Comparison of the Technical Performance of Leather, Artificial Leather, and Trendy Alternatives

Abstract

The market for biogenic and synthetic alternatives to leather is increasing aiming to replace animal-based materials with vegan alternatives. In parallel, bio-based raw materials should be used instead of fossil-based synthetic raw materials. In this study, a shoe upper leather and an artificial leather, and nine alternative materials (Desserto®, Kombucha, Pinatex®, Noani®, Appleskin®, Vegea®, SnapPap®, Teak Leaf®, and Muskin®) were investigated. We aimed to compare the structure and technical performance of the materials, which allows an estimation of possible application areas. Structure and composition were characterized by microscopy and FTIR spectroscopy, the surface properties, mechanical performance, water vapor permeability, and water absorption by standardized physical tests. None of the leather alternatives showed the universal performance of leather. Nevertheless, some materials achieved high values in selected properties. It is speculated that the grown multilayer structure of leather with a very tight surface and a gradient of the structural density over the cross-section causes this universal performance. To date, this structure could neither be achieved with synthetic nor with bio-based materials.

Full text

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Coatings2021,11,226.https://doi.org/10.3390/coatings11020226www.mdpi.com/journal/coatings
Article
ComparisonoftheTechnicalPerformanceofLeather,Artificial
Leather,andTrendyAlternatives
MichaelMeyer*,SaschaDietrich,HaikoSchulzandAnkeMondschein
FILKFreibergInstitutegGmbH,MeißnerRing1,09599Freiberg,Germany;
sascha.dietrich@filkfreiberg.de(S.D.);haiko.schulz@filkfreiberg.de(H.S.);
anke.mondschein@filkfreiberg.de(A.M.)
*Correspondence:michael.meyer@filkfreiberg.de
Abstract:Themarketforbiogenicandsyntheticalternativestoleatherisincreasingaimingtore‐
placeanimal‐basedmaterialswithveganalternatives.Inparallel,bio‐basedrawmaterialsshould
beusedinsteadoffossil‐basedsyntheticrawmaterials.Inthisstudy,ashoeupperleatherandan
artificialleather,andninealternativematerials(Desserto
®
,Kombucha,Pinatex
®
,Noani
®
,Apple‐
skin
®
,Vegea
®
,SnapPap
®
,TeakLeaf
®
,andMuskin
®
)wereinvestigated.Weaimedtocomparethe
structureandtechnicalperformanceofthematerials,whichallowsanestimationofpossibleappli‐
cationareas.StructureandcompositionwerecharacterizedbymicroscopyandFTIRspectroscopy,
thesurfaceproperties,mechanicalperformance,watervaporpermeability,andwaterabsorptionby
standardizedphysicaltests.Noneoftheleatheralternativesshowedtheuniversalperformanceof
leather.Nevertheless,somematerialsachievedhighvaluesinselectedproperties.Itisspeculated
thatthegrownmultilayerstructureofleatherwithaverytightsurfaceandagradientofthestruc‐
turaldensityoverthecross‐sectioncausesthisuniversalperformance.Todate,thisstructurecould
neitherbeachievedwithsyntheticnorwithbio‐basedmaterials.
Keywords:leather;syntheticleather;apparel;consumergood;leatheralternative

1.Introduction
Acirculareconomyaimsatreusingconsumedmaterialsandideally,productcycles
becomeclosedaccordingtothecradle‐to‐cradleprinciple[1,2].“Bio‐based”meanstheuse
ofbiogenicrawmaterialstomanufactureavarietyofproductsinsteadoffossilgas,coal,
orpetroleumaspartofthebioeconomy.Lastly,“biodegradable”meansthatamaterial
canbedegradedintheenvironmentbymicroorganismsandphysicochemicalimpact.Re‐
cently,thesocietiesofthecountriesoftheGlobalNorthhaveexperiencedastrongchange
intheirmindsetduetothediscussionaboutclimatechange,finitenessofresources,the
overutilizationofecosystems,andthepollutionoftheenvironmentbynon‐degradableor
harmfulsubstances.Thisaffectsespeciallytheconsumergoodsindustryandthedesign‐
ersofnewmaterialsaimtoreplacefossil‐basedpolymerswithbiogenicandfullybiode‐
gradablematerialswhilebeinganimal‐freeandwithouttheuseofanyharmfulsub‐
stances.Ideally,thenewmaterialsaremadefromdomesticwaste,sawdust,ororganic
garbage[3–5].
Leatherisabio‐basedandbiodegradablematerialwithatraditionnearlyaslongas
mankind.Forcenturies,itwasusedasastrongandlong‐lastingmaterialwithabroad
spectrumofmaterialsproperties.Leatherwasusedasprotectiveanddecorativeclothing
forsportsgoodsandastechnicalmaterial,e.g.,fortransmissionbelts,buckets,oraswine‐
skin.Untilthemiddleofthe19thcentury,leatheroccupiedthematerialspropertygapof
aflexiblematerialbesidesstone,metal,andwoodashardmaterialsandvarioustextiles,
whichwerenotwaterproof.Processingallowedadjustingtheleatherpropertiesfroma
Citation:Meyer,M.;Dietrich,S.;
Schulz,H.;andMondschein,A.;
ComparisonoftheTechnicalPerfor‐
manceofLeather,Artificialleather,
andTrendyAlternatives.Coatings
2021,11,226.
https://doi.org/10.3390/coat‐
ings11020226
Received:25January2021
Accepted:11February2021
Published:13February2021
Publisher’sNote:MDPIstaysneu‐
tralwithregardtojurisdictional
claimsinpublishedmapsandinsti‐
tutionalaffiliations.

Copyright:©2021bytheauthors.
LicenseeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsand
conditionsoftheCreativeCommons
Attribution(CCBY)license
(http://creativecommons.org/licenses
/by/4.0/).

