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Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International® and the PITA Annual Review , both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

Functional surfaces, films, and coatings with lignin – a critical review

Paper Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

RSCAdvances

REVIEW

Citethis: RSCAdv.,2023, 13,12529

Received22ndDecember2022

Accepted3rdMarch2023

DOI:10.1039/d2ra08179b

rsc.li/rsc-advances

1.Introduction

View Article Online View Journal | View Issue

Functionalsurfaces, ﬁlms,andcoatingswithlignin

JostRuwoldt,* FredrikHeenBlindheimandGaryChinga-Carrasco

Ligninisthemostabundantpolyaromaticbiopolymer.Duetoitsrichandversatilechemistry,many applicationshavebeenproposed,whichincludetheformulationoffunctionalcoatingsand films.In additiontoreplacingfossil-basedpolymers,theligninbiopolymercanbepartofnewmaterialsolutions. Functionalitiesmaybeadded,suchasUV-blocking,oxygenscavenging,antimicrobial,andbarrier properties,whichdrawonlignin'sintrinsicanduniquefeatures.Asaresult,variousapplicationshave beenproposed,includingpolymercoatings,adsorbents,paper-sizingadditives,woodveneers,food packaging,biomaterials,fertilizers,corrosioninhibitors,andantifoulingmembranes.Today,technical ligninisproducedinlargevolumesinthepulpandpaperindustry,whereasevenmorediverseproducts areprospectedtobeavailablefromfuturebiorefineries.Developingnewapplicationsforligninishence paramount – bothfromatechnologicalandeconomicpointofview.Thisreviewarticleistherefore summarizinganddiscussingthecurrentresearch-stateoffunctionalsurfaces, films,andcoatingswith lignin,whereemphasisisputontheformulationandapplicationofsuchsolutions.

Ligninisthesecondmostabundantbiopolymeronearth,aer cellulose.Naturalligninissynthesizedfromthethreemonolignolprecursors,namely p-hydroxyphenyl(Hunit),guaiacyl(G unit),andsyringyl(Sunit)phenylpropanoid.1 Ligninfrom sowoodconsistsprimarilyofGunits,whereashardwood lignincontainsbothGandSunits.2 Moreover,ligninfrom annualplants,suchasgrassorstraw,cancontainallthree monolignolunits.

RISEPFIAS,Høgskoleringen6B,Trondheim7491,Norway.E-mail:jostru.chemeng@ gmail.com

DrJostRuwoldtisaresearch scientistatRISEPFI,Norway. HegraduatedwithaPhDin ChemicalEngineeringfromthe NorwegianUniversityofScience andTechnology(NTNU)in 2018,andanMScinChemical andBioprocessEngineering fromHamburgUniversityof Technology(TUHH)in2015.His currentworkincludeslignin technology,thermoformingof woodpulp,andbiomass conversionandutilization.InadditiontohisworkatRISEPFI,he isavisitingresearcherandlectureratTUBerlin,Germany.

Technicalligninistheproductofbiomassseparation processesandhencediﬀersfromnaturalorpristinelignin,asit isfoundinlignocellulosebiomass.3 Thecompositionand propertiesoftechnicalligninarelargelydeterminedbytheir botanicalorigin,extractionprocess,purication,andpotential chemicalmodication.4 Presently,therearesome50–70million tonstechnicalligninavailablefrompulpingorbiorenery operations.Mostisburnedtoproduceenergyinbiorenery processesandonlyapprox.2%issoldcommercially.5 Technical ligninisolatedfrompulpingprocessesincludesKra andsoda ligninfromalkalipulping,lignosulfonatesfromsultepulping, andorganosolvligninfromsolventpulping.6 Thetwomain typesoftechnicalligninarelignosulfonates(approx.1million

DrFredrikHeenBlindheimis aPostdoctoralresearcherat RISEPFI.HereceivedaPhDin OrganicChemistryatthe NorwegianUniversityofScience andTechnology,specializingin medicinalchemistryandthe developmentofsmall-molecule bacterialkinaseinhibitors.In hiscurrentposition,heworks withchemicalmodication, quantication,andcharacterizationoftechnicalligninsfor greenapplicationsinindustry.Hismaininterestsareinorganic synthesisandspectroscopicanalysis.

tonsperyear)andkra lignin(<100000tonsperyear).In addition,theadventofhydrolysisandsteam-explosionlignin havecreatednewtypesoftechnicallignin.7,8 Theuseofionicliquidsorsupercriticalsolventshavefurthermoreyieldedthe productsionosolvligninandaquasolvlignin,respectively,with newandinterestingfeatures.9,10

Ligninispolyaromaticandduetothisstructure,itisless hydrophilicthanpolysaccharidicbiopolymers, e.g.,cellulose, hemicellulose,starch,alginateorchitosan.11 Itishence apromisingcandidateinvariousapplications,including:(i) reductionofwettabilityofhydrophilicmaterials,(ii)additionof functionalities,suchasprotectionfromUVlight,antioxidant andantimicrobialproperties,and(iii)tailoringofmaterialsand formulations, e.g.,forcontrolledsubstancerelease,adsorption, orantifoulingmechanisms.12–16 However,chemicalmodicationisrequiredformostapplicationsoflignin.Suchmodicationsfrequentlymakeuseoflignin'shydroxylgroups,for example,bygraingreactionsduringphosphorylation,sulfomethylation,esterication,oramination.17 Thearomatic moietiesinlignincanfurthermorebetargetedfor, e.g.,replacingphenolinformaldehyderesins.18 Atlast,thecarboxylgroups inligninmayalsoserveasreactivesitesforpolyesters.19

Interesthasalsobeenstrongfortheuseoftechnicalligninin polymericmaterials, e.g.,forthermoplasticsorthermosets.20 Processabilityoflignininthermoplasticscanbedonewithout modication,asligninisaninherentlythermoplasticmaterial.21,22 Lignin'sglasstransitiontemperaturecanrangefrom about60–190°Candmaydependonmanyfactors,including thebotanicaloriginandpulpingtype,moisturecontent,and chemicalmodication.23,24 Lignincanalsobechemically modiedtoimprovetheapplicationofligninasspecialty chemicalsorinpolymericmaterials.25–27 Additionally,the utilizationofligninasmacromonomer, i.e.,thermoset precursor,canbedoneaspartofpolyurethanes,polyesters, epoxideresins,andphenolicresins.11 End-usesincludethe productionofrigidorelasticfoams,rigidandself-healing materials,adhesives,biocomposites,andcoatings.19,28–33

DrGaryChingaCarrascowas borninChileandmovedtoNorwayin1987.Hegraduatedwith aCand.scient.degreeincell biology(1997)andDringin chemicalengineering(2002).He wasoneoftworecipientsofthe NorwegianWoodProcessing AssociationAward2019for nanocelluloseresearchand winnerofthe2021 – TAPPI's InternationalNanotechnology DivisionMid-CareerAward.Heis AssociateEditoroftheBioengineeringJournal,andEditor-in-Chief oftheSection – NanotechnologyApplicationsinBioengineering. Currently,heisleadscientistatRISEPFIintheBiopolymersand Biocompositesarea.

Onelong-heldbeliefisthatligninprovideswater-proongin thewoodcellwalltosupportwater-transport.34 Despiteyielding acontactanglebelow90°,whichwouldberequiredtoposeas ahydrophobicmaterial,variousresearchershaveshownthat lignincanreducethewettabilityandwater-uptakeofwoodand pulpproducts.18,35–37 Hence,bothtechnicalandchemically modiedligninhavebeenproposedasadditivesforpackaging materials.38 Reductionofwettabilityof ber-basedpackingis aparticularlyinterestingapplication,consideringenvironmentalandsocietaldriversregardingreductionofsingle-use plasticsandenvironmentalpollution.Lignincouldthusform thebasisofcoatingsorimpregnationblends,providedthatthe lignin-coatingcomplieswithfoodcontactrequirements.One exampleforlignin-blendsisthecombinationwithstarch duringsurface-sizingofpaper,whichcanimproveextensibility andreducewettingofthestarch-matrix.35,39 Layer-by-layer assemblywithmultivalentcationsorpolycationicpolymers hasalsobeendone,whichcanimprovethestrengthand hydrophobicityofcellulose.40,41

Otherapplicationsoflignin,itsderivativesandmixtures includetheuseforcontrolled-releasefertilizers,antifouling membranes, reretardancy,dyesorption,wastewatertreatment,andcorrosioninhibitors.14–16,42–44 Onepublicationeven reportedanunintentionalbutyetadvantageouscoatingofcoir bers,wherethelignindelayedoxidationandthermaldegradationofthe bersinapolypropylenecomposite.45

Majordriversforusingligninareeconomicalaspectsby attributingvaluetoaby-productfrompulpingorbiorenery operations,andsustainabilitybyreplacingfossil-basedmaterialswithbiopolymers.Manyapplicationscanthusbenetfrom theinclusionoflignininfunctionalsurfaces, lms,andcoatings.Themechanismofactionandapplicationmodecan herebydiﬀergreatly.Thisreviewthereforerepresentsaneﬀort tostructureandsummarizerecentprogress,whereemphasisis putonboththeprocessand naluseforlignininsurfacesand coatings.

2.Fundamentals

2.1.Structureandcompositionofnaturallignin Ligninispartofthelignin-carbohydratecomplexes(LCC)that arefoundincellwallsofplantsandwoodymaterials,asillustratedinFig.1.Thecellulose bersaretightlyboundto acomplexnetworkofhemicelluloseandlignin,andthethree biopolymersprovidestrengthandstabilitytothecellwalls.In additiontoprovidingstructuralintegrity,ligninhelpsbuilding hydrophobicsurfaceswhichareimportantintransportchannelsforwaterandnutrients.46

Thecomplexligninnetworkconsistsofthethree4-hydroxyphenylpropyleneunits,ormonolignols,formedfromthe parentcompounds p-coumaryl-(p-hydroxyphenyl,H-unit), coniferyl-(guaiacyl,G-unit)andsinapylalcohol(syringyl,Sunit),seeFig.2.47 Themonolignolsdiﬀeronlyinthepresence orabsenceofoneortwoaromaticmethoxygroups ortho tothe hydroxylgroup.Thesearesynthesized invivo fromthearomatic aminoacidphenylalanine,formedintheshikimicacidpathway inplants.48 Theresultantmonolignolsundergoavarietyof

Fig.1 Compositionoflignocellulosicbiomassandthestructuralrolesofcellulose,hemicellulose,andlignin.46

Fig.2 Monolignolstructureandpositionsindicatedbybluenumbersandletters.

radicalcross-couplingreactionswhichresultsinthecomplex, andvaried,ligninnetwork.Theratiosofthethreemonolignols inligninfromdiﬀerentsourcescanvaryquitesignicantly, hardwoodligninscontainG-(25–50%)andS-units(50–70%), sowoodligninscontainmostlyG-units(80–90%),whilegrass lignincontainsmixturesofS-(25–50%),G-(25–50%)andHunits(10–25%).49

Themonolignolcomposition(H:G:Sratio)canalsovary betweentissuetypesinthesameorganism,whichhasbeen illustratedinthecorkoak, Quercussuber.Ligninfromthexylem (1:45:55)andphloem(1:58:41)diﬀerlessincomposition thanthetwocomparedtothephellem(cork-part,2:85:13).

Thesediﬀerencesaﬀecttheoccurrenceofspecicinterunit linkages,whereanincreaseinS-unitsleadtoanincreasein alkyl–arylether(b-O-4)bonds:68%incork,71%inphloem, 77%inxylem.50

Thediﬀerenceinabundanceofthethreemonolignolslead tomanydiﬀerenttypesofinterunitlinkagesinlignin,specicallybetweenangiosperm(hardwoodandgrass)andgymnosperm(sowood)lignin.51 Themostcommoninterunitlinkage isthe b-O-4alkyl–aryletherbond(Fig.3),whichoccursbetween 45–50%or60–62%ofphenylpropyleneunit(C9 units)in

sowoodsandhardwoods,respectively.Asthisisthemost commonlinkage,manydelignicationprocessestargetthis speciclinkage.Forsowoods,the5–5linkageisalsoimportant,andhasanabundanceof18–25%per100C9 units,while thislinkageoccursonlyaround3–9%inhardwoods.52

Intheprocessofisolatingtechnicallignins,boththelabile aryl-alkyland b-O-4bondsaremostpronetocleavage.53 This resultsintechnicalligninshavingmorecondensedandvariable structuresthannativelignin,andawidevarietyinmolecular weight(Mw).Massaveragevalues(Mw)of1000–15000gmol 1 forsodalignin,1500–25000gmol 1 forKra lignin,and1000–150000gmol 1 forlignosulfonateshavebeenreported, dependingonbotanicaloriginandprocessconditions.54 Native ligninisavirtuallyinnitemacromerthatisbothrandomlyandpoly-branched.55 Thebondsbetweentheligninand surroundinghemicelluloseandcellulosefoundinLCChave recentlybeenreviewed.56 Allsowoodlignin,and47–66%of hardwoodlignin,isreportedlyboundcovalentlytocarbohydrates,andmainlytohemicellulose.Themostcommontypesof linkagesfoundinLCCsarebenzylether-,benzylester-,ferulate ester-,phenylglycosidic-anddiferulateesterbonds.57 Notethat duetothehighdegreeofvariabilityininter-unitandLCC

Fig.3 Commonlinkagesbetweenmonolignolsidentiﬁedinlignins.51,52

linkages,Fig.4shouldonlybetakenasanillustrativeexample. Theligninmacromoleculeispolydisperseandmayexhibit variouslinkagesandfunctionalgroups.4 Inotherwords,lignin shouldbeconsideredasstatisticalentitiesratherthandistinct polymers.

2.2.Isolationoftechnicallignin

Lignocellulosicbiomassconsistsofcellulose(30–50%),hemicellulose(20–35%)andlignin(15–30%),wheretheligninactsas a “glue” withintheLCCs.58,59 Theactuallignincontentofthe biomassishighlyinuencedbyitsbotanicalorigin e.g.,28–32%

Fig.4 AdaptationofAdler'srepresentationofsoftwood(spruce)ligninwithcolor-codedmonolignols: p-hydroxyphenyl(H-unit)inblack, guaiacyl(G-unit)inblueandsyringyl(S-unit)inred.55

isfoundinpineandeucalyptuswood,whileswitchgrass containsonly17–18%lignin,60 andlessthan15%istypically foundinannualplants.61 The rststepinligninvalorizationis biomassfractionation,wherethecellulose,hemicellulose,and ligninareseparatedfromeachother.Severaltechniqueshave beendeveloped,whichcanbegroupedintosulfurandsulfurfreepulpingfromthepaperandpulpindustry,andbioreneryprocessesthataimtoproduceofmaterials,chemicals, andenergyfrombiomass.59 Thelattermayspecicallybe designedtoisolateligninofhighpurityandreactivity,whereas pulpingoriginallyproducedligninasaby-orwaste-product.An overviewisgiveninFig.5.

Thethreeindustrialextractionmethodsforligninarekra, sulteandsodapulping.Inaddition,organosolvpulpinghas beendevelopedtoextractligninandseparatethepulp bers. Commercializationofthisprocesshasnotyetbeendone,but interesthasrisenrecentlyinthistechnology,asorganosolv pulpingproducesatechnicalligninofhighpurityandreactivity.Severalothermethodsalsoexist,butthesearemainly usedinlab-scaleandarereferredtoasbioreneryconcepts,or “pretreatments” 62 IntheKra pulpingprocess,thelignocellulosicbiomassismixedwithahighlyalkalinecookingliquid containingsodiumhydroxide(NaOH)andsodiumsulte (Na2S),atelevatedtemperaturesof150–180°C.Fromthe resultingblackliquor,kra lignincanbeprecipitatedoutby loweringthepHtoaround5–7.5.46 IntheLignoBoostprocess, thisprecipitationisdonebyadding rstCO2 andthensulfuric acid.Kra ligninhasasulfurcontentof1–3%,ishighly condensed,containslowamountsof b-O-4linkages,andis frequentlyburnedforenergyandchemicalrecoveryatthe mills.63–65 IntheKra process,ligninisfragmentedthrough aaryletheror b-aryletherbonds,whichresultsinincreased phenolicOHcontentintheresultantlignin.66 Thesulte processisanotherspecializedpulpingtechnique,whichutilizes acookingliquorcontainingsodium,calcium,magnesiumor ammoniumsulteandbisultesalts.65 Treatmentsaretypically conductedat120–180°Cunderhighpressures,whichgives lignosulfonatesthatcontain2.1–9.4%sulfur,mostlyinthe benzylicposition.67 Lignosulfonatesarecleavedmainlythrough

sulfonationatthe a-carbon,whichleadstocleavageofaryletherbondsandsubsequentcrosslinking.68 BothKra and sulteblackliquortypicallycontainsignicantamountsof carbohydrateandinorganicimpurities.64 Thesodaanthraquinoneprocessismostlyappliedinthepaperindustry onnon-woodymaterialslikesugarcanebagasseorstraw.64 The materialistreatedwithanNaOHsolution(13–16wt%)athigh pressuresandtemperaturesof140–170°C,whereanthraquinoneisaddedtostabilizehydrocelluloses.64,69 Theresulting sodaligninissulfur-freeandcontainslittlehemicelluloseor oxidizedmoieties.Organosolvligninisproducedinanextractionprocessusingorganicsolventsandresultsinseparationof dissolvedanddepolymerizedhemicellulose,celluloseas residualsolids,andligninthatcanbeprecipitatedfromthe cookingliquor.65 Varioussolventcombinationsarepossible, suchasethanol/water(Alcellprocess)ormethanolfollowedby methanolandNaOHandantraquinone(Organocellprocess), whichwillaﬀectstructureoftheresultantmaterials.51 Common forallorganosolvligninsisthattheirstructuresarecloserto thatofnaturallignins,inparticularcomparedtoKra ligninor lignosulfonates.Theyareadditionallysulfur-freeandtendto containlessthan1%carbohydrates.65

Severalothermethodsofbiomassprocessinghavebeen developedthataretargetedatligninextraction,ratherthan producingcellulose bers,whereligninisasabyproduct.64 Milledwoodlignin(MWL)canbeproducedtocloselyemulate nativelignin,butattheexpenseofprocessyields.70 Thismethod isconsideredgentlebuttimeconsuming,oenrequiringweeks ofprocessing,makingitviableonlyinalaboratorysetting.58 Othertechniquesthataimtoproducenativeligninanalogues includecellulolyticenzymaticlignin(CEL)andenzymaticmild acidolysislignin(EMAL).TheCELprocedurewasdevelopedas animprovementoftheMWLprocess,wherehigheryieldswere obtainedwithoutincreasingmillingduration.46 Byaddingan additionalacidolysisstep,Guerra etal. wereabletoagain improveontheyield,whilestillproducingligninthatclosely resembledthenativestructure.70 Thephysicochemical pretreatmentsaimtoreduceligninparticlesizethrough mechanicalforce,extrusion,orother.Thesetechniquesinclude

Fig.5 Ligninextractionprocessesandtheirproducts.3

steamexplosion,CO2 explosion,ammonia berexpansion (AFEX)andliquidhotwater(LHW)pretreatments.46 Ionic liquidshavealsobeensuccessfullyusedforligninisolation. Fivecationswithgoodsolubilizingabilitieswereidentied:the imidazolium,pyridinium,ammoniumandphosphonium cations,whilethetwolargeandnon-coordinatinganions[BF4] and[PF6] werefoundtodisruptdissolutionofthelignin.46 The chosenextractivemethodwillnotonlyaﬀectthecharacteristics oftheresultinglignin,butalsotheamountthatisextracted. Severalmethodshavebeendevelopedforthedelignicationof sugarcanebagasse, e.g.,milling,alkalineorionicliquid extraction,whereyieldsof17–32%wereobtaineddependingon themethodofchoice.71

2.3.Chemicalmodication

Chemicalmodicationoftechnicalligninsiswellexploredand includeahugevarietyoftechniques(seeFig.6forillustrative examples).Technicalligninshavebeenmodiedbyamyriadof techniques,suchasesterication,phenolationandetherication.6 Urethanizationwithisocyanateshasbeenexplored towardspolyurethanproduction,72 andallylationofphenolic OHgroupsenabledClaisenrearrangementintothe ortho-allyl regioisomerwhichisofinterestforitsthermoplasticproperties.73 Thesolubilityandchargedensityoftechnicalligninscan beaﬀectedbysulfomethylationorsulfonation,17,74 andmethylationofthephenolicOHgroupshaveledtoligninwithan increasedresistancetoself-polymerization17 Thethermal stabilityofligninshasalsobeenimprovedbysilylatingthe

hydroxylgroupswithTBDMS-Cl,andtheresultingmaterial couldbeincorporatedintolow-densitypolyethylene(LDPE) blendsformingahydrophobicpolymermatrix.75 Ligninis aversatilescaﬀoldfordiﬀerentmodicationsdependingonthe desiredapplication.Fortheproductionofepoxyresins,epoxidationwithepichlorohydrinisacommontechnique.This approachhasalsobeencombinedwithCO2 xationresultingin cycliccarbonatesbeingincorporatedinthelignin.76

2.4.Analysistechniques

Techniquestoassessligninsandlignocellulosicbiomasshave longbeenatopicofgreatinterest,bothforquantitativeand qualitativecharacterization.Suchtechniquesarealsocriticalto probeandassesschemicalmodications.Asummaryof commonmethodsisgiveninTable1.

Diﬀerenttechniquesareoencombinedtoprovideabetter overallpicture.Forexample,chemicalmodicationoflignin maybeprobedintermsofmolecularweight, i.e.,byusingsizeexclusionchromatography,andabundanceoffunctional groups,asdeterminedbyFTIRor2DNMRanalysis.Thetechniqueofchoicecandependonfactorssuchasthetargetgroups ofinterest,butalsoonavailabilityandcost.Thepolydisperse natureoftechnicallignincansometimesmakeaccurate measurementsdiﬃcult.Thisismanifested,forexample,inthe incompleteionizationofphenolicmoietiesduringtitrationor UVspectrophotometry,asthecongurationandsidechainsof phenolicmoietiesinducevaryingdegreesofresonance stabilization.

Table1 Characterizationtechniquestoassessligninsquantitativelyandqualitatively

MethodDescriptionLimitationsRef.

FTIRspectroscopyPopulartechniquetocombinewith chemometricmethodssuchas principalcomponentregression (PCR)orpartialleastsquares(PLS) regression.Havebeenusedfor successfullydetermininglignin contentinbiomasssamples

1H/13C2DNMRExtremelydetailedinformation aboutinter-unitlinkagescanbe obtained.Hasallowedforthe assignmentandquanticationof over80%oflinkagesinligninoil fromreductivecatalytic fractionationofpinewood

31PNMRDiﬀerentiationofthephenolicOH contentofthethreemonolignolsis possiblefromexperimentsaer derivatizationoftheOHgroups

UV-visspectroscopyCruderdeterminationofphenolic OHcontentispossibleby comparingthediﬀerencesin absorptionatspecicmaxima betweenneutralandalkaline solutions

Simultaneousconductometricand acid–basetitrations

Sizeexclusionchromatography (SEC)

Gaschromatography – pyrolysis (GC-Py)

Coherentanti-StokesRaman scattering(CARS)microscopy

Time-of-ightsecondaryionmass spectrometry(ToF-SIMS)

Afastandcheapalternativetowetchemicalmethodsfordetermining bothphenolicOHgroupand carboxylicacidcontents

Populartechniqueforobtaining weightandnumberaverage molecularweights, Mw and Mn,and forfurthercalculatingthe polydispersityindex(PI)ofsamples

Analysisofbiomasscomposition, quanticationofvolatiles,bio-oil andbiochar.Canbecoupledwith TGAandFTIR.

Label-freemethodwithhigh sensitivityandchemicalselectivity forimagingofligninin e.g. plant cellwalls

Visualizationofmonolignol distributiononplantsamplecrosssections

3.Formulationsandapplicationsof lignin-basedsurfacesandcoatings

Thecoatingsandsurfacemodicationsinthisreviewmost oenfullloneoftwopurposes.Firstly,theymayseekto protecttheunderlyingsubstrate, e.g.,frommechanicalwear, chemicalattack(corrosion),orUVradiation.Secondly,theyadd functionalitysuchasantioxidant,controlledsubstancerelease, orantimicrobialproperties.Reducedwettingand

Calibrationrequiredwithsamples ofknownconcentrations.Large dataset(trainingandtestsets) neededforreliablequantication. Trainingsamplesandprediction samplescannotdiﬀergreatly.

Analysesaresensitivetosample preparationtechniques

NMRexperimentsareexpensive, instrumentsfoundatspecialized institutionsanduniversities.Both experimentsanddataprocessing canbehighlytime-consuming

FullderivatizationofOH-groupsis essentialforproperquantication. Inversegateddecouplingpulse sequenceneededforquantication: reducedsensitivityandincreases relaxationtimeofanalysis

Lessaccuratethan 31PNMR. Aﬀectedbyincompleteionizationof functionalgroups.Presenceofother ionizablegroupscanaﬀectresults

HeterogeneityofCOOH-andOHgroupsdistortsinectionpoint. LimitedtoquanticationofCOOHandOH-groups(andpossiblyother ionizablegroups)

Timeconsumingcalibration required.Samplesmustbewithin linearrange.Acetylationisoen usedforincreasedsolubilitypriorto analysis

78and79

78and80

81and82

78and83

Variationsininherentmetal contentsgreatlyaﬀectsthepyrolysis reactionofthebiomass 84and85

Interferencefromotheraromatic compounds(phenylalanine, tyrosine)candistortimage.Not suitedforimagingalltissuetypes 58and86

Pronetoartefact-generationarising fromsamplepreparation.

Fragmentationofligninduring imagingduetohigh-energyion bombardment

hydrophobizationarefrequentlymentionedforlignin,34,77,78 whichwouldnormallyfallintothesecondcategory,unlessthe purposeistoprotecttheunderlyingsubstratefromdegradation bywater.Thediﬀerentapplicationswillbediscussedmorein detailinthischapter.

Theend-useusuallydeterminesthemanner,inwhich mixturesandcoatingsmustbeformulated.Inprinciple,four diﬀerentapproachescanbedistinguished,whichare(1) applicationofneatlignin,(2)blendsofligninwithotheractive

Overviewofdiﬀerentapplicationmodesforproducingfunctionalsurfacesandcoatingswithlignin.

orinertmaterials,(3)theblendingoflignininthermoplastic materials,and(4)theuseofligninasaprecursorforsynthesizingthermosetpolymers.Anoverviewofthediﬀerent approachesforformulationandapplicationisgiveninFig.7. Thesewillbediscussedinmoredetailfurtheron.

Whilesurfacelayerorcoatingareusuallyappliedonto anothermaterial,therearealsoimplementationsthatinclude ligninaspartoftheoverallbase-matrix.Examplesforthelatter includelignin-derivedbiocarbonparticlesforCO2 captureor wastewatertreatment,polyurethanefoams,andligninasan internalsizingagentinpulpproducts.36,43,79,80 Thepredominant wayofusinglignininfunctionalsurfacesisbyblendingwith othersubstances.Suchformulationsoenincludeagents, whichareestablishedforaparticularapplication, e.g.,starch forpapersizingorclayforcontrolled-releaseureafertilizers.81,82

Formulationsinpolymersynthesisusuallydrawonspecic functionalgroupsthatarefoundinlignin,forexample,the hydroxylgroupsaspolyolreplacementinpolyurethaneorthe aromaticmoietiesasphenolreplacementinphenolformaldehyderesins.18,33

3.1.Surfacesandcoatingswithneatlignin

Applyingtechnicalligninbyitselfisasimpleapproach,asno co-agentsarerequired.Whilesomedegreeofadhesiontothe substrateisoengiven,pressureandheatmaybeappliedin addition.Publicationspertainingtothistopiccanbegrouped intotwocategories, i.e.,fundamentalresearchstudyingthe formationandpropertiesoflignin-based lmsandcoatings,as wellasappliedresearch,whichisusuallyfocusedonaspecic end-use.

3.1.1.Fundamentalresearch. Afundamentalstudywas performedbyBorrega etal.,whopreparedthinspin-coated lmsfromsixdiﬀerentligninsamplesinaqueousammoniummedia.83 The lmsexhibitedhydrophilicitywithcontact

anglesrangingfrom40–60°.Despitewidelydiversecompositions,thesolubilityinwaterwasfoundtobetheparameter governingthepropertiesofthethin lms.Similarresultswere obtainedbyNotleyandNorgren,whofoundthatlignincoatings preparedfromdiiomethaneorformamideyieldedevenlower contactanglesatabout20–30°.34 Theapproachwasfurther renedbySouza etal.,whotreatedthespin-coatedlignin lms via UVradiationorSF6plasmatreatmentinaddition.84 While theUVtreatmentreducedthecontactanglefromabout90°to 40°,theplasmatreatmentproducedsuperhydrophobicsurfaces withcontactanglesexceeding160°.Thelatterwasalsoshownto inducemajorsurfacerestructuringwithastrongincorporation ofCFx andCHx groups,whichwouldaccountforthelarge increaseincontactangle.Coatingswithlignin-basednanoparticlescanalsobemadebyevaporation-inducedselfassembly,whosepropertiesandmorphologyarestronglygovernedbythedryingconditionsandevaporationrate.85 An exampleoftheobtainedmorphologiesisshowninFig.8.Based onthesereports,researchisgenerallyconcurringonthefact thatligninbyitselfisnotahydrophobicsubstance.Harsh treatments,chemicalmodications,or ne-tuningofsurface morphologyarenecessarytoinvokehydrophobicity.

Spin-coated lmsofmilled-woodligninhavefurthermore beeninvestigatedforenzymeadsorption.86 Similarly,the adsorptionofproteinsoncolloidalligninhasbeenstudiedby Leskinen etal.,whoproducedproteincoronasonthelignin particles via self-assembly.87 Theauthorsfurthershowedthat thisdepositionwasgovernedbytheaminoacidcompositionof theprotein,aswellasenvironmentalparameterssuchasthepH andionicstrength.Theuseofligninforprotein-adsorptionis aninterestingimplementation,asitcanprovidediﬀerent surfacechemistriesthanitslignocellulosiccounterparts.Still, thecompatibilitywith invivo environmentsisquestionable,as biodegradationisnotgivenhere.

Fig.7

3.1.2.Appliedresearch. Anexampleforappliedresearch wouldbepaperandpulpproducts,whichcanberenderedless hydrophilicbysurface-sizing.Applicationofthelignincanbe done via anaqueousdispersionoralternativelybyimpregnationaerdissolutioninasolvent.35,88 Asimilarapproachwas usedtotreatbeechwoodwithligninnanoparticle via dipcoating,whichimprovedtheweatheringresistanceofthe wood.89 Suchdip-coatingmaypreservebreathabilityofthe substrateduetotheporousstructure.Inthiscontext,thepatent applicationWO2015054736A1shouldbementioned,which disclosesawaterproofcoatingonarangeofsubstrates includingpaper.90 Inthisinvention,theligniniscoatedonto thesubstrateaeratleastpartialdissolution,followedbyheat oracidtreatment.However,asdiscussedabove,theligninby itselfisnotahydrophobicmaterial.Whilelignin-nanoparticles mayalterthesurfacemorphologyofpulpproducts,an improvementinlong-termwater-resistancemaybemostly determinedbyaﬀectingmass-transferkinetics.

Depositionoflignosulfonatesonnylonhasbeendemonstrated,whichimprovedtheultravioletprotectionabilityofthe fabric.91 Thisdepositiontookplacefromaqueoussolutionand underheating,reportedlyyieldingachemicalbondingof lignin'sOHgroupstotheNHgroupsofnylon6.Suchbonding wouldindeedbenecessary,asthelignosulfonatewouldotherwisebeeasilywashedaway.

Zheng etal. coatedmicrobrillatedcellulosewithKra ligninandsulfonateKra lignin,whichpromoted reretardancyofthematerial.42 Atlast,iron-phosphatedsteelwas renderedmoreresistanttocorrosionaerspraycoatingwith lignin,whichwas rstdissolvedinDMSOandothercommerciallignin-solvents.92 Whileproveninthelab,thesetwoapplicationsmustbeconsideredwithcare,asunmodiedligninis abrittlematerial,whichcanlimitthelong-termdurabilityof suchproducts.

3.2.Theuseofchemicallymodiedlignin

Chemicalmodicationofligninisfrequentlydonetoimprove orenabletheprocessabilityinblendswithmaterials.Inaddition,chemicalmodicationmayaddoralterfunctionalitiesas requiredinspecicapplications.

3.2.1.Lignin-esterderivatives. Estericationofligninwith fattyacidshasbeeninvestigatedbyseveralauthors.This approachbearspotential,asitcombinedtwobio-derived (macro-)molecules.Thelignincontributesabackbonefor graingandmayimprovedispersibilityandadhesionofthe fattyacidsonlipophobicsurfaces.Thefattyacidscaninturn rendertheligninmorehydrophobic,improvingthewater barrier, e.g.,onpapersubstrates.Toimprovethereactionyield, reactiveintermediatesarefrequentlyused.Severalpublications havestudiedtheuseofligninesteriedwithfattyacid-chlorides ashydrophobizationagentsforpaperandpulpproducts.78,93 Thecoatingaﬀectedboththesurfacechemistryand morphology,asillustratedinFig.9.Theresultisusually adecreaseinwater-vaportransmissionrate(WVTR),oxygen transmissionrate(OTR),andanincreaseinaqueouscontact angle.Oxypropylationwithpropylenecarbonatehasbeenused asanalternativeestericationapproach,whichyielded asimilarhydrophobizationandbarriereﬀectonrecycled paper.94 Adownsideofoxypropylationistheuseoftoxicreactants, i.e.,propyleneoxide,andtherequirementforhighpressureduringthereaction.Whilefattyacidchloridesdonotneed highpressures,thesechemicalsarehighlycorrosiveandrequire theabsenceofwater.Allmentionedaspectscanstandinthe wayofcommercialimplementation.

Hua etal. reactedsowoodKra ligninwithethylene carbonatetoconvertphenolichydroxylunitstoaliphaticones,95 astheseareconsideredmorereactive.Thesampleswerefurther esteriedwitholeicacidandspin-orspray-coatedontoglass, wood,andKra pulpsheets.Theauthorsshowedthathydrophobicsurfaceswithcontactanglesrangingfrom95–147°were possible.Thepulpboardsfurthermoreshowedamoreuniform surfaceaerthecoating.Estericationwithlauroylchloride wasalsousedbyGordobil etal.,whostudiedtheirapplication aswoodveneerbypress-moldinganddip-coating.96

Whilethefeasibilitytotreatwoodandwood-basedproducts wasdemonstratedonatechnologicallevel,thecomparisonto establishedtreatmentagentsisfrequentlylacking.Forexample, linseedoilisanestablishedwood-treatmentagent,which undergoesself-polymerizationinthepresenceofair.Papersizingagentscanbebasedoncompoundsthataresimilarin

Fig.8

functiontofattyacids,suchasresinacidsoralkenylsuccinic anhydride.Consideringtheseexamples,thequestionsarises whethermodifyingligninbearsanadvantageoverusing establishedcoatingsorsizingagents.Inthelightofthis discussion,theacid-catalyzedtransestericationofligninwith linseedoilshouldbementioned.77 Accordingtotheauthors, asuberin-likelignin-derivativewasproduced,whichintroduced hydrophobicityonmechanicalpulpsheets,whilebeingmore compatiblewiththe bersthanlinseedoilalone.Theproposed processissimpleinsetupandreactants,whichfacilitatesease ofimplementation.Inaddition,theligninisprescribedakey function, i.e.,actingasacompatibilizerbetweenthe bersand thetriglycerides.

Atlast,controlled-releasefertilizerswithlignin-fattyacid gra polymershavebeenproposed.Wei etal. crosslinked sodiumlignosulfonatewithepichlorohydrin,followedby estericationwithlauroylchloride.97 Sadehi etal. reacted lignosulfonatewithoxalicacid,proprionicacid,adipicacid, oleicacid,andstearicacid.98 Themodiedligninwasfurther usedtospray-coatureagranules.Bothimplementations showedenhancedhydrophobicityandtheabilitytocoaturea forslowerreleaseofnitrogen.Still,itwouldbeimportantto comparesuchapproacheswithestablishedcoatingorblendsof ligninandnaturalwaxesortriglycerides,whichdonotrequire anelaboratedsynthesis.

3.2.2.Enzymaticmodication. Enzymaticmodicationof ligninhastheadvantageofcomparablymildreactionsconditions,whichcanhaveapositiveimpactonprocesseconomics. Onthedownside,enzymesarecomparablyexpensiveand imposeshighertechnologicaldemands.Inaddition,thevariety oflignin-compatibleenzymesissomewhatlimited.Enzymatic treatmentcaninduceanumberofchangestolignin,suchas oxidation,depolymerization,polymerization,andgraingwith othercomponents.99 Forexample,Mayr etal. coupledlignosulfonateswith4-[4-(triuoromethyl)phenoxy]phenolusing laccaseenzymes.100 Aersuccessfulcoupling,thelignosulfonate lmsexhibitedreducedswellingandanincreasein aqueouscontactangle.Fernandez-Costas etal. performed laccase-mediatedgraingofKra ligninonwoodasapreservativetreatment.101 Whilethereactionitselfwasdeemed asuccess,thedesiredantifungaleﬀectwasonlyobtainedaer inclusionofadditionaltreatmentagents,suchascopper.Itis hencequestionableifenzymaticallycoupledligninposesas acompetitivewood-treatmentagent,asthelignincouldalsobe

usedinwood-varnishformulationswithahighertechnological maturity.

3.2.3.Otherapproaches. Avarietyofothermodications hasbeenproposedtodevelopcoatingsfromlignin.For example,Dastpak etal. reactedligninwithtriethylphosphateto spray-coatiron-phosphatedsteelforcorrosionprotection.44 CoatingofaminosilicagelwithoxidatedKra ligninwasperformedbyelectrostaticdeposition,whichimprovedthe adsorptioncapacityfordyesfromwastewater.102 Wang etal. phenolatedlignosulfonate,followedbyMannichreactionwith ethylenediamineandformaldehydetoproduceslow-release nitrogenfertilizers.103 The nalproductexhibitedelevated contactangles,however,anincreasedsurfaceroughnesslikely alsocontributedtothiseffect,asthephenolatedandaminated ligninexhibitednanoparticlestructures.Adifferentapproach wastakenbyBehinandSadeghi,whoacetylatedligninwith aceticacidtocoatureaparticlesinarotarydrumcoater.104 The useoflignininslow-releasefertilizerscanbeuseful,aslignin canhaveasoil-conditioningeffect.However,biodegradability alsomustbeconsidered,whichcanbenegativelyaffectedby chemicalmodication.

Self-healingelastomersweresynthesizedbyCui etal.,who graedligninwithpoly(ethyleneglycol)(PEG)terminatedwith epoxygroups.31 Theauthorsconcludedthatanewmaterialwas developedwithpotentialapplicationforadhesives,butthe ultimatestresswascomparablylowat10–12MPa.Thematerial wasnamedasaself-healingelastomer;however,theappearanceandrheologicalpropertiessuggestathixotropicgel instead.

3.3.Blendsofligninwithothersubstances

Inthecontextofthisreview,thelargestnumberofpublications wasfoundforlignin-blendswithothersubstances.Theadvantageofthisapproachliesintheeaseofimplementation, exibilityforlateradjustments,andpotentialsynergieswithother co-agents.Theligninandotheradditivesmaybemixedright beforeorduringsurfacemodication,hencenotrequiring lengthypreparationssuchasthesynthesisofchemically modiedligninorapre-polymer.Tofacilitatebetteroverview, thissectionwassubdividedintoseveralsub-section,whichwere distinguishedbytheapplicationareaorformulation-approach.

3.3.1.Cellulose bersandotherwood-basedproducts. The useofligninincombinationwithcellulose bers, brils,or derivativeshasreceivedconsiderableattention,asthiscanyield

Fig.9 SEMimagesof(a)uncoatedpaperboardand(b)paperboardcoatedwithlignin-fattyacidester.This ﬁgurehasbeenadapted/reproduced fromref.78withpermissionfromElsevierB.V.,copyright2013. 12538 | RSCAdv.,2023, 13,12529–12553©2023TheAuthor(s).PublishedbytheRoyalSocietyofChemistry

all-biobasedmaterialsandcoatings.Forexample,eucalyptus Kra ligninandcelluloseacetatewerecombinedinsolution andcastontobeech-wood,whichproducedaprotectivecoating similartobark.37 However,theauthorsdidnotdeterminethe mechanicalpropertiesoftheproduct,whichwouldbeimportanttoaddress,asthepotentialbrittlenesscouldimpartpracticaluse.Ontheotherhand,thebiodegradationofligninis indeedmorechallengingthanthatofcelluloseandhemicellulose,105 whichmayhencecontributetoanimprovedresistanceagainstcertainfungiandbacteria.Inaddition,theligninbasedveneermayaddfunctionalitiessuchaswater-repellence, UV-protection,andimprovedabrasionresistance,106 butstill acomparisonwithestablishedtreatmentagentsislacking.

Cellulosenanobrils(CNF)and(cationic)colloidallignin particleswascastinto lmsbyFarooq etal.,yieldingimproved mechanicalstrengthascomparedtotheCNFalone.107 AschematicoftheproposedinteractionsisgiveninFig.10.The authorsconcludedthattheligninparticlesactedaslubricating andstresstransferringagents,whichadditionallyimprovedthe barrierproperties.Thediscussedeﬀectscouldalsobeinduced bytheligninactingasabinder,hence llinggapsandproviding anoveralltighternetwork.36,88 Riviere etal. combinedligninnanoparticlesandcationicligninwithCNF,however,the oxygenbarrierandmechanicalstrengthwerelowerthanthe CNFwithoutaddedlignin.108 Thiseﬀectwaslikelydueto adisruptionofthebindingbetweenCNFnetworks.Thepolyphenolicbackboneofligningenerallyprovideslessopportunitiesforhydrogenbondingthancomparedtothecellulose macromolecule.Theauthorsworkonsolventextractionof ligninfromhydrolysisresiduesisnoteworthy,however,andthe workshowedpromisingpotentialforantioxidantuse.

LCCwerecombinedwithhydroxyethylcellulose,producing free-standingcomposite lms.109 Inthisstudy,theadditionof LCCenhancedtheoxygenbarrierpropertiesandcouldalso improvethemechanicalstabilityandrigidity.Abettereﬀectof LCCwasnotedthancombininglignosulfonateswithhydroxyethylcellulosealone.Synergiescouldhencearisefromcarbohydratesthatarecovalentlybondontothelignin.

AninterestingapproachwastakenbyHambardzumyan etal.,whoFenton'sreagenttopartiallygra organosolvlignin

ontocellulosenanocrystals.110 Theproductwascastintothin lms,whichshowednanostructuredmorphologieswith increasedwaterresistanceandtheabilitytoformselfsupportedhydrogel-lms.Inanotherpublication,Hambardzumyan etal. simplymixedthecellulosenanocrystalswith lignininsolution,aerwhich lmswerecastontoquartzslides anddriedbyevaporation.111 Theauthorsfoundthatoptically transparent lmswithUV-blockingabilitycouldbeproduced.It wasconcludedthatincreasingtheCNFconcentrationallowed forbetterdispersionoftheligninmacromolecules,dislocating the p–p aromaticaggregatesandhenceyieldingahigher extinctioncoeﬃcient.

Anelaborateworkonlignin-starchcomposite lmswas conductedbyBaumberger.39 The lmswereproduced via oneof twomethods:(1)powderblendingofthermoplasticstarchand lignin,followedbyheatpressingandrapidcooling,and(2) dissolutioninwaterordimethylsulfoxidefollowedbysolventcastingandsolventevaporation.Theauthorconcludedthat theligninactedeitheras llerorasextenderofthestarch matrix,wherethecompatibilitywasfavoredbymediumrelative humidity,highamylopectin/amyloseratios,andlowmolecular weightlignin.Lignosulfonatesformedgoodblendsand impartedahigherextensibilityontothestarch lms,likelydue tobenecialinteractionsbetweensulfonicandhydroxylgroups. Non-sulfonatedlignin,ontheotherhand,improvedwaterresistancetoagreaterextent.

Threerecentstudieshavefoundthatincorporatinglignin intoamoldedpulpmaterialscanreducethewettabilityofthe material,aswitnessedbyanincreaseincontactangleor adecreaseinwater-uptake.8,36,88 Theadvantageofsuchimplementationisthathightemperatureandpressurewillpromote densication,asthelignincan owintocavities.Highdensities ofupto1200kgm 3 werereported,wheretheuptakeofwateris hinderednotonlybylimitingmass-transport,butalsoby conningtheswellingofcellulose bers.88

Variousresearchershaveincludedligninintheformulation ofpaper-sizingagents.Inoneimplementation,Javed etal. blendedKra ligninwithstarch,glycerol,andammonium zirconiumcarbonatetoproduceself-supporting lmsand paperboardcoatings.112 Themechanical lmstabilitywasbetter

Fig.10 SchematicillustrationofproposedinteractionbetweenCNFanddiﬀerentligninmorphologies.107

whenusingammoniumzirconiumcarbonateasacross-linking agent,inadditiontoreducingthewater-transmissionrate.Both theligninandtheammoniumzirconiumcarbonatealso reducedleachingofstarchwhenincontactwithwater.In asecondpublication,theauthorfurtherdevelopedtheformulation'suseinpilottrials.81 Johansson etal. coatedpaperboard, aluminiumfoil,andglasswithmixturesoflatex,starch,clay, glycerol,laccaseenzyme,andtechnicallignin.113 Theauthors foundthattheoxygenscavengingactivitywasgreatestfor lignosulfonates,ascomparedtoorganosolv,alkaliorhydrolysis lignin.Thiseﬀectwasexplainedbyagreaterabilityofthelaccasetointroducecross-linkingonthelignosulfonatemacromolecules.Inanotherpublication,Johansson etal. also combinedlignosulfonateswithstyrene-butadienelatex,starch, clay,glycerol,andlaccaseenzyme.114 Theresultsshowedthat bothactiveenzymeandhighrelativehumiditywerenecessary forgoodoxygenscavengingactivity.Laccase-catalyzedoxidation oflignosulfonatesfurthermoreresultedinincreasedstiﬀness andwater-resistanceofthestarch-based lms.Winestrand etal. preparedpaperboardcoatingsusingamixtureoflatex,clay, lignosulfonates,starch,andlaccaseenzyme.115 The lms showedimprovedcontactanglewithactiveenzymeandoxygenscavengingactivityforfood-packagingapplications.Whilethe resultsforpaper-sizingwithadditionofligninshowpromising potential,foodpackagingapplicationsmayimposeadditional requirements.Forexample,stabilityofthecoatingsmaynotbe giveninenvironmentsthatcontainbothmoistureandlipids.In addition,tothebestofourknowledge,nostudyaddressedthe migrationofsizing-agentsintofood.Still,theutilizationof ligninasoxygenscavengerispromising,asthisutilizesoneof ligninsinherentproperties,whicharefoundinfewother biopolymers.

Asanalternativetotechnicallignin,Dong etal. applied alkalineperoxidemechanicalpulpingeﬄuentinpaper-sizing, whichcomprised20.1wt%ligninand16.5wt%extractives basedondrymatterweight.116 Blendedwithstarch,theeﬄuent improvedthetensileindexandreducetheCobbvalueofpaper, whileprovidingcontactanglesof120°andhigher.Such implementationcan,however,alsoaggravatecertainproperties

ofthepaper,asagingandyellowingmaybepromotedbythe presenceofacidsandchromophores.

Layer-by-layerself-assemblywasusedbyPeng etal. to producesuperhydrophobicpapercoatedwithalkylatedlignosulfonateandpoly(allylaminehydrochloride).40 Alternatively, Lit etal. depositedsuchlayersoncellulose bersbycombining lignosulfonateswiththedivalentcoppercationinsteadof apolycation.41 Asimilareﬀectonsurfacemorphologywas noted,whilecontactangleswithinthehydrophobicregime couldbeachieved.Theutilizationoflignosulfonate-polycation assembliesforcellulosehydrophobizationissomewhatcounterintuitive,sincepolyelectrolytecomplexestendtobehydrophilicandcanswellinwater.Thelong-termstabilityofsuch coatingsinwaterishencequestionable,stillforshortcontact timesthemodulationofsurfaceroughnessandchemistrycan bebenecial.

SolventcastingwasemployedbyWu etal.,usingionic liquidstodissolvecellulose,starch,andlignin.117 Thebiopolymerswerecoagulatedbyadditionofthenon-solventwater, furtherbeingprocessedinto exibleamorphous lms.The processappearssimilartotheproductionofcelluloseregenerates.Utilizingotherbiopolymersthancellulose, i.e.,lignin, hemicellulose,andstarch,isaninterestingapproachfor netuningthedesired lmproperties.

Zhao etal. usedevaporationinducedself-assemblyoflignin nanoparticlesandCNF,whichweresubsequentlyoxidizedat 250°Candthencarbonizedat600–900°C.79 Thesenano-and micro-sizedparticlescouldbeusedforCO2 adsorption,where synergisticeﬀectsbetweentheCNFandligninnanoparticles werenoted.AnillustrationoftheparticlesisshowninFig.11. Agrochemicalformulationswithlignin-basedcoatings predominantlyinvolvefertilizerformulations, i.e.,for controlledreleaseofnutrients.Thelignincanbepartof acoating,whichthenactsasamass-transferbarrierthatdelays thedissolutionofnutrients.82,118,119 Thefocusisusuallyonurea asnitrogenfertilizerorcalciumphosphateassuperphosphate fertilizers.82,118–120 Anadvantageofusinglignin,apartfrom beingbiodegradableandwater-insoluble,isthepotential functionassoilamendment.121

Fig.11 Lignin-basedporousparticlesobtainedbyoxidationandcarbonizationforcarboncapture.SEMimagesofligninparticlescarbonizedat low(a)andhigh(b)pre-oxidationrate,yieldingtwodistinctmorphologies.This ﬁgurehasbeenadapted/reproducedfromref.79withpermission fromElsevierB.V.,copyright2017.

Twoapproachescangenerallybedistinguished,basedon eithertheuseofneatorchemicallymodiedlignin.Properties suchaswater-permeabilityandnitrogenorphosphorrelease canbepositivelyaﬀected;however,chemicalmodicationmay impairbiodegradation.Withthatsaid,theworkofFertahi etal. shouldbenoted,whocoatedtriplesuperphosphatefertilizers withmixturesofcarrageenan,PEG,andlignin.118 Thelatterhad beenobtainedfromalkalipulpingofolivepomace.Thethree mentionedcoating-materialsareinprincipleallbiodegradable. BlendingligninwithcarrageenanorPEGimprovedthe mechanicalstabilityofthe lmscomparedtoligninalone, whilealsoincreasingtheswellingofthecoatings.Similar blendswerestudiedbyMulder etal.,whofoundthatglycerolor polyolssuchasPEG400couldimprovethe lmformingproperties.120 Thewaterresistance,ontheotherhand,wasimproved byusinghighmolecularweightPEGorcrosslinkingagentssuch asAcronalorStyronal(commercialname).Onthedownside, thebiodegradabilitywillbenegativelyaﬀectedbysuchcrosslinkingagents,especiallyacrylatesorstyrene-based chemistries.

ChemicalmodicationofligninforcoatingofsuperphosphatefertilizerswasalsoconductedbyRotondo etal., 119 where thetechnicalligninwaseitherhydroxymethylatedoracetylated. Apartfromutilizingtoxicchemicalsinthesynthesis,these modicationsalonedonotposeasadetrimenttobiodegradability.However,theRotondo etal. alsosynthesizedphenolformaldehyderesintocoatthefertilizercores,whichcouldbe troubling,astheauthorsbasicallysuggestedaddingplasticsto thesoil.Zhang etal. furthermoremodiedligninbygraing quaternaryammoniumgroupsontoit.82 Whilethequaternary ammoniummayconvenientlybindanionsandaddnitrogento thesoil,someofitsdegradationproductsarehighlytoxicand henceconcerning,unlessthegoalistoaddbiocidestothesoil. AsimilarapproachwasdonebyLi etal., 14 whosynthesized multifunctionalfertilizers.First,alkaliligninandNH4ZnPO4 weremixedanddissolvedtoproducefertilizercores,which werefurthercoatedwithcelluloseacetatebutyrateandliquid paraﬃn.Asecondcoatingwasthenappliedasasuperabsorbent,whichwasbasedonalkaliligningraedwithpoly(acrylic acid)inablendwithattapulgite.Boththeparaﬃnandpoly(acrylicacid)gra shouldhavebeenavoidedduetoenvironmentalincompatibilities.

