Industrial Crops and Products 95 (2017) 574–582

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Industrial Crops and Products

journal homepage: www.elsevier.com/locate/indcrop

Nanocellulose in packaging: Advances in barrier layer technologies

Ana Ferrer, Lokendra Pal, Martin Hubbe ∗
North Carolina State University, Dept. of Forest Biomaterials, Campus Box 8005, Raleigh, NC 27695-8005, USA

a r t i c l e i n f o a b s t r a c t

Article history: The review aims at reporting on recent developments in nanocellulose-based materials and their appli-
Received 30 October 2015 cations in packaging with special focus on oxygen and water vapor barrier characteristics. Nanocellulose
Received in revised form 9 October 2016 materials, including cellulose nanocrystals (CNC), nanofibrillated cellulose (NFC), and bacterial nanocel-
Accepted 6 November 2016
lulose (BNC), have unique properties with the potential to dramatically impact many commercial markets
Available online 15 November 2016
including packaging. In addition to being derived from a renewable resource that is both biodegradable
and non-toxic, nanocellulose exhibits extremely high surface area and crystallinity and has tunable sur-
Keywords:
face chemistry. These features give nanocellulose materials great potential to sustainably enhance oxygen
Cellulose nanocrystals
Nanofibrillated cellulose
and water vapor barrier properties when used as coating, fillers in composites and as self-standing thin
Microfibrillated cellulose films.
Packaging © 2016 Elsevier B.V. All rights reserved.
Barrier films

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574
2. Barrier properties within the nanocellulose film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575
3. Technologies for nanocellulose-based materials production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578
3.1. Layer-by-layer (LbL) assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578
3.2. Electrospinning (ES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579
3.3. Composite extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579
3.4. Casting from solution and evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579
3.5. Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579
4. Safety and biodegradability issues in nanocellulose applications within packaging field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579
4.1. Biodegradability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .580
4.2. Cytotoxicity and genotoxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580
4.3. Ecotoxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580

1. Introduction fossil-derived synthetic plastics, which gives rise to concerns from

both economical and environmental perspectives (Johansson et al.,
Nowadays, the use of packaging materials can be justified for 2012). It is true that the materials cited above show high strength
preventing the deterioration of food and drink, cosmetics, health- and barrier properties; however, they also have some drawbacks
care and other consumer goods due to physical, biochemical, such as unsustainability, fragility (for instance in the case of glass)
and microbiological factors. In addition, they should also pro- and sometimes they are really heavy, and that increases energy
vide sufficient barrier against oxygen, water vapor, grease, and costs for shipping (Bayer et al., 2011; Reis et al., 2011; Rodionova
microorganisms, among others (Nair et al., 2014). The packaging et al., 2011). The continued use of petroleum-based products will
industry currently uses materials based on glass, aluminum, tin, and mean eventually a decrease in availability and thus an increase in
the price of the raw materials. Additionally, because of their lack
of biodegradability, petroleum-based products can generate sig-
∗ Corresponding author. nificant waste disposal problems in some areas (Johansson et al.,
E-mail address: hubbe@ncsu.edu (M. Hubbe). 2012).

http://dx.doi.org/10.1016/j.indcrop.2016.11.012
0926-6690/© 2016 Elsevier B.V. All rights reserved.
 A. Ferrer et al. / Industrial Crops and Products 95 (2017) 574–582 575

Fig. 1. The structure of cellulose.