Coatings2021,11,2262of15


hardboard‐likeappearance,e.g.,assoleleathertoverysofttouchtextile‐likegloveleath‐
ers.Tomanufactureshoes,leatherismadehydrophobic,andaswashleather,itabsorbs
muchliquid.Thebasicstructureisananimalskin,whichcanbedescribedasanon‐woven
withdifferentdensitygradientsingrain,papillaryandreticularlayer(Figure1A).
Duringindustrialization,alternativematerialswereinvented,firstandforemostthe
oilclothesmadefromtextiles(linen,cotton),whichweresoakedorcoatedwithboiled
linseedoilandaddedwithfillers,siccatives,andpigments[6].Thenextlevelofcoating
textileswasachievedbytheuseofnaturalrubberbutonlythediscoveryofvulcanization
ledtonon‐stickyfilms[7].Moreandmorenewmaterialsemergedwiththeinventionof
additionalsyntheticpolymers,whichallowedthereplacementofleatherinmanyappli‐
cations.Syntheticpolymersenabledcustomizedandhigh‐performancesolutionsthatout‐
performedleatherbyfarfortechnicalapplicationsingears,conveyorbelts,orvessels.
Alternativesweredevelopedaswellindesign‐drivenapplicationssuchasupholstery,
shoes,andclothing.Often,itisaimedtoimitateleatherbythesematerials.

Figure1.Theprincipalstructureofleather(A)andartificialleather(B).Leatherstillreflectsthe
histologicalstructureofskinwithgrainmembrane(a),papillarylayer(b),reticularlayer(c),and
hairchannels(d).Artificialleatheriscomposedofatopcoat(e),acompactlayer(f),afoamedmid‐
dlelayer(g),andthetextilesupport(h)(drawingsbyEnnoKluever,FILK).
Leathershowsanumberofuniqueproperties,whicharehighlyvaluedforpurposes
suchasstrengthandelasticity,watervaporpermeability,abrasionresistance,durability,
andlongevity.Inthepast,syntheticmaterialscompetingwithleathertriumpheddueto
lowerprices,theyareofteneasiertobeprocessedandcanbemanufacturedasacontinu‐
ousmaterialaccordingtoindustrialneedsinroll‐to‐rollproductionlines.However,
leatherisstillpopularduetoitsbeneficialproperties,naturalappearance,andatouchof
noblematerial.
Syntheticalternativesusuallyconsistoftextilesupportcoveredbytwoormoresyn‐
theticpolymerlayers(Figure1B).Nowadays,oftenpolyestertextilescoatedbyPVCor
polyurethanefilmsareused,makingthemacompletelyfossil‐basedmaterial.Thesurface
opticcanbedesignedleather‐likebyembossingagrainstructure.Manydifferentterms
areusedtodescribethesematerialsinthemarket,e.g.,artificialleather,syntheticleather,
leatherette,imitationleather,fauxleather,man‐madeleather,bondedleather,pleather,
textileleather,orpolyurethane(PU)‐leather.Meanwhile,theusageofthesetermsisre‐
strictedintheEuropeanstandardEN15987.Here,wewillusetheterm“artificialleather”
todescribesyntheticmaterialsimitatingtheopticalappearanceofleather.
Inrecentyears,concernsoversustainabilityinanyfieldofindustrialproductionhave
ledtoapressingrationaletoenhancetheuseofnaturalmaterialsandreplacenon‐renew‐
ablefossil‐basedrawmaterials.Althoughleatherisbio‐basedandrenewable,thesecon‐
siderationsdidnotleadtoarenaissanceofleather.Instead,leathergotevenmoreunder
pressureduetoongoingdiscussionsoverthegreenhousegasemissionofcattlebreeding,

Coatings2021,11,2263of15
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thesustainabilityofleatherproduction,andanimalwelfare.Atthesametime,anincreas‐
ingnumberofpeoplewanttoeatconsciouslymeat‐freeortodowithoutanyproductsof
animaloriginentirely.Alltheseneedsposenewchallengesincultureandmaterialdevel‐
opment[3].
Onestrategypursuesthedevelopmentofalternativenature‐based,animal‐freefi‐
brousmaterials.Thismaterialistrama,thebulkmaterialofsomemushroomfruitbodies
(e.g.,Fomesfomentarius,Phellinusellipsoideus).Theextraordinarysoftfeelofthedrymyce‐
liummakesitapreciousmaterialforcupsandhandcraftaccessoriesandalreadytheIce‐
manuseditasamaterialincombinationwithleather[8,9].Muskin®isanexampleofthis
material.Duetothecomplicatedharvest,therestrictedavailabilityofthementioned
fungi,andthelimitedareasthatcanbeobtaineditseemstobefarfrombeingableto
replaceleather.
Furthernewwaysarepavedbyusingbiotechnologicalprocessestoproducefiber‐
basedmaterials.Namely,fungiandsymbiosisofbacteriaandyeastareusedtoproduce
fibrousnetworksaimingtoimitatethefibrousstructuresimilartoananimalskinassingle
materialsorassupportforacoatinglayer.Micro‐cellulosicfibernetworksareproduced
bybacteria(e.g.,Acetobacterxylinum),themyceliumfibernetworksoffungihyphaecon‐
sistofchitin,cellulose,andproteoglycans[5,10,11].Thesemyceliagrowonorganicwaste
[11,12].
Inasecondstrategy,itistriedtoreducethenon‐renewablecontentofartificialleather
byreplacingpartsofthesyntheticcomponentpolyvinylchloride(PVC)orpolyurethane
(PUR)ofsyntheticcoatingswithagriculturalwaste‐derivedproductsasfillingmaterial,
suchasgrain,applepomace(Vegea®,Appleskin®),ormilledcactusleaves(Desserto®).
Athirdwaytoreplaceallfossil‐basedrawmaterialsinacoatedtextilehasbeenex‐
ploredinPinatex®.Renewablefibersofpineappleleavesareprocessedintonon‐woven
supportcoatedwithpolylacticacid(PLA)producedfromcornstarch[13].
Regardlessofthetypeofmaterial,itcanbeleather,artificialleather,oratrendyal‐
ternativeacoupleofphysicalandmechanicallimitsareusuallydefinedandhavetobe
achieved.Theselimitsmustbeevaluatedinregardtothestressesassociatedwiththepro‐
duction,processing,anduseofthematerials.Ingeneral,examinationstoqualifymaterials
andtoquantifytheirpropertiesneedtobeperformedaccordingtostandardizedtesting
procedures.
Here,wepresentacomparativestudyofleatherandalternativematerials,whichare
usedforsimilarfinalapplications,focusedonmaterialstructures,physical,andmechan‐
icalperformance.Additionally,thematerialswerescreenedforhazardoussubstancesby
establishedstandardizedtestmethodswithrespecttoshoe,glove,andapparelgoodsap‐
plications.Materialsintendedforautomotiveandupholsteryapplicationsareexplicitly
notincludedinthisstudy,becausetheyhavetomeetmanysuperiorspecifications.
Wefocusedonmaterialperformance.Otherimportantaspectssuchastheoriginof
therawmaterial(renewableoroil‐based),thecarbonfootprint,theenvironmentalfoot‐
print,traceability,andbiodegradabilityarenotdealtwithinthisstudy.Alternativema‐
terialsofdifferentsourcesweretestedincomparisontoacommonshoe‐upperleatheras
areferenceandaconventionalPUR‐coatedtextile(artificialleather)asusedforfootwear
asasecondreference.Allmaterialsarecommerciallyavailableandhavealreadybeen
appliedinvarioustypesoffinalproducts.Thematerialsweretestedaccordingtotheap‐
propriateinternationallyharmonizedandacceptedspecificationsforshoe,glove,andap‐
parelgoods[14–16].
2.MaterialsandMethods
Ninematerials,whichareofferedasanalternativeforleatherandrepresentdifferent
principalstructures,havebeeninvestigatedbylightmicroscopy;theirphysicalproperties
weremeasuredandtheirchemicalcompositionswereanalyzed(Table1).Theregistered

