Cellulose Nanomaterials—Binding Properties and Applications

24 Pages·2017·5.09 MB·English

molecules
Review
Cellulose Nanomaterials—Binding Properties and
Applications: A Review
AliH.Tayeb1,2,*,EzatollahAmini1,2,ShokoofehGhasemi1,2andMehdiTajvidi1,2
1 SchoolofForestResources,UniversityofMaine,5755NuttingHall,Orono,ME04469,USA;
[email protected](E.A.);[email protected](S.G.);[email protected](M.T.)
2 AdvancedStructuresandCompositesCenter,UniversityofMaine,35FlagstaffRoad,Orono,ME04469,USA
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:18September2018;Accepted:13October2018;Published:18October2018
Abstract: Cellulosenanomaterials(CNs)areofincreasinginterestduetotheirappealinginherent
propertiessuchasbio-degradability,highsurfacearea,lightweight,chiralityandtheabilitytoform
effectivehydrogenbondsacrossthecellulosechainsorwithinotherpolymericmatrices. Extending
CNself-assemblyintomultiphasepolymerstructureshasledtousefulend-resultsinawidespectrum
of products and countless innovative applications, for example, as reinforcing agent, emulsion
stabilizer, barrier membrane and binder. In the current contribution, after a brief description of
salient nanocellulose chemical structure features, its types and production methods, we move
to recent advances in CN utilization as an ecofriendly binder in several disparate areas, namely
formaldehyde-freehybridcompositesandwood-basedpanels,papermaking/coatingprocesses,and
energystoragedevices,aswellastheirpotentialapplicationsinbiomedicalfieldsasacost-effective
andtissue-friendlybinderforcartilageregeneration,woundhealinganddentalrepair. Theprospects
ofawiderangeofhybridmaterialsthatmaybeproducedviananocelluloseisintroducedinlight
of the unique behavior of cellulose once in nano dimensions. Furthermore, we implement some
principlesofcolloidalandinterfacialsciencetodiscussthecriticalroleofcellulosebindinginthe
aforesaidfields. EventhoughtheCNfacetscoveredinthisstudybynomeansencompassthegreat
amountofliteratureavailable, theymayberegardedasthebasisforfuturedevelopmentsinthe
binderapplicationsofthesehighlydesirablematerials.
Keywords:nanocellulosebinders;cellulosenanofibrils;cellulosenanocrystals;biopolymericbinder;
bacterialcellulose;particleboard;energystoragedevices;papercoating;boneregeneration;biomedical
1. Introduction
BackgroundandMotivation
Serious environmental concerns about the consumption of fossil-fuel based materials, such
as green gas emissions, long degradation time, human health issues, etc., have forced technology
to seek to employ sustainable nature-friendly materials as a replacement for petroleum-derived
products. Among existing biopolymers, the straight-chain polysaccharide cellulose has long been
recognized as the most abundant renewable organic polymer in the biosphere, having excellent
mechanicalcharacteristics,economicproductionandyetfairlyfunctionalizable. Whetherintheform
offibersorderivatives,celluloseconstitutesnearlyonetrilliontonsoftheworld’sannualbiomass
production[1],andtodate,ithashadamajorimpactonthedevelopmentofnumerouseco-friendly
materials. Recently,therehasbeenaresurgenceininterestinutilizingcellulosetoproduceadvanced
materialsandundoubtedly,itwillplayacriticalroleinthefutureofthe(bio)economy. Celluloseis
a fibrous water-loving polymer, essential in the structure of plant cell walls and naturally formed
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through the plant cellular development process and cellulose biogenesis, fixed by van der Waals
forces and hydrogen bonds [2,3]. It is also found in some marine creatures (e.g., tunicates), fungi,
invertebrates,andalgae,aswellasbacteria[4]. Twodistinguishedregionswithincellulosefibrilsare
thecrystallineandamorphousparts. Byapplyingchemicalprocesses,cellulosenanocrystals(CNCs)
canbeproducedthroughtheisolationofcrystallinedomains. Mechanicaltreatmentsontheother
handgeneratecellulosenanofibrils(CNFs)[3]. Lately,utilizationofCNFsandCNCswithnanoscale
lateral dimensions has garnered increasing attention, not only for their intrinsic biodegradability
andabundance,butalsoduetounsurpassedquintessentialproperties,suchasstiffness,lowdensity,
flexibility,largeaspectratioanduniquerheology,whichare,inpart,duetotheircrystallineassembly
viahydrogenbonds[3,5–10]. Alsotheirhighchemicalfunctionalityisduetotheabundantprimary
andsecondaryhydroxyls(referredtoascellulosealcohols)onthesurface,whichcanbeeasilymodified
withbio-polymersandyieldcellulosederivativesorbygraftingtodifferentmaterials[11]. TheseOH
groupscanattracteachotherelectrostatically(viahydrogenbonds),causingthechainstobuildan
ordered structure [2]. Such hydrogen bonding throughout the polysaccharide has a key function
inbinderapplicationsrelatedtoadhesionbetweencellulosenanoparticlesandothermaterials[12],
forexample, inhybridcompositeandpaperproduction[13–16], environmentalandwater-related
areas[17],biomedicalapplications[18],aswellasenergystoragedevices[19,20].
In the present work, first we briefly introduce the structures, types and pertinent production
methods of cellulose nanomaterials and then review some of the recent studies on CN-based
binders in four separate areas, namely hybrid composites and wood panels (in light of recent
efforts to replace common carcinogenic industry adhesive ingredients, particularly formaldehyde,
by CNs), papermaking/coating processes, energy storage devices and finally, the potential usage
of nanocellulose in biomedical fields as a cost-effective and tissue-friendly binder for cartilage
regeneration,woundhealinganddentalrepair. ThekeypropertiesofCNfortheintendedbinding
applications will be delineated separately in the corresponding sections but overall their bio- and
tissue-friendly nature, high aspect ratio, excellent mechanical attributes and ability to establish
extensiveHbondswithinvariouspolymericmatricesareamongthemostimportantfeatures. Also
relevanttoitsbindingperformance,theyarecapableofbuildingawiderangeofCN-basedhybrids
viaself-ordirected-assembly[3,10,13,21,22].
2. ChemistryofNanocelluloseandItsProductionMethods
Cellulose is a linear-chain homopolymer composed of repeating ringed anhydro-D-glucose
monomers with the general formula of (C H O ) , (n < 20,000). Each repeating unit is rotated
6 10 5 n
180degreesaroundtheaxisofthecellulosebackbonerelativetotheneighboringring,linkingtogether
viaaβ-(1,4)-glyosidicbond[21,23,24]; theC1atomofthefirstringisattachedtotheC4oneofan
adjacentglucosethroughacovalentoxygenbond.