Atlast,adiﬀerentapplicationwasexploredbyNguyen etal., i.e.,theencapsulationofphoto-liablecompoundswithalignin coatinglayer.122 Inparticular,theauthorsemulsiedthe insecticidedeltamethrininacornoilnanoemulsionwith polysorbate80andsoybeanlectinasemulsier.Thedroplets werefurthercoatedwithchitosanandlignosulfonate.The lignincontributedherebytoboththeUV-protectionofthe emulsiedinsecticide,aswellastoitscontrolledrelease.This approachispositiveinseveralregards,asonlybiobasedagents wereusedintheformulation,thelignosulfonateswerenot chemicallymodied,andtheapplicationdrewonsomeof lignin'sinherentproperties.

3.3.2.Biomaterialsandbiomedicalapplications. A biomaterial, i.e.,amaterialintendedforuseinoronthehuman body,mustcomplywithcertainrequirements.Thisimpliesthat

thematerialshouldbebiocompatibleandshouldnotcausean unacceptableeﬀectonthehumanbody.123 However,thedenitionofbiocompatibilityhasbeendebatedintheliterature.In additiontoamodieddenitionof “biocompatibility”,Ratner proposed “biotolerability” todescribebiomaterialsinmedicine.124 Biocompatibilitywasdenedby “theabilityofamaterial tolocallytriggerandguidenonbroticwoundhealing,reconstructionandtissueintegration”,whilebiotolerabilitywas proposedtobe “theabilityofamaterialtoresideinthebodyfor longperiodsoftimewithonlylowdegreesofinammatory reactions”.Novelbiomaterialsdevelopedforbiomedicalapplicationscouldbedenedbythesetermswiththetargetof limited broticreactions,125 andligninmaybewithinthisgroup ofbiomaterials.Ligninisamaterialderivedfrombiobased resources,withattractivepropertiesforbiomedicaluse, primarilywithantioxidantandantibacterialcharacteristics. Theantioxidantpropertyofligninisdependentonthephenolic hydroxygroupscapableoffree-radicalscavenging.Theantimicrobialeﬀectisalsocausedbythephenoliccompounds.126 As expected,theantibacterial,antioxidantandcytotoxicproperties mayalsodependonthetypeoflignin.127,128 Forexample,kra ligninhasbeenfoundtohavelessantibacterialproperties comparedtoorganosolvligninduetothelargermethoxyl contentinorganosolvlignins.127

Severalauthorshaveattemptedtodrawonlignin'santibacterialandantiviralproperties,whichcouldbeusefulinsurfaces forbiomaterialsandbiomedicalapplications.Antimicrobial coatingswere,forexample,preparedbyLintinen etal.via deprotonationandionexchangewithsilver,129 asshownin Fig.12.Jankovic etal. alsodevelopedsuchsurfacesby ashfreezingadispersionoforganosolvligninandhydroxyapatite withorwithoutincorporatedsilver.130 Aerfreezing,the samplesweredriedbycryogenicmultipulselaserirradiation, producinganon-cytotoxiccomposite,whichwerefurthertested ontheirinhibitoryactivity.Asimilarapproachwastakenby Erakovi´ c etal.131 Theauthorspreparedsilverdopedhydroxyapatitepowder,whichwasthensuspendedinethanolwith organosolvligninandcoated via electrophoreticdeposition ontotitanium.131 Thiscompositeshowedsufficientreleaseof silvertoimposeantimicrobialeffect,whileposingnon-toxicfor healthyimmunocompetentperipheralbloodmononuclearcells attheappliedconcentrations.However,theuseofsilverhas causedsomeenvironmentalconcernsthatshouldbe addressed.132 Asanalternative,copperhasbeenreportedwith betterantibacterialeffectthansilver,whichtoourknowledgeis currentlyunexploredinantimicrobiallignincomplexes.133

Lignin-titaniumdioxidenanocompositeswerepreparedby precipitationfromsolutionandtestedfortheirantimicrobial andUV-blockingproperties.134 Theauthorsconcludedthatthe lignincouldfunctionasthesolecappingandstabilization agentforthetitaniumdioxidenanocomposites.Betterperformanceofthenanocompositesforantioxidant,UV-shielding, andantimicrobialpropertieswasreported,ascomparedto theligninortitaniumdioxidealone.

Kra ligninandoxidizedKra ligninwereprocessedinto colloidalligninparticlesandcoatedwith b-casein,whichwas furthercross-linked.29 Thisworkaimedtoproducebiomaterials ©2023TheAuthor(s).PublishedbytheRoyalSocietyofChemistry

Simpliﬁedschematicoftheproductionofsilver-dopedligninnanoparticles.129

andbio-adhesives,wherethecolloidalligninactedasthescaffoldintendedforthesynthesisofbio-compatibleparticles. However,noassessmentofbiocompatibilityofthegenerated complexeswasperformed.Hence,thequestionremains whetherthisapproachmaybesuitableforbiomedical applications.

AccordingtoDominguez-Robles,therearevariousadditionalbiomedicalapplications,inwhichlignincouldbe promising, e.g.,ashydrogels,nanoparticlesandnanotubes,for woundhealingandtissueengineering.135 Theinterestinlignin forbiomedicalapplicationswasalsoemphasizedbythe increasingamountofpublicationsrelatedtoligninappliedas afunctionalmaterialfortissueengineering,drugdeliveryand pharmaceuticaluse.135,136 However,thereportedstudieson ligninforbiomedicalapplicationsisstilllimitedandvarious challengeswillneedtobeovercometoadvanceinthisarea. Theseareespeciallyregardingrelevantassessmentoflignins toxicologicalproleandbiocompatibility.Nottomentionthe largevariabilityofligninswhenitcomestothesourceoflignin, thefractionationprocessesandposteriormodication,which mayaﬀectthechemicalstructure,homogeneity,andpurity.

3.3.3.Wastewatertreatment. Diﬀerentresearchershave formulatedlignin-basedmaterials,whichweredesignedforthe puricationofdyecontainingwastewater.16,102,137 Suitabilityfor suchapplicationsisprincipallyderivedfromthechemical similarities,whichexistbetweenligninandmanydyes, i.e.,an abundanceofheteroatomsandaromaticmoieties.Such adsorbentscanbeproduced via depositiononsilicagel,102 coatingontocarbonparticles,16 carbonization,43 sorptionand co-precipitation.138 Developmentofsuchmaterialsisgenerally positive,asitdrawsontheuniquecompositionoflignin.

Adiﬀerenttechnologicalapproachwithinthesamearea wouldbemembranes.Layer-by-layerassemblyisafrequently usedtechnique,inwhichpolyanioniclignosulfonatesor sulfonatedKra ligninarecombinedwithapolycation. Multiplebilayersaremadebystepwiseapplicationofthepolyelectrolytes, e.g.,byimmersioninasolutionofpolyanion rst, followedbydryingandimmersioninasolutionofpolycation. ThisapproachwasusedbyShamaei etal. asanantifouling

coatingformembranes,improvingthetreatmentofoily wastewater.139 Gu etal. usedlignosulfonatesandpolyethyleneimineonapolysulfonemembrane,whichsuccessfully repelledtheadsorptionofproteins.15 Whilethepreparationis straight-forward,thelong-termstabilityofsuchcoatingsalso mustbedemonstrated.Boththeligninand(poly-)cationwere water-soluble,sothecoatingwouldbepointlessifwashedaway withtheretentate.

3.3.4.Packagingapplications. Packagingapplicationscan benetfromlignin-containingsurfacesinvariousregards. Improvementsinthewater-resistance,waterandoxygen barrier,andmechanicalstrengthofcellulose-basedsubstrates havebeenreported.35,107,108,140 Amoredetailedsummaryofthese materialsisgiveninSection3.3.1.Theligninmayalsoserveas anoxygenscavenger.Toimplementthis,severalauthorshave formulatedcoatingsthatincludebothligninandlaccase enzyme.113–115 Atlast,theantibacterialandUV-shieldingpropertiesofligninhavebeenmentionedasbenecialcontributors.83,107,110 Whilethestudiesdemonstratefeasibilityon atechnologicallevel,thereareotherfactorsthatmustbe consideredaswell.Long-termstabilityandmigrationofthe coatingsisrarelyaddressed,despitethisbeingacrucial parameterinfoodpackaging.Inotherwords,onemustbesure thatnodetrimentalsubstancesaretransferredtothefood. Somefoodsreleasebothwaterandfat,forwhichligninisin theoryagoodmatch,asitissolubleinneither.Packagingof non-foodsgenerallyposeslessharshrequirements.The requirementsonpricepervolumearegreater;however,the pricingoftechnicalligninshouldbecompetitive.

3.3.5.UV-protection. Thepolyaromaticbackboneoflignin providesextendedabsorptionatsub-visiblewavelengthsof light.UV-shieldingapplicationshencedrawononeofthe intrinsicpropertiesoflignin.Oneexamplewouldbethe developmentofnaturalsunscreens via hydroxylationoftitaniumoxideparticles,duringwhichlignosulfonateswere added.141 Unsurprisingly,itwasconcludedthatthelignin enhancedtheUV-blockingeﬀectofthetitaniumoxideparticles. Anotherpublicationexploredsunscreens,wherenanoparticlesizeligninwasaddedtocommerciallotionformulations.142

Fig.12

TheUVabsorbancewasimprovedbybothnanoparticle formationandpretreatmentwiththeCatLigninprocess.The latterwasexplainedbypartialdemethylationandboostingof chromophoricmoieties.Whileligninmayconvenientlyreplace fossil-basedandnon-biodegradableUVactives,otherfactors alsoneedtobetestedforsuchaproducttobecomefeasible, e.g.,non-hazardousness,safetyforhumancontact,andskin tolerance.

Otherapplicationsthatcanprotfromthispropertyinclude UV-protectiveclothing,91 packagingmaterials,83,107 agrochemicalformulations,122 andpersonalprotectiveequipment.134 It shouldbementioned,however,thatenhancedUVabsorbance isnotalwaysbenecial,asitcanalsoleadtofasterdegradation ofthelignin-containingmaterials.12

3.4.Ligninaspartofthermoplasticmaterials

Ligninisathermoplasticmaterialwithglass-transition temperaturesintherangeof110–190°C.24 Assuch,itis straightforwardtousetechnicalligninasa llermaterialin, e.g.,thermoplasticsorbitumenadmixtures.38 Potentialadvantagesoflignininthermoplasticpolymercoatingshavebeen discussedbyParitandJiang,21 i.e.,byaddingUV-blockingand antioxidantactivityasrequiredinpackagingapplications.In general,theadditionoflignininthermoplasticscanincrease stiffness,butattheexpenseofextensibility.38 Chemicalmodication(alkylation)mayberequiredtoimprovebothtensile stiffnessandstrengthofolenicpolymers.26 Ontheotherhand, lignin'samphiphilicmake-upcanimpartadvantages, e.g.,by improvingtheadhesionofpolypropylenecoatings.143 Another examplewouldbetheuseoflignininbiocompositesfrom polypropyleneandcoir bers.45 Whilenosignicanteffecton tensilestrengthofthecompositeswasfound,addinglignin reportedlydelayedthethermaldecomposition.

Coatingswithpolymersarefrequentlyusedtoprotectthe mechanicalintegrityoftheunderlyingsubstrate.Foradded lignintobeadvantageous,themechanicalcharacteristicsofthe polymerblendmusthencebeimproved.Whilepublicationsin thisareafrequentlyfocusontheaddedfunctionalities,some alsoreportedimprovementsinthemechanicalstrengthofthe coatings.12,26 Onthedownside,theadditionofligninisoen limitedtolowratiosandchemicalmodicationmaybe required.12 Thesefactorscanlimittheoverallsustainability

gain,whichbiopolymershaveoverfossil-basedpolymersand llers.

Atlast,slow-releasefertilizerscanbepreparedfromthermoplasticsandlignin.Li etal. blendedpoly(lacticacid)(PLA) withKra ligninsamples,someofwhichhadbeenchemically modiedbyestericationorMannichreaction.144 Ureaparticles werethencoatedbysolventcastingordip-coating,wherethe alkylatedligninyieldedimprovedbarrierpropertiesandbetter compatibilitywithPLA.Microscopeimagesofthecoatedurea particlesareshowninFig.13.Whilelignin-PLAblendscan potentiallybemorebiodegradablethanlignin-basedresins,the biodegradationinsoilmaystillbeinsuﬃcient.Ourrecommendationishencetofavorblendsofunmodi edligninwith biopolymers,suchasstarch,cellulose,orcarrageenan,asthis willnotcontributetomicroplasticspollution.

3.5.Ligninasaprecursortothermosets

Thefourmostcommonapplicationsoflignininthermosetsare polyurethane,epoxideresins,phenolicresins,andpolyesters.11 Unsurprisingly,formulationsoflignin-basedthermosetcoatingsareoenderivedfromsuchchemistries.Thelignincan alsoberenderedcompatiblewithotherformulations, e.g.,with polyacrylatesbygraingwithmethacrylicacid.145 Suchgraing reactionsareindeedinstrumentaltoovercomesomeofthe traditionalchallengesoflignin;146 however,theycanalsobe accompaniedbyunwantedside-eﬀects,suchaspoor biodegradability.

3.5.1.Lignin-basedpolyurethanecoatings. Ligninutilizationinpolyurethanesisdoneaspolyolreplacement,where lignin'shydroxylgroupsarereactedwithisocyanategroups actingascross-linker.Theligninmayevenbesolubleinthe polyol,whichaidsstraight-forwardsubstitution.Ligninderivatizationtoimprovethecompatibilityandperformance includeshydroxyalkylation(e.g.,withpropyleneoxide, propylenecarbonate,orepichlorohydrin),estericationwith unsaturatedfattyacids,methylolation,anddemethylation.28

Chen etal. blendedalkaliligninandPEG,whichwerefurther polymerizedwithhexamethylenediisocyanateinpresenceof silicaaslevelingagent.147 Experimentswerelimitedto60wt% lignin,ashigherratiosyieldedanembrittlement.Themixtures wereprocessedinto lms,whichshowedsomepotentialfor biodegradation.Theseresultsindeedcorroboratedbyother authors,whichalsostatethatligninincorporationin

polyurethanesyieldedalimiteddegreeofbiodegradability.148 A diﬀerentapproachwasmadebyRahman etal., 149 whosynthesizedwaterbornepolyurethaneadhesiveswithaminatedlignin. ThetensilestrengthandYoung'smodulusimprovedwith increasingratiosofaminatedlignin,whichcouldbeduetoan increasedcross-linkingdensity.Still,theoverallpercentagesof lignininthecoatingswerecomparablylow,astheauthors addedonlybetween0–6.5mol%lignin.Itiscurioustonotethat theauthorsproclaimedbetterstoragestabilityofaminated lignindispersions,yetonlytheweatheringresistanceofthe nalcoatingwasmeasured.

Someofthechallengeswithlignininpolyurethanematerials includereactivityandahighcross-linkingdensity.Duetothe latter,polyurethaneformulationsarefrequentlylimitedtolow percentagesoflignin,typically20–30wt%atmax,ashigher ratioscanyieldbrittleandlow-strengthmaterials.150 One approachistoincreasethedegreeofsubstitutionisdepolymerizationoflignin,butotherchemicalmodicationsorfractionationsmaybeequallyapplicable.Inthiscontext,thework byKlein etal. shouldbementioned,whoreportedpolyurethane coatingswithligninratiosofupto80%.151 Acomparablylow curingtemperatureof35°Cwasused,whichcouldalsoentail incompletereaction.Curiously,thereisnodataonthe mechanicalstrengthofthe lms.Inaddition,theauthors measurementsofhydroxylgroups via ISO14900and 31p-NMR arewidelydivergent.Intwootherpublicationsbythesame author,theantioxidantpropertiesandantimicrobialeﬀectof such lmswerestudied.13,152 Inadiﬀerentstudy,methyltetrahydrofuranwasusedtoextractthelow-molecularweight portionfromKra lignin.153 Theauthorsusedbetween70to 90wt%lignininthe nalformulationatNCO/OHmolarratios of0.16–0.04.Whileprovidingagoodadhesivestrength,the lmselasticmodulusiswithinthesamerangeofthefractionatedlignin,whereasnoinformationonthematerialstrength wasprovided.Itwouldthusappearthattheelevatedcrosslinkingdensitymaybecircumvented, i.e.,simplybyreacting onlyasub-fractionoftheavailablehydroxylgroupsoflignin. Still,ithasyettobedemonstratedthatsuchcoatingsarealso competitiveinmechanicalstrengthandabrasionresistance.

3.5.2.Lignin-basedphenolicresincoatings. Ligninmay alsobeusedasaphenol-substituentinphenol-formaldehyde resins.20 ThisapproachwasutilizedbyPark etal. toproduce cardboardcompositesbyspraycoating.154 Theauthorsreported thatligninpuricationbysolventextractionyieldedbetter resultsthanbyacidprecipitation.Substitutingwith20–40wt% ligninsurprisinglyacceleratedthecuringkinetics,comparedto thelignin-freecase.Thecoatedcardboardshowedlowerwater absorption;however,thecontactanglewasalsolower,which couldbeduetoachangeinsurfacechemistryandmorphology. Itwouldbeinterestingtostudyevenhigherdegreesofsubstitutionandtodelineatewiththemechanicalstrength.Still,it appearsthatcoatingswithlignin-phenol-formaldehydehaveso farbeenaimedatprovidingawater-barrier.Forexample,the workbyRotondo etal. coatedsuperphosphatefertilizerswith hydroxymethylatedligninresins,119 whichsignicantlyslowed thephosphaterelease.

3.5.3.Lignin-basedepoxyresincoatings. Similartolignincontainingpolyurethanes,epoxyresinsalsotargetareaction withthehydroxylgroups.Inanalogytothat,chemicalconditioningsuchasdepolymerizationcanpotentiallyimprovethe nalmaterial.Forexample,Ferdosian etal. testeddifferent ratiosofdepolymerizedKra ororganosolvlignininconventionalepoxyresinformulations.155 Theauthorsshowedthat largeamountsofligninretardedthecuringprocessparticularly inthelatestageofcuring.Attherightdosage(25%),theligninbasedepoxyexhibitedbettermechanicalpropertiesthanthe neatformulation,whileimprovingadhesiononstainlesssteel. Botheffectsappearplausibleconsideringligninsmacromolecularandpolydispersecomposition.Inthiscontext,arecent patentbyAkzoNobelshouldalsobementioned,which describestheuseofligninandpotentialepoxycrosslinkerfor functionalcoatings.156 AdifferentapproachwaschosenbyHao etal.,whocarboxylatedKra lignin rst,followedbyitsreaction withPEG-epoxy.157 Thecoatingspossessedalignincontentof 47%.Inaddition,theself-healingabilitywasdemonstratedby transestericationreactioninpresenceofzincacetylacetonate catalyst.

Crosslinkingofnanoparticlesisaninterestingapproach,as thecoagulationtonanoparticlesmayfavoradiﬀerentratioof functionalgroupsatthesurfacethaninthebulklignin.In addition,thisapproachcanproducecompositematerials, whichexhibitdiﬀerentcharacteristicsthanahomogeneous polymer.Forinstance,Henn etal. combinedligninnanoparticleswithanepoxyresin, i.e.,glyceroldiglycidyl ether,totreatwoodsurfaces.106 Thecoatingsshowednanostructuredmorphology,whichstillpreservedthebreathability ofthewood,hencedrawingadvantagefromlignin'snanoparticleformation.Zou etal. coprecipitatedsowoodKra lignintogetherwithbisphenol-a-diglycidylethertoproduce hybridnanoparticles.158 Theparticleswereeithercuredin dispersionforfurthercationizationordirectlytestedintheir functionaswoodadhesives.Theuseoflignin-basednanoparticlesincurableepoxyresinsishencepromising,asitcan generatenewfunctionalities,butthematurityofthistechnologystillneedstobeadvanced.

3.5.4.Lignin-basedpolyestercoatings. Whiletheuseof lignininpolyestercoatingsistechnologicallyfeasible,few publicationswerefoundtothistopic.Onereasonforthiscould betheslowreactionkineticsofdirectesterication.Coupled withlignin'sstructureandchemistry,polyester-basedcoatings wouldbelessstraightforwardthanpolyurethanesorepoxy resins,whichinvolvehighlyreactivecouplingagents.Asdiscussedpreviously,chemicalmodicationofligninmayimprove thiscircumstance,forexamplebydepolymerizationorintroductionofnewreactivesites.Assuch,oxidativedepolymerizationandsubsequentmembranefractionationhasbeen suggestedtoproducearawmaterial,whichcanbeutilizedin subsequentpolyestercoatings.159 Asecondexamplewouldbe solvent-fractionatedlignin,whichhasbeencarboxylatedby estericationwithsuccinicanhydride.19 AsillustratedinFig.14, themodiedligninreportedlyunderwentself-polymerization, wherethegraedcarboxylgroupsreactedwithresidual

hydroxylgroupsonthelignin.Developmentinthisareahas potential,aspolyesterstendtoexhibitbetterbiodegradability thanpolyolens.

3.5.5.Lignin-basedacrylatecoatings. Lignin-basedacrylatesrelyonthegraingofacrylatemoieties,asthesearenot inherenttolignin.Forexample,methacrylationofKra lignin wasdonetoproduceUV-curablecoatings.145 Theauthors concludedthatincorporatingligninintotheformulation improvedthermalstability,curepercentage,andadhesive performance.Anelaboratestudyonagingoflignin-containing polymermaterialswasconductedbyGoliszek etal.30 The authorsgraedKra ligninwithmethacrylicanhydrideand furtherpolymerizedtheproductwithstyreneormethylmethacrylate.Lowamountsoflignin(1–5%)showedincorporation intothenetwork,whereashigherconcentrationsshowed aplasticizingandmoreheterogeneouseffect.Highlignin loadingsalsoenhancedthedetrimentaleffectsofaging,which mayseemcounterintuitive,asotherreportsfrequentlystate aUV-protectiveabilityoflignin.Still,increaseabsorptionofUV lightcanalsoamplifythedetrimentaleffectsthereof.A combinationofepoxyandacrylatewasusedtodevelopdualcuredcoatingswithorganosolvlignin.160 Theligninwas rst reactedwithepoxyresinandsubsequentlywithacrylatetoform aprepolymer.Inasecondstep,theprepolymerwasmixedwith initiatorsanddiluenttobecoatedontotinplatesubstrates.All inall,lignin-basedacrylatecoatingsappeartohavereached sufficienttechnologicalmaturity,yettheadvantageofadding ligninissometimesunclear.

3.5.6.Otherapproaches. Szabo etal. graedKra lignin with p-toluenesulfonylchloride,whoseproductwasthengraftedontocarbon bers.161 Theresultssuggestedanimproved sheartoleranceofthemodiedcarbon bersinepoxyor cellulose-basedcomposites.SilylationwasfurthermoreperformedofKra lignin,whichwasfurtherco-polymerizedwith

polyacrylonitrile.162 Theauthorsconcludedthatsilylation improvedthecompatibilityforsurfacecoatingsand lms.

4.Ligninintechnicalapplications –acriticalcommentary

Thedevelopmenthigh-valueproductsfromligninhasbeen atopicofgreatinterestforsomeyears,andisstillgaining popularity.163 Added-valueapplicationsarebeingpursued, rangingfromasphaltemulsiersorrubberreinforcingagents, totheproductionofaromaticcompounds via thermochemical conversion.164 Thequestionarises,however,ifincludinglignin inacoatingcanreallyleadtoabetteroverallproduct? Comparisonwithstate-of-the-artformulationsisfrequently omitted,benchmarkinglignin-basedsolutionsonlytoareferencecasewithlowperformance. “Attributingvaluetowaste” is oneoftheprimarymotivationsbehindlignin-orientedresearch. Forexample,bioethanolproductionfromlignocellulose biomassoengivesrisetoalignin-richbyproduct.Theoverall economicsofsuchbioreneriescouldbeimprovediftheligninrichresiduecouldbemarketedatavalue.Still,toestablish anewproductonthemarket,thisproductalsoneedsto competewithexistingsolutionsintermsofperformanceand price.Thispointisoenoverlookedinliterature,inparticular concerninglignin-basedsurfacesandcoatings.

Harnessinglignin'sinherentpropertiesiskey,asthiscan createsynergiesandyieldanadvantageoverotherbiopolymers. Itcomestonosurprisethatthedominantuseoftechnical ligninisinwater-solublesurfactants,aspolydispersityisakey featurehere.3 Ashasbeenpointedout,theperformanceof surfactant-blendsoenoutperformssinglesurfactantsinrealworldapplications,sincethemixturecanpreserveitsfunction overawiderrangeofenvironmentalconditions.Asecond exampleofkeypropertieswouldbelignin'spolyphenolic

Fig.14 Schematicofproducinglignin-basedthermosettingpolyestercoatings.This ﬁgurehasbeenadapted/reproducedfromref.19with

structure,whichisnotfoundincommonpolysaccharides. LigninhashencebeeninvestigatedasaUVblockingadditivein e.g.,sunscreenproductsorpackaging.165 However,compatibilityoftheresultingproductwithhuman-orfood-contactis addressedinsuﬃcientlybymanyauthors.Asimilarsituation wasgivenincaseofligninasantioxidantadditivein cosmetics,166 wherethedarkcolorandsmellmaylimitthe nal use.

Comparedtocelluloseorhemicellulose,ligninhasahigher carbon-to-oxygenratio.Duetothisanditspolyaromaticstructure,itwouldindeedbeabetterrawmaterialforproducing carbonaceousmaterials.Researchonactivatedcarbon, graphiticcarbon,andcarbon bershasindeedbeingconducted.167 Akeysteptowardlignin-basedcarbon berproduction wasidentiedasremovalof b-O-aryletherbonds.60 Inaddition, thecharringabilityofligninhasbeenproposedasabenetin reretardants.168 Still,lignin-based reretardantsoenuse chemicalmodications,suchasphosphorylation.Ifchemical modicationisnecessary,thequestionarisesifsuchchemistriesreallyneedtobebasedonlignin,sinceotherbiomacromoleculesmaypossessahigherreactivityandnumberof reactivesites.

Lignincanbereadilyprecipitatedfromsolutionintonanoparticlesandnanospheres.Variousapplicationshavebeen suggestedbasedonthis,suchasfunctionalcolloidsand compositematerialswithusesin ameretardancy,foodpackaging,agriculture,energystorage,andthebiomedical eld.169 A morespecicexamplewouldbenanoparticulatedligninin poly(vinylalcohol) lmswithincreasedUVabsorption.170 While thistechnologyappearsstraight-forward,its nalusehasyetto beproven.

Atlast,technicalligninisusuallythermoplastic,exhibiting glass-transitiontemperaturesintherangeof110–190°C.24 The useofligninaspolymeric llerorinthermoplasticblendsis hencepromising.Insomecases,chemicalmodicationmaybe necessarytoimprovecompatibility, e.g.,withpolyolens;26 however,theuseassimple llermaterialwouldnotnecessitate modication.Additionalstrengthcouldalsobederivedfrom addedcellulose bers,whichcouldpotentiallybenetfrom addedligninascompatibilizer.

Insummary,oneneedstobuildontheinherentpropertiesof lignin,suchaspolydispersity,poly-aromaticity,ahigherC/O ratiothanforpolysaccharides,andthermoplasticity.Onlyby utilizingcharacteristicsthatsetligninapartfromother biopolymers,cansolutionsbedevelopedthatareinnovative andmarketcompetitive.Chemicalmodicationisausefultool fortailoring;however,eachprocessingstepwilladdan economicalandenvironmentalcosttothe nalproduct.In otherwords,thesimplestapproachisoenthebest – somethingthatisfrequentlydisregardedwhendevelopingcomplex synthesisprotocolsforlignin.

5.Summaryandconclusion

Functionalsurfacesandcoatingscanbeformulatedinavariety ofways,whichincludestheuseofneat,chemicallymodied, blended,andcross-linkedlignin.Thisreviewprovides

asummaryofthecurrentdevelopmentsinresearch,where focuswasplacedontheformulationand nalapplications. Overall,coatingswithneatligninorblendsofligninwith otheractiveingredientsappearthemostpractical.Reduced wettingisherebyachieved,asthelignincanalterthesurface morphology,hindermass-transport,andconneswellingof enclosed bers.Theligninitselfisnotconsideredahydrophobicmaterial,becausethecontactangleisusuallybelow90°. Ontheotherhand,hydrophobicitycanbeinducedbyplasma surfacetreatment,blendingwithotheragents,orchemical modication.Forthelatter,graingorestericationoflignin withalkyl-containingmoietiesisafrequentlytakenapproach. Chemicalmodicationmayalsobeusedtoimprovethe compatibilitywitholenicthermoplastics.Additionoflignin can ne-tunethecharacteristicsthermoplasticsandimprove adhesiontoothermaterials.Onthedownside,embrittlement frequentlylimitsthistechnologytolowpercentagesoflignin. Thermosetcoatingswithlignincanbebasedonchemistries suchaspolyurethanes,phenolicresins,epoxyresins,polyesters, andpolyacrylates.Varioussynthesisrouteshavebeenproposed inliterature,whichcanbenettosomedegreeoftheinherent propertiesoflignin.

Boththeformulationandprocessingdependonthe nal applicationofthecoatingorsurfacefunctionalization.Theuse ofligninwithcellulose-basedsubstratesisfrequentlysuggested,asthiscanyieldall-biobasedmaterials.Lignincan improvetheresistancetowettingofpaperandpulpproducts.In addition,itcanaddUVprotectionandoxygen-scavenging capabilitiesinpackagingapplications.Lignin-basedsurfaces havealsobeenproposedforadsorbentsforwastewatertreatment,woodveneers,andcorrosioninhibitorsforsteel.The biomedical eldhasalsoexploredlignin-basedbiomaterials, whichdrawonitspotentialantimicrobialproperties.Agreat numberofpublicationsalsoreportsonagriculturaluses,where alignin-basedcoatingmayaccountforslowerreleaseoffertilizer.Atlast,general-purposepolymercoatingscanbetailored via theinclusionoflignin,andtheresistancetofoulingof membranescanbeimproved.Allmentionedapplicationswere discussedcriticallyinthisreview,placingemphasisonthe benetthataddingligninmayprovide.Whileintroductionof functionalitiesmaybepossible,publicationsfrequentlydonot comparetoawell-performingreferencecase,hencelimitingthe assessmentofthetruepotential.Inaddition,theratiooflignin inthermosetcoatingsisusuallyquitelow.Higherlevelsmaybe achievedaerchemicalmodication,butsuchsynthesiscan alsohavenegativeimplicationsontheeconomicandenvironmentalcostofthe nalproduct.

Inconclusion,theadvancementoffunctionalsurfacesand coatingswithligninhasyieldedpromisingresults.However, therealsomustbeabenetofusinglignincomparedtoother biopolymersorexistingpetrochemicalsolutions.Onlybyharnessinglignin'sinherentproperties,cansolutionsbedeveloped thatarecompetitiveandvalue-creating.Theseproperties includelignin'spolyphenolicstructure,ahigherC/Oratiothan, e.g.,polysaccharidebiopolymers,itsabilitytoself-associateinto nano-aggregates,anditsthermoplasticity.Thesepropertiesare

utilizedinsomeofthereviewedliterature,henceprovidingthe groundfornewandpromisingtechnologyinthefuture.

Authorcontributions

JostRuwoldt:conceptualization,writingoriginaldra,review, editing&visualization.FredrikHeenBlindheim:writing& visualization.GaryChinga-Carrasco:writing,review,editing, visualization,supervision.

Conﬂictsofinterest

Theauthorsdeclarenoconictofinterest.

Acknowledgements

TheauthorsthanktheResearchCouncilofNorwayforfunding partofthiswork.

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Volume 10, Number 3, 2024 Fit-for-Use

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

Paper Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

nanomaterials

Article Fit-for-UseNanoﬁbrillatedCellulosefromRecoveredPaper

AnaBalea 1 ,M.ConcepcionMonte 1 ,ElenaFuente 1 ,JoseLuisSanchez-Salvador 1 ,QuimTarrés 2 , PereMutjé 2 ,MarcDelgado-Aguilar 2 andCarlosNegro 1, *

Citation: Balea,A.;Monte,M.C.; Fuente,E.;Sanchez-Salvador,J.L.; Tarrés,Q.;Mutjé,P.;Delgado-Aguilar, M.;Negro,C.Fit-for-Use NanoﬁbrillatedCellulosefrom RecoveredPaper. Nanomaterials 2023, 13,2536.https://doi.org/10.3390/ nano13182536

AcademicEditor:HirotakaKoga

Received:4August2023

Revised:6September2023

Accepted:8September2023

Published:11September2023

Copyright: ©2023bytheauthors. LicenseeMDPI,Basel,Switzerland. Thisarticleisanopenaccessarticle distributedunderthetermsand conditionsoftheCreativeCommons Attribution(CCBY)license(https:// creativecommons.org/licenses/by/ 4.0/).

1 DepartmentofChemicalEngineeringandMaterials,UniversityComplutenseofMadrid,AvdaComplutense s/n,28040Madrid,Spain;helenafg@ucm.es(E.F.)

2 LEPAMAPResearchGroup,UniversityofGirona,MariaAurèliaCapmany,6,17003Girona,Spain; pere.mutje@udg.edu(P.M.);m.delgado@udg.edu(M.D.-A.)

* Correspondence:cnegro@ucm.es;Tel.:+34-913944242

Abstract: Thecost-effectiveimplementationofnanofibrillatedcellulose(CNF)atindustrialscale requiresoptimizingthequalityofthenanofibersaccordingtotheirfinalapplication.Therefore,a portfolioofCNFswithdifferentqualitiesisnecessary,aswellasfurtherknowledgeabouthowto obtaineachofthemainqualities.Thispaperpresentstheinfluenceofvariousproductiontechniques onthemorphologicalcharacteristicsandpropertiesofCNFsproducedfromamixtureofrecycled fibers.Fivedifferentpretreatmentshavebeeninvestigated:amechanicalpretreatment(PFIrefining), twoenzymatichydrolysisstrategies,andTEMPO-mediatedoxidationundertwodifferentNaClO concentrations.Foreachpretreatment,fivehigh-pressurehomogenization(HPH)conditionshave beenconsidered.Ourresultsshowthatthepretreatmentdeterminestheyieldandthepotentialof HPHtoenhancefibrillationand,therefore,thefinalCNFproperties.Theseresultsenableoneto selectthemosteffectiveproductionmethodwiththehighestyieldofproducedCNFsfromrecovered paperforthedesiredCNFqualityindiverseapplications.

Keywords: recycledﬁbers;nanocellulose;enzymaticpretreatments;TEMPO-mediatedoxidation; reﬁning;high-pressurehomogenization

1.Introduction

Papermakingisoneofthemostsustainableindustriescontributingtothecircular economy[1].Recoveredpaperiswidelyrecognizedasanefficientandeco-friendlycellulosesourceforpaperandboardproduction.InEurope,55.9%ofrawmaterialscome fromrecoveredpaper,whichcorrespondstoapaperrecyclingrateof70.5%[2].InSpain, thesevaluesareevenhigher(92%and71%,respectively).Theuseofrecoveredpaperas rawmaterialpresentsseveralchallenges,duetotherelativelylowqualityofthesecondary fibersafterseveralrecyclingcycles,whichmainlyaffectthemechanicalpropertiesofthe recycledpaperduetoshorteningandhornificationofthefibersduringeachrecycling process.Toenhancethequalityoftherecycledfibersandachievethedesiredproperties inthefinalproduct,papermakersusefiberrefiningandstrengtheningadditives.Inthis context,extensiveresearchhasbeencarriedoutinthelastdecadetoexplorethepotential ofnanocellulose-basedproductsaspaper-reinforcedadditives.Alotofdifferentnanomaterialshavebeendevelopmentduringthelastthreedecades,suchascarbonnanotubes,2D mono-elementalgraphene-likematerials,and2D β-Indiumsulfidenanoplates[3,4].Some ofthesehavebeencombinedwithnanocellulosetoimprovepaperpropertiesortodevelop newmaterials[5].ItiswellknownthatCNFsimproveinterfiberbonding,significantly increasingthemechanicalpropertiesoftherecycledproducts[6].However,theuseof CNFisstilllimitedatindustrialscale.Thisismainlyduetothehighcostofnanocellulose productsandtosomelimitsintheirapplicationmainlyduetotheireffectondrainage andthedispersiondegreeofthe3Dnetworkashasbeenrecentlydemonstrated[7].To

minimizetheselimitations,newapproachesarebeingdevelopedbasedontheinsitu productionanduseofﬁt-for-useCNFs.

AlthoughCNFscanbeobtainedfromawidevarietyofcellulosesourcessuchas wood(hardwoodandsoftwood),seedfibers(cotton,coir,etc.),bastfibers(flax,hemp,jute, kenaf,ramie,etc.),grasses(bagasse,bamboo,etc.),marineanimals(tunicate),algae,fungi, invertebrates,andbacteria[8],thetrendforinsituproductionisfocusedontheutilization ofvirginandsecondarypulps,dependingontheplantlocation[6].Althoughvirginpulps havebeenwidelystudiedforCNFproductionatindustrialscale,furtherknowledgerelated totheuseofrecycledpulpisstillnecessary.

ThemethodsusedtoobtainCNFsincludeavarietyoffabricationtechniques,eachof whichiscategorizedusingdifferentlabelsliketop-downandbottom-upapproaches,oraccordingtotheirnature,whichincludesphysical,chemical,andbiologicalmethods,oreven dependingonwhethertheyinvolvespinningornon-spinningtechniques[9].Spinning methodscanbefurthercategorizedintoelectrospinningtechniques,whichutilizeelectric voltageforfibermorphologycontrol;andalternativespinningapproacheswhichemploy forceslikepressurizedair(bubbleelectrospinning)orcentrifugalforces(centrifugalspinning)[10].Amongallofthem,physicalmethods(top-downapproaches)basedonintensive mechanicalpressure(high-pressurehomogenization,microfluidization,grinding,refining, ormilling)arecommonlyusedtoobtainCNFsfromvirginandsecondarycellulosefibers. Priortothismechanicaltreatment,variouspretreatments,includingchemical,enzymatic, mechanical,orcombinedapproaches,canbeemployedtoenhancenanofibrillationand theseparationoftheindividualnanofibers.Furthermore,thepretreatmentscontributeto reducetheenergydemandofthesubsequentmechanicaltreatment[11].

Ontheotherhand,thefinalpropertiesoftheCNFsareinfluencedbyseveralfactors, includingthecelluloserawmaterialused,thetypeandintensityofthepretreatment,and thetypeandseverityofthemechanicaldefibrillationprocess.Thesevariablescollectively determinethefinalcharacteristicsoftheproducedCNFs[12].Differentpropertiesare requiredbasedonthefinalapplication.Forexample,themedicalandelectronicfields requireCNFswithhighpurity,fibrillationyield,andhomogeneity[13,14]andtransparent nanopaperproductionrequireshightransmittance[15],butthesepropertiesarenotalways keyforreinforcingsomecompositesorpaperproducts[6].

ThereareseveralstudiesrelatedtotheuseofrecoveredpapertoobtainCNFsatlab scale.Someresearchershaveemployeddifferenttypesofrecycledcellulosicmaterials, suchasrecyclednewspapers[16];recycledpulp[17];wastepaper[18,19];oldcorrugated boards[20,21]orrecycledmilk-containerboard[22].Inthesestudies,CNFswereobtained throughultrafinegrindingorsonicationasmechanicaltreatments,withouttheapplication ofanypretreatment.OnlyUkkolaetal.(2021)pretreatedthepulpsinadeepeutectic solventsolutionbasedoncholinechlorideandureatoobtainnanofoamsfromcrosslinked CNFs[22].Recentstudieshavefocusedonutilizingdeinkingpulp(DIP)asrawmaterial toobtainCNFs.LeVanetal.(2018)successfullyobtainedCNFsbyemployingTEMPOmediatedoxidationfollowedbyhomogenizationinadigitalhomogenizermixer[23]. Angetal.(2020and2021)producedCNFsthroughacombinationofmechanicalpretreatment(PFIrefining)andhigh-pressurehomogenization(HPH)[24,25].Zambranoetal. (2021)obtainedCNFssolelythroughultrafinegrindingusingDIPasrawmaterial[26]. Baleaetal.(2019)producedCNFsfromtwodifferenttypesofrecycledpulps,oldnewsprint (ONP)andoldcorrugatedcontainer(OCC),atdifferentTEMPO-mediatedoxidationlevelspriortothehomogenizationprocess[6].Thestateoftheartshowsafragmentated knowledgefromwhichisnotpossibletoproducefit-for-useCNFs.

Theeffectofdifferentpretreatmentsandhomogenizationconditionshavebeenstudiedforvirginbleachedsoftwoodandhardwoodchemicalpulps[27]andthermomechanicalpulp[28]whichalsodeterminedtheenergyappliedforthenanoﬁbrillationprocess. However,tothebestofourknowledge,theimpactoftreatmentintensity,involvingvariouspretreatmentmethodsandhomogenizationconditionsappliedtorecoveredpaper-

board,onthepropertiesoftheresultingCNFsandtheassociatedenergyconsumption remainsunexplored.

Recently,Angetal.(2021)producedseveralCNFnanopapersfromDIPusingdifferent combinationsofmechanicalrefining(10,000,30,000,and50,000revolutionsinPFI)and HPHat1000bar(1passand3passes)[24].Theyfoundthatthenanofibrillationefficiency forrecycledfiberswaslowerthanthatforvirginbleachedeucalyptuskraftpulpdue tohornificationduringpapermaking.Therefore,resultsfromvirginpulpscannotbe transferredtorecycledpulp,sincethepulpcompositionisdifferent,aspaperboardcontains moreligninandashes,andthefibersarehornifiedduetothedryingprocess.Therefore, recycledpulpswillbehavedifferentlyduringCNFproduction.

Intermsofenergyconsumption,Jossetetal.(2014)conductedanenergy-related studyonthedirectfibrillationofrecyclednewspaperandwheatstrawthroughagrinding process[16].TheycomparedtheCNFsobtainedfromtheserawmaterialswithCNFs derivedfromableachedwoodpulp(ECF)byevaluatingthemechanicalpropertiesand specificsurfaceareasofthefibrillatedmaterials.Thestudyfoundthattheenergyinputs requiredtoachieveoptimalmechanicalpropertiesinthepreparedfilmsfromrecycled newspaperwerehighercomparedtotheECF,rangingfromapproximately4to5kWh/kg. AnotherstudybyOzolaetal.(2019)estimatedthattheproductionofCNFsfromrecycled pulp,withoutdeinking,hadhigherspecificenergyconsumption(0.865kWh/kgwaste) comparedtotheproductionofotherproductssuchaseggpackagingorcardboard[29]. Overall,thesestudiesdemonstratethattheproductionofCNFsfromdifferenttypesof recycledpapersisfeasibleintermsofimprovingthequalityofthefinalproduct.However, itisimportanttoconsidertheenergyconsumptionassociatedwiththespecificproduction processesforeffectiveresourcemanagementandsustainability.

Thisresearchaimstogeneratenewknowledgeontheeffectsofdifferenttreatment combinationsontheCNFpropertiesobtainedfromrecoveredpaperboard.Fivedifferent pretreatmentsareconsidered,includingamechanicalpretreatmentthroughPFIrefining (20,000revolutions),twoenzymatichydrolysisapproaches(80mg/kgand240mg/kg), andTEMPO-mediatedoxidationundertwodifferentconditions(5molNaClO/kgand 15molNaClO/kg).ThestudyexploresfivedifferentconditionsfortheHPHmechanical processforeachpretreatmentmethod.Furthermore,theenergyrequirementsassociated withthedifferenttreatmentprocessesareassessed.Thisinformationwillfacilitatethe selectionofafibrillationprocessbasedontherequiredCNFproperties.

2.MaterialsandMethods

2.1.Materials

OCCwasselectedastherecycledcellulosesourceasitisoneofthemostsignificant typesofrecoveredpapersforrecycling,anditislistedintheEuropeanListofStandard GradesofRecoveredPaperandBoard(EN643)asGroup1inordinaryqualities(1.05). Linerandfluting,usedinproportionsof35and65%forOCCpulppreparation,were kindlyprovidedbySAICA(Zaragoza,Spain).Thisfluting/linerratioisthemostcommon forcardboardboxes.ThechemicalcompositionoftherecycledpulpispresentedinTable 1 Acommercialmonocomponentenzymecocktail,Novozym476,waskindlysuppliedby NovozymesA/S(Bagsværd,Denmark).Thisenzymecocktailcontains2%(w/v)ofendoβ-1,4-glucanases,withanenzymeactivityof341U/mL.Theotherreagentsutilizedinthis researchwerepurchasedfromSigmaAldrich(Madrid,Spain).

2.2.CelluloseNanoﬁberProduction

OCCpulpwaspreparedfromamixtureof35%linerand65%flutingOCCpaper bydisintegrationat3%at3000rpmfor20minusingastandardized2Llabpulper.The resultingpulpwasthenadjustedtothedesiredpretreatmentconsistencybydilutingitwith distillatedwaterorfilteringitthroughaqualitativepaperfilterusingaBuchnerfunnel. TheproductionofCNFsinvolvesatwo-stepprocess,beginningwithapretreatmentstage

followedbymechanicaldeﬁbrillationusingHPH.Differentpretreatmentswereexplored, andﬁvelevelsofHPHenergywereappliedduringtheprocess.

Table1. Chemicalcompositionofrecycledpaperboardpulp.

CompoundPercentage(wt%)

Cellulose56.8 ± 0.4

Hemicelluloses14.0 ± 0.2

Solublelignin4.6 ± 0.2

Klasonlignin11.6 ± 0.2

Extractives1.7 ± 0.1

Ashes11.3 ± 0.4

2.2.1.Pretreatments

TheﬁvedifferentpretreatmentsconsideredinthestudyaresummarizedinTable 2.

Table2. PretreatmentsstudiedtoproduceCNFsfromrecycledpaperboard.

Pretreatment Identiﬁcation DescriptionIntensityConditions

Mec

ReﬁningatPFIreﬁner ISOstandard5264-2 (2011) 20,000revolutions

Enz_80 Enzymatichydrolysis

Doseofenzyme: 80mg/kg

PC=5%

pH=7

Time=4h

Enz_240240mg/kg

TEMPO_5

Denaturalization (80 ◦ C,15min) Pulpwashing

T=50 ◦ C

5molNaClO/kg

TEMPO_1515molNaClO/kg

TEMPO-mediated oxidation Pulpwashingwith waterandﬁltration uptopH=7

Allthedosesareexpressedasdryagentperkgofdrypulp.PC:Pulpconsistency.

PC=1%

pH=10

T=25 ◦ C

ThemechanicalpretreatmentwasconductedinaccordancewiththeISOstandard 5264-2(2011)[30];theenzymatichydrolysisprocedurefollowedthemethodologyoutlined inTarrésetal.(2016)[31]andTEMPO-mediatedoxidationwasperformedfollowingthe methoddescribedbySaitoetal.(2007)[32].

2.2.2.High-PressureHomogenization

Inthisresearch,atotalof25differenttypesofCNFswereproduced.Thenanoﬁbrillationofthesampleswascarriedoutbypassingthepretreatedpulpsuspensionsat1%of consistencythroughaPANDAPlus2000laboratoryhomogenizer(GeaNiroSoavi,Italy) usingdifferentpressuresequencesasdetailedinTable 3

AllCNFsproducedwerenamedusingthefollowingnomenclature:Pretreatment_doseCNF-HPHsequencenumber.Asanexampleofthis,CNFsproducedthroughenzymatic hydrolysispretreatmentusinganenzymedosageof80mg/kg,followedbyaHPHsequenceofthreepassesat300bar,threepassesat600bar,andthreepassesat900bar,are designatedEnz_80-CNF-5.

TheenergyconsumptionoftheHPHprocesswascontinuouslymonitoredusinga CircutorCVM-C10measuringdevice(Barcelona,Spain),providingreal-timereadingsof

theconsumedenergy.TheenergyconsumptionwasrecordedaftereveryHPHcycleand wasthennormalizedtotheprocesseddrymass.

Table3. HPHconditionstoproduceCNFs.

HPHSequenceNumber

NumberofCycles

2.3.CharacterizationofthePulps

2.3.1.ChemicalComposition

Thecompositionanalysisofbothunpretreatedandpretreatedpulps(Table 4)was conductedaccordingtoTAPPIT204forextractivedetermination,TAPPIT211forashdetermination,andNREL/TP-510-42618foradditionallaboratoryanalytical procedures[33–35] Briefly,theextractivesweredeterminedusingtheSoxhletextractionmethodwithacetone assolvent.Theashcontentwasdeterminedthroughcalcinationat525 ◦ C.Forlignin analysis,thefollowingstepswerefollowed:(i)acidhydrolysisof0.3gofdrypulpusing 3mL of72%H2 SO4 for1hat30 ◦ C,(ii)dilutionwith84gofdistilledwatertoprevent phaseseparationbetweenhighandlowconcentrationacidlayers,and(iii)autoclavingat 121 ◦ Cfor1h,followedbyfiltrationtodetermineKlasonligninbydryingandweighing thefiltrationcake.Thepercentageofsolublelignin,cellulose,andhemicelluloseweredeterminedbyanalyzingthefiltrate;solubleligninwasobtainedbymeasuringtheabsorbance ofthefiltrateat240nmusingaUV–visspectrophotometer.Celluloseandhemicellulose wereanalyzedusinghigh-performanceliquidchromatography(HPLC)ofthefiltrateafter neutralizationwithCaCO3 andmicrofiltrationthrougha0.2 μmfilter.

Table4. Compositionofpretreatedpulps.

ExtractivesAshesSolubleLigninKlasonLigninCelluloseHemi-Cellulose

Thebehaviorofpretreatedpulpsisinﬂuencedbythefunctionalgroupspresentonthe surfaceofcellulose.Therefore,conductimetrictitrationwasusedtodeterminethecontent ofcarboxylgroupspergramofthepretreatedpulps,byfollowingthemethoddescribed inpreviousstudies.Inthismethod,adriedsampleweighingbetween0.05and0.1gwas suspendedin15mLof0.01NHClsolutionandstirredfor10minandthentakentoa conductivitysensor.Thetitrationwasperformedbyconsecutivelyadding0.1mLof0.05N NaOHsolutiontothesuspensionuntilanoticeableincreaseinconductivitywasobserved. Asthealkalineutralizesthechlorohydricacid,theconductivityinitiallydecreases.Once theacidgroupsareneutralized,theconductivityremainsconstant.Finally,theexcessof NaOHleadstoanincreaseinconductivity.Thenumberofcarboxylgroupswascalculated fromthetitrationcurve,whichrelatesconductivityvs.amount(meq)ofNaOHadded, followingthemethodologydescribedbyHabibietal.(2006)[36].

2.3.2.Morphology

Themorphologicalanalysisencompassedseveralparameters,includingaverage lengthweightedinlength(lengthw ),diameter(d),coarseness,andﬁnepercentage(%). ThesemeasurementsweredeterminedusingimageanalysisperformedwithaMorFi Compactanalyzer(TechPap,Gières,France).However,astheresolutionofMorFiimages waslimitedandwewereunabletoaccuratelymeasurenanoﬁbers,theaspectratioofthe pretreatedsampleswasestimatedusingthemeasurementofthegelpoint(GP)basedonthe methodologydevelopedbyVaranasietal.(2013)andSanchez-Salvadoretal.(2021)[37,38].

Furthermore,opticalimagesofthepretreatedpulpsweretakenusingaZeissAxio Lab.A1opticalmicroscopeandacolormicroscopecameraZeissAxioCamERc5s(Carl ZeissMicroscopyGmbH,Göttingen,Germany).Subsequently,theopticalimageswere processedusingtheversion1.53iofImageJsoftwarepackagetoenhancetheclarityofthe imagesforfurtheranalysis.