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trademarksarespecifiedinthetable.Additionally,ashoeupperleatherandapolyure‐
thane‐coatedtextileasartificialleatherweretestedasareference.TheNoani®samplewas
obtainedasabeltconsistingofseveralmaterials,whichhadbeencombinedbysewing.
Thephysicalcharacterizationcomprisedstandardizedmeasurementsofthickness,
tensilestrength,tearstrength,flexresistance,watervaporabsorption,andwatervapor
permeability[17–22].Thecrosssections,surfaces,andthereversesideofthematerials
wereportrayedbylightmicroscopyatdifferentmagnifications.Chemicalconstitution
andadditiveswereinvestigatedbyFTIR(fouriertransforminfraredspectroscopy)and
thermaldesorptionGC/MS(gaschromatography/massspectroscopy).FTIRspectrawere
measuredusingadiamondATR(attenuatedtotalreflection)techniquewith16scansina
rangeof4000–650cm−1.Evaluationofthespectrawasbasedonownandvariouscommer‐
cialdatabases.VolatileandharmfulsubstancesweremeasuredaccordingtoVDA278[23]
(VDA–VerbandderAutomobilindustrie,Germany).Atotalof5mgto10mgofmaterial
wereheatedto120°Cfor60min,andallvolatilecompoundswerecollectedfromthe
evaporatebycoolingwithliquidnitrogentominus100°C.Thesubstanceswerevaporized
fromthetrapfor5minat280°C,separatedandcharacterizedbyGC/MS(GC:50°Cfor2
min,25K/minto160°C,10K/minto280,10minat280°C,columnUltra2(5%phenyl‐
methylsiloxane),50m×320μm×0.52μm,flow1.3mL/minconstantpressure,MS:29–
450m/z).
Table1.Samplesandtheircomposition.
NameColorComposition(fromtoptobackside)
Shoe‐upper
leatherbrownFull‐grainbovineleather;typicalleatherstructure;finishedwithaverythintopcoat
Muskin®brownFinelyfibrous,porous,andnaturallygrownmaterialinasinglelayer;naturalfibersbasedonpolysaccha‐
rides;withoutanycoatingortextilebacking
KombuchabrownDensecompactmaterialbasedonpolysaccharides,containstalcum,someheterogeneousinclusionsofun‐
clearorigin
PURcoated
textilegrey
Coatedtextilewiththincompacttopcoat(PUR),alayerunderneath(PUR)filledwithheterogeneousparti‐
clesmadeofmodifiedcelluloseandatextilecarrierbasedonpolyester,materialmadebycoagulationpro‐
cess
Desserto®beigeCoatedtextilewithacompactlayer(PUR)andpartiallyfoamedlayer(PUR)filledwithheterogeneousparti‐
clesoforganicorigin,textilecarrierbasedonpolyester,materialmadebyareversecoatingprocess
Appleskin®blackCoatedtextilewiththincompactlayers(PUR),afoamedlayer(PUR)filledwithorganicparticlesandaPUR
impregnatedtextilecarrier,materialmadebycoagulationprocess
Vegea®blackCoatedtextilewithacompactlayer(PUR),apartiallyfoamedlayer(PUR)filledwithsomeparticles,anda
cellulose‐basedtextilecarrier
TeakLeaf®russet
Coatedandlaminatedleaves,topcoat:atransparentfilmbasedonpolyolefinwax,leaves,laminationon
backofleaveswithtwonon‐woventextilelayers(lightbrown–cellulose‐basedfiberswithacrylate‐based
binderandwhite‐polypropylenefibers)
Pinatex®blackNon‐wovenmaterialmadeofcellulose‐basednaturalfibers,coatedwithathinpolymericlayer(similarto
polyurethaneacrylate)
SnapPap®brownDensesinglelayermaterialwithanon‐wovenstructuremadeofcellulosicfibersandimpregnatedwith
acrylate‐basedpolymer
Noani®mixed
Compositematerialmadeupofthreeindividualmainlayers:Upperlayerpolyestermicrofibermaterial,
corelayerleatherfiberboard,backsidelayercoatedtextilewithacompactlayer(PUR),afoamedlayerPVC,
andatextilecarrier
Forevaluation,10well‐trainedpanelistsassessedthematerialstoevaluatetouchand
feel.Nomaterialrelatedinformationwasprovidedtotheminadvanceoftheevaluation.
Thematerialswerepresentedtothepanelistsalwaysinthesameordereitherbylayingit
directlyonatableboardorplacingitonasoftPURfoamof4mmthickness.Thesurfaces
ofthematerialswereblindlytouchedwithoutanystretchingorfolding.Thetouchand
feelpropertiesofallmaterialswerereferencedtoleather—temperaturesensation(warmer