AsillustratedinFigure1,theelectrostaticattractionsbetweenoxygenandhydrogenatomsof
the adjacent rings, inducing intra-molecular hydrogen bonding, cause more stabilized glycosidic
linkage motifs, linear-chain configuration, in addition to feeble solubility in polar solvents [24].
Also, inter-chain hydrogen bonds (occurring between polymer chains), along with van der Waals
forces,promoteparallelstacking,knownascelluloseelementaryfibrils,whichcanfurtheraggregate
intomicron-scalepredecessors[24]. Theseintra-andinter-chainnon-covalentattractionsarevital
forthestabilityandfirmstructureofcellulose, thelatterofwhichisessentialforplantsandsome
marinecreatures. Dependingontheprimaryandsecondaryalcoholsonthesurface;C2-OH,C3-OH
andC6-H OH,cellulosemayhavedifferentdegreesofhydrogenbonding. Forexample,regenerated
2
cellulose(celluloseII)whichisproducedviaamercerizationprocess,demonstratesalteredhydrogen
bondingmotifs,withastrongerandmoresolidstructurecomparedtonaturalcellulose(celluloseI)[25].
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Figure1.Cellulosestructureandintra/intermolecularhydrogenbondingpattern(top).Exampleimages
Figure 1. Cellulose structure and intra/intermolecular hydrogen bonding pattern (top). Example
summarizingthetopicalfocusofcurrentreview,whereintherecentdevelopmentofnanocellulose
images summarizing the topical focus of current review, wherein the recent development of
materialsandtheirbinding-relatedapplicationsinparticleboards,papercoating,electronicdevicesand
nanocellulose materials and their binding-related applications in particle boards, paper coating,
biomedicalscience(e.g.,incartilageregeneration)iscovered(bottom).Imageslabeledas“Electronics”
electronic devices and biomedical science (e.g., in cartilage regeneration) is covered (bottom). Images
and“Biomedical”arereprintedwithpermissionfrom[26].Copyright2009Elsevierand[27].Copyright
labeled as “Electronics” and “Biomedical” are reprinted with permission from [26]. Copyright 2009
2015AmericanChemicalSociety,respectively.
Elsevier and [27]. Copyright 2015 American Chemical Society, respectively.
Withincellulosefibrils,therearetwodiscretedomains:highlyordered(crystalline)anddisordered
Within cellulose fibrils, there are two discrete domains: highly ordered (crystalline) and
(amorphous)structures. Byapplyingdifferenttechniques,twocommonlyusedformsofnanocellulose,
disordered (amorphous) structures. By applying different techniques, two commonly used forms of
cellulosenanocrystals(CNCs)andcellulosenanofibrils(CNFs)canbeseparatedfromthecellulosic
nanocellulose, cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) can be separated from
source. Athirdform,bacterialcellulose(BC)issynthesizedbysomeGram-negativebacterialspecies
the cellulosic source. A third form, bacterial cellulose (BC) is synthesized by some Gram-negative
anddiffersfromthetwoothertypesinthatitisnaturallyformedbyabottom-upprocessasopposed
bacterial species and differs from the two other types in that it is naturally formed by a bottom-up
tothetop-downapproachesusedtoproduceCNCandCNF[7]. Collectively,alltypesofcellulose
process as opposed to the top-down approaches used to produce CNC and CNF [7]. Collectively, all
nanomaterialscanbecalledcellulosenanoparticles(CNs). Inadditiontotheiruniquephysiochemical
types of cellulose nanomaterials can be called cellulose nanoparticles (CNs). In addition to their
features(whichwillbediscussedlaterinthisreview),CNshavehighpotentialassubstituentsfor
unique physiochemical features (which will be discussed later in this review), CNs have high
syntheticpetroleum-basedbindersandadhesives. Inthenextsection,wefocusonthepreparation
potential as substituents for synthetic petroleum-based binders and adhesives. In the next section, we
techniquesalongwiththepropertiesofeachgroup.
focus on the preparation techniques along with the properties of each group.
2.1. CelluloseNanofibrils(CNFs)
2.1. Cellulose Nanofibrils (CNFs)
CNFsarenano-scalefibrilswithhighaspectratio;theirwidthvariesfrom2to60nm(beingseveral
CNFs are nano-scale fibrils with high aspect ratio; their width varies from 2 to 60 nm (being
micrometersinlength)andareformedasaresultofcellulosechain-stacking,inducedbyhydrogen
several micrometers in length) and are formed as a result of cellulose chain-stacking, induced by
bonds [28]. These nanoparticles (also known as nanofibrillated cellulose, NFC, or microfibrillated
hydrogen bonds [28]. These nanoparticles (also known as nanofibrillated cellulose, NFC, or
cellulose, MFC) which consist of both crystalline and amorphous domains, can be produced by
microfibrillated cellulose, MFC) which consist of both crystalline and amorphous domains, can be
liberatingthefibrilsfromtheintegralmicrofiberbundlesthroughvigorousmechanicalfibrillation
produced by liberating the fibrils from the integral microfiber bundles through vigorous mechanical
processes [21,29]. By far, many reviews have discussed their exceptional characteristics, such as
fibrillation processes [21,29]. By far, many reviews have discussed their exceptional characteristics,
biodegradability, excellent mechanical properties, high surface area, light weight, etc. [3,5,6,21,30].
such as biodegradability, excellent mechanical properties, high surface area, light weight, etc.