The global consumer packaging demand is currently in the range potential applications across several industrial sectors and allows
of US$400b-$500b, and it is worth highlighting that it is one of the development of innovative materials, as well as the enhance-
the fastest growing markets (Nair et al., 2014). In this sense, the ment of conventional materials properties. The nanoscale cellulose
development and search for new materials, products, and processes can be utilized, in fact, as filler, in composites manufacture, as coat-
have become a necessity and should be based on the principles of ing and as self-standing thin films, achieving very interesting and
sustainability, industrial ecology, eco-efficiency, and green chem- promising properties. These properties, together with fundamental
istry (Abdul Khalil et al., 2012). With the increased environmental aspects such as its renewable nature, non-food agricultural-based
concerns over sustainability and end-of-life disposal challenges, sources, biodegradability and/or biocompatibility, low cost and low
materials derived from renewable resources have been strongly energy consumption have attracted a lot of attention, which is moti-
advocated as potential replacements. In this sense, cellulose, whose vated also by the general interest towards a sustainable economy
structure is shown in Fig. 1, accounts for approximately 40% of lig- able to overcome today’s dependency on fossil sources (Li et al.,
nocellulosic biomass (Li et al., 2015). According to the literature, 2015).
this polymer is the most abundant renewable organic material in This paper aims to give an overview of the use of nanocellulose
the biosphere, and its annual production is estimated to be more for packaging purposes as well as to summarize the recent develop-
than 75 billion tons (French et al., 2004; Habibi, 2014). From all ments in various barrier films based on nanocellulose with special
this annual cellulose production, only around 2 billion per year is focus on oxygen and water vapor barrier properties.
used for industrial conversion and human consumption, including
all the possible uses (Keijsers et al., 2013). 2. Barrier properties within the nanocellulose film
A large amount of cellulose-based materials have been used
by the packaging industry for different purposes such as wrap- In order to understand the role of nanocellulose in the field
ping materials and containers, primary and secondary packages of packaging, it is worth describing the diffusion process of the
and flexible and rigid packaging (Lee et al., 2008). In fact, there are molecules through this material (nanocellulose). The diffusion of
countless benefits when cellulose is used for paper-based pack- molecules between two adjacent volumes separated by a thin film
aging, as for example, lightness in weight, low cost, and most of solid polymer or membrane happens basically in three steps
important, sustainability (Nair et al., 2010). Unfortunately, there (Nair et al., 2014). In the first step, the sample surface adsorbs the
are some drawbacks inherent in the use of regular paper prepared diffusing molecule. In a second step, the diffusing molecule will go
from lignocellulosic fibers. These include paper’s low barrier abil- through the film or membrane. Finally, in a third step, the diffusing
ity against water, water vapor, oxygen or oil, and these deficiencies molecule will be desorbed from the film or membrane surface on
need to be addressed in order to produce high-quality packages the other side. In many cases it is satisfactory to regard the adsorp-
that meet various specifications (Hyden, 1929; Pal et al., 2008). tion and desorption steps as being rapid relative to the diffusion
The packaging industry presently uses mainly unsustainable through the film. Thus, according to this three-step process, the
coatings of wax, plastics, or aluminum, and many other materi- gas permeability through a nanocellulose film will mainly depend
als in order to manufacture competitive packages based on paper. on the dissolution of gas and its rate of diffusion in the film. Thus,
In addition to all these unsustainable materials, the packaging one can express the permeability of gas molecules through various
industry also uses cellophane, which is the only cellulosic material types of material, including thin films, as follows (Eq. (1)):
currently used as a film for packaging, as it provides an effective gas
P =D×S (1)
barrier mainly in dry conditions. However, and despite the obvi-
ous benefits of employing a product based on photosynthetically where P is the permeability, D is the diffusion coefficient, and S is
renewable cellulose, the production of cellophane is harmful to the the solubility coefficient.
environment; the viscose route to cellophane production gener- The permeability coefficient P can be obtained from the appli-
ates harmful byproducts and uses sulphur-based chemicals (Hyden, cation of Henry’s law of solubility to Fick’s law of diffusion (Eq.
1929). (2)),
In light of the issues just considered, nanofibrillated cellulose
q×l
(NFC), cellulose nanocrystals (CNCs) and bacterial nanocellulose P =D×S = (2)
A × t × p
(BNC) have emerged as key components that should be seri-
ously considered for the production of cellulose-based materials where q is the amount of material passing through the film, l is the
for packaging purposes (Hoeger et al., 2013; Stelte and Sanadi, flow length or thickness, A is the cross-sectional area, t is time, and
2009). Nanocellulose can be extracted from various plant resources p is the pressure difference between the two sides of film.
through mechanical and chemical operations (Klemm et al., 2011). It is worth mentioning that the gas molecules should be first
A classification of nanocellulosic materials is given in Table 1. dissolved in the membrane or film before the diffusing step occurs.
Cellulosic nanomaterials have large specific surface areas and Although it is well known that the film surface influences the
also the ability to form hydrogen bonds. This hydrogen bonding permeating gas molecules, the rate of molecule diffusion in the
ability allows the material to create a strong, dense network, which membrane/film or, also called bulk flow, is the most important
makes them very hard for various molecules to pass through (Nair factor in the molecular migration (Lagaron et al., 2004). Also, in
et al., 2014). This property is excellent for barrier applications, and other words, bulk flow is commonly known as the movement of
this is what the packaging industry is looking for. Nanocellulose has molecules from an area of high concentration to an area of low
concentration.
576 A. Ferrer et al. / Industrial Crops and Products 95 (2017) 574–582

Table 1
The family of nanocellulose materials (Data extracted from (Klemm et al., 2011)).