2.4.CharacterizationoftheCNFs

CharacterizationoftheCNFsinvolvedthemeasurementofseveralproperties,includingnanofibrillationyield,transmittanceat600nm,cationicdemand(CD),andaspectratio oftheCNFsuspensions,followingthemethodsdescribedinpreviousstudies[6,27].To determinethenanofibrillationyield,CNFsuspensionwasdilutedto0.1wt.%andcentrifugatedat4500 × g for30mintoisolatenanofibrillatedcellulosefromnon-nanofibrillated components,whichsettledinthesediment.Theweightofthesedimentwasmeasuredto calculatethenanofibrillationyield.AUV–visShimadzuspectrophotometerUV-160Awas usedtomeasurethetransmittanceoftheCNFsuspensionsattwodifferentwavelengths. TheanioniccharacterofCNFscorrelateswiththeirCD,whichwasdeterminedviaback titration,withpolydiallyldimethylammoniumchloride(PDADMAC)andpolyvinylsulfate potassiumsaltusingaMütekPCD04particlechargedetector(BTGInstruments,Weßling, Germany).TheaspectratiooftheCNFswasdeterminedusingarecentlydeveloped methodologybySanchez-Salvadoretal.(2021)[38].Thisapproachmodifiedthetraditional GPmethodologybydyingthefiberswithcrystalviolet,enablingthevisualizationofthe fibrilsedimentationandoptimizingthesedimentationtimetoensurecompletesettling.

AnopticalmicroscopewasusedtoobtainimagesfromCNFhydrogelsforqualitative supportofourresults.Inthiscase,theparametersandmethodusedforobtainingthese imageswereconsistentwiththoseutilizedforthepretreatedpulps.Additionally,the opticalimagesunderwentprocessingwiththeImageJsoftwarepackagetoseamlessly eliminatethecontinuousbackground.Thiswasachievedbyapplyingthe“Process› SubtractBackground”commandfromthemenu,withthespeciﬁedparametersincludinga rollingballradiusof50pixels,alightbackground,andaslidingparaboloid.

FortheobservationofCNFs,aJEOLJEM1400PLUStransmissionelectronmicroscopy(TEM)device(Tokyo,Japan)wasutilizedattheSpanishNationalCentreof ElectronicMicroscopy(CNME),followingthemethodologydevelopedbyCampanoetal. (2020)[39].TheTEMmicroscopeoperatedat100kVacceleratingvoltage,withanOrius SC200CCDcameramanufacturedbyGatan(Pleasanton,CA,USA),featuringaresolution of2048 × 2048pixelsandapixelsizeof7.4microns.

WiththeTEMimagesobtained,projectedfractaldimension(D2)wasmeasured.D2 givesanideaofthe“treestructure”ofthesample,whichcaninfluencethemechanical propertiesofthematerial[40].Theacquiredimageswerealsoprocessedandanalyzedwith ImageJsoftware,editingimagestoachievegooddefinitionandhighcontrastinCNFsand convertingtheseinto8-bitimages.Theywerebinarizedthroughathresholdandcorrected withaclosefilter.Elementsintheimageswereselectedindividuallyandcopiedintoa separatefile.Thefractalanalysiswasperformedwiththefractalboxcountplugin.To reduceprocessingtimes,thisprocedurewasautomatizedthroughascript[41,42].

3.ResultsandDiscussion

3.1.PretreatmentEffectsonFiberMorphologyandCarboxylContentofthePulp

Table 4 showstheeffectofthedifferentpretreatmentsonthechemicalcomposition ofthepulp.Thewashingstagefollowedtheenzymaticpretreatmentstoremovepart oftheashes,whichexplainsthereductioninashcontent.Thisreductioninashescould notbedetectedbySanchez-Salvadoretal.(2022)becauseofthelowashcontentofthe virginpulps[27].Parketal.(2022)observedthatashinrecycledpaperhasahighafﬁnity forcellulaseenzymesandattachedtothem,reducingenzymeefﬁciency[43].Thus,ash attachedtotheenzymeisremovedduringpulpwashing.InthecaseofTEMPOpretreatments,theincreaseinashcontentisattributedtotheproductionofNaClasbyproduct duringtheoxidationprocessandtheadditionofNaBr,bothofwhicharenotcompletely removedduringwashing[44].Partoftheextractivescouldbedegradedorsolubilizedby theenzymes,asshowninTable 4.Consequently,thisledtoanincreaseinthepercentage ofcellulose.However,excessiveamountsofenzymesorNaClO,particularlyatahigh TEMPO-mediatedoxidationdegree(TEMPO_15),couldresultintheexcessivedegradation ofcellulosechains,leadingtotheformationofsolubleoligosaccharidesorsugars,which weresubsequentlyremovedduringthewashingstage.Hemicelluloseseemstoremain intactafterthepretreatmentprocess.

Figure 1 showsthecarboxylcontentofthepretreatedpulps.Thetypeofpretreatment anditsseveritydeterminetheﬁnalquantityofcarboxylgroups.Mechanicalandenzymatic pretreatmentsdonotcontributetothegenerationofcarboxylgroups.Incontrast,the numberofcarboxylgroupsgeneratedthroughTEMPO-mediatedoxidationincreaseswith thedosageoftheoxidant,althoughnottothesameextentastheNaClOdosage.This discrepancyarisesbecauseaportionoftheNaClOisconsumedintheoxidationofother componentsfoundinrecoveredpapers,suchasdissolvedligninandcolloidalmaterial[45]. Therefore,onlyafractionoftheNaClOisavailabletooxidizehydroxylgroupstocarboxyl groups.Furthermore,thereisasaturationphenomenon;whenthedosageofNaClO surpassesathreshold,theoverdosedNaClOremainsinthemediumandcausessecondary reactionsinsteadofeffectivelyoxidizingcellulose.

Figure1. Carboxyliccontentoftherecycledﬁbersaftereachpretreatment.

Table 5 andFigure 2 presentthemorphologicalparametersofthefibersandtheoptical imagesofthepretreatedfibersproducedbydifferentpretreatments.Table 5 showsfibers andfineswhicharedetectablebytheMorFianalyzer,whosedetectionlimitisaround5 μm (forlength).

Table5. Morphologicalcharacteristicsofthepulpspretreatedwithreﬁning,enzymatichydrolysis, andTEMPO-mediatedoxidationpriortotheHPH.

Lengthw :lengthweightedinlength.

Figure2. Opticalimagesoftheinitialrecycledpulpwithoutpretreatment(a)andthepulpsafter differentpretreatments:mechanicalpretreatment(reﬁning)(b);enzymatichydrolysiswith80mg/kg (c);enzymatichydrolysiswith240mg/kg(d);TEMPO-mediatedoxidationwith5NaClO/g(e);and TEMPO-mediatedoxidationwith15mmolNaClO/g(f).

Figure 2 showstheopticalimagesoftheoriginalandpretreatedpulps.Theoriginal fibers(Figure 2a)exhibitedasmoothsurface,withoutnotableexternalfibrillation.In contrast,theMecpulpimage(Figure 2b)showsanextremelyhighdegreeoffibrillation, whichfavorsfine-and-fiberinteraction,aswellastheformationofsmallernetworks. Duringmechanicalrefining,fiberhornificationisreversedsincefibersrecoverpartoftheir swellingability,whichincreasestheircoarseness[46].Fibrillationwasalsoobservedin theEnz_80-andEnz_240-pretreatedpulps,albeittoalesserextent(Figure 2c,d).This factwasalsoobservedbyotherauthors[27],soenzymeshavebeenproposedasan alternativetorefiningtoreducethehornificationeffect.ItisnoticeablethatEnz_240 inducedgreaterfibrillationcomparedtoEnz_80,resultinginareductioninfiberlength (Table 5).Incontrast,TEMPO-mediatedoxidationledtothelowestexternalfibrillation, asthisreactionspecificallyoxidizesthecellulosechains,conferringelectrostaticrepulsive forcesoninterfibrilsthatreducetheenergyrequiredfornanofibrillation(Figure 2d,e),but withoutchangingthefibermorphologyduringthechemicaltreatment.However,athigh dosesofNaClO,someexternalmicrofibrillationonthefiberscanbeobservedduetothe highelectrostaticrepulsiveforcesamongchainsduetothegeneratedcarboxylgroupsin combinationwithhydrodynamicforcesduringpulpwashing.ThisproducessomeCNFs

fromthesmallestfines,asshownbythecleanbackgroundoftheopticalimageinFigure 2f. TheseCNFsarenotvisiblewithanopticalmicroscope.Furthermore,somedegradationof theamorphouspartofthefibersbyside-reactionscancontributetotheexternalfibrillation (Figure 2f),whichisinaccordwiththeabove.

Furthermore,theTEMPO-mediatedoxidationprocessresultedintheproductionof asignificantquantityoffinesandmicrofibersthatdecreasedthevaluesofcoarseness anddiameter.Someofthemicrofibersweretoosmalltobeclearlyobservedwithan opticalmicroscopeandappearedasdotsinFigure 2e.TheopticalimagesofTEMPOmediatedoxidizedpulpsrevealthetreatedfibersretaintheirstructurepriortoundergoing mechanicalnanofibrillation.However,theseimagesdonotshowtheelectrostaticrepulsive forcesoccurringwithinthefibers,whichareresponsibleforinducingnanofibrillationwhen shearingforcesareapplied.

3.2.EffectofthePretreatmentonthePropertiesoftheCelluloseNanoﬁbers

Table 6 showstheresultsofthequantitativecharacterizationofdifferentCNFsproducedusingthehighestintensityofHPHthatcorrespondstoSequence5,asindicatedin Table 3.GPwasusedtodeterminetheaspectratiooftheﬁbersusingcrowdingnumber (CN)theory[37,38,47].

Table6. Propertiesofcellulosenanoﬁbersproducedbydifferentpretreatmentsandfollowedby Sequence5HPH(3passesat300bar+3passesat600bar+3passesat900bar).

(GP) Mec16.215.2202173 Enz_8028.026.323391 Enz_24030.028.9228109 TEMPO_541.438.5110847 TEMPO_1562.359.819087.3

Theaspectratioofthenanofibersintheobtainedhydrogelsdependsonthepulp pretreatment.Themechanicallypretreatedfibers(refinedpulp)showedthehighestaspect ratio,approximately173.Figures 3 and 4 providevisualqualitativeevidenceofthesignificantlyhighaspectratioofthispretreatedpulpcomparedtotheothers.Theyareoptical (Figure 3)andTEMmicrographs(Figure 4)ofthetwenty-fiveCNFsproduced.EachCNF producedwasanalyzedbycapturingtwentyimages,andeachimagein Figures 3 and 4 wasselectedasarepresentativefromthosetwentyimages.

Figures 3 and 4 display,inthecaseofCNFsobtainedbyrefining(Mec),thepresence ofhighlyfibrillatedfiberslongerthan500 μm,withastructurethinnerthanthoseobtained throughenzymaticdegradationfollowedbyHPH.Opticalimagesprovideahighervisual field,butwithlowerresolutionthanTEMimages.Forexample,thefluffshowninFigure 3 mustbeinterpretedwiththeinformationgivenbyTEMimages,whichshowthelevelof nanofibrillationachieved.Figure 5 showstheD2ofrefiningpretreatedpulpatdifferent HPHsequencescomparedtotheotherpretreatments.UndersoftHPH,itispossibleto observeahighD2value,near2.0,duetothefibrilsjoiningtothecellulosebackbone, formingbundles.Increasingtheintensityofhomogenization,adecreaseisobservedinthe D2,associatedwithfibrilsseparationfromthemainstructure.

BothenzymatichydrolysisandTEMPO-mediatedoxidationpretreatmentsresultina decreaseinfiberlengthand,consequently,aspectratiocomparedtomechanicallytreated pulp.Enz_80CNFsuspensioncontainsasubstantialamountofshortandthickfibersasD2 showsinFigure 5,withalowervaluethanforMec.Figure 3 showsthelengthandFigure 4 showsthehigherthicknessoffibersthatremainedaftertheHPHprocess.Amoreintense enzymaticpretreatment,Enz_240,reducesthethickness,asevidencedbyFigure 4,andthe

amountofthesefibersthatremainafterHPH,asshownbytheyieldandtransmittance values(Table 6),resultinginincreasedaspectratiovalues.Thiseffectonthicknessisnot observedinpretreatedpulp,becauseitistheresultofthecombinedeffectsofhydrolysis andHPH.Theinternalnanofibrillationcausedbytheenzymefavorsthedeconstructionof fiberinnanofibersduringHPH.ThiseffectisalsoobservedinFigure 5,inwhichtheD2 increaseswiththeseverityofHPH,whichindicatestheformationofmicrofibrilsaround themainstructureswiththemaintreatment.Ontheotherhand,theorangecolorofEnz-80 andEnz-240CNFsinFigure 6,whichisevendarkerthanMecCNFs,couldbeduetothe degradationproductsfromenzymatichydrolysis,whichcanproduceoligosaccharides fromtheamorphouspartofthecellulosechainsdecreasingthepolymerizationdegreeof cellulose[48].

Figure3. OpticalmicroscopyimagesofCNFs(scalebar:500 μm).

Figure4. TEMimagesofCNFs(scalebar:2 μm).

Figure5. Projectedfractaldimension(D2)calculatedfromTEMimagesatdifferentHPHsequences.

Thus,theaspectratioofTEMPOCNFsisnotablylowerthantheothersandisdecreasedwiththeNaClOdose.Figure 3 showsthatonlyafewmicrofibrilsinTEMPOare detectedbyopticalmicroscopyafterHPH(shownassmalldots)andthesebecomefewer andfewerastheintensityofhomogenizationincreases,asD2indicateswithitsdecrease duetofiberindividualization.However,Figure 4 showsaCNFnetworkbeingresponsible forformingagel,asevidencedbythephotographinFigure 6 forTEMPO-15.Thefactthat noneoftheobtainednanocellulosesweretransparentindicatesthatallofthemhavesome microfibrilsornanocelluloseaggregatesthatdispersedvisiblelight.However,Figure 6 shows,theopacityofthegelsuspensionwaslowerinthecaseofTEMPO-15,asexpected fromthelowernumberofmicrofibersandhigheramountofnanofibrillatedcelluloseand nanocrystals.ThethinandshortnanofibersobservedinFigure 4 forTEMPO-15-CNF-5 accountforthelowaspectratiovaluesoftheseCNFs(Table 6).Thesensitivityofcellulose tomechanicalforcesduringtheHPHprocesswasincreasedbythecelluloseoxidationand theremovalofligninduetotheeffectofNaClOduringTEMPO-mediatedoxidation.The softyellowcolorofTEMPO-pretreatedCNFsindicatesligninremovalduringthetreatment. Infact,atthefirststageofthereaction,NaClOismainlyconsumedinligninoxidation[45]. Asaresult,anintenseHPHtreatmentcausedthehighestcuttingeffect,whichaccounts forthelowestaspectratio(around7)obtainedforTEMPO_15-CNF-5,suggestingthat thesufficientlyintenseTEMPO-mediatedoxidationfollowedbyanintenseHPHprocess producescellulosenanocrystals(CNCs),whichhaveloweraspectratiosthanCNFs[42]. AccordingtoSerra-Pararedaetal.2021,CNCshaveD2valuesunder1.4–1.5,whereasCNFs havehigherD2[42].Withthiscriterion,asFigure 5 shows,TEMPO_15-CNF-5isinthe CNCscalewhileTEMPO_15-CNF-3andTEMPO_15-CNF-4rangefromCNCstoCNFs.

3.3.EffectofHomogenizationSequenceonthePropertiesandMorphologyofCelluloseNanofibers Thehomogenizationpressureandcycleswerevariedinthisstudyfortheisolationof differentCNFqualities.ThefinalgoalistooptimizetheproductionofCNFsbasedonthe desiredmorphologyandpropertiesforspecificapplicationsinthefuture.

Figures 3 and 4 showthesignificantinfluenceofpretreatmentonthenanofibrillation processofthepulpsunderdifferentHPHintensities.TheMecpulpsufferedminimal changesduringHPHprocess,exceptforsomeinternalfibrillationasshowninFigure 4. Althoughthefiberwallstructureafterenzymatictreatmentsexhibitedlimitedsignsof externalfibrillation,thisfibrillationnoticeablyincreasedwiththeintensityofsubsequent mechanicaltreatmentusingHPH(Figure 3).Furthermore,Figure 4 showssomeinternal

Figure6. PhotographsofobtainedCNFhydrogels.

fibrillation.ThemostsignificantimpactofHPHonmorphologywasobservedinTEMPO_5 andTEMPO_15CNFs.Inbothcases,thenumberofvisibleelements(Figure 3)decreased withincreasingHPHintensitywhenobservedthroughopticalmicroscopy.Thisreduction wasattributedtotheintenseinternalnanofibrillationresultingfromthecombinationof shearingforcesandelectrostaticrepulsiveforces.

Figure 7 showshowthepretreatmentdeterminesthepotentialofHPHtoenhancethe propertiesofCNFs.Inallcases,CNFsproducedsolelythroughmechanicalpretreatment andHPHexhibitedthelowestvaluesofyield,transmittance,andCD,butthehighest valuesofaspectratio,measuredbyGP.Mechanicalpretreatmentcausedexternalfibrillation andincreasedtheaccessibilityofcelluloseforsubsequenttreatments.However,ithad limitedimpactontheyieldandtransmittanceofCNFs.Theenergyconsumptionforeach HPHsequencedependsontheappliedpretreatment.EnzymatichydrolysisandTEMPOmediatedoxidationaremoreefficientinreducingenergyconsumptionforhomogenization asshownatFigure 7.ThemostefficientwasTEMPO_15,asexpectedfromthehighanionic groupsintroducedonthefibersthatgenerateelectrostaticrepulsiveforcesamongcellulose chainshelpingnanofibrillation.Nanofibrillationyieldandtransmittanceincreasednotably withenergyconsumptionandwithNaClOusedinTEMPO-mediatedoxidation.However, differencescausedfortheenzymedosewerenotsignificantexceptforaspectratio.This couldindicatethattheeffectoftheenzymeislimitedbyothervariables,forexample,due topooraccessibilityoftheenzymetothecellulosechainsduetothelowerswellingability andthepresenceoflignin.Figure 6 showsthatEnz_80andEnz_240CNFshavedifferent behaviorwithwater.ThelattertendstoreleasewaterwhenitisdepositedinaPetridish. TheinteractionwithwateriskeyforGPdeterminationbecauseitaffectsthesettlingprocess. ThisdiffersfromtheresultsobtainedforvirginpulpsbySanchez-Salvadoretal.(2022) whoobservedanotableincreaseintransmittance,yield,andcationicdemandbyincreasing thedoseofenzymethreefold[27].Thisevidencestheeffectofrawmaterialcompositionin CNFproductionbyenzymatichydrolysis.

TheCDofCNFsisnotsolelyattributedtothepresenceofanionicgroupsonthe nanofibrilsurface,butitisalsoinfluencedbythesurfaceareaavailableforPDADMAC adsorption,whichdependsontheexternalspecificsurfaceoftheCNFs.Thisisevidentfrom theincreaseinCDwiththeintensityofHPHtreatmentandwiththeleveloffibrillation. However,themainfactoraffectingCDisthepretreatmentitself.Itisnotpossibletoachieve ahighCDforTEMPOCNFswiththestudiedcombinationsofpretreatmentfollowedby HPH,duetothesubstantialnumberofanionicchargesgeneratedbyTEMPO-mediated oxidation.TEMPOCNFsalsoexhibitedhigheryieldandtransmittancecomparedtoothers.

However,similaryieldsandtransmittancetoTEMPO_5CNFscanbeobtainedusing enzymatichydrolysisfollowedbyamoreintenseHPHtreatmentbutwitharound50% higherenergyconsumption.Evenmechanicalrefiningoftherecycledpulpfollowedby themostintenseHPHstudiedcanproduceCNFsofthesameorderofmagnitudein termsofyieldandtransmittance,althoughwithalowervaluethanthelessfibrillated TEMPO_5CNF.ThesefindingsdifferfrompreviousstudiesonAspenpulp[28],where TEMPO-mediatedoxidationfollowedbyhighlyintenseHPHenabledayieldof100%and atransmittancearound95%,whichweresignificantlyhigherthanthevaluesobtainedwith recoveredpaper.Inthiscase,pulpcompositionisdifferentduetothehigherpercentageof ashesandthepresenceofhornifiedfibers,whichaffectstheircapacityforbeingfibrillated andtheirinteractionwithwaterandchemicalsandsomedissolvedandcolloidalmaterial. TheeffectofHPHonaspectratioisclearlyinfluencedbythepretreatment.Inthe caseofamechanicalpretreatment,theaspectratioofCNFsincreasedwiththeenergyconsumptionofHPH.Asmalleffectonaspectratiowasobservedforenzymaticpretreatments andanegligibleeffectwasobtainedinthecaseofTEMPO_5.However,theaspectratio decreasedwithHPHintensityforTEMPO_15CNF.TheseCNFsformedahydrogelwhose consistencyincreasedwithHPHintensity,whichindicatesthattherewasa3Dnetworkof CNFsinteractingwithwater.IftherealaspectratioofalltheCNFsinTEMPO_15wasas lowasthatshowninTable 6 andFigure 7,theformationofanetworkwouldbeunlikely.

Theaspectratiodecreased(Figure 7)whiletheconsistencyofthegelincreased(Figure 6). Thisisduetothepresenceofnon-settledparticlesduringthesedimentationexperiments. Tobettermeasuretheaspectratio,different,verydilutedsuspensionswerepreparedand thepeakofsedimentwasmeasuredafteralongtime(attheendofsedimentationprocess). InthecaseofTEMPO_15CNF,thereweresomeverystablenanoparticlesinthesuspension thatdidnotsettleevenafterseveralweeks,andtheﬁnalaspectratiowasnotrealsinceit correspondedtothesettleablematerial.TheincreaseinHPHintensityincreasedthedegree ofnanoﬁbrillationandCDvalues,whichimpliesthatahigheranionicchargeisavailable togeneraterepulsiveforcesamongparticles,whichcaninteractmorestronglywithwater, formingastablegel.

Figure7. Nanoﬁbrillationyield(a),transmittance(b)cationicdemand(c)andaspectratio(d)ofthe differentCNFsuspensions.

4.Conclusions

Bycarefullyselectinganappropriatepretreatmentandlevelofseverity,aswellasan optimalHPHintensity,differentCNFqualitiescanbeproducedfromrecoveredpaperboard. BasedonthepretreatmentandHPHintensity,theresultscanbeusedtopredicttheexpected qualityofCNFs.Recycledpaperboardpulpallowsonetoobtaincost-effectivecoloredand turbidhydrogelswithanotablenanofibrillationyield(63%)andtransmittance(60%),which canbeusedasreinforcingaidsinboard-recyclingmills.Thehornificationofthefibersand thepresenceofashesandligninaffectstheefficiencyofenzymatichydrolysis,withadose of80mg/gofpulpbeingenoughtoobtainthemaximalperformanceoftheproducedCNF. ThecharacteristicsoftheproducedCNFsandtheimpactofHPHonnanofibrillationare bothsignificantlyinfluencedbythepulppretreatment.Thisenablesonetochoosethebest

(a)

(c)

productionstrategywiththehighestyieldintermsofproducedCNFfortherequiredCNF qualityinvariousapplications.

AuthorContributions: Conceptualization,C.N.,A.B.,E.F.andM.D.-A.;Methodology,C.N.,M.C.M., A.B.,E.F.,M.D.-A.andQ.T.;Software,J.L.S.-S.;FormalAnalysis,A.B.andJ.L.S.-S.;Investigation,A.B., J.L.S.-S.andM.C.M.;Writing—OriginalDraftPreparation,A.B.,M.C.M.andE.F.; Writing—Review andEditing,C.N.,E.F.,A.B.,M.D.-A.,Q.T.andJ.L.S.-S.;Visualization,A.B.,M.C.M.andE.F.;Supervision,C.N.andM.D.-A.;ProjectAdministration,C.N.andP.M.;FundingAcquisition,C.N.andP.M. Allauthorshavereadandagreedtothepublishedversionofthemanuscript.

Funding: TheauthorsaregratefulfortheﬁnancialsupportbytheSpanishMinistryofScienceand InnovationtotheprojectsCTQ20217-85654-C2-1-RandCTQ2017-85654-C2-2-R(NANOPROSOST) andPID2020-113850RB-C21andPID2020-113850RB-C22(CON-FUTURO-ES)andtheCommunityof Madrid(projectS2018/EMT-4459“RETOPROSOST2-CM”).

DataAvailabilityStatement: Datacanbemadeavailableuponrequesttothecorrespondingauthor.

Acknowledgments: TheauthorsarethankfultotheSpanishNationalCentreofElectronicMicroscopy forthesupportduringTEMimagesacquisition.MarcDelgado-AguilarandQuimTarrésareSerra HúnterFellows.

ConﬂictsofInterest: Theauthorsdeclarenoconﬂictofinterest.Thefundershadnoroleinthedesign ofthestudy;inthecollection,analyses,orinterpretationofdata;inthewritingofthemanuscript;or inthedecisiontopublishtheresults.

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FROM

Volume 10, Number 3, 2024

PAPERmaking!

Facile Strategy for Boosting of Inorganic Fillers Retention in Paper

Paper Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

polymers

Article

FacileStrategyforBoostingofInorganicFillersRetention inPaper

KlaudiaMa´slana 1, *,KrzysztofSielicki 1 ,KarolinaWenelska 1, * ,TomaszK˛edzierski 1 ,JoannaJanusz 2 , GrzegorzMaria´nczyk 2 ,AleksandraGorgon-Kuza 2 ,WojciechBogdan 2 ,BeataZieli´nska 1 andEwaMijowska 1

1 DepartmentofNanomaterialsPhysicochemistry,FacultyofChemicalTechnologyandEngineering, WestPomeranianUniversityofTechnologyinSzczecin,PiastowAve.45,70-311Szczecin,Poland; krzysztof-sielicki@zut.edu.pl(K.S.);tomasz.kedzierski@zut.edu.pl(T.K.);bzielinska@zut.edu.pl(B.Z.); emijowska@zut.edu.pl(E.M.)

2 ArcticPaperKostrzynSA,ul.Fabryczna1,66-470KostrzynnadOdra,Poland; joanna.janusz@arcticpaper.com(J.J.);grzegorz.marianczyk@arcticpaper.com(G.M.); wojciech.bogdan@arcticpaper.com(W.B.)

* Correspondence:klaudia.maslana@zut.edu.pl(K.M.);kwenelska@zut.edu.pl(K.W.)

Citation: Ma´slana,K.;Sielicki,K.; Wenelska,K.;K˛edzierski,T.;Janusz,J.; Maria´nczyk,G.;Gorgon-Kuza,A.; Bogdan,W.;Zieli´nska,B.;Mijowska, E.FacileStrategyforBoostingof InorganicFillersRetentioninPaper. Polymers 2024, 16,110.https:// doi.org/10.3390/polym16010110

AcademicEditor:SelestinaGorgieva

Received:21November2023

Revised:18December2023

Accepted:25December2023

Published:29December2023

Abstract: Achievingthedesiredpropertiesofpapersuchasstrength,durability,andprintability remainschallenging.Papermillsemploycalciumcarbonate(CaCO3 )asafillertoboostpaper’s brightness,opacity,andprintability.However,weakinteractionbetweencellulosefibersandCaCO3 particlescreatesdifferentissuesinthepapermakingindustry.Therefore,thisstudyexploresthe influenceofvariousinorganicadditivesascrosslinkerssuchasmesoporousSiO2 nanospheres,TiO2 nanoparticles,h-BNnanoflakes,andhydroxylatedh-BNnanoflakes(h-BN-OH)oninorganicfillers contentinthepaper.Theywereintroducedtothepaperpulpintheformofapolyethyleneglycol (PEG)suspensiontoenablebondingbetweentheinorganicparticlesandthepaperpulp.Ourfindings havebeenrevealedbasedondetailedmicroscopicandstructuralanalyses,e.g.,transmissionand scanningelectronmicroscopy,X-raydiffraction,Ramanspectroscopy,andN2 adsorption/desorption isotherms.Finally,theinorganicfillers(CaCO3 andrespectiveinorganicadditives)contentwas evaluatedfollowingISO1762:2001guidelines.Conductedevaluationsallowedustoidentifythe mostefficientcrosslinker(SiO2 nanoparticles)intermsofinorganicfillerretention.Papersheets modifiedwithSiO2 enhancetheretentionofthefillersby~12.1%.Therefore,webelievethesefindings offervaluableinsightsforenhancingthepapermakingprocesstowardboostingthequalityofthe resultingpaper.

Keywords: cellulose;crosslinker;ﬁllers;polymer

1.Introduction

Theprocessofpapermakinginvolvesconvertingrawmaterials,likewoodfibersor recycledpaper,intoaproductthatmeetsqualitystandardsandmarketdemands.Oneofthe mainchallengesinthisprocessisachievingthedesiredpropertiesinthefinalproduct,such asstrength,durability,brightness,opacity,andprintability,whichenhancethestrengthand stiffnessofthepaper,avoidingpoorfolding,tearing,andcrackingresistance.Toachieve this,papermillsoftenusefillerssuchasCaCO3 [1].However,theweakinteractionbetween cellulosefibersandCaCO3 particlescanleadtoarangeofproblemsinpapermaking[2,3]. Thisproblemisrelatedtothecellulosefibers’structuralproperties,whichhaveahighly crystallinestructurewithalowsurface-area-to-volumeratio.Asaconsequence,alimited availabilityofactivesitesforhydrogenbonds,bothbetweenfibersandwithCaCO3 particles,isobserved[4,5].TheinteractionbetweencellulosefibersandCaCO3 particles affectspapermassdrainageandretention.Thepresenceoffillerscaninterferewithwater flowthroughthepapermachine,leadingtolongerdryingtimesandincreasedenergy consumption[6].

Variousstrategieshavebeendevelopedtoovercometheseobstaclestoenhancethe affinityofthepapermasstoCaCO3 .Oneofthestrategiesistheadditionofcrosslinkers,whichcanimprovetheinteractionbetweenthecellulosefibersandCaCO3 particles. Crosslinkersarechemicalcompoundsthatcanformbondsbetweenthecellulosefibers, creatingamorestablenetworkthatcanbetterresistthedisruptiveeffectsoffillers[7,8].The additionoffillerssuchasSiO2 [9],TiO2 [10],h-BN[11],andh-BN-OH[12]hasbeenshown tobeeffectiveinimprovingtheaffinityofthepapermasstoCaCO3 .Thesecompounds canformbondswithboththecellulosefibersandCaCO3 particles,creatingastronger networkthatimprovesthestrength,stiffness,andprintabilityofthepaper.Therefore, usingcrosslinkersinpapermakingisanimportantstrategyforimprovingthequalityand performanceofpaperproducts.

Silicondioxide(SiO2 )canformcovalentbondswithboththecellulosefibersand CaCO3 particles.TheSiO2 particlescanalsoserveasaconnectorbetweenthecellulose fibersandCaCO3 particles,helpingtostrengthentheinter-fiberbondingandimprovethe stiffnessandstrengthofthepaper.Inadditiontoitsroleasacrosslinker,SiO2 canalso improvethedrainageandretentionofthepapermass.TheSiO2 particlesarehydrophilic, whichmeansthattheycanhelpabsorbwaterandimprovetheflowofthepapermass throughthepapermachine.Thiscanleadtofasterdryingtimesandincreasedproductivity. SiO2 canbeaddedtothepapermassinvariousforms,includingcolloidalsilica,precipitated silica,andsilicafume[9,13].

Titaniumdioxide(TiO2 )asacrosslinkercanalsoimprovetheopticalpropertiesof paperproducts.TiO2 isawhitepigmentthatcanreflectandscatterlight,leadingtoa brighterandmoreopaquepaper.Thiscanbeparticularlybeneficialinapplicationswhere highbrightnessandopacityareexpected.Inaddition,TiO2 canalsoimprovethedrainage andretentionofthepapermass—similartoSiO2 .Thiscanleadtofasterdryingtimesand increasedproductivity.TiO2 canbeaddedtothepapermassinvariousforms,including rutile,anatase,andnano-sizedTiO2 particles.Rutileandanatasearetwocrystallineforms ofTiO2 ,withrutilebeingthemostcommonlyusedforminpapermakingduetoitshigher refractiveindexandopacity.Nano-sizedTiO2 particleshaveasmallerparticlesizeandcan provideadditionalbenefitssuchasimprovedprintabilityandinkadhesion[10,14].

Hexagonalboronnitride(h-BN)isalayeredmaterialthatconsistsofhexagonally arrangedboronandnitrogenatoms.Ithasahighsurfaceareaanduniquesurfacechemistry thatmakeitattractiveforvariousapplications.Whenaddedtothepapermass,h-BN canformcovalentbondswithboththecellulosefibersandCaCO3 particles,improving theinteractionbetweenthemandcreatingamorestablenetwork.Theh-BNparticles canalsoserveasaconnectorbetweenthefibersandfillers,enhancingthestiffnessand strengthofthepaper.h-BN-OHisaderivativeofh-BNthathasbeenfunctionalizedwith hydroxylgroups.Thehydroxylgroupsincreasethesurfaceenergyandwettabilityofthe h-BNparticles,allowingthemtointeractwiththecellulosefibersandCaCO3 particles moreeffectively.Thehydroxylgroupscanalsoprovideadditionalchemicalfunctionality, allowingforfurthermodificationsandimprovementsinthepaperproperties[12,15].

Inthiswork,theimpactofdifferentinorganicfillersoncalciumcarbonatecontentin thepaperwasinvestigated.Forthisreason,mesoporousSiO2 nanoparticles,TiO2 nanoparticles,h-BNnanoflakes,andhydroxylatedh-BNnanoflakes(h-BN-OH)havebeenexplored asadditives.Theyhavebeenintroducedtothepaperpulpintheformofapolyethylene glycol(PEG)mixturetoinducebondingbetweentheinorganicstructuresandpaperpulp components.Variouscharacterizationmethodswereemployedtodeterminethechemical structureandmorphologyofpreparedsamples,includingTEM,SEM,XRD,Ramanspectroscopy,andTGA.Theashcontent(residualsolidparticlesafterthecombustionprocess) wasevaluatedaccordingtoISO1762:2001.Itallowedustoselectmesoporoussilicaasthe mostefficientfiller,enhancingtheretentionofthefillersby12.1%inrespecttounmodified papersheets.

2.ExperimentalPart

2.1.Chemicals

Eucalyptus-derivedcellulosefibers,wood-derivedcellulosefibers,chemicalthermomechanicalpulpcellulose(CTMP),cationicstarch,calciumcarbonate,andpotassiumamyl xanthate(PAX)werederivedbyArcticPaperKostrzynSA.Polyethyleneglycol2000(PEG, M=1800–2200g/mol)waspurchasedfromCarlRoth,Karlsruhe,Germany.Titanium dioxidewaskindlysuppliedbyGrupaAzotyPolice(Police,Poland).Tetraethylorthosilicate(TEOS)andboronnitride(BN,~1 μm,98%)werepurchasedfromSigmaAldrich (Poznań,Poland).Ammoniasolution(NH4 OH,25%),sulfuricacid(H2 SO4 ,95%),nitric acid(HNO3 ,65%),andpotassiumpermanganate(KMnO4 )weredeliveredfromChempur (Piekary ´ Sl˛askie,Poland).

2.2.SynthesisofSiliconDioxide(SiO2 )

SiO2 nanoparticleswerepreparedaccordingtoamodiﬁedStöbermethod[16];5.8mL ofTEOSand150mLofethanolwereinitiallymixedinaroundbottomﬂaskandstirredfor 10minatroomtemperature(RT).Next,7.8mLofammoniasolutionwasadded,andthe mixturewasfurtherstirredfor12h.Afterthat,theproductwasseparatedbycentrifugation, washedafewtimeswithethanol,anddried.

2.3.SynthesisofHexagonalBoronNitride(h-BN)

Atotalof0.2gofbulkboronnitridewasplacedinaﬂask,andthen,200mLof ethanolwasadded.Afterward,theobtainedmixturewassonicatedusinganultrasonic homogenizerfor12h.Theﬁnalproductwaswashedwithdistilledwateranddriedat 80 ◦ C.

2.4.SynthesisofOxidizedHexagonalBoronNitride(h-BN-OH)

Toprepareh-BN-OH,amodifiedHummersmethodwasapplied[12].Briefly,0.2g ofh-BNpowderwasplacedinaround-bottomedflask,andthen,13.5mLofH2 SO4 and 4.4mLofHNO3 wereadded.Themixturewasstirredtoobtainahomogenousdispersion. Afterthat,1.2gofKMnO4 waspartiallyintroduced,andfinally,themixturewasheatedto 90 ◦ Candkeptatthistemperaturefor12h.Next,themixturewascooledtoRT,filtrated, andwashedafewtimeswithdistilledwateruntilthepHvalueapproached7.Finally,the productwasdriedat80 ◦ Covernight.

2.5.PreparationofPEGSolution

Intheexperiments,thedifferentpolyethyleneglycol(PEG)variants(PEG2000,PEG 4000,PEG10000,andPEG20000)weretestedtochoosetheoptimalone.Duringthe preparationofpapersheetsusingtheRapid-Köthenmachine(Lodz,Poland)different performancecharacteristicsofthepaperwiththetestedPEGvariantswereevaluated.The ﬁnalselectionofPEG2000wasbasedonitssuperiorperformanceduringthepapersheet preparationprocess.PEG2000exhibitedthebestsolubilityinthechosensolventandits integrationwithpaperpulp.AnappropriatePEG2000amountwasaddedto1Lofdistilled water,andwiththeuseofamagneticstirrer,itwascompletelydissolved.Theamountof PEG2000wascalculatedin1tonofdrypulptoobtainaconcentrationof1kgofPEG/ton ofdrycellulose(typicalcommercialprocedure).

2.6.PreparationofPaperSheets

Asareferencesample,asheetofpaperwithouttheadditionofinorganicfillerswas prepared.Thereferencepapersamplecontainedonlystandardcommercialcomponents usedfortheproductionofpapersheets.Paperwithagrammageof80g/m2 wascreated bycombiningthreetypesofcellulosefibers:short-fibercellulosepulp(eucalyptusderived), long-fibercellulosepulp(birch-derived),andchemicalthermomechanicalpulpcellulose (CTMP)withamassratioof70/20/10,respectively.Thecellulosicmasswasmixedina plasticcontainerusingamechanicalstirrerfor15min.Subsequently,PAXandacationic

starchsolution(3.8%)wereaddedtothemixturewith2and5kgpertonofdrycellulose fibers,respectively.Finally,CaCO3 andappropriatecrosslinkers(SiO2 ,TiO2 ,h-BN,and h-BN-OH)dispersedinPEGsolutionwereintroducedtothesystem.Todoso,1kgof crosslinkersper1tonofdrycellulosefiberswasfirstdispersedin100mLofPEGsolution andsonicatedtoobtainahomogeneousmixture.Thepreparedmixturewasmechanically stirredwithcellulosicmassfor15min.PapersheetswereformedusingaRapid-Köthen AutomaticSheet-FormingMachine(Lodz,Poland),followingtheguidelinesofPN-ISO 5262–2.Thismethodofproducingmodifiedpapersheetsusingthementionedequipment replicatestheconditionsfoundinlarge-scaleproduction,facilitatingeasyscalabilityof theentireprocess.Thepreparedpapersheetsweredenotedasreferenceforthesample withoutanycrosslinker,andPEG/SiO2 ,PEG/TiO2 ,PEG/h-BN,andPEG/h-BN-OHfor sampleswhereadditionalPEGsuspensioncontainingSiO2 ,TiO2 ,h-BN,andh-BN-OH wereadded,respectively.Thereferencewaspreparedbythesameprocedurebutwithout theadditionofPEG/inorganicfillermixture.

2.7.Characterization

High-resolutiontransmissionelectronmicroscopy(HR-TEM)(Washington,DC,USA) imagingwasperformedwiththeFEITecnaiF20microscopeatanacceleratingvoltageof 200kV.Theimagesweretakendirectlyonsample-drop-castCugridswithcarbonfilm. Ascanningelectronmicroscope(SEM)(VEGA3,TESCAN)(Brno,CzechRepublic)was usedtodeterminethemorphologyofthepreparedsheets.Thechemicalbondingofthe structuresinthepapersheetswasexaminedusingRamanspectroscopy(InViaRenishaw, Wotton-under-Edge,UK)equippedwithanexcitationwavelengthof785nm.Itisanideal methodtostudythestructuralpropertiesofthenanomaterials.Thephasecompositionwas determinedbyX-raydiffraction(XRD)patternsbyusinganAeris(MalvernPanalytical, Malvern,UK)diffractometerusingCuKα radiation.ThecontentofCaCO3 wasdetermined inaccordancewiththeInternationalOrganizationofStandardization(ISO1762:2001).The thermogravimetricanalysiswasconductedusinganSDTQ600Thermogravimeter(TA Instruments,NewCastle,DE,USA)underairflowof100mL/min.Ineachcase,the sampleswereheatedfromroomtemperatureto600 ◦ Catalinearheatingrateof10 ◦ C/min. Thesamplesweremeasuredinanaluminacruciblewithamassofabout5.0mg.N2 adsorption/desorptionisothermswereacquiredatliquidnitrogentemperature(77K) usingaMicromeriticsASAP2460(Norcross,GA,USA).TheBrunauer–Emmett–Teller(BET) anddensityfunctionaltheory(DFT)methodswereadoptedtocalculatethespecificsurface areaandporesizedistribution.

3.Results

TEMimagesofSiO2 ,TiO2 ,h-BN,andh-BN-OHarepresentedinFigure 1.SiO2 andTiO2 revealdistinctmorphologies.SiO2 (Figure 1A)exhibitsparticleswithspherical morphology,showcasingvisibleporositywithinthesilicastructure.Highporositydirectly leadstothehighsurfaceareaofSiO2 .Similarly,TiO2 (Figure 1B)nanoparticlesdisplaya sphericalorquasi-sphericalmorphology,relativetoSiO2 ,andexhibitobservableporosity withevidentpores.Forh-BN,theTEMimageillustratesaﬂatandtwo-dimensional(2D) sheet-likestructure.Itisalayeredmaterialwithahexagonallatticeresemblinggraphene, showcasingathinandplanar2Dnature.Hydroxylatedh-BN(h-BN-OH,Figure 1D)also portraysthis2Dplanarstructure,withthepresenceofhydroxyl(OH)groupsaltering surfacecharacteristicsbutnotthefundamentalstructuralmorphology.

Figure1. TEMimagesofcrosslinkers:(A)SiO2 ,(B)TiO2 ,(C)h-BN,and(D)h-BN-OH.

XRDpatternsofallcrosslinkers(SiO2 ,TiO2 ,h-BN,andh-BN-OH)areshownin

Figure 2.Bothh-BNandh-BN-OHshowreﬂectionscorrespondingtoboronnitride (2θ =~26.7◦ , 41.6◦ ,43.7◦ ,54.9◦ ,75.6◦ ;ICDDPDFno.00-034-0421).Forh-BN-OH,aclear shiftofpeakstowardthehigheranglesisobservedincomparisontoh-BN.Forexample,the signalat~26.39◦ ,correspondingtothe(002)planeofh-BN,isshiftedto26.83◦ .Thisisdue totheexpansionofcrystallographicstructurebytheincorporationofthe-OHfunctional groupsintothelattice.SiO2 exhibitsonebroadpeakcenteredataround23◦ ,whichcanbe assignedtotheamorphousstructureofsilicaoxide[17,18].TiO2 iscomposedoftwocrystal phases:anatase(ICDDPDFno.04-014-8515)andrutile(ICDDPDFno.00-021-1276).There are~83.9%ofanataseand~16.1%ofrutileinthesample[19].

N2 adsorption/desorptionisothermsacquiredatliquidnitrogentemperatureare presentedinFigure 3.TheisothermsforTiO2 ,SiO2 ,h-BN,andh-BN-OHaretypeII isotherms,wherethereisawiderangeofporesizes[20].FromTEM,itisclearthat poresarepresent.ThehighestcontentisinSiO2 ,whichcanresultinahighsurfacearea. ThehighestsurfaceareawasdeterminedfortheSiO2 ,whichis275.4m2 /g.h-BN-OH exhibitsalargerspeciﬁcsurfaceareacomparedtoh-BN,whichis38.7m2 /gand19.8m2 /g, respectively.Thisisduetotheexpansionofindividualh-BNlayersbyhydroxylgroups andthecreationofalargersurfaceareathatisaccessibleforN2 adsorption.Thelowest speciﬁcsurfaceareafromallcrosslinkerswasmeasuredfortheTiO2 (10.8m2 /g).Asimilar dependencecanbeobservedforthetotalporevolume(Figure 3b).Themeasuredtotalpore

volumeswere0.248,0.035,0.011,and0.008cm3 /gforSiO2 ,h-BN-OH,h-BN,andTiO2 , respectively.DatacollectedfromtheN2 adsorption/desorptiontestarecollectedinTable 1.

Figure2. XRDpatternsofcrosslinkers:TiO2 ,SiO2 ,h-BN-OH,andh-BN.

(a) (b)

Figure3. (a)N2 adsorption/desorptionisothermsofcrosslinkers,TiO2 ,SiO2 ,h-BN,andh-BN-OH, and(b)poresizedistribution.

Table1. BETsurfacearea,microporevolume,andmedianporewidthofsamples.

Next,theimpactofdifferentcrosslinkersonthemorphologyofthepreparedpaper sampleswasevaluatedviaSEM(Figure 4).Thereferencesampleexhibitsthepresence ofCaCO3 betweenthecellulosefiberswithunevendistribution.ThePEG/SiO2 and PEG/TiO2 papersamples(Figure 4B,C)showamoreevendistributionofCaCO3 particles alongcellulosefiberscomparedtothereferencesample.InthematerialwithPEG/SiO2 , theamountofCaCO3 betweenthefibersismuchhighercomparedtothereferencesample. Papersampleswithh-BNandh-BN-OHexhibitagglomeratedCaCO3 particles.Thismay leadtoincreasedpermeabilityofCaCO3 throughcellulosefibers,whichwillresultina reducedcontentofcalciumcarbonateinthesample.

Figure4. SEMimagesof(A)referencepaperandpapercontainingdifferentcrosslinkers: (B)PEG/SiO2, (C)PEG/TiO2 ,(D)PEG/h-BN,and(E)PEG/h-BN-OH.

Figure 5 presentstheRamanspectraofpapersamples(reference,PEG/SiO2 ,PEG/TiO2 , PEG/h-BN,andPEG/h-BN-OH).Celluloseisalinearpolymermadeupofrepeatingglucoseunitslinkedby β-1,4-glycosidicbonds.Itexhibitsahighdegreeofcrystallinity,with bothcrystallineandamorphousregionsinitsstructure.Ramanspectroscopyissensitivetovariousvibrationalmodespresentincellulose.ThemainRaman-activemodes forcellulosefibersincludeC-CandC-Obonds.Thesevibrationsareobservedaround 1095cm 1 ,offeringvaluableinformationaboutthemolecularbondswithinthecellulose structure[21,22].Next,out-of-planeringbending(C-C-CandC-O-C),observedintherange of400to600cm 1 ,providesinsightsintothespatialarrangementandflexibilityofthecelluloserings.Additionally,RamanpeaksassociatedwithdeformationsintheCH2 andCH3 groupsareprominentintherangeof1300to1470cm 1 [23](orangezones).Furthermore, RamanspectroscopystudiesrevealedthedistinctivepeaksofCaCO3 correspondingto twodifferentforms:calcite(greenzones)andvaterite(bluezones).Thepeakat1085cm 1 signifiescalcite,whilethepeaksat1080and1090cm 1 representvaterite,specifically

correspondingtotheAginternalmodederivedfromthev1 symmetricstretchingmodeof thecarbonateionineachmaterial.Thev4 in-planebendingmodeofcarbonateisobserved at712cm 1 forcalciteand739–749cm 1 forvaterite.Itisnoteworthythatthecharacteristic peakforvateritewasnotdetectedinthecaseofPEG/h-BNandPEG/hBN-OH.However, thereasonofthelackofthispeakisnotclear.

Figure5. Ramanspectraofreferencepaperandpaperwithdifferentcrosslinkers(PEG/SiO2 , PEG/TiO2 ,PEG/h-BN,andPEG/h-BN-OH).

Figure 6 showstheX-raydiffractionpatternsofthereference,PEG/SiO2 ,PEG/TiO2 , PEG/h-BN,andPEG/h-BN-OHpapersamples.Inthepatternsofallsamples,twocharacteristicphaseswereidentified,i.e.,celluloseandcalciumcarbonate(CaCO3 ).Thethree observedpeaks(broadpeaks)at2θ =16◦ ,22◦ ,and35◦ correspondtocellulose.However,a seriesofreflectionsat2θ equalto~23◦ ,29.4◦ ,36◦ ,39.4◦ ,43.2◦ ,47.5◦ ,and48.5◦ arecharacteristicforCaCO3 (ICDDno.00-005-2586).TheXRDdiffractogramsalsoshowtwopeaks (lowintensity)at~30.9◦ and31.6◦ ,whichcanbeattributedtoothercalciumcarbonate polymorphs(e.g.,aragoniteandvaterite)(ICDDno.01-075-9984,00-024-0030).Thereflections,whichcorrespondtothecalciumcarbonate,arenarrowandintense(comparedto thecellulosepeaks,thepeaksarewiderandoflowerintensity),whichindicatesthehigh crystallinityoftheusedCaCO3 .Alackofasignificanteffectofthefillersontheintensity andlocationofindividualreflectionswasfound,whichmaybeattributedtothesmall amountoftheadditives.

Todefinethethermalbehaviorofthepapersamples,thermogravimetricanalysis (TGA)wasapplied.TheTGAresultsforthereferenceandthePEG/SiO2 ,PEG/TiO2 , PEG/h-BN,andPEG/h-BN-OHpapersarepresentedinFigure 7.Forallsamples,three significantweightdecreasesarenoticeable.First,weightlossisobservedat90 ◦ Candis attributedtotheevaporationoftheadsorbedwaterormoisturepresentinthesamples. Furthermore,twostagesofcellulosedegradationareobserved.Thefirstdecomposition stagestartsat250 ◦ C.Celluloseconsistsofglucosemoleculeslinkedtogetherby β-1,4glycosidicbonds[24].Duringthisstage,glycosidicbondsbreak,resultingintherelease ofvolatileproductsandvariousvolatileorganiccompounds,suchasaceticacidand levoglucosan[25,26].Next,aseconddecompositionstagestartingat350 ◦ Cisobserved. Duringthisstage,additionalvolatileproductsareformedascellulosedecomposes.Carbon dioxide(CO2 )andcarbonmonoxide(CO)arereleasedatthisstageasaresultofthe

breakdownofmorecomplexcellulosestructures.Startingwiththereference,agradual masslosswithincreasingtemperatureleadstoaresidueofapproximately26%,indicating theashcontentinthereferencesample.InthecasesofPEG/SiO2 ,PEG/TiO2 ,PEG/h-BN, andPEG/h-BN-OH,thethermalproﬁlesdisplayanalogousmasslosscurves,resultingin residualmassesofabout27.0%,25.7%,25.5%,and24.8%,respectively.

Figure6. X-raydiffractionpatternsofreferencepaperandpaperwithdifferentcrosslinkers: PEG/SiO2 ,PEG/TiO2 ,PEG/h-BN,andPEG/h-BN-OH.

Figure7. TGAplotsofreferencepaperandpaperwithcrosslinkers:PEG/SiO2 ,PEGTiO2 ,PEG/h-BN, andPEG/h-BN-OH.

Todeterminetheashcontentinthepaper,theguidelinessetbytheInternational OrganizationofStandardization(ISO1762:2001)wereapplied.Theprocedurewasthrough sampleignitionat525 ◦ C.Theashcontentinthereferenceandmodiﬁedpapers(PEG/SiO2 , PEG/TiO2 ,PEG/h-BN,andPEG/h-BN-OH)arepresentedinTable 2

Table2. DataobtainedfromtheInternationalOrganizationofStandardization(ISO)standard(ISO 1762:2001)ashcontentforallsamples.

BasedontheresultspresentedinTable 2,theadditionofSiO2 nanoparticlesincreased theashcontentinthepapersignificantly.ThismeansthatthisfacilestrategyboostsCaCO3 retentiononcellulosefibersby12.1%inthepresenceofPEG/SiO2 asananofiller,which canberelatedtotheabundanceofactivesitesavailableforCaCO3 bonding.Theopposite trendcanbeobservedforPEG/TiO2 ,PEG/h-BN,andPEG/h-BN-OH,whereashcontent is8.1,11.1,and17.5%lowercomparedtothereferencepaper,respectively.