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orcolder),deformationinz‐direction(softerorharder),roughness,slipperiness/blocking
behavior,pleasantorunpleasant,naturalorartificialtouch,high‐orlow‐qualityfeeling.
3.ResearchResults
3.1.MaterialCompositionandStructure
Allmaterialsarecomposedoffibers.However,chemicalconstitution,arrangement,
fibersize,andcoatingfollowdifferentprincipalconcepts.Theresultsofthelightmicro‐
scopicinvestigations,thermaldesorptionanalysis,andFTIRspectraallowustoidentify
thecompositionsofthematerials(Table1).
Aselectionofmicroscopicpicturesofthecrosssectionsrepresentingthedifferent
principalstructuresisshowninFigure2,andthedifferentsurfacedesignsarepresented
inFigure3.Allmicroscopicpicturesareaddedinhighresolutioninthesupplementsas
wellasanexampleofaFTIRspectrumofDesserto®,aPURcoatedmaterial(seeSupple‐
mentaryMaterials).
ThestructuresofDesserto®(Figure2A),Vegea®,andAppleskin®reflectthetypical
compositionofPUR‐coatedartificialleatherusedfor,e.g.,shoeorupholsteryapplication.
Thesupportoftheinvestigatedsamplesconsistedofknittedorwovenpolyestertextiles,
exceptthatofVegea®,whichwasmadeofcellulose.Thetextileswerecoatedwithfoamed
polyurethane‐basedmiddlelayerscontainingorganicfillersbasedoncellulose.Thema‐
terialsarefinishedwithpolymer‐basedtopcoats.Thesurfacesofthematerialswerepartly
embossedtoachievealeather‐likeopticandtoadjustthehaptic(Figure3E–H).EvenTeak
Leaf®fallsintothestructuralcategoryofacoatedtextile.Here,thetextilesupportonthe
reversesideisbuiltbytwonon‐wovenlayers.Themiddlelayeriscellulose‐based,the
fibersarestucktogetherwithanacrylicacid‐basedpolymericbinder,andthebasicsup‐
portonthereversesideispolyester‐based.Onthetop,theTeakLeaf®materialiscoated
byatransparentwaxypolyolefinfilm.Theleafofteakmainlyfulfillsdesiredopticalneeds
(Figure3D).


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Figure2.Leather(A)andamaterialwithanaturallygrownfiberstructure(Muskin®)(B);struc‐
tureofatypicalcoatedtextile(exampleDesserto®(C);(a)topcoat;(b)foamedmiddlelayer;(c)
textilesupport);non‐wovenmadeofnaturalfibers(examplePinatex®)(D).Thescalesbetween
Muskin®andtheothersamplesdifferby2.5times.
Pinatex®(Figure2D)andSnapPap®consistofacellulose‐basedfibrousnon‐woven.
ThefibersofSnapPap®arebondedbyanacrylicacid‐basedpolymericbinder.Incontrast,
theinvestigatedsampleofPinatex®iscoatedwithathinpolymericlayer.Thefibrous
structuresofthenon‐wovenfabricofbothmaterialsarevisibleatthesurfaceduetothe
thincoatinglayer(Figure3I,K).
Muskin®(Figure2B)andKombuchaaresingle‐layermaterialswithoutanytextile
supportandwithoutatopcoat.Bothconsistofpolysaccharides.Muskin®appearsvery
porouscomposedoffinebrownfiberswithafiberorientationperpendiculartothesur‐
face.Thesefibersappearatthesurfaceasafinelawnwithoutadistinctstructure(Figure
3B).Incontrast,Kombuchaconsistsofonecompactlayer,whichcontainssomeinclusions
andtalcumandshowsaglossybrownsurface(Figure3C).
ItwasfoundthatNoani®wasnotasinglematerialbutcraftedofthreedistinctlayers.
Thetopisformedbyanembossedmicrofibermaterial,themiddlelayerconsistsofleather
boardmaterialandthebacksideisaPUR/PVC‐coatedtextilethatconsistsofthreelayers
typicalforconventionalartificialleather.Theuseofaleatherboardmaterialissurprising
becauseNoani®isreferredtoasaveganproductandthelabel“PETAapproved”isem‐
bossedinthematerial.BecauseofthisandsincetheNoani®sampledoesnotrepresenta
singlematerial,itwillnotbefurtherdiscussedwithrespecttostructuralcategories.Re‐
gardlessoftheactualcomposition,allengineeredmaterialstrytomimicanaturalappear‐
ance(Figure3).

Figure3.Surfacesoftheinvestigatedmaterials:Naturallygrownmaterial(Leather(A),Muskin®
(B),Kombucha(C),TeakLeaf®(D));embossedsurfacesofcoatedtextiles(PUR‐coatedtextile(E),
Desserto®(F),Appleskin®(G),Vegea®(H));fiberstructuresofnon‐wovennaturalfibers(Pinatex®
(I),SnapPap®(K)),andmicro‐fibermaterial(Noani®(L),embossed).
3.2.TouchandFeelProperties
ThesurfaceofMuskin®andtheuppermaterialofNoani®feelpleasant.Duetothe
presenceofveryfinefibers,bothmaterialscreateavelvetyfeelingsimilartosuedeleather.
Thematerialswithsyntheticsurfacesasinartificialleathers(Desserto®,Appleskin®,

