechanical properties and increased the glass transition and thermal resistance. However, adding clay in the nanocellulose reduces the transparency of NC film [75]. 1.7.1. ROLE OF CLAYS INTO THE NANOCELLULOSE NETWORK The packaging industry prefers low-cost additives and eco-friendly materials that should be universally accepted for easy processability by industry. Layered inorganics used as additives are two-dimensional, usually about 1 nm in thickness and several microns in length depending on the types of silicates or clays. As already discussed the tortuosity of NC film is increased through clay particles sitting in the diffusion path of the film, and the barrier performance and potential of the given nanocomposite is thus improved [83]. Figure 10-Structure of MMT. The image obtained from [84] 1.7.2. STRUCTURE OF NANOCELLULOSE COMPOSITE There are three types of structure in composites, namely phase separated nanocomposites, intercalated nanocomposites and exfoliated nanocomposites as shown in Figure 11. The method of fabrication of composites, the configuration of clay platelets and the fibril size of the nanocellulose control the structure of the composite and its barrier performance. In phase separated nanocomposites, clay platelets exist in a tactoid structure in the composite where the polymer and clay platelets are not mingled. As a result, clay platelets are aggregated in the composite resulting in the formation of a macroscopic composite with
- 30. poor barrier performance. In the second type of composite, the clay platelets are intercalated with polymer fibres. The nanocellulose fibres penetrate the interlayer region of the nanoclay platelets resulting in the formation of an ordered structure with a multilayer of alternative NC/clay layers at nanometre scale. In the exfoliated structure, the clay platelets are well delaminated and distributed in the interior of the nanocellulose region of the nanocomposite and extensive nanocellulose penetration into the layer of clays is observed [83, 85]. Figure 11 shows the orientation of clay platelets in nanocomposites and how this affects barrier mechanism demonstrated in figure 12. Figure 11-Structure of nanocomposite. The image obtained from Alexandre, M et al , 2000 [86] 1.8. FABRICATION METHODS NC films/composites can be used in various applications hence the demand for NC films/composites is rapidly increasing. The market for NC is predicted to reach USD 700 million by 2023 as NC demand increases at a compound annual growth rate of more than 33%, especially increasing in the pulp and paper sector [41]. However, the commercialization of these products is limited by the existing methods for preparing NC films/composites. The key issue is processing time, and therefore, a rapid and flexible process to handle high NC suspension concentration is required to prepare free-standing NC films with properties such as strength, barrier properties and roughness required for specific functionalities.
- 31. Methods used for the fabrication of free-standing nanocellulose films/ composites are solvent casting, spin coating, Roll to Roll (R2R) coating, layer by layer assembly, vacuum filtration and spraying. Vacuum filtration is now considered as a conventional laboratory method for NC film fabrication. 1.8.1. SOLVENT CASTING Solvent/Solution casting is a laboratory scale technique for NC film fabrication. In this method, NC suspension is poured on a glass or plastic Petri dish. The solvent is evaporated from the poured suspension on the substrate under standard laboratory conditions [87]. Freeze-drying of the cast suspension is another option., Normally, the time taken for forming the films varies from a minimum of 3 days to a week, depending on the concentration of NC suspension and the thickness of suspension [74]. The higher the water content of the NC suspension the more time is required for evaporation. The suspension consistency normally used in solvent casting varies from 1.13 wt. % to 2 wt. % NC content. Although, this method is a quite simple method, controlling the uniformity of the NC film thickness and basis weight is quite challenging. After drying, films generally tend to shrink and wrinkle, resulting in poor uniformity of the NC film that affects its strength and other characteristics. Good distribution of the NC in the suspension is also required to achieve quality films and sheets, as it controls the properties of the films and sheets [65]. Figure 12-Mechanism of Barrier Performance in Nanocomposites. This image obtained from Garusinghe et al, 2018 [54]