The hydrogen bonds between the hydroxyl groups prompt a highly ordered conformation of the
[3,5,6,21,30]. The hydrogen bonds between the hydroxyl groups prompt a highly ordered
cellulosechainsinelementaryfibrilsthatfurtherfacilitateadhesionwithotherpolymericcomponents,
conformation of the cellulose chains in elementary fibrils that further facilitate adhesion with other
e.g.,withligninorproteins,toformnetworksinaqueousmedia[12,31]. Sinceanenergy-intensive
polymeric components, e.g., with lignin or proteins, to form networks in aqueous media [12,31]. Since
mechanical grinding is required to separate the stacked fibrils, different types of pretreatments,
an energy-intensive mechanical grinding is required to separate the stacked fibrils, different types of
suchasalkaline[32,33],radiation[32],chemical[34,35],andenzymatic[36,37]aretypicallyapplied
pretreatments, such as alkaline [32,33], radiation [32], chemical [34,35], and enzymatic [36,37] are
prior to the fibrillation process to remarkably lower the cost and energy. For this reason, CNF
typically applied prior to the fibrillation process to remarkably lower the cost and energy. For this
manufacturing at an industrial scale was not feasible until recently [38]. A production technique
reason, CNF manufacturing at an industrial scale was not feasible until recently [38]. A production
whichhasbeenusedfrequentlyispre-oxidationusing(2,2,6,6,-tetramethylpiperidin-1-yl)oxidanyl,
technique which has been used frequently is pre-oxidation using (2,2,6,6,-tetramethylpiperidin-1-yl)
abbreviated as TEMPO, to reduce the grinding cycles prior to applying the vigorous mechanical
oxidanyl, abbreviated as TEMPO, to reduce the grinding cycles prior to applying the vigorous
mechanical fibrillation by microfluidization or homogenization. The resulting nanoparticles give a
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gel-like appearance at low solid concentrations (1–2%) in water [38,39], and once formed into a
fibrillation by microfluidization or homogenization. The resulting nanoparticles give a gel-like
transparent film, display high mechanical integrity, limited oxygen transmission rate as well as low
appearanceatlowsolidconcentrations(1–2%)inwater[38,39],andonceformedintoatransparentfilm,
thermal expansion (~2.7 ppm K−1. m) [39–41]. Translucent thin films from microfibrillated cellulose
displayhighmechanicalintegrity,limitedoxygentransmissionrateaswellaslowthermalexpansion
((2~02–.710p0p nmmK i−n1 d.imam) [e3t9er–)4 w1]e.reT raalsnos liuncterondtuthceind bfiylm Oskfarmomuram eitc raol.fi [b4r2i]l.l aStoe dfacre, lrleusleoasrech(2er0s– 1h0a0ven umseidn
CNF as reinforcing agents in different composites such as hybrid plastics, papers and thin packaging
diameter)werealsointroducedbyOkamuraetal.[42].Sofar,researchershaveusedCNFasreinforcing
films. For instance, the use of CNF in the papermaking wet-end results in papers with higher strength
agentsindifferentcompositessuchashybridplastics,papersandthinpackagingfilms. Forinstance,
[43].
theuseofCNFinthepapermakingwet-endresultsinpaperswithhigherstrength[43].
22..22.. CCeelllluulloossee NNaannooccrryyssttaallss ((CCNNCCss))
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dthuericnlega tvhaeg celeparvoacgeses p.rocess.

FFiigguurree 22.. ((aa)) SScchheemmaattiicc ooff cceelllluulloossee rreeppeeaattiinngg uunniitt wwiitthh tthhee ββ--((11,,44))--ggllyyccoossiiddiicc lliinnkkaaggee,, ddootttteedd lliinneess
iinnddiiccaattee iinnttrraammoolleeccuullaarr hhyyddrrooggeenn bboonndd;; ((bb)) hhyyppootthheettiiccaall ccoonnffiigguurraattiioonn ooff oorrddeerreedd ((ccrryyssttaalllliinnee)) aanndd
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aincdruitcicinagl lteuvneal,btlhee syufrofarmce ocrhdeemreisdt,ryb i[r9e]f.r iOngnecne tthpeh acsoenscienntcroaltliooind aolfs tyhsetseem csry[5s3ta].llAinen unmeebdelreso fexstcuededie sa,
hcraivtiecable leenveclo, nthdeuyc tfeodrmon oprdroedreudc,i nbgirnefarninogcreynstt aplhlianseesc einll ucloolsloeiadnadl siytsstaepmpsli c[5a3ti]o. nAi nnuamvabreire toyf osftufideiledss,
shuacvhe abseehny cborindducocmtedp oosnit pesro[d54u]c,ibnagr nriaenroficlrmysst,aellliencetr coenlliucldoesev iacneds [i5ts5 ]a,papnltiicmatiicorno binia al fivalmries,tye mofu fliseilodns
sstuacbhil iazse rhsy,bertcid.[ 3co,7m,5p6,o5s7i]t.esF o[5r4e]x, abmarprliee,rb feiclmauss, eeloefctthroenirich idgehvsitcieffsn [e5s5s],a nadntsimurifcarcoebaiarel afi[l5m8s],, aesmwuelsllioans
lsotawbirloizuegrhs,n eetscs. ([~3,27,n5m6,5)7a]n. dFodre nexsiatmyp(1l.e6, gb/eccamu3s)e[ o4]f, tChNeirC hsihgahv setiaftftnreascst eadnadt tseunrtfiaocne faorreua s[e58in], flaes xwibellel
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suitable candidate as green reinforcing materials in composites [23,60,61].
2.3. BacterialCellulose(BC)
2.3. Bacterial Cellulose (BC)
Anothergroupofcellulosenanoparticlesdifferingfromthefiber-likeCNFandneedle-shapeCNC
isbacAtenroiathlecre lglurolouspe (oafl scoelrleufleorsree dnatonoapsamrtiiccrloesb idalifnfearninogce flrluomlos teh,eo rfibbieorc-elilklue loCsNe)F. Baancdt enreiaeldcleel-lsuhlaopsee
(CBNCC) ciosn btaaicntserriiablb coenll-ushloaspee (naalsnoo firebfeerrsreldin ktoe dasto mgeitchroerbitahlr onuagnhocaelnlueltowsoe,r koro fbhioycderlolugleonseb).o nBdasc.teIrtiaisl
fcoerllmuleodseb y(BvCar) iocuonstmaiincsr oroirbgbaonni-ssmhaspaen dnabnaoctfeibriearlss plienckieeds (teo.gg.e,tAhceert otbharcotuergxhy lain nuemtw,Posreku doofm honyadsr)o[g6e2n],
tbhoantdasr.e Ifto iusn fdorwmheedr ebcya rvbaorhioyudsr amteicferromoregnatnaitsimonss atankde bpalactceeri[a6l3 –s6p5e]c.ieUs n(leik.ge., oAthceertonbaacntoerc exllyulilnousems,,
BPCseuisdopmroodnuasc)e d[62th],r othuagth aareb fioou-fnadbr wicahteiroen caasrsbeomhbyldyrpatreo cfeesrsmferonmtatlioowns- mtaoklee cpullaacrew [6e3ig–h65t]s.u Ugnarliskseu octhhaesr
nDa-gnloucceollsueloanseds,i sBiCso liast epdroadsuacnedex othprooluygsahc cah abriiod-feaabtritchaetiaoinr- nausstreimenbtlym epdroiaceinsst efrrfoamce [l6o6w].-mThoelerecufolarer
uwnedigehrst tsaungdainrsg siutschb iaoss yDn-tghluetciocsper oanceds sisi sisiomlaptoerdt aanst atno beextotperoltyusnaeccthhaerciduelt uart inthge caoinr-dniutitornienant dmaeldteiar
tinhteedrfeascier ed[66n]a. nTohfiebrreilfsorsetr uucntduerersatannddcirnygs tiatlsl inbiitoysywnhthicehtimc apyroleceasds tois niemwpporrotapnetr ttioes ,bfeuttnecrt iotunnaeli ttihese,
acunldtuarpinpgli ccaotniodnitsiofonr asnudc halbtearc ttehreia dl-edseirreivde dnannaonfoibcreilllsu slotrsuecstu[6r7e] .aFnidg ucrryes4taslhlionwitys awhhyicdhr omgealys lheeaedt otof
bnaewcte rpiraolpceerlltuielso,s efu(nbcottihonitasli3tiDes,s taruncdt uarpepalnicdataiomnsi cfroors csoupcihc ibmaacgteer)iatlh-daterwivaesdp rnoadnuocceedlluvlioaseas st[a6t7i]c.