Insummary,basedontheaboveresults,itcanbeinferredthattheadditionofporous SiO2 intheformofaPEGsuspensionisthemostpromisingcrosslinkerincreasingthe affinityofCaCO3 tocellulosefibers.Thisconclusionissupportedbynumerousanalyses, suchasTGA,andISO1762:2001tests,whichshowedthattheinorganicfillerscontent aftertheintroductionofPEG/SiO2 increasedby12.1%,whichprovesthepotentialof thisfacilestrategyinpracticalapplicationinthepaperindustry.Thismaybebecause SiO2 possessesthelargestspecificsurfaceareaamongothercrosslinkers(TEMandN2 adsorption/desorption),increasingaffinitytocellulosefibers,resultinginhigherinorganic fillercontentretention.

4.Conclusions

Insummary,thisstudyinvestigatedtheinfluenceofvariouscrosslinkers(TiO2 ,SiO2 , h-BN,andh-BN-OH)ontheinorganicfillercontentinpapersamples.Ramanspectroscopy andXRDconfirmedthatthechemicalstructureofthesamplesremainedinsignificantly changedupontheadditionofthesecompounds.Thisstudyfurtherdelvedintothe moleculardynamicsofcellulosethroughvariousRamanpeaksassociatedwithspecific vibrationalmodes.Additionally,Ramanspectroscopyidentifiedcharacteristicpeaksfor CaCO3 ,distinguishingbetweencalciteandvateriteforms.Moreover,theintroductionof h-BNandh-BN-OHledtotheformationofagglomeratesandunevendistributionofCaCO3 particlesoncellulosefibers,resultingindecreasedashcontentaftertesting.Amongthe crosslinkers,SiO2 suspendedinaPEGsolutionemergedasapromisingcandidateduetoits excellentaffinitytocelluloseandhighsurfacearea,enhancinginorganicfillercontentinthe paper.Thesefindingscontributetoadeeperunderstandingofhowdifferentcrosslinkers impactthecompositionandstructureofpapersamplesandallowtheboostingofthe contentofinorganiccompoundsinthepapersheet,reducingthecontributionofcellulose.

AuthorContributions: Methodology,K.W.andT.K.;Investigation,W.B.;Writing—originaldraft,K.M. andK.S.;Visualization,B.Z.;Supervision,E.M.;Projectadministration,A.G.-K.;Fundingacquisition, J.J.andG.M.Allauthorshavereadandagreedtothepublishedversionofthemanuscript.

Funding: ThisresearchreceivedfundingfromtheNationalCentreforResearchandDevelopment (Poland):POIR.01.01.01-00-0272/19-00.

InstitutionalReviewBoardStatement: Notapplicable.

DataAvailabilityStatement: Thedatapresentedinthisstudyareavailablefromthecorresponding authoruponreasonablerequest.

ConﬂictsofInterest: Theauthorsdeclarenoconﬂictofinterest.

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FROM

Volume 10, Number 3, 2024

PAPERmaking!

Evaluation of oxygen delignified fibers with high water absorbency, as a greener alternative to fully bleached fibers for tissue paper

Nord.PulpPaperRes.J.2024;aop

ChemicalPulping

CláudiaV.G.Esteves*

Evaluationofoxygendelignified fiberswithhigh waterabsorbency,asagreeneralternativetofully bleached fibersfortissuepaper

https://doi.org/10.1515/npprj-2024-0024

ReceivedApril5,2024;acceptedJuly2,2024; publishedonlineAugust27,2024

Abstract: Thepotentialofoxygendelignified fiberstoreplace fullybleached fibersintissueproductswasinvestigatedon softwoodpulps.Theabsorption,mechanicalpropertiesand softnessoflaboratorytissuehandsheetsfromonecommercial fullybleachedpulpand fiveunbleachedoxygendelignifiedlab pulpswerecompared.Thepulpswithdifferentlignincontent andtotal fiberchargewereevaluatedwithandwithoutPFI refining.Thepulpssubjectedtooxygendelignificationresulted inpulpswithmuchhighertotal fiberchargecontentthatledto higherswellingandhigherwetstrengthwhencomparedtothe commercialfullybleachedpulp.Someunbleachedoxygen delignifiedpulpsshowedgreatpotentialintheabsorption capacity,whileothersshowedamuchhigherwettensile strengthwhencomparedtothecommercialpulp.Comparedto thecommercialbleachedpulp,asimilarsoftnessforahigher wetanddrytensileindexintheunbleached fiberswasobserved fortheoxygendelignifiedpulps.Unbleachedpulpssubjected toanextendedoxygendelignificationprovedtobeasuitable alternativetofullybleachedpulpsintissuegrades,depending onthedesiredproperty(absorptionorwetstrength).

Keywords: absorptioncapacity; ﬁbercharges;highlignin content;oxygendeligniﬁcation;wetstrength

1Introduction

Tissueproductsarepresentinoureverydaylivesfordiﬀerent purposes,suchaspersonaluseasfacialtissue,toiletpaper, napkins,householdtowels,packingtissuepaper,among others.Theprimaryrequiredpropertiesaregenerallygood liquidabsorptioncapacity,smoothness,bulkand,strength

*Correspondingauthor:CláudiaV.G.Esteves,SustainableMaterialsand PackagingDepartment,RISEResearchInstitutesofSweden,Bioeconomy andHealth,DrottningKristinasväg61,SE-11428,Stockholm,Sweden, E-mail:claudia.esteves@ri.se

(deAssisetal.2018).Thosepropertiesarehighlydependenton theproductionprocessandrawmaterialandtheywillvary accordingtothe ﬁnaluse.Whileabsorbencyandsoftnessare themostrequiredqualitiesfortoiletpapers,napkinsorfacial tissue(Kimetal.1994),fortowelpapers,besidestheabsorption capacity,tensilestrengthisalsoacrucialpropertytotake intoconsideration,especiallywettensilestrength(Gigacand Fišerová2008).

Theproductionandconsumptionoftissueproductshave beenconstantlygrowingovertheyears,withtheprediction thattheywillcontinuetoincrease,accordingtoCEPI(Central EuropeanPaperIndustries)statistics.Theawarenessaround healthandhygieneissues,populationgrowthandtheimproved standardoflivingaresomeofthecausesoftheincreased demandfortissue.However,theincreaseintissueconsumptionleadstosomeenvironmentalissues.

Nowadays,fullybleached fibersaretheonestypicallyused intissueandhygieneproductsduetotheirgoodproperties: high-levelbrightness,goodwaterabsorption,softnessandlower impuritycontent(Beutheretal.2010;Fišerováetal.2019; Kullander2012;Rebolaetal.2021).However,fullybleached fibersrequirebleachingprocessesthatnegativelyaffectthe environmentandcompromisesustainability.Consumer awarenessforsustainabilityisrisingeverydayandthereisan increasingpreferenceforproductswithlowerenvironmental impact,suchasrecycledorunbleachedproducts(Britoetal. 2023;Feberetal.2020;Halleretal.2020;Zambranoetal.2022).

Unbleached fiberscanbeagreatalternativetofully bleached fibersduetotheirlowercarbonfootprint(Jouretal. 2013;Manetal.2020).Dependingonthe finalproductproperties,unbleached fiberscanprobablybetailor-madeand eventuallyreplacethefullybleached fibersusednowadays. Studieshaveshownthatunbleachedoxygendelignifiedpulps canhaveagreatswellingabilityduetothe fibercharge increaseduringtheiroxidationreactions(Esteves2022;Mai 2021;Zambranoetal.2022;Zhangetal.2005).Besidesthat, oxygendelignificationtendstoresultin fiberswithhigher curlandahighernumberofkinks(Estevesetal.2021a;Mohlin andAlfredsson1990),whichcanbebeneficialforthebulkof the finalproduct(Rebolaetal.2021).Thisworkaimsto

maximizetheincreaseinthetotal fiberchargeofthe fibers subjectedtooxygendelignificationassessingkraftcooked pulpwithdifferentstartingkappanumbersandinvestigating thepotentialofusingthosepulpsasareplacementforfully bleached fibers.

2Materialsandmethods

2.1Materials

Amixtureofsoftwoodchipsfromanindustrialmill(80% Spruceand20%Pine)wasusedinthisstudyforkraftcooking andoxygendeligniﬁcation.Aneverdriedcommercialfully bleachedpulp(referencepulp)fromthesamemillwasused forcomparison.Forfuturecomparisons,itisimportantto highlightthatthefullybleachedpulpwasproducedindustriallyand,thereforesubjectedtomuchharshertreatments thanthepulpsproducedinthelaboratory.

2.2Methods

2.2.1Kraftcooking

Threeseriesofkraftcooksweredoneinthisstudytoobtain pulpswithdiﬀerentkappanumbers,twowereproducedin onerecirculatorydigesterandonewasproducedinautoclave.

Recirculatordigester:Thewoodchipswerekraftcooked inarecirculateddigesterwithadrychipscapacityof2kg, controlledtemperatureandaforcedliquor flow.Thewood chipswereimpregnatedinwaterunder0.5MPanitrogen pressurethedaybeforethecooking.Thewaterwasthen removedandcookingliquorwasaddedtogivealiquor/wood ratioof4.5l/kg.Theimpregnationstepwasperformed for30minat100 °Candthecookingstepwasperformedat 160 °C.Afterthecooking,thesteam flowwasstopped,and thespentliquorwasdrainedoff andcollectedforanalysis.

Autoclave:Thetrialswereperformedinsteelautoclaveswithavolumeof2.5dm 3 ,whichwereloadedwith

250g(o.d.)woodchips.Withavacuumpump,theairinside thevesselswasremovedfor30minandafterthattime,the cookingliquorwassuckedintotheautoclaves.Thechips were ﬁ rstsubjectedto0.5MPanitrogeninjectionforabout 30min,andthenreleasedbeforestartingthecook.Forthe impregnationstep,theautoclaveswereplacedinasteamheatedglycolbathat100 °Cfor30min,andat160 °Cforthe cookingstep.Rotationandslightinclinationoftheautoclavesensuredgoodmixinginside.Afterthecookingstep, theautoclaveswerecooleddowninawaterbathfor10min, andthenthespentliquorwasdrainedo ﬀ thechipsand collectedforanalysis.

Thetrialswereperformedwithaneffectivealkaliof 22%,asulfidityof30%andwithanionicstrengthof0.1M. ThetrialswerestoppedatdifferentHfactors(cookingtimes) toachievedistinctkappanumbers.

Thedelignifiedchipswerewashedwithdeionizedwater for10hinself-emptyingmetalcylindersandsubsequently defibratedandscreenedinanNAFwaterjetdefibrator (NordiskaArmaturfabriken).Theshiveswerecollected, driedat105 °C,andweighed.

2.2.2Oxygendeligniﬁcation

Oxygendelignificationwascarriedoutwith60gofovendried(o.d.)pulp,placedinpolyethylenebagswiththe requiredamountofNaOH,MgSO4,andwaterataconsistencyof12%.Thebagswereclosedbyheat-welding,kneadedinitiallybyhandandtheninavibrationalshaker.The pulpwasthenremovedfromthebagsandtransferredto pressurizedteflon-coatedstainless-steelautoclaves.The autoclaveswereclosed,pressurizedwithoxygenandplaced inanelectricallyheatedglycolbath,withrotationataslight inclination.Thetrialswereperformedwithsingleordouble oxygenstageswithnowashinginbetween.Thesingleoxygenstageswerepressurizedwith0.7MPaofoxygenandthe reactionwascarriedat100 °C.Forthedoublestagesofoxygendelignificationthepressureusedinthe firststagewas 1MPaandthetemperaturewas85 °C,whileforthesecond stagethepressurewas0.5MPaand110 °C.Aftertheoxygen delignification,thepulpswerewashedwithdeionizedwater and filtrated.

TheconditionsforeachtrialareillustratedinFigure1. PulpsweredenominatedKX_OY,whereXisthekappa numberofthecookedpulpandYisthekappanumberafter theoxygendeligniﬁcationstage.

2.2.3Re

ﬁning,papermaking

Thepulpsampleswerereﬁnedattwodiﬀerentlevels(2000 and4,000rpm)inaPFI-millaccordingtoISO5264–2and 2

Figure1: Illustrativerepresentationoftheperformedkraftcookingandoxygendelignificationtrials.Theblackarrowsrepresentthecookingextension andthewhitearrowstheoxygendelignification.Thewhitesinglearrowsarerelativetothesingleoxygenstage,whilethetwowhitearrowsoverlapping arerelativetothedoubleoxygenstage.ThesamplesaredenominatedKX_OY,whereXisthekappanumberofthecookedpulpandYthekappanumber afteroxygendelignification.

handsheetswerepreparedaccordingtoISO5269–1with deionizedwater,toagrammageof60g/m2 forthestandard handsheetsandagrammageof20g/m2 forthetissuehandsheets,withnopressing.

2.3Analysisofpulpandpaperproperties

Table1and2presentthediﬀerentpulpandpapercharacterizationstandards.

Measuringtissuepropertiesusuallydependsonseveral factorsthatareoftenpoorlycontrolledsuchasthesample size,thenumberofplies,amongothers.Inthepresentstudy, thepaperswereproducedandevaluatedinthesameconditions:labtissuehandsheetsof20g/m2 forthecommercial fullybleachedpulpandfortheoxygendeligniﬁedpulps.

Table : Standardsusedforthepulpcharacterization.

MethodsStandards KappanumberISO

Table : Standardsusedforthestandardandtissuehandsheet characterization.

MethodsSamplesStandards

Grammage

Thicknessandbulk

Basketimmersiontest

Capillarityrise(Klemm)

Drytensilestrength

Dryandwettensilestrength

TissueSoftnessanalyzer(TSA)

g/m

RISEinternalstandard ainsteadof  gthetestwasperformedwith  gofsample, binsteadof  min thetestwasperformedfor  min.

2.3.1Softnessevaluation

Thesoftnessofthetissuehandsheetswasevaluatedbythe TissueSoftnessAnalyzer(TSA).TSAcollectsdatafromseveral sensorsintheinstrument,butthreeparametersareconsidered tobeofmostimportance:softness(TS7),surfacesmoothness (TS750)andsheetstiﬀness(D).Theequipmentusesarotating lamellarfanonthepaper’ssurfacetoestimatethesample’s softness.BothTS7andTS750aredeterminedfromthesound spectrumcapturedbymicrophonesoneithersideofthesample.

Thehighertheresponsefromthesensortheloweristhepaper softness.Thetopmicrophonealgorithmwasusedinthiswork.

2.3.2Total ﬁberchargeincreasecalculation

Toquantifyhowmuchtheincreaseinthe fiberchargeafter oxygendelignificationwascomparedtothekraftcooking,a regression fitwasmadeforthekraftcookedpulpsasa reference,givenbyEquation(1).

TTFCcook ( y) = 0 88x + 58 9(1)

TTFCCook isthe Theoreticalvalueforthe Total Fiber Chargeofakraftcookedpulpforagivenkappanumber.The ﬁberchargeincreaseobtainedbyoxygendeligniﬁcationis thencalculatedaccordingtoEquation(2):

TFCoxygen TTFCcook TTFCcook (2)

whereTFCOxygen isthetotal fiberchargeofpulpafteroxygen delignificationtokappanumber “x”,andTTFCCook isthetheoreticaltotal fiberchargeofpulpafterkraftcookinguntilthe kappanumber(x)wasthesameastheoxygendelignifiedpulps.

3Resultsanddiscussion

3.1Deligniﬁcation

Thecookingtrialswereaimedtoobtainthreedi ff erent kappanumbers:ahighkappanumber(109),amediumhighkappa(61)andalowkappanumber(29) – Table3. Thekappanumbersaimedforarebasedonresultsfrom previousstudies(Esteves2022;Estevesetal.2021a; Mai2021)whereitwasseenthatthehigherthestarting kappanumberisaftercookingthehigherwillbethe fi ber chargeincrease.Havingkraft-cookedpulpsbetween kappa109and29willalsoallowustoevaluatethe in fl uenceofthelignincontentonthe fi nalabsorption andstrengthpropertiesinconnectionwiththe fi ber charge.

Thepulpwiththehighestkappanumber(K109)was de fi bratedinaSproutWaldronrefi nerbeforetheoxygen deligni fi cation.Theoxygentrialswereaimedtohave extendeddelignifi cation(morethan60min)withahigh alkalicharge(morethan3%NaOH)andanendpHbetween 10and11.5.Thegoalwastoachievethehighestpossible total fi bercharge.However,forthelowercookedkappa number(K29)theendpHwashigherthanitwasaimedat sincethealkalichargewastoohighforthedelignifi cation e ffi ciency.

Table : Summaryofthekraftcookingandoxygendelignificationtrials. Cookingtemperaturewas  °C,sulfidity  %,effectivealkali  %and liquor-to-woodratio   l/kg.Forthesingleoxygentrialsthepressurewas   MPaandthetemperaturewas  °C,whileforthedoubleoxygen trialsthepressurewas  MPaandthetemperaturewas  °Cforthe first stageand   MPaand  °Cforthesecondstage.

SamplesH factor – Kappa no. Totalyield (cooking stage)(%) Residual alkali(g/l) K

SamplesTime, min Alkali charge, % Kappa no. Totalyield (oxygen stage)(%) EndpH K

3.2Total ﬁbercharge

Itisknownthatthecarboxylicacidgroupsarethemain chargedgroupsfoundinthechemicalpulp fibers(Dangetal. 2006;Zhangetal.2005).However,thechargessuffermodificationsduringthedifferentpulpingprocessesthatcan eventuallyleadtotheirdecline.Figure2illustratesthe differentbehaviorsthatoccurduringKraftcooking,oxygen delignification,andbleaching.

Thelineardecreaseinthechargedgroupsasthelignin contentisreducedduringkraftcookingisclear(Figure2) anditiswelldocumented(Buchertetal.1997;Chaietal.2003; Dangetal.2006;Esteves2022).Thisiscausedbythelignin removalduringthedeligniﬁcationandbysomexylan dissolution(Buchertetal.1997;Chaietal.2003;Dangetal. 2006;Estevesetal.2021a)sincetheyarethemaincomponentswherethechargedgroupsarepresent.

Foroxygendelignification,thetotal fiberchargedevelopmentisnotstraightforward,buttheincreaseintotal fiber chargewhencomparedtothekraftcookedpulpsforthesame lignincontentisevidentinthehigherkappanumbers.The highervaluesforthechargecontentwereobtainedforthe pulpwiththehighestkappanumberfromthecooking,which wasexpected.Thisismainlyduetotheoxidationreactionsin ligninthatleadtoahighercontentofnewchargedgroupson thepulpduetothehigherlignincontent.Thesegroupsare largelyfromthemuconicacidstructurescreatedinlignin (Dangetal.2006;Snowmanetal.1999;Yangetal.2003),but

Figure2: Total fiberchargefornonrefined pulpsatdifferentkappanumbersafterkraft cooking(blackline)andoxygendelignification (green,orangeandbluelines)atdifferent alkalicharges,givenas%NaOHinthe figure. Theoxygentrialspresentedingreenwere performedonkraftcookedpulpwithaninitial kappa(iKa)numberof29,whiletheoxygen trialspresentedinorangewereperformedon kraftcookedpulpwithaninitialkappanumber (iKa)of61andtheoxygentrialspresentedin bluewereperformedonkraftcookedpulp withaninitialkappanumber(iKa)of109.The commercialfullybleachedpulpis denominatedasreferenceinthe figure(black square). C.V.G.Esteves:Oxygendeligni

alsofromoxidizedcarbohydrates(Taoetal.2011;Zhangetal. 2006;Zhaoetal.2016).Forthelowerkappanumber(K29)the increasein fiberchargeswasalmostinsignificantafteroxygendelignification.Thiswasnotasurprisesincethelignin contentwasreducedsignificantlyduringthekraftcooking step,andthepotentialtomaximizethechargeswasreduced.

Previousstudieshavereportedthatthe fibercharge increasefromoxygendelignificationwillbedependenton severalfactors,suchasinitiallignincontent,alkalicharge applied,delignificationdegreeandtheextensionofthereaction(Esteves2022;Estevesetal.2021a;Zhangetal.2006). FromFigure2thevariationinthoseparameterscanbe observed.Pulpswithdifferentstartingkappanumber(109, 61and29)willhavedifferent finalchargecontentafter extendedoxygendelignificationdependingontheconditionsintheoxygenstage(alkalichargeandreactiontime).

Table4presentsthe finaltotal fiberchargeobtained afteroxygendelignificationandtheincreasedamountof total fiberchargeifthepulpwouldbeonlycookedtothe samekappanumbers,calculatedbyequation(1)and(2).

Table : Fiberchargeincreaseforthestudiedpulps.Thesamplesthat obtainedthehighestincreaseof ﬁberchargeswithineachkraftcooking seriesarehighlightedinbold.

SamplesKappa number Total ﬁbercharge, meq/kg Fiberchargeincrease,% K

_O Noincrease K_O K_OO K_O K_O

Previousstudies(Esteves2022;Estevesetal.2020)have shownthataminimumincrease(>60%)inthe fibercharge isvitalforthechangeinpulpproperties.FromTable4the samplesthatpresentedthehighest fiberchargeincrease wereselectedandconsideredforfurtherstudies(refining andpapertesting).Thesamplesfromthecookedpulpwith kappanumber29didn’thaveasignificantincreaseinthe fibercharge,achievingonly23%increase,nevertheless,the samplewasalsoconsideredforfurthertrialstoevaluatethe influenceofthelignincontentonthe finalproperties.

3.3Waterretentionvalue

Thewaterretentionvalue(WRV)wassignificantlyhigherfor theoxygendelignifiedpulpscomparedtothecommercial fullybleachedpulp(reference) – Figure3A.Thisbehavior wasalreadyexpectedsincetheoxygendelignified fibers presentedahighertotal fibercharge(highercontentof carboxylicacidgroups)whichleadstoanincreasein fiber swelling(Estevesetal.2020;Sjöstedtetal.2015;Zhaoetal. 2016).Thepulpswiththehighestkappanumberwerethe oneswiththehighestwaterretentionvalues,whilethe referencefullybleachedpulppresentedthelowestcharge andthelowestWRV – Figure3BandC.

Figure3Bshowsthewaterretentionvaluecorrelation alongthekappanumberwhenpulpsareunrefinedor refinedwith4000revolutions.Asexpected,theWRVincreaseswithrefininganddecreaseswithkappanumber.A similarrelationcanbeseeninFigure3CwheretheWRVis plottedagainstthetotal fibercharge.Aspreviouslysuggested,WRVishighlydependentonthetotal fibercharge (Estevesetal.2021b).

Thesigniﬁcantincreaseinthewaterretentionvalueis verybeneﬁcialtotheabsorptionandstrengthpropertiesof

Figure3: WaterretentionvalueforunbleachedoxygendelignifiedpulpsandfullybleachedpulpasafunctionofA)PFI-refining,B)kappanumberandC) total fibercharge.

thepulps,however,itmightimpactthepaperdryingsection, leadingtohigherenergyrequirements.However,theincreaseinthewaterretentionvaluealsoleadstoadecreasein thereﬁningrequiredtoacertaintensilestrength,whichin turnwillleadtolowerenergyconsumption.

3.4Absorptionandcapillarityproperties

Toevaluatetheabsorptionandcapillaritypropertiesofthe studiedpulps,thebasketmethodandKlemmtestwere used,respectively.Theabsorptioncapacitycanbede fi ned asameasurementofthe fi berabilitytoabsorbwater(g water/g fi ber)untilitgetssaturated(SchuchardandBerg 1991).Theabsorptionquanti fi cationalsoconsidersthe fi ber capacityofholdingwater,sincethereisadewatering periodbeforemeasuringthewetweight.Theabsorption capacityfortheanalyzedsamplesispresentedinFigure4. Asexpected,theabsorptioncapacityisdirectlyrelatedto thepaperbulkdespitethelargespaninkappanumber, rangingfrom0(REF)to47.

ThecommercialfullybleachedpulpandthepulpsK61_O andK61_OOpresentedverysimilarresultsfortheabsorptioncapacity(Figure4)whiletheoxygendeligniﬁedpulps withthehighestkappanumber(K109_OandK109_OO)and thelowest(K29_O10)afterthecookingsteppresentedlower absorptioncapacity.

Absorbencycanbesigni fi cantlyincreasedthrough theincreaseintheproductbulk(SedinandVomho ff 2017).Bulkisthereforeanimportantpropertyoftissue paper,asitregulatesthepaperabsorption,paper networkporosityandsoftnesssensation.Theoxygen deligni fi edpulps(K61_O25andK61_OO18)presented

Figure4: Absorptioncapacityforunbleachedoxygendeligniﬁedpulps andfullybleachedpulpasafunctionofbulk(20g/m2 laboratorytissue handsheetswereused).Decreaseinbulkforeachpulpsampleisobtained byincreasedreﬁning.

similarabsorptioncapacityasthereferencepulpwith slightlyhigherbulk.Theoxygendeligni fi edpulps K109_O47,K109_OO35andK29_O10presentedamuch lowerbulkwhencomparedtotheotherpulpsand, consequentlylowerabsorptioncapacity.Fortheunrefi nedsamples,K109_O47andK29_O10,thebulkandabsorptioncapacityareverysimilar,eventhoughthelignin contentissigni fi cantlydi ff erent.

Contrarytowhathappenswiththewaterretention value(Figure3A),whenrefiningisappliedthewaterabsorptioncapacityofthepulpsdecreases – Figure4.Whilethe waterretentionvalueismainlyrelatedtothewaterabsorptioninthe fiberwall,theabsorptioncapacityisrelated tothewaterabsorptionwithinthe fibernetwork.The externalandinternal fibrillationcreatedbytherefining leadstoagreaterswellingpotentialbyincreasingthewater penetrationbetweenthe fibrilsseenaswaterretention

Figure5: Watercapillarityrisedistanceforunbleachedoxygen deligniﬁedpulpsandfullybleachedpulpasafunctionofbulk(20g/m2 laboratorytissuehandsheetswereused).Decreaseinbulkforeachpulp sampleisobtainedbyincreasedreﬁning.

value(Hartman1985;Kang2007),butsimultaneously,the increasein ﬁbrillationalsoleadstobetter ﬁberbonding whichpromotesadenserpaperstructure,leadingtoalower absorptionvalues(Lumiainen2000).

Figure5showsthecapillaryrisebehaviorofthe diﬀerentpulpsobtainedbytheKlemmmethod.Similarto theresultsinFigure4,thepulpsK61_O25andK61_OO18 presentedthemostsimilarresultswhencomparedtothe referencefullybleachedpulp.

Capillaryrisedependsonthecohesionandadhesion forcesofthewatermoleculesthatwillrisealongthe fiber wallsorwithinthepores(MoraisandCurto2022).Thesize ofthoseporeswilldefinethecapillarypressureinside the fibersandthe fibernetwork,determininghowfastthe

waterwillrise.Microporesarepresentinnative fibers,but duringpulpingthelignin–hemicellulosematrixisgradually removedcreatingsomeporesinthecellwall,calledmacropores(Brännvalletal.2021).Themicroandmacropores willinfluencethewaterabsorptioncapacityofthe fibers, oncetheyinfluencethe fibercapillarypressure(Beuther etal.2010).Biggerporestendtodecreasethecapillaryforces andtheholdingcapacity(JoutsimoandAsikainen2013), whilesmallerporesarecapableofretainingmoreliquidat higherpressuresthanlargerporesduetogreatercapillary force(BrodinandTheliander2012).

When fi bersundergore fi ning,thecapillarypressure tendstodecrease,probablyduetointernalandexternal fi brillation.As fi berinteractionsgetstronger,thesheet thicknessisreducedbytheincreaseinsheetdensifi cation, andconsequently,theopenvoidsavailableinthesheetin whichwatercanbeheldwillbereduced(Kullander2012). Thedecreaseinthecapillarypressurewiththeincreasein fi brillationwassuggestedbefore(Moraisetal.2021).The higherthebulk,thehigherthevolumeofporesinsidethe papernetwork.

Figure6showsthediff erentspeedsinthecapillaryrise forthestudiedpulps.Forunre fi nedpulps,theoxygen deligni fi edK29_O10andK61_O25hadthefastestwaterrise (Figure6A).Thepulpswithslowerwaterrisingwerethe oxygendelignifi edpulpswithhigherkappanumbers, K109_O47andK109_OO35.Thefastercapillarityriseseen fortheK61_OandK61_OOisprobablyduetothemoreopen structureofthepapernetworksincebothpulppresented thehighestbulk.

Figure6: RateofwatercapillarityriseforunbleachedoxygendelignifiedpulpsandfullybleachedpulpforA)unrefinedandB)4000PFI-refinedpulps. (20g/m2 laboratorytissuehandsheetswereused).

8 C.V.G.Esteves:Oxygendeligniﬁed ﬁbersasagreeneralternativefortissuepaper

Whenreﬁningisapplied,thefastestwaterriseisforthe commercialfullybleachedpulp,whiletheoxygendeligniﬁed pulpslandinlowerwaterrising,especiallyfortheK29_O10.

3.5Mechanicalproperties

3.5.1Drytensileproperties

Thereferencefullybleachedpulppresentedamuchlower tensileindexthantheunbleachedoxygendelignifiedpulps –Figure7A.Thedifferenceinthetensileindexisevenmore significantforunrefinedpulps.Thisiscausedbythedifferent chemistryofthefullybleachedandunbleached fibersandthe harsherindustrialtreatmentthatthefullybleachedpulpwas subjectedto.Theoxygendelignifiedpulpsobtainedahigher tensileindex.Alltheunrefinedoxygendelignifiedpulps,obtainedsimilartensileindex(between74and83Nm/g)and higherbulk(1.55–1.45cm3/g)whencomparedtothereference fullybleachedpulprefinedwith2000PFI-revolutions(72Nm/g and80g/cm3).Forhigherrefininglevels,theunbleachedpulps continuedtopresenthighertensileindexvaluesthanthe referencepulp.Theresultsshowedthatoxygendelignification pulpsobtainsimilarabsorptionpropertieswithmuchhigher tensilestrengththanfullybleachedpulps,withlowerrefining energyrequired.

Theincreaseinpaperstrengthwassurelyachieved bytheincreaseininternalandexternal fi brillationinthe re fi ningstep.Re fi ningbreakssomeinnerbondsinthe fi berswall,makingthemmore fl exibleandswellable,

leadingtobetter fi berbonding(Gharehkhanietal.2015; Kullander2012;MaloneyandPaulapuro1999).However, forthefullybleachedpulps,theincreaseinthetensile indexseemstobemorein fl uencedbytheincreaseof sheetdensi fi cationduetore fi ning,ratherthantheincreaseinthe fi berbondingstrength.Oxygendeligni fi ed pulpspresentedhigherstrengthforsimilarbulk comparedtothereferencepulp.Thissuggeststhataless bondedareaisneededtoaccomplishacertainstrength. Nordström(2014)andEstevesetal.(2021b)havepreviouslyreportedsimilarresults,wherepulpswithhigher lignincontentobtainedahighertensileindextoagiven lowdensity.

FromFigure7B,itisinterestingtohighlightthatthe oxygendeligniﬁ edpulpshaveasigniﬁ cantlyhigher strengthforsimilarabsorptioncapacity.Asseenforthe otherproperties,thesamplesK61_O25andK61_OO18presentedthebeststrengthandabsorptionperformance,as showninFigure4.

3.5.2Wettensileproperties

Cellulose-basedmaterialssuchastissueproductsare highlysensitivetohumiditysincecellulosemoleculeshold alargenumberofhydroxylgroups.Paperis,therefore,a hydrophilicmaterial;whenincontactwithwateror moisture,itstartstoswell,breakingdownthehydrogen bondsandlosingitsstructure.Therefore,thewetstrength becomescriticalfortissuepapersexposedtowaterand moisture.

Figure7: TensileindexforthereferenceandoxygendeligniﬁedpulpsasafunctionofA)bulk(60g/m2 laboratorytissuehandsheetswereused)and B)absorptioncapacity(20g/m2 laboratorytissuehandsheetswereused).Theincreaseintensilestrengthforeachpulpsampleisobtainedbyincreased reﬁning.

C.V.G.Esteves:Oxygendeligni

Wettensilestrengthisoneoftheimportantproperties oftissueproductssincetheyshouldberesistantand maintaintheirstructurewhenwetted.Figure8presents thewettensileindexfortheoxygendeligniﬁedpulpsand thereferencefullybleachedpulp.Thewettensileindexfor

Figure8: Wettensileindexforthereferenceandoxygendeligniﬁed pulpsasafunctionofA)bulk,B)waterretentionvalueandC)absorption capacity.(20g/m2 laboratorytissuehandsheetswereused).

thereferenceunreﬁnedpulpwasnotpossibletodetermine duetothelackofwetstrengthsincenowetstrengthadditiveswereusedinthisstudy.

Thereferencefullybleachedpulpafter2000and4000 PFIrevolutionsdidnotpresentanincreaseinthewettensile strength(Figure8A).Theunbleachedpulps,ontheother hand,hadanalmostlinearincreaseofthewettensileindex withthereﬁning,obtainingbetween200and400%higher wettensilethanthereferencefullybleachedpulpforsimilar bulkvalues.

Whenthewettensileindexisseenasafunctionofthe waterretentionvalue,therelationisalmostlinearforthe unbleachedpulps(Figure8B).Thehigherthepulpswelling thehigherthewettensileindex.

Figure8CshowssimilarbehaviourasseeninFigure8A, sincebulkandabsorptioncapacityhavealinearrelationship. Forsimilarabsorptioncapacity(around7g/g),thepulpswith thehigherkappanumber(K109_O47andK109_OO35)present thehighestwetstrength(2.5Nm/g).

Theincreasein fi ber fi brillationand fi berbonding, thatisexpectedfromre fi ning,seemstovanishwhen thepaperiswettedforthecommercialfullybleached pulp.Theunre fi nedunbleachedpulpspresentedwet tensilestrengthinthesamerangeasthecommercial fullybleachedpulpwhenitwasre fi nedwith2,000and 4,000revolutions,withmuchhigherbulk.Thehigherwet strengthseenfortheunbleachedpulpsshowsagreat potentialfortowelpapers,forexample,wheretheycan stillhaveahighabsorptioncapacityandhigherstrength.

Whenthestrengthvaluesareplottedasafunctionof thekappanumbertherelationisclear:higherlignincontentinthepulpsleadstohigherwettensilestrength (Figure9).Someoldlaboratorystudieshavealsoshowna connectionbetweenhighlignincontentandincreasedwet strength(Gunnarsson2012).Figure9Bshowsthefairly linearrelationofthepulps ’ relativestrengthandlignin content.

Theseresultsshowthepotentialofusingpulpswithhigh lignincontenttoincreasethewetstrengthofthetissue products.Theequilibriumbetweenwetstrengthandabsorptioncapacityofunbleachedtissueproductscanbe alteredbytheoxygendeligniﬁcationconditionsandbythe startingkappanumberofthepulps.

3.6Softness

Softnesscanbedescribedasthehumanperceptionof somethingthatisbulkyanddelicatewithnosharpedges. However,itisalsoaquitecomplexphenomenontoevaluate andquantify.Softnessdependsonseveralfactors,suchas

10 C.V.G.Esteves:Oxygendeligniﬁed ﬁbersasagreeneralternativefortissuepaper

Figure9: Strengthpropertiesforthereferenceandoxygendelignifiedpulpsatdifferentlevelsofrefiningasafunctionofkappanumber.A)Wettensile indexandB)relativestrength.(20g/m2 laboratorytissuehandsheetswereused).ThenumbersonthetopofthesymbolsingraphA)arethetotal fiber chargeofthesamples.Thevaluesareinmeq/kg.

individualsensibility,bulk, ﬂexibility,thickness,among others.Softnessisconsideredoneofthemostessential propertiesoftissueproducts(deAssisetal.2018).

ThestudiedsampleswereevaluatedusingtheTissue SoftnessAnalyzer(TSA),whichisclaimedtogivea perceptionofthesample ’ ssoftness(Figure10).ThesoftnesscanbeestimatedbytheTS7parameter,wherelower TS7valuesareattributedtohighersoftness(deAssisetal. 2020).

Theunbleachedpulps(K61_O25andK61_OO18)presentedsimilarTS7valuesforslightlyhigherbulkasthe referencepulpwhennore ﬁ ningwasapplied.Thehighest andlowestkappasamplesalsoobtainedasimilarTS7 valueasthereferencebutforlowerbulkvalues.TheincreaseinTS7withre ﬁ ningwasexpectedsincethehigher strengthisusuallyassociatedwithlowersoftness(Debnathetal.2021;Scottetal.1995).Thus,anincreased strengthresultsinimpairedsoftnessandviceversa (Figure10BandC).

Anincreaseinthepaperstrengthisgenerallyfollowed byadecreaseinsoftness(Salemetal.2023).However,itcan beseenthatunbleachedpulpspresentedsimilarsoftness levelswithmuchhigherwetanddrystrength(Figure10B andC).Figure10Bshowsthatoxygendeligni fi edpulps, whennotre fi ned,presenthigherwetstrengthforlower TS7values(highersoftness).Whenthepulpsarere fi ned, thewetstrengthsigni fi cantlyincreaseswithoutvaryingthe TS7factorsignifi cantly(K109_O47andK109_OO35).Quite

similarbehaviorwasseenforthedrystrength – Figure10C. Theoxygendelignifi edpulpsevenshowedslightlybetter softnessatspeci fi ctensilestrengththancommercially bleachedpulp.Unfortunately,thesoftnessmeasurements throughtheTSAequipmentcomewithasigni fi cantlylarge standarddeviationforsomesamples,whichgivessome uncertaintyaboutthesoftnesscomparisonbetweenthe samples.

4Conclusions

Thisworkshowedthepotentialofusingoxygendelignified pulpsasareplacementforfullybleachedpulps.Comparedto thecommercialfullybleachedpulp,similarabsorptionand muchhighertensilestrengthwereobtainedfortheoxygen delignifiedpulps,especiallyforunrefinedpulps.Twoofthe oxygendelignifiedpulps(K61_O25andK61_OO18)achieved similarabsorptioncapacityandfasterwaterrisingwith higherbulkwhencomparedtocommercialpulp.Significantly higherwetstrengthandhigherstrengthratiowereobtained fortheoxygendelignifiedpulps,especiallyforthepulpswith thehighestlignincontent.Theoxygenpulpsshowedsimilar softnesslevelsforhighertensilestrength.Lignincontentwas seentohaveanimpactontheWRVbutnoeffectonabsorptioncapacityorcapillaryrising,whichonlydependedonbulk. Asexpected,whentheoxygendelignificationstartswitha kraftcookedpulpwithalowkappanumbertheincreasein

C.V.G.Esteves:Oxygendeligni

Figure10: Softnessforunbleachedoxygendeligniﬁedpulpsandfully bleachedpulpasafunctionof,a)bulk,b)wetandc)drytensileindex (20g/m2 laboratorytissuehandsheetswereused).

the fiberchargesisnotmaximizedasinthehigherkappa numberpulpsandthereforenosignificantbenefitwasseen. Thehigherwetstrengthandsimilarabsorptioncapacity seenfortheoxygenunbleached fiberswillbesuitablefor differenttypesoftissueproducts,makingthemamoresustainableoptionthanthefullybleached fibers,sincenobleachingwouldbenecessary.Nevertheless,otherconsiderations mustbeconsideredandstudiedfurther,suchasthepossibly higherenergyconsumptioninthedryingpapersection.

Acknowledgments: Theauthorgratefullyacknowledges “ SödraSkogsägarnasStiftelseförForskning,Utvecklingoch Utbildning ” forthe fi nancialsupportofthisproject2022–297.TheauthorwouldalsoliketoacknowledgeMikaela Kubatforthecooking,oxygendelignifi cationand fi ber chargemeasurements.FredrikAdåsandLadanFouladiare alsoacknowledgedfortheirguidanceandcommentsonthe tissuetesting.ElisabetBrännvallandAronTysénare acknowledgedforreadingandcommentingonthe manuscript.

Researchethics: Notapplicable.

Authorcontributions: Theauthorhasacceptedresponsibilityfortheentirecontentofthismanuscriptandapproved itssubmission.

Competinginterests: Theauthorstatesnocon ﬂ ictof interest

Researchfunding: SödraSkogsägarnasStiftelseförForskning, UtvecklingochUtbildning,2022–297.

Dataavailability: Therawdatacanbeobtainedonrequest fromthecorrespondingauthor.

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Comparison of Degradation of Lignincontaining Wastewaters in the Presence of Different Microbial Consortia

Paper

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Introduction

Keywords

Chem. Biochem. Eng. Q.

Shinella Cupriavidus Bosea Serratia Yersinia

et al.

Chem. Biochem. Eng. Q.

Materials and methods

Materials

Shinella Cupriavidus Bosea Bacillus Rhodococcus Serratia Yersinia Astragalus membranaceus Fritillaria cirrhosa, Angelica sinensis

Culture media and model wastewater

Domestication of the microorganisms

Chem. Biochem. Eng. Q.

Analysis methods

Wastewater treatment and conditions optimization

– Experimental design for wastewater treatment conditions

Chem. Biochem. Eng. Q.

Statistical analysis

Results and discussion

Conditions optimization and analysis of wastewater degradation effect

Effect of treatment time on degradation

3.1.2 Effect of temperature on COD removal

– Degradation of lignin-containing wastewater by different microorganisms

Chem. Biochem. Eng. Q.

– Effect of treatment time on lignin-containing wastewater treatment (a: traditional Chinese medicine wastewater; b: papermaking wastewater)

– Effect of temperature on lignin-containing wastewater treatment (a: traditional Chinese medicine wastewater treatment by J-6; b: traditional Chinese medicine wastewater treatment by J-1; c: papermaking wastewater treatment by J-6; d: papermaking wastewater treatment by J-1)

Effect of pH on COD removal

Effect of dissolved oxygen on COD removal

Chem. Biochem. Eng. Q.

Effect of wastewater concentration on COD removal

Chem. Biochem. Eng. Q.

– Effect of different factors on the treatment of lignin-containing wastewater (a: effect of pH on traditional Chinese medicine wastewater treatment; b: effect of pH on papermaking wastewater treatment; c: effect of DO on traditional Chinese medicine wastewater treatment; d: effect of DO on papermaking wastewater treatment; e: effect of wastewater concentration on traditional Chinese medicine wastewater treatment; f: effect of wastewater concentration on papermaking wastewater treatment)

Effect of nitrogen source addition on papermaking wastewater treatment

Chem. Biochem. Eng. Q.

et al. et al.

– Effect of nitrogen source addition on papermaking wastewater treatment (a: average removal efficiency of COD under different nitrogen addition; b: effluent ammonia nitrogen concentration under different nitrogen addition)

Consortium structure and diversity analysis

Analysis of consortium diversity and composition

Chem. Biochem. Eng. Q.

– Bacterial community diversity in different samples

Sphingomonas Rhodoplanes Lactobacillus Pseudomonas Serratia Ralstonia

Rhodoplanes Pseudomonas Serratia Lactobacillus Ralstonia

Bacteroides

Rhodoplanes

Sphingomonas Rhodoplanes

Rhodoplanes

Sphingomonas

Chem. Biochem. Eng. Q.

– Bacterial community composition of different wastewater treatment systems (T: traditional Chinese medicine wastewater; P: papermaking wastewater)

Serratia Ralstonia Pseudomonas Lactobacillus Rhodoplanes Geobacter

Serratia Sphingomonas

Pseudomonas Lactobacillus Ralstonia

et al.

Serratia

Chem. Biochem. Eng. Q.

– Microscopic examination of lignin degradation microbial consortia in different wastewater treatment systems (a: traditional Chinese medicine wastewater treatment by J-1; b: papermaking wastewater treatment by J-1; c: traditional Chinese medicine wastewater treatment by J-6; d: papermaking wastewater treatment by J-6)

Microorganism correlation analysis

Serratia

Rhodoplanes Sphingomonas, Pseudomonas

Serratia Geobacter Lactoba-

Biochem. Eng. Q.

– Correlation between different species in the lignin-containing wastewater treatment systems

cillus Serratia Geobacter Lactobacillus

Conclusion

DECLARATION OF COMPETING INTEREST

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

ACKNOWLEDGMENTS

This research was supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ20B060004, Zhejiang Shuren University project (2019R017) and Zhejiang Shuren University Basic Scientific Research Special Funds (2021XZ016).

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Which Wastepaper Should Not Be Processed?

Paper

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Essential Guide to Aqueous Coating

sustainability

Article

WhichWastepaperShouldNotBeProcessed?

EdytaMałachowska 1,2, *,AnetaLipkiewicz 1 ,MarcinDubowik 1 andPiotrPrzybysz 1,2

1 NaturalFibersAdvancedTechnologies,42ABlekitnaStr.,93-322Lodz,Poland

Citation: Małachowska,E.; Lipkiewicz,A.;Dubowik,M.; Przybysz,P.WhichWastepaper ShouldNotBeProcessed?

Sustainability 2023, 15,2850. https://doi.org/10.3390/su15042850

AcademicEditors:JianxinJiang, JunHuandHailongYu

Received:26November2022

Revised:13January2023

Accepted:2February2023

Published:4February2023

* Correspondence:edyta_malachowska@sggw.edu.pl;Tel.:+48-22-59-385-45

2 InstituteofWoodSciencesandFurniture,WarsawUniversityofLifeSciences-SGGW,159Nowoursynowska Str.,02-787Warsaw,Poland

Abstract: Inthe21stcentury,numerouseconomicandenvironmentalinitiativeshavesignificantly increasedpaperrecycling,whichcontinuestoexpandduetoenvironmentalawareness.Withincreasingrecyclingrate,low-qualitypaperfractionsmaybeincludedintheprocess,leadingtothe overproductionofverylow-valuepapersthatcannotbereprocessed.Moreover,theproductionof paperfrompoor-qualitywastepapercanresultintheintroductionofchemicalsfromtherecycled paperintotherecyclingloopandunintendedspreadofchemicalsubstances.Therefore,reliable andconsciousselectionofrecycledpulpisimperative.Tothisend,thepresentstudyverifiedthe propertiesofrecycledpulpcriticalfortheassessmentofitspapermakingabilityfortheproduction ofhigh-qualitysanitarypaper.Followinganexaminationofsamples,itwasfoundthatthekey parametersthatinfluencethepapermakingabilityofwastepaperincludepresenceofimpurities, contentofextractivesubstances,freeness,andlengthoffiber.Onthisbasis,typesofwastepaperthat, attheverybeginning,didnotportendwellforobtainingpaperproductswithhighpotentialfor utilitywereeliminated.

Keywords: wastepaper;recycling;papermakingability;wastemanagement

1.Introduction

Recoveredfibrousrawmaterialsfrompaperproductshavelongbeenusedforsecondarypulpproduction[1–4].Forseveralcenturies,productsofinadequatequality,unsuitableforfurtheruseorwithdrybreaks,havebeenusedasinputmaterials.Withgrowing demandforpaper,however,theprocessingofusedpaperproducts,calledwastepaper,has becomeincreasinglysignificantforobtainingfibrousrawmaterials.Currently,thepulpand paperindustryusessecondarypulpasanindispensableproductioninput.Theglobalpaper recyclingratestandsatapproximately58%[5–8].Insomeofthemoredevelopedcountries, therecyclingrateofwastepaperhasreachedashighas70–75%ofthetotalamountof wastepapergeneratedatthenationalscale[9].Inparticular,Europestandsoutwiththe highestpaperrecyclingrateintheworld(72%),followedbyNorthAmerica,whereas Asia,LatinAmerica,andAfricahavethelowestrecyclingrates[10].Manycountrieshave unceasinglysoughttoincreasethisratebyimplementingmeasurestooptimizeactivities fromthebeginningofpaperproductionuntilitsuse,collection,andrecycling.

Paperanditswasteareeasilydegradable,andtheresultingcellulosefiberscanbe recycleduptoseventimes[11,12].Therefore,usefulpaperwastemustberecycled.Efficient wastepaperrecyclingplaysasignificantroleinbuildingasustainableenvironment,offering severalbenefitsoverthepaperlifecycle[13,14].Sensiblepaperrecyclingsavesenergy, water,andlandfillareas.Moreover,wastepaperrecyclingsavesvirginfiberinputduring paperproduction.Asopposedtochemicalpulpingofwoodfromvirginpulp,wastepaper pulpinggeneratesnowoodwasteordissolvedchemicalsforenergygeneration[15].Moreover,paperrecyclingreducestheemissionsofCO2 ,NO2 ,andSO2 intheairanddecreases waterpollutioncausedbychlorinecompoundsfrombleachingandchemicals[16].In

general,recyclingandretreatmentofpaperimpactsustainability,particularlytheeconomy, environment,andsociety.

Whilstthepaperrecyclingratecanundoubtedlybefurtherincreasedinmostcountries, thequalityofwastepapermayeventuallydecreaseasincreasingnumbersofmarginalpaper fractionsaregatheredforrecycling.However,increasingthepaperrecyclingraterequires theuseofrecycledfibersfromhigh-qualitypaper.Thisiscrucialsincethemechanical andprintingpropertiesofthematerialdeteriorateeverytimeitisrecycledowingto thelimitednumberofrecyclingrounds[17–21].Thesefibersareshortenedandtheir swellingandflexibilitypropertiesdeteriorate.Poor-qualitywastepapernotonlyreducesthe papermakingabilityofpulpbutisalsoassociatedwithincreaseduseofchemicaladditives andfillers,whichproducelargequantitiesofby-products,creatingmajorenvironmental andeconomicchallenges[22].Moreover,poor-qualityrecycledfibersincreasethecontent ofinjurioussubstancesinpaper[23–25].Inthiscontext,systematicanalysisofpaper pulpisessentialtoprovideabasisforfurtherevaluationofthequalityofwastepaperas aresourceaswellastoidentifyappropriateprocessingmethodsforensuringthatthe resultingrecycledpulpmeetstheexpectationsofpapermakersand,ultimately,consumers. Thisisparticularlyapplicabletosanitarypapers,whichmustbenotonlystrongbutalso absorbentandsoft.Inaddition,consumersrequireacertainqualityofwhitenessand brightness.Thus,recyclablewastepapercanbesegregatedintovariousgradestofacilitate theproductionofhigh-qualityproducts.Bydividingtherecyclablepaperwasteaccording toitspropertiesandcomparingthemwiththecharacteristicsofthedesiredfinalproduct, paperwastecanberecycledmoreefficiently.Inaddition,ahighlysortedpaperstream facilitatestheproductionofhigh-qualityendproducts;savesprocessingchemicals,water, andenergy;andreducestheamountofsludgeandrejectsgeneratedduringwastepaper processing.Removingcontaminantsfromwastepaper,ensuringpapermakingability,and maintainingconsistentqualitywhilstlimitingtheenvironmentalfootprintofproductshave becomeamajorgoalandchallengeintheindustry.

Thesegoalsandchallengeswereexploredbymanystudiesonpaperrecyclingat theturnofthelastdecade[26–30].Previousresearchmainlyfocusedonthedevelopmentoftechnologiesthatenablethereuseoffibersfromwastepaper,suchasbleaching methods[31–35] offibersanddeinkingtechnologies[36–39].Publicationsonpaperrecyclingpeakedin2000,whichmayhavebeendictatedbytheEuropeandeclarationonpaper recoveryissuedinthatyear[40].Duringthenextdecade,publicationscontinuedtofocus onfiberrecoveryissues[41–45].Inrecentpublications,thetopicofrecoveryofbiomaterialsandbio-refineryfeedstockfromwastepaperprevailed[46–49]since,consideringthe principlesofcirculareconomy,thisisstrategicformitigatingtheongoingclimatechange.

However,theavailableliteraturelacksthoroughanalysesofthepapermakingability ofspecifictypesofwastepaper,alongwiththeverificationofthepossibilityofusingthem fortheproductionofspecifictypesofpaper.Allofthisismoresignificantsince,inpractice, thequalityandcompositionofwastepaperdeliveredtopapermillsoftendifferfrom expectations,andthespecifictypesofwastepaperdonotmeettherequirementsinthe respectivestandards.Therefore,inthepresentstudy,wedeterminedthepapermaking abilityofawiderangeofwastepaperprocessedinaselectedpapermillandverifiedthe possibilityofusingthemfortheproductionofhigh-qualitysanitarypaper.Theresearchin thearticlecontributesfurthertothemethodsofassessmentofwastepaperforthepaper production.