- 32. Casting is also a common laboratory scale method for producing clay–nanocellulose composites. Using a suitable base surface, the key factor is proper dispersion of the clay in the suspension which is required to avoid the stacking and aggregation of clay in the composites. Improper dispersion leads to exfoliation, with resultant poor barrier performance. Therefore, an additional step is required to improve the dispersibility of clay platelets in the NC suspension, aiming at an intercalated composite [87, 88]. The clay dispersibility can be promoted by intensive agitation such as by stirring and shear mixing. Recently, ultra-sonication has been used to provide a good dispersion of nanoclay in suspension and break the nanoclay clusters or aggregates. However, this may not be a solution for large scale production of composites [54]. Several examples of cast films are worth noting. Cast composites from vermiculite nanoplatelets in homogenized NC showed good oxygen and water vapour barrier properties and achieved good tensile strength and stiffness[77]. A cast composite of cellulose foam with homogenized NC and Tween 80- 2.5 wt.% MMT achieved a WVP of 216000 g µm m-2 day-1 KPa-1 , a strength of 13.1±1.7 MPa and a Young's modulus of 0.455 GPa [88]. The low WVP and high strength was due to the composite containing a lower loading of MMT. The composite from the casting of poly (vinyl alcohol) PVA- Poly acrylic acid (PAA) – NFC – 50 wt. % Sodium MMT achieved a WVP of 105 g µm m-2 day-1 kPa-1 a strength of 190±11 MPa and a modulus of 15.9±0.3 GPa [89]. The cast composite showed better WVP and good strength due to the presence of biodegradable polymers and high content of MMT. The biodegradable polymer (PAA) promoted the dispersion of clay particles and crosslinking between PVA and PAA [56]. NFC – vermiculite composite prepared via two pressure homogenization stages followed by solution casting was reported to have a WVP of 60 g µm m-2 day-1 kPa-1 a tensile strength of 244±24 MPa with a modulus of 17.3±0.4 GPa [77]. The processing time for forming the composite film via casting consumed days and is considered as a huge constraint for commercialization. In solvent casting, NC suspension and clay platelets are mixed together for better dispersion and allow swelling of the clay layers. Therefore, the clay platelets are retained onto the surface of the poured suspension as a single layer and causes exfoliation of platelets in the sheet. The exfoliated composite gives poor barrier performance against water vapour and oxygen due to its poor maze structure in the diffusion pathway. Therefore, an additional step is required in improving the dispersibility of clay platelets into NC suspension to get an intercalated composite [90, 91]. The clay dispersibility could be promoted by an intensive stirring and shear mixing. Recently, ultra-sonication has been used to provide a good dispersion of nanoclay in suspension and break the nanoclay clusters or aggregates. However,
- 33. it may not be a solution for large scale production of composites [54]. Solution blending followed by casting has drawbacks of being a time-consuming process, challenging to form a composite with exfoliation of clay platelets. It requires an additional process to disperse clay to produce the composite at laboratory scale. 1.8.2. SPIN COATING Spin coating is a technique for producing freestanding thin nanocellulose film with nanometric thickness. Spin-coated thin NC films have been used for investigating the interaction of biological molecules with NC[92]. In this method, high-speed spinning is applied to remove the excess NC suspension, thus leaving ultra-thin NC films. This is not a scalable method and is not suitable to meet the commercial requirements of NC film as per the current demand [87], but, it is a perfectly suitable method to make films for looking at biomolecule interaction with cellulose fibrils at laboratory scale. Spin coating was also used to produce cellulose nanomaterial layers as a film on the polypropylene. A 1 µm thin layer of CNC coated on a UV treated Poly Propylene (PP) film reduced oxygen permeability by 39% compared to uncoated PP[93]. Spin-coated CNC films promote their gas barrier functionality and multilayered coated CNC films can be used in gas separation and as barrier material [94]. 