Type of nanocellulose Synonyms Typical sources Formation and average size

Nano- or Microfibrillated cellulose, Wood, sugar beet, potato tuber, Delamination of wood pulp by
microfibrillated nanofibrils and microfibrils, hemp, flax mechanical pressure before and/or
cellulose (NFC/MFC) nanofibrillated cellulose after chemical or enzymatic treatment
Diameter: 5–60 nm
Length: several micrometers
Nanocrystalline Cellulose nanocrystals, Wood, cotton, hemp, flax, Acid hydrolysis of cellulose from many
cellulose (NCC) crystallites, whiskers, rodlike wheat straw, mulberry bark, sources
cellulose, microcrystals ramie, Avicel, tunicin, cellulose Diameter: 5–70 nm
from algae and bacteria Length: 100–250 nm (from plant
celluloses); 100 nm to several
micrometers (from celluloses of
tunicates, algae, bacteria)
Bacterial nanocellulose Bacterial cellulose, microbial Low-molecular weight sugars Bacterial synthesis
(BNC) cellulose, biocellulose and alcohols Diameter: 20–100 nm
Different types of nanofiber networks

The fact that nanocellulose has good barrier properties, espe- present a high tortuosity for the diffusion of gases. The concept
cially in terms of oxygen, can be due to the dense network structure of tortuosity is represented in Fig. 2.
that is formed by nanofibrils. In the case of nanocrystalline cellu- NFC films generally show better results in terms of barrier
lose, the density can be attributed to the small and uniform particle properties (more specifically, less oxygen permeability) compared
sizes. In the case of nanofibrillated cellulose, the densification is to CNC films, even though the latter have higher crystallinity. It
enhanced by the inherent flexibility of the wet material. According is also true that the solubility is very similar for both NFC and
to these explanations, the pores that might exist within the films CNCs. However, in the case of the NFC films, the oxygen molecules
serve as the major path for permeating oxygen molecules (Nair can penetrate more slowly and with more difficulties. This fact
et al., 2014). NFC, due to its inherent flexibility, would be able to can be explained by the structural organization within the films.
form a denser film, sealing up most of the spaces between the fibrils More and higher entanglements can be observed in the case of the
down to a molecular scale. By contrast, CNC particles are inher- NFC films, leading to a high density and a tortuous diffusion path
ently rigid, so it is reasonable to expect there to be a lower density (Belbekhouche et al., 2011).
of packing and a higher porosity within a film composed only of Nanocellulose may play a different role with respect to gas
CNC. The concept of combining CNC (for high crystallinity) with NFC permeability when it is used as a reinforcement in a polymer
(for high conformability, favoring high film density) merits research matrix. The extent to which tortuosity can account for reductions
attention. in permeability through nanocellulose-containing polymer films
The definition of the term “pores” is worthy of receiving more is a topic that merits further research attention. In such compos-
attention in future research. At one extreme, the term can refer to ite structures, the content of nanocellulose within the polymer
an unobstructed passage, or network of passages, through which matrix is often rather low, often in the range of 0.5–20%. For exam-
gas or liquid molecules can pass from one side of the film to the ple, Follain et al. (2013) reinforced a poly-caprolactone film with
other. The sizes of such pores can be specified according to typical 3–12% of isocyanate-grafted CNC, resulting in increased resistance
size by using the IUPAC definitions of macropores, mesopores, and to the diffusion of water. This increased resistance was attributed
micropores. In addition, future modeling studies need to take into to tortuosity. However, other factors that might account from such
account the contributions to overall transport that can be attributed observations include any tendency for the nanocellulose to improve
to the diffusion of O2 , H2 O, or other molecules of interest through film uniformity or to make film tough enough to decrease the fre-
the amorphous polymeric matrix phases. If one compares cellulose quency of cracks or other defects. Due to their fibrillar or columnar
fibers to nanofibrils, one can conclude that the nanofibrils might shapes, nanocelluloses would not be expected to provide effective
form more complex and smaller pores compared to their counter- blockage of diffusion paths when present at relatively low levels in
parts (cellulose fibers) due to significantly higher surface area and a polymer matrix.
high aspect ratio. This kind of network that the nanofibrils can orig- In summary, one can conclude that the barrier properties for
inate basically decreases the permeability by increasing the density oxygen are high for nanocellulose. However, the same trend can-
within the film (Syverud and Stenius, 2009). not be observed for the water vapor barrier properties, as they
It is well known that both nanofibrillated cellulose and bacte- tend to be low for unmodified nanocellulose material (Nair et al.,
rial nanocellulose contain a high proportion of crystalline regions in 2014). One can attribute this to the high affinity that exists between
combination with disordered regions. Crystallinity values for vari- water and the nanocellulose fibrils. Nevertheless, if one compares
ous types of nanocellulose have been reported ranging from 40 to nanocellulose fibrils and cellulose fibers in terms of water vapor
90%, showing cellulose nanocrystals (CNCs) as having the highest barrier properties, there is a strong reducing effect on this barrier
crystallinity (higher than the nanofibrillated cellulose, NFC) due to property in the case of the nanocellulose, and this can be mainly
the strong acid used for their production, which hydrolyzes disor- attributed to its size and swelling constraints formed due to rigid
dered cellulose (Guo and Catchmark, 2012; Nair et al., 2014). The network within the films. However, when the relative humidity is
crystalline regions are essentially impermeable to gas molecules high, the typical structure of the nanocellulose fibrils within the
(Lagaron et al., 2004; Saxena et al., 2010). However, because of the film can be ruined because of the high swelling, and then the bar-
impermeable nature of cellulose crystalline regions, the length of rier properties for both oxygen and water vapor can be lost (Spence
the effective flow path of the air, water vapor or gas molecules et al., 2010).
through a nanocellulose film can be significantly greater than the The literature shows different examples for the nanofibrillated
measured film thickness (Lagaron et al., 2004; Saxena et al., 2010). cellulose (NFC) and nanocrystals (CNCs) regarding their applica-
In other words, a dense, nonporous film of nanocellulose may tion in barrier layers. According to the data that can be found
in the literature, NFC could be considered as a strong gas barrier
 A. Ferrer et al. / Industrial Crops and Products 95 (2017) 574–582 577

