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

Approaching super-hydrophobicity from cellulosicmaterials: A ReviewJunlong Song and Orlando J. Rojas

KEYWODS: Super-hydrophobic surfaces, Cellulose,Roughness, Surface energy

SUMMARY: The development of super-hydrophobicsurfaces has been inspired by natural systems and uses anumber of novel (nano) technologies. Superhydro-

phobicity has stimulated much scientific and industrialinterest because of applications in water repellency, self-cleaning, friction reduction, antifouling, etc. Despite itsinherent hydrophilicity, cellulose has unparalleledadvantages 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 withthe non-renewable materials typically used. This reviewincludes a comprehensive survey of the progress achievedso far in the production of super-hydrophobic materials

based on cellulose. Emphasis is placed on the effects ofcellulose surface chemistry and topography, both ofwhich affect hydrophobicity. Overall, this contributionsummarizes some of the aspects that are critical toadvance this evolving field of science.

ADDRESSES OF THE AUTHORS:

Junlong Song: Jiangsu Provincial Key Lab of Pulp andPaper Science and Technology, Nanjing ForestryUniversity, Nanjing, Jiangsu, P.R. China 210037;Orlando J. Rojas: Departments of Forest Biomaterialsand Chemical and Biomolecular Engineering, NorthCarolina State University, Box 8005, Raleigh, NC 27695-8005 USA; Department of Forest Products Technology,Faculty of Chemistry and Materials Sciences, AaltoUniversity, P.O. Box 16300, Aalto FIN-00076 Finland.

Wettability is a fundamental property of solid surfacesthat plays important roles in household, industrial andadvanced applications. Super-hydrophobic surfaces, i.e.,surfaces with a water contact angle (WCA) larger than150° and a sliding angle (SA) less than 10°, have drawngreat scientific and industrial interest due to their

relevance in water repellency, self-cleaning, frictionreduction 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 assatellite dishes, solar energy panel, photovoltaics, exteriorarchitectural glass, automobile windshields, swimmingcoatings, and surfaces with special heat and mass transfer

properties (Cheng et al. 2006; Ma, Hill 2006; Nakajima etal. 2001). The reduction of friction is of major importancein 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 filminfiltration and to limit surface exposure to corrosivemedia (Barkhudarov et al. 2008; Wang et al. 2011b; Yuanet al. 2011; Zhang et al. 2011). Super-hydrophobic

surfaces have been applied to prevent bacterial adhesion(Crick et al. 2011); to manufacture intelligentmicrofluidic 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 humidatmospheres (Farhadi et al. 2011; Yang et al. 2011; Yin etal. 2010).

Generally, both the surface chemistry and the surfaceroughness affect hydrophobicity (Genzer, Efimenko2006; Nakajima et al. 2001) and the interplay betweenthese properties have been the subject of active researchduring the last decades. Techniques to endow the surfacewith nano-scale roughness, such as etching and

lithography, sol-gel processing and electrospinning have been implemented while the surface chemistry has beenmodified by using strategies involving physical andchemical adsorption. Important progress achieved in thisfield has been reported in several excellent review papers(Dorrer, Rühe 2008; Genzer, Efimenko 2006; Ma, Hill2006; Nakajima et al. 2001).

A distinctive feature is the fact that substrates based onnon-renewable materials, including minerals andsynthetic polymers, are being substituted as platforms todevelop super-hydrophobic surfaces. This is motivated in

part by the recent impetus of sustainability andenvironmental conscience. Therefore, efforts to induce

hydrophobicity in bio-based materials, includinglignocellulose, are growing steadily and are beingreported more often. Here lignocellulose is referred to asmaterials 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 arereadily available, light-weighted, abundant andrenewable.

The know-how available to attain low and ultra-lowenergy surfaces from mineral and polymeric materials hasundergone several major advances. However, the case oflignocelluloses has proved to be more challenging; this isin part because of the influence of hydrophiliccomponents that are present in complex and highlyhierarchical 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 bysurface and bulk modifications. For example, animal glueand so-called “surface” and “internal” sizing use woodrosin, alkenylsuccinic anhydride (ASA) and alkylketenedimers (AKD) (Hubbe 2006), which are commonadditives in papermaking. Nevertheless, surface andinternal sizing fall short to meet the new demands ofsuper-hydrophobic materials. Furthermore, while

processes to modify paper surfaces are widely known,



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

production of super-hydrophobic materials based oncellulose fibers are less developed.

Many developments to obtain super-hydrophobicsurfaces based on cellulose have been reported in recentyears, for example in cotton fabrics and paperboard. Inorder 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 thelatest 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 greaterthan 150°combined with a low roll-off angle based on thedefinition, some developments to enhance the wettabilityclose to that criterion are also addressed.

Wettability and super-hydrophobicity

Definitions

Super-hydrophobic surfaces are abundant in nature, asexemplified 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 ofsuper-hydrophobicity, attempts to understand associated

phenomena have been applied only recently. Scanningelectron microscopy (SEM), transmission electronmicroscopy (TEM) and atomic force microscopy (AFM)are some of the tools that have enabled the observation ofnano-scale features on the surfaces and to unveil some ofthe secrets behind super-hydrophobic development.Bottom line is the fact that the combination of low

surface energy and nano-roughness are effective inattainment of super-hydrophobicity. Based on related

principles, many methods and techniques are now beingdeveloped following bio-mimicry.

Wettability and repellency are important properties ofsolid surfaces from both fundamental and practicalaspects. The wettability of a solid surface is acharacteristic property of materials and depends stronglyon both the surface energy and roughness (Cassie, Baxter1944; Genzer, Efimenko 2006; Koch, Barthlott 2009;

Nakajima et al. 2001; Sun et al. 2005; Wenzel 1936).Some fundamental concepts relevant to wettability andsuper-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 givensolid. (cf. Fig 1) When the probing fluid is water, themeasured contact angle is the “water contact angle”

(WCA). The WCA is specific to any given system and isdetermined by the interactions across the three-phase line.Hydrophilic, water-loving materials are substances withWCA lower than 90° while hydrophobic ones are water-hating, displaying a WCA higher than 90°. As statedearlier, super-hydrophobic characteristics are attainedwhen the water contact angle is greater than 150°.The Young equation is the most important yet thesimplest relationship to describe the balance of forcesacting on a liquid droplet spreading on a surface.

Fig 1. Schematic illustration of Young’s (a), Wenzel (b) andCassie (c) regimes.

The contact angle (θ ) (cf . Fig1a) of a drop is related tothe interfacial energies acting between the solid-liquid(γSL), solid-vapor (γSV ) and liquid-vapor (γ LV ) interfacesand is described by Eq 1:

[1]

There is a limitation in the application of the Youngequation in that it is strictly valid only for surfaces thatare atomically smooth, chemically homogeneous, and donot change by possible interactions with the probingliquid.

According to Young Equation, the highest contact anglewill be achieved under the condition of lowest γSV , i.e.,the material with the lowest possible surface energy. Asa reference, the surface energy of some moieties ofinterest decreases in the following order: -CH2->-CH3->-CF2-CF2H>-CF3 (Genzer, Efimenko 2006). The lowestsurface energy value yet recorded, 6.7 mJ/m2, is obtainedfor a surface with regularly aligned closest-hexagonal

packed -CF3 groups, which leads to a calculated WCA ofabout 120° (assuming water and standard conditions in

Eq 1) (Nishino et al. 1999). Strikingly, natural surper-

hydrophobic surfaces as those mentioned previouslyshow larger values. How is then such a super-hydrophobic behavior possible?