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Vegea®,PUR‐coatedtextile)showasoftfeelingandcanbedeformedinthez‐direction.
However,thetouchofthesematerialsappearsartificiallywithastickytendency.Thesur‐
faceofPinatex®andTeakLeaf®appearssynthetic,too.Inaddition,Desserto®,Pinatex®,
andSnapPap®areevaluatedtofeelrough.ThesurfaceofKombuchaappearedsticky.
3.3.Thickness
Thethicknessofthematerialswasdeterminedbysurveyingthepreparedmicro‐
scopiccrosssections.Theoverallthicknessofallmaterialswasfoundtorangefrom0.29
mmto6.22mm,whichalsoshowsthevarietyofthematerialtypes(Table2).Thethickness
ofleather,thePUR‐coatedtextile,andthetrendyalternativesDesserto®,Appleskin®,
Vegea®,andPinatex®areinatypicalrangeformaterialsusedforshoes,gloves,andap‐
parelgoodsapplications.ThematerialsKombucha,SnapPap®,andTeakLeaf®appear
verythinandincontrast,Muskin®isverythickforthementionedapplications.Inconse‐
quence,alreadyfromtheresultsofthethicknessmeasurements,significantdifferencesin
thematerialpropertiescouldbeexpected,e.g.,indeformationproperties.
Table2.Physicalproperties.
Physicalproperties
ThicknessTensileStrengthTearRes.WVPWVAFlexRes.
ISO17186‐AISO3376ISO3377‐1ISO14268ISO17229ISO32100
(mm)(N/mm2)(N/mm)(mg/(cm2×h))mg/cm2FlexCyclesto
Grade≥2
Naturallygrown
material
Leather1.9339.582.94.68.4>200,000
Muskin®6.220.20.510.46.010,000
Kombucha0.299.75.10.19.210,000
Coatedtextile
PURcoattext.1.3710.2171.11.4200,000
Desserto®0.8820.837.20.52.530,000
Appleskin®1.141418.40.41.750,000
Vegea® 0.959.416.60.63.050,000
TeakLeaf®0.5712.230.70.10.1100
Non‐wovensofplant
fibers
Pinatex®1.434.5312.53.8150,000
SnapPap®0.5724.97.510.33.75,000
(WVP:watervaporpermeability;WVA:watervaporabsorption;TearRes:tearresistance).
3.4.TensileStrengthandTearStrength
Themostimportantmechanicalpropertiesformaterialsusedforshoes,gloves,and
apparelgoodsaretensilestrengthandtearstrength[17,18].Theresultsforbothparame‐
tersvaryoveraverywiderange,whereasthecategoryofnaturallygrowntissuesshows
thebroadestrange.
Leatherasthegrownskintissueshowsaveryhighmechanicalstability(tensile
strength,tearstrength),representingthehighestvalueforthetensilestrengthandtear
strengthwithinthatsurvey.Tensilestrengthexceedsthespecificationof>15N/mm2for
chrometannedupperleatherforshoes(ISO20942)[14].Incontrast,thevaluesoftheMus‐
kin®sampleareextremelylow.
Thecoatedtextilesshowtensilestrengthsof9upto20N/mm2.Thetensilestrength
ofcoatedtextilesdependsmainlyonthepropertiesofthesupportingfabric.Theresults
showareasonablechoiceoffabricfortheintendeduseoftheseengineeredmaterialsre‐
gardlessofthenaturalorartificialoriginofthefabricfibers.
Thetensilestrengthsofthenon‐wovenmaterialsmadeofnaturalplantfibersrange
from4upto25N/mm2.Strengthdependsonfiberpropertiesandfiberbonding.Despite
asatisfyingtensilestrength,thetearresistanceofSnapPap®islowduetotheshortfiber
length,whichcannotbeleveragedbythepolymericbinder.Exceptforthenon‐woven
materials,tearstrengthfollowsthesametendencyastensilestrength.

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3.5.FlexResistance
Materialsforshoesmustresistintensivebendingandconvexandconcavedefor‐
mationduringusage.Theflexometertestisusedtoassessthelong‐timeresistanceagainst
bending.Grade0isthebestratingindicatingthatthematerialitselfandthecoatinglayers
showsnocracksbyflexing.Agradeof≤2(onlyverysmallcracksintoplayerofthecoat‐
ing)isusuallyacceptedtopassthattest.Whenagradeof>2isobserved,flexinghastobe
stoppedandthenumberofflexcyclesisnoted.Leather,Pinatex®,andPUR‐coatedtextile
fulfilledthespecificationof>80.000flexcyclesaccordingtoISO20942.Theflexresistance
ofTeakLeaf®,SnapPap®,andMuskin®materialwasfoundtobeinsufficientfortheaimed
applications.
3.6.WaterVaporPermeabilityandWaterVaporAbsorption
Apleasantwearingcomfortofshoes,gloves,orapparelisrelatedtothewatervapor
permeability(WVP)ofthematerial,whichallowstransportingthehumidityofthebody
throughtheclothingmaterialtoitssurface.Thecomfortisalsoenhancedbytheabilityof
thematerialstoabsorbwatervapor.Comfortablewatervaporpermeabilitylimitsare
specifiedinISO20942tobe>>0.8mg/(cm2×h).Leather,Muskin®,andSnapPap®exceed
thewatervaporpermeabilitybyfar,andPinatex®andPUR‐coatedtextilestillfulfillthe
ISO20942requirement.TheWVPofallothermaterialsisinsufficient.
Watervaporabsorption(WVA)ofKombucha,leather,andMuskin®ishigh,presum‐
ablyduetotheirpolarnatureofthenaturalpolymers.Theothermaterials,especially
thosethatcontainasignificantamountofsyntheticpolymerwithlesspolarityshowmuch
lowerwatervaporabsorption.
3.7.HarmfulSubstances
Thematerialswereexaminedforpotentiallyhazardoussubstancesbymeansofther‐
maldesorptionanalysis.Inseveralsamples(Appleskin®,Pinatex®,Desserto®,Vegea®,
SnapPap®,TeakLeaf®)syntheticandbiogenicrawmaterialshadbeencombined.How‐
ever,theprocessingoffossil‐basedrawmaterialsoftenrequirestheapplicationofsol‐
vents,crosslinkingagents,orplasticizerstoachievesuitablematerialproperties.
Alltestedmaterialsemittedvolatileorganiccompoundswhenappliedtothethermal
desorptionscreeningprocedure.Restrictedsubstanceswereidentifiedinthesamplesof
PUR‐coatedtextile(reference),thesimilarlyconstructedmaterialsDesserto®,Appleskin®,
andVegea®,butalsoinPinatex®.ThePUR‐coatedtextilecontainedconsiderableamounts
ofdimethylformamide(DMFa)andtolueneandtracesofN,N‐dimethylacetamide.InAp‐
pleskin®,butanoneoximeandtracesofDMFaweredetected.Desserto®containedthefive
restrictedsubstancesbutanoneoxime,toluene,freeisocyanate,folpet(anorganicpesti‐
cide),andtracesoftheplasticizerDiisobutylphthalate(DIBP).Toluenewasdetectedin
thesampleofVegea®andDIBPinthatofPinatex®.
4.Discussion
Thematerialsthathavebeentestedinthisstudyareusedtomanufacturefashionable
consumergoodsasshoes,bags,clothes,andaccessories.Inthisregard,aspectsof(1)func‐
tionalpropertiesand(2)theappearancehavetobediscussed.Whiletheconstructionof
thematerialmainlyinfluencesthefunctionalproperties,theappearanceismarkedlya
resultofthesurfaceproperties.Bothgroupsofpropertiesvariedonaverybroadscale,
thoughthematerialsareofferedforsimilarfinalapplications.Analyzingthecomposition,
theconstruction,thesurfaces,andthefeelallowedustocomparethematerialsinregard
totheirpossibleperformance.Forthispurpose,thematerialswereinvestigatedbystand‐
ardizedtestingproceduresforleathersincethematerialsareofferedasaleatheralterna‐
tive.
4.1.Structure