2.MaterialsandMethods

2.1.Materials

Thefollowingmaterialswereselectedfromthepapermillforresearch:

• Whitewastepaper,includingproductsofbleachedpulps,scrapsofwood-freepaper, littleprinted,noglue,nowaterproofpaper,andnocoloredpaper(ranked3.04accordingtotheEN643‘ListofEuropeanstandardtypesofwastepaper’[50])(seven samples).

• Mixedwastepapercomposedofunsortedwastepaper,formallyclassifiedastheentire spectrumofthesecondtypeofpaper,thatismedium-sizedvarieties.Theseincluded newspapersandprintedofficewastepaper,amongothers(ranked3.19accordingto theEN643‘ListofEuropeanstandardtypeswastepaper’)(fivesamples).

Thewastepaperwascrushedmanually(piecesofapproximately2–5cm)andmixed toensurethatthesamplewasmixedhomogeneously.Thewastepaperpreparedinthisway wasplacedinthedescribedPPfoilbags,whichweresubsequentlystoredinbarrelswith tightcoverstoprotectthesamplesfrommoistureandcontamination.Aftermechanical shredding,thewastepapersampleswerepackedintightcontainersandstoredataconstant temperatureofapproximately15 ◦ C.

2.2.Non-FibrisedSubstancesandChemicalAnalysisofPulp

Todeterminetheamountofnon-fiberizedsubstancesinthetestedwastewater,the rewettedpulpsamples(22.5gdryweightsamplessoakedinwaterfor24h)weresubjected todisintegrationusingalaboratoryJACSHPD28Dpropellerpulpdisintegrator(Danex, Katowice,Poland)with23,000revolutionsfollowingISO5263-1(2004).Non-defibered substancesversusfibersandwaterwereremovedusingamembranescreener(PS-114; Danex,Katowice,Poland)atanamplitudeof25mmandafrequencyof2Hz.Thescreener isequippedwithagapscreen(gapwidth=0.50mm).

Toidentifythedissolvedsubstancesinthetestedwastepapersamples,theassociation betweentheamountofoxygenrequiredfortheoxidationoforganicsubstancesinthetested samplesandthatofsolublesubstanceswasused.TheMERCKCODtestwasappliedto quantifythesesubstances.Organiccompoundswereoxidizedfollowingthemanufacturer’s instructions.Theamountofoxygenconsumedtooxidizethesesubstanceswasdetermined usingaspectrophotometer(UV-1280;Shimadzu,Japan)at620nm.

Analysesofthechemicalcompositionofcellulosicpulpincludedthequantiﬁcationof ash,extractives,holocellulose,andlignin.Ashcontentwasdeterminedusingagravimetric methodincompliancewiththeTappiT211standard(ashinwood,pulp,paper,and paperboard;combustionat525 ◦ C).Theamountsofextractivesweredeterminedaccording totheTappiT204standard(solventextractivesfromwoodandpulp).Holocellulosecontent wasdeterminedaccordingtoTappiUsefulMethod249(celluloseinpulp).Asindicatorsof lignincontentinthepulp,kappanumbersfordriedpulpswereexaminedaccordingtoISO 302(2015).Theaveragepolymerizationdegreeofcellulose(DP)inthepulpwasdetermined usingviscometryfollowingISO5351(2010).Allchemicalanalyseswereperformedin triplicateforeachpulpsample.

2.3.AnalysisofFibreandPulpProperties

Pulpswerecharacterizedintermsoffiberdimension,finecontent,waterretention value(WRV),andfreeness.ThedimensionsoffibersweremeasuredaccordingtoISO 16065-2:2014usingtheMorfiCompactBlackEditionapparatus(Techpap,Grenoble,France). WRVwasdeterminedaccordingtoISO23714:2014.Freenesswasmeasuredusingthe Schopper–Rieglerapparatus(Thwing-AlbertInstrumentCompany,WestBerlin,NJ,USA), accordingtoPN-ENISO5267-1(2002).Allanalyseswereperformedonbothunbeatenand refinedpulpsamples.

2.4.PulpReﬁning

Beforeprocessing,thepulpwassoakedinwaterfor24h.Then,wastewaterwas treatedintheDanexJACSHPD28Dpropellerpulpdisintegrator(Danex,Katowice,Poland) accordingtoPNENISO5263-1(2006)with23,000revolutions.Thereﬁningprocesswas performedintheDanexJACPFID12XPFImill(Danex,Katowice,Poland),withasingle batchofdriedpulp(22.5g),accordingtoPN-ENISO5264-2(2011).Pulpwasreﬁnedto 30 ± 1 ◦ SR.

2.5.PreparationofPaperSheets

Laboratorypapersheetswerepreparedfromunbeatenandreﬁnedpulpsamplesthat hadbeenpreviouslydisintegrated,asdescribedinSection 2.4 Sheetsofpaperwereformed usingtheRapid–KoethenapparatusinaccordancewithPN-ENISO5269-2(2007).Each papersheethadthebasisweightof80g m 2 (ISO536:2012).Onlysheetswithbaseweights between79and81g · m 2 wereusedforfurtherinvestigation.

Thepapersampleswereconditionedat23 ◦ Cand50%relativehumidityaccordingto ISO187:1990foraminimumof24hbeforeexamination.

2.6.AnalysisofPaperProperties

ContaminantsonthetestedpapersurfaceswereassessedusingtheKeyenceVHX6000microscope(Keyence,Belgium)equippedwiththeVH-Z20UTlens(20/200 × magnification).TheOP-72402adapter(ringshape)wasusedforsampleillumination.Image analysis-basedmeasurementsofelementswasperformedusingthemicroscopesoftware. ThesurfacetextureofsampleswastestedunderamicroscopeinaccordancewithISO 25178:2016GeometricalProductSpecifications(GPS).Briefly,basedonthree-dimensional microscopicphotographsofcoatedsurfacepapers,roughnessprofileswereprepared. Specifically,roughnessprofileswereobtainedfromthesurfaceprofilesbyseparatingthe long-wavecomponents(wavinessandshapedeviations)withan λcprofilefilter.The λcprofilefilterdeterminesthetransitionfromroughnesstowaviness,thatis,random orclose-to-periodicinequalities.Thebasicroughnessparameters(SaandSz)describing surfacemicrogeometryandthoserelatedtospecificprofilefeatureswereobtainedfrom theroughnessprofilephotographs.Forroughnessprofiles,Sarepresentsthearithmetic meandeviationoftheroughnessprofilealongthesamplinglength,whilstSzrepresents themaximumroughness(themaximumheightoftheprofileindicatestheabsolutevertical distancebetweenthemaximumprofilepeakheightandmaximumprofilevalleydepth alongthesamplinglength).

Furthermore,theroughnessofpapersurfacewasdeterminedinaccordancewithISO 8791-2:2013withtheTMI58-27BendtsenRoughnessTester(Kontech,Lodz,Poland).Air permeabilitywasdeterminedaccordingtotheISO5636-3:2013withtheTMI58-27Bendtsen RoughnessTester(Kontech,Lodz,Poland).Opticalparametersweredeterminedusingthe X-riteExactspectro-densitometerinaccordancewithISO2470-1:2016.

TheprioritystrengthpropertiesofpapersweredeterminedusingtheZwick005 ProLinetestingmachine(ZwickRoell,Ulm,Germany)coupledwiththetestXpertIIIsoftwareinaccordancewithISO1924-2:2010.Thefollowingtensilepropertiesofpaperwere examined:

• IB :breakinglength[m]

• FB :tensileforceatbreak[N]

• σT b :width-relatedforceatbreak[N m 1 ]

• σT W :forceatbreakindex[Nm g 1 ]

• εT :strainatbreak[%]

• WT b :energyabsorption[J m 2 ]

• WT W :energyabsorptionindex[J · g 1 ]

• Eb :tensilestiffness[N m 1 ]

• Ew :tensilestiffnessindex[Nm g 1 ]

• E*:Young’smodulus[MPa]

Detailedstatisticalanalysiswasperformedonindividualresearchseriestodetermine thefollowingbasicindicators:arithmeticmean,extendeddeviation,andpercentagerelative error.

3.ResultsandDiscussion

Contentsofnon-ﬁberizedsubstancesinwastepaperpulpsamplesaresummarized inTable 1.Whitewastepaper,describedasnumber3.04inaccordancewiththePN-

EN643:2004standard,containsscrapsofwood-freepaperwithlittleprint,noglue,no waterproofpaper,andnocoloredpaper.Cursoryanalysisofwastepaperrevealedthatmost ofthetestedsamplesdidnotmeettheassumedrequirementstoagreaterorlesserextent. Specifically,thiswastepapercontainedcertainamountsofwoodpaperandpapercolored inmass,andmostofthemcontainedasignificantamountofheavilyprintedpaper.Based ontheresultsoftheseanalyses,thedeliveredwastepapertothemillshouldbeformally characterizedundercategory3.02,thatis,mixedwastepaperwithscrapsofprintingand writingpaper,slightlydyedinmass,containingatleast90%ofwood-freepaper,orunder category3.03,thatis,wastepapercontainingbookbindingscrapsofwood-freepaper(itmay contain10%woodpaperatthemost),withlittleprintandgluebutnopapercoloredinmass. Insummary,thetreatmentofwhitewastepaperaspurelytype3or04isasimplification usedinthetradeofwastepaper.Infact,thistypeofpaperexhibitsaspectrumofproperties.

Table1. Non-ﬁberizedsubstancesandchemicalcompositionofwastepaperpulps. Wastepaper

White1.1 2.5519.971.602.6767.6749.67376

White1.2 0.0118.121.471.9563.2649.58339

White1.3 0.0016.710.223.2878.335.34840

White1.4 2.6117.601.131.9866.685.28832

Mixed1.5 1.0523.681.102.4670.7745.50531

White2.1 0.7127.270.751.7273.6845.32660

White2.2 0.7924.601.572.4476.7319.38673

Mixed2.3 0.4837.871.416.7470.5839.62376

Mixed2.4 2.4022.921.243.6076.5239.59412

Mixed2.5 1.3527.481.241.3576.9540.15377

Mixed3.1 1.7125.430.500.5971.3816.25611

White3.2 0.3516.900.410.6579.5913.60643

Theseresultsconﬁrmedthechemicalcompositionoftheexaminedwastepaperpulp, withoutsimpledependenciesonthetypeofwastepaper.Thedegreeofpolymerization affectsthestrengthpropertiesofpaper;thisisduetothefactthatwithadecreasein polymerizationdegree,themechanicalstrengthofcellulosedecreases.Basedontheresults ofouranalyses,despitethehighdegreeofcellulosepolymerization,theselectedwastepaper pulpsdidnotachievethehigheststrengthproperties.Therefore,presenceofimpurities andmanyotherfactorsmayaffectthestrengthofpaperderivedfromrecycledpulp.

Todeterminethecriticalparametersforobtaininghigh-qualityrecycledpulpand ensuringsuitabilityfortheproductionofspecifictypesofpaper,papermakingabilitywas assessed.Thisanalysiscoversawidespectrumoftestsaimedatdeterminingwhethera givenfibrousrawmaterialissuitableforuseinpaperproduction.Papermakingabilityis determinedbasedonmultipleparametersofpulp,includingthechemicalcompositionand propertiesofthefinishedpaper.Theassessmentofpapermakingabilityallowscomprehensivecomparisonofthepropertiesofrecycledpulpsanddeterminationofthetypesof wastepaperthatmaybeuseful.Mostimportantly,thisassessmentidentifiesthetypesthat cannotcertainlybeusedtoobtainproductsofhighutilityvalue.

Toassesspapermakingability,pulpwasrefined,theprimarypurposeofwhichis themaximumdevelopmentofsurfaceareaboundinthepaper.Refiningincreasesthe elasticityofcellulosefibers,therebyimprovingthestrengthandstructural-dimensional propertiesofthefinishedpaper.Simultaneously,however,itcontributestothereduction ofpulpdewatering,whichadverselyaffectstheefficiencyofthepapermakingmachine. Moreover,therefiningprocessishighlyenergyintensive,accountingfornearly50%of theelectricityconsumptionofthepapermill.Previousstudiesandindustrialexperiments haveestablishedthattheoptimumcost,efficiency,andpaperproductpropertiescouldbe

obtainedatalubricityofapproximately30 ◦ SR.Therefore,thepapermakingabilityofthe selectedpulpswasdeterminedafterrefiningfortheSchopper–Rieglerfreenessof30 ± 1 ◦ SR,whichwasexpectedtoensurethemaximumstrengthofpulpsandcontributetoeasy dehydration.Infurtheranalysesofpapermakingability,thepulpsamplesthatdidnot promiseproductswithexcellentfunctionalpropertiesduetotheircharacteristicsatthevery beginningwererejected.Therefore,fromamongstallconsideredwastepapertypes,the pulpsthatwerecharacterizedbyafreenessof>30 ◦ SRintheunrefinedstatewererejected. Inaddition,pulpsamplesinwhichthecontentofextractivesubstancesexceeded1.30% wereexcluded.Owingtotheirhighchemicalreactivityaswellasthesignificantviscosity andadhesionoftheircomponents,extractivesmayhinderthepapermakingprocessinthe formofso-calledresindifficultiesandmaycreateresinstains,visibleasspotsinthefinished products.Inaddition,pulpsampleswithfiberlengthof<900 μmwereexcludedasthey increasestaticstrengthbutdecreasetearresistance.Incontrast,theintermediatefraction lowersthestaticstrengthbutimproveshedynamicstrength.Meanwhile,thelong-fiber fractionimprovesallstrengthpropertiesofthepaper.Alongerfiberlengthincreasesthe tearresistanceandextensibilityofthepaper,whichareparticularlydesirableinthecase ofsanitarypapers.Inthepresentstudy,whitewastepaper1.3,1.4,2.1and3.2andmixed wastepaper1.5mettheabovecriteria.

Consideringthepracticaltechnologicalaspectsofsubsequentprocessesonapaper machine,themostimportantchangesoccurringasaresultoftherefiningprocess,in additiontofromfreeness,includethedevelopmentofswellingdegree(WRV)ofthe recycledpulp.Thegreaterthedegreeofpulpswelling,thegreaterthecompactnessofthe structureoftheobtainedpaper,whichsignificantlyimprovesitspropertiesbutreducesits dynamicstrength.Afterrefining,WRVofthetestedpulpsincreasedby28–61%(Table 2), achievingthehighestvaluesforwhitewastepaper.Mixedwastepaper1.5wasmuchmore keratinizedandachievedamuchlowerdegreeofpulpswelling.Similarly,regardingfiber length,highervalueswererecordedforwhitewastepaper(Table 3),suggestingthatthe paperobtainedfromthesetypeswillachievehigherstrengthparametersattheoutset.To date,thehighestcontentofthefinefractionamongstrefinedpulpshasbeenrecordedfor white2.1andmixed1.5wastepaper(Table 2),whichisconducivetothequalitypaper derivedfromthesepulpsaswellastotheprocesseconomyresultingfromthesavingof bulkadditives.Overall,ouranalysesofunrefinedandrefinedpulpsshowedthatthetype ofwastepaper(impuritycontent),similartochemicalcomposition,doessignificantlyaffect fiberandpulpproperties.

Table2. Characteristicsofwastepaperpulps.

WastepaperFineContentFineContentWRVSchopper–RieglerFreeness [%inArea][%inLength][%][◦ SR]

UnrefinedRefinedUnrefinedRefinedUnrefinedRefinedUnrefinedRefined White1.1 21.05-62.24-130.9-36-

White1.2 23.63-65.94-136.3-56-

White1.3 10.0315.2734.7327.89103.8164.621731

White1.4 12.209.4137.3920.71101.4159.081829

Mixed1.5 14.5515.8749.2041.03103.6134.172229

White2.1 12.3910.9554.9941.93102.3151.862230

White2.2 13.32-47.31-101.3-20-

Mixed2.3 16.63-51.08-91.1-22-

Mixed2.4 13.42-44.98-97.6-22-

Mixed2.5 26.78-66.60-107.2-35-

Mixed3.1 17.63-46.68-103.7-20-

White3.2 10.7118.4734.5332.27105.8185.651630

Resultsofmicroscopicanalysisofimpuritiesonthesurfaceofthetestedpaperare presentedinFigure 1.Signiﬁcantamountsofimpuritieswereobservedonthesurfaceof papersproducedfrommixedwastepaperduetothehighercontentofimpuritiesinthe pulp.

White wastepaper 1.1

Whitewastepaper 1.2

White wastepaper 1.3

White wastepaper 1.4

Mixed wastepaper 1.5

White wastepaper 2.1

White wastepaper 2.2

Mixed wastepaper 2.3

Mixed wastepaper 2.4

Mixed wastepaper 2.5

Mixed wastepaper 3.1

White wastepaper 3.2

Figure1. Microscopicimagesofwastepapersurfaces.

Table3. Fiberproperties.

WastepaperMeanLength-WeightedFibreLengthMacro-FibrillationIndexMeanFibreCoarseness [μm][%][mg m 1 ]

UnrefinedRefinedUnrefinedRefinedUnrefinedRefined

White1.1 994-1.11-0.16-

White1.2 925-1.24-0.15-

White1.3 102110060.510.650.100.10

White1.4 112610380.500.540.090.10

Mixed1.5 9798800.860.650.130.10

White2.1 10609721.220.860.150.13

White2.2 960-0.82-0.11-

Mixed2.3 884-0.86-0.11-

Mixed2.4 825-0.88-0.10-

Mixed2.5 985-1.06-0.15-

Mixed3.1 886-0.48-0.11-

White3.2 102210010.690.750.140.12

Whenprocessingpaper,theroughnessofthematerial’ssurfaceisaveryimportant parametersinceitaffectsmanypropertiesofthematerialaswellasvariousothercharacteristicsoftheproductsuchasappearance,aesthetics,andfunctionalvalue,whichare particularlyimportantinthecaseofsanitarypapers.Hence,basedonthethree-dimensional microscopicanalysisofdifferentpapers,roughnessprofileswereprepared.Specifically, profileswereobtainedbyseparatingthelong-wavecomponentsofthesurfaceprofile (wavinessandshapedeviations)witha λcprofilefilter.The λcprofilefilterdeterminesthe transitionfromroughnesstowaviness,thatis,randomorclose-to-periodicinequalities. Therefore,thebasicroughnessparameters(Sa andSz )describethesurfacemicrogeometry andlinkittospecificprofilefeatures(Table 4).Inroughnessprofiles,Sa representsthe arithmeticmeandeviationoftheroughnessprofilealongthesamplinglength,whilstSz representsthemaximumroughness(themaximumheightoftheprofileindicatestheabsoluteverticaldistancebetweenthemaximumprofilepeakheightandmaximumprofile valleydepthalongthesamplinglength).

Table4. Structuralandopticalpropertiesofpapers.

WastepaperSa Sz

[μm][μm][mL min 1 ][mL/min][%]

UnrefinedRefinedUnrefinedRefinedUnrefinedRefinedUnrefinedRefinedUnrefinedRefined

White1.1 3.15-36.77-991-183-66.38-

White1.2 3.08-32.95-315-169-67.24-

White1.3 3.783.6441.2141.564956215428825285.1891.14

White1.4 4.033.4046.7045.563971191329829983.0580.43

Mixed1.5 3.833.1242.7838.872905150827724764.1165.89

White2.1 3.513.0040.8130.85264314596027478.0776.70

White2.2 3.57-42.87-4407-303-74.58-

Mixed2.3 3.99-39.47-4456-56-68.39-

Mixed2.4 3.98-36.95-3440-59-70.36-

Mixed2.5 3.72-40.74-1458-59-56.75-

Mixed3.1 4.01-48.05-5009-323-70.70-

White3.2 3.742.3745.0632.41401979731918976.4978.44

Microscopicanalysisofpaperroughnessshowednosignificantdifferencesdepending onthetypeofwastepaperusedtoproducepapersheets,despitethedifferencesinthe amountofimpuritiesbetweenwhiteandmixedwastepaper(Table 4).Additionalroughness measurements,performedfollowingtheBendtsenprocedure(Table 4),confirmedthatthe typeofwastepaperuseddidnotalterroughnessparameters.Notably,inmostcases,the processofrefiningsmoothedthesurfaceofthepapersproducedonlytoasmallextent.

Thiswasalsotrueforopticalparameters.Interestingly,comparedtomixedwastepaper, whitewastepaperdidnotexhibithigherwhitenessindices.Theopticalcharacteristicsare presentedinTable 4 andFigures 2 and 3

Anotherimportantpropertythatcontributessignificantlytothefunctionalproperties ofpaperisairpermeability.Moreover,airpermeabilityisanimportantindicatorforthe productionprocesscontrolofsanitarypapers,sinceitreflectstheporosityoftheproduct andthusitsabsorbanceandtensileproperties.Mostofthestudiedpapersproducedfrom unrefinedpulpswerecharacterizedbyhighairpermeability,whichdecreasedsignificantly afterrefining(Table 4).However,whetherthisisdisadvantageousdependsonthepurpose ofapplication.

Thetensilepropertiesofpapersheetsareimportantformanufacturingandprocessing[51,52].Therefore,theeffectsofspecificpulpparametersandimpuritycontentson thestrengthofpaperswereanalyzed,andtheresultsaresummarizedinTables 5 and 6 Regardingstrengthproperties,papersproducedfromunrefinedwhitewastepaper2.1and 3.2achievedthebestresultsintheconductedtests(Table 5).Refiningsignificantlyimproves thepapermakingabilityofwastepaperpulp,increasingthestrengthoftheobtainedpapers. Fromrefinedpulp,thebestresultswererecordedforwhiterecycledpapers1.3and3.2 (Table 6),whichwasexpectedconsideringtheirexcellentpulpandfiberproperties(Tables 2

Figure2. Lightreﬂectioncurveofwhitewastepaper.

Figure3. Lightreﬂectioncurveofmixedwastepaper.

and 3).Hightearresistanceandelasticityrenderthesepapersthemostattractiveforusein theproductionofsanitarypaper.Theloweststrengthwasrecordedfromrecycledpaper1.5, whichmaybeduetothelowestﬁberlengthofitspulp.

Table5. Tensilepropertiesofpaperproducedfromunreﬁnedpulp.

WastepaperIB

White1.1 250029.1195624.61.6320.50.258255,50032132320

White1.2 290034.3231928.41.6625.70.315266,40032622425

White1.3 285033.1222027.71.6924.00.300309,10038592809

White1.4 230026.5182922.61.4918.10.224256,10031572328

Mixed1.5 230026.7180422.51.5919.60.244258,00031592344

White2.1 245028.3192423.8 2.0626.70.331 260,60032242367

White2.2 255029.0201225.01.6823.00.285272,80033912481

Mixed2.3 180020.8141216.61.3010.50.130216,60026781970

Mixed2.4 205023.6159019.91.5317.00.213219,90027562000

Mixed2.5 225026.3176021.91.6619.60.244233,20029002120

Mixed3.1 235028.0189223.31.7521.70.267261,10032072376

White3.2 315036.6248030.5 1.7026.20.329 321,40040312922

Abjectresultsaremarkedwithcolor(thebesttensilepropertiesonthegreen,theworstonthered).

Table6. Tensilepropertiesofpaperproducedfromreﬁnedpulps.

WastepaperIB

White1.3 705077.1533569.03.02103.21.33514,400

Abjectresultsaremarkedwithcolor(thebesttensilepropertiesonthegreen,theworstonthered).

Ofnote,inafewcases,pulpswithsimilarpropertiesintheunreﬁnedstateexhibited markedlydifferentpropertiesinthereﬁnedstate,suchaswastepaper1.4and1.5.Therefore, thoroughestimationofthepapermakingabilityisessential.

4.Conclusions

Analysesconductedinthepresentworkwereaimedatverifyingthespecificproperties ofwastepaperpulponthebasisofwhichtheirsuitabilitytoproduceaproductwith satisfactoryusabilitycharacteristicscanbeinitiallyassessed.However,althoughthepresent workdoesnotallowforidentifyingthepulpthatisreliableintermsofpapermaking, itallowsforrevealingthepulpthatisnotsuitable.Inotherwords,theusefulnessof recycledpulpcanbeassessedathighprobabilitybydeterminingthefreenessandcontent ofextractivesubstances.Basedontheobtaineddata,itwasconductedthat,samples containinglargeamountsofextractives(above1.30%)anddissolvedsubstancescannot yieldproductswithahighaddedvalue.Also,sampleswithveryhighinitialSchopper–Rieglerfreenessshouldnotbeintroducedintothesystem,astheydonotallowforachieving theexpectedresultsandmayinduceanumberoftechnologicaldifficulties.Similarly,pulp sampleswithfiberlengthof<900 μmdonotallowgoodpaperstrength,andespecially satisfactorytearresistance,soessentialtosanitarypapers.Moreover,strengthproperties cannotbedeterminedwithouttherefiningprocess.Overall,comprehensiveevaluation ofthepapermakingabilityofwastepaperisimperative.Undeniably,ourfindingswill behelpfultofurtheranalyzethevalidityofrecycledmaterialsandreducetheimpactof wastepaperontheenvironment.

AuthorContributions: Conceptualization,E.M.andP.P.;methodology,A.L.,M.D.andE.M.;data processing,M.D.andA.L.;literaturereview,M.D.;writing—originaldraftpreparation,E.M.;writing— reviewandediting,E.M.;supervision,P.P.;fundingacquisition,P.P.Allauthorshavereadandagreed tothepublishedversionofthemanuscript.

Funding: ThisresearchwasfundedbytheNationalCenterofResearchandDevelopmentinPoland, grantnumberPOIR.01.01.01-00-0084/17.

InstitutionalReviewBoardStatement: Notapplicable.

InformedConsentStatement: Notapplicable.

DataAvailabilityStatement: Thedatasetsgeneratedduringand/oranalyzedduringthecurrent studyareavailablefromthecorrespondingauthoronreasonablerequest.

ConﬂictsofInterest: Theauthorsdeclarenoconﬂictofinterest.

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Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T E P O P T

Production of wood based panel from recycled wood resource: a literature review

Paper

Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

European Journal of Wood and Wood Products (2023) 81:557–570

https://doi.org/10.1007/s00107-023-01937-4

REVIEW ARTICLE

Production of wood-based panel from recycled wood resource: a literature review

Duy Linh Nguyen1,3 · Jan Luedtke2 · Martin Nopens2 · Andreas Krause2

Received: 17 February 2022 / Accepted: 28 January 2023 / Published online: 17 February 2023 © The Author(s) 2023

Abstract

This article presents and discusses the available studies on utilization of waste wood (WW) resource for wood-based panel production. The cited literature indicated that the majority of WW research was from Europe and conducted mainly on recycled material from particleboard. In addition, particleboard was presented as the first option of wood-based panel product manufactured from waste wood. There was a lack of research on the recycling of plywood. Physical and chemical contaminants fluctuated strongly between low- and high-quality recycled wood mixes depending on their origins. Findings from studies also noticed that wood-based panels (e.g., particleboard) could be produced from 100% WW. However, the physical and mechanical properties of wood-based panel drop with the high proportion of WW content due to the decrease in slenderness ratio and increase in contaminants. Moreover, formaldehyde emission content of particleboard and Oriented Strand Board (OSB) manufactured from WW particles increases when the WW percentage increases. Contrary, the formaldehyde amount decreases with the increase in recycled fiber content in fiberboards. Notably, the properties and emission of recycled wood composite products could be improved by applying high-tech sorting technologies, appropriate chipping techniques, pretreatment steps and formaldehyde-free binders during waste wood handling and production process.

1 Introduction

Post-consumer waste wood is a valuable feedstock for energetic and material sector. Its volume has been increased together with the rapid urbanization and industrialization. Based on Eurostat data in 2014, Europe generates annually about 60 million tons of waste wood collected from diﬀerent sectors. Germany is the country in Europe collecting the highest number of waste wood per year, accounting for about 6.6 million tons in 2016 (Purkus et al. 2019). Italy, UK and France generate roughly 4 million tons per year, whereas Belgium, Austria, Spain and Poland produce around 2 million tons in 2014 (Silvio 2018). In addition, Sweden, Norway and Denmark collect nearly 1.0 million tons per

* Duy Linh Nguyen linh.nguyen@uni-hamburg.de

1 Institute of Wood Science, University of Hamburg, Leuschnerstrasse 91C, 21031 Hamburg, Germany

2 Thuenen Institute of Wood Research, Leuschnerstrasse 91C, 21031 Hamburg, Germany

3 Faculty of Forestry, Nong Lam University of Ho Chi Minh City, Hamlet 6, Linh Trung Ward, Thu Duc District, 70000 Ho Chi Minh City, Vietnam

year (Sekundaerrohstoﬀe 2018). According to the United State Environmental Protection Agency (EPA), USA generated about 18.1 million short tons of waste wood in 2018 collected from municipal solid waste streams.

Waste wood originates from various resources. Therefore, it is not a homogeneous material due to its complexity of wood types, applications and sources (Bergeron 2014). In addition, waste wood is also considered as a highly sophisticated material in terms of chemical and physical composition (Edo et al. 2016). Various physical and chemical contaminants exist in the waste wood resources causing problems for recycling processes and inﬂuencing the properties of recycled products. Nowadays, mechanical processes (e.g. sieving, magnet or eddy current) can sort physical contaminants in waste wood such as plastic, metal, textile, etc. However, chemical contaminants that are coming from substances of wood preservatives, paints, glues, etc. are not easy to eliminate from waste wood mechanically. Thus, the management of these contaminants from inputs plays an important role in cascading and re-using this valuable resource eﬀectively (Besserer et al. 2021). From this point of view, management using waste wood ordinances has been established and used in many countries. However, there is a lack of uniform waste wood ordinance among countries

nowadays. Germany, Austria and Switzerland apply the same ordinance divided into four categories depending on the characteristics of the waste wood, namely AI, AII, AIII and AIV, whereas France, Belgium, Netherlands and Luxemburg classified their waste wood categories with A, B, C and D (Jan 2019). UK, Sweden, Estonia and Spain have their own waste wood ordinances. In addition, many countries have not established waste wood ordinance. In this sense, it is difficult to trade different collected waste wood assortments between European countries.

Despite the diﬀerence in waste wood management, the recycling rate of waste wood varies from country to country in material and energy uses. For instance, in European countries, for decades, energy utilization of waste wood exceeds material utilization, accounting for 60–95%. Sweden, Switzerland, Norway, Netherlands, and Finland are the top leading European countries, which share high waste wood portion for energy purpose, ranging from 85 to 95% (BAV e.V. 2021). In the sector of material use, Italy ranks in ﬁrst position among European countries with a 42%, share of waste wood in panel production, followed by Austria with 33% (Silvio 2018).

As resources, fossil and renewables are limited in their availability, though for different reasons, the European Green Deal laid the political basis for a shift from linear to circular economy. Products entering the cycle must be designed in a way that supports circular utilization (Fig. 1). However, repeated utilization of resources through recycling requires a thorough cleaning process to prevent contaminants that may have entered the resources cycle decades ago from being carried over and accumulated. As the use of natural resources gains massive interest, their efficient use together with consumer protection is of high concern. Many authors have therefore been working on the characteristics of different waste wood resources and their utilizations. Some focused on the origins and contaminants of waste wood materials (Tables 1, 2). The others concentrated on the application

of waste wood to the production of wood-based panel as well as its effects on the physical and mechanical properties and formaldehyde emission of final products (Tables 3, 4). Due to the variety of materials and the effects of possible contaminations, this paper presents an overview of the conducted waste wood research with the goal to answer the following questions:

1. Where do the waste wood materials come from and what are the target composites?

2. What are the challenges that recyclers are facing during the conversion of waste wood?

3. What is the concentration of recycled wood material in the new composite products and its consequences?

2 Methods

The stated questions were addressed by collecting peerreviewed articles, proceedings of conferences and reports of research projects relating to waste wood material and its utilization from the following scientiﬁc websites:

• ScienceDirect (https://www.sciencedirect.com/)

• Google Scholar (https://scholar.google.com/)

• WorldCat (https://www.worldcat.org/)

• SpringerLink (https://link.springer.com/)

• Taylor&Francis Online (https://www.tandfonline.com/)

• ACS Publications (https://pubs.acs.org/)

• Web of Science (https://www.webofscience.com)

The following keywords were used:

• Waste wood contaminants

• Waste wood composites

• Recycled wood, formaldehyde emission

• Secondary wood resources

• Wood residues utilization

The term waste wood mentioned in the searched articles is restricted to used/secondary or recycled wood. Research articles, dealing with by-products from sawmills or the like are not included in this article.

Fig. 1 Existing resources utilization pathways (solid

and approaches

The collected publications for the review article were analyzed and categorized dealing with the research questions focusing on the recycling of waste wood materials from wood-based panel products to produce wood-based panel only. Therefore, wood plastic composite-based publications and the research papers that focused on waste wood materials from solid wood with and without CCA treated will not be discussed and shown up in the Supplementary information.

3 Results and discussion

3.1 Waste wood origin and target composite type

3.1.1 Waste wood origin

Table 1 includes twenty-eight research articles classified into two main categories namely origin and target composite type presenting the origin of waste wood materials conducted in the last twenty years. About half of the studies in Table 1 was conducted from 2015 to 2019. The results indicated that a large proportion of waste wood stream originates mainly from recycling center companies or combustion power plants. In this waste stream, construction and demolition (10 references) and furniture (9 references) dominate the origin uses of waste wood,

followed by packaging (6 references), whereas municipal counts for only one interest (Lesar et al. 2018). This finding is also consistent with the research of Mantau and Doering (2018) and Van Benthem et al. (2007) about waste wood streams in Europe.

In the sector of waste wood type, most studies dealt with/focused on wood-based panel (17 references). It is surprising that particleboard was found more than fiberboard, plywood and OSB in waste wood type of woodbased panel industry. According to FAO (2018), plywood production accounted for the largest volume of woodbased panel globally, followed by fiberboard, particleboard and OSB. Therefore, it was expected that more secondary/recycled plywood would be found in the waste wood type rather than particleboard and fiberboard. However, the finding in this research is controversial to the

References Origin

WW type Original use

Contaminant analysis

PBFBOSBPlywoodSolid woodC & DMunicipalFurniturePackagingPhysicalChemical

Schild et al. (2019)

Faraca et al. (2019)xxxxxxxxx

Azambuja et al. (2018a)xxxxx

Azambuja et al. (2018b)xxxxx

Hameed et al. (2018a)xx

Laskowska and Maminski (2018)x

Hong et al. (2018)x

Lesar et al. (2018)xxxxxxxxxxx

Robey et al. (2018)xx

Hameed et al. (2018b)xx

Zamarian et al. (2017)xxxxx

Edo et al. (2016)xxxx

Roﬀael et al. (2016)x

Andrade et al. (2015)xxx

Costa et al. (2014)

Martins et al. (2007)xx

Nagalli et al. (2013)xxxx

Lykidis and Grigoriou (2011)x

Mirski and Dorota (2011a)xx

Mirski and Dorota (2011b)x

Suﬃan et al. (2010)x

Lykidis and Grigoriou (2008)x

Yang et al. (2007)x

Wang et al. (2007)x

Mantanis et al. (2004)x

Jermer et al. (2001) xx

Tolaymat et al. (2000)xx

Krzysik et al. (1997)x

Table 1 Origin of waste wood materials

Table 1 (continued)

ReferencesOrigin

Target composite type and research topic

Country Wood based panel Composition Strength properties FE PBFBOSBPure WWMix

Schild et al. (2019)Canadaxxx

Faraca et al. (2019)Denmark

Azambuja et al. (2018a)Brazilxxx

Azambuja et al. (2018b)Brazilxxx

Hameed et al. (2018a)Swedenxxx

Laskowska and Maminski (2018)Polandxxx

Hong et al. (2018)Koreaxxxx

Lesar et al. (2018)Germany, UK, Finland, Slovenia

Robey et al. (2018)USA

Hameed et al. (2018b)Swedenxxx

Zamarian et al. (2017)Brazilxxx

Edo et al. (2016)Sweden

Roﬀael et al. (2016)Germanyxxxx

Andrade et al. (2015)Portugalxxx

Costa et al. (2014)Portugalxxxx

Martins et al. (2007)Portugalxxxx

Nagalli et al. (2013)Brazil

Lykidis and Grigoriou (2011)Greecexxxx Mirski and Dorota (2011a)Polandxxxx

Mirski and Dorota (2011b)Polandxxx

Suﬃan et al. (2010)UKxxx

Lykidis and Grigoriou (2008)Greecexxxx

Yang et al. (2007)Taiwanxxx

Wang et al. (2007)Taiwanxxxx

Mantanis et al. (2004)Portugalxxx

Jermer et al. (2001)Sweden, German, Netherlands

Tolaymat et al. (2000)USA

Krzysik et al. (1997)Polandxxx

FAO statistic about the production volume and recycling amount of plywood. On the other hand, more plywood and OSB were expected in waste wood research rather than particleboard and fiberboard as the waste wood stream came mostly from construction and demolition. Moreover, it could be that plywood and OSB are often used for exterior applications or as formwork. Therefore, they could be contaminated by preservatives and are usually not suitable for material utilization anymore.

The majority of the found research was conducted based on the waste wood materials collected in Europe (18 references) and South America (4 references). Only six articles were found in other countries such as Canada (Schild et al. 2019), Korea (Hong et al. 2018), USA (Robey et al. 2018; Tolaymat et al. 2000), Taiwan (Yang et al. 2007; Wang et al. 2007). This shows that Europe is more concerned with the waste wood recycling topic than other continents due

to established recycling programs, policies and regulations (e.g., European Union Commission Decision 2009/894/EC; Zero Waste Europe 2014).

3.1.2 Target composite type

Relating to the target composite type, the possibilities of using different waste wood percentages to produce woodbased panels were investigated. Particleboard (14 references) is by far the most favorable product to be made from waste wood materials. The number of fiberboards (Hong et al. 2018; Roffael et al. 2016; Mantanis et al. 2004; Krzysik et al. 1997) and OSB (Schild et al. 2019; Mirski and Dorota 2011a, b) articles together account for seven references. There was no research found using waste wood materials for the production of plywood. The reason could be the impossibility of processing waste wood materials into veneers

European Journal of Wood and Wood Products (2023) 81:557–570

for plywood production. This can be done only from wood logs. On the other hand, there are more advantages in low cost, simple treatment process (mechanical e.g., chipping instead of chemical methods), and less technical barriers for waste wood during the conversion of wood-based panel (particleboard, fiberboard, OSB, plywood) into particles for particleboard production compared to conversion of old fiberboard into fiber or old OSB and plywood into strands. In general, recycled plywood can be processed into strands for the production of OSB when they are well collected and sorted. However, waste wood streams are normally a mixture of wood-based panel products together. The difficulty in the conversion process of inhomogeneous waste wood types into proper strand size and shapes hinders the usage of this resource in the three-layer OSB panel production compared to virgin wood.

3.2 Challenges in waste wood conversion and recycled composites products

3.2.1 Contaminants in waste wood and sorting technologies

3.2.1.1 Contaminants in waste wood Material flows during recycling must ensure that the products contain only nonhazardous contamination levels. The type and threshold of contaminations described in the national legal framework determines whether waste wood may be reused for material purposes or can only be thermally utilized. To use the waste wood efficiently by sorting out as little uncontaminated wood as possible is one of the most challenging steps during recycling. Physical contaminants or material impurities in WW are usually plastic, metal, glass, textiles, concrete or stone (Edo et al. 2015; Vaermeforsk 2012; Krook et al. 2006) that may originate from different material sources depending on the end-use of wood products or the waste wood collection plant/process. Chemical contaminants on the other hand may come from wood treatments, which were applied in order to improve wood products appearance (e.g., coating pigments, paints, oils), strengthen properties (e.g., gluing agents), prevent biological decay (e.g., wood preservatives) or fire resistance (e.g., flame-retardants) indicated by Johan et al. (2007).

Table 2 shows the research conducted in Europe and America on the analysis of physical and chemical contamination in waste wood. The waste wood materials were collected from different sources such as combustion plant, construction site and recycling companies. Physical and chemical impurities of waste wood fluctuate considerably from the findings. It can be seen in Table 2 that physical contaminants were found from 1 to 3% basic dry weight of total material content (Faraca et al. 2019; Lesar et al. 2018; Edo et al. 2016; Jermer et al. 2001). A higher proportion of

non-wooden material was found in low quality mixed recycled wood (e.g., hazardous waste wood) rather than in high quality material (e.g., clean/non-hazardous waste wood) (Lesar et al. 2018). These ﬂuctuations might be relevant to types, sources, fractions and seasons in year, collection and sorting process as well as management of waste wood at recycling facilities. Lesar et al. (2018) also indicated that companies with sophisticated sorting systems showed low content of non-wooden compounds in their waste wood materials. Nowadays, manual sorting, size, sink-ﬂoat, gravity, magnetism, surface tension, and electric conductivity are the most popular sorting methods, which can help to sort out up to 96% of physical impurities in waste materials (Lahtela and Kaerki 2018).

The chemical elements in waste wood originate from various substances accumulated from preservatives, adhesives, pigments, paints, coatings or lacquers. Wood preservatives (e.g., Chromated Copper Arsenate CCA) can be found in many waste wood samples of the collected articles. The amount of Cr, Cu and As varies widely depending on various origins of incoming recycled wood. For example, the waste wood materials from Europe (Faraca et al. 2019; Lesar et al. 2018; Edo et al. 2016; Jermer et al. 2001) tend to contain less CCA than the ones from America (Robey et al. 2018; Tolaymat et al. 2000). It can be explained by the fact that the CCA has been banned in Europe since 2006 as wood preservative (EU Directive 2006/139/CE), whereas it is still allowed in USA. Therefore, these CCA values are lower than in USA. Moreover, Jermer et al. (2001) also found that the amount of arsenic, copper and chromium in German waste wood are lower than in Sweden. This may be a result of the German wood waste ordinance which limits strictly those chemical impurities at lower values compared to Sweden [e.g., As and Cd (2 mg/kg), Cu (20 mg/kg), Cr and Pb (30 mg/kg), Hg (0.4 mg/kg), Cl (600 mg/kg)].

In addition, the amount of Pb, Hg, Cd and Cl varied significantly depending on waste wood sources. Pb was found at high level (up to 2900 ppm) in waste wood from Sweden (Edo et al. 2016) whereas Cl was found (up to 1191 ppm) in waste wood mix of Sweden, Germany and Netherlands (Jermer et al. 2001). The reasons for these phenomena could be due to the difference in waste wood quality among countries depending on company size, collecting seasons and deliveries of waste wood. These elements are commonly used in pigments, paints, coatings, lacquers for wood floor and furniture treatment and were found more in Swedish waste wood (Fjelsted and Christensen 2007; Jermer et al. 2001). Furthermore, Pb and Cd also originated from heat stabilizers in PVC products (Mesch 2010; Krook et al. 2004). Another reason could be due to the waste wood fraction variations. Faraca et al. (2019) proved that the fractions of waste wood affected the amount of contaminants. For instance, the amount of Cl and Pb in waste wood increases

Table 2 Contaminants

in waste wood

ReferencesCountrySource

Contaminants analysis

Physical/material contaminants

%wt. dry basic of total material content (1)

Variation in (1)

Stone (%)Plastic (%)Metal (%)Textile (%)Other (%)

Faraca et al. (2019)DenmarkRecycling center1–21–892–990–1

Lesar et al. (2018)Germany, Slovenian, Finish, UK Recycling companies1–2.96

Robey et al. (2018)USARecycling Facilities

Edo et al. (2016)SwedenCombustion power plant 1.119–4414–2514–22

Nagalli et al. (2013)BrazilConstruction site28.8–75.748.3–69.22.9–11.1

Jermer et al. (2001)Sweden, Germany, The Netherlands

Combustion plant< 1

Tolaymat et al. (2000)USARecycling facilities

References Contaminants analysis

Chemicals/trace element

mg/kg dry wood (ppm) CrCuAsPbHgCdClPCPPCBPAH

Faraca et al. (2019)0.5–1501–5000.03–7.00.1–1200.01–0.510–5–1.010–3–10–1 10–5–10

Lesar et al. (2018)3–591–251–11697–802

Robey et al. (2018)7.0–94.63.7–3482.0–150

Edo et al. (2016)1.5–3133.6–32000.10–2701.80–29000.5–10.5–10.07–0.13

Nagalli et al. (2013)

Jermer et al. (2001)9–730–641–410–1530.06–0.520.12–1.2291–1191

Tolaymat et al. (2000)10–29,00039–1600

when the waste wood particles, which are lower than 4 mm (fine fraction) increases (Edo et al. 2016; Vaermeforsk 2012; Jermer et al. 2001). This is probably due to the crushing and chipping process resulting in more surface coating materials removed from the waste wood surface increasing the amount of heavy metal and Cl in the fine fraction after sieving. There are limited studies conducted on finding organic compounds of waste wood in the recycling process. Faraca et al. (2019) was the only reference found in analyzing PCP, PCB and PAH of waste wood materials. The finding showed that those substances are mainly coming from old furniture. In the past, PCB was used as plasticizers in the coating ingredients of paints and flame retardant (Butera et al. 2014; Jartun et al. 2009a, b). Nowadays, these substances are slowly replaced by other ingredients in paints and coating recipes applied to wood surface treatment. Therefore, high quality waste wood was found containing less of these components and complies with European standards for organic pollutants.

3.2.1.2 Sorting technologies Different sorting technologies have been developed to detect and eliminate chemical contaminants in waste wood particles such as atomic absorption spectroscopy (CV, GF, or HG-AAS), inductively coupled plasma spectrometry (ICP-OES, ICP-MS), energy dispersive X-ray fluorescence (ED-XRF) and near infrared (NIR) spectroscopy (Mauruschat et al. 2016; Fellin et al. 2014, 2011; Hasan et al. 2011a, b; Williams 1976). It is noticed that atomic absorption spectroscopy and inductively coupled plasma spectrometry are the methods used to detect the chemical contaminants of waste wood or biomass in the laboratory indicated by EU Commission decision 2009/894/EC. Other sorting techniques such as energy dispersive X-ray fluorescence (ED-XRF) and near infrared (NIR) spectroscopy have recently shown advantages in the sorting process of waste wood due to fast detection and high sorting efficiency. Plastics and wood preservatives can be detected and sorted by these techniques easily. For example,

European Journal of Wood and Wood Products (2023) 81:557–570

Hasan et al. (2011a, b) stated that the application of EDXRF in online sorting could eliminate 92–96% of wood preservatives (CCA) and alkaline copper quaternary (ACQ) in recycled wood at recycling plants. This method also showed high efficiencies with certain limitations for the elemental analysis of six different groups (origin, type, material, visually detected pollution, pollutant macro category and pollutant specification) from wood residues in wood recycling plants (Fellin et al. 2014). On the other hand, Fellin et al. (2011) stated the positive results on the application of infrared spectroscopy for the detection of pollutants in wood residues. Moreover, Mauruschat et al. (2016) indicated that near infrared (NIR) spectroscopy and automatically pneumatic nozzles can distinguish four types of plastic granulate in WPC. In addition, this study also showed the possibility to distinguish between untreated and treated wood at different moisture contents containing inorganic and organic preservatives. However, these technologies need more investigations/improvements prior to being used on an industrial scale besides focusing on the development of new sorting techniques.

Principally, the detection and sorting of most contaminants in waste wood could be conducted by appropriate methods. However, the waste wood after sorting and processing could contain certain contaminants. Depending on types and contents, those physical and chemical contaminants in waste wood resources will be classiﬁed whether they are problematic for the later wood-based panel products.

3.2.2 Properties of wood-based panel produced from waste wood

Table 3 presents information about the properties of woodbased panels made from recycled wood. In this part, some factors inﬂuencing physical and mechanical properties of particleboard, ﬁberboard and OSB produced from recycled wood such as treatment process of waste wood (e.g., hydrothermal process), waste wood mixing ratio, and adhesives type will be addressed.

3.2.2.1 Particleboard Different investigations based on hydrothermal treatment processes were conducted at various schedules to separate waste wood into particles and use them for the production of particleboard (Andrade et al. 2015; Lykidis and Grigoriou 2011, 2008). The findings indicated that the particleboards manufactured from treated waste wood particles show stable dimensions. However, the mechanical properties (MOE, MOR and IB) and physical properties (thickness swelling and water absorption) of the panel decrease when the temperature increases. This finding is also consistent with the results of Michanickl (1996a) and Boehme and Michanickl (1998). It can be explained by the degradation of holocellulose and lignin in recycled wood

resulting in reduction of the mechanical properties of boards due to the temperature increase (Yilgor et al. 2001). Moreover, these ﬁndings correspond to the research of Goldstein (1973) that the treatment temperature range should be between 110 and 170°C for recycled particles of particleboards. Additionally, Lykidis and Grigoriou (2011, 2008) concluded that the panel made from hydrothermally treated waste wood particles at around 150 °C shows better quality than others.

The effects of different waste wood ratio on the physical and mechanical properties of particleboard were investigated by Azambuja et al. (2018a, b), Laskowska and Mamiński (2018), Zamarian et al. (2017), Martins et al. (2007) and Suffian et al. (2010). The research results stated that it is possible to use 100% recycled wood particles in the production of panel products with UF as binder. In general, the higher the wood mix ratio applied, the lower the mechanical properties (MOE, MOR and IB) and the higher the hygroscopic properties (thickness swelling and water absorption) of panel achieved. The reasons could be due to the decrease in slenderness ratio of waste wood particles formed during the chipping process, which leads to the limitation of contact area between the particles (Arabi et al. 2011). In addition, the physical contaminants from surface coating materials (e.g., polypropylene, polyethylene, polyvinylchloride) of different waste wood-mix resources ( e.g., construction and demolition, furniture, packaging) cause negative effects on glue bonding of panel production, resulting in the decline of strength properties of boards. Moreover, Czarnecki et al. (2003) confirmed that waste wood particles containing PF resin could hinder UF curing due to its alkaline character resulting in reducing MOE, MOR and IB of recycled particleboard. On the other hand, Azambuja et al. (2018b) proved that the strength properties of produced particleboard containing up to 50% of waste wood mix are comparable to the one made from fresh wood particles and their values meet the standard of panel type P2. In general, the properties of the particleboards can be controlled based on the waste wood ratio in wood mixture.

