PAPER CHEMISTRY
Approaching super-hydrophobicity from cellulosic
materials: A Review
Junlong Song and Orlando J. Rojas
KEYWODS: Super-hydrophobic surfaces, Cellulose,
Roughness, Surface energy
SUMMARY: The development of super-hydrophobic
surfaces has been inspired by natural systems and uses a
number of novel (nano) technologies. Superhydrophobicity has stimulated much scientific and industrial
interest because of applications in water repellency, selfcleaning, friction reduction, antifouling, etc. Despite its
inherent hydrophilicity, cellulose has unparalleled
advantages as a substrate for the production of superhydrophobic materials. This is because of its abundance,
biodegradability and unique physical, chemical and
mechanical properties that compare advantageously with
the non-renewable materials typically used. This review
includes a comprehensive survey of the progress achieved
so far in the production of super-hydrophobic materials
based on cellulose. Emphasis is placed on the effects of
cellulose surface chemistry and topography, both of
which affect hydrophobicity. Overall, this contribution
summarizes some of the aspects that are critical to
advance this evolving field of science.
ADDRESSES OF THE AUTHORS:
Junlong Song: Jiangsu Provincial Key Lab of Pulp and
Paper Science and Technology, Nanjing Forestry
University, Nanjing, Jiangsu, P.R. China 210037;
Orlando J. Rojas: Departments of Forest Biomaterials
and Chemical and Biomolecular Engineering, North
Carolina State University, Box 8005, Raleigh, NC 276958005 USA; Department of Forest Products Technology,
Faculty of Chemistry and Materials Sciences, Aalto
University, P.O. Box 16300, Aalto FIN-00076 Finland.
Wettability is a fundamental property of solid surfaces
that plays important roles in household, industrial and
advanced applications. Super-hydrophobic surfaces, i.e.,
surfaces with a water contact angle (WCA) larger than
150° and a sliding angle (SA) less than 10°, have drawn
great scientific and industrial interest due to their
relevance in water repellency, self-cleaning, friction
reduction and antifouling (Genzer, Efimenko 2006; Ma,
Hill 2006; Ma et al. 2005). The interest in self-cleaning,
a unique property of super-hydrophobic surfaces, has
been driven by the need to fabricate surfaces such as
satellite dishes, solar energy panel, photovoltaics, exterior
architectural glass, automobile windshields, swimming
coatings, and surfaces with special heat and mass transfer
properties (Cheng et al. 2006; Ma, Hill 2006; Nakajima et
al. 2001). The reduction of friction is of major importance
in many material applications (Choi et al. 2006;
Schneider et al. 2008). Contribution to corrosion
prevention is also recognized as an attribute of superhydrophobic surfaces, which are effective to prevent film
infiltration and to limit surface exposure to corrosive
media (Barkhudarov et al. 2008; Wang et al. 2011b; Yuan
et al. 2011; Zhang et al. 2011). Super-hydrophobic
surfaces have been applied to prevent bacterial adhesion
(Crick et al. 2011); to manufacture intelligent
microfluidic devices (Chunder et al. 2009); to improve
blood compatibility and anticoagulation (Hou et al. 2010;
Leibner et al. 2009), and to reduce ice growth in humid
atmospheres (Farhadi et al. 2011; Yang et al. 2011; Yin et
al. 2010).
Generally, both the surface chemistry and the surface
roughness affect hydrophobicity (Genzer, Efimenko
2006; Nakajima et al. 2001) and the interplay between
these properties have been the subject of active research
during the last decades. Techniques to endow the surface
with nano-scale roughness, such as etching and
lithography, sol-gel processing and electrospinning have
been implemented while the surface chemistry has been
modified by using strategies involving physical and
chemical adsorption. Important progress achieved in this
field has been reported in several excellent review papers
(Dorrer, Rühe 2008; Genzer, Efimenko 2006; Ma, Hill
2006; Nakajima et al. 2001).
A distinctive feature is the fact that substrates based on
non-renewable materials, including minerals and
synthetic polymers, are being substituted as platforms to
develop super-hydrophobic surfaces. This is motivated in
part by the recent impetus of sustainability and
environmental conscience. Therefore, efforts to induce
hydrophobicity in bio-based materials, including
lignocellulose, are growing steadily and are being
reported more often. Here lignocellulose is referred to as
materials derived from plant fibers that contain cellulose,
lignin, and heteropolysaccharides. This includes wood,
non-wood plants, agricultural residues, grasses, etc.
(Rowell 2002). Compared with nonrenewable materials,
lignocellulose has distinctive advantages because they are
readily available, light-weighted, abundant and
renewable.
The know-how available to attain low and ultra-low
energy surfaces from mineral and polymeric materials has
undergone several major advances. However, the case of
lignocelluloses has proved to be more challenging; this is
in part because of the influence of hydrophilic
components that are present in complex and highly
hierarchical structures. Applications such as printing,
packaging, food storage and others that demand certain
barrier properties are at the center of interest.
The partial water resistance required in writing,
printing, and packaging materials, has been achieved by
surface and bulk modifications. For example, animal glue
and so-called “surface” and “internal” sizing use wood
rosin, alkenylsuccinic anhydride (ASA) and alkylketene
dimers (AKD) (Hubbe 2006), which are common
additives in papermaking. Nevertheless, surface and
internal sizing fall short to meet the new demands of
super-hydrophobic materials. Furthermore, while
processes to modify paper surfaces are widely known,
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 216
PAPER CHEMISTRY
production of super-hydrophobic materials based on
cellulose fibers are less developed.
Many developments to obtain super-hydrophobic
surfaces based on cellulose have been reported in recent
years, for example in cotton fabrics and paperboard. In
order to discuss these cases and also the broader aspects
of super-hydrophobic cellulose, this review begins with a
brief discussion of the fundamentals of superhydrophobicity and then follows with a report of the
latest advances related to chemical grafting, sol-gel
process, nanoparticles modification and pulsed laser and
plasma deposition, among others. Even though, superhydrophobic surface requires water contact angles greater
than 150°combined with a low roll-off angle based on the
definition, some developments to enhance the wettability
close to that criterion are also addressed.
γLV
γSV
(a)
θ
Liquid
γSL
Solid
(b)
Liquid
Solid
(c)
Liquid
Wettability and super-hydrophobicity
Solid
Definitions
Super-hydrophobic surfaces are abundant in nature, as
exemplified by the wings of butterflies (Chen et al. 2004;
Genzer, Efimenko 2006), the feet of water striders (Gao,
Jiang 2004), and the leaves of plants (Cheng et al. 2006;
Koch et al. 2009). Despite the ubiquitous presence of
super-hydrophobicity, attempts to understand associated
phenomena have been applied only recently. Scanning
electron microscopy (SEM), transmission electron
microscopy (TEM) and atomic force microscopy (AFM)
are some of the tools that have enabled the observation of
nano-scale features on the surfaces and to unveil some of
the secrets behind super-hydrophobic development.
Bottom line is the fact that the combination of low
surface energy and nano-roughness are effective in
attainment of super-hydrophobicity. Based on related
principles, many methods and techniques are now being
developed following bio-mimicry.
Wettability and repellency are important properties of
solid surfaces from both fundamental and practical
aspects. The wettability of a solid surface is a
characteristic property of materials and depends strongly
on both the surface energy and roughness (Cassie, Baxter
1944; Genzer, Efimenko 2006; Koch, Barthlott 2009;
Nakajima et al. 2001; Sun et al. 2005; Wenzel 1936).
Some fundamental concepts relevant to wettability and
super-hydrophobicity are addressed briefly in the
following paragraphs.
A key definition is that of the contact angle, i.e., the
angle at which the liquid/vapor interface meets a given
solid. (cf. Fig 1) When the probing fluid is water, the
measured contact angle is the “water contact angle”
(WCA). The WCA is specific to any given system and is
determined by the interactions across the three-phase line.
Hydrophilic, water-loving materials are substances with
WCA lower than 90° while hydrophobic ones are waterhating, displaying a WCA higher than 90°. As stated
earlier, super-hydrophobic characteristics are attained
when the water contact angle is greater than 150°.
The Young equation is the most important yet the
simplest relationship to describe the balance of forces
acting on a liquid droplet spreading on a surface.
217 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
Fig 1. Schematic illustration of Young’s (a), Wenzel (b) and
Cassie (c) regimes.
The contact angle (θ) (cf. Fig1a) of a drop is related to
the interfacial energies acting between the solid-liquid
(γSL), solid-vapor (γSV) and liquid-vapor (γLV) interfaces
and is described by Eq 1:
   SL
cos( )  SV
[1]
 LV
There is a limitation in the application of the Young
equation in that it is strictly valid only for surfaces that
are atomically smooth, chemically homogeneous, and do
not change by possible interactions with the probing
liquid.
According to Young Equation, the highest contact angle
will be achieved under the condition of lowest γSV, i.e.,
the material with the lowest possible surface energy. As
a reference, the surface energy of some moieties of
interest decreases in the following order: -CH2->-CH3->
-CF2-CF2H>-CF3 (Genzer, Efimenko 2006). The lowest
surface energy value yet recorded, 6.7 mJ/m2, is obtained
for a surface with regularly aligned closest-hexagonal
packed -CF3 groups, which leads to a calculated WCA of
about 120° (assuming water and standard conditions in
Eq 1) (Nishino et al. 1999). Strikingly, natural surperhydrophobic surfaces as those mentioned previously
show larger values. How is then such a superhydrophobic behavior possible?
The explanation lies on the contribution of the
roughness of surfaces. It is very difficult to find in nature
a surface perfectly flat as assumed in Young’s approach.
So in most cases roughness ought to be considered (cf.
Fig 1b). Wenzel (1936) proposed a model to describe the
contact angle θ’ of a rough surface by modifying Young’s
equation as follows:
r ( SV   SL )
cos( ') 
 r cos( )
[2]
 LV
In Eq 2, the parameter r, is a roughness factor, defined
as the ratio of the actual area of the rough surface to the
geometric projected area. Since r is always larger than
unity, the surface roughness enhances hydrophilicity in
PAPER CHEMISTRY
the case of hydrophilic surfaces. Similarly, it enhances
the hydrophobicity of hydrophobic ones. In other words,
in the Wenzel regime roughness amplifies both
hydrophilicity and hydrophobicity.