1.8.3. ROLL TO ROLL COATING The Roll to Roll Coating (R2R) has been used to produce NC film [65] and NC barrier layers on cellulose substrates to enhance the mechanical and barrier properties [95]. In making films this way, NC is coated evenly on pre-treated substrates, normally plastic films. The spreading of NC is controlled and adheres to the substrate followed by drying under a controlled environment. The NC films are then detached from the substrate and pressed. The NC film prepared via this method has no shrinkage. The average roughness of NC film made this way is 400 nm on the side exposed to the plastic substrate and 800 nm on the free side. This is a scalable method for producing translucent, smooth and densely packed NFC film. The time required for making the film is 15 minutes [65]. The OTR of NC film was less than 0.1 cm3 /m2 /d (0% RH; 15 μm thickness) and tensile strength of 200 Mpa, while water vapour barrier was not reported. The coating weight of NC using R2R can vary from 0.1 to 400 g/m2 . The quality of NC film can be controlled by various parameters such as NC solid concentration, wettability of NC suspension on the substrates and velocity of webs etc. 1.8.4. LAYER BY LAYER ASSEMBLY This method deposits macromolecules and nanoparticles containing charged groups in solution to form a film with variation in thickness from nanometre to micrometres. This method can be used to produce thin functional films and for coating of nanocellulose for
- 34. specific functionality. One application used a thickness of the film of 5 µm [96] and reported a surface roughness of 9 nm. It is not a scalable process for commercial production of NC films. In the past three decades, Layer–By-Layer assembly carried out at mild reaction conditions has been used as a rapid technique for surface engineering of cellulose fibre sheets. The size and shape of the substrate do not affect this process. The film produced via this process can be flexible in composition, with tailorable thickness and stable performance. The thickness of the composite can be engineered via multiple coatings on the surface of base substrates. The multilayer coating of MMT on the base substrates affects the strength of the composites and can cause aggregation of MMT Layer-by-layer coating of MMT on biodegradable polymer sheet enhanced its barrier performance including giving a low oxygen permeability [97]. The surface coating of MMT on sisal fibre cellulose microcrystals via this method was utilised to develop a flame retardant material [98]. 1.8.5. VACUUM FILTRATION Vacuum filtration is the most common method for forming NC films, as it used in cellulosic fiber sheets such as paper and paper board. In the laboratory method, NC suspension is poured into a column which contains a metal mesh as a filter at the bottom. A wet film of NC fibres is formed on the mesh. The sequences involved in the preparation of NC film via vacuum filtration are shown in Figure 13. The retention of NC on the filter depends on the mesh size and its pore size; however, smaller mesh size generally increases the drainage time in the filtration process. The commonly used filters in practice [99] are tetrafluoroethylene (PTFE) membrane (pore size 0.1 μm), woven filter fabrics (pores – 55 µm, 109 µm, 150 µm) [100] and Millipore filter membrane (Pore- 0.65 µm) [101]. The wet NC film is formed on the mesh and subjected to air or drum drying. The time required to prepare films is varied to 10 minutes[43] to 4 hours [102]. Reasons for variation in the time for the NC film prepared via vacuum filtration is dewatering time, the pore size of the filter medium, the pore volume of the NC film and thickness of the NC film formed on the filter medium. Normally, filtration time also increases with NC suspension concentration [43]. Laboratory scale production of homogenized NC film via filtration requires approximately 10 minutes to produce a film with basis weight of 60 g/m2 . It is a conventional method to produce NC films. However, it has a limitation in that the drainage time increases with basis weight of the film. The increase of thickness and basis weight of the film with drainage time is exponential. The thickness of NC film produced via the filtration varies from 50 to 100 µm. The notable issue in this method is the separation of the film from the filter after couching. Sometimes a filter mark is imprinted on the NC film affecting the uniformity of the