Fig. 2. Schematic representation of increased diffusion path within the nanocellulose films. Figure is replotted from (Nair et al., 2014).

Table 2
Oxygen permeability of nanocellulose film compared to those made from commercially available petroleum based materials and other polymers (Data extracted from (Nair
et al., 2014)).

Material Oxygen permeability Conditions References

(cc ␮m/m2 day kPa)

NFC 0.6 65% RH Österberg et al., 2013

23 ◦ C
NFC (carboxymethylated) 0.0006 0% RH Aulin et al., 2010
23 ◦ C
NFC (carboxymethylated) 0.85 50% RH Aulin et al., 2010
23 ◦ C
Cellophane 0.41 0% RH Wu and Yuan, 2002
25–80 ◦ C
Polyethylene (PE) 50–200 50% RH Lange and Wyser, 2003
23 ◦ C
Polyvinylidene chloride (PVdC) 0.1–3 50% RH Lange and Wyser, 2003
23 ◦ C
Polyvinyl alcohol (PVOH) 0.20 0% RH Lange and Wyser, 2003
23 ◦ C
Ethylene vinyl alcohol (EVOH) 0.01–0.1 0% RH Lange and Wyser, 2003
23 ◦ C

material. As a matter of fact, films made of NFC with a thickness not observed for the water vapor barrier properties. Table 3 shows
around 21 ␮m show very high air and oxygen barrier proper- the water vapor transmission rate (WVTR) of NFC films and com-
ties, as for example, the oxygen transmission rates (OTR) were mercially available films from petroleum based materials and other
around 17 ± 1 ml m−2 day−1 . If one compares these values with polymers. The strong hydrophilic nature of nanocellulose could be
those shown by synthetic polymers such as ethylene vinyl alco- the explanation for this difference in performance (Nair et al., 2014).
hol (3–5 ml m−2 day−1 ) or polyvinylidene chloride coated polyester However, this is not the only reason that we can find in order to
films (9–15 ml m−2 day−1 ) of approximately same thickness, one explain these results. Also, the compactness of the film will sig-
can conclude that the values shown by the NFC are very competi- nificantly affect the WVTR as less amount of water can penetrate
tive (Syverud and Stenius, 2009). In addition, there are reports, such through the film, which is the case of the films made of NFC.
as that of Rodionova et al. (2011), which have demonstrated that Lignin is a major component of the most widely available cellu-
the barrier properties can be tunable for the NFC. As a matter of losic resources, and it is even possible to skip the delignification step
fact, Rodionova and co-workers showed that NFC films (both pure when preparing nanofibrillated cellulose by extensive mechanical
and partially acetylated) can be utilized for modified atmosphere shearing (Spence et al., 2010). Lignin is also a factor to be consid-
packaging (Rodionova et al., 2011). Table 2 reviews the oxygen per- ered as it can create films with surfaces more hydrophobic, which
meability of NFC films compared with those from commercially means higher water contact angles but less water vapor barrier
available petroleum-based materials and other polymers. properties. In addition to this, when lignin is present, the compact-
In summary, NFC films are competitive with commercial films ness of the film is clearly reduced compared to films made of NRC.
made of synthetic polymers in terms of oxygen and air barrier This observation can be explained by the fact that lignin might hin-
properties especially in dry conditions; however, this trend was
578 A. Ferrer et al. / Industrial Crops and Products 95 (2017) 574–582

Table 3
Water vapor transmission rates of NFC films compared to commercially available petroleum based materials and other polymers (Data extracted from (Nair et al., 2014)).

