Comparative Study of BrightnessWhiteness Using Var

ComparativeStudyofBrightness/WhitenessUsingVariousAnalyticalMethodsonCoatedPapersContainingColorantsBurakAksoy,MargaretK.JoyceandPaulD.FlemingDepartmentofPaperandPrintingScienceandEngineering,WesternMichiganUniversity,Kalamazoo,MI,49008ABSTRACTFluorescentWhiteningAgents(FWA),dyesandpigmentedinksareusedextensivelyinthepaperindustrytoenhancetheappearanceandopticalperformanceofcoatedanduncoatedpapers.Thereareseveraldifferentsystemsinuseforthedescriptionandspecificationofcolorandopticalpropertiesofpaper,usingcolorimeters,spectrophotometersandbrightnessmeters.CIEandTAPPIarethemostcommonlyusedcolorsystemsinthepaperindustry.Inthisstudy,threedifferentcolors(blue,blackandred),andFWAwereaddedintocoatingformulationsfromthreetofiveadditionlevels.CoatedLWCpapersweremeasuredfortheiropticalpropertieswithtwodifferentspectrophotometersandabrightimeter.TheCIEwhitenessformulation(1996)wasanalyzedonslightlycoloredpapers.Itwasfoundthatinsomecases,calculatedCIEwhitenessvaluesincreasedwithincreasingamountofcolorsinthecoatinglayer,althoughthepapersappeareddarkerorreddertotheobservers.SmalldeviationsinmeasuredCIEtristimulusfunctionsX,Y,andZoftencausedsignificantchangesincalculatedCIE,andGanzwhitenessvalues.HunterwhitenessvaluesalsofollowedthesamepatternasCIEandGanzwhiteness,althoughitislesssensitivetosmallchangesinthetristimulusvalues.Theresultsshowthedeficienciesinthecurrentmeasurementtechniquesforassessmentofbrightnessandwhitenessofwoodcontainingpapersandcoatingscontainingopticalbrighteningagents.INTRODUCTION:Otherthanphysicalpropertiesofapaper,itsappearanceisveryimportant,especiallytotheprinterandtothefinalreader.Today,manyfreesheetandgroundwoodcontainingoffset,gravureandinkjetgradescontainFWAs(1).Appearanceisalsoimportantinseveralotherpapergradessuchastissuepapers.Forthisreason,therehavebeenmanyeffortstoproducepaperathighwhitenesslevels.Thebleachingofpulp,additionoffillersintothepaper,coatingthepaper,andadditionofFWAsservetoincreasethewhitenessofpapers.However,theircontributiontowhitenessand/orbrightnessofthepaperandpaperboardislimitedduetostrengthandporosityissues.HighwhitenessonpaperscanbeachievedonlywiththeproperusageofFWAs(2).Whitenessandbrightnessaremanifestationsofopticalpropertiesandtheycannotbemeasureddirectly.Evaluationofwhitenessintermsofitsvisualwavelengthscanbemadebyvisualappearanceorbyinstrumentalmeasurements.Onlythephysicalproperty,thespectralreflectanceofasamplecanbemeasured.Thereareseveraldifferentsystemsinuseforthedescriptionandspecificationofcolorandopticalpropertiesofpaper,usingcolorimeters,spectrophotometersandbrightimeters.CIEwhiteness(3)andTAPPIbrightness(4)arethemostcommonlyusedmeasurementsinthepaperindustry.However,CIEwhiteness,andTAPPIbrightnessaredifferentconceptsandTAPPIbrightnessmayleadtomisleadingassessmentsofopticalpropertiesofpapersthatcontaindyesand/orFWA’s.Likewise,CIEwhitenessmaygiveadifferentimpressionofrelativewhitenessesoftwosamplesrelativetowhatahumanobserverwouldconclude.Secondly,measuredspectralreflectanceisnotastandardfixedquantity.Itisinfluencedbyavarietyoffactors.Geometryofthemeasurementdevice,aperture,lightsource,filtersandmeasurementsetupallinfluencethedatathatareacquiredfromaninstrument.Thesefactorsmaydifferfromoneinstrumenttoanother.Thus,comparingdatafromatleasttwodifferentinstrumentscouldbeanappropriateapproachtoenablemillsandtheircustomerstohavemoreaccurateandreliabledataforopticalanalysesofpaperthatcontainsdyeorFWA.Besidestheconfigurationofthemeasuringdevice,thebasepapermayalsocomplicatetheusefulnessofthemeasurementtechniquesandinterpretationoftheopticaldata.ThisisespeciallytrueforLWC(LightWeightCoatedPaper)papersthatcontainlargeamountsofmechanicalpulp.Thehighamountsofligninpresentinthisgradecausetheproducttoagerapidly,whichresultsintheyellowingofthepaperwithtime.LWCpapersmayalsoexhibitdifferingpropertiesanddifferentshadesofcolorsfromonemanufacturertoanother,dependingonthewoodspecies,proportionsofthewoodspecies,andtheratioofmechanicalpulptochemicalpulpusedintheproductionofthebasepaper.Generallyspeaking,paperscontaininghighamountsofdark,mechanicalwoodfibers,influencetheopticalpropertiesoftheappliedcoatingmorethannon-woodcontainingpapers.Allthesefactorsmakethedevelopmentofastandardopticalmeasurementmethodforthisgradeverydifficult.Asaresult,therehasnotbeenaSWOP(5)standardadoptedforthisgrade(6).Forthisreason,aLWCgradepaperwasselectedforthisstudy.Theworkhereinwasperformedtobetterunderstandthedifficultiesandcomplicationsfacedwhentryingtoperformbrightnessandwhitenessmeasurementsonthesepapersinanattempttomeetthecommercialspecificationsoftheprinter.TECHNICALCONSIDERATIONS:Whenlightstrikesanobjectitmaybetransmitted,scattered,reflectedorabsorbedandallthesemayoccurseparatelyorincombination.Theobjectappearswhiteifittotallyreflectsthelight,andscattersdiffusivelyatallwavelengthsofthevisiblespectrum.Theobjectappearscoloredifsomewavelengthsofthelightarereflectedwhiletheothersareabsorbed.Itisblackiftheobjectabsorbsallthewavelengthsofthelightinthevisiblespectrum(1,7).Byanotherdefinitionwhiteisthea