When the surface contains small protrusions, which
cannot be filled by the (probing) liquid and are instead
filled with air (cf. Fig1c), the wettability enters the socalled Cassie-Baxter regime (1944). Cassie proposed an
equation describing the contact angle θ’ at a surface
composed of solid and air and used the wetted area
fraction f, that is, the one in contact with the liquid. More
specifically, f was defined by Σa/Σ(a+b), where a and b
represent the contact area with the liquid a and the air b,
respectively. Thus, the water contact angle θ was
expressed according to Eq 3, which assumes a water
contact angle with air of 180°.
cos( ')  f cos  (1  f )cos180  f cos  f 1
[3]
In Eq 3, the fraction (1-f) describes the area in contact
with air. Note that Eq 3 only applies to cases where the
liquid touches just the top of the surface. If partial
penetration through surface grooves occurs, a more
complex equation is required, which has been described
by Werner et al. (2005). Because pores are filled with air,
which is extremely hydrophobic, the contact angle always
increases, relative to the behavior observed on a flat
substrate having an identical chemical composition.
Hence surface topography has a very profound effect on
material wettability. In this case, roughness works as an
amplifier only for hydrophobicity.
It has been shown that there is a critical value of f,
below which the Cassie regime exists and above which
the Wenzel regime is thermodynamically more stable
(Bico et al. 1999; 2001; Callies, Quéré 2005; Lafuma,
Quéré 2003; Quéré et al. 2003). The corresponding
transition occurs at a certain critical wetting angle (θc)
defined by:
cos(c ) 
f 1
r f
[4]
where f and r were defined in reference to Eq 2 and Eq 3.
Hence at contact angles larger than θc air pockets are
present beneath the drop, which, in turn, exists in the
Cassie regime. Quéré and coworkers (Quéré 2004; Quéré,
Bico 2003) also pointed out that the Wenzel regime was
the equilibrium state of the Cassie regime.
A molecular dynamics study (Yang et al. 2006) of liquid
droplets in contact with self-affine fractal surfaces also
provided some evidence that the contact angle for nanodroplets depends strongly on the root-mean-square
surface roughness amplitude but is nearly independent of
the fractal dimension Df of the surface.
The second requirement for super-hydrophobicity
(water must not stick to the surface) is closely related to
the contact angle hysteresis Δθ of the surface — the
difference between the largest (advancing) and smallest
(receding) stable contact angle θadv − θrec. The contact
angle hysteresis is extremely important in the selfcleaning applications. The maximum lateral force Flat that
a distorted droplet can buildup depends on θadv and θ rec as
Eq 5 (Furmidge 1962):
Flat∝cosθrec− cosθadv
[5]
which can be approximated for small hysteresis as Flat∝
Δθsinθ.
It has to be noted that super-hydrophobic surfaces can
be wetted by organic liquids easily. Super-oleophobic
surfaces denote those surfaces with a contact angle
greater than 150 degrees with organic liquids that have
appreciably lower surface tensions than that of water
(Tuteja et al. 2007). Compared with super-hydrophobic
surfaces, super-oleophobic surfaces are much less
discussed in the literature; recently, however, they have
received increased attention since they have a great
impact on a wide range of phenomena, including
biofouling, loss of self-cleaning ability, and swelling of
elastomeric materials. Calculations suggest that creating
such a surface would require a surface energy lower than
that of any known material. In this regard, Tuteja et al.
(2007) proposed a third factor, the re-entrant surface
curvature, which combined with chemical composition
and roughness, can be used to design surfaces that display
extreme resistance to wetting from a number of liquids
with low surface tension, including alkanes, such as
decane and octane. Super-oleophobic surfaces have been
achieved by using fluoro-decylpolyhedral oligomeric
silsesquioxanes (POSS) coatings, which display contact
angles greater than 160° for various non-polar fluids
(Meuler et al. 2011).
Generic methods toward super-hydrophobicity
According to the previous discussion on superhydrophobicity, there is only one way to access this
property: to exploit the combined effects of surface
chemistry and morphology of a give surface. Materials
with low surface energy include fluorocarbons, silicones,
and some organic and inorganic materials. Among low
surface energy materials, fluorocarbons are the most
frequently used. They have also attracted industrial
interest. Silicones (Artus et al. 2006; Dou et al. 2006;
Guo et al. 2006; Jin et al. 2005; Khorasani, Mirzadeh
2004; Khorasani et al. 2005; Lee et al. 2005; Liu et al.
2006; Ming et al. 2005) and organic materials (Jiang et al.
2004; 2005; Lee et al. 2004; Lu et al. 2004; Zhao et al.
2005) are other two widely used low surface energy
materials. Although fluorocarbons show the lowest
surface energy known, it is interesting to note that they
are rarely present in the organic materials produced by
nature.
The fluorine/carbon atomic ratio is a key aspect to
define hydrophobicity, as has been demonstrated by
Hsieh et al. from studies with carbon nanofiber arrays
(Hsieh et al. 2006). As expected, as more F atoms are
introduced, lower surface energy and higher contact angle
are achieved. The packing density of related moieties is
another factor influencing the surface energy and its
stability. Genzer and Efimenko (2000) demonstrated
elegantly this concept after creating long-lived superhydrophobic polymer surfaces through mechanically
assembly monolayer. F(CF2)y(CH2)xSiCl3 molecules were
deposited from the vapor phase on a stretched elastomeric
poly(dimethyl siloxane) (PDMS) substrate; after
deposition the strain was released making the film to
return to its original size, causing the grafted
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 218
PAPER CHEMISTRY
Low surface
energy
Fluorocarbon
s
Silicone
s
Organic
materials
Inorganic
materials
Etching &lithography
Superhydrophobic
surface
Layer-by-layer & colloidal assembly
Surface
roughness
Sol-gel
processing
Electrochemical reaction / deposition
Electrospinning, phase separation
Rapid expansion of supercritical
solution`
…
Fig 2. Strategies used to attain super-hydrophobicity by
combining low surface energy coatings and surface roughness.
F(CF2)y(CH2)xSiCl3 molecules to form a densely
organized mechanically assembled monolayer. The water
contact angle, compared to the un-stretched surface was
found to increase by up to 30 degrees.
We now turn our attention to methods to produce nanoroughness. The statistical roughness parameters are
uniquely related to each other over the different
measuring techniques and sampling sizes, as they are
purely statistically determined. However, they cannot be
directly extrapolated over different sampling areas as they
represent transitions at the nano-, micro-to-nano and
micro-scale levels. Therefore, the spatial roughness
parameters including the correlation length and the
specific frequency bandwidth should be taken into
account, which allow for direct correlation of roughness
data at different sampling sizes (Samyn et al. 2011b).
Nano roughness can be created through etching and
lithography, sol-gel processing, layer-by-layer and
colloidal assembly, electrochemical reaction and
deposition, electrospinning and phase separation. These
approaches were addressed in detail in a review article by
Ma and Hill (2006). In addition, it can be noted that rapid
expansion of supercritical solution as reported by Werner
et al. (Quan et al. 2009; Werner et al. 2010), utilizes the
high solvating power of supercritical carbon dioxide to
produce micro or nano-particles that can render the
surface with nano-roughness. It is worth noting, however,
that these approaches have been usually applied to hard
surfaces and the question is then if they can be adapted to
cellulosic materials. In sum, Fig 2 indicates the methods
available to attain super-hydrophobic surfaces by using
two combined strategies, one related to changes in
surface energy and the other one related to changes in
surface roughness.
Recent progress to approach super-hydrophobic
cellulosic materials
In the previous section, the general principles of superhydrophobic surfaces and the approaches toward superhydrophobicity were introduced. However, what about
the progress toward surperhydrophobic cellulosic
materials? Based on the principles of superhydrophobicity it is now clear that the effects of low
surface energy and nano-roughness topography are
required. In general, surface energy reduction and
roughness creation can be accomplished simultaneously
or in sequence.
Compared with efforts to create nano-roughness,
surface energy reduction is much simpler and
219 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
straightforward. Chemicals with low surface energy, such
as fluorine- and silicone- containing polymers as well as
hydrocarbons can be applied to cellulose by grafting,
adsorption, chemical vapor deposition, etc. On the other
hand, the creation of nano-roughness may be the more
demanding step toward super-hydrophobic cellulose.
Thus, surface energy reduction is usually addressed
somewhere during the application of techniques to endow
the materials with roughness.
The approaches used in recent years toward superhydrophobicity of cellulosic materials can be classified
into two categories, based on the generation of roughness:
(1) Roughness offered by coating cellulosic substrates,
which include:
a) Chemical grafting to modify the surface chemistry
and surface morphology of cellulosic fiber/surface
simultaneously.
b) Sol-gel processes to render cellulose fiber/surface
with porous outer-layer and to reduce surface
energy by post-treatment or by mixing precursors
with low surface energy side chains.
c) Nanoparticle deposition, for example by using
metal, metal oxide, mineral and polymers that
modify the morphology of the cellulosic
fiber/surface, followed by surface energy
reduction by post-treatment.
d) Chemical vapor deposition.
(2) Roughness offered by regeneration or fragmentation
of cellulosic materials, which among others include:
e) Electrospinning and elecrospraying
f) Use of nanocellulose (cellulose nanocrystals and
nanofibrillated cellulose)
g) Use of cellulose composites.
Modification of roughness by coating
Chemical grafting. Cellulose (C6H10O5)n is a long-chain
polymeric polysaccharide of glucopyranoses repeating
units linked together by β-1,4 glycosidic bond (cf. Fig 3).
It forms the primary structural component of green plants.
The primary cell wall of green plants is made of cellulose
while the secondary wall contains cellulose with variable
amounts of lignin.
The hydroxyl groups of cellulose can be partially or
fully reacted with various species to provide cellulose
derivatives with useful properties. This provides an
opportunity to modify the surface of cellulose/lignocellulosics by chemical reactions. Through derivatization
reactions, polymers with special structures and functions
can be introduced to modify the surface and attain the
desired properties. This has been the subject of intense
studies and a review paper which addresses the subject in
detail is available (Cunha, Gandini 2010).
Due to the abundance of OH groups in cellulose,
chemicals with low surface energy, such as fluorinecontaining substituents (Cunha et al. 2006; Cunha et al.