- 35. film [65]. Although this method can achieve excellent barrier and mechanical properties [43], preparation time is a bottleneck for scaling up the process for commercialization of NC film. Figure 13 - Vacuum filtration for NC film fabrication. Reprinted adapted with permission from Sehaqui et al 2010 [103] 1.8.6. NANOCOMPOSITE VIA FILTRATION Vacuum filtration is a conventional cellulose fibre / cellulose based composite sheet making process in which the suspension of cellulose nanofibrils with nano-inorganics is collected on the filter to form the composites. The wet composite on the filter are couched and followed by pressing and drying process. In the mixing stage, inorganics are well distributed into the 3D matrix of fibrous network. The MMT–NC composites have been produced via vacuum filtration and reported low oxygen and water vapour permeability [54, 56, 82]. The WVP of MMT –NC composites via filtration are reported to be 13.3 ± 2.0 g µm/m2 .day.KPa with 16.7 wt. % of MMT loading in the composites. Increasing MMT in 0.2 wt. % of NC suspension increase drainage time in the filtration process and high constraint for commercialization. A composite film from 25 wt. % Clay loaded in Eucalyptus NC and P. radiata NC and prepared via vacuum filtration resulted in WVTR of the composites ranging from 183 to 197 g/m day. [100]. Increasing the clay loading from 0 wt.% to 25 wt.% reduced WVTR from 299 to 197 in P. radiata NC composites and from 285 to 183 in Eucalyptus NC composites [104]. The processing time for composites was not reported. 1.8.7. IMPREGNATION PROCESSES This method has been used to develop composites as samples for evaluating mechanical, thermal and optical properties. In this method, a thin cellulose film initially produced via membrane filtration is then laminated with nanomaterial. The composite was promoted for its dimensional stability, flame retardancy function and its strength [105]. The barrier performance of the composites via impregnation was not reported. 1.8.8. HOT PRESSING
- 36. Hot pressing is used to develop nanocellulose – inorganics composite for barrier applications. Hot pressing is a final step applied after filtration. It is performed at a temperature of 105°C and pressure of 150 bar. Time consumption of pressing step is 3 to 4 hours including cooling and heating of the composites. The WVP of Trimethyl ammonium –modified nanofibrillated cellulose and layered silicate ( TMA-NFC/Mica R120) was 2200 g µm m-2 day - 1 KPa-1 at a condition of 23 ° C and 50% RH. The strength of this hot-pressed composite was 104±5 MPa with Young's modulus of 10.3±1.1 GPa [104]. The WVP of a TMA-NFC/Vermiculite composite was 1400 g µm m-2 day -1 kPa-1 , with a strength of 135±5 MPa and a modulus of 10.3±1.1 GPa [106]. 1.8.9. SPRAY COATING Spray coating is a bench scale technique for fabricating nanocellulose film. This method is used to develop free-standing films by spraying on to a solid surface either permeable [69, 107] or impermeable [108] . Spraying allows contour coating and the coating is performed contactless with the surface [109]. In addition to that, the coating process is not influenced by the surface’s topography [109]. Films produced by spraying are stronger than cast film [110]. Figure 14 shows the sequences involved in the spraying process. In spraying process, the mechanism consists of the formation of spray jet of the liquid and then atomization of the sprayed jet into the mist of the liquid. The atomization involves two mechanisms namely liquid lamella disintegration and disintegration of the liquid jet. The fine droplets formed from atomization coalesce on the contact surface and a film is formed on the base surface. Spraying of these biopolymers onto the solid surface causes coalescence of sprayed droplets due to the film-forming properties of the polymers via hydrogen bonding [111]. The film that is formed on the surface can be peeled from the substrates after drying. Figure 14 - Mechanism of spraying