cFuigltuivrea t4i osnhomwest hao hdy.drogel sheet of bacterial cellulose (both its 3D structure and a microscopic image)
that was produced via a static cultivation method.
Molecules2018,23,2684 6of24
Molecules 2018, 23, x FOR PEER REVIEW 6 of 24

FFiigguurree 44.. ((aa)) PPrroodduucceedd bbaacctteerriiaall cceelllluulloossee iinn ssttaattiicc ccuullttiivvaattiioonn;; ((bb)) ssccaannnniinngg eelleeccttrroonn mmiiccrroossccooppee iimmaaggee
ooff aa BBCC nneettwwoorrkk ((rreepprriinntteedd wwiitthh ppeerrmmiissssiioonn ffrroomm [[6666]].. CCooppyyrriigghhtt 22001144 SSpprriinnggeerr))..
DDeessppiittee hhaavviinngg ssiimmiillaarriittiieess ttoo CCNNFF aanndd CCNNCC,, tthhee nnaannooffiibbrriill bbuunnddlleess iinn BBCC hhaavvee ssoommee uunniiqquuee
pprrooppeerrttiieess ssuucchh aass mmoorree ccrryyssttaalllliinniittyy,, hhiigghheerr ddeeggrreeee ooff ppoollyymmeerriizzaattiioonn ((uupp ttoo 88000000)),, aass wweellll aass aabbiilliittyy
ttoo ffoorrmm aann eexxttrreemmeellyy uullttrraaffiinnee wweebb ssttrruuccttuurree,, tthhaatt mmaakkeess iitt ddiiffffiiccuulltt ttoo ddiissppeerrssee iinn wwaatteerr [[6644,,6688,,6699]]..
BBCC aallssoo hhaass ffiibbeerrss wwiitthh aa hhiigghh eellaassttiicc mmoodduulluuss ooff 7788 GGPPaa [[7700]],, ((cclloossee ttoo tthhaatt ooff ggllaassss ffiibbeerrss,, aanndd ggrreeaatteerr
tthhaann nnaattuurraall nnaannoossccaallee ffiibbeerrss)),, aanndd ppoosssseesssseess aa llaarrggeerr wwaatteerr hhoollddiinngg ccaappaacciittyy ccoommppaarreedd ttoo ppllaanntt--
ddeerriivveedd cceelllluulloossee [[7711]].. AAddddiittiioonnaallllyy,, tthhee lliigghhtt wweeiigghhtt,, bbiiooddeeggrraaddaabbllee aanndd nnoonn--ttooxxiicc bbaacctteerriiaall cceelllluulloossee
iiss pprroodduucceedd aass ppuurree cceelllluulloossee aanndd aavvooiiddss tthhee ttrraaddiittiioonnaall cchheemmiiccaall ttrreeaattmmeennttss rreeqquuiirreedd ffoorr lliiggnniinn oorr
hheemmiicceelllluulloossee eelliimmiinnaattiioonn [[99,,7722]]..
SSeevveerraall ssttuuddiieess hhaavvee iinnddiiccaatteedd tthhee ppoossssiibbllee uussaaggee ooff bbaacctteerriiaall cceelllluulloossee aass aa ssttrreennggtthheenniinngg aaggeenntt,,
ttrraannssppaarreenntt fifillmm aanndd bbiinnddeerr iinn hhyybbrriidd nnaannooccoommppoossiitteess [[7733––7766]].. LLeeee eett aall.. mmaaddee aa rroobbuusstt ssiissaall fifibbeerr
pprreeffoorrmm uusisnigngb abctaecrtiearlicael llucelollsueloasseb inasd ebrianndders hoanwde dsthhoewreesdu lttihneg croemsupltoisnigte hcoadmbpeotsteitrep ehrafdor mbeatntceer
apnedrfomrmecahnacne icaanldp mroepcehratnieiscacl opmroppaerertdiesto coamcpoamremdo tnol ya ucsoemdmpoonlylym uesrepdo plyoly(amcreyrl aptoeldy- e(pacorxyildaitseedd-
seopyobxeidaniseodil -bsaosyebdeapno lyomil-ebrass(epdo lyp-oAlyEmSOe)rs[ 77(p].oAlyl-sAoEthSeOc)o m[7b7i]n. aAtiolsno ofthBeC caonmdbcionnadtiuocnti nogf pBoCly maenrds
(ceo.gn.d,upcotliynagn iplionley,mpoelrys p(yer.gro.,l ep)ohlaysarneisliunlete, dpionlyapfyurlrlyolbei)o hdaesg rraedsaubltleedin itner -ac ofnunlleyc tbediodstergurcatudraeb,lseu iitnatbelre-
fcoornaneercotgedel sstarnudctutrraen, sspuaitraebnltec foomr apeorsoitgeesls[ 7a8n]d. transparent composites [78].