2007; Östenson et al. 2006; Yuan et al. 2005), silicone
and hydrocarbon polymers (Garoff, Zauscher 2002;
Pasquini et al. 2006) can be easily introduced by
esterification with acid or anhydride (cf. Fig 4).
PAPER CHEMISTRY
OH
OH
O
HO
HO
OH
HO
O
O
O
OH
DP
-2
2
OH
Non-reducing end
OH
O
HO
O
OH
Cellobiose
(2009) used a similar approach to produce superhydrophobic and self-cleaning cellulose surfaces via
surface-confined grafting of glycidyl methacrylate by
using atom transfer radical polymerization combined with
post-modification reactions.
Fabbri and coworkers (2004) proposed a new pathway
to introduce fluorine-containing polymers onto cellulose.
Their work involved organometallic compounds (one or
two metals bound to carbon) reacting with OH groups of
cellulose (cf. Fig 6). Unreacted metal-to-carbon bonds
were exploited to graft numerous molecular structures
derived from alcohols and amines.
A polymer containing an azide group was employed by
Li et al. (2010a) to covalently attach amphiphilic triblock
copolymers to cotton via UV irradiation. Amphiphilic
triblock azide copolymers containing poly(ethylene
glycol) (PEG) and poly(2,2,3,4,4,4-hexafluorobutyl
acrylate) blocks were synthesized through room
temperature reversible addition fragmentation chain
transfer polymerization using redox initiation (cf. Fig 7).
These polymers were successfully applied to fabricate
super-hydrophobic cotton fabrics by using a facile UV
irradiation approach.
The introduction of fluorinated polymer chains endowed
cotton with a WCA of 155°. Since the fluorinated
polymer chains were covalently attached on the surface,
the super-hydrophobic cotton fabric possessed high
stability and chemical durability.
It is reasonable to expect for the hydrophilic PEG
blocks to locate in the interface between the block
copolymer coating and the cotton fibers, which was
suggested to occur via H-bonds between PEG and the
hydroxyl groups of cellulose; also, it is expected that the
fluorinated blocks aggregate in the outermost side of the
coating, which contribute to the super-hydrophobicity of
the modified substrate.
OH
HO
O
OH
Reducing end
Fig 3. Chemical structure of cellulose.
O=C-R
OH
OH
OH
Cellulose
O
O
R C Cl
or
O
R C OH
O=C-R
OH
O
Cellulose
Fig 4.Scheme for esterification reactions of cellulose. R is a
long fluorine-containing or hydrocarbon chain.
Nyström et al. reported on a method to improve the
surface density of fluorine-containing moieties by using
branched “graft-on-graft” architectures (cf. Fig 5)
followed by post-functionalization leading to fluorinated
brushes (Nyström et al. 2006). In their work, the first
route (route I in Fig 5) involved reaction of the hydroxyl
groups on the cellulose substrate with pentadecafluorooctanoyl chloride. The coverage of fluorinate groups was
observed to be limited and so the exact water contact
angle was difficult to measure as it changed with contact
time. In fact, the surface first showed superhydrophobicity with WCA 150°; but after 20 min the
contact angle decreases to 128° and below 90° after 50
min. This indicates that the cellulose surface was not
fully covered by the fluorinated material. In order to
improve the surface-coverage and thereby the
hydrophobicity, a functional monomer was grafted from
the cellulose substrate to enable attachment of an
increased amount of fluorine groups. In route II (Fig 5),
two fluorine groups were grafted to the surface and the
contact angle increased to 154° and somewhat more
irreversibly.
To
further
improve
the
superhydrophobicity, a branched-like structure was used to
attach four fluorine groups per hydroxyl group on
cellulose (route III in Fig 5). This step improved the
WCA to extremely high values, up to 170°. Nyström et al
C 7 H15
O
O
Br n
HO
Br n
O
OH
O
Br n
O
O
O
O
O
O
Cellulose Surface
O
O
O
Cellulose Surface
O
O
O
(iv)
(iii)
C7 H15
II
O
(i)
Cellulose Surface
Cellulose Surface
(ii)
O
(ii)-(iv),(i)
Br m
C7 H15
O O
OH
O
O
O
Cellulose Surface
O
C 7H 15
O
(i)
Br n
O
O
O
C 7H 15
O
Cellulose Surface
I
O
C7 H15
O
O
O
O
O
Br
O
III
O
O
O
O
Br
m
O
C7 H 15
Cellulose Surface
Fig 5. Routes for “graft-on-graft” architectures to produce super-hydrophobic cellulose (adapted from (Nyström et al. 2006) with
permission from RSC Publishing).
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 220
PAPER CHEMISTRY
OH
O
OH
HO
O
HO
R
O + Mt R
R
O
OH
OH
R
Mt
R
O
O
OH
HO
O + RH
O
HO
O
OH
OH
R
Mt
R
O
O
OH
HO
O + 2R'OH
O
HO
O
OH
OH
OR'
Mt
OR'
O
O
OH
HO
O + 2RH
O
HO
O
OH
OH
Fig 6. Schematic illustration of the modification of cellulose with
organometallic compounds (Fabbri et al. 2004).
SC12H 23
H3 C
O
O
113
O
PEG5000-CTA
H3 C
APM/MA(1:8), BPO/DMA
S
1,4-dioxane, RT
S
SC 12 H 23
O
O
O C 0.72 O C 0.28 S
47
O
O
113
O
PEG5000-b-P(MA-co-APM)
CH3
HFA, BPO/DMA
1,4-dioxane, RT
H3 C
S
N3
SC 12 H 23
O
O
S
O C 0.72 O C 0.28
O C 59
47
O
O
O
CH3
F2 C
PEG5000-b-P(MA-co-APM)-b-PHFA
CF2
F3 C
113
S
O
N3
Fig 7. Synthesis of PEG5000-b-p(MA-co-APM)-b-PHFA which
were then applied on cellulose (adapted from (Li et al. 2010a)
with permission from Elsevier).
Fig 8. Modification of cellulose by amphiphilic triblock
copolymers containing azide groups: SEM images of original
(a) and modified (b) cotton fabric together with respective AFM
images (c and d). Reprinted from (Li et al. 2010a) with
permission from Elsevier.
221 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
The surface morphologies of the original and the
modified cotton fabrics were investigated by SEM and
AFM. Fig 8a and 8b shows highly textured, microscale
fibers on the cotton fabric while the modified substrate
reveals a coating layer consisting of the copolymers with
some embossments caused by the aggregation of the
fluorinated polymer blocks, Fig 8d. In addition, SEM top
view of the modified cotton fabric shows that a layer of
polymer was armored on the as-prepared superhydrophobic surface (Fig 8b).
In order for the polymer to anchor onto the cellulosic
materials covalent and hydrogen bonding were suggested.
Samyn and coworkers (Samyn et al. 2011a; Stanssens et
al. 2011) synthesized a block polymer with amide groups
(Fig 9). The amide blocks exhibited good adhesion to the
cellulosic fibers, which was proposed to be the result of
hydrogen bonding between the electronegative nitrogen
in amide and the hydrogen of the hydroxyl groups in
cellulose. The hydrophobic polystyrene blocks selfassembled outside of the fibers, rendering the system
with low surface energy and nano-roughness.
Another method, admicellar polymerization, can be
applied. It is a versatile method to form ultrathin
polymeric films on a surface. This method consists of
three main steps: Firstly, an admicelle in the form of a
surfactant bilayer is formed on the substrate’s surface
followed by the addition of an organic monomer that
preferentially diffuses by its hydrophobicity into the
hydrophobic core of the admicelle, a process called
adsolubilization (Tragoonwichian et al. 2009). Finally,
polymerization of the monomer in the admicelle is
initiated through the addition of a suitable initiator such
as ammonium persulfate. After the completion of the
reaction, the weakly bound surfactants are washed away,
leaving a thin polymer film coating on the surface. This
method was employed by Tragoonwichian and coworkers
to modify cellulose fibers (Tragoonwichian et al. 2009).
The sodium salt of dodecylbenzenesulfonic acid (DBSA)
was used as surfactant and first coated 2-hydroxy-4acryloyloxybenzophenone (HAB) on a cotton fabric. At
last, methacryloxymethyltrimethyl-silane (MSi) was used
to initiate the polymerization to form a thin layer. The
structure of HAB and MSi is illustrated in Fig 10. In this
work admicellar polymerization produced a bifunctional
cotton fabric possessing both UV-protection and waterrepellent property due to the UV adsorption of HBA and
hydrophobicity of silane block. The resultant fabrics were
found suitable for outdoor use where both UV-protection
and water repellency are required, for example in coating
materials and protective clothing.
Polyhedral oligomeric silsesquioxane (POSS) consisting
of an inorganic silicon-oxygen cage with general
composition SiO3/2 and organic substituents R capping
the silicon atoms of the cage, were incorporated with
fluorine-containing organic polymers to improve thermal
and other properties of fibers (Gao et al. 2010; Wang et al.
2011a; 2013). Terpolymer of POSS, methyl methacrylate
(MMA) and 2,3,3,3-tetrafluoro-2-(1,1,2,3,3,3-hexafluoro2-(perfluoropropoxy) propoxy) propyl methacrylate
[(HFPO)3MA] were synthesized successfully (Fig 11) to
then coat cotton fabrics. The coated fibers possessed
excellent water and oil repellency.
PAPER CHEMISTRY
H H2
C C
m
O
H H
C C
O
n
O
H H2
C C
NH3
H2O
m
O
H H
C C
N
H
n
O
Fig 9. Imidization of poly(styrene-maleic anhydride) into
poly(styrene-maleimide) in the presence of ammonium
hydroxide (Samyn et al. 2011a).
CH2
(a)
CH2
O
H
O
(b) H 3 C
O
CH 3
Si CH3
CH 3
O
OH
O
Fig 10. Chemical structures of (a) 2-hydroxy-4-acryloyloxybenzophenone (HAB) and (b) methacryloxymethyltrimethylsilane (MSi).
Fig 11. Preparation of P(POSS-MMA-(HFPO)3MA). Reprinted
from (Gao et al. 2010) with permission from Elsevier.