- 37. Compared with laboratory and conventional methods for NC film making, spraying is a novel and fast method to create a coating layer either on cellulosic substrates or solid surfaces. Spraying of nanocellulose on a paper substrate has been performed to enhance its barrier and mechanical properties [112][107]. Similarly, spraying NC on nylon fabric was attempted to make free-standing nanopaper as barrier materials [69]. The spray coating of chitin and cellulose nanofibers onto poly lactic films has been performed to enhance barrier performance and mechanical properties [113] . In this thesis, the preparation of free-standing NC film via spraying is the focus. This method can be classified into two approaches - spraying on a permeable substrate or spraying on an impermeable substrate. 1.8.9.1. SPRAYING OF NC ON PERMEABLE SUBSTRATES Spraying can provide a good range of coat basis weight significantly higher than filtration. The spraying process is independent of the NC suspension concentration provided the suspension remains within the sprayable viscosity. Spraying of NC is a method that has been investigated for creating barrier layers on cellulosic fibrous substrates [107] by spray coating an homogeneous layer of NC on paper substrates [107]. Up to 2 wt. % NC was used for spraying, thus reducing the water to be removed in subsequent drying. 1.8.9.2. SPRAYING OF NC ON IMPERMEABLE SUBSTRATES Spraying of NC on smooth and polished solid surfaces has been investigated to produce free-standing NC film. Recently, spraying of micro-fibrillated cellulose (MFC) on a preheated metal surface was used to produce MFC film with 3D structures [108]. The basis weight and thickness of the film varied from 59 to 118 g/m2 and 46 to 68 µm respectively. The limitations in this method were cracks and wrinkles formed on the surface of the NC film. In this investigation, the amount of water was reduced but spraying with both 4.5 wt. % and 9 wt. % NC caused the formation of a disturbed spray jet due to the gel nature of the MFC. Consequently, the cracked film was produced [108]. The replication and evaluation of surface roughness of spray coated NC film from the base surface was not reported. In another investigation, ethyl cellulose was well dispersed in water with addition of plasticizer and the liquid was sprayed on Teflon plates. The spraying produced a uniform and reproducible cellulose film with tensile strength better than cast films [110]. The spraying of cationic chitin fibres and anionic cellulose nanofiber on poly lactic acid (PLA) films was investigated to promote barrier performance. The multilayer spray coating of these fibrous materials on PLA reduced O2 permeability but had a lower maze than when spray coated with only chitin nanofiber or cellulose nanofiber [113]. 1.8.9.3. LIMITATIONS IN SPRAYING OF NC ON PERMEABLE SURFACE Using spray coating [69], an NC suspension was sprayed on a permeable nylon fabric substrate having opening size of 20.4 µm, 14 % open area and 470 mesh size and then
- 38. vacuum filtration was used to remove the excess water from the spray coated NC layer on the nylon fabric. This spraying method is a time-consuming process as it took 10 to 20 minutes. The report detailed that the NC films were made with 5 pass homogenized NC suspension with fixed consistency of 2 wt. %. The surface roughness of the NC film and the effect of suspension consistency on the properties of the NC film were not reported. The thickness and basis weight of the NC film were varied from 10µm to 72 µm and 13.7 g/m2 to 124 g/m2 respectively by varying the speed of the conveyor in the experimental set up from 0.5 to 3 m/min. The constant MFC slurry flow from the nozzle of spray system was 0.75 kg/min. After spraying, the time taken for filtration varied from 15 sec to 90 sec. After dewatering, the wet MFC film was placed between new nylon fabrics and two blotting papers and two pass compressed with a 3 kg roll, and vacuum dried at 90° C. The time taken for drying a 55 g/m2 film and another thicker film were 10 and 25 mins respectively. The nylon fabric imprints on spray-coated nanopaper was reported, however these markings were not evaluated. The effect of the base surface on the properties of NC film was not investigated [69]. Figure 15 shows a typical operation of the spray coating process. Spraying process produced high thickness and basis weight of the NC film or barrier layers on the cellulosic substrates compared to vacuum filtration. Spraying homogenized micro-fibrillated cellulose onto a nylon fabric moving on a conveyor belt set at a speed of 0.5 m/min, yielded 124 g/m2 of the NC film. The spray coated films have a Young’s Modulus of 18 GPa and its air permeability is reported to be < 0.7 nm Pa-1 s-1 [69]. Spraying has not so far been used to make discrete NC film. 1.8.9.4. SPRAY COATING FOR COMPOSITES LAYER DEVELOPMENT Recently, the spraying of nanocellulose with inorganics onto permeable substrates such as paper and cardboard has been found to be a simple approach for developing composites barrier layers. This approach permits the coating of a larger surface area and reduces operation time [107][114].