Material Water vapor Average thickness of Conditions References

transmission rate the film (␮m)
(WVTR) (g/m2 day)

NFC 234 42 50% RH Rodionova et al., 2011

23 ◦ C
NFC (acetylated for 0.5 h) 167 46 50% RH Rodionova et al., 2011
23 ◦ C
Cellophane 350 27 85% RH Joshi et al., 1997
ambient
Polyvinylidene chloride (PVdC) 3.07 12.7 100% RH Steven and Hotchkiss, 2002
27 ◦ C
Polyethylene (PE) 16.8 18.3 100% RH Steven and Hotchkiss, 2002
27 ◦ C
Plasticized polyvinyl chloride (PVC) 118.56 12.7 100% RH Steven and Hotchkiss, 2002
27 ◦ C

der the hydrogen bonding creating more hydrophobic pores which to act as a water vapor seal on the surfaces of sausages, thus extend-
facilitate the water vapor transmission (Spence et al., 2010). ing their shelf life. Because of such promising results, in addition to
In spite of the poor water vapor barrier properties that the NFC the “generally regarded as safe” status of some BNC materials (FDA,
films show, there are some authors that have been working on 2006), there is strong motivation to consider BNC for food-contact
the potential improvement of these specific barrier properties. For packaging applications.
instance, Sharma and co-workers reduced by 50% the water vapor In recent literature, one can find different processes theo-
permeability of NFC films by heating them at 175 ◦ C for 3 h com- retically applicable for the production of nanocellulose-based
pared to untreated NFC films. This achievement could be explained materials; however in this review, we will comment very briefly
by the increase in hydrophobicity, as evidenced by the increase only on those that appear as most promising for manufacturing
in water contact angle and reduced porosity by heat treatment packaging materials based on nanocellulose, namely, layer-by-
(Sharma et al., 2014). Further, researchers have shown improve- layer assembly, electrospinning, composite extrusion, casting
ment in water vapor and gas barrier properties by developing evaporation, and coating.
nanocellulose composite films (Isogai, 2013).
On the other hand, very few studies have been carried out for 3.1. Layer-by-layer (LbL) assembly
CNC films. Belbekhouche and co-workers studied and compared
the gas barrier properties between NFC and CNC films. They found Layer by layer (LbL) assembly is a generic technique for
out that the oxygen permeability was lower for the NFC films com- thin film coating of functional materials onto surfaces (Decher,
pared to the CNC films, and this can be attributed to the higher fibril 1997). This technique basically allows the creation of multicom-
entanglements and higher film density that happen within the NFC ponent films on solid supports by controlling adsorption from
films (Belbekhouche et al., 2011). solutions or dispersions (Decher, 1997). In this way it possible
to preserve the properties that are very important for packag-
3. Technologies for nanocellulose-based materials ing applications, as for example gas barrier (Aulin et al., 2013;
production Marais et al., 2014.) and wet-strength (Ankerfors et al., 2016).
To achieve the effects cited above, the layers of nanocellulose
The diameter of the cellulose nanofibrils is in the range of were alternated with poly-(etheneimine) (Aulin et al., 2013),
2–50 nm, and the lengths are up to several micrometers, depending cationic starch or polyamido-amine wet-strength resin (Ankerfors
on the raw material and process used for their extraction (Hoeger et al., 2016). Other researchers have prepared layer-by-layer films
et al., 2013; Stelte and Sanadi, 2009; Hubbe et al., 2008). As it of nanocellulose alternating with chitosan (de Mesquita et al.,
has been mentioned earlier, this material has extraordinary opti- 2010; Li et al., 2013) or various cationic polyelectrolytes such as
cal and mechanical properties, which allows the generation of a poly-(ethyleneimine), poly(allylamine hydrochloride), and poly-
huge variety of high-performance materials (Abraham et al., 2011; (diallyldimethylammonium chloride) (Wågberg et al., 2008; Marais
He et al., 2012; Kaushik and Singh, 2011). For the conversion of et al., 2014). In principle the self-assembly of nanomaterials, in
cellulosic fibers into nanofibrils, it is required to use intensive response to electrostatic attraction, can be envisioned as a mecha-
mechanical treatments (Uetani and Yano, 2011); however, a chem- nism potentially leading to a dense, ultra-thin nanocellulose layer
ical pretreatment such as enzymatic treatment (Henriksson et al., during the alternating steps of the layer-by-layer building process.
2007) may be applied to reduce the energy demand, depending This technique can also create multiple-layer structures of very
on the raw materials and process. In the literature, one can find thin alternating films based primarily on electrostatic interactions
several methods used for the production of cellulose nanofibrils, and hydrogen bonds between a polyanion and a polycation (Jean
such as refining and high-pressure homogenizers (Chinga-Carrasco et al., 2009). This approach shows different advantages as, for exam-
and Syverud, 2012; Karande et al., 2012), microfluidizers (Ferrer ple, simplicity and versatility, thickness control at the nanoscale
et al., 2012), and grinders (Wang et al., 2012). By contrast, sulfuric and the potential of coating easily on tridimensional objects, such
acid or hydrochloric acid are needed in order to prepare cellulose as small bottles, cups and trays.
nanocrystals (Saxena et al., 2010). In view of the multiple steps required to implement the types
Bacterial nanocellulose, which is lignin-free and does not of layer-by-layer structures in the above-cited articles, there also
require chemical treatments or intensive milling to achieve nano- has been strong interest in self-assembled structures. For example,
scale dimensions, so far has received much less research attention Liu and Berglund (2012) prepared nanocomposites with mont-
as a way to prepare barrier films for packaging (Dobre et al., 2012; morillonite clay nanoplatelets and nanofibrillated cellulose. This
Padrao et al., 2016). Padrao et al. showed that a film of BNC was able combination, facilitated by the co-addition of chitosan, yielded a
 A. Ferrer et al. / Industrial Crops and Products 95 (2017) 574–582 579