The explanation lies on the contribution of theroughness of surfaces. It is very difficult to find in naturea 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 thecontact 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 thegeometric projected area. Since r is always larger thanunity, the surface roughness enhances hydrophilicity in

cos( ) SV SL

LV

( )cos( ') cos( )SV SL

LV

r r

Liquid

Solid

Liquidθ

γSL γSV

γLV (a)

Solid

(b)

Liquid

Solid

(c)



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the case of hydrophilic surfaces. Similarly, it enhancesthe hydrophobicity of hydrophobic ones. In other words,in the Wenzel regime roughness amplifies bothhydrophilicity and hydrophobicity.

When the surface contains small protrusions, whichcannot 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 anequation describing the contact angle θ’ at a surfacecomposed of solid and air and used the wetted areafraction f , that is, the one in contact with the liquid. Morespecifically, 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 wasexpressed according to Eq 3, which assumes a watercontact 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 theliquid touches just the top of the surface. If partial penetration through surface grooves occurs, a morecomplex 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 alwaysincreases, relative to the behavior observed on a flatsubstrate having an identical chemical composition.Hence surface topography has a very profound effect onmaterial wettability. In this case, roughness works as anamplifier 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 correspondingtransition occurs at a certain critical wetting angle (θ c)defined by:

[4]

where f and r were defined in reference to Eq 2 and Eq 3.Hence at contact angles larger than θ c air pockets are

present beneath the drop, which, in turn, exists in theCassie regime. Quéré and coworkers (Quéré 2004; Quéré,Bico 2003) also pointed out that the Wenzel regime wasthe equilibrium state of the Cassie regime.

A molecular dynamics study (Yang et al. 2006) of liquiddroplets in contact with self-affine fractal surfaces also

provided some evidence that the contact angle for nano-droplets depends strongly on the root-mean-squaresurface roughness amplitude but is nearly independent ofthe fractal dimension D f of the surface.

The second requirement for super-hydrophobicity(water must not stick to the surface) is closely related tothe contact angle hysteresis Δθ of the surface— thedifference between the largest (advancing) and smallest(receding) stable contact angle θ adv − θ rec. The contactangle hysteresis is extremely important in the self-cleaning applications. The maximum lateral force F lat thata distorted droplet can buildup depends on θ adv and θ rec as

Eq 5 (Furmidge 1962): F lat ∝ cosθ rec− cosθ adv [5]

which can be approximated for small hysteresis as F lat ∝

Δθsinθ .It has to be noted that super-hydrophobic surfaces can

be wetted by organic liquids easily. Super-oleophobicsurfaces denote those surfaces with a contact anglegreater than 150 degrees with organic liquids that have

appreciably lower surface tensions than that of water(Tuteja et al. 2007). Compared with super-hydrophobicsurfaces, super-oleophobic surfaces are much lessdiscussed in the literature; recently, however, they havereceived increased attention since they have a greatimpact on a wide range of phenomena, including

biofouling, loss of self-cleaning ability, and swelling ofelastomeric materials. Calculations suggest that creatingsuch a surface would require a surface energy lower thanthat of any known material. In this regard, Tuteja et al.(2007) proposed a third factor, the re-entrant surfacecurvature, which combined with chemical compositionand roughness, can be used to design surfaces that display

extreme resistance to wetting from a number of liquidswith low surface tension, including alkanes, such asdecane and octane. Super-oleophobic surfaces have beenachieved by using fluoro-decylpolyhedral oligomericsilsesquioxanes (POSS) coatings, which display contactangles greater than 160° for various non-polar fluids(Meuler et al. 2011).

Generic methods toward super-hydrophobicityAccording to the previous discussion on super-hydrophobicity, there is only one way to access this

property: to exploit the combined effects of surfacechemistry and morphology of a give surface. Materials

with low surface energy include fluorocarbons, silicones,and some organic and inorganic materials. Among lowsurface energy materials, fluorocarbons are the mostfrequently used. They have also attracted industrialinterest. Silicones (Artus et al. 2006; Dou et al. 2006;Guo et al. 2006; Jin et al. 2005; Khorasani, Mirzadeh2004; 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 energymaterials. Although fluorocarbons show the lowestsurface energy known, it is interesting to note that theyare rarely present in the organic materials produced by

nature.The fluorine/carbon atomic ratio is a key aspect todefine hydrophobicity, as has been demonstrated byHsieh et al. from studies with carbon nanofiber arrays(Hsieh et al. 2006). As expected, as more F atoms areintroduced, lower surface energy and higher contact angleare achieved. The packing density of related moieties isanother factor influencing the surface energy and itsstability. Genzer and Efimenko (2000) demonstratedelegantly this concept after creating long-lived super-hydrophobic polymer surfaces through mechanicallyassembly monolayer. F(CF2)y(CH2)xSiCl3 molecules weredeposited from the vapor phase on a stretched elastomeric

poly(dimethyl siloxane) (PDMS) substrate; afterdeposition the strain was released making the film toreturn to its original size, causing the grafted

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

1cos( )c

f

r f



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Fig 2. Strategies used to attain super-hydrophobicity bycombining low surface energy coatings and surface roughness.

F(CF2)y(CH2)xSiCl3 molecules to form a denselyorganized mechanically assembled monolayer. The watercontact angle, compared to the un-stretched surface wasfound 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 differentmeasuring techniques and sampling sizes, as they are purely statistically determined. However, they cannot bedirectly extrapolated over different sampling areas as theyrepresent transitions at the nano-, micro-to-nano andmicro-scale levels. Therefore, the spatial roughness

parameters including the correlation length and thespecific frequency bandwidth should be taken intoaccount, which allow for direct correlation of roughnessdata at different sampling sizes (Samyn et al. 2011b). Nano roughness can be created through etching and

lithography, sol-gel processing, layer-by-layer andcolloidal assembly, electrochemical reaction and

deposition, electrospinning and phase separation. Theseapproaches were addressed in detail in a review article byMa and Hill (2006). In addition, it can be noted that rapidexpansion of supercritical solution as reported by Werneret al. (Quan et al. 2009; Werner et al. 2010), utilizes thehigh solvating power of supercritical carbon dioxide to

produce micro or nano-particles that can render thesurface with nano-roughness. It is worth noting, however,that these approaches have been usually applied to hardsurfaces and the question is then if they can be adapted tocellulosic materials. In sum, Fig 2 indicates the methodsavailable to attain super-hydrophobic surfaces by usingtwo combined strategies, one related to changes in

surface energy and the other one related to changes insurface roughness.

Recent progress to approach super-hydrophobiccellulosic materialsIn the previous section, the general principles of super-hydrophobic surfaces and the approaches toward super-hydrophobicity were introduced. However, what aboutthe progress toward surperhydrophobic cellulosicmaterials? Based on the principles of super-hydrophobicity it is now clear that the effects of lowsurface energy and nano-roughness topography arerequired. In general, surface energy reduction and

roughness creation can be accomplished simultaneouslyor in sequence.