- 39. MFC -SiO2 composites have been developed via spray coating with operation time on a base sheet of almost 310 cm2 of less than 30 minutes for spraying, independent of SiO2 concentration. SiO2 was added in the range 0 to 33 wt. % in the spraying suspension, tailoring the properties of the composite. The coating layer weight on the cellulosic substrates was controlled by varying the velocity of the conveyor in the experimental setup and the inorganics content in the NC suspension. These composites produced a material impermeable to air, confirming its good barrier performance [114]. Similarly, a composite made up of graphite and MFC as an electrode were developed via spraying [115]. Spraying of cellulose nanofibrils with nanoclay (MMT) on a Kraft paper and printing paper as base cellulosic substrates were developed and increased its barrier properties against oxygen and water vapour and tensile strength. The operation time for producing the composite barrier layers on the paper were 30 sec and 50 sec for clay suspension varying from 0 wt. % to 5 wt. %. At 3 wt. % and 5 wt. % clay composite layer with basis weight of 22 and 29 g/m2 the oxygen barrier permeance was 88600±2000 and 36400±1100 cm3 m-2 day respectively. The water vapour barrier performance of these composites was reported to be 14 and 8 gm-2 day. The time consumed for 19 and 29 µm thickness of barrier layer on paper Figure 15-Spray coating to prepare NC film. The image obtained from Beneventi et al, 2015 [69]
- 40. was less than 30 sec in the spraying process [58]. In this process, the water in the sprayed suspensions was removed by vacuum filtration. Peeling the layer as a film from the base surface was a challenge. Surface treatment of the substrates such as wetting, and corona discharge treatment was required to enhance the wettability between substrates and cellulose nanofibrils. NC concentration was varied between 1 wt. % and 2 wt. % NC but the effect of NC suspension concentration on the properties of composite layers was not investigated. Some surface non-uniformity of composite layers such as agglomeration was observed. Layered spray coating of cationic chitin fibres and anionic cellulose fibres on the PLA films has also been investigated. This method provided alternative multi-layer coating of these fibres on PLA film to tune its barrier properties and mechanical properties. The OP of composite layer with PLA film is 22 cm3 µm/m2 /day/kPa at 50% RH compared to that of the plain PLA film which was OP 70 cm3 µm/m2 /day/KPa at 50% RH. The WVTR of the composite layer with PLA film is reported to be 60 g water/m2 /day at 50 % RH, slightly lower than the WVTR of pure PLA film [113]. 1.8.9.5. SPRAY COATING FOR DEVELOPING FREE-STANDING NANOCOMPOSITE Recently, spraying is a rapid process to produce NC film and its operation time less than 30 mins [69]. Spraying is an effective method for producing composites and therefore implemented to make NC–MMT Composite. Generally, free standing NC films have been prepared using either vacuum filtration or casting. Both methods are time-consuming processes and required from 10 minutes to 3 days for the film to form and drying. Wrinking of the film during drying also places limitations on these processes. The operation time for nanocomposite via vacuum filtration required minimum of 24 hours to 4 days. Additionally, there are shortcomings in the separation of the film from the filter. The handling of the peeled wet film was difficult before being dried. This is also a conventional method which has limitations in tailoring the basis weight and thickness of the composites. 1.8.9.6. PROS AND CONS OF THE DIFFERENT METHODS In summary, casting methods are too slow and the quality of the film, because of wrinkling, is a major difficulty. In filtration methods, the time required is excessive, limiting the range of suspension concentrations permissible. In addition, peeling is a challenge and filter marking is a problem. While spraying appears a good option because it is quicker, studies on spraying methods for fabricating NC sheet have not explored sprayable limits of NC suspension. The effect of base surface on the surface properties of NC film has not so far been explored, and the effect of NC suspension and spraying parameters on the uniformity of NC films has not been investigated.
- 41. 1.8.9.7. IDEAL REQUIREMENT OF PREPARATION OF NC FILMS When compared to synthetic plastics films, the manufacturing rate of NC films is very slow [75]. Rapid manufacturing of NC films will facilitate their applications in packaging, as air and water filters, membranes and films for tissue engineering applications. Ideally, for commercial preparation of NC films methods must be able to • Increase film performance in water vapour and oxygen barrier properties via preparation method • Tailor the thickness and basis weight of the NC film via processing method without affecting operation time • Engineer the strength and bulk properties of NC film by varying the NC suspension concentration without affecting operation time in the method • Produce a film with an outstanding uniformity • Permit composites with the addition of nanoparticles not affecting the process variables in the method • Produce NC film in a single step to avoid further processing such as dewatering, vacuum drying, couching of the wet film • Produce a smooth nanocellulose film and engineer their roughness for developing potential material without any additional treatment in the methods 1.9. RECYCLABILITY OF NANOCELLULOSE FILMS Sustainable packaging requires recyclable packaging films and minimizing the carbon footprint and environmental pollution. Safety and sustainability are the important requirement in engineering food packaging materials [5]. The toxicity of cellulosic fibres is very low based on low genotoxicity against hepatocytes and aquatic species. Like cellulose macro fibres, NC is also biodegradable and does not enter or interfere into the food chain [5][116]. Nanocellulose has high surface area which contributes to high microbial degradation. Therefore, nanocellulose can be recyclable and used in sustainable barrier materials. Cellulose nanofibers are obtainable from virtually inexhaustible resources that are renewable and widely available around the world. Nanocellulose satisfies the sustainable packaging concepts defined by the Sustainable Packaging Alliance (SPA). Table 10 shows the sustainability potential of nanocellulose in packaging applications.