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Basedonthestructuraldesignandwithrespecttofunctionalproperties,theinvesti‐
gatedsamplescanbegroupedintothreecompletelydifferentmaterialconcepts,which
are(a)grownanimal‐freematerials,(b)multi‐layeredcoatedfabricsasforartificialleather
combinedwithplant‐basedadditives,and(c)non‐wovenfabricswithorwithoutasurface
finish.
AsderivedfromtheresultsoftheFTIRanalysis,thefunctionallayersofthecoated
fabricsaremainlyPURbased.Incontrast,fullybio‐basednaturallygrownmaterialsin‐
vestigatedinthisstudydidnotnearlyfulfillthemechanicalrequirementsexpected,e.g.,
forshoeuppermaterials.Thequestionarises,whyanimal‐freematerials(a),whichare
directlymanufacturedbypreservingagrownstructurecombinedwithmoreorlessin‐
tensiveprocessing,showonlypoormechanicalresistancecomparedtoleather.
Theresultingstructuresofgrownnaturalmaterialsareintendedtobeanalternative
forthefibrousstructureofanimalskin[11,24,25].Atypicalrepresentativeisthefungus‐
basedmaterialMuskin®takenfromPhellinusellipsoideus.Themicroscopicpicturesshow
thatthemyceliumiscomposedofhyphaethatrepresentthestructure‐formingcompo‐
nent.Ourmeasurementsshowedmainlypolysaccharidesaschemicalstructures.There‐
fore,itcanbeassumedthatchitinformsthehyphae.TheFTIRspectrumexactlyfitsthat
ofotherinvestigations[26].Thelatterinterpretedthespectrainmoredetailandalsoas‐
signedproteins,lipids,andnucleicacids.However,themechanicalstabilityofthemyce‐
liumislimited.Thismaybecausedbythelimitedstabilityofthehyphaeitself,andthe
processedmyceliumasshownelsewhere[11,27].Thepicturesofthecrosssectionshow
thatthefibersareorientedperpendiculartothesurface.Thisalsoleadstopoormechanical
stability.Presumably,themechanicalperformancecouldbeimprovedifthefiberswere
alignedalongthedirectionofthemechanicalload.
Incontrast,KombuchaisatraditionalJapanesebeverage,whichispreparedbya
symbiosisofbacteriaandyeastsmetabolizingsugarsintoorganicacids,ethanol,andcar‐
bondioxide.Inparallel,bacteria(e.g.,Acetobacterxylinum)aresecretinghighmolecular
weightpolysaccharides,whichleadtoagel‐likeconsistency.Theintensivelygrowingmi‐
croorganismscanbeharvestedandthesecretedpolymersareusedasabiogenicstructure‐
formingmaterialafteradryingstep[10].ThemechanicalstabilityoftheKombuchama‐
terialsamplewasmuchhigherthanthatofMuskin®butmissedtherequirementsforshoe
uppermaterialsaswell.WhileMuskin®showsalooseandopenstructure,whichallows
watervaportodiffusethroughthematerial,Kombuchaisverytightbutitabsorbshigher
amountsofwater.Bothmicroorganism‐basedmaterialsappearhomogenousintheircross
sections.
Therawmaterialofleather,theanimalskiniscomposedofcollagen,astructure‐
formingprotein.Leatherisbuiltfromintertwinedfibrilsandfibers,whichareaddition‐
allycross‐linkedbyleathertanning.Thefibrousstructureofleathershowsagradientin
itsmaterialdensityfromthegraintothereverseside.Thefinallayerontopiscomposed
ofverythinandtightcollagenfibers.Thegrownskintissueshowsaveryhighmechanical
stability(tensilestrength,tearstrength),whichisby100to1000timeshigherthanthatof
themicroorganism‐basedmaterials[5,10,11,25,26,28].Thestrengthofleathercanbere‐
latedtothestabilityofthecollagenfibersthemselvesandtotheweavingandcrosslinks
betweenthefibers.
Fromthebiologicalpointofview,hyphaeofmicroorganismsandtheanimalskin
fulfillverydifferenttasksintherespectiveorganisms.Theperformanceofnaturalmate‐
rialshadbeensummarizedinthepastinAshbyplots.Figure4providesanimpressive
overviewofacoupleofdifferentnaturalmaterialsandtheirmechanicallimits[29].Nat‐
uralflexibleloadtransferringmaterialsappearwithadensityof~1Mg/m3andrangefrom
0.1MPatomorethan1000MPa(markedred).Thetensilestrengthofparenchymatous
planttissuesislow(markedblue).Collagen‐basedmaterialssuchasskinandtendonap‐
peartobeverystable(redellipse).Highstabilityisalsoobservedinstructuresofplants,
whichhavetotransferload(wood,cellulosefibers).

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Itcanbededucedfromtheseconsiderationsthatgrownnaturalmaterialswillonly
exhibitflexibleandmechanicalresiliencewhenloadtransferringstructuresareused.The
animalskincoversabroadsetofaimedfunctions.Ithastoprotectthebodyforalong
timeagainstmechanicalimpact,itisflexibletoallowmobility,andoftenithastoregulate
thetemperatureandwaterbalance.Plantfibershavetoabsorbtheweightoftheplant
againstgravitationandinloaddirection.
Incontrast,thetissue‐likestructuresofmicroorganismsappearasparenchymatous
materials,basedonfibernetworks,whichprimarilyofferactivecellsamatrixformetab‐
olism(bacteria,fungi)andtransportationofnutrients(hyphae).Therefore,theyappearat
thebottomofFigure4.Toovercomethesemechanicaldeficiencies,itwasproposedto
stabilizethefibernetworkofMuskin®bycrosslinkingagents[25,26],ortoadjustthesoft‐
nessofKombuchamaterialsbytheadditionofplasticizingagents[10].However,thiscon‐
tradictsthemultiscaleidea,thevariationofthedensityandtheorientationofthefibers
alongtheirloaddirection,whichwouldpresumablybetterhelptoovercometheobserved
limitations.