(a)
(b)
(c)
(c)
Fig 12. FE-SEM images of POSS coated cotton fabrics
surfaces. Insets are photos of liquid droplets on the
corresponding samples: cotton fabric coated with P1 (a), cotton
fabric coated with P2 (b), cotton fabric coated with P3 (c),
cotton fabric coated with P4 (d). P1~P4 are terpolymers with
increasing amount of POSS content. Reprinted from (Gao et al.
2010) with permission from Elsevier.
The WCA was observed to be a function of the POSS
content. The contact angles were increased as the content
of POSS in the terpolymer increased from 6.4 to
13.4 wt% from 140 to 152° and from 127 to 144°, in the
case of water and natural oil, respectively. The superhydrophobicity achieved by this POSS-based terpolymers
may be due to the self-aggregation of the polymer chains
grafted on the POSS cage. FE-SEM images of POSS and
coated cotton fabrics surfaces are shown in Fig 12. It is
clearly shown that with the increase of POSS (as
indicated by P1, P2, P3 and P4 for increased POSS
content), the nano-roughness generated by selfaggregation of the POSS-based terpolymers increased
markedly. The insets provided in Fig 12 are images of
liquid droplets on the corresponding samples.
In other efforts, electrostatic and van der Waals forces
were utilized to adsorb polyelectrolytes on the surface of
fibers. Lingström et al. (2007) treated individual wood
fibers with polyelectrolyte multilayers consisting of two
different polymer combinations, namely polyallylamine
(PAH)/poly acrylic acid (PAA) (PAH/PAA) and
polyethylene oxide (PEO)/PAA. They found that multilayer deposition not only increased the hydrophobicity
but also the strength of paper made from the treated
fibers. In this case, however, the treated fibers were not
strictly superhydrophobic. This layer-by-layer (LbL)
deposition method has been used by several other groups
to create super-hydrophobic surfaces based on substrates
different than lignocellulose (Ji et al. 2006).
Sol-gel process. The sol-gel process is a wet-chemical
technique widely used in the fields of materials science.
Such method is used primarily for the fabrication of
materials (typically metal oxides) starting from a
colloidal solution (sol) that acts as the precursor of an
integrated network (gel) of either discrete particles or
network polymers. Sol-gel processes have been used to
produce super-hydrophobic surfaces from a variety of
alkylsilane precursors, for example: tetraethoxysilane or
tetraethyl orthosilicate (TEOS) (Ding et al. 2006; Huang
et al. 2009; 2011b; Ivanova, Zaretskaya 2010; Latte et al.
2010), methyltriethoxysilane (MTES) (Latte et al. 2010;
Mahadik et al. 2010; Rao et al. 2010), tetramethoxysilane
(TMOS) (Latte et al. 2009; Nadargi et al. 2010), methyl
trimethoxy silane (MTMS) (Rao et al. 2006; 2009; Xu et
al. 2011), methyltrichlorosilane (MTCS) (Shirgholami et
al. 2011), trimethylchlorosilane (TMCS)(Rao et al. 2009),
isobutyltrimethoxysilane (Xiu et al. 2008), phenyltrimethoxysilane (Ph-TMS)(Rao et al. 2010), fluoroalkylsiloxane (Ivanova, Zaretskaya 2010) and sodium silicate
(which is also called water glass) (Li et al. 2008b; Shang
et al. 2010) and an industrial waterproof reagent
[(potassium methyl siliconate) (PMS)] (Li et al. 2008a),
etc.
Taking the case of TEOS as an example, the sol–gel
reactions of alkylsilane can be described by the following
sequence (Bae et al. 2009)):
(1) alkylsilane hydrolysis in acid or basic conditions:
Si-(OC2H5)4 + 4H2O → Si-(OH)4 + 4C2H5OH
(2) Alcohol condensation
Si-(OH)4+Si-(OC2H5)→≡Si-O-Si≡ + 4C2H5OH
(3) Water condensation
Si-(OH)4 + Si-(OH)4→≡Si-O-Si≡ + 4H2O
Alkylsilane initially hydrolyze in acid or basic
conditions to form silicic acid and then the formed silicic
acid condensates with another alkylsilane or with itself to
form a cross-linked polysiloxane network; thus, a gel is
formed. The hydroxyl groups of polysiloxane can react
with the hydroxyl group over the surface of a fiber and
therefore the gel actually can be covalently attached to
the fiber support.
In a similar way as that for TEOS, water glass and
potassium methyl siliconate can hydrolyze in the
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 222
PAPER CHEMISTRY
(c)
Hydrophobic
groups
DTMS
O
O
O
O
O
O
O
O
O
O
O
O
TEOS
Fig 13. Chemical structures of DTMS (a) and TEOS (b) and a
proposed schematic diagram of sol–gel films formed on the
substrate (cellulose acetate)(c)
OH
OH
Cellulose
OH
Cellulose
OCH 2 CH 3
DMF OCN (CH 2) 3 Si OCH2 CH 3
DMTL
OCH 2 CH 3
O
is
y s Cl
ol
:H
dr
hy tO H
id
S
E
A c ter: + EO
DT
wa
F
P
OH
O
Si (CH2 )2 (CF2) 7CF3
OH
OH
Si (CH2 )2 (CF2) 7CF3
O
OCH 2CH3
O HN (CH2 )3 Si O
OH
O
O HN (CH2 )3 Si O
O
O
O
Cellulose
OCH 2CH3
Acid hydrolysis
O HN (CH2 )3 Si OCH 2CH 3
Water:EtOH:HCl
OCH 2CH3
OH
OCH 2CH3
O HN (CH2 )3 Si OCH 2CH 3
OCH 2CH3
Ac
id h
O
Wa
yd
roly
te r
:E t
sis
+ OH :H
TE
Cl
OS
O
OH
O HN (CH2 )3 Si O
OH
OH
OH
O HN (CH2 )3 Si O
O
O
Cellulose
223 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
(b)
(a)
Cellulose
presence of acid to produce silicic acid by the following
reactions:
Na2SiO3 + H2O + 2H+→Si-(OH)4 + 2Na+
CH3-SiO3K+ H2O + H+→CH3-Si-(OH)3 + K+
The condensation reactions are the same as those
discussed before in the case of TEOS. Some sol-gel
precursors bear one or more organic chains, such as
methyl, ethyl, phenyl and fluoroalkyl. These chemicals
not only generate gel porous structures to render a nanoscale roughness but also introduce the respective low
energy groups. In some reports two or more precursors
are mixed to create roughness and reduce the surface
energy via simple dip coating process (Rao et al. 2006;
2009; 2010; Xu et al. 2011). Post-treatment with long
alkyl chains silanes is also reported. For example, Ding
and coworkers (Ding et al. 2006; Miyauchi et al. 2006)
modified electrospun hydrophilic cellulose acetate with a
simple sol-gel coating of dodecyl(trimethoxy)silane
(DTMS) and TEOS, as shown in Fig 13. Thus, sol-gel
films were formed on the rough fibrous mats after
immersion in the sol-gel. Through post-treatment of
silane with low energy chains, a better surface
hydrophobicity was obtained since the self-assembly of
long hydrophobic chains on the surface formed
aggregates that enhanced the nano-scale roughness. This
sol-gel process can also be accomplished in a gas-solid
reaction (Cunha et al. 2010b).
Cunha et al. (2010a) described a reactive precursor for
sol-gel processes which contain a cyanate group. A precursor (3-isocyanatopropyl) triethoxysilane (ICPTEOS)
reacted heterogeneously with the hydroxyl groups of
cellulose in a DMF solvent. The reactions was followed
by acid hydrolysis (and condensation) of the appended
siloxane moieties as such, and in the presence of either
tetraethoxysilane
or
1H,1H,2H,2H-perfluoro-decyltriethoxysilane. The reaction scheme is shown in Fig 14.
Polycarboxylic acids were employed as cross-linker
between cellulose fibers and silica coatings (Huang et al.
2011b; Liu et al. 2011). In contrast with inorganic acid
and monoacids, polycarboxylic acids act not only as
catalyst in the preparation of the sol, but also as crosslinker between cellulose and the silica gel. It was
observed that active hydrogen in citric acid remained
after the hydrolysis reaction of water glass. This
indicated the residual carboxylic acid groups in citric acid
could continually react with hydroxyl groups of cellulose
fibers to form ester bonds with sodium hypophosphite as
the catalyst. The residual carboxylic acid groups could
also react with Si–OH. The formed silica gel can be
regarded as a structure with a polymeric network crosslinked by citric acid groups. As a result, the compatibility
and adhesion between the fiber surface and the silica sol
as well as the hydrophobic durability are improved. It can
be noted that residual hydroxyl groups in polysiloxane
are readily reactive with the hydroxyl groups in cellulose;
thus, the use of a crosslinker, e.g. cyanate, epoxy or
polycarboxylic acid to bind cellulose and the silica layers
is not necessary.
HO
O
OH
O HN (CH2 )3 Si O
OH
OH
OH
O HN (CH2 )3 Si O
OH
O
HO
Si O
O
Si O
OH
OH
Si O
O
Si O
O
Fig 14. Scheme of the chemical modification of the cellulose
fibers with (3-isocyanatopropyl) triethoxysilane (ICPTEOS) and
the acid hydrolysis treatments of the modified cellulose fibers.
Redrawn from (Cunha et al. 2010a).
As far as the actual application, an interesting approach
was presented by Taurino et al. (2008) who introduced an
air-brush applicator to spray sol-gel solutions on given
substrates. This method has the advantage of being
simple and less expensive, compared to other
conventional techniques. The air-brushing apparatus
consists of a bottle with the sol-gel solution and an airbrush applicator (Fig 15). The morphology and surface
energy of the coated surfaces can be adjusted by the air
pressure and the precursors in the container. Sol-gel
solution of TEOS, tetraethyl orthotitanate (TEOT), and
tetra-n-propyl zirconate (TPOZ) mixed with α,ωtrialkoxysilane-terminated PFPE were demonstrated to
produce super-hydrophobicity on a variety of substrates
with little or no pretreatment (Taurino et al. 2008).
PAPER CHEMISTRY
I. Hydrolysis and condensation
O - Na +
O Si
O - Na +
+
O
Water glass
O
OMe
Si OMe
OMe
O
H+
O
O
GPTMS
OH
OH
O
HO
O
O
Si
O
OH
O
Si
O O
Si
O O
O
Cellulose
Cellulose
O
O
O
Si
O
OH
O
O
OH
Fig 15.Air-brushing apparatus for application of sol-gel
solution of TEOS: substrate (a), sol–gel solution (b), air-brush
applicator (c). Reprinted from (Taurino et al. 2008) with
permission from Elsevier.