Compared with efforts to create nano-roughness,surface energy reduction is much simpler and

straightforward. Chemicals with low surface energy, suchas fluorine- and silicone- containing polymers as well ashydrocarbons can be applied to cellulose by grafting,adsorption, chemical vapor deposition, etc. On the otherhand, the creation of nano-roughness may be the moredemanding step toward super-hydrophobic cellulose.

Thus, surface energy reduction is usually addressedsomewhere during the application of techniques to endowthe materials with roughness.

The approaches used in recent years toward super-hydrophobicity of cellulosic materials can be classifiedinto two categories, based on the generation of roughness:(1) Roughness offered by coating cellulosic substrates,which include:

a) Chemical grafting to modify the surface chemistryand surface morphology of cellulosic fiber/surfacesimultaneously.

b) Sol-gel processes to render cellulose fiber/surfacewith porous outer-layer and to reduce surface

energy by post-treatment or by mixing precursorswith low surface energy side chains.c) Nanoparticle deposition, for example by using

metal, metal oxide, mineral and polymers thatmodify the morphology of the cellulosicfiber/surface, followed by surface energyreduction by post-treatment.

d) Chemical vapor deposition.

(2) Roughness offered by regeneration or fragmentationof cellulosic materials, which among others include:

e) Electrospinning and elecrosprayingf) 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 repeatingunits 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 cellulosewhile the secondary wall contains cellulose with variableamounts of lignin.

The hydroxyl groups of cellulose can be partially orfully reacted with various species to provide cellulosederivatives with useful properties. This provides anopportunity to modify the surface of cellulose/ligno-cellulosics by chemical reactions. Through derivatizationreactions, polymers with special structures and functionscan be introduced to modify the surface and attain thedesired properties. This has been the subject of intensestudies and a review paper which addresses the subject indetail 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), siliconeand hydrocarbon polymers (Garoff, Zauscher 2002;Pasquini et al. 2006) can be easily introduced byesterification with acid or anhydride (cf. Fig 4).

Fluorocarbon Silicone

Sol-gel

Etching &li thography

Layer-by-layer & colloidal assembly

Electrochemical reaction / deposition

Electrospinning, phase separation

Rapid expansion of supercritical

Low surfaceenergy

Surfaceroughness

…

Organic

Inorganic

Superhydrophobicsurface



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Fig 3. Chemical structure of cellulose.

Fig 4.Scheme for esterification reactions of cellulose. R is along fluorine-containing or hydrocarbon chain.

Nyström et al. reported on a method to improve thesurface 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 firstroute (route I in Fig 5) involved reaction of the hydroxylgroups on the cellulose substrate with pentadecafluoro-octanoyl chloride. The coverage of fluorinate groups wasobserved to be limited and so the exact water contactangle was difficult to measure as it changed with contacttime. In fact, the surface first showed super-hydrophobicity with WCA 150°; but after 20 min thecontact angle decreases to 128° and below 90° after 50min. This indicates that the cellulose surface was notfully covered by the fluorinated material. In order toimprove the surface-coverage and thereby thehydrophobicity, a functional monomer was grafted fromthe cellulose substrate to enable attachment of anincreased amount of fluorine groups. In route II ( Fig 5),

two fluorine groups were grafted to the surface and thecontact angle increased to 154° and somewhat moreirreversibly. To further improve the super-hydrophobicity, a branched-like structure was used toattach four fluorine groups per hydroxyl group oncellulose (route III in Fig 5). This step improved theWCA to extremely high values, up to 170°. Nyström et al

(2009) used a similar approach to produce super-hydrophobic and self-cleaning cellulose surfaces viasurface-confined grafting of glycidyl methacrylate byusing 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 ortwo metals bound to carbon) reacting with OH groups ofcellulose (cf. Fig 6 ). Unreacted metal-to-carbon bondswere exploited to graft numerous molecular structuresderived from alcohols and amines.

A polymer containing an azide group was employed byLi et al. (2010a) to covalently attach amphiphilic triblockcopolymers to cotton via UV irradiation. Amphiphilictriblock azide copolymers containing poly(ethyleneglycol) (PEG) and poly(2,2,3,4,4,4-hexafluorobutylacrylate) blocks were synthesized through roomtemperature reversible addition fragmentation chain

transfer polymerization using redox initiation (cf. Fig 7 ).These polymers were successfully applied to fabricatesuper-hydrophobic cotton fabrics by using a facile UVirradiation approach.

The introduction of fluorinated polymer chains endowedcotton with a WCA of 155°. Since the fluorinated

polymer chains were covalently attached on the surface,the super-hydrophobic cotton fabric possessed highstability and chemical durability.

It is reasonable to expect for the hydrophilic PEG blocks to locate in the interface between the blockcopolymer coating and the cotton fibers, which wassuggested to occur via H-bonds between PEG and the

hydroxyl groups of cellulose; also, it is expected that thefluorinated blocks aggregate in the outermost side of thecoating, which contribute to the super-hydrophobicity ofthe modified substrate.

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

Br m 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 OO=C-R O=C-R

or



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

Fig 7. Synthesis of PEG5000-b-p(MA-co-APM)-b-PHFA whichwere 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 AFMimages (c and d). Reprinted from (Li et al. 2010a) withpermission from Elsevier.

The surface morphologies of the original and themodified cotton fabrics were investigated by SEM andAFM. Fig 8a and 8b shows highly textured, microscalefibers on the cotton fabric while the modified substratereveals a coating layer consisting of the copolymers withsome embossments caused by the aggregation of the

fluorinated polymer blocks, Fig 8d . In addition, SEM topview 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 cellulosicmaterials covalent and hydrogen bonding were suggested.Samyn and coworkers (Samyn et al. 2011a; Stanssens etal. 2011) synthesized a block polymer with amide groups( Fig 9). The amide blocks exhibited good adhesion to thecellulosic fibers, which was proposed to be the result ofhydrogen bonding between the electronegative nitrogenin amide and the hydrogen of the hydroxyl groups incellulose. The hydrophobic polystyrene blocks self-

assembled outside of the fibers, rendering the systemwith 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 ofthree main steps: Firstly, an admicelle in the form of asurfactant bilayer is formed on the substrate’s surface

followed by the addition of an organic monomer that preferentially diffuses by its hydrophobicity into thehydrophobic core of the admicelle, a process calledadsolubilization (Tragoonwichian et al. 2009). Finally,

polymerization of the monomer in the admicelle isinitiated through the addition of a suitable initiator such

as ammonium persulfate. After the completion of thereaction, the weakly bound surfactants are washed away,leaving a thin polymer film coating on the surface. Thismethod was employed by Tragoonwichian and coworkersto 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. Atlast, methacryloxymethyltrimethyl-silane (MSi) was usedto initiate the polymerization to form a thin layer. Thestructure of HAB and MSi is illustrated in Fig 10. In thiswork admicellar polymerization produced a bifunctionalcotton fabric possessing both UV-protection and water-

repellent property due to the UV adsorption of HBA andhydrophobicity of silane block. The resultant fabrics werefound suitable for outdoor use where both UV-protectionand water repellency are required, for example in coatingmaterials and protective clothing.

