Approaching super-hydrophobicity from cellulosic materials: A Review

PAPER CHEMISTRY

Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013 216

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. Superhydro-

phobicity has stimulated much scientific and industrial

interest because of applications in water repellency, self-

cleaning, friction reduction, antifouling, etc. Despite its

inherent hydrophilicity, cellulose has unparalleled

advantages as a substrate for the production of super-

hydrophobic 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 27695-

8005 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 super-

hydrophobic 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,

PAPER CHEMISTRY

217 Nordic Pulp & Paper Research Journal Vol 28 no. 2/2013

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 super-

hydrophobicity 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, super-

hydrophobic 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.

Wettability and super-hydrophobicity

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 water-

hating, 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.

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:

[1]

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 surper-

hydrophobic surfaces as those mentioned previously

show larger values. How is then such a super-

hydrophobic 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:

[2]

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

cos( ) SV SL

LV

( )cos( ') cos( )SV SL

LV

rr

Liquid

Solid

Liquid θ

γSL γSV

γLV (a)

Solid

(b)

Liquid

Solid

(c)

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 so-

called 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°.

[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:

[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 nano-

droplets 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 self-

cleaning 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 super-

hydrophobicity, 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 super-

hydrophobic 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

cos( ') cos (1 )cos180 cos 1f f f f

1cos( )c

f

r f

PAPER CHEMISTRY


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 nano-

roughness. 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 super-

hydrophobic surfaces and the approaches toward super-

hydrophobicity were introduced. However, what about

the progress toward surperhydrophobic cellulosic

materials? Based on the principles of super-

hydrophobicity 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

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 super-

hydrophobicity 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/ligno-

cellulosics 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 fluorine-

containing 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).

Fluorocarbons Silicones

Sol-gel processing

Etching &lithography

Layer-by-layer & colloidal assembly

Electrochemical reaction / deposition

Electrospinning, phase separation

Rapid expansion of supercritical solution`

Low surface

energy

Surface

roughness

…

Organic materials Inorganic materials

Superhydrophobic surface

PAPER CHEMISTRY


Fig 3. Chemical structure of 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 pentadecafluoro-

octanoyl 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 super-

hydrophobicity 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 super-

hydrophobicity, 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

(2009) used a similar approach to produce super-

hydrophobic 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.

Fig 5. Routes for “graft-on-graft” architectures to produce super-hydrophobic cellulose (adapted from (Nyström et al. 2006) with permission from RSC Publishing).

O

OH

OH

HO OO

OH

OH

OH

HO

OO

OH

O

OH

HOO

OH

OH

HO

HO

Non-reducing end

DP2

-2

Cellobiose Reducing end

O O

C7H15

Cellulose Surface

Cellulose Surface

OH

Cellulose Surface

O O

Br

(ii)

(i)

Cellulose Surface

O O

Br

O

O

O

n

Cellulose Surface

O O

Br

O

O

HO OH

n

Cellulose Surface

O O

Br

O

O

O

n

(iii) (iv) (i)

O

C7H15

O

O

C7H15

Cellulose Surface

O O

Br

O

O

O

nO

O

Brm O

O

O

O

C7H15

O

C7H15

O

O

Br

O

O

O

O

C7H15

O

OC7H15

m

(ii)-(iv),(i)

I

II

III

Cellulose

OH OH OH

R C Cl

O

R C OH

O

Cellulose

O OH O

O=C-R O=C-R

or

PAPER CHEMISTRY


Fig 6. Schematic illustration of the modification of cellulose with organometallic compounds (Fabbri et al. 2004).

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.

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 super-

hydrophobic 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 self-

assembled 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-4-

acryloyloxybenzophenone (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 water-

repellent 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-hexafluoro-

2-(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.

O

OH

OH

HO OO

OH

O Mt

R

R

R

+

OH

HO

O

OH

O

HO OO

OH

O

OH

HO

MtR

R

RH+

O

OH

O

HO OO

OH

O

OH

HO

MtR

R

2R'OH+

O

OH

O

HO OO

OH

O

OH

HO

MtOR'

OR'

+ 2RH

APM/MA(1:8), BPO/DMA

1,4-dioxane, RTO

OS S

H3C

O

SC12H23

113

OO

SH3C

S

OCO

O

CH3

CO

O

N3

113 0.72 0.28 47

SC12H23

1,4-dioxane, RT

HFA, BPO/DMA

OOH3C

S

OCO

O

CH3

CO

O

N3

113 0.72 0.28 47

S

SC12H23

CO

O

F2CCF2

F3C

59

PEG5000-b-P(MA-co-APM)-b-PHFA

PEG5000-b-P(MA-co-APM)

PEG5000-CTA

PAPER CHEMISTRY


Fig 9. Imidization of poly(styrene-maleic anhydride) into poly(styrene-maleimide) in the presence of ammonium hydroxide (Samyn et al. 2011a).