O
Si
O
II. Cross-linking
OH +
HO
O
Si
O O
Si
O O
Si
O O
O O
Si
O O
HO O
Si
Si
O O
O
Si
O O
O O
Si
O
O O
Si
Si
HO O
O O
Si
O O
Fig 16. Water glass coupled with GPTMS reacting with
cellulose. Redrawn from (Shang et al. 2010).
(a)
(b)
Fig 17. Scheme showing a cotton fabric coated by the silica
hydrosol (a) and by silica hydrosol followed by HDTMS
modification (b). Reprinted from (Xu et al. 2011) with
permission from Elsevier.
Fig 18. Cotton fabrics without treatment coated with silica
nanoparticles (a) (reprinted from (Bae et al. 2009) with
permission from Elsevier) and a paper surface coated with
silica particles using multi-layer technique (b) (reprinted from
(Yang, Deng 2008) with permission from Elsevier).
In order to improve the bonding between silica coating
and cellulose fibers, 3-glycidyloxypropyl-trimethoxysilane (GPTMS) was used as crosslinker (Shang et al.
2010). In their work, durable super-hydrophobic cellulose
fabrics were prepared from water glass and noctadecyltriethoxysilane (ODTES) and 3-glycidyloxypropyltrimethoxysilane (GPTMS) was used as
crosslinker in the sol-gel method (cf. Fig 16). The result
showed that the addition of GPTMS resulted in a better
fixation of the silica coating from water glass on
cellulose.
Deposition of nanoparticles on cellulosic surfaces.
Some nanoparticles (NPs), such as those based on SiO2,
ZnO, CaCO3 and TiO2 can be applied to the surface of
cellulosic substrates to produce roughness in the
micro/nano scale. By using this method, post-treatment
with fluorine-containing compounds (Xu et al. 2011;
Yang, Deng 2008; Yu et al. 2007), silicones and
functionalized organic materials can be used to reduce
the surface energy and give super-hydrophobicity. This
protocol is illustrated in Fig 17.
SiO2 NPs. SiO2 NPs are usually prepared using the
Stöber method (Stöber et al. 1968). The principles to
produce SiO2 NPs are similar as those described in the
sol-gel section. According to the mechanism of silane’s
hydrolysis and condensation, the reaction time, catalyst
and ratio of water/alcohol are primary factors to control
the size of nanoparticles.
Silica NPs can attach onto cotton fibers by simple dip
coating (Zhou et al. 2012), by spraying alcohol
suspensions of SiO2 nanoparticles (Ogihara et al. 2012)
or by heat treatment (Xu et al. 2011), resulting in the
formation of a chemical bonds between hydroxyl groups
of cellulose and silica. Thus, covalently attach to
cellulose is produced without the requirement of binders.
The surface of cellulose fiber carries negative charges in
aqueous solution, and the NPs prepared are usually
negatively charged as well. Therefore, the coverage of
silica NPs on cotton fibers without treatment is quite poor
(Bae et al. 2009; Yu et al. 2007), as shown in Fig 18a. In
order to increase the coverage and also to enhance the
roughness created by NPs, multilayers of silica NPs
deposited from solution can be combined with cationic
polyelectrolytes. For example, Yang and Deng (2008)
developed a facile method for preparing a super-hydrophobic paper surface using multi-layer deposition of
polydiallyldimethylammonium chloride (polyDADMAC)
and silica particles, as shown in Fig18b. Zhao et al. (2010)
assembled multilayer by alternately immersing the
treated fabric into 1.0 wt% aqueous solution of cationic
poly(allylamine hydrochloride), PAH and a solution
containing 1.0 wt% SiO2 NPs. This electrostatic layer-bylayer (LbL) assembly is based on alternative adsorption
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 224
PAPER CHEMISTRY
on charged substrates of oppositely charged polyelectrolytes, inorganic nanoparticles, macro-molecules or
even supramolecular systems. It is a versatile technique
to build up multilayered composite films in a controlled
manner (Aulin et al. 2010a; Eita et al. 2012; Karabulut et
al. 2012; Karabulut, Wågberg 2011; Tanaka, Ödberg
1992). Charged fibers are suitable as solid supports to
conduct LbL since the assembly is independent of the
size and topography of the substrates and uniform multimultilayers can be formed with different spatial
structures. Fig 19 includes smooth surfaces and cotton
fibers and the changes in the nano topography after
application of coating layers.
Xue et al. (2009) developed another LbL protocol by
modifying the surface of silica NPs with amino- and
epoxy- functional groups, respectively. Alternate coating
of amino- and epoxy-functionalized silica nanoparticles
on epoxy-functionalized cotton textiles were used to
generate a dual-size surface roughness, followed by
hydrophobization (schematically illustrated in Fig 20) to
produce super-hydrophobic surfaces. The static water
contact angle of the most super-hydrophobic sample
prepared reached 170°.
ZnO nanorods (NRs). ZnO NRs can be prepared on
cellulose through a hydrothermal route. A seed layer of
ZnO was obtained by solution-based (Xu, Cai 2008) and
magnetron sputtering (Xu, Cai 2008; Lim et al. 2010)
methods. In the solution method, zinc salt was dissolved
in ethanol under vigorous stirring. Then a solution of
hydroxide (0.15 M) in ethanol was slowly added and
stirred. ZnO gradually deposited on the fibers and thus
ZnO nanocrystals grew slowly on their surfaces. In the
magnetron sputtering method, the substrate was treated
by radio frequency magnetron sputtering (base pressure
10−3mTorr, Argon plasma, rf power 100 W, working
pressure 10 mTorr, deposition rate 12 nm/min). ZnO NRs
were fabricated via a hydrothermal route with a growth
solution containing an equimolar aqueous mixture of 25
mM zinc salt and hexamethylenetetramine and 0.1 g of
poly(ethylenimine) (PEI) (Xu, Cai 2008; Lim et al. 2010).
The seeded layer-coated fabric was immersed in 50 ml of
the growth solution contained in a glass bottle sealed
with an auto-cleavable cap and heated at 90°C for 6 h.
PEI was used to enhance the aspect ratio of the NRs.
SEM images of ZnO NRs are included in Fig 21.
Compared with SiO2 NPs, ZnO NRs have a much
higher aspect ratio. Therefore, cellulose treated with ZnO
NRs can yield lower roll-off angles and better waterrepellent properties compared to SiO2 NPs of similar
diameter. This is due to the discontinuous three-phase
contact line of NRs (Xu, Cai 2008).
TiO2 NPs. TiO2 is widely used in paper industry to
produce better paper opacity (Farrokhpay 2009; Marques
et al. 2006; Nelson, Deng 2008; Park et al. 2007). NanoTiO2 can improve significantly the brightness and opacity
of paper materials because of its high index of refraction
compared to cellulose fibers or other fillers. TiO2 NPs
were used to generate nano-roughness required for superhydrophobicity as well (Xue et al. 2008). TiO2 sol was
prepared by a typical controlled hydrolysis process. For
example, Xue et al. used a solution (Solution A) prepared
225 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
by slow addition of 8.5 ml of tetrabutyltitanate into 20 ml
of anhydrous ethanol under continuous stirring.
A second solution, Solution B, was prepared by mixing
1.5 ml of deionized water, 5 ml of acetic acid, and 20 ml
of anhydrous ethanol together with stirring. Upon slow
addition (1 h) of Solution A into Solution B the reaction
mixture was stirred for 24 h at room temperature and as a
result, a transparent bright yellowish TiO2 sol was
obtained (Xue et al. 2008). The as-prepared TiO2 was
applied to cellulose to increase its roughness. Hydrophobization was needed in order to lower the surface
energy of the TiO2-coated substrate.
Due to the extremely large surface area-to-particle size
ratio, nanosized-TiO2 pigments were poorly retained
when mechanically mixed with fibers. Thus, Huang and
coworkers (Huang et al. 2011a) reported a type of paper
with super-hydrophobic and photocatalytic properties
after modification with TiO2 NPs and a silane molecule,
3-(trimethoxysilyl)propyl methacrylate (MPS). MPSmodified nano-TiO2 particles were added to the fiber
suspension before the process of beating to inhibit
agglomeration of TiO2 on the surface of the fibers.
Kettunen et al. (2011) used a chemical vapor deposition
method to coat a thin TiO2 film on lightweight native
nanocellulose aerogels to offer a novel type of functional
material that shows photo-switching between watersuperabsorbent and water-repellent states. Highly porous,
nanocellulose aerogels were here first formed by freezedrying from the corresponding aqueous gels (Jin et al.
2011). In this straightforward, one-step process the
nanocellulose aerogels were exposed to titanium
isopropoxide vapor at 190°C at pressures of 1–5 kPa for
2 h. Well-defined, nearly conformal TiO2 coatings with
thicknesses of ca. 7 nm were obtained.
Fig 19. SEM images of untreated cotton fabric (a and b) and
cotton fabrics assembled with (PAH/SiO2)n multilayers: n = 1
(c), n = 3 (d), n = 5 (e), and n=7 (f). Reprinted from (Zhao et al.
2010) with permission from Elsevier.
PAPER CHEMISTRY
Fig 20. Schematic illustration of alternate coatings to prepare super-hydrophobic cotton. Reprinted from (Xue et al. 2009) with
permission from Elsevier.
Fig 21. SEM images of ZnO nanorods-coated cotton fibers (c).
A high magnification image of ZnO nanorods on the cotton fiber
is shown in (d). Scale bar in the inset represents 100 nm.
Reprinted from (Lim et al. 2010) with permission from Elsevier.
The LbL method was employed to deposit
TiO2/poly(acrylic acid) (PAA) on fiber surfaces in order
to increase the coverage of TiO2 NPs and the roughness
of the cellulose fibers (Li et al. 2010b; Ogawa et al. 2007).
The process included hydrophobization with long chain
silane molecules (Li et al. 2010b).
Liquid flame spray (LFS) coating is a technique which
generates NPs and can be used to simultaneously coat
substrates (Stepien et al. 2011; Teisala et al. 2010). A
LFS setup is schematically illustrated in Fig 22.