33.. NNaannoocceelllluulloossee aass CCooaattiinngg MMaatteerriiaall
3.1. TraditionalPaperCoatings
3.1. Traditional Paper Coatings
The potential of using CNF in paper coatings has been intensely studied for the formation
The potential of using CNF in paper coatings has been intensely studied for the formation of a
of a continuous layer on paper surfaces to induce suitable qualities such as mechanical strength,
continuous layer on paper surfaces to induce suitable qualities such as mechanical strength, fire
fire retardancy and water/gas impermeability [79–85]. Because the properties of coated surfaces
retardancy and water/gas impermeability [79–85]. Because the properties of coated surfaces depend
dependhighlyontheCNFdistributionandthecorrespondinglayerthickness,anappropriatecoating
highly on the CNF distribution and the corresponding layer thickness, an appropriate coating
formulation is critical to achieve the desired features. CNF was applied to a paper surface via
formulation is critical to achieve the desired features. CNF was applied to a paper surface via two
two different methods: roll coating and size press, resulting in coat weights of 14 and 3 g/m2,
different methods: roll coating and size press, resulting in coat weights of 14 and 3 g/m2, respectively
respectively[86]. Athin,wellspreadlayerofCNFcanalsobeattainedbyspraycoating. Thepossible
[86]. A thin, well spread layer of CNF can also be attained by spray coating. The possible challenge
challengehoweveristhepreparationoflowsolidcontentsolutionthatmaybothaffectthedrying
however is the preparation of low solid content solution that may both affect the drying process
process and/or lead to breakage of the web [86]. Another issue is the high viscosity of the CNF
and/or lead to breakage of the web [86]. Another issue is the high viscosity of the CNF suspension,
suspension, even at low solids, and the absorption of water by the base paper which can lead to
even at low solids, and the absorption of water by the base paper which can lead to localized
localizedthickeningofthecoatingmaterialandcreateunevenspreading. Arecentworkhasshown
thickening of the coating material and create uneven spreading. A recent work has shown that the
thattheuseofonly4%carboxymethylcellulose(CMC)basedonthedryweightoftheCNFcanleadto
use of only 4% carboxymethyl cellulose (CMC) based on the dry weight of the CNF can lead to a
asubstantialdropinviscosityandbettercoverageofthepapersurface[87]. CNFhasbeenalsocoated
substantial drop in viscosity and better coverage of the paper surface [87]. CNF has been also coated
onpaperviafoamcoatingamixtureof2.9%CNFandananionicsurfactantunder80–95%compressed
on paper via foam coating a mixture of 2.9% CNF and an anionic surfactant under 80–95%
air. Theresultingdouble-layerhadacoatweightof1–2.6g/m2 [88]. Othersmeanwhilehavereported
compressed air. The resulting double-layer had a coat weight of 1–2.6 g/m2 [88]. Others meanwhile
theapplicationofCNFatlowsolidsandmoderatespeeds,andlatelyithasbeenshownpossibleto
have reported the application of CNF at low solids and moderate speeds, and lately it has been shown
possible to apply CNF at high speeds (10 m/s) and moderate solids (up to 5%) using a cylindrical
laboratory coater (CLC). Coat weights in double layers as high as 10 g/m2 were reported [89].
Molecules2018,23,2684 7of24
applyCNFathighspeeds(10m/s)andmoderatesolids(upto5%)usingacylindricallaboratory
Molecules 2018, 23, x FOR PEER REVIEW 7 of 24
coater(CLC).Coatweightsindoublelayersashighas10g/m2werereported[89].
3.2. CNF as Binder in Coating Layers
3.2. CNFasBinderinCoatingLayers
The application of CNF as binder in conventional paper coating processes is a relatively new
TheapplicationofCNFasbinderinconventionalpapercoatingprocessesisarelativelynew
concept that has garnered a great deal of attention. Owing to its strong hydrogen bonding and the
concept that has garnered a great deal of attention. Owing to its strong hydrogen bonding and
ability to establish a uniform layer on hydrophilic substrates, CNF was suggested as a green binder
the ability to establish a uniform layer on hydrophilic substrates, CNF was suggested as a green
to improve the properties of coating layers on paper/paperboard and reduce the level of latex
binder to improve the properties of coating layers on paper/paperboard and reduce the level of
[15,86,88,90–92]. Figure 5 presents the hypothetical positive role of CNF in the coating process in
latex[15,86,88,90–92]. Figure5presentsthehypotheticalpositiveroleofCNFinthecoatingprocessin
which it helps the coating material to bind better on the fibrous surface and form a less defective and
whichithelpsthecoatingmaterialtobindbetteronthefibroussurfaceandformalessdefectiveand
smoother surface. A low ratio of CNF was applied to attach clay onto paper surfaces and the obtained
smoothersurface. AlowratioofCNFwasappliedtoattachclayontopapersurfacesandtheobtained
CNF-clay layer produced a significant raise in surface smoothness and print density, almost identical
CNF-claylayerproducedasignificantraiseinsurfacesmoothnessandprintdensity,almostidentical
to when polyvinyl alcohol (PVA) was used as binder [91]. In another study, a CNF-latex mixture was
towhenpolyvinylalcohol(PVA)wasusedasbinder[91]. Inanotherstudy,aCNF-latexmixturewas
employed as a potential binder in color coating and it was revealed that the presence of CNF
employedasapotentialbinderincolorcoatinganditwasrevealedthatthepresenceofCNFincreased
increased the picking strength while slightly reducing the paper surface energy [90].
thepickingstrengthwhileslightlyreducingthepapersurfaceenergy[90].
To address this issue other methods such as inkjet or flexographic printing can be used where
ToaddressthisissueothermethodssuchasinkjetorflexographicprintingcanbeusedwhereCNF
CNF is employed as a binder in coating formulas. Enzymatically pre-treated CNF and TEMPO-
isemployedasabinderincoatingformulas. Enzymaticallypre-treatedCNFandTEMPO-oxidized
oxidized CNF were investigated as alternative binders (for partial latex replacement) to bind calcium
CNFwereinvestigatedasalternativebinders(forpartiallatexreplacement)tobindcalciumcarbonate
carbonate particles in coating formulations. The results indicated a successful replacement of 20% of
particlesincoatingformulations. Theresultsindicatedasuccessfulreplacementof20%oflatexwith
latex with CNF in coating colors with no evidence of printing or runability issues [92]. Nevertheless,
CNFincoatingcolorswithnoevidenceofprintingorrunabilityissues[92]. Nevertheless,anadverse
an adverse effect on the energy consumption during the drying process was seen and attributed to
effectontheenergyconsumptionduringthedryingprocesswasseenandattributedtothehigher
the higher water content of the coating layers. A pertinent study indicated a blend of CNF and starch
water content of the coating layers. A pertinent study indicated a blend of CNF and starch could
could significantly improve the coating layer properties and produce a low-linting surface, indicating
significantlyimprovethecoatinglayerpropertiesandproducealow-lintingsurface,indicatingtherole
the role of CNF to link the starch molecules onto the paper surface. In conventional coating processes,
ofCNFtolinkthestarchmoleculesontothepapersurface. Inconventionalcoatingprocesses,different
different coat weights of 8–12 and 0.5–2 g/m2 are used for pigment and starch, respectively [86]. CNFs
coatweightsof8–12and0.5–2g/m2 areusedforpigmentandstarch,respectively[86]. CNFshave
have also been utilized to bind graphite to cellulose nanocomposites [93]. Also, addition of CNF in
alsobeenutilizedtobindgraphitetocellulosenanocomposites[93]. Also,additionofCNFinpaper
paper coating could increase the printing pick resistance, improve the strengths and modifying the
coatingcouldincreasetheprintingpickresistance,improvethestrengthsandmodifyingtherheology
rheology of the coated layer [10,94]. However, a key issue for using CNF in the coating process is its
ofthecoatedlayer[10,94]. However,akeyissueforusingCNFinthecoatingprocessisitsrheology,
rheology, that even at low solid content (<2%) gives a viscous suspension that hardly loses its water
thatevenatlowsolidcontent(<2%)givesaviscoussuspensionthathardlylosesitswater[95].