Polyhedral oligomeric silsesquioxane (POSS) consistingof an inorganic silicon-oxygen cage with generalcomposition SiO3/2 and organic substituents R cappingthe silicon atoms of the cage, were incorporated withfluorine-containing organic polymers to improve thermaland 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) tothen coat cotton fabrics. The coated fibers possessedexcellent water and oil repellency.

O

OH

OH

HO OO

OH

O Mt

R

R

R

+

OH

HO

O

OH

O

HO OO

OHO

OH

HO

Mt

R

R

RH+

O

OH

O

HO OO

OH

O

OH

HO

Mt

R

R

2R'OH+

O

OH

O

HO OO

OH

O

OH

HO

Mt

OR'

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



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Fig 9. Imidization of poly(styrene-maleic anhydride) intopoly(styrene-maleimide) in the presence of ammoniumhydroxide (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). Reprintedfrom (Gao et al. 2010) with permission from Elsevier.

Fig 12. FE-SEM images of POSS coated cotton fabricssurfaces. Insets are photos of liquid droplets on thecorresponding samples: cotton fabric coated with P1 (a), cottonfabric coated with P2 (b), cotton fabric coated with P3 (c),cotton fabric coated with P4 (d). P1~P4 are terpolymers withincreasing amount of POSS content. Reprinted from (Gao et al.2010) with permission from Elsevier.

The WCA was observed to be a function of the POSScontent. The contact angles were increased as the contentof POSS in the terpolymer increased from 6.4 to13.4 wt% from 140 to 152° and from 127 to 144°, in thecase of water and natural oil, respectively. The super-hydrophobicity achieved by this POSS-based terpolymersmay be due to the self-aggregation of the polymer chains

grafted on the POSS cage. FE-SEM images of POSS andcoated cotton fabrics surfaces are shown in Fig 12. It isclearly shown that with the increase of POSS (asindicated by P1, P2, P3 and P4 for increased POSS

content), the nano-roughness generated by self-aggregation of the POSS-based terpolymers increasedmarkedly. The insets provided in Fig 12 are images ofliquid droplets on the corresponding samples.

In other efforts, electrostatic and van der Waals forceswere utilized to adsorb polyelectrolytes on the surface of

fibers. Lingström et al. (2007) treated individual woodfibers with polyelectrolyte multilayers consisting of twodifferent 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 treatedfibers. In this case, however, the treated fibers were notstrictly superhydrophobic. This layer-by-layer (LbL)deposition method has been used by several other groupsto create super-hydrophobic surfaces based on substratesdifferent 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 ofmaterials (typically metal oxides) starting from acolloidal solution (sol) that acts as the precursor of anintegrated network (gel) of either discrete particles ornetwork polymers. Sol-gel processes have been used to

produce super-hydrophobic surfaces from a variety ofalkylsilane precursors, for example: tetraethoxysilane ortetraethyl orthosilicate (TEOS) (Ding et al. 2006; Huanget 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 etal. 2011), methyltrichlorosilane (MTCS) (Shirgholami etal. 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; Shanget 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 – gelreactions of alkylsilane can be described by the followingsequence (Bae et al. 2009)):

(1) alkylsilane hydrolysis in acid or basic conditions:Si-(OC2H5)4 + 4H2O→ Si-(OH)4 + 4C2H5OH

(2) Alcohol condensationSi-(OH)4+Si-(OC2H5)→≡Si-O-Si≡ + 4C2H5OH

(3) Water condensationSi-(OH)4 + Si-(OH)4→≡Si-O-Si≡ + 4H2O

Alkylsilane initially hydrolyze in acid or basicconditions to form silicic acid and then the formed silicicacid condensates with another alkylsilane or with itself toform a cross-linked polysiloxane network; thus, a gel isformed. The hydroxyl groups of polysiloxane can reactwith the hydroxyl group over the surface of a fiber and

therefore the gel actually can be covalently attached tothe fiber support.In a similar way as that for TEOS, water glass and

potassium methyl siliconate can hydrolyze in the

H

C

H2

C

H

C

H

C

O OO

m n

NH3

H2O

H

C

H2

C

H

C

H

C

N OO

m n

H

OH

CH2

O

OOH

H3C O Si

CH2

O

CH3

CH3CH3

(a) (b)

(a) (b)

(c) (c)



PAPER CHEMISTRY


presence of acid to produce silicic acid by the followingreactions:

Na2SiO3 + H2O + 2H+→Si-(OH)4 + 2Na+

CH3-SiO3K+ H2O + H+→CH3-Si-(OH)3 + K +

The condensation reactions are the same as thosediscussed before in the case of TEOS. Some sol-gel

precursors bear one or more organic chains, such asmethyl, ethyl, phenyl and fluoroalkyl. These chemicalsnot only generate gel porous structures to render a nano-scale roughness but also introduce the respective lowenergy groups. In some reports two or more precursorsare mixed to create roughness and reduce the surfaceenergy via simple dip coating process (Rao et al. 2006;2009; 2010; Xu et al. 2011). Post-treatment with longalkyl chains silanes is also reported. For example, Dingand coworkers (Ding et al. 2006; Miyauchi et al. 2006)modified electrospun hydrophilic cellulose acetate with asimple sol-gel coating of dodecyl(trimethoxy)silane(DTMS) and TEOS, as shown in Fig 13. Thus, sol-gelfilms were formed on the rough fibrous mats afterimmersion in the sol-gel. Through post-treatment ofsilane with low energy chains, a better surfacehydrophobicity was obtained since the self-assembly oflong hydrophobic chains on the surface formedaggregates that enhanced the nano-scale roughness. Thissol-gel process can also be accomplished in a gas-solidreaction (Cunha et al. 2010b).

Cunha et al. (2010a) described a reactive precursor forsol-gel processes which contain a cyanate group. A pre-cursor (3-isocyanatopropyl) triethoxysilane (ICPTEOS)reacted heterogeneously with the hydroxyl groups ofcellulose in a DMF solvent. The reactions was followed

by acid hydrolysis (and condensation) of the appendedsiloxane moieties as such, and in the presence of eithertetraethoxysilane 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 acidand monoacids, polycarboxylic acids act not only ascatalyst in the preparation of the sol, but also as cross-linker between cellulose and the silica gel. It wasobserved that active hydrogen in citric acid remainedafter the hydrolysis reaction of water glass. Thisindicated the residual carboxylic acid groups in citric acidcould continually react with hydroxyl groups of cellulosefibers to form ester bonds with sodium hypophosphite asthe catalyst. The residual carboxylic acid groups couldalso react with Si – OH. The formed silica gel can beregarded as a structure with a polymeric network cross-linked by citric acid groups. As a result, the compatibilityand adhesion between the fiber surface and the silica solas well as the hydrophobic durability are improved. It can

be noted that residual hydroxyl groups in polysiloxaneare 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 aproposed schematic diagram of sol –gel films formed on thesubstrate (cellulose acetate)(c)

Fig 14. Scheme of the chemical modification of the cellulosefibers with (3-isocyanatopropyl) triethoxysilane (ICPTEOS) andthe 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 anair-brush applicator to spray sol-gel solutions on givensubstrates. This method has the advantage of beingsimple and less expensive, compared to otherconventional techniques. The air-brushing apparatusconsists of a bottle with the sol-gel solution and an air-

brush applicator ( Fig 15). The morphology and surfaceenergy of the coated surfaces can be adjusted by the air

pressure and the precursors in the container. Sol-gelsolution of TEOS, tetraethyl orthotitanate (TEOT), andtetra-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

C e l l u l

o s e

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

C e l l u l o

s e

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

C e l l u l o

s e

O

O

HN (CH2)3 Si O

OCH2CH3

O

O

HN (CH2)3 Si O

OO

OH

C e l l u l o

s e

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

A c i d

h y d r o l y s i s

w a t e r : E

t O H : H C l

+

P F D T

E O S

Acid h ydrolysis

Water:EtOH:HCl

A c i d h y d r o l y s i s

W a t e r : E t O H : H C l +

T E O S



PAPER CHEMISTRY


Fig 15.Air-brushing apparatus for application of sol-gelsolution of TEOS: substrate (a), sol –gel solution (b), air-brushapplicator (c). Reprinted from (Taurino et al. 2008) withpermission from Elsevier.