In the LFS process the precursor fluids diluted in water
or alcohol are fed together with the combustion gases into
a specially designed spray gun. Instantly after exiting the
burner nozzle the precursor solution is atomized to
micron-sized droplets by the high-velocity gas flow.
Liquid droplets evaporate in a hot and turbulent flame
and subsequent reactions of the precursor vapor lead to
formation of NPs of desired material. Afterwards the NPs
grow larger in the flame by condensation, coagulation,
coalescence and agglomeration. The final particle size
can be controlled, e.g. via total mass flow rate of the
precursor (product of the precursor solution feed rate and
its concentration) or by adjusting the collecting or
depositing distance. The main exhaust gases formed as
by-products in the LFS process are normally water vapor
and carbon dioxide. Low waste flows combined with
relatively simple and inexpensive equipment make the
LFS process comparatively cheap and environmentally
friendly.
The major advantage of such process is the broad
spectrum of metal or metal oxide NPs that can be created
using different liquid precursors. For example, Stepien et
al (2011) dissolved titanium (IV) isopropoxide (TTIP) or
Fig 22. Schematic image of the liquid flame spray set-up for
coating paperboard. Hydrogen is used as a nebulizing gas.
Reprinted from (Stepien et al. 2011) with permission from
Elsevier.
(a)
(b)
Fig 23. SEM images of samples coated with TiOx (a) and SiOx
NPs (b). Reprinted from (Stepien et al. 2011) with permission
from Elsevier.
tetraethylorthosilicate (TEOS) precursor in isopropanol
(IPA) to adjust the wetting properties of paperboard with
nanosized TiOx or SiOx coatings produced by the LFS
process. SEM images of TiOx or SiOx coated paperboard
were shown in Fig 23.
Ag NPs. Silver and silver ions have been reported to be
effective against nearly 650 trends of bacteria (Dubas et
al. 2006) due to their ability to attach to the their cell
membranes, penetrate and attack the respiratory chain of
the cell, hence leading to the bacteria’s death (Morones et
al. 2005; Rai et al. 2009). Silver NPs (Ag NPs) appear to
exhibit enhanced antibacterial activity due to their
increased surface area, which provides larger contact
possibilities with microorganisms per surface unit. In
addition, Ag NPs are also known to release silver ions
thereby enhancing their bactericidal activity (Morones et
al. 2005; Rai et al. 2009). Ag NPs have been incorporated
onto various polymeric fibers and the resultant fibrous
composites have reported successful antibacterial activity
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 226
PAPER CHEMISTRY
(Hong et al. 2006; Parikh et al. 2005; Son et al. 2004b;
Xu et al. 2006). Generally, there are two methods to
deposit Ag NPs onto polymeric fibers. The first method
consists of preparing colloidal Ag NPs by reducing Ag
salts, e.g. AgNO3, using reducing agents such as NaBH4,
citrate or sugars. This synthesis is commonly followed by
immersion of fibers into the colloidal Ag NP solution
hence directly depositing the NPs onto the fiber’s surface
through electrostatic interactions (Dong, Hinestroza
2009), hydrogen bonding (Dong et al. 2008), or through a
layer-by-layer deposition (Dubas et al. 2006). An
alternative route is an in situ method, where Ag ions are
initially adsorbed onto fiber surfaces and then posttreated with UV radiation (Chen, Chiang 2008), heat
treatments (Ifuku et al. 2009), chemical reduction (Dong,
Hinestroza 2009), ultrasound (Perelshtein et al. 2008) or
microwave (Chen et al. 2008) to reduce Ag ions into Ag
NPs.
Besides the production of antibacterial fabrics, Ag NPs
can be used as a facile and effective method to prepare
super-hydrophobic cellulose (Khalil-Abad, Yazdanshenas
2010). For example, Ag NPs were produced on cotton
fibers by treatment with aqueous KOH and AgNO3,
followed by reduction treatment with ascorbic acid in the
presence of a polymeric steric stabilizer, generating a
dual-size surface roughness. Further modification of
particle-containing cotton textiles with octyltriethoxysilane (OTES) led to hydrophobic surfaces. The
procedures are schematically illustrated in Fig 24 while
SEM images of Ag NPs coated fabric are presented in
Fig 25.
The surfaces prepared showed a sticky hydrophobicity:
a static water contact angle of 151° was measured for a
10 μl droplet of water that did not slid off even when the
sample was held upside down. The modified cotton was
found to have potent antibacterial activity toward both
Gram-positive and Gram-negative bacteria.
CaCO3 NPs. CaCO3 is a mineral which is widely used
in the paper industry. CaCO3 NPs can be prepared by the
reaction of sodium carbonate with calcium chloride in the
presence of oleic acid and heptadecafluorodecyltrimethoxysilane (Wang et al. 2010) or obtained from
commercial precipitated calcium carbonate (PCC)
particles (Hu et al. 2009). The advantages of CaCO3
include its low cost and nontoxicity. Hu et al. (2009) used
a polymer latex agent as polymer binder in order to
increase the adhesion of CaCO3 NPs to cellulose fibers.
Commercial PCC, hydrophobic stearic acid and polymer
latex particles were used for surface roughness control,
surface hydrophobic modification agent, and polymer
binder, respectively. A simple coating via dipping was
used to produce papers with high WCA and water
resistance. It was found that the surface pretreatment of
PCC with fatty acid salt prior mixing with polymer
binder played an important role for improving the WCA.
The combination of surface coating with dipping
treatment further increased the WCA and water resistance
of the paper. A WCA near 150° over modified paper
surface was achieved. The morphology of the surface of
the paper coated using modified PCC with different
polymer content can be observed in Fig 26 as a function
of latex content.
227 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
Organic NPs. Organic NPs have also been also
employed to create the required nano-roughness and low
surface energy. The literature available that addresses
cellulosic substrates is very limited, though. Quan et al.
utilized alkyl ketene dimer (AKD) NPs, which were
generated from rapid expansion of supercritical solutions
(RESS) to produce super-hydrophobic cellulose fibers
(Quan et al. 2009). AKD is a common sizing agent used
in the paper industry (Asakura et al. 2006; Lindfors et al.
2007; Lindström, Glad-Nordmark 2007a; b; Lindström,
Larsson 2008; Ohga et al. 2008) and is a crystallizing
wax with a melting point between 40 and 60°C,
depending on the dimer carbon chain length (see the
chemical structure of AKD in Fig 27). AKD acts by
increasing the contact angle of the fibers to levels of
around 110°. The RESS process utilizes the high
solvating power of supercritical carbon dioxide (SC-CO2)
to make fine (nano to micron-sized) particles.
Fig 24. Schematic illustration of the processes involved in the
fabrication of cotton textiles with surface-bound silver particles.
Reprinted from (Khalil-Abad, Yazdanshenas 2010) with
permission from Elsevier.
(a)
(b)
(c)
Fig 25. SEM images of a cotton fiber (a), Ag NP-covered cotton
fiber (b), and Ag NP-covered cotton fiber after OTES
modification (c). Reprinted from (Khalil-Abad, Yazdanshenas
2010) with permission from Elsevier.
Fig 26. Morphology of paper surfaces coated with modified
PCC with different polymer content. Reprinted from (Hu et al.
2009) with permission from Elsevier.
PAPER CHEMISTRY
After dissolving the AKD in SC-CO2, an extremely fast
phase transfer from the supercritical to the gas-like state
takes place during the expansion into atmospheric
condition. Because of the high super-saturation in SCCO2, extremely small particles can be formed in the
RESS process (Quan et al. 2009; Werner et al. 2010).
One advantage of the RESS process is the ability to
produce solvent-free products without the need for
additional solvents or surfactants to induce precipitation.
Since the solvent is a dilute gas after expansion, the
RESS process offers a highly pure final product that
enable a much more convenient way in making superhydrophobic paper or other solid substrates.
Finally, electro-spinning, which is normally used to
produce nanofibers, can be used to spray polymer
solutions onto a substrate to generate micro/nano particles,
as in electrospraying. Electrospinning cellulosic fibers
will be covered in later sections. Sarkar and coworkers
(2010) employed this technique to electro-spray
polyvinylidene fluoride and fluorinated silane molecules
onto cellulosic materials (cotton fabric or paper) with
different concentrations for differential superhydrophobicity and hydrophilicity on each side of the
surfaces. They found that a smaller particle diameter
enhanced super-hydrophobicity when a constant surface
roughness and surface chemical composition were used.
Chemical vapor deposition (CVD). Chemical vapor
deposition (CVD) is a chemical reaction which
transforms gaseous molecules, called precursor, into a
solid material, in the form of thin film or powder, on the
surface of a substrate (Alf et al. 2010; Asatekin et al.
2010). The CVD approach is regarded as a simple and
effective method to deposit super-hydrophobic coatings
onto substrates. For vapor deposition, the reaction
temperature in the chamber is required to be higher than
the boiling point of the precursor. For example, silane
precursors of trichloromethylsilane (TCMS) and
dimethyl-dichlorosilane (DMDCS) require a temperature
higher or close to the boiling point of the precursor, 66°C,
and 68–70°C, respectively (Li et al. 2007; Oh et al.
2011). Cotton fabrics or filter papers were placed in a
sealed chamber for a set time, into which a vapor of
precursor was introduced. The precursor was absorbed
onto the cotton fibers’ surface and penetrated into the
fibers and reaction between the halide and hydroxyl
groups took place. An efficient reaction enhanced the
content of covalently bound silicone to the fibers’
surfaces. After the deposition, condensation of silane
polymers was required. This step was conducted in an
aqueous solution of pyridine (1 M) at room temperature
to hydrolyze the remaining Si–Cl bonds (Li et al. 2007).
Finally, subsequent polymerization of Si–OH was
accomplished in an oven at 150°C for 10 min, which
resulted in a nano-scaled silicone coating tightly attached
to the surface. Therefore, this method can be regarded as
a special sol-gel process in which the precursor attaches
to the surface in gaseous phase rather than in solution.
One disadvantage of this process is that the wet tensile
strength can be reduced, which is caused by the
generation of hydrogen chloride during reaction that
degrades the acid-sensitive cellulose fibers (Oh et al.
2011).