[95].

Figure 5. Schematic of hypothetical CNF effect to improve the coating material bonding: (a) the
Fdigefuercet iv5e. Ssuchrfeamceatwici thofn hoyCpNotFh;e(tbic)asla mCNeFsu erfffaeccet wtoit himCpNroFvien ctohrep ocroaattiionng. material bonding: (a) the
defective surface with no CNF; (b) same surface with CNF incorporation.
TobetterunderstandtheroleofCNFasabinderincoatingprocesses,knowledgeofthepossible
interTaoc tbioenttserb uentwdeeresntannadn tohcee lrloulleo osef CaNndF apsa ap ebrinsduebrs itnra ctoeaitsinegs spernotcieasl.seIst, iksnkonwolwedngeth oaft thceel pluolsossieblteo
incetellrualcotsieonasd hbeestiwoneetna kneasnpolcaeclelufrloosme manodle cpualpaerrt osusebvsetrraatlem isil leimsseetnetriaslc.a Ilte sis( Tkanbolew1n). tHhaytd rcoeglleunlobsoen tdos
caenlldulvosaen addehresWioana ltsakfoesr cpelsacaer efrocomn smidoelerecudlathr etod soemveirnaal nmtiilnlitmeremteor lseccaulelasr (Tfoarbclees 1c).o Hntyrdibruogtienng btoontdhse
aanddh evsaino ndsetrr eWngatahls, yfoetrcietsi sanreo tcfounlslyiduenredde rtshteo oddomwhinicahnto inneteirsmooflceecnutlraarl fiomrpceosr tcaonncteritboutthinegb itno dtihneg
asdtrheensgiothn [s1t6re].nAgtlhso, ypehty ist icisa lneont tfaunlglyle umnednetrtshtoaotdre lwiehsicohn omneec hisa nofic caelnfitbrarill liamtipoonrt(awnicthe itno mthicer obninsdcianlge )
sctarennbgotho s[t1c6e].l lAullosose p-cheylsluiclaols eenbtoanndgilnemgeinntp tahpaetr raenlidesp oanp emrbecohaardnsictaol faibgrriellaatteioxnte (nwt.itThhinis mphicernoonm sceanloe)n
coacnc ubrosodstu creinllgultohseeb-ceealtluinlgosper boocensdsinugse idn tpoappreorm anotde pthaepefirbbeorairndtse rtloo cak ginregapt hexenteonmt. eTnhais[9 p6h].enomenon
occurs during the beating process used to promote the fiber interlocking phenomena [96].
Molecules2018,23,2684 8of24
Table1. Differentcellulose-celluloseinteractionsrelativetomoleculardistance,dataadoptedfrom
literature[97,98].
AdhesionMechanism BondLength BondStrength(KJ/mol)
Diffusion <2000µm -
Physicalentanglement 0.01–1000µm -
VanderWaals 0.5–1nm 8.4–21
Acid-BaseInteraction 0.1–0.4nm -
Hydrogenbond 0.235–0.27nm 4.2–188
Covalentbonding 0.15–0.45nm 147–628
3.3. SuperhydrophobicityInducedbyCNF
Superhydrophobicityisafundamentalattributeofsolidsurfacesthatplaysanessentialrolein
manyhouseholdandindustrialapplications. Bydefinition,anysurfacewithawatercontactangle
greaterthan150degreesisconsideredassuperhydrophobic[99]. Amongmanysubstrates,suchas
plastic,glass,metals,etc. producingwater-repellingcellulose,e.g.,hydrophobicpaper,hasattracted
agreatdealofattentionowingtoitscosteffectiveness,wideavailabilityandinpartfortherecent
impetusofenvironmentalconscience[100]. Therefore,fast-growingeffortstoproducehydrophobic
bio-basedsubstrates,suchaspapers,arebeingreportedmoreoften.
Someofthecommonmethodstofabricatehydrophobicpapersareplasmaetching[101,102],ink-jet
printing[103],graftinghydrophobicmolecules[104]aswellasdepositionofnano-materials[105],or
hydrophobicchemicalssuchasalkylketenedimer(AKD)[106].Ondraetal.[107]createdanextremely
waterrepellentsurface(watercontactangelof174◦)byapplyingAKDtoaglasssurface,despitethe
factthattheintrinsiccontactangleofsmoothAKDhasbeenreportedas109◦. Theyascribedthisextra
hydrophobicity to the fractal growth of AKD crystals, leading to formation of a micro-size highly
roughsurface[107]. Anotherapproachistouseprecipitatedcalciumcarbonate(PCC)toprovidesuch
amicro-scaleroughnessforthesubsequentAKDdeposition[108,109]. HoweverPCCparticlesform
looseattachmentstothepapersurfaceandadditionalchemicalsarenecessarytoadherethemtofibers.
Totacklesuchissue,CNFwassuggestedasabindertoachievethedesiredhydrophobicityonthe
paper[15,110]. InastudybyArbatanetal. CNFwasappliedtolinkPCCpigmentsontothepaper
surface,providingtherequiredroughsurfacepriortothesizinglevel. Inthatwork,adip-coating
approachwasusedtoformaPCClayerandthenAKDwasappliedtoachieveasuperhydrophobic
surface[110]. ArelatedstudyshowedthepositiveeffectofCNFtolinklatexonpapersurfacesduring
the coating process, however, its presence reduced the paper surface energy to some degree [90].