Fig 10. Chemical structures of (a) 2-hydroxy-4-acryloyloxy-benzophenone (HAB) and (b) methacryloxymethyltrimethyl-silane (MSi).

Fig 11. Preparation of P(POSS-MMA-(HFPO)3MA). Reprinted from (Gao et al. 2010) with permission from Elsevier.

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 super-

hydrophobicity 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 self-

aggregation 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 multi-

layer 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), phenyltri-

methoxysilane (Ph-TMS)(Rao et al. 2010), fluoroalkyl-

siloxane (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

HC

H2

CHC

HC

O OO

m nNH3

H2O

HC

H2

CHC

HC

N OO

m n

H

OH

CH2

O

OOH

H3CO Si

CH2

O

CH3

CH3CH3

(a) (b)

(a) (b)

(c) (c)

PAPER CHEMISTRY


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 nano-

scale 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 pre-

cursor (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-decyl-

triethoxysilane. 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 cross-

linker 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 cross-

linked 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.

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)

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 air-

brush 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,ω-

trialkoxysilane-terminated PFPE were demonstrated to

produce super-hydrophobicity on a variety of substrates

with little or no pretreatment (Taurino et al. 2008).

O OO O O

OO O OO O O

Hydrophobic groups

DTMS

TEOS

(a)(b)

(c)

OH

Cellulose

OH OH

OH

Ce

llulo

se

O

O

OCN (CH2)3 Si

OCH2CH3

OCH2CH3

OCH2CH3

HN (CH2)3 Si OCH2CH3

OCH2CH3

OCH2CH3

O

HN (CH2)3 Si OCH2CH3

OCH2CH3

OCH2CH3

O

OH

Ce

llulo

se

O

O

HN (CH2)3 Si O

OH

OH

O

Si

OH

O

(CH2)2(CF2)7CF3

HN (CH2)3 Si O

OH

OH

O

Si

OH

O

(CH2)2(CF2)7CF3

OH

Ce

llulo

se

O

O

HN (CH2)3 Si O

OCH2CH3

O

O

HN (CH2)3 Si O

OO

OH

Ce

llulo

se

O

O

HN (CH2)3 Si O

OH

OH

O

HN (CH2)3 Si O

OH

OH

O

Si

Si

O

O

SiHO O

O

OH

OH

O

O

Si OHO

O

DMFDMTL

Acid

hydr

olysis

water:E

tOH:H

Cl

+

PFDTEOS

Acid hydrolysis

Water:EtOH:HCl

Acid hydrolysis

Water:EtOH:HCl+

TEOS

PAPER CHEMISTRY


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.

Fig 16. Water glass coupled with GPTMS reacting with cellulose. Redrawn from (Shang et al. 2010).

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-trimethoxy-

silane (GPTMS) was used as crosslinker (Shang et al.

2010). In their work, durable super-hydrophobic cellulose

fabrics were prepared from water glass and n-

octadecyltriethoxysilane (ODTES) and 3-glycidyloxy-

propyltrimethoxysilane (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-hydro-

phobic 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-by-

layer (LbL) assembly is based on alternative adsorption

SiO +O Si

O OMe

OMe

OMe

O Si

O OSi

O

Si

OSi

O

HO O

O

O O

OO

O- Na+

O- Na+

H+

OH

Cellu

los e

OH

OH

+ O Si

O OSi

O

Si

OSi

O

HO O

O

O O

OO

OH

Cellu

los e

O O Si

OSi

O

Si

OSi

O

HO O

O

O O

OO

OH

O O Si

OSi

O

Si

OSi

O

HO O

O

O O

OO

OH

I. Hydrolysis and condensation

II. Cross-linking

Water glass GPTMS

(a) (b)

PAPER CHEMISTRY


on charged substrates of oppositely charged poly-

electrolytes, 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 multi-

multilayers 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

10−3

mTorr, 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 water-

repellent 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). Nano-

TiO2 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 super-

hydrophobicity 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

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. Hydro-

phobization 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). MPS-

modified 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 water-

superabsorbent and water-repellent states. Highly porous,

nanocellulose aerogels were here first formed by freeze-

drying 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.