Fig 27.Chemical structure of AKD. R1 and R2 represent straight
carbon chains of lengths varying between 16 and 18 C atoms
(Quan et al. 2009).
Laser beam
(a)
Port with
Quartz window
Target Carousel
Rotating
target
(b)
Laser plume
Substrate
Sample stage
(c)
Fig 28. Schematic of a Pulsed Laser Deposition system
(redrawn from http://www.andor.com/physics/) (a). PTFE
deposited on a cotton fiber by PLD (b) and high magnification
SEM micrograph showing the granular morphology and
nanostructure of the PTFE film (c). Figures (b) and (c) are
reprinted from (Daoud et al. 2006) with permission from
Elsevier.
Laser assisted chemical vapor deposition. Chemical
vapor deposition is the simplest method for deposition
and has the advantage of easy controllability. However,
one drawback of chemical vapor deposition is that the
polymer used has to be readily vaporized. Pulsed Laser
Deposition (PLD) enables the deposition of materials
with high melting point.
PLD is a dry process, very versatile and has several
advantages in the formation of thin films of functional
materials. The schematic of PLD is observed in Fig 28a.
A laser is utilized as energy source to evaporate a target
material that is placed on a carousel. The laser plume
deposits onto the surface of the substrate which is placed
on the sample stage. Through PLD, it is possible to
control the film thickness digitally at the angstrom-scale.
It is also possible to deposit a hetero-structure or multilayer film whose structure differs from layer to layer by
changing the ablation target appropriately. Among
available organic materials, polytetrafluoroethylene
(PTFE) has been the main target in PLD because it
exhibits poor solubility in all solvents and has low
surface adhesion. Note that conventional wet processes
such as spin-coating methods are not applicable for the
formation of thin PTFE films. Daoud et al. (2006) used
PLD to deposit a uniform 50-70 nm layer PTFE on
cellulosic cotton substrates and achieved a WCA of 151°.
The morphology of fiber surface is shown in Fig 28b-c to
illustrate the effect of such nano deposition.
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 228
PAPER CHEMISTRY
Plasma assisted chemical vapor etching and
deposition. Plasma-based technologies are exciting
alternatives for cellulose and paper modification. Plasma
is ionized gas consisting of a mixture of free electrons,
ions and neutral atoms (depending on the degree of
ionization) and is formed when sufficient energy is put
into a gas. Under laboratory conditions, ionized gas is
generated by using high electric fields, known as cold
plasma or electric discharge. Ionized molecules with high
energy can be bombarded on the surface of a given
substrate. Most of the chemistry occurs on the surfaces
within the reactor due to the high energy ion bombardment. High energy ions cause relatively unselective
fragmentation of the surface adsorbed species and a
deposition or etching processes results, depending on the
power and molecules involved (Tendero et al. 2006).
Through plasma-assisted CVD technique, physical
heterogeneities (roughness) of fibers can be minimized by
selective etching of the amorphous cellulose domains in
oxygen plasma (Balu et al. 2009). The low surface energy
of these etched paper surfaces can be obtained by
depositing a thin film of pentafluoroethane (PFE) (Balu et
al. 2008; 2009) or fluorotrimethyl silane (FTMS)
(Navarro et al. 2003; Vilaseca et al. 2005) from the
plasma environment.
Atomic layer deposition (ALD) technique, similar with
CVD, is a chemical process to deposit a thin layer or film
by sequential use of a gas. Al2O3 and SiO2 coatings
deposited on cellulose-base materials have been reported
to function as gas and vapor barriers (Hirvikorpi et al.
2010; 2011a; 2011b; 2011c). Cotton fibers were modified
with ZnO to make them conductive (Jur et al. 2011). In
the ALD process, a metal organic precursor (e.g.
bis(diethylamido) silane, trimethylaluminium and diethyl
zinc) and a reactant (e.g. water) are sequentially exposed
to a surface desired to be modified, resulting in a
complementary sequence of self-limiting reactions. ALD
allows preparation of dense and pinhole-free inorganic
films. ALD is expected to be a promising method to
produce super-hydrophobic surfaces with topography
control.
Roughness offered by regeneration of cellulosic
materials, cellulose nanocrystals and composites
The previous section dealing with chemical grafting,
nanoparticle deposition and chemical vapor deposition,
introduced the roughness required to attain superhydrophobicity, by self-assembled polymer, nanoparticles
etc. In this section, the roughness required for superhydrophobicity is produced by the cellulosic material
itself, i.e., by regeneration or nanofibrillation. These
techniques include electrospinning and electro-spraying
and application of cellulose nanocrystals and aerogels.
Electrospinning. In electrospinning, high voltage,
typically 5-30 kV, is applied between a spinneret(s) and a
collector, and therefore a static electric field is established
between. A so-called Taylor cone forms due to the
competing forces of the static electric field and the
liquid’s surface tension. For liquids with a finite
conductivity, charged droplets are dispersed from the tip
of the Taylor cone and are delivered to the target. If the
liquid consists of a polymer melt or a polymer solution
229 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
and the concentration of that polymer is sufficiently high
to cause molecular chain entanglement, a fiber, rather
than a droplet, is drawn from the tip of the Taylor cone
(Huang et al. 2003; Li, Xia 2004).
Electrospinning of cellulose has been extremely
challenging because of its high crystallinity, which
prevents its dissolution in common solvents.
Manufacturing of electrospun cellulose fibers has been
reported through direct electrospinning of cellulose
solution in N-methylmorpholine-N-oxide (NMMO)
hydrate (Kim et al. 2006), lithium chloride (LiCl)/N, Ndimethyl acetamide (DMAc) (Kim et al. 2005; 2006),
Urea/NaOH solution (Qi et al. 2010) and ionic liquids
(Quan et al. 2010; Viswanathan et al. 2006). However,
cellulose solutions of NMMO or LiCl/DMAc must be
electrospun at elevated temperatures and all three
resulting cellulose fibers require post-treatment with
water in order to completely remove residual solvents.
An alternative approach to the direct electrospinning of
cellulose derivatives solution which is easy to dissolve,
for example cellulose acetate (CA)(Han et al. 2008; Liu
and Hsieh 2002; Son et al. 2004a; b; Vallejos et al.). CA
can be electrospun in a mixture of acetone/water or
methylene chloride/ethanol (Han et al. 2008; Liu, Hsieh
2002; Yoon et al. 2009; Son et al. 2004a; b; Song et al.
2012).
Binary roughness can be fabricated through electrospinning. Micro/nano roughness can be achieved by the
entanglement of electrospun fiber in the mat. In addition,
nano pores can be created during electrospinning due to
the rapid solvent evaporation. Furthermore, the pore
structure can be adjusted by altering the ratio of
methylene chloride/ethanol as reported by Yoon et al.
(Yoon et al. 2009). Fig 29 demonstrates the micro fiber
and nano porous structure as a function of ratio of
methylene
chloride/ethanol.
Super-hydrophobic
electrospun CA mat have been achieved by treating the
fibers with a CF4 plasma (Yoon et al. 2009).
Cellulose composites. Fine solid particles such as silica,
carbon black, and clay when adsorbed at liquid-liquid
interfaces can act as stabilizers for emulsions, foams, and
water droplets replacing surfactants. Emulsions stabilized
with such surface active particles are called “Pickering
emulsions”. Bayer et al. (2009) utilized related
approaches to fabricate nano-rough cellulosic surfaces.
Emulsions were formed by dispersing cyclosiloxanes in
water stabilized by layered silicate particles and were
subsequently modified by blending into a zinc oxide nano
fluid. The polymer matrix was a blend of cellulose nitrate
and fluoroacrylic polymer pre-compatibilized in solution.
Coatings were sprayed cast onto substrates from polymer
blends dispersed in modified Pickering emulsions. As a
result, super-hydrophobic cellulose-based bionanocomposite films were formed without post-treatment.
Cellulose nanocrystals and nanofibrillated cellulose.
Cellulose nanocrystals (CNC) (Rånby 1949; 1951, Rånby,
Ribi 1950,) are prepared by removing the amorphous
domains and keeping the crystalline regions of cellulose
(Beck-Candanedo et al. 2005; Cranston, Gray 2006;
Edgar, Gray 2003; Fu et al. 2012; Habibi et al. 2010;
Moon et al. 2011). Aulin et al. (2009) took advantage of
the nano-size and properties of cellulose nanocrystals to
PAPER CHEMISTRY
Fig 29. SEM images of cellulose acetate fibers electrospun with
different MC/EtOH (v/v) ratios: 80/20 (a), 90/10 (b) and 100/0
(c). Reprinted from (Yoon et al. 2009) with permission from
Elsevier.
produce super-hydrophobic surfaces by assembling
multilayers of poly(ethyleneimine) (PEI) and colloidal
CNCs followed by hydrophobization with (tridecafluoro1,1,2,2,-tetrahydrooctyl-)trichloro-silane
in
heptane
solution.
Micro- or nanofibrillated cellulose (NFC), another form
of nanocellulose (Turbak 1983) can be isolated through
high-shear mechanical disintegration and homogenization
of cellulose fibers (Adbelgawad et al. 2013; Ferrer et al.
2012a; Henriksson et al. 2008; Spence et al. 2010a;
2010b; Zimmermann et al. 2010; Zimmermann et al.
2004). Porous aerogels of NFC were prepared by freezedrying (cf. Fig 30) followed by chemical vapor
deposition with 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (PFOTS) which was used to uniformly coat the
aerogel and to tune their wetting properties towards nonpolar liquids (Jin et al. 2011; Aulin et al. 2010b) or by
functionalizing the native cellulose nanofibrils of the
aerogel with a hydrophobic but oleophilic titanium
dioxide to produce floating, sustainable, reusable, and
recyclable oil absorbents (Korhonen et al. 2011). Recent
work by Mertaniemi et al. reported using air-brush
techniques with solvent-based NFC onto the surface,
followed by quick drying, and a chemical modification
performed either before or after the airbrushing, to
produce super-hydrophobic surfaces (Mertaniemi et al.
2012).
Contact angle hysteresis
superhydrophic surfaces
of
cellulose-based
For self-cleaning applications a low sliding angle or low
contact angle hysteresis is an important requirement.
Therefore, any wetting analysis requires the
determination of both the advancing and receding contact
angles, as was defined in previous sections of this review.
