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DOI10.1126/science.1075707,9762002297Scienceetal.RobertR.Matheson,Jr.,SoftCoatings20thto21stCenturyTechnologicalChallengesinThiscopyisforyourpersonal,noncommercialuseonly..clickingherecolleagues,clients,orcustomersby,youcanorderhighqualitycopiesforyourIfyouwishtodistributethisarticletoothers.herefollowingtheguidelinescanbeobtainedbyPermissiontorepublishorrepurposearticlesorportionsofarticlesthisinformationiscurrentasofMarch23,2010Thefollowingresourcesrelatedtothisarticleareavailableonlineatwww.sciencemag.orghttp//www.sciencemag.org/cgi/content/full/297/5583/976versionofthisarticleatincludinghighresolutionfigures,canbefoundintheonlineUpdatedinformationandservices,foundatcanberelatedtothisarticleAlistofselectedadditionalarticlesontheScienceWebsiteshttp//www.sciencemag.org/cgi/content/full/297/5583/976relatedcontent7articlesontheISIWebofScience.citedbyThisarticlehasbeenhttp//www.sciencemag.org/cgi/collection/mat_sciMaterialsSciencesubjectcollectionsThisarticleappearsinthefollowingregisteredtrademarkofAAAS.isaScience2002bytheAmericanAssociationfortheAdvancementofScienceallrightsreserved.ThetitleCopyrightAmericanAssociationfortheAdvancementofScience,1200NewYorkAvenueNW,Washington,DC20005.printISSN00368075onlineISSN10959203ispublishedweekly,exceptthelastweekinDecember,bytheScienceonMarch23,2010www.sciencemag.orgDownloadedfromwhereRisthereflectanceofthesample,H9270isthedistanceonthesurfacebetweentwopoints,H9275istheangularfrequency,kisthewavenumber,fisthefocallengthoftheincidentradiation,andH9268isthermsheightofthesurface.A2DanalysisoftheopticshasbeencarriedoutbyOgilvy35.Whitehouseconcluded34forundulationswithlengthscalegreaterthanthewavelengthoftheincidentradiationthatthesurfaceappearedglossyiftheprobabilitydensityoftheslopesonthesurfacewasstrictlyconfinedtoanarrowangle.Biocompatibility.Finally,biologicalinteractionswithasurfacehavealsobeenfoundtodependonitstopography.AgoodreviewofthetopologicalcontrolofcelladhesionandactivityonasurfacehasbeenmadebyCurtisandWilkinson36,andamoregeneralreviewoftheroleofpolymerbiomaterialsmayalsobefound37.Suchconsiderationsarerelevantforanumberofinvivoandinvitroapplications,suchasbiologicalsensors,hipreplacements38,andmorecomplextissueimplantssuchasreplacementbone,wherethegrowthofcellswithintheartificialstructureistobeencouraged.Forexample,thesizeandmorphologyofcrystalsatthesurfaceofoctacalciumphosphate–coatedcollagenhavebeenshowntoaffecttheinteractionofcellswiththesurface,asillustratedinFig.4.Thelargerscaletopographywasfoundtoleadtolessfavorablespheroidalcellsthatformedfewerintercellularconnections39.Insomecases,thetopographyofasurfacemaybecarefullycontrolledtopromotecelladhesion40,41.ConclusionThetopographyofasurfaceisadirectresultofthenatureofthematerialthatdefinesit.Theanalysisofthetopographyofasample,madepossibleonthenanoscalebythedevelopmentofAFMtechniques,needstobecarefullyconsideredinordertorelatethecomplexityofa2Dsurfacetothematerialsproperties.Theresultwillbethebettercontrolofanumberofproperties,suchasopticalfinish,andoftheinteractionofasurfacewithasecondarymaterial,whetherthatbeanadhesive,asecondarycomponentofacomposite,orabiologicalspecies.ReferencesandNotes1.J.D.Affinitoetal.,ThinSolidFilms291,631996.2.J.A.DeAro,K.D.Weston,S.K.Buratto,U.Lemmer,Chem.Phys.Lett.277,5321997.3.Y.Nabetani,M.Yamasaki,A.Miura,N.Tamai,ThinSolidFilms393,3292001.4.M.Sferrazzaetal.,Phys.Rev.Lett