promising combination of high strength, toughness, fire-resistance, 3.5. Coating

and resistance to oxygen permeation. Layering involving the
organic and inorganic nano-components was evident in the com- This is a technique that is very commonly used in order
posite. to improve many properties of packaging materials (Farris and
Piergiovanni, 2012; Pal et al., 2008). When coating is used in the
packaging industry, the main aim that needs to be pursued is
3.2. Electrospinning (ES) including thin layers, which may be either external or sandwiched
between two substrates (Li et al., 2015). These layers normally show
Electrospinning (ES) is a broadly used technology for electro- thicknesses that range from tenths nanometers to a few microm-
static fiber formation that utilizes electrical forces to produce eters. The excellent dispersibility of nanocellulose in water makes
polymer fibers with diameters ranging from 2 nm to several it very attractive as an aqueous coating that can be applied as a
micrometers using polymer solutions of both natural and syn- pure nanocellulose thin layer or as a composite with other tradi-
thetic polymers (Bhardwaj and Kundu, 2010). The viscous flow is tional coating materials. In addition to this, currently there is a trend
responsible for the alignment of the individual polymer chains. to try to reduce the thickness of oil-based plastic films by using a
In the literature there are a lot of works showing that ES can be thin layer of functional and high performing bio-based material. In
used to combine different polymers with nanocellulose, overcom- this sense, nanocellulose appears to be an ideal material due to its
ing problems of compatibility and preserving the most interesting high crystallinity, among other properties. In fact, there are some
properties of the biopolymer. More specifically, studies have been reports in the literature that have successfully tested the use of
focused on the production of nanocellulose composites with both nanocellulose for coating. Among all the examples that can be found
bio-based and synthetic polymers. In addition to this, the thermal in the literature, Aulin and co-workers coated carboxymethylated
and mechanical properties need to be enhanced, and basically this microfibrillar cellulose on papers, and they reported a low oxygen
could be achieved by the change in crystallinity and the alignment permeability of the films at low relative humidity values (Aulin
of nanocellulose along the fiber length (Park et al., 2007). As an et al., 2010). The values were comparable to those shown by con-
example, cellulose microfibrils were introduced into poly(ethylene ventional synthetic films such as ethylene vinyl alcohol. However,
oxide) fibers by electrospinning (Fortunato et al., 2012), and in the same trend was not observed at high relative humidity values,
2010, Peresin and co-workers electrospun poly(vinylalcohol) with as the microfibers were swelled and plasticized (Aulin et al., 2010).
cellulose nanocrystals, obtaining reinforced nanofibers (Peresin
et al., 2010).