It is known that two key factors to obtain ultra-water repellency are the low surface energy plus
nano-roughness[111]andreviewedworksinthissectionprovideevidencethatusingCNFasabinder
maycontributetoboththeseaspects. ThecitedworkregardingthePCCattachmenttopaperviaCNF
wasanexampleofchangeinsurfacemorphologywhereasinthecaseofCNF-latexmixture,itcould
alterthesurfacefreeenergy. Despitemajoradvancesinobtaininglow/ultra-lowenergyinterfaces
frompolymericmaterials,therearestilllimitedstudiesonthecaseofnanocellulose. Suchanapproach,
ifprovedsuccessful,canhighlybenefitpackagingandfoodstorageindustriesinwhichwaterbarrier
propertiesareatthecenterofinterest.
4. NanocelluloseinEnergyStorageDevices
There is a high demand for low-cost green energy storage devices, with large energy density,
recyclability and a harmless core disposal. To achieve these goals, employment of biodegradable
polymerssuchascellulosehasbeensuggestedtobeadoptedinbatteriesaselectrodebinder,battery
bodyandevenelectrolytematerial[112]. Electrodebindersshouldbeelectrochemicallystableand
are used to attach active materials to the composite electrode. They can affect the irreversible
capacitylossesandprovidestabilitytoelectrodecyclesbywithstandingdimensionalchangesduring
charging-dischargingperiodswhilekeepingacceptableenergydensity[112–114]. Anotherimportant
Molecules2018,23,2684 9of24
aspectofbindersistheirsolubility. Polyvinylidenedifluoride(PVDF)isacommonstrongelectrode
binder,howeveritisonlysolubleinnonpolarsolventswhicharemostlyflammable,expensiveand
pollutant. Replacingsuchmaterialwithwater-solubleones,canbothreducetheenvironmentalimpact
andproductioncost[115]. Hereweonlyfocusonthebinderapplicationofnanocelluloseinenergy
storageMdoelevcuilcese 2s0.18, 23, x FOR PEER REVIEW 9 of 24
4.1. Nanboicnedlelur,l ohsoewaesvFerl eixt iibs loenElyle scotrluobdlee Binin ndoenrpolar solvents which are mostly flammable, expensive and
pollutant. Replacing such material with water-soluble ones, can both reduce the environmental
CNF,isarenewablepolymerandcanself-assembleintoacontinuousfilmwithhighstiffness,
impact and production cost [115]. Here we only focus on the binder application of nanocellulose in
transpaerneenrcgyy, sfltoerxaigbei lditeyviacensd. lowthermalexpansion[6,7,116]. Thereforethereisagreatpotentialfor
CNFutilizationasabinderinenergystoragedevices(e.g.,electrodesinLi-ionbatteries),alternativeto
4.1. Nanocellulose as Flexible Electrode Binder
petroleum-basedpolymers[117–119]. Here,someoftherecentadvancementsinthedevelopmentof
CNF-basedCbNinFd, eisr sa irnenbeawtatebrley pcoolmympeor naenndt csaanr seelrfe-avsiseewmebdle. into a continuous film with high stiffness,
transparency, flexibility and low thermal expansion [6,7,116]. Therefore there is a great potential for
In a study on lithium ion batteries, electrolyte saturated paper and dispersed nanofibrillated
CNF utilization as a binder in energy storage devices (e.g., electrodes in Li-ion batteries), alternative
cellulosewereusedasseparatorandelectrodebinder,respectively. Theachievedbindingqualitywas
to petroleum-based polymers [117–119]. Here, some of the recent advancements in the development
similartoPVDF[120]. Similarly, aCNFnetwork(asbinder)wasplacedaroundgraphiteplatelets
of CNF-based binders in battery components are reviewed.
viaasimpleInw aa tsetur-dbya osend litchaisutmin gionm beatthteordiest,o eilnecdtruoclyetep osartousriattye,dfl peaxpiebri laitnyd adnisdpehrisgehd cnhanaorfgibinrigllactaedp acityto
graphiteceallnuolodsee .wTehree urseesdu latsi nsegpsaeralfto-srt aanndd ienlegcteroledcet rboinddeesr,s rheospwecetdivealyg.r Tehaet ealcehcietrvoedch beinmdiicnagl qpuearlfitoyr mance,
closetowthaas tsiomfilcaor ntov PeVnDtioF n[1a2l0P]. VSiDmFil-abrlays, ead CNelFe cntertowdoerks ([a9s3 b]i.nder) was placed around graphite platelets
via a simple water-based casting method to induce porosity, flexibility and high charging capacity to
An MFC-polypyrrole (PPy) electrode composite was made through coating microfibrillated
graphite anode. The resulting self-standing electrodes showed a great electrochemical performance,
cellulose on polypyrrole (via in situ polymerization) to obtain self-standing conductive paper for
close to that of conventional PVDF-based electrodes [93].
a more ecofriendly binder system [118]. Also, an environmentally-benign flexible battery cell has
An MFC-polypyrrole (PPy) electrode composite was made through coating microfibrillated
beensucgeglleuslotesed otnh prooulygphyrirnolceo (rvpiao rina tsiiotun pooflyCmNerFiziantitoon)a tsoi nobgtlaeinp saeplfe-srtalnadyienrg tcooncdouucptilvee ipnatpoetrh feore ale ctrode
activemmaotreer ieaclosf.riTenhdelyp broincdeesrs siynstveoml v[1e1d8]a. Aselsqou, aenn etinavlirwonamteernfitalltlrya-btieonnigann fldextihblee abtattateinrye cdelfil hlmass b(e2e5n0 µmin
thicknesssu)gdgeissptelda ytherdouagchc einpctoarbploercaytioclni nogf CpNerFf oinrtmo aa nsicnegluep poanpepr rloayloern tgoe cdoutepsltei ningto[ 1th2e1 e].leIcntraoddei fafcetirveen tstudy,
materials. The process involved a sequential water filtration and the attained films (250 µm in
CNFwasusedincathodesformulationmadeofLiFePO -carbonblack-CNFviafiltrationtechnique
4
thickness) displayed acceptable cycling performance upon prolonged testing [121]. In a different
andtheresultingcellulose-basedelectrodesdemonstratedahighYoung’smodulusanddischarge
study, CNF was used in cathodes formulation made of LiFePO4-carbon black-CNF via filtration
capacityteocfh1n0iq0uMe aPnada tnhde r1e1s0ulmtinAg hcgel−lu1l,orsees-bpaescetdiv eelelyctr[o1d2e2s] .dLemikoenwstirsaet,eads as hhiogwh nYoinunFgi’gs umreod6u,laufsr aenedst anding
flexibledSiis/chCaNrgCe c-bapaasceidty aonf o10d0e MwPaas anpdro 1d10u mceAdhign−1,a retesprnecatirvyelsyy [s1t2e2m]. Lcioknewsiisseti, nags sohfonwann ion cFeiglluurleo 6s,e a, carbon
nanotubfreesesatannddisnigli cfolenxinbalen oSpi/CarNtiCc-lbeasseads bauniolddei nwgabsl opcrkosd.uTcehde isnt roan tgerantatrayc hsmysetenmt ocfoSnisipstainrtgi colef stothe
nanocellulose, carbon nanotubes and silicon nanoparticles as building blocks. The strong attachment
porousnanocellulose-carbonnanotubesmatrixresultedinahighspecificcapacitanceandexcellent
of Si particles to the porous nanocellulose-carbon nanotubes matrix resulted in a high specific
charge-dischargecyclingperformanceapplicableforlithium-ionbatteries.