Adhesive types also strongly affect the properties of particleboards. Several investigations focused on PF, TF and PMDI adhesives instead of UF for the improvement of particleboard properties using 100% waste wood particles (Hameed et al. 2018a; Laskowska and Mamiński 2018; Yang et al. 2007; Wang et al. 2007). The key findings illustrated that the MOE, MOR, and IB increase and TS and WA decrease when the percentages of those glues in waste wood mixture increase. This may be due to the differences in bonding properties (e.g., impregnation/absorbing ability only on surface or deeply inside middle lamella of wood) of UF, PF, TF and PMDI adhesives with wood particles/ fiber during the curing process affecting the physical and mechanical properties of produced panels. Better bonding

Properties of wood-based panels made from waste wood

Table 3

ParticleboardAzambuja et al. ( 2018a ) xxUF25; 1007500.49–1.304.6–7.20.18–0.7615.2–26.744.4–79.4

Azambuja et al. ( 2018b ) xxUF25; 507500.70–1.493.7–8.40.48–0.9214.2–27.943.2–81.3

Hameed et al. ( 2018a ) xUF, TF, PMDI1006402.10–2.1210.1–11.00.40–0.428.9–16.226.9–51.5

xUF, PF20; 40; 60; 80; 1006500.10–2.01.5–17.00.01–0.552.0–13.0

Laskowska and Mamiński ( 2018 )

xxUF10; 25; 50; 75; 1007001.45–1.9610.2–13.10.60–0.9611.6–16.034.8–42.7

Zamarian et al. ( 2017 )

Andrade et al. ( 2015 )xxUF0; 25; 50; 75; 1006000.61–0.844.4–9.00.37–0.8930.592.5

Martins et al. ( 2007 )xUF50; 70; 1006120.52–1.003.4–8.90.16–0.42

Lykidis and Grigoriou ( 2011 ) xUF1006801.78–2.699.2–14.20.38–0.5419.4–24.079.8–86.3

Suﬃan et al. ( 2010 )xUF1006502.5613.80.6135.763.6

xUF1006502.14–2.589.5–17.20.18–0.9426.2–59.179.0–119.6

Lykidis and Grigoriou ( 2008 )

Yang et al. ( 2007 )xPF100700; 8001.73–5.3311.1–29.00.12–0.422.0–11.0

Wang et al. ( 2007 )xPF, PMDI1008002.08–3.4511.4–27.90.56–0.737.0–18.1

FiberboardHong et al. ( 2018 )xUF10; 20; 307001.60–2.3010.0–18.00.08–0.2218.2–53.023.2–92.8

Roﬀael et al. ( 2016 )xUF, PMDI0; 33; 67; 1007300.40–0.5414.7–19.649.7–75.3

Mantanis et al. ( 2004 ) xUF2575032.4–37.80.60–1.027.0–8.2

Krzysik et al. ( 1997 )xPF7010003.38–4.1811.7–37.70.41–0.597.1–12.513.2–25.5

OSB Schild et al. ( 2019 )xPF0; 25; 50; 1006008.10–12.6528.0–33.50.38–0.5928.5–39.070.0–77.0

Mirski and Dorota ( 2011a ) xMUPF0; 25; 50; 75; 1006001.55–6.759.9–36.90.32–0.6426.9–33.6

Mirski and Dorota ( 2011b ) xPMDI0; 25; 50; 75; 1006001.40–7.1510.2–44.10.49–0.8821.6–32.9

OSB Oriented Strand Board, WW waste wood, WBP wood-based panel, UF urea formaldehyde, TF tannin formaldehyde, PF phenol formaldehyde, PMDI polymeric methylene diphenyl isocyanate, MUPF melamine urea phenol formaldehyde, MOE modulus of elasticity, MOR modulus of rupture, IB internal bond, TS thickness swelling, WA water absorption Target compositeReferences WW typeAdhesivesWW

European Journal of Wood and Wood Products (2023) 81:557–570

ability will result in better board properties. Laskowska and Maminski (2018) and Yang et al. (2007) stated that boards made from PF showed better properties than UF boards, whereas Wang et al. (2007) indicated that panels produced from PMDI showed higher properties than PF ones. Furthermore, Hameed et al. (2018a) demonstrated that the combination of TF and PMDI at the ratio of 30%:70% and 40%:60% in particleboard manufactured from waste wood material complied with the standard values of type P2 strength.

3.2.2.2 Fiberboard In the sector of fiberboard produced from the mixture of waste wood fiber and virgin wood, Hong et al. (2018), Roffael et al. (2016), Mantanis et al. (2004) and Krzysik et al. (1997) found that the strength properties (MOE, MOR and IB) of the panel decrease tendentially with the increase in waste wood fiber content. It could be explained by the fact that the handling process (e.g., hammering, cooking, refining) of recycled fiberboard into fiber resulted in shortening the fiber length of recycled fiber leading to the reduction in mechanical properties. There was a controversial finding in hygroscopic properties of investigated fiberboard. Hong et al. (2018) and Krzysik et al. (1997) found that TS and WA of investigated fiberboard increased with the higher proportion of recycled fiber content, whereas Roffael et al. (2016) and Mantanis et al. (2004) stated that TS and WA were improved and decreased when more recycled fibers are used. However, the difference could be explained by the fact that the adhesive content in recycled fiberboard contributes to the increase in TS and WA. Moreover, the interaction of cross-linking of the lignocellulose fibers with existing UF-pre-polymers in UF resin could be a reason for this effect (Andrews et al. 1985). Another possibility may be the effects of contaminants from adhesives, coating layers or surface laminate types (e.g., polyethylene terephthalate) in recycled fiberboard. Therefore, the findings indicated that it is only feasible to substitute 20% (Hong et al. 2018) to 25% (Mantanis et al. 2004) of recycled fiber in the UF wood mixture to produce fiberboards, reaching mechanical and physical properties comparable to virgin wood fibers.

At industrial scale, fiberboard recycling is facing a major problem of effectively collecting, sorting and disintegrating the wood fibers. Recycled fiberboards from off-cuts, machining errors, and transport and storage losses contain different types of wood adhesives and coating surface materials. These cause difficulties in applying appropriate technologies (e.g. mechanical, thermo-hydrolytic and chemical) to disintegrating fiberboard waste wood into reclaimed fiber completely in a single step used for fiberboard production. Therefore, the combination of mechanical, thermal and chemical technologies is requested. However, this combination will lead to the quality reduction in recovered fibers (Irle et al. 2019; Buschalsky and Mai 2021).

In addition, most collected waste wood resources at recycling centers or companies are mixtures of different wood types such as particleboard, fiberboard, plywood and OSB. Fiberboard accounts for about 5–15% of the amount of these waste wood resources and normally is not easy to separate from the mixture by traditional sorting methods (Fechter 2021). This amount will generate challenges (e.g. dust during chipping process of waste wood into particles and higher consumption of adhesives at gluing stage) when using it for the manufacture of industrial particleboards. For the time being, sorting technologies are developed that can sort out most of the fines from the waste wood mixture. Great effort is made to increase the recovery of MDF by different technologies besides improved sorting such as steaming at high pressure, ohmic heating or microrelease.

3.2.2.3 OSB For the production of OSB from waste wood, Mirski and Dorota (2011a, b) stated that 75% of recycled wood particles could replace virgin ones in the core layer of OSB with MUPF and PMDI, complying with the mechanical and physical property values of standard EN 300. On the other hand, Schild et al. (2019) indicated that the substitution up to 100% of unsorted waste wood particles in the core layer of OSB with PF is possible and the MOE, MOR and IB of the boards comply with standard requirements, except for TS and WA. However, MOE and MOR decrease when the waste wood content increases, whereas IB, thickness swelling and water absorption increase with the increase in waste wood proportions. These eﬀects could be due to the inhomogeneous distribution of strands and particles and particles contaminants (e.g., wood preservatives, resonated waste wood particles, paints) in the face and core layers of OSB resulting in high-density variations in core layer and the whole board.

3.2.3 Formaldehyde emission

Table 4 shows the summarized data of studies conducted on formaldehyde emission of wood-based panels (particleboard, OSB, ﬁberboard) produced with various waste wood ratio and adhesives types.

Tendentially, the amount of formaldehyde emission of particleboard and OSB produced from waste wood particles increases with higher proportion of waste wood mixture. Martins et al. (2007) indicated that particleboards produced from higher waste wood ratio (from 50 to 100%) and same UF content showed higher formaldehyde emission. Mirski and Dorota (2011a) found the same tendency in the production of OSB from recycled wood. The reason for this is probably due to the former concentration of formaldehyde included in the glue of recycled wood. In contrast, Hong et al. (2018) and Roﬀael et al. (2016) found that the amount of formaldehyde emission of ﬁberboards made from waste

Adhesives (%)WW ratio (%)Density (kg/m 3 ) Formaldehyde emission

Desiccator (mg/L)

Flask (mg/1000 g o.d) (EN 717–3)

Perforator (mg/1000 g o.d) (EN 120)

References WW type

PBFBUnknownType%Chamber (ppm or mg/m 3 air) (EN 717–1)

Costa et al. ( 2014 ) xUF71006502.90–8.50

Martins et al. ( 2007 ) xUF750; 70; 1006122.20–6.96

Lykidis and Grigoriou ( 2011 ) xUF8;121006803.68–14.40

Lykidis and Grigoriou ( 2008 ) xUF71006501.61–10.26

Wang et al. ( 2007 ) xPF PMDI 6 4 1008000.03–0.89

Table 4

Target composite

ParticleboardHameed et al. ( 2018b ) xUF, TF, PMDI1006400.06–0.593.10–13.305.30–145.4

OSBMirski and Dorota ( 2011a ) xMUPF50; 25; 50; 75; 100 6004.87–6.21

Hong et al. ( 2018 ) xUF1210; 20; 307000.80–1.50

Roﬀael et al. ( 2016 ) xUF PMDI 10 0.5;1.0 0; 33; 67; 1007301.90–11.802.70–122.0

Formaldehyde emission of wood-based panel products produced from waste wood UF urea formaldehyde, PF phenol formaldehyde, TF tannin formaldehyde, PMDI

Fiberboard

European Journal of Wood and Wood Products (2023) 81:557–570

wood fiber decreases with the increase in recycled fiber content. This formaldehyde reduction could be related to the amount of urea pre-polymers, urea and ammonia generated during the degradation of amino-plastic resin in recycled fiberboard reacting with formaldehyde as formaldehyde scavengers. Another explanation might be the release of melamine during hot-pressing acting as formaldehyde scavengers (Sugita et al. 1990; Martin et al. 1992). Furthermore, the findings of Costa et al. (2014), Martins et al. (2007) and Mirski, and Dorota (2011a) indicate that formaldehyde emission of particleboards and OSB (middle layer) produced from 100% recycled wood meets the standard values of EN 120 (< 8.0 mg/100 g o.d).

The type of adhesives affects the formaldehyde emission strongly. According to Hameed et al. (2018b), the particleboard produced from UF showed higher amount of formaldehyde content than TF/PMDI. Moreover, the amount of formaldehyde content reduced notably when the ratio of TF/PMDI increased. Wang et al. (2008) added that formaldehyde release of particleboards made from waste wood decreases linearly when the PMDI/PF ratio increases. The same tendency was found in the study of Roffael et al. (2016) with UF and PMDI for fiberboard production from secondary wood fibers.

On the other hand, the hydrothermal process of waste wood particles contributed to reduce the amount of formaldehyde emission. Moreover, the formaldehyde content of thermally treated wood particles is comparable or almost the same as the formaldehyde content of the virgin ones. Waste wood particles treated at 150 °C with 30% water retention/20 mins; 45% water retention/10 mins; and 60% water retention/8 mins (Lykidis and Grigoriou 2011) and 6 bar/156 °C/45 min (Lykidis and Grigoriou 2008) reduce considerably the formaldehyde content in the produced particleboards compared to control panel and comply with emission class E1. Roﬀael (1995), Michanickl (1996a, b) and Dix et al. (2001a, b) found the same. It can be explained by the fact that the increase in temperature during hydrothermal treatments speeds up the degradation of adhesives in waste wood particles and therefore, urea and other derivatives of hardened urea-formaldehyde will be activated as formaldehyde catchers (Roﬀael and Kraft 2005).

It can be noticed that the formaldehyde emission of wood-based panels manufactured from waste wood could be reduced using thermal hydrolysis process to handle waste wood particles or formaldehyde-free adhesives.

4 Conclusion

Over the last two decades, many eﬀorts have been put into studying the properties of waste wood resource and its application on material use. Evidently, waste wood is not

a homogeneous material. Therefore, there are still some limitations that need to be overcome before waste wood can be used as raw material for wood composites production. Considering the research questions put forward in this review of waste wood utilization, the following conclusions can be drawn:

• It is not surprising that most of previous investigations focused on recycled wood of construction and demolition, furniture and packaging since they are the most popular waste wood stream resources. The potential municipal waste resource was missing in the research.

• There are not enough published data and results available based on research with material derived from plywood and OSB as compared to particleboard and ﬁberboard even though most of the wood-based panel products in the world are plywood. It is controversial between production volume, usage and recycling.

• Due to the rather strict national and/or Europe-wide regulations controlling recycling topics, European institutions and European research institutes are currently the forerunners in waste wood studies. However, the European member states are lacking a common legislation scheme about the recycling of wood regarding classiﬁcation and thresholds. Waste wood resources in other continents such as Asia, Africa and Australia would be of high research interest in the future. In addition, more studies about waste wood ordinances should be conducted especially for countries outside Europe.

• The advantages in technical and mechanical treatment process of waste wood into particles indicated that particleboard was the primary option for the production of wood-based panel compared to ﬁberboard and OSB. The present literature analysis has conﬁrmed that currently, there appears to be hardly any research on the use of waste wood materials in the production of plywood.

• Physical and mechanical contaminants of waste wood resources would not be a problem for wood composites recycling if they were well managed. This management can be done beforehand at recycling companies or facilities via steps of collection, separation and sorting into certain grades. In general, every contaminant could be detected and eliminated by appropriate sorting techniques. On the other hand, the focus on improvement of sorting methods will bring the future perspective values for cleaning waste wood mix. Moreover, changing ingredients of coating pigments, paints, preservatives etc., which contain less harmful substances contributing to reduce contaminants in recycled wood, would be an option as well.

• Particleboard and the core layer of OSB panel products could be substituted up to 100% by waste wood particles. However, the contaminants and the low slenderness ratio

of recycled wood particles will result in the reduction of physical and mechanical properties of the panel products. Those disadvantages could be overcome by applying modern sorting techniques to eliminate contaminants and appropriate chipping techniques in order to increase the slenderness ratio of recycled wood particles. Further investigations are needed at the moment for the improvement of fiberboard properties made from 100% recycled fibers since only up to 25% of waste wood fiber can be utilized in the fiberboard wood mixture achieving comparable physical and mechanical properties with fiberboard from virgin wood.

• Using waste wood for the production of wood-based panel increases the risk of formaldehyde emission in products of particleboard and OSB, except for ﬁberboard. However, this risk can be addressed by applying pretreatment steps to reduce formaldehyde emission (e.g., hydrothermal process) or using formaldehyde-free adhesives (e.g., PMDI)

Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s00107-023-01937-4

Author contributions All authors contributed to the study conception and design of this review paper. Papers search, data collection, analysis and interpretation were performed by NDL, JL, MN and AK. NDL wrote the ﬁrst version of this manuscript. All authors commented and revised on previous versions of the manuscript. At the end, all authors proofread and approved the ﬁnal manuscript to be published.

Funding Open Access funding enabled and organized by Projekt DEAL. German Academic Exchange Service (Deutscher Akademischer Austauschdienst [Grant no. 91691453]) funded this research.

Data availability The datasets collected, generated and analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Conflict of interest On behalf of all authors, the corresponding author states that there is no conﬂict of interest.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

Recent Advances in Natural Fibre-Based Materials for Food Packaging Applications

Paper

Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

polymers

Review

RecentAdvancesinNaturalFibre-BasedMaterialsforFood PackagingApplications

HarikrishnanPulikkalparambil 1 ,SandhyaAliceVarghese 1 ,VaneeChonhenchob 1,2 ,TarineeNampitch 1 , LerpongJarupan 1,2 andNathdanaiHarnkarnsujarit 1,2, *

Citation: Pulikkalparambil,H.; Varghese,S.A.;Chonhenchob,V.; Nampitch,T.;Jarupan,L.; Harnkarnsujarit,N.RecentAdvances inNaturalFibre-BasedMaterialsfor FoodPackagingApplications. Polymers 2023, 15,1393.https:// doi.org/10.3390/polym15061393

AcademicEditor:SwarupRoy

Received:22December2022

Revised:24January2023

Accepted:23February2023

Published:10March2023

1 DepartmentofPackagingandMaterialsTechnology,FacultyofAgro-Industry,KasetsartUniversity,50Ngam WongWanRd.,Latyao,Chatuchak,Bangkok10900,Thailand

2 CenterforAdvancedStudiesforAgricultureandFood,KasetsartUniversity,50NgamWongWanRd.,Latyao, Chatuchak,Bangkok10900,Thailand

* Correspondence:nathdanai.h@ku.ac.th;Tel.:+662-562-5045;Fax:+662-562-5046

Abstract: Packagingisoneofthemajordomainsinthefoodprocessingindustrythatreduceswaste andenhancesproductshelflife.Recently,researchanddevelopmenthavefocusedonbioplasticsand bioresourcestocombatenvironmentalissuescausedbythealarminggrowthofsingle-useplastic wastefoodpackaging.Thedemandfornaturalfibreshasrecentlyincreasedbecauseoftheirlow cost,biodegradabilityandeco-friendliness.Thisarticlereviewedrecentdevelopmentsinnatural fibre-basedfoodpackagingmaterials.Thefirstpartdiscussestheintroductionofnaturalfibresin foodpackaging,withafocusonfibresource,compositionandselectionparameters,whilethesecond partinvestigatesthephysicalandchemicalwaystomodifynaturalfibres.Severalplant-derivedfibre materialshavebeenutilisedinfoodpackagingasreinforcements,fillersandpackagingmatrices. Recentinvestigationsdevelopedandmodifiednaturalfibre(physicalandchemicaltreatments)into packagingusingcasting,meltmixing,hotpressing,compressionmoulding,injectionmoulding,etc. Thesetechniquesmajorlyimprovedthestrengthofbio-basedpackagingforcommercialisation. Thisreviewalsoidentifiedthemainresearchbottlenecksandfuturestudyareasweresuggested.

Keywords: naturalfibres;foodpackaging;fibremodifications

1.Introduction

Bothsmall-andlarge-scalefoodindustriesaregrowingcontinuously,withfood packagingbeinganintegralaspecttoreducespoilageandextendproductshelflife[1].The globalproductionofplasticsisprojectedtoreach1100milliontonnesby2050,with36%of theoutputcurrentlyusedinthepackagingindustryand85%ofthisendsupinlandﬁlls.

Figure 1 showsagraphicalrepresentationofplasticwastegenerationbyseveralindustrial sectorsin2015[2].Thisdiscardedwastepollutestheenvironment.Plasticpackagingis nowusedtoproducecomplexgeometrieswithfunctionalsnapﬁtsanddecorationsbut single-useplasticscauseextremeecologicalissues.Highproductionvolumes,shortusage timeanddealingwiththedisposalofend-lifeplasticshavebecomepressingissues.Rates ofrecyclingforconventionalsingle-usepackagingsuchasglass,plastic,paper,aluminium, andotheralloysarelow,withpaperandpaper-basedpackagingmaterialsrecycledaround 20%ofthetime,whileotherssuchasplasticarerecycledatsubstantiallylowerrates[3].

(measured in tonnes per year)

Figure1. Plasticwastegeneratedbydifferentindustrialsectors[2].

Packagingmaterialhasavitalroleinproductfunctionality,efficiency,processing parametersandenvironmentallyfriendlycustomersatisfaction[4,5].Petroleum-based conventionalplasticpackagingismostlynon-biodegradable,withrisingandunstable pricesduetofluctuationsintheavailabilityofpetroleumsources.Packagingdisposalis nowaprimaryconcernthreateningtopollutewatersupplies,sewersystems,riversand lakes[6].Overtime,plasticproductsfragmentintomicro-andnano-sizedparticlesthat causeserioushealthissues[7].Microplasticshavebeendetectedin15humanbiological componentsincludingbreastmilk,bronchoalveolarlavagefluid,blood,lung,liver,kidney, spleen,placenta,meconium,skin,hair,head,face,hand,saliva,colectomyspecimens, faecesandsputum[8].Babiesingest553to4,550,000microplasticparticles/daythrough feedingbottles[9].Thismicroplasticexposuredirectlyimpactsthedigestive,reproductive, centralnervous,immuneandcirculatorysystemsduringearlydevelopmentalstages.

Pollutioncausedbyplasticsrequireswastemanagementactionbyinnovation,improvedproductandpackagedesignandincreasedrecycling.Thisrequiresorganised legislativeactionsandinternationalcooperation.Thestatisticsindicatethattheutilisation ofplasticresourcesinthetakeawayfoodindustryhasmushroomed[10].Chinaisthe largestconsumeroftakeawayfood,generating350kT/dayofplasticfoodpackaging,with 40billionfoodboxesdiscardedperdayin2019[11].Recently,theCOVID-19crisiscaused a2.2%reductionintheuseofplasticsin2020butthevolumesoftakeawaypackaging andconsumptionofplasticmedicalequipmentincreasedaseconomicactivityresumedin 2021.Thisupwardprogressionoftheuseandconsumptionofplasticsmustbecritically addressed[12,13].

Thisstudyinvestigatedalternativestotraditionalsyntheticplasticpackagingby adoptingasustainable,renewableandbiodegradableapproach[14,15].Naturalfibres arecommonlyusedasreinforcementincompositematerials[16].Theyplaypivotalroles inattainingsomeofthespecificneedsincompositepreparations.Recently,theutilisationofnaturalfibreshasincreasedbecauseofecologicalconcerns;theyarelightweight, naturallydegradable,CO2 neutralandreadilyavailableasrenewablematerials[17,18]. Mostimportantly,duetotheirvulnerabilitytolivingorganisms,theyarebiodegradable anddonotimpacttheecosystem[19].Consequently,incorporatingnaturalfibresinto

thepolymermatricesimprovesthedisposalofcompositematerial[20–22].Varghese etal.[23]investigatedtheuseof Ceibapentandra naturalfibresinpoly(3-hydroxybutyrateco-3-hydroxyvalerate)-basedpackagingapplications.Theyfoundthattheincorporation ofnaturalfibresacceleratedthedegradationofpackagingmaterials,whichshowedgood antibacterialcapacityagainst Staphylococcusaureus andeffectivelypreservedthefreshness ofstrawberriesforalongerperiod.Naturalfibresareecologicallyfriendlybutnegative packagingaspectsincludetheirdominanthydrophilicityandlowmechanicalproperties underhumidconditions.Therefore,usesofnaturalfibresinseveralpackagingapplications arelimited.Recently,naturalfibreshavebeenutilisedwhentherecoveryofconventional plasticsisnoteconomicallyfeasible,controllableorviableandone-time-usepackaging materialsarepreferrable.Naturalfibre-reinforcedcompositescanbereused,unlikecardboardboxes.Saraivaetal.[24]developednaturalfibre-reinforcedcompositematerialfrom spongegourdresidueandcompareditspackagingefficacywiththatofcardboardboxes. Theresultsshowedthatthedevelopednaturalpackagingmaterialwaspreferableafter fourcyclesofuse.

Thisreviewaimedtoinvestigateadvancementsintheresearch,developmentand utilisationofnaturalfibre-basedcompositesforfoodpackagingapplications.Sources, compositionsandrecentproductiontechniquesofseveralnaturalfibresinfoodpackaging werediscussed.Physicalandchemicalmodificationsofnaturalfibresthatimprovetheir suitabilityforfoodpackagingwerealsoexplored.

2.NaturalFibresinPackaging

Naturalfibresareabundantlyavailableasbiodegradableandrenewablenatural materials[25]andtheyhaverecentlyreceivedhugeattentionfromtheglobalresearch community[26,27].Naturalfibrescanbedividedintothreecategoriesbyorigin:animalbased,mineralbasedandplant-based[28].Plant-basednaturalfibreswerethemainfocus ofthisreviewbecauseoftheirabundantavailabilityatlowcost.Plant-basednaturalfibres arelignocellulosicinnaturewiththeirbasicconstituentsincludinglignin,hemicellulose andcellulose.Animal-basedfibresmostlyconsistofproteins,e.g.,woolandsilk.Mineralbasedfibresareformedasaresultofgeologicalprocesses,suchasasbestosandbasalt.In plant-basednaturalfibres,bothprimaryfibresobtaineddirectlyfromplantsandsecondary fibresobtainedasbyproductsafterutilisationofprimaryfibresareused.

2.1.SourceandCompositions

Naturalprimaryplantfibresincludehemp,kenaf,sisalandcotton,whilesecondary fibresincludebagasse,coir,pineapple,agaveandoilpalm[29–31].Naturalfibreshave longbeenexploitedinthepreparationandmanufactureofropesandtextiles,forexample, flax,hemp,cottonlintorsisal.Somefibreshavesecondaryapplicationsinfoodpackaging. Figure 2 showssomeofthecommonlyavailablenaturalfibresandtheirsources,while Table 1 showstheoriginandpropertiesofnaturalfibres.Someofthemajorvarietiesof naturalfibresarediscussedinmoredetailbelow.

2.1.1.Hemp(Cannabissativa)

Hempisoneofthemostwidelyutilisednaturalfibresaftersisalasreinforcementfor composites[61].HempisgrownwidelyintheEU,China,thePhilippinesandCentral Asia.Theplantsarecultivatedfromseedandcangrowupto5minheight.Crops cultivatedforfibrearedenselysownandproduceplantsaveraging2–3minheightwith almostnobranching.Hempfibreshaveantibacterialproperties[62–64]emanatingfrom cannabinoids,alkaloids,otherbioactivecomponentsorlignin[65].Khanetal.[66]studied theantibacterialpropertiesofhemphurdpowderagainst E.coli usingretted,semi-retted andnon-rettedhemphurdpowderwithdifferentparticlesizes.Thefibreswerekeptat 160 ◦ Cfor2hto eliminateself-contaminationssuchashumidityandthermalhistory.These authorsfoundthathempwasanecofriendlyfoodpackagingmaterialsuitableformeat, saladsandready-madefoodproducts.Teixeiraetal.[67]studiedthetemperatureeffects onmechanicalstrengthofhempfibres.Theyfoundthattensilestrengthincreasedby18% oncethefibreswereexposedto100 ◦ Cfor24h.However,whenexposedto200 ◦ Cfor24h, tensilepropertiesdecreasedandthefibresbecamefragileandbrittle.

2.1.2.Sisal(Agavesisalana)

Sisalisoneofthemostcommonlyusednaturalfibresgrownintropicalandsubtropical regionsofNorthandSouthAmerica,Africa,theWestIndiesandtheFarEast.Moreover, thehydroalcoholicextractobtainedfromsisalleavespossessessignificantantimicrobial activityagainst Aspergillusniger and Candidaalbicans [68,69].Pulikkalparambiletal.[70] examinedthereuseofdiscardedpolypropylene(PP)-baseddisposablefacemaskswith sisalandhempfibremats.TheyusedhotcompressionmouldingtosandwichthePPmasks andnaturalfibres.Theresultingcompositesshowedexcellentmechanicalpropertieswith antimicrobialactivitiesagainst S.aureus

2.1.3.Kenaf(Hibiscuscannabinus L.)

KenafisamemberoftheMalvaceaefamilyandherbaceousﬁbrecrops.Kenafplants aregrownthroughouttheyearwithashortharvesttimeinWestAfrica,IndiaandChina[71]. Theycangrowupto2.5–4.5mtallwithstemsupto1–2minlength.Kenafseedsandleaves areusedinfoodproductsastheyarerichinnutritionalandphytochemicalcompounds[72].

2.1.4.Bamboo(Bambusavulgaris)

Bamboo(Bambusavulgaris)growsinAsia-Pacific,African,EuropeanandNorthand SouthAmericanregions.Bambooreachesmaturityin3yearswhenitstensilestrength iseffectivelycomparabletomildsteel.Mosobamboohasagrowthrateof2inchesper hour.Somebamboospeciesreachaheightof60feetin3months.Therefore,cutting downthiswooddoesnotaffecttheecologicalandnaturalbalancemuch[73].Bamboois consideredthemostunder-utilisednaturalfibreandisabundantlyavailableinSoutheast Asiancountries.Theannualglobalavailabilityofbamboofibresis30milliontonneswith amaturitycycleofonly3–4years.Bamboofibreshaveexcellentmechanicalstrength. Thespecificstiffnessandstrengthiscomparabletoglassfibres[74].Afrinetal.[75] reportedstrongantibacterialpropertiesofbamboofibresagainst E.coli and S.aureus due toretainedlignin.AnotherpossiblereasonwasthepresenceofH2 O2 thatdamagedthe DNAsequencingof E.coli.However,theybelievedthatthiswasnotpossibleasH2 O2 was thermallydecomposedduringtheextractionprocess.

2.1.5.Jute

Juteisoftenneglectedbutconsideredasoneofthemostimportantfibres.Juteisinthe TiliaceaefamilywiththescientificnameCorchoruscapsularisbecauseitisextractedfrom Corchorusplants.JutefibresaremostlyfoundintheMediterraneanbutrecentlythefinest growthfibrescomefromBangladesh,India,China,Nepal,Thailand,IndonesiaandBrazil. Jutefibresarebrittleandcangrow2–3.5minheight.Theypossessveryhighlignincontent (12–16%)andthushavelowelongationatbreak.Jutefibrespossessuniqueproperties which,ifutilisedeffectively,cansolveproblemsinthetextileandfoodpackagingfields. TheincorporationofjutefibresinPLAmatricesimprovedbothoxygenandwatervapour barrierproperties.

2.1.6.Flax(Linumusitatissimum)

FlaxiscommonlygrowninmoderateclimaticregionssuchasIndia,Argentina,SouthernEurope,ChinaandCanada[28].Flaxplantscangrowtoheightsof80to150cmin lessthan110days.Fibresfromflaxbastgrowbetween60and140cmlongwithdiameters rangingfrom40to80 μm.Theyaremembersofthebastfamily.Thebastfibresarecollected fromthefibrousbundleslocatedintheinnerbarkofaplantstem.Themajorcomponentsof flaxfibresarepectin,hemicellulose,cellulose,andlignin.Therearealsosmallamountsof wax,oilandwater.Theincorporationofflaxfibresincreasesstrengthandstiffness,which canbefurtherimprovedbymodificationwithamalleatedcouplingagent[76].

2.1.7.BananaPlants

Bananaplantsaremostlygrownintropicalcountrieswheretheyareconsideredas anagriculturalcrop.Bananabastfibreisalingo–cellulosicmaterialandextractedas awasteproductofbananaplantcultivation.Bananafibrehasgreatspecificstrength whichiscomparabletoconventionalmaterialssuchasglassfibre[77].Ranaetal.[78] manufacturedbananafibre-reinforcedpolyvinylalcohol(PVA)resinandevaluatedthe mechanicalstrengthofthesecomposites.TheyconcludedthatPVAcompositeswith reinforcedbananafibrecouldbeusedasbiodegradablefoodpackaging.Theyhadgood biodegradationwithadequatehandlingstrength.

2.1.8.Ramie(Boehmerianivea (L)Gaud.)

RamieisaperennialhardyshrubbelongingtotheUrticaceaefamily.Ramieisconsideredoneoftheoldestvegetablefibreswhichhasbeenutilisedforthousandsofyears, specificallyasmummyclothsinEgyptfrom5000–3000BC.Ramiewasinitiallygrownin China,whiletodayramiefibreismainlygrowninBrazil,India,China,thePhilippines, SouthKorea,TaiwanandThailand.ThefibresarepopularlyknownasRhea,Kunkura, Pooah,Kunchoor,Puya,steelwireandChinagrassindifferentpartsofIndia.Thereishigh demandforramiefibresduetotheirperformanceandaestheticproperties.Ramiefabrics effectivelyabsorbmoisture,transmitheat,andaremoreresistanttomildewthanother cellulose-basedfibres.

Natural Fibre Origin WorldProduction (× 103 Tonnes)

AbacaLeaf700.83114–130418–48612–13.8BananaStem2001.3580–250529–7598.201–3.5 BambooStem10,000910-50335.911.4 CoirFruit1001.15100–460108–2524–615–40 CottonLintFruit18,5001.6-287–5975.5–12.63–10 FlaxStem8101.5-345–150027.6–801.2–3.2 JuteStem25001.46-393–80010–301.5–1.8 HempStem2151.48-550–900701.6 KenafStem7701.4812504.3OilpalmFruitAbundant0.7–1.55150–50080–2480.5–3.217–25 RamieStem1001.0–1.5520–80400–100024.5–1281.2–4.0

Ricehusk Fruit/ grain Abundant-----

RoselleStem250----SisalLeaf3801.4550–300227–4009–202–14

2.2.NaturalFibreSelectionParametersasPackagingMaterial

Thefunctionoffoodpackagingistoprotectthefoodcontainedinsidethepackaging fromphysical,chemicalandbiologicalhazards(oxygen,moisture,light,microbialcontaminationandinsects)[80].Packagingmaterialsdependgreatlyuponthetypeoffood. containedinsidethepackagingsuchasmeat,fruits,vegetablesandready-to-eatfood.The packagingmustmaintainthesafetyandqualityofthecontainedfood.Otherfunctionsincludingpropercontainment,convenience,andinformationregardingthefoodarerequired,

whilemostimportantlythepackagingmustlookaestheticallypleasing.Table 2 listssome ofthecategoriesinvestigatedwhenstudyingthepropertiesofpackagingmaterials[81].

Oneprimaryconcernisthestructuralaspectsofthepackagingmaterialincluding tensileandtear,strength,bending,compression,punctureandfoldingparametersthat needtowithstanddifferentloadingconditionsduringstacking,transferandtransportation. Thestrengthofnaturalfibre-reinforcedcompositesrelatestotwofactors:(a)thestiffness andstrengthofthenaturalfibresand(b)compatibilitybetweenthefibresandthematrix. Strengthandstiffnessdependonthearrangementofcellulosicfibrilsinthemicrofibrils presentinthefibres,whilethemechanicalpropertiesofnaturalfibresarecontingentonthe partofthetreeorplantfromwhichthefibrehasbeencollected.Crystallineandamorphous fibrecharacteristicsdifferbetweenpartsofthetreeandbetweentrees.Previousstudies suggestedthatfibreswithfeweramorphouscontentssuchashemicellulose,ligninand pectinpossessbettermechanicalproperties[82,83].

Theconcentrationofnaturalfibresincorporatedintocompositesalsoimpactsthe mechanicalproperties.Anincreaseinfibrecontentincreasesmechanicalpropertiesupto anoptimalconcentrationbeyondwhichthemechanicalstrengthstartstoshowadecrease trend[84].Taoetal.[85]studiedtheeffectofjuteandramiefibreloadinginPLAcompositesandfoundthat30%naturalfibrecontentinPLAprovidedtheoptimalmechanical properties.Anotherfactoraffectingthepropertiesofnaturalfibre-reinforcedcomposites iscompatibility,whereinteractionbetweenthefibresandthematrixplaysanimportant roleinuniformlydistributingtheappliedloadintothematrix.Kamarudinetal.[86] reportedthatPLA/kenafcompositesshowedexcellentmechanicalstrengthatupto40% fibreloadingduetogoodfibre–matrixinterfacialinteraction.Beyondthiscriticalfibre loadingvalue,poorfillermatrixcompatibilityresultedinearlierfractureofthecomposite. Severalsurfacemodificationtechniques(bothphysicalandchemical)havebeenstudiedto improvethecompatibilityofnaturalfibresandmatrixmaterials.

Compositespreparedfromnaturalfibresshowpromiseappliedasfoodpackaging materials.However,theroleoffoodpackagingmaterialsisnotlimitedtoprotecting productsfromphysicalandmechanicaldamageduringdistribution[87].Foodpackaging mustalsocontrolthetransferofwatervapour,oxygenand/orcarbondioxide,which impactratesofoxidation,microbialdevelopmentandphysiologicalreactionsoffood degradation.Plasticsarecommonlypermeabletosmallvolatilessuchasgases(O2 ,CO2 ), watervapour,organicvapoursandliquids[88,89]andwaterabsorptionbarrierproperties areessentialbasicrequirementswhenpackingfood.Themoisturebarrierpropertyis importantinfoodpackagingasthispreservesthetextureinbothdryandmoistfoodand controlsmicrobialgrowthofaerobicspoilage.Importantparametersresponsibleforthe controlwatervapourpermeability(WVP)arefibrecontentandsize,fibre/matrixadhesion andcrystallinityandplasticisationofthematrix[90–92].Thedispersionoffibresinthe matrixcanevokeimpermeabilityduetothetortuosityeffect.However,theWVPofnatural fibre-reinforcedcompositessignificantlyincreasesduetothehygroscopicnatureofthe fibresandpoordispersioninthematrix.Bycontrast,hydrophilicpolysaccharidematrices havelowWVPproperties.Sirvioetal.[93]observedthatincorporationofupto50wt%of cellulosemicrofibrilsinalginatefilmsdecreasedtheWVPduetoanincreaseintortuosity. However,theaggregationandpercolationofsmallnaturalfibresinpolymermatricescan resultfrompoorfibre/matrixadhesion,leadingtovoidsinthepolymerswhichencourage thetransportofwatermolecules[94].OtherwaystoimprovetheWVPofnaturalfibres includecoatingwithPLA[95].Dasetal.[96]observedenhancedvapourpermeability andabsorptioncapacityof15.9%and48.1%,respectively,inshreddedbetelnutcomposite sheetscomparedwithcardboardsheets.Foodmustalsobeprotectedfromoxygenand carbondioxide,whichcausemanydegradationreactions.HighlevelsofCO2 inchilieslimit theKrebscycle,whereaslowlevelsofO2 decreasetheactivationofcytochromeoxidase, polyphenoloxidase,glycolicacidoxidaseandascorbicacidoxidase[97].

Migrationintofoodisanotherparametertobeconsideredwhenchoosingmaterialsfor foodpackagingapplications[98].Toxicologicalsubstancessuchaspesticideresiduessuch

asherbicides,fungicidesandinsecticidesandotherpollutantsintheenvironmentssuch aspolycyclicaromatichydrocarbonsmightbepresentinnaturalfibres.Specialadditives suchasplasticisers,includingphthalates,arealsoaddedinformulationsofpackaging materials.Theseunnaturalsubstancesmightmigrateintofoodduringstoragecausing spoilage.Temperature,activationenergyandmicrostructuresofthepackagingfilmscan alsorestrictdiffusionofunwantedsubstancesintofood[99,100].Thus,itisnecessaryto decontaminatenaturalfibresduringextractionbeforecompositepreparation.

Finally,thebiodegradability/compostabilityofthepackagingmaterialsisalsoof interesttoconsumers.Usingfullbioplasticsasmatricesandnaturalfibresproduces eco-friendlypackagingwithlowcarbonfootprints.Biodegradationratesofnaturalfibre compositesdependonthenatureofthefillers,reinforcementsandthematrixaswellasthe compositeratios.

PropertyExamples

Structuralproperties

Barrierandabsorptionproperties

Tensilestrength,tearproperties,compressionproperties,bendingstiffness, edgecrushresistance,burststrength,punctureresistance,foldingendurance, wetstrengthanddelamination

Oxygenpermeability(OP),watervapourpermeability(WVP),Volatile permeabilityandwaterabsorptioncapacity

ManufacturabilityandmanufacturingqualityUniformityofthickness,densityandmoisturecontent

MigrationintofoodToxicologyparametersandmigrationstudies

Non-structuralfunctionalityAbrasionresistanceandstaticandkineticfriction

Degradability/compostabilityCompostabilityinbiodegradationtestsanddisintegrationtests

3.PhysicalandChemicalModiﬁcationsofFibresforFoodPackaging

Numerousstudieshaveinvestigatednaturalfibre-reinforcedpolymersbecauseof theirimprovedstrengthandstiffnesswiththelowcost,biodegradability,renewabilityand abundancyofnaturalfibres[101,102].However,mixingnaturalfibrewithapolymermatrix commonlycausespoormechanicalcompositepropertiesbecauseof(a)poorcompatibility betweenthepolarhydrophilicnaturalfibreandthenon-polarhydrophobicpolymermatrix and(b)non-homogeneousdispersionoffibreandwoodpowderinthePPmatrix[103,104]. Themodificationofnaturalfibrescanreducetheirhydrophilicityand,thereby,increasetheir compatibility.Theremovalofunwantedwaxandincreasingsurfaceroughnessincreases thecontactsurfaceareaofthefibres,whichinturnincreasesthetransferofstressuniformly intothematrix.Severalchemicalandphysicaltreatmentstoimprovethecompatibilityof naturalfibresareasfollows.

3.1.ChemicalModiﬁcationTechniques

Compositesreinforcedwithchemicallytreatednaturalfibresgenerallyshowenhanced mechanicalpropertiesbecauseofimprovedinterfacialadhesionbetweenthefibreandthe matrix[82].Numeroussurfacetreatmentmethodsareavailableforthemodificationof naturalfibres.Alkaline(NaOH)treatment/mercerisationisthesimplest,mosteffectiveand commonlyusedchemicaltreatment.Traditionally,mercerisationisatechniquetomodify surfaceofcotton.Strongcausticsodasolutionsareusedtotreatmaterialsfor1–3min undertensionandlowtemperatures,followedbywashing.Holdingcottonfabricunder tensioninthecausticsolutionhelpstomaintainitsoriginaldimensions.Asaresult,the fibreshavemorearoundedstructureinthecrosssection,reflectinglighttoimprovelustre. Mercerisedcottonalsoinvolvesachangeincrystallinestructureanddegreeofcrystallinity, therebyreducingstressesandincreasingthestrengthoftheweakpointsinthefibre[105].

ImmersioninNaOHaqueoussolutionremoveshemicellulose,lignin,pectinandother impuritiesfromthefibresurface.Thisresultsinaroughersurface,whichinturnimproves themechanicalinterlockingofthefibreswiththematrix.Borahetal.[106]foundthat alkalitreatmentofbetelnutfibresbeforecompositeformationimprovedtensionstrength by18%,elongationatbreakby6%,bendingstrengthby11%andimpactstrengthby18%. Thereactionbetweenthefibreandalkalisolutioncanberepresentedbytheequation below[107].

Theutilisationofoxidisingagentssuchashydrogenperoxide(H2 O2 )innaturalfibres canalsoeliminatethecementingsubstancesfromsurfaceofthefibres,whichhinder adhesionwiththepolymermatrix.Fornaturalfibres,alkalitreatmentresultsinthe formationofanalkali-resistantlinkagebetweenligninandhemicellulosethatmayimpede theremovaloflignin.UsingH2 O2 breaksthesebondsanddelignifieslignocellulosic fibre,whichenhancestheinterfacialadhesionwiththepolymermatrices[108,109].The acetylationoffibrealsoimproveshydrophobicity,whichenhancesinterfacialadhesionby reducingthemoistureabsorptionofthecellulosecomponents.Thisincludestreatmentsof aceticorpropionicacidatelevatedtemperatureswithorwithoutthecombinationofan acidcatalyst[110].Acetylationofthecellulosecomponentssubstituteshydroxylgroups ofthecellwall,whichincreasesthehydrophobicityofthenaturalfibres.Thismethod improvescompatibilitywiththepolymermatrixbydecreasingwaterabsorption.

Finally,couplingagentsalsoreduceinherentincompatibilitybetweenthepolymer matrixandnaturalfibres,whichenhancestheinterfacialadhesion.Polymersconsistof bifunctionalgroupswhicheffectivelyreactwithboththefibreandthematrix.Organofunctionalsilanecouplingagentsformcovalentbondswiththehydroxylgroupsofcellulose. Alkoxygroupsarehydrolysable.Moisturefacilitateshydrolysisandformssilanolswhich furtherreactwiththehydroxylgroupsofthefibre.Consequently,stablecovalentbonds areformedwiththecellwallthatarechemisorbedontothefibresurface[110–112].This chemisorptioncommonlyimprovesthedegreeofcrosslinkingatinterface,whichimproves affinityoforganophilicpolymers[110].Addinghydrocarbonchainsbythemodificationof naturalfibrewithsilanesmodifiestheirwettabilityandreduceswateruptakeascovalent bondingformscross-linkingbetweenthefibreandthematrix.

However,theuseofalkalineoranyotherchemicaltreatmentsadverselyimpacts productsustainability,withgreentechniquespreferredfornaturalfibremodification. Smithetal.[113]preparedasustainablegreencompositebasedonagavefibre(Agavetequilana) modifiedwithpoly(3-hydroxybutyrate)(PHB)inthepresenceofasmallquantity(0.1phr) oforganicperoxidethroughone-stepreactiveextrusionprocessing.Resultsshowedthat 25wt%agavefibrewith0.1phrperoxideimprovedflexuralstrengthby46%,impact strengthby45%andheatdeflectiontemperature(HDT)by39%comparedwithneat PHB.Thesefindingssuggestedthatthepresenceofperoxideprovidesacost-effectiveand sustainablealternativetopetroleum-basedconventionalplasticsforfoodpackaging.

Mohantyetal.[114]chemicallymodiﬁeddatepalmleaf(DPL)usingacrylicacid andtestedthedispersionandcompatibilitywithpolyvinylpyrrolidonecompositesfor packagingapplications.Preparedbiocompositesreinforcedwith26wt%DPLﬁbreloading showedpromiseforuseaswater-andchemical-resistanthydrophobicpackagingmaterials. Nazrinetal.[115]studiedtheincorporationofnanocellulosetoenhancetheproperties ofthermoplasticstarch(TPS),polylacticacid(PLA)andpolybutylenesuccinate(PBS)for foodpackaging.TheyreportedthattheadditionofnanocelluloseinTPSimprovedthelow waterbarrierandtensileproperties,whiletheadditionofnanocelluloseintoPBSandPLA enhancedtheoxygenbarrierpropertiesandmechanicalstrength.

Naturaljuteﬁbreincorporatedwitharedgrapepomaceextract(RGPE)hasbeen developedforactivepackaging.TheRGPEwasderivedfrompressurisedliquidextraction (PLE)andenhancedsolventextraction(ESE)techniques[116].Thepackagingshowed excellentantibacterialactivitiesagainst E.coli, S.aureus and Pseudomonasaeruginosa.The RGPEextractfromPLEusingC2 H5 OH:H2 O(asasolvent)had11majorphenoliccom-

Fibre OH + NaOH → Fibre O Na+ + H2 O

pounds.Jara-Palaciosetal.[117]foundquercetin-3-O-glucosideasthemostabundant compoundinRGPEextractedbyC2 H5 OH:H2 O.Wangetal.[118]modifiedhempfibre withalysine-graftedN-halamineorganicasanantibacterialagentinhempfibreusing amildSchiffbasereaction.Thematerialstotallyeliminated Staphylococcusaureus and Escherichiacoli in5min,whiletheinhibitionzoneincreasedto18.4mm.

3.2.PhysicalModiﬁcationTechniques

Previousstudiesmainlyfocusedontheeffectsofcouplingagentsandcompatibilisers totailorthemechanicalpropertiesofnaturalfibre-reinforcedcomposites.Mostchemical treatmentsweresuccessfulandresultedinincreasedthermalandmechanicalproperties. However,somemajorproblemsassociatedwithchemicaltreatmentsarethehighcost andpollutionfromthedisposalofthechemicalsaftertreatment[119].Plasmatreatment introducesfunctionalgroupsontonaturalfibresthatformstrongcovalentbondswith thematrix,leadingtoastrongfibre/matrixinterface.Plasmatreatmentissimple,shortduration,consumeslittleenergy,andlowcost.Thetechniquerequiresnowaterorany potentiallyhazardouschemicals.Surfaceetchingimprovesthesurfaceroughnessofnatural fibresandresultsinbetterinterfacialinteractionwiththematricesthroughmechanical linking[120–124].

3.2.1.ColdPlasmaTreatments

Coldplasmatechniquesaredry,cleanprocesseswithlessenvironmentalconcerns. Suchamodificationoccursonlyonthesurfacewithnointerferenceonthebulkproperties. Figure 3 showsaschematicrepresentationoftheeffectsofplasmaandcationisingprocesses ofcellulose-constitutingcottonfibres[125].Sinhaetal.[122]studiedtheinfluenceofphysicaltreatmentonthemorphology,wettabilityandimpactofthefinestructureoffibreson interfacialadhesionofnaturalfibre-reinforcedcomposites.Theyfoundthatplasmatreatmentreducedfibrehydrophilicityduetothedecreaseinphenolicandsecondaryalcoholic groupsandoxidationofthebasicstructuralligninandhemicellulosescomponents.Plasma treatmentimprovedfibre/matrixadhesion,asrevealedbyscanningelectronmicroscopy (SEM)morphology.Figure 4 demonstratestheetchedsurfacesandincreasednumbersof newoxygenfunctionalgroupspresentonthesurfaceofsisalandcoconutfibresrevealed usingSEManalysis[126].

Figure4. Theeffectofplasmatreatmenton(a)thesurfaceroughnessofﬁbresand(b)theinterfacial interactionbetweentheﬁbreandmatrix[126].

Combiningchemicaltreatmentswithphysicalplasmatreatmentswasstudiedonflax fibresbyGiepardaetal.[127]tounderstandthesynergisticeffects.Theauthorsused silanisationandplasmatreatmentbothindividuallyandincombination.Theresults revealedanincreasedthermalstabilitywithasignificantimpactonfibrediameterand specificsurfacearea.Erwinetal.[128]studiedliquidplasmatreatmentoncoirfibre withmicrowaveplasmaintheliquid.Themediumswerewaterandsodiumbicarbonate (NaHCO3 )solution.Theinterfacialshearstrengthofthecoirfibre–epoxymatrixincreased afterliquidplasmatreatmentwithbothwaterandsodiumbicarbonatebecauseofchemical adhesionwhichfacilitatedmechanicalinterlocking.

3.2.2.SteamExplosion

Steamexplosionisanotherphysicalmodificationtechniquefornaturalfibres.Thisinvolvesheatingthefibresatahightemperatureandpressure,causingmechanicaldisruption ofthecellularmaterialthatundergoesfibrillation.Theselectionofsteamingtemperature andexposuretimeisveryimportanttoachieveoptimalfibreproperties.Hanetal.[129] studiedtheeffectsofsteamtreatmentonwheatstrawunderdifferentpressuresandtimes. Theyreportedthatthetreatmentenhancedthedimensionalstabilitywiththeremovalof lignin,ashandextracts.

4.ProductionTechnology

Naturalfibre-reinforcedhybridcompositesarenowextensivelyappliedtodealwith technologicalproblems[130–132].Table 3 listscompositesmadeofnaturalfibresthathave widespreadapplicationsinareaswherethecostofreinforcementslimitstheutilisation ofconventional,lightweight,reinforcedplasticmaterials[133–135].Cabedoetal.[136] comparedalmondshell,ricehuskandseagrassasfillersinPHB/fibrecompositesprepared bythemelt-blendingprocess.Theystudiedtheinfluenceoffibretypeandfibrecontenton morphology,thermal,mechanicalandbarrierproperties,compostabilityandprocessability. Theyconcludedthatallthreefibresweresuitableforthedevelopmentoffullycompostable biocompositesforpackagingapplications.Rawietal.[75]studiedtheeffectsofcompressionmouldingparametersonthemechanicalpropertiesofbamboofabric,poly(lacticacid) (PLA)compositesforpackagingapplications.Theyreportedthatthecompositeswiththe highestcompressionpressureof1.01MPaat3minexhibitedasuperiortensilestrengthof 80.71MPaandflexuralpropertiesof124MPa.They[137]alsocomparedpolypropylene (PP)andbamboofabricPLAcompositestoinvestigatetheuseofenvironmentallyfriendly

compositesforpackagingapplications.Thefindingsindicatedthatbamboofabric/PLA compositesenhancedPLAimpactstrengthby117%,withcomparativelylowerimpact strengthobservedforPP/bamboofibrecomposites.Thermalstabilityintermsoftheheat deflectiontemperature(HDT)ofPPandPLAmatriceswasincreasedbytheadditionof bamboofabric.Thehighheatresistancepropertyofcompositesissuitableforpackaging applications.Nabels-Sneidersetal.[138]studiedlaminationtechnologyofcasthemppaper withbio-basedplasticsusingacompressionmouldingprocesstoreplaceconventionalplasticsandsolvetheexistingwastedisposalproblems.Theycomparedpolyhydroxyalkanoate (PHA),polylacticacid(PLA),polybutylenesuccinate(PBS)andpolybutylenesuccinate adipate(PBSA)laminatespreparedatthreedifferentcompressionpressures.Thedesired pressureonporouscastpaper,impregnationandexcellentlayeradhesionwasproposed intheirstudy.Jietal.[139]preparedchitosan-basedcompositefilmsreinforcedbyramie fibreandlignin,asshowninFigure 5,forfoodpackagingapplications.Theadditionof 20%ramiefibreand20%ligninimprovedthemechanicalpropertiesandwaterresistance byupto29.6%and41%,respectively.Foodpackagingstudiesshowedextendedshelflife inmeatproductssuchaschickenbreastscomparedwithfruitssuchascherrytomatoes.

Tawakkaletal.[140]usedathymolextractwithkenafﬁbrestostudythemigrationof thymolextractfromPLA/kenafcomposites.Melt-blendingwasusedtopreparetheﬁlms inaninternalmixer(155 ◦ Cfor8minand50rpm)followedbyheatpressing.Thematerials weremeltedbypreheating(150 ◦ Cfor3min)withoutapplyingpressureandthenpressedat thesametemperaturefor2minwithaforceof20kNbeforequenchcoolingto30 ◦ Cunder pressure.Tawakkaletal.[141]alsostudiedtheantimicrobialactivityofPLA/kenaf/thymol against Escherichiacoli bacteriaandnaturallyoccurringfungi.Filmswithhigherthymol concentrationsandhigherkenafloadingexhibitedexcellentantibacterialpropertiesagainst fungalgrowthduetothereleaseofthymolintotheheadspacesurroundingthesamples; however,theshelflifeafterstoragefor3monthsatambienttemperatureshowedonly aslightdecreaseinantimicrobialproperties.