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

Fig 17. Scheme showing a cotton fabric coated by the silicahydrosol (a) and by silica hydrosol followed by HDTMSmodification (b). Reprinted from (Xu et al. 2011) with

permission from Elsevier.

Fig 18. Cotton fabrics without treatment coated with silicananoparticles (a) (reprinted from (Bae et al. 2009) withpermission 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 coatingand cellulose fibers, 3-glycidyloxypropyl-trimethoxy-silane (GPTMS) was used as crosslinker (Shang et al.2010). In their work, durable super-hydrophobic cellulosefabrics were prepared from water glass and n-octadecyltriethoxysilane (ODTES) and 3-glycidyloxy-

propyltrimethoxysilane (GPTMS) was used ascrosslinker in the sol-gel method (cf. Fig 16 ). The resultshowed that the addition of GPTMS resulted in a betterfixation 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 ofcellulosic substrates to produce roughness in themicro/nano scale. By using this method, post-treatmentwith fluorine-containing compounds (Xu et al. 2011;Yang, Deng 2008; Yu et al. 2007), silicones andfunctionalized organic materials can be used to reducethe 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 thesol-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 controlthe size of nanoparticles.

Silica NPs can attach onto cotton fibers by simple dipcoating (Zhou et al. 2012), by spraying alcoholsuspensions of SiO2 nanoparticles (Ogihara et al. 2012)or by heat treatment (Xu et al. 2011), resulting in theformation of a chemical bonds between hydroxyl groupsof cellulose and silica. Thus, covalently attach tocellulose is produced without the requirement of binders.The surface of cellulose fiber carries negative charges in

aqueous solution, and the NPs prepared are usuallynegatively charged as well. Therefore, the coverage ofsilica NPs on cotton fibers without treatment is quite poor(Bae et al. 2009; Yu et al. 2007), as shown in Fig 18a. Inorder to increase the coverage and also to enhance theroughness created by NPs, multilayers of silica NPsdeposited 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 solutioncontaining 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

C e

l l u l o

s e

OH

OH

+ O SiO O

Si

OSi

OSi

O

HO O

O

O O

OO

OH

C e

l l u l o

s 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 oreven supramolecular systems. It is a versatile techniqueto build up multilayered composite films in a controlledmanner (Aulin et al. 2010a; Eita et al. 2012; Karabulut etal. 2012; Karabulut, Wågberg 2011; Tanaka, Ödberg

1992). Charged fibers are suitable as solid supports toconduct LbL since the assembly is independent of thesize and topography of the substrates and uniform multi-multilayers can be formed with different spatialstructures. Fig 19 includes smooth surfaces and cottonfibers and the changes in the nano topography afterapplication of coating layers.

Xue et al. (2009) developed another LbL protocol bymodifying the surface of silica NPs with amino- andepoxy- functional groups, respectively. Alternate coatingof amino- and epoxy-functionalized silica nanoparticleson epoxy-functionalized cotton textiles were used togenerate a dual-size surface roughness, followed by

hydrophobization (schematically illustrated in Fig 20) to produce super-hydrophobic surfaces. The static watercontact 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 ofZnO was obtained by solution-based (Xu, Cai 2008) andmagnetron sputtering (Xu, Cai 2008; Lim et al. 2010)methods. In the solution method, zinc salt was dissolvedin ethanol under vigorous stirring. Then a solution ofhydroxide (0.15 M) in ethanol was slowly added andstirred. ZnO gradually deposited on the fibers and thusZnO nanocrystals grew slowly on their surfaces. In the

magnetron sputtering method, the substrate was treated by radio frequency magnetron sputtering (base pressure10−3mTorr, Argon plasma, rf power 100 W, working

pressure 10 mTorr, deposition rate 12 nm/min). ZnO NRswere fabricated via a hydrothermal route with a growthsolution containing an equimolar aqueous mixture of 25mM 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 ofthe growth solution contained in a glass bottle sealedwith 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 muchhigher aspect ratio. Therefore, cellulose treated with ZnO

NRs can yield lower roll-off angles and better water-repellent properties compared to SiO2 NPs of similardiameter. This is due to the discontinuous three-phasecontact line of NRs (Xu, Cai 2008).

TiO2 NPs. TiO2 is widely used in paper industry to produce better paper opacity (Farrokhpay 2009; Marqueset al. 2006; Nelson, Deng 2008; Park et al. 2007). Nano-TiO2 can improve significantly the brightness and opacityof paper materials because of its high index of refractioncompared to cellulose fibers or other fillers. TiO2 NPswere 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. Forexample, Xue et al. used a solution (Solution A) prepared

by slow addition of 8.5 ml of tetrabutyltitanate into 20 mlof anhydrous ethanol under continuous stirring.

A second solution, Solution B, was prepared by mixing1.5 ml of deionized water, 5 ml of acetic acid, and 20 mlof anhydrous ethanol together with stirring. Upon slowaddition (1 h) of Solution A into Solution B the reaction

mixture was stirred for 24 h at room temperature and as aresult, a transparent bright yellowish TiO2 sol wasobtained (Xue et al. 2008). The as-prepared TiO2 wasapplied to cellulose to increase its roughness. Hydro-

phobization was needed in order to lower the surfaceenergy of the TiO2-coated substrate.

Due to the extremely large surface area-to-particle sizeratio, nanosized-TiO2 pigments were poorly retainedwhen mechanically mixed with fibers. Thus, Huang andcoworkers (Huang et al. 2011a) reported a type of paperwith super-hydrophobic and photocatalytic propertiesafter modification with TiO2 NPs and a silane molecule,3-(trimethoxysilyl)propyl methacrylate (MPS). MPS-

modified nano-TiO2 particles were added to the fibersuspension before the process of beating to inhibitagglomeration of TiO2 on the surface of the fibers.

Kettunen et al. (2011) used a chemical vapor depositionmethod to coat a thin TiO2 film on lightweight nativenanocellulose aerogels to offer a novel type of functionalmaterial 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 thenanocellulose aerogels were exposed to titaniumisopropoxide vapor at 190°C at pressures of 1 – 5 kPa for

2 h. Well-defined, nearly conformal TiO2 coatings withthicknesses of ca. 7 nm were obtained.

Fig 19. SEM images of untreated cotton fabric (a and b) andcotton 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) withpermission from Elsevier.