.78,36931997.5.E.Scha¨ffer,T.ThurnAlbrecht,T.P.Russell,U.Steiner,Europhys.Lett.53,2182001.6.D.G.Bucknall,G.A.D.Briggs,MRSSymp.Ser.NanopatterningUltralargeScaleIntegrationBiotechnol.705,1512002,L.Merhari,K.E.Gonsalves,E.A.Dobisz,M.Angelopulss,D.Herr,Eds.7.S.Walheim,E.Scha¨ffer,J.Mlynek,U.Steiner,Science283,5201999.8.J.Heier,E.Sivaniah,E.J.Kramer,Macromolecules32,90071999.9.T.ThurnAlbrecht,J.DeRouchey,T.P.Russell,Macromolecules33,32502000.10.A.Karimetal.,Macromolecules31,8571998.11.X.P.Jiang,H.P.Zheng,S.Gourdin,P.T.Hammond,Langmuir18,26072002.12.G.GoldbeckWoodetal.,Macromolecules35,52832002.13.V.N.Bliznyuk,K.Kirov,H.E.Assender,G.A.D.Briggs,Polym.Preprints41,14892000.14.F.Dinelli,H.E.Assender,K.Kirov,O.V.Kolosov,Polymer41,42852000.15.C.Rauwendaal,PolymerExtrusionHanser,Munich,1985.16.S.J.Liu,Plast.RubberComposites30,1702001.17.B.Monasseetal.,Plast.ElastomeresMag.53,292001.18.Y.Oyanagi,Int.Polym.Sci.Technol.24,T381997.19.A.Guinier,XrayDiffractioninCrystals,ImperfectCrystals,andAmorphousBodiesW.H.Freeman,SanFrancisco,1963.20.V.N.Bliznyuk,V.M.Burlakov,H.E.Assender,G.A.D.Briggs,Y.Tsukahara,Macromol.Symp.167,892001.21.W.M.Tong,R.S.Williams,Annu.Rev.Phys.Chem.45,4011994.22.P.Meakin,Fractals,ScalingandGrowthFarfromEquilibriumCambridgeUniv.Press,Cambridge,1998.23.C.M.Chan,T.M.Ko,H.Hiraoka,Surf.Sci.Rep.24,31996.24.E.M.Liston,L.Martinu,M.R.Wertheimer,J.AdhesionSci.Technol.7,10911993.25.Q.C.Sun,D.D.Zhang,L.C.Wadsworth,TappiJ.81,1771998.26.V.Bliznyuketal.,Macromolecules32,3611999.27.N.Zettsu,T.Ubukata,T.Seki,K.Ichimura,Adv.Mater.13,16932001.28.R.S.Hunter,R.W.Harold,TheMeasurementofAppearanceWiley,NewYork,ed.2,1987.29.H.Dav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uitenoticeablyattemperatureswellbelowthosewhererapidflowandlevelingareachieved3.Acceptablesolventsubstitutionbasicallyamountstousingtheliquidformofsubstancesthatarenaturallypresentasgasesintheatmospheresuchaswaterandcarbondioxide.Liquidorsupercriticalcarbondioxideislimitedtoindustrialapplicationsbecauseoftherequirementsforhighpressure.Wateriseasiertousewidelyasacoatingsolvent,butitisnotapanacea.Oneexampleofaproblemthatcomeswithwateristheinevitablywidevariationindryingtimesthataccompaniesapplicationinenvironmentsofdifferentrelativehumidity.Becauserelativehumiditycanchangealmosthourly,thisisaseriouscomplication.Infact,virtuallyallwaterbornecoatingstodaycontainquitesubstantiallevelsoforganiccosolvents.TheVOCcontentofwaterbornecoatingsisgreatlyreducedascomparedtothatofolder,conventionalsolventbornecoatings,butitisnotfullyeliminated.Amajoractivityinmoderncoatingdevelopmentisthesearchforbalancedchemistrythatwillpushbacktheselimitsonenvironmentallymorefavorablelacquerswhileretainingtheattractivesimplicity,thesyntheticcontrol,andthelowcostofthetechnology.VOCreleaseisnottheonlyenvironmentalimpactfactorthatisimportantfordrivingchangeincoatingtechnology.IntheUnitedStates,regulationsonsocalledhazardousairpollutantsHAPsareimportant4.Thisisanexplicitlistofsolvents,typicallyaromatic,thatareusedinlargequantitiesandareeitherknowntocauseorsuspectedofcausinghumanhealthproblemswithchronicexposure.Avarietyofother,similarlylocalconstraintsforparticularingredientsexistaroundtheglobe.Oneverywidelyexperiencedrestrictionisthatonheavymetals.Therearemanyhistoricalexampleswherefairlylargeamountsofparticularmetalshavefounduseinsoftcoatings5.Someexamplesaretheuseofleadforanticorrosionincathodicelectrocoatcoatings,ofhexavalentchromiuminmetalcoatings,ofbothleadandcadmiuminvariouspigments,ofdivalenttininantifoulingmarinecoatings,andevenofmercuryasanantifungalagentforsomeinteriorpaints.Incommonwithotherareasofmaterialsprocessing,coatingtechnologynowhastolookforalternativeingredientswithoutuncontrollable,longtermenvironmentalconsequences.Nosimilarlygeneraltechnicalsolutionshaveyetbeenfound,althoughprogressisbeingmade,particularlywithrespecttoanticorrosioncoatings.BeatingBacktheEnvironmentChemicalandmechanicalresistancetoenvironmentalinsultisacommonfeatureofmanycoatingsystemsandakeyreasonfortheirapplication.Biologicalattacksareclassicproblemsencounteredovertheyears,andtheircatalogdefinesthecurrentfrontier.Underwatercoatingsthatcanresisttheattachmentanddegradationofaqueousorganismssuchaswormsandbarnaclesareneededforshippingandforstructures.Exteriorcoatingsthatcanresistparticularinsect,bird,andplantexcretionsarefrequentlyneededinlocalgeographies.Interiorcoatingsthatcanresistmildew,otherfungaldamage,molds,andbacteriaarefrequentlydesired.Thegeneralchallengeisnearlyalwaysthesamespecificresistancetoadefinedclassofbiologicalinsultwithoutnonspecifictoxicityorirritation.Itisnaturaltoworktowardthissetofobjectiveswithadditivestailoredtoeachtask.Experienceshowsthisnaturalpathtobeexpensiveandusuallyimperfect,butoccasionallyfruitful.Still,itisprobablyfairtosaythatnoexamplesexistwheretheperformanceofthebroadlytoxic,heavymetal–basedadditiveshasbeenachievedwiththemorespecificmoderntools.AnewideaistoproducecounteragentsinsitubytappingthechemicalreactionsthatmustaccompanybiologicalattackorevensimpleweatheringintheactiveenvironmentsnearEarthssurface6.Forexample,biologicaldamagemaybeaccompaniedbyhydrolyticscissionofcoatingcomponents.Ifthosecanbedesignedtohydrolyzeintoantiseptic,antifouling,orantiwhateverproductstailoredtothetask,thenperhapsaneffectivesolutioncanbefound.MaximizingControlThroughMolecularArchitecturesItisimportanttolookbeyondenvironmentalattackonthecoatingitself.Theclassicalroleofcoatingsistoprotectsomethingelsefromtheenvironment.Thisprotectioncanbemechanicalorchemicalinnature.Varnisheshavebeendevelopedparticularlyforthesepurposes,becauseinsituorontheworkcrosslinkingisaveryeffectivetechniqueforaugmentingthecoatingsmaterialpropertieswhileavoidingcompromisesinapplication.Anextremelyactiveareaofdevelopmentinmoderncoatingsisdirectedtoimprovingthecontrolofsuchreactions.Inevitably,thesemustbecarriedoutinavarietyofworkenvironments.Nearoneextremearecoatingsusedincontrolledchambers,asfortheradiationorthermalcuringofacoatingusedtosealaconnectionbetweenelectroniccomponents.Thevariablesofconcernmaybefilmthicknessatvariouspoints,surfacecleanliness,andsmalltemperaturegradientsarisingfrommaterialsofdifferentthermalconductivity.Neartheotherextremearethegrosslyfluctuatingenvironmentsthatcanbefoundinarailroadlocomotiveshed,wherehumidity,temperature,airflow,applicationrate,surfacepreparation,andmaybeevensurfacematerialareallvariables.Oneideaformeetingthesechallengesiswww.sciencemag.orgSCIENCEVOL2979AUGUST2002977MATERIALSSCIENCESOFTSURFACESonMarch23,2010www.sciencemag.orgDownloadedfromtotakegreatcareinsynthesizingthevarnishingredientssothatnoespeciallytroublesomereactionscaneveroccur.Somesidereactionsareunderstandablymoredeleteriousthanothers,andthegoalistoleavenoopportunityforthemosttroublesome,nomatterwhatconditionsmightappear.Thisleadstotheuseofcontrolledpolymerizationtechniquesexemplifiedbutcertainlynotlimitedtogrouptransferpolymerization7foracrylicmaterials,rigorousexclusionofnonfunctionalunabletoparticipateincuringmatrixmaterials,andoptimizingmolarmassdistributionstoavoiduntimelyimmiscibilityduringcureandsimilarstrategies.Thespecificfieldofautomotivecoatinghasbeenintheforefrontofthisactivitybecauseoftheextremelyhighperformancestandardsandpowerfuleconomicincentivesfoundinmassproducingautomobiles.Someofthenewsyntheticandanalyticaltechniquesbeingintroducedforcontrollingandmonitoringautomotiveenamelshavebeendescribed8.Decorativecoatingsinparticularneedtoincorporatepigments,dyes,reflectivemetal,andmicaflakesformanyapplications.Onecommontechniqueforeffectivelydistributingsuchparticlesistocovertheirsurfaceswithdispersantsthataidintheirdispersioninthebulkofthecoatingandpreventreagglomerationunderthevarietyofcircumstancesthatmightariselater.Exquisitecontrolofthemolecularstructureisneededinordertoachievegooddistributionoftheparticles,minimalmobilityonceappliedtoasurface,theabilitytoresistforcesthatdrivereagglomeration,andcompatibilitywiththebulkcoatingandyetnotinduceproblemswithadhesion,application,orlongtermperformance.Becausepigmentsareveryfrequentlythemostexpensiveingredientsinadecorativecoating,itisimportanttousethemefficiently.Additionally,assolventconcentrationandvarietyaredecreasedbecauseoftheenvironmentalpressurespreviouslycited,opportunitiesformanagingdispersionproblemsbymodifyingthecoatingmediumthecoatingvehicledecrease.Smallwonderthatthechemistryusedtomakemoderndispersantsprovidesanexceptionallyclearpictureofthestateoftheartinmolecularcontrolincoatings.Techniquesformakingblockcopolymerseachblockdesignedforaffinitytoeitherasurfaceorthesolventenvironmenthavebeendevelopedandcommercializedandarestillbeingimproved.Thepatentartisextensiveandgrowing,butthatfromC.HosotteFilbert9isarepresentativeexample.FunctionalCoatingsAfourthmodernfrontierintheworldofsoftcoatingscanbedescriptivelycalledpostcurereactivityforvarnishes,orperhapsinusereactivityforlacquers.Suchreactionshavebeenrecognizedforalongtimeinexamplessuchasthelongtermoxidationofalkydvarnishesandmanylacquersbasedonnaturalproducts.Historically,thesehavebeenviewedastroublesomeinstabilities.However,ithasbeenlearnedthatsomeinstancesofpostcurechemistryhaveadvantages,withoneexamplebeingtheslowcondensationandinterchangeofsiloxanebondsinorganosilaneenamels10.Thesecanacttorelaxstressesthatotherwisegrowuncompensatedinlightandoxidationstressedexteriorcoatings.Heterogeneouscoatingsthatreacttocracksorfracturesbyreleasingpostcurerepairingredientshavebeenpostulated11.Evenmoresophisticatedusesincontrolledreleaseorothertransportcontrolproblemscanbesketchedtoday.Itshouldbenotedthatagreatmanyinstancesexistinwhichcoatingsareusedtomanipulategenerallytoretard,delay,orpreventthetransportandexchangeofmaterials.Atmosphericoxygencontactingfood,carbondioxideexitingcarbonatedbeverages,thereleaseofpharmaceuticalsintothebody,electricalchargeleakingintoadevicecomponent,heatexitinganisothermalenvironment,orwaterandionicmaterialscontactingcorrosionsusceptiblemetalsareexampleswherethetransportcharacteristicsofcoatingsareimportantindeterminingperformance.Thelongtermcapabilityofacoatingtoimproveorattheleastreacttocompensateforadecliningtransportcharacteristicmaybejustasusefulasthesameabilitytooffsetdecliningmechanicalcharacteristics.IndustrialScaleChallengesAfinalclassofproblemsdrivinginnovationinmoderncoatingscanbefoundinthecostsandlimitationsoftheheatingstepinenamelprocessing.Notsurprisingly,theseproblemsincludethecapitalandenergycostsassociatedwithheatingobjectswithlargethermalmasses,damagetoheatsensitivesubstrates,andtheinventoryproblemsthataccumulatewithlongcycletimesinanyprocess.Themostdirectapproachistoreducetherequiredbakingtemperatureand/ortime.Thereisscopeherefornovelchemicalreactionsandcatalystinnovations,bothofwhichcommandattentiontoday.Alternatively,ifthecuringreactionscanbeactivatedbyamechanismotherthansimpleheating,thenproblemscanbeminimizedwithoutlosingthecureinducedimprovementsincoatingperformance.Muchcurrentworkisdirectedtoradiationcurablewithultravioletlight,electronbeams,andevenvisiblelightcoatingsandeffortstoextendtheircurrentembodimentstocomplexarticlesandlongtermuse12.Powdercoatingsandliquidcoatingsarebothobjectsofstudyandinnovation.Themajorchallengefacedinsuchdevelopmentarisesfromlimitationsontheuniformityofcureforincompletelytransparentcoatingsshadowedareasdonotreceivethesamefluxofradiationorcoatingsoncomplexshapes.Examplesofspecificnewproductsarisinginresponsetooneoranotherofthesefivegeneraldevelopmentareascanbefoundinmanyplacesandfrommanydevelopmentlaboratories.Perhapsnoexampleexiststhatillustratesall,butthereisatleastonethatcomesclose.AneedexistsintheautomotiveworldforapaintingsystemwithlowerenvironmentalemissionsparticularlyVOCs,improvedresistancetoenvironmentaldamageparticularlymechanicalscratching,outdoordurabilityapproachingadecade,corrosionresistanceofthecoatedmetalforthesameperiodoftime,andimprovedapplicationrobustness.Thisneedhasbeenrecentlymetwithwhatmightbeconsideredanexemplarymoderncoatingsystem.FourlayersofcoatingareusedFirst,ananticorrosioncoatingnowfreeofheavymetalsisappliedbycathodicelectrodepositionsecond,apowderprimernowwithzeroVOCsthird,awaterbornelayercontainingpigmentsnowwithminimalVOCsandminimalHAPswithmodernpolymericdispersantmoleculesandfinally,anewclearcoatnowwithmorethan20lowerVOCs,greatlyimprovedscratchresistance,andexcellentresistancetoacidrain,chemicalattack,andphotochemicalexposure.Allthisisappliedwiththeuseofexistinginfrastructure,includingbothautomaticandmanualequipmentwhenrequired,atcommercialratesandwithlowertotalenergyinputandimprovedvisualquality.Theautomotiveuseenvironmentisharshforcoatings,andthisnewsystemhasonlybeguntobeused.Itisriskytoforetelldecadelongsuccesssofarinadvance,butthestepappearstobeanimportantoneintheevolutionofsoftcoatingsthatcanresistallthemajorpressures.LookingForwardThefivegeneraldevelopmentfrontiersappeartoindicatetwostrongtrendsintodayssoftcoatingdevelopment.Thefirstisatrendtowardeverimprovingcontrolofstructureatalllengthscales.Whetherincuresitedistributionsformorerobustvarnishes,molarmassdistributionsinenvironmentallyfriendlylacquers,oropticalpathdistributionsforultravioletcurables,thepathofimprovementandinnovationgoesalongthelineofimprovedstructuraluniformityandcontrol.Thesecondtrendisthehistoricallyfamiliaronetowardblurringthedistinctionsbetweenandtherebyremovingsomelimitationsfromfamiliarcoatingclasses.Whetherbyslowinusereactionsfornominallacquersandambientvarnishesorbyradiationassistedcrosslinkinginnominalenamels,thetechnologytrendistowardtheboundarieslaiddownbythedescriptors.Onecanphrasethesetrendsintheformoftwostrategicquestionsfor9AUGUST2002VOL297SCIENCEwww.sciencemag.org978MATERIALSSCIENCESOFTSURFACESonMarch23,2010www.sciencemag.orgDownloadedfrom
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