2023 , 15 ,1393

Polymers

MajorFindingsRef.

[ 138 ]

Biodegradationinacontrolledcompost at58 ◦ Cresultedinfulldegradation within40to80days,withPLAandPHA laminatesshowing40and50days, respectively.

Methodof Packaging Production

Table3. Summaryofnaturalﬁbre-reinforcedcompositesforfoodpackagingapplications.

Packaging Form

RoleofFibrein Packaging

Typeofmatrix/OtherPolymerBlend(IfAny)

FibrePrepara- tion/Treatment

PartofPlant

Source ofFibre

[ 142 ]

Oilpalmemptyfruitbunchﬁbre-based trayswerebelowtheallowablelimit speciﬁedbyCommissionRegulation (EU)No10/2011.

Theresinhadfavourablecharacteristics intermsofelasto–plasticand stress–strainbehaviour,suitablefor storageandtransportation.

Directmeltcoating

Sodiumhydroxide (NaOH)treatment

HempStraw

[ 140 , 141 ]

Solventcasting

MatrixTray

Emptyfruit bunchOilpalmemptyfruit bunch+Formaldehyde

Oilpalm

LaminateCasting

PolyesterresinReinforcement

[ 96 ] Kenaf

BetelnutSeed

AddingkenafﬁllertothePLAenhanced thereleaseofthymolfromthePLA matrix,reducedproductioncostsand increasedmechanicalstrength. Thecompositeﬁlmsreduced Escherichiacoli inoculatedonthesurface ofprocessedslicedchickensamplesafter 30daysat10 ◦ Cbothindirectcontact andinthevapourphase.

Meltblendingand heatpressing

ReinforcementFilm

[ 143 ]

Flexuralstrengthimprovedby28% afteracetylationtreatment.

LaminateCasting

[ 144 ]

Theintroductionofplasticisersreduced brittlenessandenhancedﬂexibilityand peelabilityofﬁlms.

Solution-casting technique

Heated hydraulicpress

PolyesterFiller

Matrix Film

FillerFilm

Acetylation treatment

Plantain pseudostem

SugarpalmTrunkSugarpalm+glycerol andsorbitol

[ 137 ]

Theimpactstrengthwasenhanced by117%.

Film-stackingand compression moulding

Laminate

Reinforcement

A3.5-foldincreaseinwatervapour permeabilitywasrecorded. [ 145 ] Bamboo Stem-PLA

Wheatstraw Straw-PHBV

2023 , 15 ,1393

MajorFindingsRef.

Biocompositesreinforcedwith26wt% DPLﬁbreloadingcanbeusedaswater- andchemical-resistantpackaging materialsduetotheir hydrophobicnature.

Alkalitreatmentinthepresenceof asilane-couplingagentcausedmatrix skinformationandtheformationof ﬂower-likestructuresonthesurfaceof thefabric,suggestinggoodbonding betweenthereinforcementand thematrix.

Thehighestcompressionpressureof 1.01MPaat3minexhibitedasuperior tensilestrengthof80.71MPaand ﬂexuralpropertiesof124MPa.

Sizeandshapeirregularitiesoftheﬁbres playedadominantroleinthe ultimateproperties.

Methodof Packaging Production

Meltmixing fabrication technique

Packaging Form

RoleofFibrein Packaging

Typeofmatrix/OtherPolymerBlend(IfAny)

Table3. Cont.

FibrePrepara- tion/Treatment

PartofPlant

Source ofFibre

DatepalmLeafﬁbreAcrylicacidPolyvinylpyrrolidoneReinforcement

poly(lacticacid)(PLA)Reinforcement LaminateHotpressing

Alkalitreatment and silane-coupling agent

Film-stackingand compression moulding

Sterculiaurens

References

5.ConclusionsandFuturePerspectives

Inthelastdecade,numerousstudieshavebeenconductedontheutilisationofnatural fibrestoreplaceconventionalpolymerapplications.Severalfibreshavebeenextracted fromplantresourcessuchashemp,sisal,kenaf,bamboo,jute,flax,bananaandramie. Chemicalextractionsandtreatmentsincludingalkalinesolutions,oxidisingagentsand couplingagentshavebeendemonstratedtopurifyandimprovethestrengthoffibre. Alternatively,physicaltreatmentssuchascoldplasmatreatmentsandsteamexplosion enhancethepropertiesandpurificationsofextractedfibre,removinglignin,ashandother substanceswhileincreasingdimensionalstability.Thesefibreshavebeenrecentlyutilised infoodpackagingasmatrices,fillersandreinforcementsbysolutioncasting,meltmixing, hotpressing,compressionmoulding,injectionmoulding,etc.Theincorporationofthese naturalfibreseffectivelyimprovedthemechanicalstrengthofthepackaging.Further developmentofnaturalfibre-basedfoodpackagingmaterialscanbeproposedasfollows:

• Thevalorisationofnaturalfibresinthefoodpackagingsectorexhibitedpromisingresults. However,along-lastingsupplyofrawmaterialsisessentialtoensuresustainability.

• Environmentallyfriendlyextraction/purificationisidealfortheproductionofuniformqualityfibres.Themodificationofnaturalfibresneedstoaddressenvironmentalissues impliedbychemicalmethods.

• Naturalfibresensurethesafetyandprotectionoffoodbyenhancingthemechanical propertiesoffoodpackagingtoresistphysicaldamage.However,severalotherfactors mustbeconsidered.Thepackagingmustbedesignedtoovercomedegradation reactionsandalsobeabletoregulategasandwaterbarrierproperties.Theselectionof naturalfibrescombinedwiththeuseofappropriatemodificationmethodscanprevent theformationofdefectsthatwoulddegradethemechanicalproperties,whilealso enhancingpackagingpermeability.

• Thedecontaminationofnaturalﬁbresshouldcomplywiththeregulationsonfood contactmaterialstoguaranteethehealthoftheconsumer.Thisaspectischallenging whenusingnaturalﬁbresduetothepresenceoftoxicologicalsubstancessuchas pesticidesthatcouldmigratetofoodfromthepackagingmaterials.

• Consumerwillingnesstopurchaseeconomicallycompetitivefullybiocompositealternativesisstilluncertain.Thecostofbiocompositesinfoodpackagingmaterialsneeds toberegulatedtoimprovethedemandinlocalmarkets.Thefutureuseofnatural ﬁbresishighlyrecommendedforpackagingmaterialsduetotheircost-effectiveness andavailabilitythroughouttheyear.

AuthorContributions: Conceptualisation,H.P.andN.H.;methodology,H.P.andN.H.;validation, N.H.;investigation,H.P.andN.H.;writing—originaldraftpreparation,H.P.andN.H.;writing— reviewandediting,H.P,S.A.V.,V.C.,T.N.,L.J.andN.H.;supervision,N.H.;fundingacquisition,N.H. Allauthorshavereadandagreedtothepublishedversionofthemanuscript.

Funding: ThisresearchwasfundedbyKasetsartUniversityResearchandDevelopmentInstitute, KURDIgrantno.FF(KU)17.65withﬁnancialsupportfromtheOfﬁceoftheMinistryofHigher Education,Science,ResearchandInnovation;andtheThailandScienceResearchandInnovation throughtheKasetsartUniversityReinventingUniversityProgram2022.

InstitutionalReviewBoardStatement: Notapplicable.

DataAvailabilityStatement: Notapplicable. ConﬂictsofInterest: Theauthorsdeclarenoconﬂictofinterest.

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FROM THE PUBLISHERS OF PAPER

Volume 10, Number 3, 2024

PAPERmaking!

A critical review of test methods and alternative scientific approaches to compliance and safety evaluation of paper and board for food contact

Paper Technology

International® PITA Annual Review

Essential Guide to Aqueous Coating

TYPE Review

PUBLISHED 11July2024

DOI 10.3389/fchem.2024.1397913

Acriticalreviewoftestmethods

OPENACCESS

EDITEDBY EmmanouilTsochatzis, EuropeanFoodSafetyAuthority(EFSA),Italy

REVIEWEDBY RaquelSendon, UniversityofSantiagodeCompostela,Spain FátimaPoças, EscolaSuperiordeBiotecnologia-Universidade CatólicaPortuguesa,Portugal

*CORRESPONDENCE

AngelaStörmer, angela.stoermer@ivv.fraunhofer.de †Theseauthorshavecontributedequallyto thiswork

RECEIVED 08March2024

ACCEPTED 21May2024

PUBLISHED 11July2024

CITATION

StörmerA,HetzelLandFranzR(2024),Acritical reviewoftestmethodsandalternativescientiﬁc approachestocomplianceandsafety evaluationofpaperandboardforfoodcontact. Front.Chem. 12:1397913. doi:10.3389/fchem.2024.1397913

©2024Störmer,HetzelandFranz.Thisisan open-accessarticledistributedundertheterms ofthe CreativeCommonsAttributionLicense (CCBY).Theuse,distributionorreproductionin otherforumsispermitted,providedtheoriginal author(s)andthecopyrightowner(s)are creditedandthattheoriginalpublicationinthis journaliscited,inaccordancewithaccepted academicpractice.Nouse,distributionor reproductionispermittedwhichdoesnot complywiththeseterms.

andalternativescientiﬁc approachestocomplianceand safetyevaluationofpaperand boardforfoodcontact

AngelaStörmer*†,LisaHetzelandRolandFranz† FraunhoferInstituteProcessEngineeringandPackaging,Freising,Germany

Paperandboardarewidelyusedasfoodcontactmaterials.Forsuchsensitive applications,consumersafetyregardingthetransferofchemicalcomponents andcontaminantstothefoodneedstobeestablished.Suchsafetyassessments arebecomingincreasinglychallengingnotonlyduetointentionallyadded substancesbutalsonon-intentionallyaddedsubstances.IntheEuropean Union,compliancetestingandsafetyevaluationofpaperinfoodcontactare largelybasedonnationallegislationandstandards.Theunderlyingtestsare conventionalmethods,oftenoverestimatingandsometimesunderestimatingthe migrationintofood.Inthisarticle,therelevantstandardtestmethodsare contrastedwithcurrentlyavailablescientificknowledge.Thescientific approachestodevelopandidentifysuitabletestmethodsarecritically reviewed.Furthermore,theoreticalpredictionsviamathematicalmodeling, withtheaimtorealisticallysimulatetransfertofood,arepresentedand discussedincomparisonwithavailablemigrationstudieswithfoods. Objectivesareto(i)summarizetheactualscientificknowledgeinthe fieldand drawconclusionsregardingthepotentialandlimitationsoftheexistingtest methodsand(ii)identifyresearchgapstowardabetterqualitativeandquantitative understandingoftransportprocessesofvolatileandnon-volatilesubstances frompaperandboardintofoods.

KEYWORDS

foodcontactmaterials,paperandboard,migration,paperextracts,foodsafety,dry foods,fattyandwettingfoods,migrationmodeling

1Introduction

Paper includingcardboard isincreasinglyusedasfoodcontactmaterial(Technavio, 2023).ThisisbecauseitseemstoalmostperfectlyservetheobjectivesoftheEU sustainabilitystrategy(CommissionEuropean,2024a; CommissionEuropean,2024b), giventhatitisbasedonrenewablerawmaterialsandislargelyrecyclableor compostable.Meanwhile,productionofpaperishighlyenergyandwaterconsumptive (CEPI,2023).Theuseofrecycled ﬁbersreducestheenvironmentalimpact(Deshwaletal., 2019; Wellenreutheretal.,2022).However,withrecycled ﬁbers,unwantedsubstancesare introducedintothematerial.Thesemaybesuchvariedthatitseemsimpossibletocover themwithrelativelysimplemethodsandensureconsumersafety,exceptforapplications

withfunctionalbarrierslikeinnerbags,barriercoatings,orindirect contactforpackinginsensitivefoodslikesalt(Biedermannand Grob,2013; Geuekeetal.,2018).

Incontrasttoplasticfoodcontactmaterials(FCM),whichare subjecttoaspeciﬁcanddetailedEUlegislation(EU,2023)ina systematicandlargelyscience-basedway,paper-basedfoodcontact materialsstilllacksuchdetailedspeciﬁcregulationsataharmonized Europeanlevel(Simoneauetal.,2016).Onemajorreasonisthatfor paper,duetotheinherentstructuralandchemicalcompositional complexity,theknowledgebaseforproperriskassessmentisnotas advancedasitisforplastics.Nevertheless,thesafetyofpaper applicationsinfoodcontactneedstobeensurede.g.,inthe EuropeanUnionaccordingtoArticle3oftheFramework Regulation1935/2004(EU,2004).

Generally,thetransferofsubstancesfrompackagingmaterialsto foodsmustbeevaluatedtoensureconsumersafetyandthisatbest, asrealisticscenarios.However,tocoveralargenumberofpossible foodsas ﬁllinggoodsandavoidanalyticaldifﬁcultieswithcomplex foodmatrices,simulantsandstandardizedcontactconditionsare typicallyused.

Foodcontactcompliancetestingandsafetyevaluationofpaper arestilllargelybasedonstandardmethods,whicharerather conventionaltestproceduresthansimulatingtransfertoreal foods.Asystematicandholisticapproachsimilartoplasticfood contactmaterialsisstillmissingbutwouldbeneededasabasisfor thefutureEUlegislationandframingbetterrulesforthepaper packagingindustry(Lestido-Cardamaetal.,2020; Kourkopoulos etal.,2022).Therequirementstoensureconsumersafetyinfood contactapplicationshavebeenadvancingduringthelastfew decades,especiallywithnon-intentionallyaddedsubstances emergingtotheforefront(Kosteretal.,2015; LeemanandKrul, 2015; EP,2016; CoE,2020; Nerinetal.,2022).Paperfoodcontact materialshavebeenreportedinamultitudeofscientiﬁcarticlesas potentialsourcesforreleasing/migratingchemicalcontaminantsof knownandunknownidentityandtoxicityintothefoodswhenin contactwiththem.Inparticular,foodcontactmaterialshaving recycledpaperqualitiesraiseconcerns(Jickellsetal.,2005; Sturaroetal.,2006; Begleyetal.,2008; Zhangetal.,2008; Gärtneretal.,2009; Vollmeretal.,2011; EFSA,2012a; Pivnenko etal.,2015; Canavaretal.,2018; Deshwaletal.,2019; Conchione etal.,2020; Zabaletaetal.,2020; Panetal.,2021).

Substanceswithonlyscarceornotoxicologicaldatacanbe evaluatedassafeonlyatlowmigrationlimits,e.g.,byapplying ThresholdofToxicologicalConcern(EFSA,2012b; CoE,2020).Test methods whichhighlyoverestimaterealmigrationinto foods maytriggereitherprematurenegativeevaluationsand unjustiﬁednon-complianceassessmentsofpapermaterialsorthe needforelaborateandcostlymigrationtestsincontactwith representativeorworst-casefoodstuffsthemselves(“food prevails”).Thisriskusuallyincreaseswithdecreasing migrationlimits.

Overall,thereisahighnumberandvariabilityofapplicabletests andevaluationmethodsinEurope.Thereisalotofdiscussioninthe scientiﬁcliterature,whichmethodstoapplyforvariouspurposes, andtherelateduncertaintiesandinterpretationgaps.

Theobjectiveofthisarticleisto(i)provideanoverviewof available,legallybinding,andnormativetestprocedures,including guidancedocumentsinEurope;(ii)compilealternativescientiﬁc

approachespublishedinthescientiﬁcliterature;and(iii)explainand criticallydiscussthepresentedtestmethodsandapproaches concerningtheirobjectives,potentials,andlimitations.The overarchingintentionistoidentifyandhighlighttheresulting conclusionsforfutureresearchtowardabetterharmonizedruleandscience-basedevaluationschemeforpaper-basedfood contactmaterials.

Thispaperfocusesonsimulatingthetransferprocessestofood. Workaboutmethodstodetectandidentifypossiblymigrating substancesaswellasbioassays,althoughbelongingtotheareaof newalternativeapproaches,areintentionallyexcludedfrom thisreview.

2Ashortviewonthediversityofpaper foodcontactapplications

Paperiscommonlyusedasapackagingmaterialforawiderange offoodproducts,includingdrygoods(e.g., flour,cereals,snacks,and pasta),bakedgoods(e.g.,breadandpastries),andfreshproduce (e.g.,fruitsandvegetables).Importantproductsforfoodprotection duringtransportandstoragearebags,boxes,andtrays.Forhightemperatureapplications,e.g.,baking,parchmentpaperorbaking cupsformuffinsareusedtopreventfoodfromstickingtosurfacesin commercialandhomebakingsettingsandteabagsorcoffee filters forhotaqueouscontact.Infoodserviceareaswithinthecommercial andresidentialsettings,disposabletableware suchascups,plates, bowls,napkins,andstraws ismadefrompaper.Otherapplications comprisewrappingmaterials includingwaxedpaper foravariety offoodproducts,suchasmeatproductsandsandwiches.Papercan beusedasitisorcoatedwithavarietyofbarriermaterialstoprevent moisture,oxygen,andaromacompoundsfromenteringorleaving thepackageandimprovegreaseresistance.Barriersmightbe coextruded films(e.g.,polyethylene),lacquers,orcoatingsfrom petrochemicalorbiobasedsources.Finally,labelsandtags applied tofoodpackagesorfoodproductsforidentificationorpromotional purposes areoftenbasedonpaper.Acomprehensiveoverviewof papercategoriesandpaper-basedfoodpackagingmaterialscanbe foundintheliterature(Simoneauetal.,2016; Deshwaletal.,2019). Inconclusion,theuseofpaperishighlydiverse,coveringawide rangeofproductsandfoodcontactapplications.Thepapertypes anddesignsdependontheparticularapplicationneedsandthe relevantregulatorystandards.Demonstratingorprovingthe chemicalsafetyofsuchlargevarietyofapplicationsposea seriouschallengetoindustrial,contract,andcontrollaboratories, particularlywhenrecycled fiberscarryingpotentiallynumerous chemicalcontaminantsenterproductionlines.Undoubtedly,this situationindicatesaneedforbetterscience-basedmethodological supporttowardthesafetyassessmentofpaperfoodcontact materials,asindicatedby Grob(2022)

3EUlegislationof ﬁber-basedfood contactmaterialsandrelated(supra) nationalprovisions

Aswithanyotherfoodcontactmaterial,paperforfoodcontact issubjecttotheoverarchingEuropeanFrameworkRegulation(EC)

No.1935/2004(EU,2004)andtheGoodManufacturingRegulation (EC)No.2023/2006(EU,2006).Inshort,theseregulationslaydown generalprinciplestoensurethatanyfoodcontactmaterialissafefor theconsumerandmanufacturedunderquality-controlled conditions.PapermaterialsarelistedinAnnex1of1935/2004as materials,whichmaybecoveredbyspecificEUmeasuresbutarenot harmonized.Therefore,Article7ofthatRegulationforeseesthat nationalprovisionscanbemaintainedoradoptedbyMemberStates intheabsenceofEU-specificmeasures.Atotalof10MemberStates (Belgium,CzechRepublic,Estonia,Greece,Germany,France, Croatia,Italy,theNetherlands,andSlovakia)havesetouttheir provisionsandsafetycriteriainnationalregulationsor recommendations.Thelatter,suchassetoutbytheGermanBfR, arelegallynotbindingbuthavealmostthestrengthoflegislationby theforceofthemarket.Asummary includingthelimitsfor specificsubstances isgiveninAnnex16of Simoneauetal. (2016).Thereisahighvarietyofrules,requirements,and substancelistswithonlylittlecongruencebetweentheMember States(Simoneauetal.,2016).TheCouncilofEuropehasrecently releasedthegeneralresolutionCM/Res(2020)9onthesafetyand qualityofmaterialsandarticlesincontactwithfood(CoE,2020), whichisapplicabletonon-harmonizedmaterialsinEU,giving detailedbutnotlegallybindingrequirementsregardingconsumer safety.Aspecifictechnicalguideonpaperandboard(CoE,2021) supplementsthisgeneralresolution.AnnexIIgivessome restrictionstospecificsubstancesoccurringwithinpaper. ComprehensiveanddetailedoverviewsofthepaperEU regulatorysituationalongwithextensivelistsofnational provisionsandmeasurescanbefoundinthebaselinestudy (Simoneauetal.,2016)andinreviewarticlesby Kourkopoulos etal.(2022) and Oldringetal.(2023).Amatterofconcerninthelast decadewasthepresenceofmineraloilcomponents,especially aromatichydrocarbons(MOAH)inpaperswithrecycled fibers, orfromprintinginksonfoodcontactmaterials.TheGerman FederalMinistryofFood,AgricultureandConsumerProtection (BMEL)proposedspecificmigrationlimitsforMOAHtobe undetectableinfoodandfoodsimulantsatdetectionlimitsof 0.5and0.15mg/kgfoodsimulant.Therespectivenationaldecree wasnotsetintoforcependingafutureEuropeanmeasure(BMEL, 2022b;a).Limitsformineraloilcomponentsinfoodarestill discussedinEU(EC,2023).

4Normativeframeworkofstandard testmethodsandguidelines

SupportingandenforcingtheEuropeanandnationallegislation forpaper,morethan20standardtestmethodsandproceduresare availableonEuropeanandnationallevels(Simoneauetal.,2016; CoE,2021; Oldringetal.,2023).Thesestandardscoverarangeof testprinciples(determinationofresidualcontent,extraction,and migration),speciﬁctargetsubstancesorsubstancegroups(suchas bisphenolA,anthraquinone,andphthalates),andotherpaper materialparameters,includingorganoleptictesting,fastnessof colors,oropticalbrighteners.

Testingthetransferofsubstancesfrompaperincludesthestepof transfer(migration,gasphasetransfer,orextraction)andaspeciﬁc analyticalmethodforthetargetsubstances(BfR,2015).For

simulatingthetransfer,conventionalproceduresareusually applied,whichshallcovertheworstcase(BfR,2015).Thefour mostimportantandrelevantbasictestproceduresarecoldwater extract,hotwaterextract,organicsolventextract,andmigration testingusingmodifiedpolyphenyleneoxide(MPPO,poly2,6diphenyl-p-phenyleneoxide,e.g.,Tenax®)asasimulant.All proceduresarepublishedasEuropeanStandards.Theextractsare carriedoutunderdefinedconditions(sampleweight,water/solvent volume,andcontacttime/temperature; Table1).GermanBfR recommendedslightmodificationstoincreasetheintra-and inter-laboratoryreproducibilityofthewaterextracts.

Coldandhotwaterextractsareconsideredtosimulatedirect aqueousfoodcontact;thecoldwaterextractrepresentsaqueous foodsandbeveragesatallapplicationsexcepthotandbaking applications.Forthesetwoapplications,thehotwaterextractis usedforwater-solubleandhydrophilicsubstances.Theorganic solventextractsimulatescontactwithfattyfoodsbyusing95% ethanolandisooctaneassolvents.Theseextractionsarecarriedout withcutsamples.

Onthecontrary,theMPPOtestisamigrationtest,inwhich theadsorbentMPPOisspreadonthefoodcontactsurface.MPPO simulatescontactwithdryfoodsandathightemperatureswithall foods(microwaveandbakingapplications).Thetestconditions areusuallytakenfromAnnexVofthePlasticsRegulation(EU)10/ 2011( EU,2011).TheEURL-FCMguidelinefortestingconditions ofkitchenwarestatesexamplesforavarietyofapplications( Beldi etal.,2023).Forhigh-temperatureapplications(microwaveand oven),therecommendedtemperaturesvary.EN14338givesa maximumtemperatureof175 °Cforthetestbutdoesnotgive adviceontheselectionoftestconditions.CoETechnicalGuide proposes2hat175 °Cforovenapplicationsand30minat150°Cfor microwave( CoE,2021 ).EURL-FCMguideline preparedbythe EUreferenceandnationalsurveillancelaboratories recommends conditionsupto2hat200°C,dependingontheapplicationinthe ovenand30minat121°Cforwarmingupordefrostingorat175°C forcookinginthemicrowave(Beldietal.,2023).Accordingto GermanBfRatcontactof2hat220 °Cnodegradationforbaking papersshouldoccur,and30minat150°Cformicrowave applicationsshouldbeapplied(BfR,2015 ).EURL-FCM guidelinedistinguishesbetweencoatedortreatedpaperarticles, whichdonotabsorbmoistureoroilandwithstandmigrationtests basedontheconditionsfromRegulation10/2011,andotherpaper articles.Fortheformer,theconditionsforplasticmaterialsare givenintable5Aoftheguideline,andforthelatter,theextraction andMPPOtestsasdescribedabove(table5B,there).Plastic migrationprocesseshavecompletely differentcharacteristics comparedwithpaper.Consequently,thecontactconditionsare notnecessarilyapplicabletopaperasdiscussedindepthbelowin Section5 .Inadditiontothesetests,CouncilofEuropeTechnical Guide( CoE,2021 )recommendsusing3%aceticacidfor estimatingthereleaseofmetalsintoacidicfoods.Generally,for thecontacttestconditions,theguidereferstoEURLFCMguideline.

Theresultsofcoldandhotwaterextractsinmg/Lareconsidered conventionallyequivalenttomigrationinmg/kgfood(CoE,2021). CoETechnicalGuidestatesthattherealratioofsurfaceareatothe amountoffoodmustbeusedorthemaximumallowablesurface-tofoodratioshouldbedeclared.Bycontrast,theEURL-FCMguideline

TABLE1RelevantEuropeanStandardsfortheextractionormigrationtestingofpaper Standard

EN645:1993ColdwaterextractSample,cutorrippedinpieces,extractedwithwater10g/200mLat 23 C±2 Cfor24h,shakingoccasionally;the ﬁltrate(ﬁlledupto 250mL)usedforanalysis CEN (1993a)

Modiﬁcation byBfR

Samplecut(notripped),shakingnotnecessary,vacuum filtrationwitha glass fiber filter(1.2μm)insteadofglassdrip(10–16μm),andpressing ofthe filtercakeincaseofhighwaterabsorption BfR (2022a)

EN647:1993HotwaterextractExtractedwithwaterat80°C±2°Cfor2h.Theprocedurewassimilarto thatofthecoldwaterextract CEN (1993b)

Modiﬁcation byBfR

Similartothatforthecoldwaterextractbutnocommentonshaking BfR (2022b)

EN15519:2007OrganicsolventextractSamplecut,extractionwithethanolorisooctane10g/200mLcontact timeandtemperaturedependingonapplication2hor24h/20 C(shortorlong-termcontact)and2h/60°C(forhotcontact),shaking occasionally; ﬁltrate(ﬁlledupto250mL)usedforanalysis

EN14338:2003Migrationusingmodiﬁedpolyphenyleneoxide(MPPO)asthe simulant

CEN/TS14234: 2002 Polymericcoatingsonpaperandboard guidetotheselectionof conditionsandtestmethodsforoverallmigration

referstoArticle17ofPlasticsRegulation10/2011andthe exemptionsforsmall(<500gormL)andverylargepackages (>10kgorL)forwhichtheconventionalratioof6dm2/kgis used.Thisconceptcanalsobeappliedtopaper(Beldietal.,2023). Thus,coldandhotwaterextractresultsarerecalculatedtothe surfaceareaofthesampleandthe ﬁlling.Thenewer(oralteratively: morerecent)organicsolventstandardEN15519:2007already requiresreportingtheresultsrelatedtothesurface.

Contrarytotheirname,thewaterandsolventextraction methodsarenotnecessarilyexhaustive(BfR,2015).The extractivepowerwilldependonthetypeandnatureofthetarget substance(organic-polar,organic-unpolar,orinorganic),its physical–chemicalproperties,anditsinteractionswiththepaper sample.Thename “extraction,” whichisusuallyreservedfor exhaustivemethods,maycauseconfusion(Oldringetal.,2023). Takingthesestandardstodepict “worstcase” migrationintofood (BfR,2015),itneedstobestatedthattheseconventionalmethods mightnotmatchtherealconditionsofuse.Confederationof EuropeanPaperIndustries(CEPI)seeshotwaterextracttesting asaclosecopyoftheintended ﬁnaluseandotherextractiontestsas mostlyoverestimating(CEPI,2019/2021).AlthoughCoETechnical Guidereferssolelytothesestandardtests,German BfR(2015) recognizesthispointandrecommendsatieredapproachusing representativerealfoodsincaseofdoubtorknown overestimationorunderestimation.Testingresultsinfoodhave priorityforthefoodregulatoryassessment.Forplastics,general testingrequirementsshallcoverworstcaseoffoodcontact applicationsandbeevenmoresevere,asspeciﬁedinAnnexesIII andVofEURegulation10/2011.Similarly,themethodsforpaper areintendedtosimulatetransferintorealfoodsconservativelyand areslightlyoverestimating.However,forplastics,tooseveretestscan showconformitybutnotdisapprovethem.Thisisthecasefor alternativemethods theso-calledscreeningmethods butmight alsobefor(overestimating)conventionaltestingresults.EU

AdsorbentMPPO(Tenax®)isspreadonthesurfaceofthesample(4g/ dm2),contactconditionsaccordingtoapplication,andsolvent extractionofMPPO

Rapidextractionmethodswithisooctaneand/or95%ethanol24hat 40°Cor50°Cdependingonthepolymerofthefoodcontactlayer;for polymerlayerthicknessesupto300μm

CEN (2007)

CEN (2003)

CEN (2002b)

Regulation10/2011initsArticle18statesthat “theresultsof speciﬁcmigrationtestingobtainedinfoodshallprevailoverthe resultsobtainedinfoodsimulant.” Thismeanssimulationor predictionofmigrationshoulddepictthesituationwithfoodas closelyaspossible,ideallymatchingwiththeupperboundmarginof thedeterminationinthefood.

Theconventionalassumptionthattheextractionvaluescorrelateto migrationintofoodholdsanon-negligibleconflictpotentialas migrationintofoodunderrealisticconditionsmaydifferfromthe extractresults.Thisbecomesobviousforcutsamples,whichpartially disintegrateduringextraction.However,thediscrepancymayoccurin bothdirections:overestimationandunderestimation.Theexampleof perfluorocompounds whichdonot fitintothesimulantschemeasa worsecase(Begleyetal.,2008) isgiveninBfRguideline(BfR,2015). Merkeletal.(2018) comparedthemigrationofprimaryaromatic amines(PAAs)fromthreepapernapkinsintofourdifferentfood categories(wet,dry,acidic,andfatty)withthecoldwaterextractresults. Inthefoodcategorypickledgherkins(aqueous–acidic),coldwater seemedtobeinsevenofninetestcasessufficientlyrepresentativeor evenoverestimatingwithameasuredtransferintofood(expressedas% ofextract)rangingfrom62%to115%,dependingmainlyonthespecific amines,butwasseverelyunderestimativeintwocaseswith224%and 271%(PAAinbothcases:aniline).Significantlyless,orevenno migrationcouldbefoundintorice(dry),buttercookies(fatty),and cucumber(wet),respectively,withtransferrangingfrom2%to79%and manynon-detects.Particularlystrikingisthedifferencebetweenthe resultsofcoldwaterextractandbuttercookies(fattyfoods):onlyin threeofninetestcases,PAAsweredetectableinthefoodand,inwhich measurabletransferwasrangingfrom2%to43%ofcoldwaterextract value,i.e.,withextremeoverestimationbytheextract.Coldwater extractisthesamplepreparationmethodproposedintherecently publishedstandardEN17163:2019(CEN,2019)fortestingPAAs. Fortheorganicsolventextract,thequestionarisesastowhether thesurface-relatedsolventextractionvaluescanbedirectly

understoodasrepresentativesofthefattyfoodstobesimulated. Lestido-Cardamaetal.(2020) showedthatthesolventextractcould bestronglyoverestimatingforlipophilicsubstancegroupslike dialkylketones.Solventextractsinisooctaneanddichloromethane werecomparedwithmigrationintotwovegetableoilsandthreefatty foods(croissant,salami,andtwokindsofcheese)underdifferent conditions.Themigrationintothesolventsandoilsexceededthe migrationintotherealfoodsbyroughlyfourordersofmagnitude, suggestingthatthesolventandoilsarefarmoreextractivethanthe realfoods.Itseemsthatsolventextractsconstituteanappropriate andcorrecttooltodeterminethemigrationpotentialofsubstances frompapersbuttheobtainedresultsinmasspersurfaceunit cannotbetakenasdirectmeasuresforconsideringrealtransfer tofood.However,thispracticeisstillapplied.The dialkylketones forwhichGermanBfRhassetamigrationlimit of5mg/kgfood areusuallydirectlycontrolledonthebasisofthe organicsolventextract.

Fromtheindustryside,CEPIreleasedanupdatedguideline regardingcomplianceworkforfoodcontactpapermaterials(CEPI, 2019/2021).Theguidelineisaddressedtoallparticipantsinthe manufacturingchainaswellastotheconsumersandregulators. CEPIguidelinereferstothecurrentstandardproceduresfortesting, includingcoldwater,hotwater,andsolventextractsaswellasmigration intoMPPO.Itgivesrecommendationsonhowtohandlelimitationsof thesestandards,likethepossibleoverestimationofmigrationthrough extracts,e.g.,casesinwhichcertainsolvent–materialcombinations couldfalselyleadtofailingresults.Thesolutionwillbeacase-by-case approach,focusingontheriskassessmentoftheusedrawmaterialsin thecontextoftheintendeduseofthematerial.

Scientiﬁcguidancefortheoreticallyassessing,measuring,and estimatingthetransferofmineraloilcomponentswaspublishedby GermanFederationforFoodLawandFoodScience(Gruberetal.,2019).

5Scientiﬁcstudiesandalternative approachesincluding migrationmodeling

5.1Introductoryremarksanddescriptionof thekeychallenges

Thetransferofamigrantfromapackagingmaterialisdetermined byitsmobility/speed(diffusionrate)insidethematerialand,inthecase ofsemisolidandsolidfoods,additionallyinsidethefood.Thesecond mainparameteristhepartitioningbetweenthepackagingmaterialand thefood,inthecaseofseverallayers,additionallybetweenthelayers. Forcompliancetestingofplastics,inmostcases,liquidsimulantsare usedwhichshallroughlyrepresentthesolubilitypropertiesofthefood forthemigrants.Concerningdiffusionandtheuseofsimulants,plastics appearasasimplermatrixcomparedwithpaper.Thesimulantliquids usuallydonotpenetratetheplasticmatrixsoakineticmigrationtest willmonitorthetime-dependentdevelopmentofthemigrant’stransfer process,ideallyinawaythatiscomparabletotheprocessesoccurringin contactwithfood.NormallymigrationfollowsFickiansecondlaw. Therefore,fromkineticdatausingcurve fitting,thephysico-chemical keyparameters;thediffusioncoefficient,DP,inpolymer;andthe partitioncoefficientpolymer-food,KP/F,canbederived.Bothare fundamentalformigrationpredictionandmodeling(Mercea,2008).

Incontrasttoplastics,theliquidsimulantsusedinthe conventionaltestmethodsinthepapersector,i.e.,coldorhot waterorsolventssuchasisooctaneand95%ethanol,penetrate uncoatedandevenoftencoatedpapertestsamples,thusheavily impairingtheirfunctionalconsistency.Thiscancausephysical disintegrationofthepaper ﬁbernetwork.Therefore,theobtained valuesareratherresultsofanextractionprocessandnotofa migrationmechanismintofood.Consequently,thisleadsin manyorevenmostcasestooverestimationsbecausethesomeasuredvaluesrepresentanequilibriumbetweentheused (extraction)solventandthepapermaterial.Thisdiffersfromreal foodcontactapplications,inwhichtheequilibriumisnotreached duringthecontacttime.Migrationcanbesloweddownbythe diffusionpropertiesofthefooditself,especiallyinthecaseof semisolidorsolidfoods.Additionally,thesolubilityofthe migrantsinrealfoodmaybelowerthaninsimulants, i.e.,partitioningismoreonthefoodcontactmaterialside. However,testsinfoodcannotbeseenasanalternativeto routinetestingastheyarenotpossibleforallsubstancesand may ﬁndtheirlimitationsintheanalyticalfeasibilityforusually verycomplexfoodmatricesoreveninthechoiceofrepresentative foodsfortheintendedapplications.Awayoutoratleasta supportingtoolisthedevelopmentanduseofpredictive mathematicalmigrationmodels,whichneedtobedesignedsuch thattheydepictrealityascloselyaspossible.

Numerousscienti fi cpapersaddressedthedevelopmentof analyticalmethodsforkeymigrantsandcontaminantsin paperforfoodcontactaswellasalternativemigrationtest procedures.Thelatteraimstode fi necrucialphysico-chemical parametersformasstransferfrompaperandto fi ndrelationships betweenpapermaterialproperties,migrants,testconditions,and migrationlevels.Inthefollowingsections,anoverviewwillbe givenofpublishedexperimentalandtheoreticalscienti fi c approachestowardabetterunderstandingandevaluationof masstransferfrompaperfoodcontactmaterialsintofoods andtheirsafetyinuse.Thisoverviewisnotquantitatively exhaustivebutclaimstoaddressthemostrelevant publicationswithregardtotheintentionofthisarticle.The conclusionsofthestudiesmaydependontheconsidered migratingsubstancesandtheirproperties,e.g.,volatility.The substancesusedintherespectivepublicationsarecompiledinthe SupplementaryTableS1

5.2Experimentaltestapproachesand key ﬁndings

Thescientiﬁceffortsinthepublishedliteraturearelargelyvaried andhavemanifolddetails.Inprinciple,theycanbedividedinto threemajorresearchdirectionswiththefollowingobjectives:

(i)Exploringanddevelopingalternativefoodsimulantssuitable tomimicfoodincontactwithpaper;

(ii)Comparisonofmigrationresultsobtainedfromsimulated migrationtestingwithrealisticmigrationlevelsinfoods themselves;

(iii)Deriving/deﬁningmigrationtestconditionstobetter simulatefoodcontact.

5.2.1Migrationintodryfoods

Migrationintodryfoodshasbeenatopicformostofthese scientificpublicationsbecausecontactwithdryfoodsrepresentsa majorapplicationofpaperinfoodcontact. UrbelisandCooper (2021) publishedacomprehensivesummaryreviewof162studies thatexaminedmigrationintodryfoodsanditssimulants,an importantdatasource.Thefocusofthisstudywastheextentof migrationintodryfoodsandnotspecificallyfrompaperpackaging butanytypeoffoodcontactmaterial.However,thelargestpartis paperrelevant.Thismightcorrelatewiththefactthatmost packagingfordryfoodsispaper.Thisreviewdealswiththe analysisofmarketfoodproducts,aswellasoffoodproductsand foodsimulantsincontactwithfoodcontactmaterial,after experimentalfortificationwithknownquantitiesofamigrant. Thediscussionontesting,informationgaps,andremaining questionscoincideswiththisreviewbutisdonefromadifferent perspective.

5.2.2MPPOasasimulantfordryfood

MPPOistheofficialEUfoodsimulant(simulantE)fordry foodsintheEUPlasticsRegulation(EU,2023).Thedevelopment anduseofMPPOasasimulantfordryfoodsgobacktothe1990s. Piringeretal.(1993) publishedthemethodforthe firsttimeasa convenientapproachfordeterminingtheoverallmigrationathigh temperaturessuchasfromnon-stickcoatingsonfryingpansand bakingpapers.Themethodwasalsoapplicabletothedetermination ofspecificmigrationoforganicsubstancesfromadhesivesinpaper foodcontactmaterial(GrunerandPiringer,1999).Sincethen,the so-calledTenaxmethodhasbeenincreasinglyusedandhasreached aEurope-wideofficialstatuswithCENEN1186:2002-Part13(CEN, 2002a)fortheoverallmigrationtestingofplasticfoodcontact materialathightemperature(>100°C)andtheimplementationof MPPOasfoodsimulantEfordryfoodsinEURegulation10/2011 (EU,2023).Inparallel,themethodwasstandardizedforpaperfood contactmaterialtesting(CEN,2003).

TheuseandappropriatenessofMPPOasasimulantfordry foodswerereviewedby VanDenHouweetal.(2018).Themajority ofcollecteddataandcitedreferencesrelatetopapermaterials.The performanceofMPPOasafoodsimulantfordryfoodsisdiscussed basedoncomparisonswithrealfoodsandotherfood-simulating adsorbents.MPPOsimulationcomparedwiththerealmigration conditionsintoseveralfoodstuffs(suchassugar, ﬂour,cakeand certainpastries,semolina,instantbabyformula,milkpowder,rice, salt,cerealsandevenmeat,chocolate,sweetmatrices,freshfruits, andvegetables)showednounderestimationsoftherealconditions. VanDenHouweconcludedthatMPPOissuitableasasimulantfor dryfoodstuffs.Moreover,intheirview,theonlysuitablesimulantfor thesimulationofdryfoodstuffs.

5.2.3MPPOsimulantversusdryfoods:recycling componentsotherthanmineraloil

In1999–2002,anearlyandessentialscientiﬁcinitiativewas takenbyEUProjectFAIRCT98–4318 “Recyclability” withinits Section2 “PaperandBoard” (RaffaelandSimoneau,2002; Castle andFranz,2003).Migrationkineticsfrom15differentpapersample typeswerecomparativelyinvestigatedbetweendryfoods(cookies, ﬂour,milkpowder,noodles,salt,semolina,souppowder,sugar,and icingsugar)andMPPOattemperaturesrangingfromroom

temperatureupto100°C.Aselectionof12surrogates[suchas acetophenone,diphenylether,diisobutylphthalate(DiBP),and diisopropylnaphthalene(DIPN)isomers; SupplementaryTable S1]ofdifferentchemicalstructuresandvolatilitieswasspiked intothepapersamplesbeforetesting.Furthermore,partition coefficientsbetweenpaperandfood(simulant)asacrucialmass transferparameterformigrationmodelingpurposeswere determined.Transferfromthepaperrapidlyreachedits equilibrium,dependingonsubstanceandtemperature,e.g.,after 1hat100°Cand2–10daysat23°C.MigrationtoMPPOwasin almostallcaseshigherthanintofoods.The finalpartitionining coefficientbetweenpaperandMPPOsimulantwasalwaysatleast oneorderofmagnitudelessthantheonebetweenpaperand foodstuff.Thus,MPPOwasfoundtobemoreseverethanfood concerningtheadsorption/uptakeofmigrantsfrompaper; therefore,itwasconsideredtobeagoodsimulantfordryfoods. Diisopropylnaphthalene,intrinsicallypresentintherecycledboard, displayedadifferentkineticbehaviorcomparedwiththespiked-in experiments,whichwasexplainedby “native” DIPNbeingpresent intheencapsulatedform(fromtherecyclingofcarbonlesscopy papers).Thisexplanationisnotnecessarilycorrectbecauseothers founddifferencesbetween “native” andfortifiedsubstancesalsofor othercomponentslikephthalates(ZülchandPiringer,2010; Bradley etal.,2015).

Aurelaetal.(1999) comparedthereleaseoftwophthalates (diisobutylphthalateanddibutylphthalate)frompaperpackagesinto (crystal)sugarwiththoseintoMPPO.Themeasuredmigrationwas similarinsugarstoredfor4monthsatroomtemperatureandin MPPOstoredfor10daysat40°Cor2hat70°C.Alkylbenzenes (C10–C13alkylchain)at30minat70°Cshowedanoverestimation factorof3.8inMPPOcomparedwithhamburgerrollsunderthe samecondition(Aurelaetal.,2002).

Baeleetal.(2020) observedastrongoverestimationforvolatile substances(1,3,5-tri-tert-butylbenzene,n-hexadecane,and n-heptadecane)byMPPO(indirectcontact,22°C,16weeks)in comparisontostarchy,low-fatfoods(noodleswithandwithout eggs,wheat,andricesemolina)butsimilarmigrationasinto chocolate.Thedifferencedecreasedwithdecreasingvolatilityof themigrants. SummerfieldandCooper(2001) comparedthe migrationofdibutylphthalate,diisobutylphthalate,and diisopropylnaphthalenefromrecycledboardintovariousdry foodsandMPPO.Inidenticalconditions(10days/40°C), migrationintoMPPOwassimilarorevenlowerthanmigration intoricebutsimilar(diisobutylphthalate)ordistinctlyhigher (diisopropylnaphthalene)whenricewerestoredfor6monthsat 20°C.Diisopropylnaphthalenemigratedinsimilarorevenhigher amountsinMPPOthanin flourandpastryat40°C.Migrationin flour storedfor6monthsat20°C wassimilartothatafter10days at40°C.

5.2.4Mineraloilcomponents(MOSHandMOAH)in marketsamples

Aseriesofpublications(2010–2016)dealtwiththemigrationof mineraloilhydrocarbons(MOH,saturated:MOSH,aromatic hydrocarbons:MOAH)frompaperfoodpackagingintofoods. Themajorfocuswasondryfoodsandtheunderlying mechanisms.Theauthorshipvariedbutcenteredaroundthe CantonalFoodControlLabofZurich.Thestartingpointwas

SwissandItalianmarketsurveys,showingthepresenceof considerableamountsofMOHinfoodboxes,whichbecauseof thepresenceofalargefractionofvolatileMOH,gaverisetosafety concernsowingtotheirpotentialtogasphasetransferintothe packedfoods(Lorenzinietal.,2010).Thiswasconfirmedbya follow-upGermanmarketsurveyof119samplesofdryfoods,such ascereals,biscuits,andrice,packedinprintedpaperboard boxes withandwithoutinternalbags andintendedforlonger storageatambienttemperature.Inthissurvey,predominantly saturatedhydrocarbons(MOSH),uptoapproximatelyC24,were foundindryfoods(Vollmeretal.,2011),showingthatsubstances withboilingpointsuptoapproximately400°Ccanbetransferredvia gasphaseatambienttemperaturesfrompaperintodryfoods.This fitswiththe findingsofJickellsetal.regardingtransferfrom secondarypackagingusingpolarcontaminants(Jickellsetal., 2005).EvenuptoC28isexpectedtobetransferredintodry foodlikericeinlong-termstorage(BiedermannandGrob,2010; 2012).Themeasurementsofreplicatesfromthesamesamplesofthe surveywererepeatedafterfurther4monthsandanother12months ofstoragetime(Biedermannetal.,2013a).Migrationincreasedfrom firstmeasurementtothirdmeasurement(onaverageby60%); however,morethanhalfofthetransferwasalreadyfoundinthe firstfewmonths.Exceptfortablesalt(non-adsorptivefoodmatrix) andnoodles(lowadsorptive;notspecifiedbutmostprobably withouteggs),themigrationrangedbetween40%and84%ofthe potential,after16monthsofstoragewithsemolina,asthemost potentadsorbent.Thedifferencesbetweenthefoodtypeswere consideredmodest.Differenceswererelatedmoretothe packagingmaterialsthantothefoods.

5.2.5Migrationstudieswithmineraloil componentsundercontrolledconditions

Theabove-summarizedstudieswerecarriedoutwithmarket samplesfromtheGermansurvey,inwhichstoragetimesand temperaturesbeforepurchasingwerenotknown(Biedermann etal.,2013a).Additionalstudieswereperformedwithcontactto thefoodinthelaboratoryorsamplestakendirectlyfromtheline after ﬁllingundercontrolledconditions.

Dimaetal.(2011) exploredpossibilitiesforadequatetestingof paperpartyplates,whicharecoveredbyathinpolyethyleneor polypropylenelayertomakethemresistliquidsfromfoods. Althoughthereisnodirectfoodcontactwithpaper,including thisstudyhereisworthwhilebecauseoftheevaluationapproach. Furthermore,thethinpolymerlayersdonotactasfunctional barriersagainstorganicmolecules.Theauthorscompared migrationofthesumofMOSHandpolyoleﬁnoligomers (POSH)ofthecoatingfrom16partyplatesusinganedibleoilas asimulantat70°C,withmigrationintoavarietyoffattyfoodsunder foreseeablecontactconditionsrangingfrom60minto1dayatroom temperatureandforsomefoodswithprecedinghotcontact.The latterwasthecaseforahotmeatloaf,whichwasfreshlyfried,placed for1hontheplate,andcooleddowntoroomtemperature.Fromthe kineticmeasurementsat70°Cincontactwithoilover120min,the timepointof30minwasfoundtoreasonablycovertheworstcase determinedinfoods.Forsubstancesotherthanmineraloil,edibleoil isacomplicatedanalyticalmatrixinmanycasestomeasure migrantsatlowlevelsandduetopenetrationintopapernot suitablefornon-coatedmaterials.

Inthreeotherstudiesofthisauthorconsortium,themigrationof MOSHfrompaperfoodcontactmaterialintodryfoods,suchas noodles,rice,andmuesli,wasinvestigated(BiedermannandGrob, 2012; Biedermannetal.,2013b; Lorenzinietal.,2013).Animportant aimwastobetterunderstandthetransfermechanismsandthe influenceofthetypeandnatureofdryfoodsonmigration.Kinetic studiesintoeggpastaandmuesliat fivedifferenttemperatures(4°C, 20°C,30°C,40°C,and60°C)upto400dayswereperformed (Lorenzinietal.,2013).Bothfoodtypesshowedquitesimilar migrationcurveswithverysteepincreasesat60°Candwithslow migrationratesatthelowtemperatures.However,migrationwas increasingevenafter300and400days.Itappearedthatallmigration pointsarelikelytoapproachthesameorsimilarmigrationlevelsin foodsataninfinitetime.

Inthefollowingstudy(Zurﬂuhetal.,2013), “conventional” migrationtestingofarecycledpapertosimulatelong-termstorage atambienttemperaturewasstudiedusingMPPO(simulantE). “Conventional” testingwasunderstoodtoapplytestconditions fromEURegulation10/2011(forplastics),i.e.,10daysat60°C; however,10daysat40°Cwerealsoapplied.Inaddition,polenta (maizesemolina)wasusedatthesameconditionsasMPPO.The resultswerecomparedwiththemigrationintotestfoods(biscuits containing18%fat,polenta,noodles,rice,breadcrumbs,and oatmeal)incontactwiththesamepaperpackagingmaterial storedformorethan9months(sameexperimentas Biedermann etal.,2013b).SimulationwithMPPOafter10daysat60°Cledto almostfull “extraction” ofmigratableMOSH(i.e.,uptoC24), overestimatingthemaximummigrationofMOSHinthereal packsby73%.Tendaysofcontactwithpolentaat60°Cgavea similarmigrationofMOSHastheaverageofthetestedfoods.At 40°C,10daysofcontactwithpolentaunderestimatedtheaverage migrationinthetestedfoods.Increasingthetemperaturenotonly acceleratedthemigrationofgivensubstancesbutalsobroadenedthe rangeofmigratingsubstancesinthedirectionoflowervolatiles.The authorsconcludedthatsimulationwithMPPOwastoo overestimativebecauseoftheadsorbentandtheaccelerated conditionsoftesting.Therefore,MPPOfailedintestingthe migrationofmineraloilfrompaperboardpackaging.Theauthors questionedthesuitabilityofsuchsimulationforthepredictionof long-termmigrationandproposeddeterminationinpaperby deﬁningconventionaltransferratestofood(70%–80%).