Unfortunately, many reports in the literature only
consider a single, "static" contact angle. A convenient
way to quantify related phenomena is simply performed
by calculating the difference between the advancing and
receding contact angles, i.e., the contact angle hysteresis.
Thus, reducing the water contact angle (WCA)
hysteresis is effective toward reducing water adhesion to
Fig 30. SEM image to demonstrate the porous structure of NFC
aerogel. Reprinted with permission from (Jin et al. 2011) with
permission from American Chemical Society.
the surface and promoting self-cleaning ability (Chen et
al. 1999; Wier et al. 2006). Table 1 includes a
compilation of values of WCA hysteresis reported in the
literature for various cellulose-based substrates. It also
includes there respective static WCA and the material
used to induce roughness on the surface as well as the
respective strategy used to lower its surface energy. A
wide range of contact angle hysteresis values exist even if
a very high static WCA is achieved. In most cases, a very
small contact angle hysteresis (<5°)is observed; however,
in some cases, relatively large WCA hysteresis are noted:
13-20° for cellulose fabrics coated with silica
nanoparticles followed by modification with hexadecyltrimethoxysilane (Xu et al. 2011) or 14-35° if methyltrichlorosilane is used (Shirgholami et al. 2011). Even
higher hysteresis was measured in cases where the
substrate was coated with polymer NPs (>50° or as high
as 112°) (Samyn et al. 2011a; Stanssens et al. 2011). An
interesting case is that of layer-by-layer deposition of
silica NPs on cotton fibers followed by treatment with
fluoroalkylsilane. In such cases the WCA were above
150°, while the hysteresis angle varied with the
deposition layers: for 1 or 3 layers, the fabrics showed
adhesive contact with the water drop and a high contact
angle hysteresis (>45°). Systems with 5 layers or more,
produced slippery super-hydrophobicity, with a contact
angle hysteresis lower than 10° (Zhao et al. 2010). The
greatest hysteresis angle was achieved in the case of
cellulose coated with Ag NPs and treated with octyltriethoxysilane; in this case the water drop also exhibit
adhesive contact with the substrate, even if it was hold
upside down.
WCA hysteresis is affected by the substrate, its
roughness and texture and the coating material as well as
the strategies used to change the surface energy of the
system. However, contact angle hysteresis and adhesive
hydrophobicity is a subject still not fully understood
(Chen et al. 1999; Gao, McCarthy 2006; Wier et al.
2006). To complicate matters, poorly covered cellulose
substrates are often difficult in accurate measurements of
WCA and hysteresis (Xu, Cai 2008; Khalil-Abad,
Yazdanshenas 2010).
Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 230
PAPER CHEMISTRY
Table1. Static water contact angle (SWCA) and WCA hysteresis as reported for different cellulose-based substrates upon roughness
and surface energy modification with different nanoparticles (NPs) and/or surface treatments.
Substrate
Roughness
Surface energy
SWCA
Hysteresis
Cotton
Ag NPs
Octyltriethoxysilane
139-151
did not slide off
Amphiphilic fluorinated triblock azide
copolymers
155
<5
Fluorinated end brushes
170
Polymer
Polymer
Fluorinated end brushes
Stearic acid/ perfluorodecyltrichlorosilane
Hexadecyltrimethoxy-silane
100-120
79-120
Up to 152
Cotton
Paper
Paper
Cotton fabrics
“graft-on-graft’’
architecture
Polymer NPs
Polymer NPs
POSS
cellulose fabrics
Silica NPs
cellulose fabrics
Wood fiber
Silica NPs
Silica NPs
cotton fibers
LbL Silica NPs
Fluoroalkylsilane
Cotton textiles
Porous silica
Methyltrichlorosilane
145-170
Paper
RESS
AKD
Up to 173
Cellulose
Cotton fabric or
electro-spraying
paper
Pulp fiber
TiO2 NPs
Cotton fabrics
Cotton fabrics
Cotton fabrics
ZnO NRs
SiO2 NPs
ZnO NPs
Polyvinylidene fluoride and
fluorinated silane
3-(trimethoxysilyl) propyl
methacrylate
n-dodecyltrimethoxysilane
n-dodecyltrimethoxysilane
n-dodecyltrimethoxysilane
158-170
145-152
Up to 150
>150
Reference
(Khalil-Abad,
Yazdanshenas 2010)
(Li et al. 2010b)
Very small, once
(Nyström et al. 2006)
tilted
82-112
(Samyn et al. 2011a)
50-98
(Stanssens et al. 2011)
4-7
(Gao et al. 2010)
3-5
(Xue et al. 2009)
13-20
(Xu et al. 2011)
<5
(Yang, Deng 2008)
>45 (1-3 layers);
<10 (above 5
(Zhao et al. 2010)
layers)
14-35
(Shirgholami et al. 2011)
Very small, once
(Quan et al. 2009)
it tilted
~160
5-6
(Sarkar et al. 2010)
Up to 154.2
~3
(Huang et al. 2011a)
~161
159
153
9
29
9
(Xu, Cai 2008)
(Xu et al. 2010)
(Xu et al. 2010)
Durability of super-hydrophobic cellulose
Due to the high surface roughness required to endow
materials with super-hydrophobicity, the respective
surfaces are generally easily damaged by scratching or
erosion. In nature, a damaged surface can be repaired by
regenerative processes (Genzer, Efimenko 2006).
However, synthetic super-hydrophobic surfaces, bioinspired or not, are not easily repaired from damage.
Techniques such as wet chemical procedures may be
difficult to apply in order to repair damaged roughness
and surface chemistry. Spray and chemical vapor
deposition may have advantages over wet chemical
procedures as they are easy to apply in order to recover
the required roughness and low surface energy (Basu,
Paranthaman 2009; Verho et al. 2011). Mechanical
durability offered by hierarchical roughness ensures that
stable Cassie state remains even after some surface
features are worn away. Such morphology involves
robust micro-scale bumps that provide protection to a
more fragile nano scale roughness that is superimposed
on the larger pattern (Verho et al. 2011).
A number of applications involve surfaces that are
subject to different degrees to abrasive forces, for
example, outdoor facades or window panes, solar panels,
liquid handling devices used in microfluidics, etc. The
interaction between super-hydrophobic surface and water
is a key factor influencing most of the application
(Zimmermann et al. 2007). Compared with other
synthetic or inorganic superhydrophobic surfaces, superhydrophobic surfaces based on lignocellulose may have
231 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013
Fig 31. The relationship between contact angle and immersion
time at different pH values on super-hydrophobic cotton fabric.
Reprinted from (Li et al. 2008a) with permission from the
American Chemical Society.
some disadvantage as far as the chemical stability due to
its inherent hydrophilicity.
The chemical durability of superhydrophobic cotton was
evaluated by washing and abrasion (Wang et al. 2013;
Zhou et al. 2012). Based on their experiment, the coated
fabric that incorporate fluorinated decyl-polyhedral
oligomeric silsesquioxane (FD-POSS) and fluorinated
alkyl silane into the poly(3,4-ethylenedioxythiophene)
(PEDOT) layer could withstand at least 500 cycles of
standard laundry and 10 000 cycles of abrasion without
apparently changing the super-hydrophobicity. In another
experiment, the PD-POSS was substituted with silica
nanoparticles. The coated fabrics withstood at least
600 cycles of standard laundry and 8000 cycles of
PAPER CHEMISTRY
abrasion without apparently changing the superhydrophobicity.
The chemical durability of super-hydrophobic cotton
fabrics varies with pH of media. Fig 31 demonstrates that
the change of WCA against immersion time in aqueous
solutions with different pH by Li et al. (Li et al. 2008a).
The super-hydrophobic features involved the assembly of
a precursor silanol (potassium methyl siliconate, PMS)
film on the cellulose surface and then polycondensation
of the (PMS) aqueous solution with CO2 at room
temperature. The resultant super-hydrophobic cellulose
showed satisfactory durability in acidic and neutral
condition but less in basic medium.
There is an expanding range of applications of superhydrophobic cellulosic materials as described in sections
3.1 and 3.2. In addition, a number of super-hydrophobic
surfaces which can be produced from cellulosic substrates
have been reported by using different methods. How the
surface chemistry and the topography influence the
stability of super-hydrophobic lignocellulosic surfaces is
still a key issue that deserves further attention. In this
regards, some topics that are to be considered in order to
control the durability of super-hydrophobic behaviors
include the impact on surface energy of end groups,
cross-linking agents, functional group and inorganic
nanoparticle density, as well as the thickness of coated
layers, etc.
Final remarks
Even through know-how in the development of superhydrophobic cellulose-based materials has undergone a
rapid progress and has attracted considerable attention in
recent years, robust and inexpensive methods are still in
their infancy; in addition, a number of questions need to
be addressed which require further research. Firstly, there
is deficient information on the effect of lignin and
heteropolysaccharides in the development of superhydrophobic materials. Lignin and heteropolysaccharides
present in the cell wall of wood fibers make up an
important fraction of their composition. Compared with
the polysaccharide components, lignin has a lower polar
surface energy and a lower dispersive surface energy and
therefore if it is used in the fabrication of superhydrophobic materials, the demand for chemicals with
low surface energy may be reduced. In addition,
environmental pressures and more efficient resource
utilization may stimulate more widespread use of residual
lignocellulose (Ferrer et al. 2012b). Ionic liquids have
been used to dissolve whole wood (Fort et al. 2007; Xie,
Shi 2006). However, the high cost and the difficult
recovery of ionic liquids may hinder its large-scaled
application.
The costs involved in the fabrication of superhydrophobic materials based in cellulose are also an issue.
As a matter of fact, very limited reports exist on
successful large-scale production of super-hydrophobic
materials from cellulose. It is proposed that wellestablished textile and papermaking processing may be
utilized in the production of cellulose-based superhydrophobic materials. In such case, one can envision a
major shift in material base and industry in the bioeconomy of the future.
Acknowledgements
JS is grateful to the National Science Foundation of China
(Grant No.31270613), Research Fund for the Doctoral Program
of Higher Education of China (20103204120005), Talents
Foundation of Nanjing Forestry University (163105003), Open
fund of State Key Laboratory of Pulp and Paper Engineering
(201034), and the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
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Manuscript received October 16, 2012
Accepted April 8, 2013
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