Fig 21. SEM images of ZnO nanorods-coated cotton fibers (c). A high magnification image of ZnO nanorods on the cotton fiberis 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 depositTiO2/poly(acrylic acid) (PAA) on fiber surfaces in orderto 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 chainsilane molecules (Li et al. 2010b).

Liquid flame spray (LFS) coating is a technique whichgenerates NPs and can be used to simultaneously coatsubstrates (Stepien et al. 2011; Teisala et al. 2010). ALFS setup is schematically illustrated in Fig 22.

In the LFS process the precursor fluids diluted in wateror alcohol are fed together with the combustion gases intoa specially designed spray gun. Instantly after exiting the

burner nozzle the precursor solution is atomized tomicron-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 toformation of NPs of desired material. Afterwards the NPsgrow larger in the flame by condensation, coagulation,coalescence and agglomeration. The final particle sizecan be controlled, e.g. via total mass flow rate of the

precursor (product of the precursor solution feed rate andits concentration) or by adjusting the collecting ordepositing distance. The main exhaust gases formed as

by-products in the LFS process are normally water vaporand carbon dioxide. Low waste flows combined withrelatively simple and inexpensive equipment make theLFS process comparatively cheap and environmentallyfriendly.

The major advantage of such process is the broadspectrum of metal or metal oxide NPs that can be createdusing different liquid precursors. For example, Stepien etal (2011) dissolved titanium (IV) isopropoxide (TTIP) or

Fig 22. Schematic image of the liquid flame spray set-up forcoating paperboard. Hydrogen is used as a nebulizing gas.Reprinted from (Stepien et al. 2011) with permission fromElsevier.

Fig 23. SEM images of samples coated with TiOx (a) and SiOxNPs (b). Reprinted from (Stepien et al. 2011) with permissionfrom 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 paperboardwere shown in Fig 23.

Ag NPs. Silver and silver ions have been reported to beeffective against nearly 650 trends of bacteria (Dubas etal. 2006) due to their ability to attach to the their cellmembranes, penetrate and attack the respiratory chain ofthe cell, hence leading to the bacteria’s death (Morones etal. 2005; Rai et al. 2009). Silver NPs (Ag NPs) appear toexhibit enhanced antibacterial activity due to theirincreased surface area, which provides larger contact

possibilities with microorganisms per surface unit. Inaddition, Ag NPs are also known to release silver ions

thereby enhancing their bactericidal activity (Morones etal. 2005; Rai et al. 2009). Ag NPs have been incorporatedonto various polymeric fibers and the resultant fibrouscomposites 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 todeposit Ag NPs onto polymeric fibers. The first methodconsists of preparing colloidal Ag NPs by reducing Agsalts, 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 solutionhence directly depositing the NPs onto the fiber’s surface

through electrostatic interactions (Dong, Hinestroza2009), hydrogen bonding (Dong et al. 2008), or through alayer-by-layer deposition (Dubas et al. 2006). Analternative route is an in situ method, where Ag ions areinitially adsorbed onto fiber surfaces and then post-treated with UV radiation (Chen, Chiang 2008), heattreatments (Ifuku et al. 2009), chemical reduction (Dong,Hinestroza 2009), ultrasound (Perelshtein et al. 2008) ormicrowave (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 preparesuper-hydrophobic cellulose (Khalil-Abad, Yazdanshenas2010). For example, Ag NPs were produced on cottonfibers by treatment with aqueous KOH and AgNO3,followed by reduction treatment with ascorbic acid in the

presence of a polymeric steric stabilizer, generating adual-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 whileSEM 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 a10 μl droplet of water that did not slid off even when thesample was held upside down. The modified cotton wasfound to have potent antibacterial activity toward bothGram-positive and Gram-negative bacteria.

CaCO3 NPs. CaCO3 is a mineral which is widely usedin the paper industry. CaCO3 NPs can be prepared by thereaction of sodium carbonate with calcium chloride in the

presence of oleic acid and heptadecafluorodecyl-trimethoxysilane (Wang et al. 2010) or obtained fromcommercial 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 toincrease the adhesion of CaCO3 NPs to cellulose fibers.Commercial PCC, hydrophobic stearic acid and polymerlatex particles were used for surface roughness control,surface hydrophobic modification agent, and polymer

binder, respectively. A simple coating via dipping wasused to produce papers with high WCA and waterresistance. It was found that the surface pretreatment ofPCC with fatty acid salt prior mixing with polymer

binder played an important role for improving the WCA.The combination of surface coating with dippingtreatment further increased the WCA and water resistanceof the paper. A WCA near 150° over modified paper

surface was achieved. The morphology of the surface ofthe paper coated using modified PCC with different

polymer content can be observed in Fig 26 as a functionof latex content.

Organic NPs. Organic NPs have also been alsoemployed to create the required nano-roughness and lowsurface energy. The literature available that addressescellulosic substrates is very limited, though. Quan et al.utilized alkyl ketene dimer (AKD) NPs, which weregenerated from rapid expansion of supercritical solutions

(RESS) to produce super-hydrophobic cellulose fibers(Quan et al. 2009). AKD is a common sizing agent usedin 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 crystallizingwax with a melting point between 40 and 60°C,depending on the dimer carbon chain length (see thechemical structure of AKD in Fig 27 ). AKD acts byincreasing the contact angle of the fibers to levels ofaround 110°. The RESS process utilizes the highsolvating power of supercritical carbon dioxide (SC-CO2)to make fine (nano to micron-sized) particles.

Fig 24. Schematic illustration of the processes involved in thefabrication of cotton textiles with surface-bound silver particles.Reprinted from (Khalil-Abad, Yazdanshenas 2010) withpermission from Elsevier.

Fig 25. SEM images of a cotton fiber (a), Ag NP-covered cottonfiber (b), and Ag NP-covered cotton fiber after OTESmodification (c). Reprinted from (Khalil-Abad, Yazdanshenas2010) with permission from Elsevier.

Fig 26. Morphology of paper surfaces coated with modifiedPCC 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 statetakes place during the expansion into atmosphericcondition. Because of the high super-saturation in SC-CO2, extremely small particles can be formed in theRESS 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 foradditional solvents or surfactants to induce precipitation.Since the solvent is a dilute gas after expansion, theRESS process offers a highly pure final product thatenable 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 polymersolutions onto a substrate to generate micro/nano particles,as in electrospraying. Electrospinning cellulosic fiberswill be covered in later sections. Sarkar and coworkers(2010) employed this technique to electro-spray

polyvinylidene fluoride and fluorinated silane moleculesonto cellulosic materials (cotton fabric or paper) withdifferent concentrations for differential super-hydrophobicity and hydrophilicity on each side of thesurfaces. They found that a smaller particle diameterenhanced super-hydrophobicity when a constant surfaceroughness and surface chemical composition were used.