4. Safety and biodegradability issues in nanocellulose

applications within packaging field
3.3. Composite extrusion
Nanotechnology implies research and development of particle
The extrusion process transform materials from solid to liquid
sizes in the 1–100 nm range. Nano-materials show novel properties
and back again with the desired thickness. This approach seems
that have different applications, owing to their composition, small
to be more versatile and simpler than LbL or ES; however poor
size and shape. Although clear benefits are expected from the on-
compatibility of nanocellulose with synthetic polymers can cause
going surge of nanotechnology products, special attention must be
process issues. The main target of this technique is to incorpo-
paid to the potential hazards that nanoparticles can have on human
rate nanocellulose (acting as a functional nanofiller) in different
and environmental health (Blaise et al., 2008).
polymeric matrices and taking advantage of all their extraordinary
Safety, biodegradability, and sustainability are issues that need
properties as, for example, high aspect ratio, specific surface area
to be addressed in all types of packaging materials, and these issues
and renewability, among others (Fortunati et al., 2014; Li et al.,
are of particular concern in food packaging (Li et al., 2015). There-
2015). Since 1990, one can find many different works related to this
fore, factors such as biodegradability, cytotoxicity, genotoxicity and
topic that explain in detail how the nanocellulose can reinforce the
ecotoxicity of nanocellulose have recently been explored.
polymeric matrix. Miao and Hamad (2013) gave an overview about
Some challenges in the development of packaging materials can
the cellulosic materials used so far and their reinforcement effects
be attributed to complexity. For instance, if the product contains
(Miao and Hamad, 2013).
cellulose, should it be nanocrystalline, highly fibrillated, bacte-
rial, or macroscopic fibers, etc.? Nanocrystalline cellulose might
be preferred in certain (but not all) extrusion applications due
3.4. Casting from solution and evaporation to the inherent uniformity and very small sizes of the individ-
ual particles. In principle, the removal of non-crystalline material
This technique is based on the use of moderate temperature and during preparation of CNC can be expected to yield material that
a solvent in order to control the nanocellulose concentration (Aulin is inherently more resistant to degradation by heat or enzymes.
et al., 2010; Khan et al., 2012; Bhardwaj et al., 2014; Lu et al., 2014; By contrast, highly fibrillated celluloses might be preferred in
Li et al., 2015). Good dispersibility of the components needs to be cases where the cellulosic component needs to provide structure
achieved, and this is basically the main drawback that can be found to a barrier layer (Spence et al., 2010; Johansson et al., 2012).
(Li et al., 2015). Rodionova et al. (2012), Pereda et al. (2014) and The cellulosic component can be further differentiated by surface
Rafieian and Simonsen (2014) provide examples where researchers chemical modifications. For instance, a lot of emphasis has been
were able to disperse cellulose nanocrystals from an aqueous solu- placed on derivatization of cellulosic surfaces to improve their
tion containing inherently water-loving matrix materials such as adhesion to hydrophobic matrix materials in composites (Habibi,
chitosan. Even though casting from solution is not a technique com- 2014). Furthermore, one can choose between many different pro-
monly used in the packaging industry, there are some works in the cessing schemes, such as melt extrusion, melt compounding,
literature that report how the nanocellulose has successfully been electrospinning, layer-by-layer deposition, solution casting, and
casted. In this sense, Svagan and co-workers casted nanocellulose aqueous-based coating of suspensions. In each case, although the
combined with a 50/50 amylopectin-glycerol blend (Svagan et al., raw materials may be regarded as safe and biodegradable on
2007, 2009). their own, it is important to ask whether the terms “safe” and
580 A. Ferrer et al. / Industrial Crops and Products 95 (2017) 574–582