Figure4.“Amaterialpropertychartfornaturalmaterials,plottingstrengthagainstdensity.(…)
Structurallyefficientmaterialsarelightandstrong.”Reprintwithpermissionfrom[29]Copyright
2021Taylor&Francis.Naturalflexibleloadtransferringmaterialsappearwithadensityof~1
Mg/m3andrangefrom1MPatomorethan1000MPa(markedred).Thetensilestrengthofparen‐
chymatousplanttissuesislow(markedblue).
Thesecondconcepttoachievehighlyfunctionalleather‐likematerialswithahigh
contentofbio‐basedcomponentstakesuptheprincipleofartificialleather.Thesemateri‐
alssimulatethestructureofleatherasmultilayermaterial.Thefunctionscanbeseparated
betweenthetextilesupport,whichhastofulfillthemechanicalfunction(tensilestrength,
tearstrength),themiddlelayer,bywhichfeelandsoftnessareadjusted,andthetopcoat,
whichtakesoverthefinalopticalappearance.Agrainstructuresimilartothatofleather
ismostlyachievedbyembossing.Therefore,toincreasethebiogenicpartofthematerials,
itwouldbeeffectivetoreplacethepolyestersupportwithnaturalfibers.Asshownin
Figure4,thenaturalfibersmayshowexcellentmechanicalstabilities.However,only

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
Vegea®andTeakLeaf®useacellulose‐basedfabricassupport.Therenewablecontentin
thesemultilayermaterialscouldalsobeincreasedbyreplacingsyntheticpolymersinthe
coatinglayers.Desserto®andAppleskin®adoptthisprinciple.ApartofPURisreplaced
byagriculturalby‐products,whichareusedasfillers.Adetailedanalysisoftheoriginof
thenaturalcomponentanditscontentinrelationtothebulkofthematerialwasnotpos‐
sible,however.Nevertheless,thebulkofthematerialsremainstoconsistofpolyurethane.
InthecaseofTeakLeaf®,anatural‐appearingsurfaceiscreatedbyimpartingthenatural
leaf,whichiscoveredbysyntheticwaxes.However,becauseofthemissingelasticfoam
layerasusedinartificialleatheranditsreplacementbytheplantleaf,theflexresistance
ishamperedandappearsnotadequatelyadjustedfromafunctionalpointofview.
Asathirdstrategytoreplaceleatherwithanimal‐freematerialsforwhichplant‐
basednon‐wovenfabricsareused.Thefossil‐basedpolyestertextilesupportisexchanged
bynaturalfiberalternativesascotton,linen,etc.Ideally,thefabricwouldbefinishedwith
abiobasedpolymer.Pinatex®,forexample,ismanufacturedfrompineappleleaffibers,
whicharelaboriouslyprocessedbeforetheyarecoatedbyathinpolymerfilmthatcanbe
eitherfossil‐basedorfromrenewableresourcestoimproveusability.Pinatex®promotes
itspolymerfinishtobepolylactide,whichcanbeproducedfullybio‐based[13,30–32].
However,ouranalysisshowedatleastaremarkablecontentofPUR/acrylateinthefinish.
Theverythinsurfacecoatingdoesnotcompletelycoverthefibrousnon‐woven,which
leadstoahardsurfacewithafibrousappearancethatwithstandstheflextest,however.
Thematerialappearsmoresimilartoatextilenon‐wovenandthelowmechanicalre‐
sistancecanbecorrelateddirectlytothelowbindercontentofthefibersofthenon‐woven
support.SnapPap®isbasedoncellulosicfibersaswell,butincontrasttoPinatex®,the
matrixisboundwithacrylicacid‐basedpolymers.Bothmaterialsneithersimulatea
leatherstructurenordotheyappearasaleathersurface.Therefore,theyshouldratherbe
estimatedascoatedorimpregnatedtextilethanasartificialleatherorleatheralternative.
Figure5showsimpressivelytheperformanceofthedifferentmaterialsincompari‐
sontothereferences.Alternativematerialshavespecificadvantages,butnoneofthema‐
terialscombineshighmechanicalstrengthandflexresistancewithhighwatervaporper‐
meabilityasinthecaseofleather.


Coatings2021,11,22612of15
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
Figure5.Comparisonofthephysicalpropertiesofthematerials.Thevaluesarenormalizedtothe
maximumvaluemeasuredinthissurveyfortherespectiveparameter.
4.2.SurfacesandAppearance
Thesurfacesofthematerialsaretheir“face.”Thisisimpressivelyshownwiththe
appliedleafinTeakLeaf
®
asaneye‐catcher.Theresultingstructuresappearopticallyin‐
terestingbutdonotfulfillthesamefunctionasthefoamlayerinengineeredcoatedtex‐
tiles.Otherplantmaterials,whichareusedinsimilarconstructionstoachieveinteresting
opticalpropertiesare,e.g.,corkbased.Theyhavenotbeeninvestigatedinthisstudybut
havetobesupportedaswellbytextilestoachievesuitablephysicalproperties[33–35].
Therefore,theTeakLeaf
®
solutionappearsmoredesign‐driventhanittakesfunctional
aspectsintoaccount.
Toachievesufficientfunctionalpropertiesofthesurfaceofflexiblematerialsoftena
finaltopcoatisapplied,whichthendeterminestheopticandpartiallyhapticandother
usefulpropertiesofthematerials,e.g.,flexresistance,abrasionbehavior,andsoilingbe‐
havior.Thewatervaporpermeabilityalsodependsonthetightnessofthetopcoatorof
watertightlayersinbetween.Thethickerthislayerthelessthematerialallowsvaporand
gaspermeation[36].
Leatherasinvestigatedusuallyshowsonlyaverythintopcoattoimprovethesoiling
behavior.Theflexresistanceisveryhigh.Thewatervaporpermeabilityisinanappropri‐
aterange.Muskin
®
andSnapPap
®
showveryopenstructures,whichallowwatervapor
perfectlytodiffuse.However,theirperformancecharacteristics,particularlythepoorflex
resistance,limitsthelong‐timeuse.Allotherinvestigatedmaterialsareverytightagainst
watervaporpermeation.Usedasashoematerial,thiswouldleadtosweatingofthefeet
andwouldreducethecomfort[37,38]butthematerialswouldbetightagainstrainifused
as,e.g.,bagorrainjacket.