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

(a) (b)

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 post-

treated 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 octyltriethoxy-

silane (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 heptadecafluorodecyl-

trimethoxysilane (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.

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.

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.

(a) (b) (c)

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 SC-

CO2, 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 super-

hydrophobic 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 super-

hydrophobicity 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).

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 multi-

layer 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.

(b) (c)

Laser beam

Port with Quartz window

Sample stage Substrate

Laser plume Rotating

target

Target Carousel

(a)

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 bombard-

ment. 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 super-

hydrophobicity, by self-assembled polymer, nanoparticles

etc. In this section, the roughness required for super-

hydrophobicity 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

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, N-

dimethyl 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 electro-

spinning. 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 bionano-

composite 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.

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.

produce super-hydrophobic surfaces by assembling

multilayers of poly(ethyleneimine) (PEI) and colloidal

CNCs followed by hydrophobization with (tridecafluoro-

1,1,2,2,-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 freeze-

drying (cf. Fig 30) followed by chemical vapor

deposition with 1H,1H,2H,2H-perfluorodecyl-trichloro-

silane (PFOTS) which was used to uniformly coat the

aerogel and to tune their wetting properties towards non-

polar 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 of cellulose-based superhydrophic surfaces

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

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 hexadecyl-

trimethoxysilane (Xu et al. 2011) or 14-35° if methyl-

trichlorosilane 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 octyl-

triethoxysilane; 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).

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 Reference

Cotton Ag NPs Octyltriethoxysilane 139-151 did not slide off (Khalil-Abad,

Yazdanshenas 2010)

Cotton Amphiphilic fluorinated triblock azide

copolymers 155 <5 (Li et al. 2010b)

Cellulose “graft-on-graft’’

architecture Fluorinated end brushes 170

Very small, once tilted

(Nyström et al. 2006)

Paper Polymer NPs Polymer 100-120 82-112 (Samyn et al. 2011a)

Paper Polymer NPs Polymer 79-120 50-98 (Stanssens et al. 2011)

Cotton fabrics POSS Fluorinated end brushes Up to 152 4-7 (Gao et al. 2010)

cellulose fabrics Silica NPs Stearic acid/ perfluorodecyltrichloro-

silane 158-170 3-5 (Xue et al. 2009)

cellulose fabrics Silica NPs Hexadecyltrimethoxy-silane 145-152 13-20 (Xu et al. 2011)

Wood fiber Silica NPs Up to 150 <5 (Yang, Deng 2008)

cotton fibers LbL Silica NPs Fluoroalkylsilane >150

>45 (1-3 layers); <10 (above 5 layers)

(Zhao et al. 2010)

Cotton textiles Porous silica Methyltrichlorosilane 145-170 14-35 (Shirgholami et al. 2011)

Paper RESS AKD Up to 173 Very small, once

it tilted (Quan et al. 2009)

Cotton fabric or paper

electro-spraying Polyvinylidene fluoride and

fluorinated silane ~160 5-6 (Sarkar et al. 2010)

Pulp fiber TiO2 NPs 3-(trimethoxysilyl) propyl

methacrylate Up to 154.2 ~3 (Huang et al. 2011a)

Cotton fabrics ZnO NRs n-dodecyltrimethoxysilane ~161 9 (Xu, Cai 2008)

Cotton fabrics SiO2 NPs n-dodecyltrimethoxysilane 159 29 (Xu et al. 2010)

Cotton fabrics ZnO NPs n-dodecyltrimethoxysilane 153 9 (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, bio-

inspired 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, super-

hydrophobic surfaces based on lignocellulose may have

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 super-

hydrophobicity.

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 super-

hydrophobic 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 super-

hydrophobic 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 super-

hydrophobic 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 super-

hydrophobic 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 super-

hydrophobic 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 well-

established textile and papermaking processing may be

utilized in the production of cellulose-based super-

hydrophobic materials. In such case, one can envision a

major shift in material base and industry in the bio-

economy 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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Approaching super-hydrophobicity from cellulosic materials: A Review