Thefooddataundercontrolledconditionsindirectpaper contactcomparedwithindirectcontact(behindpolyolefinlayers) wereseparatelypublished(Biedermannetal.,2013b).Inadditionto MOSH,specificsubstanceslikeDIPN,phthalatessuchasDiBP,and severalphotoinitiators,e.g.,benzophenone,weremeasuredastarget migrants.Thisisusefultolearnmoreaboutthetransport mechanismsfrompaper.Forthisreview,wereferonlytothe resultsobtainedfromdirectpaperfoodcontact.Thefoods (chocobiscuits,polenta,noodles,rice,breadcrumbs,andoatmeal) werestoredfor9monthsatambienttemperaturewithin-between measurementsat2and4months.Forthelevelofmigration,there wasnoseveredependencyonthefoodtype(mostly <2factor), particularlywhileconsideringsinglespecificsubstancesratherthan thewholegroupofMOSH.Thefastestandhighestmigrationwas shownintooatmeal;however,forthearomaticcompounds,MOAH andDIPNoatmealandbiscuitsweresimilar.After9months,forall sixfoods,themigrationrangedforMOSH <C24groupfrom50%to

80%oftheinitialconcentrationinpaper;however,forspecific substances,therangeswerenarrower:DIPN37%–49%(with26% forbreadcrumbs).MigrationofDiBPrangedfrom0.22mg/kgfood to0.54mg/kgfood(withan “outlier” of0.06forriceandbiscuitsas thehighest)andofbenzophenone(asthemostprominent photoinitiator)from24 μg/kgfoodto59 μg/kgfood(with 6 μg/kgasthelower “outlier” fromnoodlesandoatmealasthe highest).Theauthorscommentedthis: “Migrationseemedtobe influencedmorebytheporosityofthefoodthanbythefatcontent (forinstance,MOSHmigrationintooatmealwasclearlyhigherthan intofattybiscuits).” Thisindicatesthatadsorptionratherthan dissolutioninthefatphaseconstitutesthedrivingmechanism formigrationofsubstanceswithsufficientvolatility(accordingto theauthorswhencomparedwithC24orasimilarsubstance).The steepestincreaseforMOSHwasinthe first2monthsofstorage (approximately40%ofpotential,between22%fornoodlesand57% foroatmeal),whichincreasedto50%–80%after9months.Asa potentiallylogicalnextstep,thisauthorconsortiumstudiedthe differenceofmorevolatileversusnon-volatilemigrantsfrompaper intodryfoods(Eicheretal.,2015).Inthisstudyofmechanistic character,datafrommigrationexperimentsusingnewspaperas contactwithdryfoods(rice,polenta,bakingmix,andbreadcrumbs) andMPPOwasreported.Thenewspaperwaschosenbecauseit containedvolatileMOSH(<C24)andnon-volatile polyalphaolefines(PAO,branchedalkanes,characterizedbythe retentiontimeoftherespectiven-alkanes)fromtheprintingink. Theauthorshavedifferentiatedbetween “direct” (touching)and indirect(gasphase)contacts.Theyconcludedthatmigrationinto dryfoodsviatouchingcontactisnotnecessarilynegligibleandcould evenreachconsiderablelevels.Oneoftheseveralkeyexperiments wasacomparisonof10and20daysofcontactatroomtemperature withMPPO,polenta,andrice.Themigrationfocusedonthree substanceclasses:MOSH < C24,PAO29,andPAO35,representing increasingmolecularweightsanddecreasingvolatility.Migrationof MOSH < C24was100%onMPPO,92%forpolenta,and71%for rice.Migrationofthenon-volatilesPAO29/PAO35wasmuchlower: 46%/20%forMPPO,39%/20%forpolenta,and4%/2%forrice. Decreasingtheparticlesizeofthepolentafrom2mmto1mmledto anincreaseinPAOmigrationbyfactor2.5butnotofMOSH. Furthersizereductionto0.5mmhadonlylittleeffect.Thiswas relatedtothedensityofcontactpointsinthepaper.Migrationinto MPPOatidenticalconditionsatroomtemperature(10days)wasin thesameorderofmagnitudeaspolentayetlowerthanbakingmix andlargelyhigherthanintorice.However,inlonger-term(45days) migrationofPAOintorice(higherfatcontent)passedthatofthe finerbreadcrumbsincontrasttotheresultafter8days,inwhich migrationintobreadcrumbswashigher.Migrationinbothriceand breadcrumbswaslowerthanintopolenta.Consideringall findings, theauthorsexpressedtheirconcernastowhetherMPPOtestwould besuitableforpapertosimulatetouchingcontact:elevating temperaturemayvolatilizenon-volatilesubstancesatroom temperature,andtheparticlesizeofthefoodmaybefarfrom thatofMPPO.

TheuseofsurrogatesforMOAHwhilecomparingmigration testingwithMPPOversusthedryfoodspolentaandcouscouswas themaintopicofresearchby Jaénetal.(2022).Forthesekinetic experiments(at60°C:3,6,and10daysand70°C:2h)cardboard sampleswerepreviouslyfortiﬁedwith16aromaticmodel

substances,suchas2-methylnaphthalene,2,6-DIPN,and perylene,representingMOAHinawiderangeofmolecular masses,chemicalstructures,andmostimportantly,volatilities (boilingpointrangesfrom240°Cto467°C).Migrationreached equilibriumafter3–6daysat60°C.FormorevolatileMOAH substances,theequilibriumlevelsobtainedat60°Cwerealready reachedafter2hat70°C.Thiscoincideswiththe ﬁndingsof Aurela etal.(1999).Ingeneral,themigrationvalueswerehigherinMPPO thanincouscousandpolenta,whichwashighlydistinctiveforthe morevolatilesurrogatesandlessdistinctiveoreventhesameforthe heaviersurrogates.TheauthorsconcludedthatMPPOcanbe consideredastheworstcaseofthesimulationofmigration todryfood.

5.2.6Additionalalternativesimulantsfordryfoods InadditiontoMPPO(Tenax®)andpolenta(asamodelfood), otheradsorbentshavealsobeenstudiedaspotentialdryfood simulants: Nerínetal.(2007) performedcomprehensivekinetic migrationstudiesonthreepapersampleswithdifferentrecycled pulpcontentusingPorapak(aporouscopolymer,notfurther speciﬁedinthepublication)asasolid-foodsimulant.Target migrantswereDIPN,DiBP,anddiethylhexylphthalate.Thetest setupwasdirect(“touching”)contact.Testtemperatureswere25°C, 50°C,75°C,and100°Cwithcontacttimesrangingfrom5minto 10days.Inafewselectedcases,migrationintoMPPOandmilk powderwascarriedoutforcomparison.Porapakwasfoundtoallow solidandreproduciblemeasurementofmigrationkinetics comparablewiththoseobtainedwithMPPO.Notably,bothsolid simulantscoveredreasonably,i.e.,withaslightoverestimation,and themigrationintomilkpowderoccurredat25°Cand50°C. Fengler andGruber(2022) studiedSorb-Starasanotheralternativedryfood simulant.Sorb-StarisnotporouslikeMPPObutrod-shaped polydimethylsiloxane(20mm,ø2mm),whichishighly adsorptivetowardlowandmediumvolatilelipophilicorganic compounds.Thestudycomparedmigrationkinetics(at20°C, 40°C,and60°Cupto12days)forMOHusingSorb-Starversus MPPOin “touching” versus “gasphase” contact.Thecarbon fractionsC10–C16,C16–C20,C20–C25,C25–C35,and C35–C50wereinvestigatedtoobtainbettervolatility-resolved information.Furthermore,migrationofrepresentativesinglesubstancesurrogatesforeachfraction alkanesandaromatic compounds wascompared.MorepolarMOAHmigratedslower thanMOSH.MPPOin “touchingcontact” showedthehighest values.Undergasphasecontactconditions(withoutdirect contact),migrationratesintoMPPOwerelowercomparedwith Sorb-Star.InC25-C35,migrationwasfoundonlyinMPPOtouchingcontactandSorb-Starat60°CforMOSH.Theauthors concludedthatthemigrationbehaviorofMOHcanbedepictedby theuseofsuitablyrepresentativesurrogates,whichwillhelpeasethe analyticaltasks.Migrationtestswiththesesimulants(MPPOand Sorb-Star)at20°Cand40°Ccancoverawiderangeofreal-life migrationprocessesfrompaper-basedfoodcontactmaterialsinto foods,providedthatappropriateconditionsarechosen.

5.2.7Impactofhumidity

Asapotentiallyimportantfactor,relativehumidity(rH) which couldaffecttheextentofmigrationandthetypeofmigrantsfrom paperqualitatively wasstudiedusingMPPOby Barnkoband

Petersen(2013) and Wolfetal.(2023).In2013,benzophenone transferfromapapersampleafter30daysat34°Cunderthree differentrHconditions(43%–73%),increasedbyafactorupto 7.3withincreasingrH.Wolfetal.investigatedtheeffectofrHonthe transferof59volatileorganiccompoundsfromapaperatdifferent rHsetupsandtemperaturesbygaschromatography–mass spectroscopybycomparingpeakareasandbysensorytests. FurthermoretheycompareddirectcontactofMPPOwiththe indirectcontactwiththepapersample.Transferofvolatile substancesincreasedwithincreasingrH,alsodependingonthe polarityofthesubstances.Theauthorsconcludedand recommendedthatadefinedrHlevelneedstobeestablished beforestartingmigrationorsensoryteststoensuresufficient repeatabilityandcomparabilityofsuchtests.Ingeneral,touching contactofMPPOwithpaperledtoconsiderablyhighermigration valuesthanindirectones.Theinfluenceofhumidityfromfoodstuffs incontactortheenvironmentofstoragewasalsoreportedby Zülch andPiringer(2010) and Hauderetal.(2013)

5.2.8Otherfoods

Bradleyetal.(2014; 2015) comparedthemigrationfrompaper intoMPPOwithfreshfruit(applesandbananas),potatoes, mushrooms,andraisins,whichhavedifferentcharacteristicsthan typicaldryfoodssuchaspolenta.Storagetestswithfreshfoodswere performedunderrealistictime–temperatureconditions,e.g.,5days atroomtemperature,withraisinsandMPPOunderstandard conditionsof10daysat40°C.Targetmigrantswere contaminants,intrinsicallypresentinthepapersamplessuchas DIPNorDiBP,aswellassurrogatessuchasbenzophenoneand dodecane,previouslyspikedintothepapersamples.Themajor objectiveofthesestudieswas firsttoassesstherelationshipbetween migrationfrompaperintothefoodsversusintoMPPOandsecond tostudythemigrationofintrinsicallypresentmigrantsversusspiked ones.Migrationlevelsdependedstronglyonthenatureofthe substance.MigrationfromspikedP/Bsampleswasmore extensive(asapercentageoftheavailablemigrants)thanthatof intrinsicmigratablesubstances,suchasDIPNandDiBP.Thiswas explainedbyastrongerbondingintothe fibernetworkby manufacturingthanthespikingprocess.Thisdifferenceappears tobecomerelevantforcomplianceandfoodsafetyassessment versusrealexposureestimation.Inanycase,studyingspiked samplestendstobeconservative.Thenatureofthesubstances andofthefoodsinfluencedthemigrationlevelsmuchmorethanthe characteristicsofthepapersamples.MigrationintoMPPOwasupto afactorof62(potatoes)butatleastbyafactorof10higher comparedwiththefreshfoodsstoredfor5daysatroom temperature;however,itwascomparableoronlyslightlyhigher comparedwithraisins,whichduetotheirlongshelf-life,were storedatthesametime –temperatureconditionsasMPPO.The authorsdiscussedthepotentialuseandthelimitationsof correctionfactorstocorrelateMPPOvaluesunderstandard conditionstorealisticfoodconditions.Theyconcludedthat simplecorrectionfactorswouldapproximateonlythefood characteristicsbutwouldnotre fl ectthesubstance-speci fi c natureofchemicalmigration.Furthermore,theyaddressed ongoingdevelopmentstowardacomprehensivemigration modelforpaperthattakesintoaccountsubstance-andfoodspeci fi ccharacteristicsasmodelingparameters( Section5.3 ).

Correctionfactorswerededucedfrommigrationdataand proposedby Castle(2015)

Consideringmigrationintofattyorhumidfoods,onlyafew publicationsareavailable:dialkylketonesintosalami,cheese,and croissant(Lestido-Cardamaetal.,2020);recyclingcontaminants intobutter(ZülchandPiringer,2010);andPAAsintohumidfoods (Merkeletal.,2018).

5.3Predictivemigrationestimation andmodeling

5.3.1Comparisonofmodelinginplasticswiththat inpaper

Forplasticspackaging,migrationmodeling-basedconformity assessmenthasbeenofficiallyrecognizedsince2001(EUDirective 2001/62/EC),proposedinArticle18ofthecurrentEUPlastics Regulation10/2011(EU,2023)anddescribedinaJRCguideline (Hoekstraetal.,2015).Sincethen,thistoolhasbeenincreasingly usedbyindustry,testinglaboratories,andauthorities(e.g.,EFSA andFDA)toevaluatepolymerpackagingquicklyandinexpensively, aswellastocross-checkexperimentaldesignandresultsfor plausibility.Thediffusionoforganicsubstancesinplastics generallyfollowsFick’ssecondlaw(Crank,1975).Theplastic layerisconsideredisotropicandhomogeneouswithaninitially homogeneousdistributionofthemigrantsinthelayer.The differentialequationfromFick’slawcanbesolvedusing numericalsimulation(Roduitetal.,2005; Tosaetal.,2008; Nguyenetal.,2013)andisimplementedincommercialorfree software.Thediffusioncoefficient(s)DP ofamigrantintheplastic layer(s)andpartitioncoefficientsKbetweenthelayersarerequired asinputparameters.Inplastics,DP dependsmainlyonitsmolecular size,whichallowstheestimationofDP usingrelativelysimple formulas(Begleyetal.,2005; Piringer,2008; Welle,2013; Hoekstraetal.,2015; Merceaetal.,2018).Aspartition coefficientsintofood(simulant),ifknownvaluesarenot available,defaultvaluesforhigh(KP,F =1)orlow(KP,F =1,000) solubilityinfoodareemployed(Hoekstraetal.,2015).

Theassumptionofthehomogeneityandisotropyofthelayer losesitsvalidityinthecaseof fiber-basedpackagingmaterials.Paper mainlyconsistsofcellulose fibers,creatingaporousstructure,and maycontainotheradditives, fillers,and finishingagents.The transportmechanismcanessentiallybeunderstoodasasequence ofdesorption/evaporationstepsintothevaporphaseofthepores andadsorption/condensationofthemigratingsubstances(Aurela andKetoja,2002; ZülchandPiringer,2010).Nevertheless,various authorsconsideredpapermaterialsasaquasi-homogeneous, isotropiclayeranddescribedmigrationkineticswiththe simplifiedmodelforplastics.

5.3.2ModelsfollowingFick’ssecondlaw ofdiffusion

TheextensivekineticmigrationdatasetelaboratedinEUProject FAIR-CT98-4318(RaffaelandSimoneau,2002; CastleandFranz, 2003)wasusedtoexplorewhethertheexistingmigrationmodelfor plasticsaccordingtoFick’slawcoulddescribethemasstransferfrom paperwhileassumingthepapermatrixasahomogeneousisotropic layer.Theexperimentalmigrationcurvescouldbewell-ﬁttedbased

ontheeffectivediffusionconstantsDpaper (understoodastheoverall diffusioneffectwithinthepapermatrix)andpartitioncoefficients Kpaper/food orKpaper/MPPO.EffectiveDpaper andKpaper/food valueswere obtainedfrommigrationexperimentsintoMPPOanddryfoodsat temperatures50°C,60°C,and70°C,witheightdifferentpapertypes spikedandasetofeightmigrants.Thepartitioncoefficients confirmedthatMPPOservesasamoresevereadsorbentthan foods.FromtheDpaper values,theeffectivediffusionbehaviorof thepapersampleswasfoundtobesimilartoLDPEpolymer.In general,attemperaturesof40°Candabove,migrationwas dominatedbypartitioningduetotherelativelyrapidachievementof equilibrium.Atroomtemperature,diffusionplayedabiggerrole, especiallyforlargermolecules.Therefore,thekineticmodel appearedtobemoreusefulindescribingshort-termcontactat ambienttemperatureandabove,e.g.,fastfoods,andatlow temperatures,e.g.,chilledandfrozenfoods.Animportant finding wasthatnoconsiderablekineticdifferenceswerenotedbetweenthe differentpapermaterials,asknownfordifferentplastictypes.

ZülchandPiringer(2010) developedanadaptationofthe plasticsmigrationmodelforpaper.Theystudiedthemigration behaviorfromdifferentpapersamples,spikedwithmodel substancesandnon-spikedwithfoodstuffsandMPPOasfood simulantat 18.5°Cand22°C,andablottingpaperasthe acceptorat40°C.From fittingthemigrationcurveusingthe plasticsmultilayermodeofthemodel(Tosaetal.,2008),they foundthatinthistemperaturerangetransferfrompaperwillbebest describedbyconsideringpaperasatwo-layersystem,whichis representedbyacorelayerB1withrelativehighdiffusionratesanda thinsurfacelayerB2withdifferentmigrationbehavior.Effective diffusioncoefficientswereestimatedinanalogytotheAP value approachforplastics(Begleyetal.,2005),basedonthemolecular massofthemigratingsubstanceupto400gmol 1.Forpaper,AB1 andAB2 areusedasspecificparameterswithconstantAB1 =6.AB2 valueofthevirtualsurfacelayerdependedonthepolarity,humidity inthepaper,thewateractivityofthefood,andpropertiesofthe migrantrangingbetween 10and 1forcontactwithdryfoodand upto6.0forcontactwithbutter.Thistwo-layerapproachis particularlyrelevantinthecaseoflowtemperaturesand migrantswithhighmolecularweight.Athightemperatures,the best fitofpredictedversusexperimentalmigrationdatawas obtainedwithaone-layerapproachandacommonvalueof AB1 =AB2 =AB = 2.Theauthorsconcludedthatthe differentiationbetweenthediffusioninB1andB2isunnecessary formigrantswithlowpolarity,molecularweightsbelow350gmol 1 athightemperatures(≥40°C),andhighhumidityduetothestrongly increaseddesorptionrate.Withthismodel,theauthorspresentafull migrationmodelintofoodforfoodslikebutter,chocolate,pasta, wheat flour,andbiscuitsatlowtemperatures(5°C,22°C).In2013, thesamegroup(Hauderetal.,2013)publishedfurtherworkto betterunderstandthenecessityofthechangingthemodelbehavior fromatwo-toaone-layerapproach,dependingonthetemperature andtorefinethemodel.Thespecificdiffusionbehaviorinpaperand migrationmodelingfromrecycledboardintodryfoodstuffsusing n-alkaneswith15–35carbonatomsandothersubstancesinthe board(nospiking)wasstudied.Forthesurfaceregion(B2) determiningthediffusionrate,thediffusioncoefficientsofthese migrantsdecreasedproportionallytotheirvaporpressures.Based onthese findings,theauthorsmodifiedthediffusioncoefficient

equationforB2withfunctionalconsiderationofthevaporpressure ofthemigrantsandprovidedageneralmigrationmodelforspecific andglobalmasstransferofimpuritiesfromtherecycledboardinto thedryfood. BarnkobandPetersen(2013) studiedtheapplicability ofthepapermigrationmodelby fittingthemigrationof benzophenonefrompaperatseveralhumidities(40%to >73% rH)at34°C,usingtheapproachof ZülchandPiringer(2010) Theauthorsfoundsomedifferencesconcerningtheapplicabilityof theone-andtwo-layerapproaches,whichweresmallwithinthe qualityofthe fitsbetweenbothapproaches. Hanetal.(2016) investigatedthemigrationofphotoinitiatorsintoMPPOat 50°C–100°Candderivedeffectivediffusionandpartition coefficientsby fittingtheexperimentalcurvestoFickiansecond law. Huangetal.(2013) includedatermforthepaperporosityinto theirmodelaccordingtoFickiansecondlaw.

5.3.3Othermodels

Contrarytotheabove-describedwork, Poçasetal.(2011) reportedthatFick’ssecondlawofdiffusiongavepoor fitsin somecases.Theystudiedthemigrationofseveralsubstances withdifferentchemicalfunctionalitiesfrom fivedifferentpaper materialstoinvestigatetheinfluenceofmolecularsize,chemical characteristicsofthemigrants,andpapercharacteristics(suchas type,thickness,andrecyclingcontent).To fitthemigrationcurves, theyexploredthepotentialofWeibullmodel,whichisbasedona distributionfunctiontriggeredbytwoparameters(scaleandshape parameter).Itcanbeempiricallyappliedwithouttheuseof physical–chemicalparameters,suchasDpaper andKpaper/food Migrationfrompaperwasfoundtobemuchfasterthanthose fromplastics.Thevolatilityandpolarityofthemigrantsdetermined theirtransferintofood(simulant)andthelossesfromthesystem duetoevaporation.Theauthorsconcludedthatthissimplemodel allowsthemtodescribethepatternofmigrationcurvesforawide rangeofmigrantvolatilities. Guazzottietal.(2015) appliedthe Weibullmodelto fitkineticmigrationcurvesobtainedfrompaper, spikedwithaseriesofn-alkanesat40°Cand60°Cincontactwith MPPO,confirmingthatthismodelcaneffectivelybeusedto describeadiffusionalprocessofthepaper.Anotherstatistical approachtocorrelatephysical–chemicalpropertieswith migrationbehaviorwasrecentlypublishedby Jaénetal.(2022) usingMOAHsurrogates(experimentaldetailsaregivenin Section 5.2).Theauthorsappliedmultivariateanalysisalgorithmsto correlateandgroupthemigrationofmodelsubstancesandbuilt apartialleastsquaresregressionmodeltopredicttheworstcase MOAHmigration.Themigrationpatternsshowedstrong correlationsalongwiththevolatilityofthesurrogates.The elaboratedmodelwascapableofpredictingmigrationvalues fromthephysical–chemicalsubstancepropertiesandwasauseful tooltobefurtherexplored.

AurelaandKetoja(2002) studiedthediffusionofvolatile compoundsin ﬁbernetworksbyexperimentaldeterminationand modelingusingrandomwalksimulation,whichisbasedonthe porosityofthepapersampleandthediffusionspeedthroughthe pores assumedtobethediffusionconstantinfreeair.The compoundswerevolatilesolvents,suchasethanolandbutyl acetate.TheexperimentalandmodeledeffectiveDpaper values matchedandwereintherangeofapproximatelyE-7m2/s (E-3cm2/s)atambienttemperature.Theestimateofeffective

diffusionconstantswasbasedonlyonthatofthecompoundpresent infreeair. Laineetal.(2016) addedatermdescribingsorptiontoa one-dimensionaldiffusionequationtosimulatemigrationinto MPPOthroughcardboardinindirectcontactandsolelythrough air.Thesimulateddata ﬁttedwellwithexperimentaldataon migrationintoMPPOduringanindirectcontactinanair-ﬁlled chamberandafterpermeationthroughcardboardusingMOSHand MOAHsurrogates.

Serebrennikovaetal.(2022) describedthetransportbypartial differentialequationsfortransportingwithinthegasphaseofthe poresinthepapercoupledtothosedescribingthesorptionprocess. Thetransportprocessesaredeterminedbycomplexinteractions. Diffusioncoefficientsandsorptionconstantscannotbeeasily derivedfromexperimentsbutby fittingtheparametersofthe modeltotheexperimentaldata(solutionofaninverseproblem). Theexamplewasthediffusionofdimethylsulfoxideinastackof paper(23°C,50%rH),measuredatfourtimepointsandspatially dissolvedin fivepapersheets.Forsolvingthecomplexdifferential equations(derivedfrommodelsforwatervaportransport), fittingto theexperimentaldata,andobtainingtheparameters,theyused physics-informedneuralnetworks(PINNs)andsuccessfully comparedwith finiteelementmethods.In Serebrennikovaetal. (2024),thedimethylsulfoxideexperiment(polarcomponent), extendedto12weeks,andanotheronewithtetradecane(nonpolar)werecomparedwith fivedifferentmathematicalmodels, whichwereevaluatedbyPINN.Threeofthemwerebasedon Fick’slawofdiffusion,withandwithoutaspecifictermfor sorptionanddesorption.Inaddition,pseudo-first-order adsorptionandsecond-orderreversiblesorptionmodelswere included.TheFickianbehaviormodelsdidnot fitthedata.The best fitwasobservedforapseudo-first-orderadsorptionmodel (withoutdesorptionfromthe fiber);however,afurthersearchfora suitablefunctionwascalledfor.Ingeneral,theauthorsconcluded thatPINNsrepresentaversatilemathematicaltooleithertovalidate ortorefutethecapabilityoftheoreticalmodelstodescribe experimentaldata.

5.4Partitioncoefﬁcients

Althoughmainlythediffusionprocessesandestimationof diffusioncoefficientsDpaper areaddressedforthemigration modeling,accesstoothercrucialparameter thepartition coefficientKpaper/food isalsolimited.Thepartitioncoefficients describetheconcentrationratio(mass/volume)inthe equilibrium.Forvolatiles,apromisingapproachdeterminesthe partitioncoefficientsofpaperandfoodorMPPOversusair.From thequotientofboth,thepartitioncoefficientsKpaper/food canbe derived. Haack(2006) determinedtheadsorptionisothermsofthe volatilemodelsubstancessuchashexanolandothersintopaper material(at40°C–120°C),aswellasintothefoodstuffchocolate, cookies,andpasta includingMPPOasfoodsimulantat 100°C andcalculatedpartitioncoefficientsfromthedata.For hexanol,butylacrylate,nonanal,anddiphenyloxide,Kpaper/food valuesforthethreefoodsrangedfrom0.03to0.88andKpaper/MPPO forMPPOfrom0.02to0.08,beingstronglyonthefoodside.For butanol,Kpaper/food wasbetween1and4.3forfoodsandKpaper/MPPO was3.3.Overall,thesedatademonstratethehighoratleast

comparableadsorptivepropertiesofMPPOversusdryfoods. TheseKvalueslargelyoverlappedwiththoseobtainedbycurve fittingofmigrationkineticswithinFAIR-CT98-4318(Raffaeland Simoneau,2002; CastleandFranz,2003). Triantafyllouetal.(2005) determinedpartitioncoefficientsofadditionalsubstancesbetween paperandairat70°Cand100°C.Migrationkineticsatthese temperaturesintoTenax(Triantafyllouetal.,2002)and semolina,instantbabycream,andmilkpowder(Triantafyllou etal.,2007)uptoequilibriumarereportedfromthisgroup; however,partitioncoefficientswerenotcalculated.

WithintheGermanresearchprojectIGF19016N(Fengleretal., 2019),partitioncoefﬁcientsformineraloilcomponents(MOSHand MOAH)weredeterminedandpublishedintheguidelinedocument (Gruberetal.,2019).PartitioncoefﬁcientsofMOSHandMOAH betweenpaperandfoodrangebetween1,000(crystalizedsugaror honey)and1(chocolateorchoppednuts).

6Discussion

Fromthepublishedresults,key ﬁndings,andinterpretations summarizedunder Sections4 and 5,severaldiscussionpointsarise, whicharepresentedinthefollowing,conciseway:

Standardmethodstoestimatethetransfertofoodsare extractswithwater(coldandhot),solvents(ethanoland isooctane),andmigrationtestingintoMPPO.Froma physical –chemicalpointofview,theextractsdetermine concentrationsatpartitioningequilibriuminwaterorsolvent. Thesestandardmethodshavebeenfoundtoholdpotentialfor overestimationand,insomecases,underestimationofmigration intofoods,eitherduetodifferencesinpartitioningcoef ﬁ cientsof paperversusfoodorextractan tand/orbeingfarawayfrom reachingequilibriuminrealapplications.Therefore,the extractscannotbeconsideredtoreliablyrepresentthereal exposureoftheconsumerfrompaperfoodcontactmaterials inmostcases.However,onlyalittlestudyeffortsweremade towardwettingoraqueousandfattycontacts.

MigrationintoMPPOisusedforsimulatingdryfoods(EUfood contactmaterialsimulantE)andheatcontactinovenapplications. Forthelatter,therearedivergingprotocolsregardingthetest temperaturebetweenMemberStates,EUreferencelaboratories, andCouncilofEuropeTechnicalGuidethatwillneed harmonization.However,fortheevaluationofthesehightemperatureapplications,noscientiﬁcworkwasfound.Fora sounddecision,notonlyoventemperaturesbutalsotheusually lowertemperaturesatthedirectcontactareainrealbaking applicationsforlargefoodpieces(cakesandroast)ortheshorter timesatthesehightemperaturesforcookiesshouldbeconsidered. Furthermore,forsettingharmonizedtestconditions,temperature limitationstoMPPOshouldbetakenintoconsideration.At temperaturesof200°Candhigherandinthepresenceofoxygen fromair,MPPOstartstodegradeoxidatively,whichlimitsthe numberofpossiblereusesafterreconditioning.

Toexperimentallysimulatemigrationfrompaperintodry foods,thereisabroaddiscussioninthereviewedliteratureif MPPOissuitableatall,orsuitableunderwhichtestconditions. MPPOhasahighlyadsorptivepowerduetoitsporositywithalarge innersurfaceanditschemistry.IfcomparingMPPOanddryfoods

atidenticaltestconditions,MPPOoftenhighlyoverestimates migrationintofoodsbutmayalsobeinthesamerange (chocolate)orevenlesssevere(milkpowder).However,this conclusiondependsnotonlyonthepropertiesofthefoodbut alsoonthatofthesubstances,mainlythevolatility.Forroom temperatureapplications,migrationmayincreaseovermonthsor years,withoutreachingequilibrium.Thus,acceleratedtestsare necessary.However,becauseofthecompletelydifferenttransport mechanismofgasphasetransfer,desorption,andadsorptiononthe paper ﬁbersincomparisontoplasticpolymers,anincreasein temperaturenotonlyacceleratesthediffusionratebutalso mobilizessubstancesoflowervolatilitythatwouldnotmigrateat detectableamountsatroomtemperature.Thecombinationofthe highadsorptivepowerofMPPOandaccelerationbyincreased temperatureinmanycasesleadstoahighoverestimationof migration,asshowninmanyofthereviewedpapers.The conclusionsdiffer:someappreciatetheconservative characteristics(VanDenHouweetal.,2018),whereasothers judgeMPPOasunsuitablebecauseoftheoverestimative characteristic(Zurﬂuhetal.,2013; Eicheretal.,2015).

Oneapproachtoovercomethisproblemistode fi neacertain time –temperatureconditionformigrationtests(e.g.,30minat 70 °Cforshort-termcontacts),whichcoversthemigrationinto foodinaslightlyoverestimatingway( Dimaetal.,2011 ).Such approacheswillneedagoodstatisticalbasis.Becauseofthe manifoldin fl uencefactorsonmigration,acertain time –temperatureconditionisexpectedtobevalidfora restrictedrangeofsubstancesandfoodapplications.Another approachistode fi neconventionaltransferratesintofoods (i.e.,typicalorworsecasepercentageofconcentrationin material)andanalyzethepapermaterial( Zur fl uhetal.,2013 ). Suchaconceptignoresthein fl uenceofthematerialthicknesson thetransferrate.Furthermore,andmoreimportantly,itwill impededevelopmentsfortheimplementationofbarrier propertieswithinthepapermakingprocess.Reductionfactors appliedtotheresultofmigrationintoMPPOareafurther possibility( Castle,2015 ).Fromcomparativedataofmigration intocertainfoodgroupsorstorageapplications,typical overestimatingfactorsofMPPOtestwerederivedand conservativefactorswerede fi ned,whichshallbeappliedfor theevaluationofMPPOtestresults.However,thismustbe differentiatedbasedonthevolatilityofthemigrating substanceandthetypeoffood.Simplereductionfactorswill betoocrude.Therefore, Bradleyetal.(2014) proposedfurther developmentofmodelingasthebettersolution.Adaptingthe geometryofthesimulanttothatoffoodsusingrodsinsteadof fi neparticulateadsorbentsisanexperimentalwaytoreducethe differencestorealfoods( FenglerandGruber,2022 )orusingreal foods( Eicheretal.,2015 ; VanDenHouweetal.,2018 ).

Acceptedandvalidatedmodels whichcansimulatethe migrationoutofpaperboardsintovariousfoodsconsidering thepropertiesofthesubstances,in ﬂ uencesofhumidity,paper andfoodproperties,andtemperature willbeasolutionto overcomealltheseshortcomingsoftheexperimentaltests. However,thereisahighdemandforresearch.Inthewordsof Nguyen,themechanismsandrelationshipsarestillpoorly understood( Nguyenetal.,2017 ).Atroomtemperatureand below,anon-Fickianbehaviorwasmainlyobserved.Piringer

introducedavirtualsurfacelayertodescribetheexperimental databyFickiandiffusion( ZülchandPiringer,2010 ),allowingthe useofthesamesoftwareestablishedforplastics.Thediffusion coef ﬁ cientsareestimatedfromasemiempiricalequationusing 95%con ﬁ denceupperboundparameters.Thisandother statisticalapproaches(Weibullandpartialleastsquare regression)mightbeapplicablewaysforconformitytesting butwouldneedfurtherexplorationonapplicabilityoutside thedatasetsusedforestablishmentoftheparameters.The majorresearchfocusedonrelativelyvolatilesubstances ( SupplementaryTableS1 ).Fornon-volatilesandtransfer mechanismsotherthanthosevia thegasphase,onlylittle workispublished.

Forunderstandingtheimpactsofin fl uencingparameters, de fi ningoverestimatingparametersisnotsuf fi cient.Transport inthegasphaseoftheporesanddesorptionandadsorptionon the fi bersneedtobeconsidered.Thesehighlycomplex interactionscannotbesimplyderivedfromexperiments. However,initialstepsarealreadytaken: Hauderetal.(2013) implementedatermforthevaporpressure; Huangetal.(2013) introducedpaperporosityinthemodelingequation.For volatiles,therandomwalksimulationinthepores( Aurela andKetoja,2002 )willbeapplicablebutforlessvolatiles, adsorptionanddesorptiononthe fi berswillplayanonnegligiblerole.Computationalapproachesbymultivariate dataanalysisandneuronalnetworks,incombinationwith physicalconsiderations,arepromising.Thesecanhelp identifytheinterrelationsofvariousparametersandtestthe applicabilityofproposeddifferentialequationsandboundary conditionstoexperimentaldata.However,thetestedmodelsin Serebrennikovaetal.(2024) couldnotyetsuf fi cientlydescribe theprocessesanddemandforfurtherwork.

7Conclusion

Numerousscientificattemptshavebeenmade andarestill ongoing toexplorethedeficienciesareandthealternativescientific solutionstoovercometheshortcomingsofexistingtesting approachesanddatagaps.Thescientificeffortswerefocused,in the firstplace,onthetransferfrompaperintodryfoodsundertwo aspects:(i)howandunderwhichtime–temperatureconditions migrationintodryfoodscouldbesimulatedand(ii)whatwould beanappropriatemodeltosimulateandpredict migrationintofood.

Aspect(i):MPPOseemstobethemostsuitabledryfood simulantduetoitshighadsorptiveproperties,whichmakesita moreseveretestmediumthananydryfoodbutinmanycases,a toosevereone.Forspeci fi capplications,representativemodel foodslikepolentaoradsorbingrodsmayserveasoptions. However,whenitcomestothechoiceoftime –temperature contactconditions,thereisnotenoughclarityandtargeted precisiontomatchexactlyor,atleast,verycloselythefood contactapplicationtobesimulated.Severaloptionsarediscussed butageneralapproachseemstobedif fi cult.Mostofthemare relatedtospeci fi csubstancegroupsorvolatilityrangesand applications.Itneedstobeexploredifthehumidity conditionsintheexperimentsneedtobede fi nedornot,and

ifyes,thenatwhichlevel.Fromalegalcompliancepointofview, harshandveryseveretime –temperatureconditionscanbe selected,buttheresultsarelikelytoallowonlyprovingbut notdisapprovingcomplianceandthereforeunnecessarily disqualifypaperfoodcontactmaterials.Estimationof exposurewouldfailanyway.

Aspect(ii):Outofthedifferentmodelingapproaches,a physical–chemicalmodelbasedontheknowledgeofthe underlyingmasstransportmechanismsandprocesses,aswellas onthedeterminingparameters,suchasdiffusionconstantsand partitioncoefﬁcients,are,inouropinion,themostpromisingand sustainableapproach.However,thereisalackofqualitativeand quantitativeunderstandingofthefactorsanddeterminantsof transportandpartitioningprocessesinandfrompaper.For complianceevaluations,itwillbeadvantageousiftheplastics modelcanserveastheformattobeadaptedtopaperalongwith themainpaper-relatedcharacteristics,inwhichthevaporpressure ofmigrantsplaysanimportantrole,aswasfoundinalmostall studies.Forunderstandingtheprocesses,modelsmustbemore complex.Towardwettingorfattycontact,moreresearchis alsoneeded.

Overall,thepublishedscientiﬁcdataandcollectiveknowledgein thisarea,alongwithmodernmoleculardynamicsscience,forma promisingsolidbasisforfutureworkto ﬁlltheopendatagapsand generatetheneededknowledge,thusendingupwithamigration modelofbroadapplicabilityandgeneralacceptance.

Authorcontributions

AS:Conceptualization,fundingacquisition,writing–original draft,andwriting–reviewandediting.LH:Fundingacquisition andwriting–originaldraft.RF:Conceptualizationand writing–originaldraft.

References

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Funding

Theauthorsdeclarethat ﬁnancialsupportwasreceivedforthe research,authorship,and/orpublicationofthisarticle.Thework waspartlyfundedbyIndustrievereinigungLebensmitteltechnologie undVerpackung,85354Freising(www.ivlv.org withinIVLVproject “Migrationfrompaperandboard2”).Projectcanbefoundin projectdatabase https://www.ivlv.org/en/project/migration-frompaper-and-board-2/

Conﬂictofinterest

Theauthorsdeclarethattheresearchwasconductedinthe absenceofanycommercialor ﬁnancialrelationshipsthatcouldbe construedasapotentialconﬂictofinterest.

Publisher’snote

Allclaimsexpressedinthisarticlearesolelythoseoftheauthorsand donotnecessarilyrepresentthoseoftheirafﬁliatedorganizations,or thoseofthepublisher,theeditors,andthereviewers.Anyproductthat maybeevaluatedinthisarticle,orclaimthatmaybemadebyits manufacturer,isnotguaranteedorendorsedbythepublisher.

Supplementarymaterial

TheSupplementaryMaterialforthisarticlecanbefoundonline at: https://www.frontiersin.org/articles/10.3389/fchem.2024.1397913/ full#supplementary-material

SUPPLEMENTARYTABLES1 Substancesusedinpapermigrationstudies.

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Volume 10, Number 3, 2024

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Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

Volume 10, Number 3, 2024

Installed quickly – time saved

Compact design – space saved

Strong in use – productivity increased

Volume 10, Number 3, 2024

Safety locking device PSENmlock mini in detail

Volume 10, Number 3, 2024

Safeguarding of swing gates and sliding gates with PSENslock 2

Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

CALENDAR OF EVENTS – 2025

PITA TRAINING COURSES

Cristina Lugli

Food Contact (1 day)

11 Feb.

This course will focus on the scenario of paper and board materials and articles for Food Contact (P&B FCMs), and it is intended specifically for all operators along the paper and board supply chain: producers, converters and retailers; providing essential training for operators who wish to develop a better understanding of technical and legislative requirements in the Food Contact materials market

Jo Thorne

EPR Permits (1 day)

15 Oct.

This intensive one-day course will provide an understanding of the environmental impacts of paper making within the UK and the applicable regulatory landscape. It covers the day-to-day practicalities of compliance with these regulatory requirements, specifically including Best Available Techniques for minimising environmental impact, covering areas such as environmental management systems, waste management, control of emissions to air and water, and water abstraction/efficiency, Climate Change Adaptation and Fire Prevention Planning.

Ian Padley

Modern Papermaking (2 days)

TBA

This two-day course will provide an introduction to papermaking in the 21st Century. The course will be of relevance to any paper industry operative or supplier – no previous academic knowledge required. The course will cover all the basics: from fibres, and how they are treated and prepared; to papermachines and how they transform these fibres into the paper web. Ancillary services are also looked at in relation to papermaking, along with an introduction to finishing and converting practices

Mark Smith

Introduction to Wet End Chemistry (2 days)

TBA

This essential course provides an understanding of how colloidal materials can be used and controlled to give you desired paper properties and improved paper machine efficiency. Vital knowledge for anyone operating a paper machine.

Papierzentrum Gernsback

Fundamentals of Papermaking (2½)

TBA

This course provides a fast-track introduction to papermaking (limited to operations between ‘Mixing Chest’ & ‘Reel up’). The intention is to raise the knowledge of process, technician & support staff to an intermediate level or shared understanding regarding modern Paper Machine Operations

We also have the possibility of running the following courses, if there is sufficient interest expressed by companies or individuals (please email daven@pita.org.uk):

Peter Luimes

Fundamentals of Wastewater Treatment (2 days)

This course will focus on optimising biological treatment plants to meet BAT, reduce operating costs and generate revenue. The course will be of relevance to any paper industry site operating or considering investment in an activated sludge plant, a biological filter or anaerobic digestion plant. BAT8 covers the monitoring of ‘key process parameters and of emissions to water and air’. Make sure you are ready for it with this course!

Ian Padley

Paper Appreciation (1 day)

This course is aimed at new-starters to the paper industry, and not necessarily those working in production or technical roles. It provides a general overview of the industry, and introducers attendees to the wide range of products that can be made from cellulose fibre.

Ian Padley

Introduction to Tissue (2 days)

This Introductory course provides an insight into the modern tissue mill and the peculiarities of tissue when compared with conventional paper products. It is similar to the Modern Papermaking course, but slanted towards the hygiene sector.

Ian Padley

Holistic Tissue Quality Workshop (2 days)

This is a completely new course, aimed at reviewing how tissue process management influences quality off the machine. The workshop assumes a basic working knowledge of tissue industry process. For completely new starters we suggest our 2-day introduction to tissue as a precursor. The targets are as follows:

Tissue industry suppliers, especially for consumables from fibre to chemicals Familiarisation training for people new to operations roles and general interest for anyone wanting an overview of a balanced, multi-platform approach to tissue quality ex-machine.

Alternatively, for an experienced production/operations team, we would suggest a bespoke approach based on the customers own process and incorporating stakeholder interviews, audit and a more interactive workshop/problem solving approach. Please contact us to discuss this option.

INTERNATIONAL CONFERENCES & EXHIBITIONS

CPI LAUNCHES PAPER INDUSTRY GOLD AWARDS FOR 2025

The Confederation of Paper Industries (CPI), in partnership with the Paper Gold Medal Association (PGMA), is delighted to launch the 2025 Paper Industry Gold Awards.

The annual awards will be held on Tuesday 17th June 2025 at the prestigious and historic Stationers’ Hall in London.

Now in their 4th year, the Paper Industry Gold Awards are back once again to celebrate and recognise excellence and achievement in the UK’s Paper -based Industries. This year sees the return of seven main awards categories. The Sustainable Innovation and Net Zero awards have been combined and we are delighted to announce the launch of a brand-new award for 2025 for Young Talent, a reward that will be presented to an individual who demonstrates innovation, commitment, and leadership. The Young Talent Award seeks to inspire the next generation and highlight the Paper Industry’s dedication to fostering future leaders by positioning the industry as an attractive and dynamic career choice for ambitious young people.

The Awards were first launched in 2022 and are becoming a well- established event in the UK’s Paper-based Industries calendar.

The 2025 Awards have officially been launched today, marking an exciting new chapter in recognising outstanding achievements. This year, they feature s even distinct categories, highlighting various talents and contributions. Notably, there is a brand-new award dedicated to Young Talent, designed to celebrate the exceptional skills and creativity of emerging individuals in their respective fields.

The awards categories are:

1. Recycling Award,

2. Community Engagement Award,

3. Health and Safety Award,

4. Sustainable Innovation & Net Zero Award,

5. Skills Award,

6. Equality, Diversity, and Inclusion (EDI) Award, and

7. Young Talent Award

The Paper Gold Medal will also be awarded at the ceremony - this award will recognise an outstanding, lifelong contribution to this industry.

At the ceremony, CPI will also present certificates to the first cohort of apprentices to complete their three years in the relaunched CPI Papermaking Apprenticeship Scheme.

You can find the criteria and entry forms for these awards, and the nomination form for the Paper Gold Medal at www.paper.org.uk/awards.

The awards are free to enter and provide a great opportunity to promote the UK’s Paperbased Industries and give companies and individual staff members the recognition they deserve.

Andrew Large, the Director General of CPI, said: “The Paper Industry Gold Awards are a fantastic opportunity to recognise the outstanding contributions of individuals and companies within the UK's Paper-based Industries. We encourage nominations from across the sector to help us celebrate the innovation, dedication, and talent that drive our industry forward.

We look forward to receiving the entries and showcasing the incredible talent and dedication that define the future of the Paper Industry.”

The deadline to enter is Friday 28th March 2025

For further information contact Elisse Hare ( mailto:ehare@paper.org.uk).

Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

The following pages contain a summary of the various installations and orders from around the world of papermaking, wood panel and saw mills, and bio-power generation, received between the start of July 2024 and mid-November 2024. Paper Technology International PITA Annual Review Essential Guide to Aqueous Coating

Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

Most journals and magazines devoted to the paper industry contain a mixture of news, features and some technical articles. Very few contain research items, and even fewer of these are peer-reviewed.

This listing contains the most recent articles from three of the remaining specialist English language journals alongside one Korean journal and one Japanese journal, all of which publish original peer-reviewed research:

IPPITA JOURNAL (Peer-reviewed and other)

JAPAN TAPPI JOURNAL (English abstract only)

JOURNAL OF KOREA TAPPI (English abstract only)

NORDIC PULP & PAPER RESEARCH JOURNAL

TAPPI JOURNAL

JAPAN TAPPI JOURNAL

JOURNAL OF KOREA TAPPI

Paper Technology

International PITA Annual Review

Essential Guide to Aqueous Coating – which is now available in digital formal

Energy Saving I

Topics & Information

Introduction of Research Laboratories 157

Energy Saving Carbon Neutral

Volume 10, Number 3, 2024

Topics & Information

Introduction of Research Laboratories 157

Pulp

Topics & Information

Introduction of Research Laboratories 158

Papermaking Technology

10, Number 3, 2024

Topics & Information

Papermaking Technology II

Topics & Information

Introduction of Research Laboratories 159

Pulp and Paper Mills in Japan 107

Pulp and Paper Research Conference/Patent

Volume 10, Number 3, 2024

Topics & Information

Research Report

Volume 10, Number 3, 2024

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL® R T P O P T

Volume 10, Number 3, 2024

The general peer-reviewed scientific and engineering press consists of several thousand journals, conference proceedings and books published annually. In among the multitude of articles, presentations and chapters is a small but select number of items that relate to papermaking, environmental and waste processing, packaging, moulded pulp and wood panel manufacture. The abstracts contained in this report show the most recently published items likely to prove of interest to our readership, arranged as follows:

Page 2

Page 3

Page 4

Page 5

Page 6

Page 7

Page 9

Page 10

Page 11

Page 13

Coating

Environment

Fillers

Moulded Pulp

Nano-Science

Packaging Technology

Paper Chemicals Papermaking

Pulp Testing

Tissue

Waste Treatment

Wood Panel

Paper

Technology International® PITA Annual Review

Essential Guide to Aqueous Coating

10, Number 3, 2024

Conversion and Biorefinery

Biomass

Materials Today Communications

Environmental

Engineering & Management Journal (EEMJ)

Industrial Crops and Products

Journal of Applied Polymer Science

Journal of Bioresources and Bioproducts

Journal of Natural Fibers

Polymer

Cellulose

Progress in Organic Coatings

Waste Management

10, Number 3, 2024

Journal of Dispersion Science and Technology

Drying Technology: An International Journal

IEEE Transactions on Automation Science and Engineering

Machine Learning, Multi Agent and Cyber Physical Systems

Russian Journal of Nondestructive Testing

Environmental Science and Pollution Research

Cellulose

BioResources

Cellulose Chemistry And Technology

European Journal of Wood and Wood Products

BioResources

Journal of Materials Science & Engineering

Bio-Based Building Materials

Environmental & Social Management Journal

European Journal of Wood and Wood Products

Journal of Green Technology and Environment

Wood Material Science & Engineering

PRODUCTION ENGINEER

Your next workplace

Holmen Board and Paper is a Swedish company and a member of the Holmen Group. The Workington Mill produces premium paperboard under the brand Incada. Our paperboard is an integral part of the shopping experience for our clients’ customers. Since 2013 we have powered the mill almost entirely by fossil fuel free energy. We value our employees and products highly. Today we are approximately 340 co-workers. Our integrated pulp and paperboard mill is located to the west of the beautiful Lake District in the north of England.

Your future challenge

Are you proactive, analytical and quality focussed? Someone who enjoys developing processes and driving improvements? We are looking for a Production Engineer who can lead on all development activities on the Board Machine, including capital investments, in line with the area KPI’s and Holmen success of the business.

If you’re looking for the opportunity to be part of a great work community, looking to develop yourself, engage and make a positive impact with a pioneering customer and employee orientated business, we’d love to hear from you.

Reporting to the Production Manager - Paperboard, the successful candidate will: Drive health and safety, environmental and food safety improvements across all shifts. Proactively identify and lead on process improvement and development projects to increase

Lead and contribute to capital investment projects.

Be the area representative on cross-functional project groups and report into cross-functional meetings. Be involved in customer and quality related improvements and problem-solving activities (RCA).

The successful Production Engineer will possess:

An excellent knowledge of the pulp and papermaking processes. Have strong analytical and problem-solving skills.

The following are also desired:

Degree level education in a relevant engineering/science or papermaking discipline, or equivalent experience.

Appropriate training in Health & Safety. A proactive approach to improvement activities.

A competitive salary.

Fantastic Contributory pension plan.

Private Health Insurances. Life assurance.

33 days annual leave (inclusive of bank holidays) and the option to buy additional holidays every year.

Opportunities to develop and grow.

Full PPE and annual uniform/PPE allowance.

On site Occupational Health.

Cycle to work scheme.

Social Club with regular events throughout the year.

Family friendly procedures including enhanced maternity leave and menopause procedure.

To apply, please click the “Apply for this position” link at the end of the advert and upload your CV to arrive by 1600 hours on Friday 20th December 2024.

Holmen is an equal opportunities employer that values diversity and is strongly committed to providing equal opportunities for all candidates and employees.

Employment level: Permanent employment

Working hours: Day

Occupation degree: Full time Location: Workington

Last day of application: 20 December, 2024

Val Warren

Human Resources Advisor

T: 01900 600127

E: val.warren@holmen.com

Gary Pickering

Production Manager Paperboard

T: 01900 600351

E: gary.pickering@holmen.com

PAPERmaking! Vol10 No3 2024

making!

The e-magazine for the Fibrous Forest Products Sector

PAPERmaking!

CONTENTS:

PAPERmaking!

Functional surfaces, films, and coatings with lignin – a critical review

RSCAdvances

REVIEW

1.Introduction

Functionalsurfaces, ﬁlms,andcoatingswithlignin

2.Fundamentals

2.3.Chemicalmodication

2.4.Analysistechniques

3.Formulationsandapplicationsof lignin-basedsurfacesandcoatings

4.Ligninintechnicalapplications –acriticalcommentary

5.Summaryandconclusion

Authorcontributions

Conﬂictsofinterest

Acknowledgements

References

PAPERmaking!

nanomaterials

2.2.CelluloseNanoﬁberProduction

2.3.CharacterizationofthePulps

2.3.2.Morphology

2.4.CharacterizationoftheCNFs

3.ResultsandDiscussion

PAPERmaking!

Facile Strategy for Boosting of Inorganic Fillers Retention in Paper

polymers

FacileStrategyforBoostingofInorganicFillersRetention inPaper

1.Introduction

2.ExperimentalPart

2.5.PreparationofPEGSolution

2.7.Characterization

3.Results

4.Conclusions

PAPERmaking!

ChemicalPulping

Evaluationofoxygendelignified fiberswithhigh waterabsorbency,asagreeneralternativetofully bleached fibersfortissuepaper

1Introduction

2Materialsandmethods

2.2.2Oxygendeligniﬁcation

ﬁning,papermaking

2.3Analysisofpulpandpaperproperties

3Resultsanddiscussion

3.2Total ﬁbercharge

3.3Waterretentionvalue

3.4Absorptionandcapillarityproperties

3.5Mechanicalproperties

3.5.2Wettensileproperties

3.6Softness

4Conclusions

References

PAPERmaking!

Comparison of Degradation of Lignincontaining Wastewaters in the Presence of Different Microbial Consortia

DECLARATION OF COMPETING INTEREST

ACKNOWLEDGMENTS

PAPERmaking!

Which Wastepaper Should Not Be Processed?

sustainability

WhichWastepaperShouldNotBeProcessed?

2.2.Non-FibrisedSubstancesandChemicalAnalysisofPulp

2.5.PreparationofPaperSheets

2.6.AnalysisofPaperProperties

4.Conclusions

PAPERmaking!

Production of wood based panel from recycled wood resource: a literature review

REVIEW ARTICLE

Production of wood-based panel from recycled wood resource: a literature review

1 Introduction

2 Methods

3 Results and discussion

3.1.2 Target composite type

in waste wood

3.2.3 Formaldehyde emission

4 Conclusion

References

PAPERmaking!