- 42. Table 10 - Sustainability features of Cellulosic nanofibers [119] Cellulose fibre substrates such as paper and paper board are recyclable, renewable, and biodegradable, but show poor barrier performance against oxygen and water vapour due to the presence of large pores in the fibrous network. To improve the barrier performance of cellulose fibres substrates, lamination and extrusion with synthetic plastic have been performed, but, recycling of these coated substrates has shown to be problematical [117]. In laminated and composite mixtures of many packaging materials, the layers are strongly bonded and the recycling of this material is an effort [75]. The NC coated LDPE was mechanically recycled into a second life barrier material. The Roll to Roll coating was used to coat NC on LDPE. During recycling, the LDPE and agglomerates from NC coatings were mixed together and formed a recycled film. The OTR of 1.4 g/m2 coat weight of pure NC on virgin LDPE film and recycled film were 37±4 cc/m2 /day and 924±47 cc/m2 /day, respectively. The WVTR of pure NC on virgin LDPE film and recycled film were 0.9±0.1 g/m2 /day and 2.0±0.2 g/m2 /day at a condition of 23°C and 50% RH,
- 43. respectively. The barrier properties of recycled film was decreased after recycling and the effect of NC agglomerates was insignificant [118]. This concluded the possibility of recycling of NC coating on the LDPE film and gave an advice on the focus of recycling of free-standing NC film and its effect on the barrier performance. 1.10. SURFACE ENGINEERING OF NANOCELLULOSE FILMS The roughness of nanocellulose film is one of the important criteria for developing functional material from nanocellulose. The surface roughness of the film normally controlled by the diameter distribution of nano fibril in the film and the processing method. The minimal surface roughness (high level of smoothness) is required for constructing flexible electronics. Because the conductive ink should spread on the surface of the substrate to avoid any discontinuity in the ink layers. In addition to that, spreading and penetration of the conductive ink on the substrates should be minimal on substrates for printed electronics [119]. Spreading and penetration of conductive ink on the surface of NC film depends mainly on their surface roughness. The method of NC film preparation is another approach to tailor its surface roughness. The smooth surface and texture of the base surface like plastic material can be replicated on the nanocellulose film [120][121] either with solvent casting or roll to roll methods. The average roughness of the NC film via roll to roll method is approximately 450 nm on the base surface side and approximately 800 nm on the free side of the film [65]. The surface roughness of NC film via vacuum filtration as a conventional method is 418 nm on free side and 331 nm on the filter side [122]. Sometimes, the NC film via filtration contains filter markings on the surface of the film affecting its roughness. The average roughness of plastic as the base surface is approximately 400nm. 1.11. PERSPECTIVE AND CONCLUSION Recently, nanocellulose films and their composites have potential applications in various fields. Predominantly, these sheets have been implemented in barrier applications, air filters, and anti-microbial sheets, substrates for electronics and light emitting diodes etc., and replacing synthetic plastics in practice. The methods for production of NC film in practice has time-consuming and limited in rate of production. The properties of NC films and their composites mostly depends on the preparation methods. The rate of manufacturing of NC films should be increased to be similar of plastic films. Through a critical review of the literature, this chapter provides a great need of rapid production of NC film and its composites. The rapid process should have a flexibility in the processing and the ability to tailor the properties of pure NC film and its composite for specific functionality. The barrier and mechanical performance of NC films from spraying as new and proposed
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