Chemical vapor deposition (CVD). Chemical vapordeposition (CVD) is a chemical reaction whichtransforms gaseous molecules, called precursor, into asolid material, in the form of thin film or powder, on thesurface 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 coatingsonto substrates. For vapor deposition, the reactiontemperature in the chamber is required to be higher thanthe boiling point of the precursor. For example, silane

precursors of trichloromethylsilane (TCMS) anddimethyl-dichlorosilane (DMDCS) require a temperaturehigher 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 asealed chamber for a set time, into which a vapor of

precursor was introduced. The precursor was absorbedonto the cotton fibers’ surface and penetrated into the

fibers and reaction between the halide and hydroxyl

groups took place. An efficient reaction enhanced thecontent of covalently bound silicone to the fibers’

surfaces. After the deposition, condensation of silane polymers was required. This step was conducted in anaqueous solution of pyridine (1 M) at room temperatureto hydrolyze the remaining Si – Cl bonds (Li et al. 2007).Finally, subsequent polymerization of Si – OH wasaccomplished in an oven at 150°C for 10 min, whichresulted in a nano-scaled silicone coating tightly attachedto the surface. Therefore, this method can be regarded asa special sol-gel process in which the precursor attachesto 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 thegeneration of hydrogen chloride during reaction thatdegrades 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). PTFEdeposited on a cotton fiber by PLD (b) and high magnificationSEM micrograph showing the granular morphology andnanostructure of the PTFE film (c). Figures (b) and (c) arereprinted from (Daoud et al. 2006) with permission fromElsevier.

Laser assisted chemical vapor deposition. Chemicalvapor deposition is the simplest method for depositionand has the advantage of easy controllability. However,one drawback of chemical vapor deposition is that the

polymer used has to be readily vaporized. Pulsed LaserDeposition (PLD) enables the deposition of materialswith high melting point.

PLD is a dry process, very versatile and has severaladvantages in the formation of thin films of functionalmaterials. The schematic of PLD is observed in Fig 28a.A laser is utilized as energy source to evaporate a targetmaterial that is placed on a carousel. The laser plume

deposits onto the surface of the substrate which is placedon the sample stage. Through PLD, it is possible tocontrol 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 bychanging the ablation target appropriately. Amongavailable organic materials, polytetrafluoroethylene(PTFE) has been the main target in PLD because itexhibits poor solubility in all solvents and has lowsurface adhesion. Note that conventional wet processessuch as spin-coating methods are not applicable for theformation of thin PTFE films. Daoud et al. (2006) usedPLD 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 toillustrate the effect of such nano deposition.

(b) (c)

Laser beam

Port withQuartz window

Sample stageSubstrate

Laser plumeRotatingtarget

Target Carousel

(a)



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Plasma assisted chemical vapor etching and

deposition. Plasma-based technologies are excitingalternatives for cellulose and paper modification. Plasmais ionized gas consisting of a mixture of free electrons,ions and neutral atoms (depending on the degree ofionization) and is formed when sufficient energy is put

into a gas. Under laboratory conditions, ionized gas isgenerated by using high electric fields, known as cold plasma or electric discharge. Ionized molecules with highenergy can be bombarded on the surface of a givensubstrate. Most of the chemistry occurs on the surfaceswithin the reactor due to the high energy ion bombard-ment. High energy ions cause relatively unselectivefragmentation of the surface adsorbed species and adeposition 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 byselective etching of the amorphous cellulose domains in

oxygen plasma (Balu et al. 2009). The low surface energyof these etched paper surfaces can be obtained bydepositing a thin film of pentafluoroethane (PFE) (Balu etal. 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 coatingsdeposited on cellulose-base materials have been reportedto function as gas and vapor barriers (Hirvikorpi et al.2010; 2011a; 2011b; 2011c). Cotton fibers were modifiedwith ZnO to make them conductive (Jur et al. 2011). In

the ALD process, a metal organic precursor (e.g. bis(diethylamido) silane, trimethylaluminium and diethylzinc) and a reactant (e.g. water) are sequentially exposedto a surface desired to be modified, resulting in acomplementary sequence of self-limiting reactions. ALDallows preparation of dense and pinhole-free inorganicfilms. ALD is expected to be a promising method to

produce super-hydrophobic surfaces with topographycontrol.

Roughness offered by regeneration of cellulosicmaterials, cellulose nanocrystals and compositesThe previous section dealing with chemical grafting,

nanoparticle deposition and chemical vapor deposition,introduced the roughness required to attain super-hydrophobicity, by self-assembled polymer, nanoparticlesetc. In this section, the roughness required for super-hydrophobicity is produced by the cellulosic materialitself, i.e., by regeneration or nanofibrillation. Thesetechniques include electrospinning and electro-sprayingand application of cellulose nanocrystals and aerogels. Electrospinning. In electrospinning, high voltage,

typically 5-30 kV, is applied between a spinneret(s) and acollector, and therefore a static electric field is established

between. A so-called Taylor cone forms due to thecompeting forces of the static electric field and the

liquid’s surface tension. For liquids with a finiteconductivity, charged droplets are dispersed from the tipof the Taylor cone and are delivered to the target. If theliquid consists of a polymer melt or a polymer solution

and the concentration of that polymer is sufficiently highto cause molecular chain entanglement, a fiber, ratherthan a droplet, is drawn from the tip of the Taylor cone(Huang et al. 2003; Li, Xia 2004).

Electrospinning of cellulose has been extremelychallenging because of its high crystallinity, which

prevents its dissolution in common solvents.Manufacturing of electrospun cellulose fibers has beenreported through direct electrospinning of cellulosesolution 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 beelectrospun at elevated temperatures and all threeresulting cellulose fibers require post-treatment withwater 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; Liuand Hsieh 2002; Son et al. 2004a; b; Vallejos et al.). CAcan be electrospun in a mixture of acetone/water ormethylene chloride/ethanol (Han et al. 2008; Liu, Hsieh2002; 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 theentanglement of electrospun fiber in the mat. In addition,nano pores can be created during electrospinning due tothe rapid solvent evaporation. Furthermore, the porestructure 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 fiberand nano porous structure as a function of ratio ofmethylene chloride/ethanol. Super-hydrophobicelectrospun CA mat have been achieved by treating thefibers with a CF4 plasma (Yoon et al. 2009).Cellulose composites. Fine solid particles such as silica,carbon black, and clay when adsorbed at liquid-liquidinterfaces can act as stabilizers for emulsions, foams, andwater droplets replacing surfactants. Emulsions stabilizedwith such surface active particles are called “Pickering

emulsions”. Bayer et al. (2009) utilized relatedapproaches to fabricate nano-rough cellulosic surfaces.

Emulsions were formed by dispersing cyclosiloxanes inwater stabilized by layered silicate particles and weresubsequently modified by blending into a zinc oxide nanofluid. The polymer matrix was a blend of cellulose nitrateand fluoroacrylic polymer pre-compatibilized in solution.Coatings were sprayed cast onto substrates from polymer

blends dispersed in modified Pickering emulsions. As aresult, 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 amorphousdomains 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 ofthe nano-size and properties of cellulose nanocrystals to



PAPER CHEMISTRY


Fig 29. SEM images of cellulose acetate fibers electrospun withdifferent MC/EtOH (v/v) ratios: 80/20 (a), 90/10 (b) and 100/0(c). Reprinted from (Yoon et al. 2009) with permission fromElsevier.

Fig 30. SEM image to demonstrate the porous structure of NFCaerogel. Reprinted with permission from (Jin et al. 2011) withpermission from American Chemical Society.

produce super-hydrophobic surfaces by assemblingmultilayers of poly(ethyleneimine) (PEI) and colloidalCNCs followed by hydrophobization with (tridecafluoro-1,1,2,2,-tetrahydrooctyl-)trichloro-silane in heptanesolution.