“biodegradable” still can be used when describing the manufac- (Vartiainen et al., 2011). In their investigations, they considered
tured composite product. the worker exposures to particles in air during grinding and spray-
drying of birch cellulose. In both cases, there was not a significant
4.1. Biodegradability exposure to particles during operation. In the case of grinding, the
particles were removed because of the use fume hoods, and the
Nanocellulose and its biodegradability have been studied, as it spray dryer is not supposed to leak particles unless it is not cor-
has been used as a reinforcement agent for packaging materials, rectly operated (Vartiainen et al., 2011). The health effects of the
especially when nanoparticles were used as biodegradable rein- nanocellulose were also considered. For this purpose, the viability
forcement for other polymers (Coma et al., 1994). According to and cytokine profile of mouse macrophages and human monocyte
what can be found in the literature, the biodegradability is not lost derived macrophages were evaluated after 6 and 24 h of expo-
because of nanometric dimensions (Kummerer et al., 2011). sure (Vartiainen et al., 2011). According to their findings, there
As a matter of fact, Kummerer and co-workers reported that were not inflammatory effects or cytotoxicity on mouse and human
nanoparticles from cellulose and starch were even more rapidly macrophages (Vartiainen et al., 2011). Apart from this, there are
biodegradable than their macroscopic counterparts; such observa- studies in the literature that report about the toxicity of modified
tions contrasted with tests in which the nano-reinforcements were nanocellulose (Male et al., 2012). For example, the cited study used
non-cellulosic (Kummerer et al., 2011). oxidized/carboxylic nanocellulose from different biomass sources,
Though a great deal of packaging material ends up in landfills or and these materials were tested for their possible cytotoxicity
as roadside litter, one can expect more and more of it, in the future, by electric cell substrate impedance sensing, using Spodoptera
to be recycled into useful products. In this regard, increasing moti- frugiperda insect cells (Sf9) and Cricetulus griseus (Chinese ham-
vation can be anticipated for the preparation of packaging materials ster) lung fibroblast cells. The cells were exposed to nanocellulose,
comprised of readily repulpable materials. Most of the nanocellu- and their spreading and viability were monitored and quantified
lose products discussed in this article can be expected to be highly by electric cell substrate impedance sensing. On the basis of the
compatible with their later usage as a component in recycled paper. 50% inhibition concentration, none of the nanocellulosic materials
produced had any significant cytotoxic effects (Male et al., 2012).
4.2. Cytotoxicity and genotoxicity
4.3. Ecotoxicity
Nanomaterials show properties that are not usual for bulk mate-
rials, and this is why studies in the last years have been focused This factor is also important to consider, especially when cel-
on the possible potential health hazards of nanocellulose (Li et al., lulose nanoparticles are used for packaging purposes. As a matter
2015). Pure cellulose is generally well known as a safe food sub- of fact, there is a research group in Canada that has been studying
stance, according to the Food and Drug Administration, and longer the potential environmental risks of cellulose nanoparticles and
cellulose fibers can provide structure and texture to baked products carboxymethyl cellulose since 2010 (Kovacs et al., 2010). In their
(Li et al., 2015). Although nanoparticles present several positive research, they used rainbow trout hepatocytes and nine aquatic
sensory effects, such as smooth consistency, mouth feel and sticki- species and they could show that the hepatocytes were most sen-
ness, further studies are required in order to evaluate their potential sitive to the cellulose nanoparticles, although neither cellulose
health hazards. In this sense, there are some works that can be nanoparticles nor carboxyl methyl cellulose generated genotoxi-
found in the literature, as for example, the work developed by Ni city (Kovacs et al., 2010). According to their findings, the toxicity
and co-workers who found that cellulose nanocrystals obtained by potential and environmental risk of nanocellulose are very low.
acid-hydrolysis can be used as nanobiomaterials because of their
low cytotoxicity to L929 cells (cells extensively used for cytotoxi- 5. Conclusions
city testing), and to MTT assay (common test for cell viability) (Ni
et al., 2012). In another study, the potential genotoxicity of bacte- As is evident from the studies cited in this article, interest
rial cellulose was also tested (Moreira et al., 2009). In this work, in nanocellulose-based applications in packaging is accelerating
they used in vitro analysis and also techniques that were tested rapidly but still early in the product life cycle. A variety of promis-
and successfully effective on fibrous nanoparticles, such as comet ing strategies have been demonstrated or are in different stages
assay (single-cell gel electrophoresis) and Ames test (Salmonella of implementation. Oxygen barrier properties represent a pri-
reversion assays to evaluate the mutagenic potential of chemical mary domain of strong potential for nanocellulose-based packaging
compounds). According to their findings, bacterial cellulose is not films. On the other hand, a key need for continuing research
genotoxic under the conditions tested (Moreira et al., 2009). concerns inadequate resistance to water vapor and to moisture.
Hannukainen et al. (2012) used two differently prepared Also, the contrasts between various nanocrystalline cellulose prod-
nanofibrillar celluloses in order to evaluate their potential geno- ucts, nanofibrillated cellulose products, and bacterial nanocellulose
toxicity in human bronchial epithelial BEAS 2 B cells. Nanofibrillar materials implies that there are a great many combinations to con-
cellulose was obtained by grinding after an enzymatic pre- sider. Nanocellulose materials also can be modified by means of
treatment and by microfluidization after a TEMPO-mediated surface chemical treatments (Habibi, 2014). The various nanocel-
oxidation pre-treatment. According to the results, there was an luloses can be incorporated into packaging products and barrier
induced DNA damage in the target cells when the nanocellulose layers by diverse means, with particular promise having been
produced by grinding was used, but no induction of oxidative DNA shown for solution casting, melt-extrusion, and electrospinning
damage was seen. However, when the nanocellulose obtained by technologies, among others. Such challenges and options imply that
microfluidization was used, there was a DNA damage in BEAS there will be a strong need for continuing research in the coming
2B cells and a sporadic indication of oxidative DNA damage at years to attempt to achieve the full potential of nanocellulose in a
5 ␮g/cm3 (Hannukainen et al., 2012). Nevertheless, it is worth wide range of packaging applications.
mentioning that this method has not been widely applied to stud-
ies with fibrous nanomaterials; thus, the mechanism behind the
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