capacitance and excellent charge-discharge cycling performance applicable for lithium-ion batteries.

Figure6F.ig(uar)e S6c. h(ae)m Scahteicmvatiiscu vaislluyalslyu msummmarairzizeess tthhee mmaannuufafcatcutrue rperopcreossc eosfs thoef ftlhexeibfllee xSii-bClNeTS-iC-CNNC T-CNC
composite electrode via filtration process. Photographs of the respective electrodes under bending (b)
compositeelectrodeviafiltrationprocess. Photographsoftherespectiveelectrodesunderbending
rolling (c) and twisting (d–e). Reprinted with permission from [123]. Copyright 2015 Royal Chemical
(b) rolling (c) and twisting (d–e). Reprinted with permission from [123]. Copyright 2015 Royal
Society.
ChemicalSociety.
Molecules2018,23,2684 10of24
Besides, no adverse impact on film’s flexibility and tensile properties were observed upon
incorporation of Si nanoparticles into the CNC-CNT matrix. The attained anode could be easily
rolledandtwistedwithnoperformanceloss(Figure6b–e)[123].
However,onechallengeinsuchapplicationsistheamountofwatertrappedinthenanofibrils.
Onlylow-loadsofCNF(~4wt%)couldbeusedtoavoidpossibleadverseeffectontheperformance,
while improving the corresponding energy capacity. Water content value, more than 20−50 ppm,
can lead to lithium salt degradation and since cellulose may contain from 20,000 to 120,000 ppm
of trapped water, applying a prolonged thermal treatment is necessary to remove the excess
water[121,122,124,125].Besides,inspiteofpromisingadheringeffectofCNF,ithasarelativelylow
pliabilityaroundtheelectrodewhichcanrestrictitsutilizationinflexibledevices[126,127]. Forthis,
modifiednanocellulosessuchasTEMPO-oxidizedCNFhavebeenproposedowingtotheirpotent
structure,flexibilityandcompatibilitywithwater-basedsystems[128]. Moreover,cellulosetreatment
with ionic liquids may be used in flexible electronic devices when natural CNF is not the ideal
choice. OnestudyreportedthatbendableCNFfilmscanbeproducedforelectrochemicaldoublelayer
capacitorsoncetheyaretreatedwithhighvoltageionicliquidelectrolytes[129].Furtherinvestigationis
necessarytofullyreplacethecurrentelectrodebinderswithnanocelluloseoritsnano-scalederivatives,
butifprovedfeasible,suchalternativemayfundamentallyimproveenergyefficiency.
4.2. CNFSubstrateforSupercapacitors
Supercapacitorsareshort-termelectrochemicalenergystoragesthatareusedindeviceswithrapid
charge/dischargecyclesattheelectrolyte-electrodeinterface. Theyusuallycantolerateavastrangeof
operationalthermalstresses[130]anddependingonthestoragemechanism,theyaredividedinto
electricaldouble-layercapacitors(EDLC)andelectrochemicalpseudocapacitors[131]. Manyefforts
havebeenmadetodevelopnontoxic,lightweightandbendablesupercapacitorsforusageinportable
andstretchableelectronics. Tothatend,materialssuchaspolydimethylsiloxane(PDMS),graphene
paperandcellulosehavebeenpresentedassubstitutionsofcommercialEDLCs[132–136].
Amongcapacitorcomponents,bindershavethevitalroleofprovidingenoughmechanicalstrength
totheelectrodes. Polyvinylalcohol(PVA),carboxymethylcellulose(CMC)andpolytetrafluoroethylene
(PTFE)arecommonbinderagentsthathavebeenusedincommercialsupercapacitors,howeverdue
to non-conductivity, they may lower the electrode efficiency [137]. Alternatively, various studies
have proposed CNF in such devices to enhance the capacitors strengths and electrical properties.
For instance, composite electrodes were fabricated using CNF and conductive polymers such as
polyanilineandpolypyrrole;asilver-coatedCNFaerogelwasutilizedasasupercapacitorsubstrate
andwaselectrodepositedbypolyaniline(PANI).Uponairdrying,thecapacityofresultingassembly
was monitored and a specific capacitance of 176 mF. cm−2 along with significant resistance to
bending was reported [138]. In a similar study, a highly porous electrode substrate was obtained
usingcellulosenanocrystalsfilmcoatedbypolypyrrole. Theproducedcapacitorshowedexcellent
capacitance(massnormalized)of256F.g−1 owingtoCNC-basedporousstructureandhadbetter
cyclingfunctionanddurabilitycomparedtocarbonnanotube[139]. Andresetal. haveshownthat
CNFissignificantlyeffectivetoaffixgraphiteelectrodestotheelectronicsubstratesoncesoakedin
aqueouselectrolytes[140]. Inanotherwork,abio-friendlyionogelwasproducedthroughgelation
of microcellulose thin films with various methylphosphonate ionic liquids. The attained flexible
ionogelactedashighcapacitancedielectricsunderlowvoltagewhichisapplicableinpaper-based
supercapacitors[141]. Overall,nanocelluloseshavebeensuccessfullyemployedincapacitorsasbinder
andpotentiallycaninducebothstrengthandflexibilitytotheelectronicdevices.
5. AdhesioninNanocellulose
Foralongtimetheinherentself-adhesionofcellulosefibershasbeenrecognizedasakeyfactor
inthestrengthpropertiesofpaperandothercellulose-basedcompositematerials. Theformationof
hydrogenbondsbetweenhydroxylgroupsonneighboringcellulosesurfacesplaysamajorroleinthe