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ThemicroscopicsurfaceofSnapPap®andPinatex®showsthefibrousnon‐wovenfab‐
ric,whichtakesoverthemechanicalpropertiesofthematerial.SnapPap®andPinatex®do
notappearleather‐like.Theyshowhardsurfacesandanexposedfibrousnon‐woven
structure.Applyingathickerpolymercoatingandembossingagrainstructurewould
presumablyleadtoaleather‐likeoptic.Inthiscase,considerationsregardingwatervapor
permeabilityforcoatedtextileshavetobetakenintoaccount.
5.Conclusions
Noneofthealternativematerialsachievedthepropertiesofleatheraccordingtothe
appliedreferencevalues,althoughmanyofthemareofferedasaleatheralternative(Fig‐
ure5).Thequestionofwhyitisdifficulttoachievethesepropertiesbyalternativenatural
materialsisansweredwiththedifferentbiologicalfunctionsoftheusedmaterials.Leather
isamulti‐scalematerial,whichisdesignedbynaturetofulfillloadtransferringandmet‐
abolicfunctions.Itshowsagradientinthetightnessofthestructurecomposedofdiffer‐
entlyfinehydrophilicproteinfibers.Eachpartofthestructuretakesoveraspecificfunc‐
tion.Thereticularlayer,whichconsistsofcoarsefiberbundlesisresponsibleforthehigh
mechanicalresistance(tensileandtearstrength).Thedestructionofthefibernetworkof
thegrowntissueleadstoadecreaseofthemechanicalstabilityby10times[28].Thiscan
onlyslightlybeimprovedwhenthefibersareagainboundbybindingagents(e.g.,middle
layerleatherboardofNoani®).Themorecompactandfinerfibersofthepapillarylayerof
leatherandthegrainmembranecausetheleather‐likeappearanceandthetightstructure
ontop.Nevertheless,thewatervaporpermeabilityishighifnotightsynthetictopcoatis
applied.Thehydrophilicfibersofleathercanabsorbmuchwater,whichleadstohigh
comfortifcomparedwiththesyntheticalternatives.Thebiogenicnon‐woventextiles
Pinatex®,SnapPap®,andKombuchashowsimilarwaterabsorptionvaluesasleatherbut
lackmechanicalandflexuralstrength.Therefore,itremainsachallengeandanaimto
reproducethefunctionofthebionicstructureoftheskinwithalternativebiologicaltech‐
niquesashasalreadybeenmentionedmanyyearsago[29].Whenagriculturalby‐prod‐
uctsareaddedtopolymerlayersofartificialleathers,thebiobasedcontentofthematerials
israisedbutnophysicaladvantageoverthereferencematerialcanbemeasured.Onlya
properlifecycleanalysiswouldallowassessingtheassociatedadvantage.
SupplementaryMaterials:Thefollowingareavailableonlineatwww.mdpi.com/2079‐
6412/11/2/226/s1,FigureS1:Sampleshoeupperleather;(a)crosssection,(b)surface,FigureS2:Sam‐
pleMuskin®;(a)crosssection(25timesmagnification),(b)surface,FigureS3:SampleKombucha;
(a)crosssection(100timesmagnification),(b)surface,FigureS4:SamplePUR‐coatedtextile;(a)
crosssection,A:thincompacttoplayer,B:foamedlayer,C:textilecarrier,(b)surface,FigureS5:
SampleDesserto®;(a)crosssection,A:compacttoplayer,B:foamedlayer,C:textilecarrier,(b)sur‐
face,FigureS6:SampleAppleSkin®;(a)crosssection,A:compacttoplayers,B:foamedlayer,C:
impregnatedlayerwithatextilecarrier,(b)surface,FigureS7:SampleVegea®;(a)crosssection,A:
compacttoplayer,B:foamedlayer,C:textilecarrier(b)surface,FigureS8:SampleTeakleaf®;(a)
crosssection,A:leafwithatransparenttoplayer,B:textilelayer1,C:textilelayer2,(b)surface,
FigureS9:SamplePinatex®;(a)crosssection,non‐wovenwithacompacttoplayer,(b)surface,Fig‐
ureS10:SampleSnapPap®;(a)crosssection,(b)surface,FigureS11:SampleNoani®;(a)crosssection
(25timesmagnification),A:microfiberlayer,B:leatherboardlayer,C:textilecarrier,D:foamed
layer,E:compactlayer,(b)surface,embossedregion,FigureS12:FTIRspectrumofDesserto®asan
exampleofaPUR‐coatedmaterialwithtypicalmolecularvibrations(3359NH;3931CH2;2862CH2;
1744C=O;1635O=C–NH;1554Amid;1464CH2;1257C–O–C).
AuthorContributions:Conceptualization,S.D.,H.S.,andM.M.;methodology,H.S.andS.D.;vali‐
dation,S.D.andH.S.;investigation,S.D.andH.S.;datacuration,S.D.,A.M.,H.S.,andM.M.;writ‐
ing—originaldraftpreparation,A.M.andM.M.;writing—reviewandediting,H.S.,A.M.,andM.M.
Allauthorshavereadandagreedtothepublishedversionofthemanuscript.
Funding:Thisresearchreceivednoexternalfunding.
InstitutionalReviewBoardStatement:Notapplicable.

Coatings2021,11,22614of15


InformedConsentStatement:Notapplicable.
DataAvailabilityStatement:DataiscontainedwithinthearticleandSupplementaryMaterialsac‐
cordingtotheDataAvailabilityStatementsinsection“MDPIResearchDataPolicies”at
https://www.mdpi.com/ethics.
Acknowledgments:WethanktheGermanLeatherAssociation(VDL)forthedonationofallsam‐
ples.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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