Micro- or nanofibrillated cellulose (NFC), another formof nanocellulose (Turbak 1983) can be isolated throughhigh-shear mechanical disintegration and homogenizationof 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 vapordeposition with 1H,1H,2H,2H-perfluorodecyl-trichloro-silane (PFOTS) which was used to uniformly coat theaerogel and to tune their wetting properties towards non-

polar liquids (Jin et al. 2011; Aulin et al. 2010b) or byfunctionalizing the native cellulose nanofibrils of theaerogel with a hydrophobic but oleophilic titaniumdioxide to produce floating, sustainable, reusable, andrecyclable oil absorbents (Korhonen et al. 2011). Recentwork by Mertaniemi et al. reported using air-brushtechniques 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-basedsuperhydrophic surfaces For self-cleaning applications a low sliding angle or lowcontact angle hysteresis is an important requirement.Therefore, any wetting analysis requires thedetermination of both the advancing and receding contactangles, as was defined in previous sections of this review.Unfortunately, many reports in the literature onlyconsider a single, "static" contact angle. A convenientway 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 etal. 1999; Wier et al. 2006). Table 1 includes acompilation of values of WCA hysteresis reported in theliterature for various cellulose-based substrates. It alsoincludes there respective static WCA and the materialused to induce roughness on the surface as well as therespective strategy used to lower its surface energy. Awide range of contact angle hysteresis values exist even ifa very high static WCA is achieved. In most cases, a verysmall 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). Evenhigher hysteresis was measured in cases where thesubstrate was coated with polymer NPs (>50° or as highas 112°) (Samyn et al. 2011a; Stanssens et al. 2011). Aninteresting case is that of layer-by-layer deposition ofsilica NPs on cotton fibers followed by treatment withfluoroalkylsilane. In such cases the WCA were above150°, while the hysteresis angle varied with thedeposition layers: for 1 or 3 layers, the fabrics showedadhesive contact with the water drop and a high contactangle hysteresis (>45°). Systems with 5 layers or more,

produced slippery super-hydrophobicity, with a contactangle hysteresis lower than 10° (Zhao et al. 2010). Thegreatest hysteresis angle was achieved in the case ofcellulose coated with Ag NPs and treated with octyl-triethoxysilane; in this case the water drop also exhibitadhesive contact with the substrate, even if it was holdupside down.

WCA hysteresis is affected by the substrate, itsroughness and texture and the coating material as well asthe strategies used to change the surface energy of thesystem. However, contact angle hysteresis and adhesivehydrophobicity is a subject still not fully understood(Chen et al. 1999; Gao, McCarthy 2006; Wier et al.

2006). To complicate matters, poorly covered cellulosesubstrates are often difficult in accurate measurements ofWCA 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 roughnessand 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’’

architectureFluorinated end brushes 170

Very small, oncetilted

(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 NPsStearic acid/ perfluorodecyltrichloro-

silane158-170 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 173Very small, once

it tilted(Quan et al. 2009)

Cotton fabric orpaper

electro-sprayingPolyvinylidene fluoride and

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

Pulp fiber TiO2 NPs3-(trimethoxysilyl) propyl

methacrylateUp 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 celluloseDue to the high surface roughness required to endowmaterials with super-hydrophobicity, the respectivesurfaces are generally easily damaged by scratching orerosion. In nature, a damaged surface can be repaired byregenerative 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 bedifficult to apply in order to repair damaged roughnessand surface chemistry. Spray and chemical vapordeposition 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). Mechanicaldurability offered by hierarchical roughness ensures thatstable Cassie state remains even after some surfacefeatures are worn away. Such morphology involvesrobust micro-scale bumps that provide protection to amore fragile nano scale roughness that is superimposedon the larger pattern (Verho et al. 2011).A number of applications involve surfaces that aresubject to different degrees to abrasive forces, forexample, outdoor facades or window panes, solar panels,liquid handling devices used in microfluidics, etc. Theinteraction between super-hydrophobic surface and water

is a key factor influencing most of the application(Zimmermann et al. 2007). Compared with othersynthetic or inorganic superhydrophobic surfaces, super-hydrophobic surfaces based on lignocellulose may have

Fig 31. The relationship between contact angle and immersiontime 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 toits inherent hydrophilicity.

The chemical durability of superhydrophobic cotton wasevaluated by washing and abrasion (Wang et al. 2013;Zhou et al. 2012). Based on their experiment, the coatedfabric that incorporate fluorinated decyl-polyhedraloligomeric silsesquioxane (FD-POSS) and fluorinatedalkyl silane into the poly(3,4-ethylenedioxythiophene)(PEDOT) layer could withstand at least 500 cycles ofstandard laundry and 10 000 cycles of abrasion without

apparently changing the super-hydrophobicity. In anotherexperiment, the PD-POSS was substituted with silicananoparticles. The coated fabrics withstood at least600 cycles of standard laundry and 8000 cycles of



PAPER CHEMISTRY


abrasion without apparently changing the super-hydrophobicity.

The chemical durability of super-hydrophobic cottonfabrics varies with pH of media. Fig 31 demonstrates thatthe change of WCA against immersion time in aqueoussolutions with different pH by Li et al. (Li et al. 2008a).

The super-hydrophobic features involved the assembly ofa precursor silanol (potassium methyl siliconate, PMS)film on the cellulose surface and then polycondensationof the (PMS) aqueous solution with CO2 at roomtemperature. The resultant super-hydrophobic celluloseshowed satisfactory durability in acidic and neutralcondition but less in basic medium.

There is an expanding range of applications of super-hydrophobic cellulosic materials as described in sections3.1 and 3.2. In addition, a number of super-hydrophobicsurfaces which can be produced from cellulosic substrateshave been reported by using different methods. How thesurface chemistry and the topography influence the

stability of super-hydrophobic lignocellulosic surfaces isstill a key issue that deserves further attention. In thisregards, some topics that are to be considered in order tocontrol the durability of super-hydrophobic behaviorsinclude the impact on surface energy of end groups,cross-linking agents, functional group and inorganicnanoparticle density, as well as the thickness of coatedlayers, etc.

Final remarksEven through know-how in the development of super-hydrophobic cellulose-based materials has undergone arapid progress and has attracted considerable attention in

recent years, robust and inexpensive methods are still intheir infancy; in addition, a number of questions need to be addressed which require further research. Firstly, thereis deficient information on the effect of lignin andheteropolysaccharides in the development of super-hydrophobic materials. Lignin and heteropolysaccharides

present in the cell wall of wood fibers make up animportant fraction of their composition. Compared withthe polysaccharide components, lignin has a lower polarsurface energy and a lower dispersive surface energy andtherefore if it is used in the fabrication of super-hydrophobic materials, the demand for chemicals withlow surface energy may be reduced. In addition,

environmental pressures and more efficient resourceutilization may stimulate more widespread use of residuallignocellulose (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 difficultrecovery of ionic liquids may hinder its large-scaledapplication.

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 onsuccessful large-scale production of super-hydrophobicmaterials 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 amajor 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 Programof Higher Education of China (20103204120005), TalentsFoundation of Nanjing Forestry University (163105003), Openfund of State Key Laboratory of Pulp and Paper Engineering

(201034), and the Priority Academic Program Development ofJiangsu Higher Education Institutions.

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Manuscript received October 16, 2012Accepted April 8, 2013

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