spectral emissivity properties of reflective coatings · m. Čekon spectral emissivity properties...

M. Čekon

Spectral emiSSivity propertieS of reflective coatingS

Key WorDS

• Reflective coatings,• Low emissivity surfaces,• Spectral radiative properties,• Infrared spectroscopy.

aBStract

This article deals with the spectral radiative properties of coatings consisting of hollow ceramic microspheres. They were selected from the commercial coatings available in the Slovak and Czech Republics. The aim was to measure and compare their spectral emissivity properties with standard facing coatings by means of infrared spectroscopy. The measured data demonstrates that the coatings have the same radiative properties as a standard building coating. Two reflective measurement methods (DRIFT and ATR) were used for this purpose. These results have been compared, and the DRIFT method was finally recommended for determining the spectral radiative properties of the materials measured.

Miroslav Čekonemail:[email protected] field: influence of energy balance of lowemissivity properties, infrared spectroscopy, buildingphysics

Address:DepartmentofBuildingConstruction,FacultyofCivilEngineering,SlovakUniversityofTechnology,Radlinského11,81368Bratislava

Vol. XX, 2012, No. 2, 1 – 7

1. introDuction

There have been many discussions that building coatings canmarkedlyimprovethethermalpropertiesofbuildingconstructions.Radiativepropertiesareneglectedmanytimesinbuildingpractice.Standardbuilding surfaceshaveemissivityvaluesof around0.90,which means that they can absorb about 90% of the incominglongwave (thermal) radiation. The incident radiation on asurfaceisabsorbedon thebiggerpartsand reflectedon thesmallerparts.Therefore,wecaninfluenceheattransferstotheoutsideenvironmentandimprovethermalcomfortaswellbydecreasingtheemissivityvaluesofbuildingsurfaces[10,11,12].Asapartofintroducingnewtechnologiesandmaterials,spectralemissivitypropertiesareamongmanyotherfactors,whichhavetobetakenintoaccount.Althoughtherearealotofexpectationsandmanydoubts,theyhavestillnotbeenequallyinvestigatedinmanycases.Atthepresenttime,morepreciseevaluationandmeasurementmethods[2,13],whicharenotcommonlyused in theareaofbuildingphysics,havecome to theattentionofthebuildingindustry.

The starting point of our research comes from the fact that thereare coatings on the market whose properties guaranteed by theirproducers can reduce costs for energy demands in buildings. Inpractice, their properties extensively depend on various dynamicchanges.Thebasicprincipleswerederivedatthepeakofthespaceage,whensimilarcoatingsweredevelopedforspaceshuttleshields.However, the different dynamic effects and conditions for suchshields are not compatible with building envelopes. In general,coatingsmainlycreatethinsurfacelayers.Theircharacterdoesnothave much of an influence on the thermal resistance of buildingconstructions. However, they can markedly influence transfereffects on building surfaces and can therefore be relevant to thethermalperformanceofbuildings.In the advertising materials from many producers we can find,among other things, that coatings consisting of hollow ceramicmicrospheres can reduce heat losses in winter and heat gains inthe summer, improve thermal comfort, decrease the possibility ofthe condensation of water vapor, increase surface temperatures,etc.Thesestatementsareoftenbasedonprooflessandunverifiable

2012 SlovaK univerSity of tecHnology 1

2 Spectral emiSSivity propertieS of reflective coatingS

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assertions.They stem from the feelings andhypotheses of peoplewho have used these products for improving their internalbuildingenvironments.Moreover,somematerialscallforthetotalreplacement of standard insulation materials. These uncertaintiesmainlyoccurduetoinadequateresearchontheirproperties,whichareundertheinfluenceofvariousdynamicchanges.Research on coatings is carried out at various institutions. TheInstitute of Technology of Building Materials and Componentsat the Faculty of Civil Engineering inBrno, theCzechRepublic,is one where much research work about low emissivity coatingshas been conducted [4]. The research has focused on direct andindirectmethods,especiallywithinfraredthermographyaswellasspectroscopy. Some of the applications supported by Fourier thattransforminfrared(FTIR)spectrometersandtheDiffuseReflectanceInfraredFourierTransform(DRIFT)methodarereferencedin[4,5,6].The resultsof this researchbringus somenewknowledgebutalsosomedisadvantagesofthemeasurementtechniques.Aresearchproject [14] at theDepartment ofBuildingStructures (FCESUT)inBratislava is also aiming at investigating the thermodynamicproperties and effects of the energy balance of low emissivitybuilding surfaces. For this purpose, our first step leads us todeterminethespectralradiativepropertiesofsomebuildingcoatingsbymeansofinfraredspectroscopy.

2. tHe laBoratory meaSurement tecHniqueS anD experimental metHoDS of Spectral emiSSivity

As iswell known, themainoptical properties of opaquebuildingsurfaces are absorbance, emissivity and reflectance. We cannotmeasure emissivity directly, because the measurement devicesusually obtain these values from several parameters. Variousmeasurement techniques have been developed, which may bedivided into three loosely defined groups: calorimetric emissionmeasurements, radiometric emission measurements and reflectionmeasurements[1].Moreover,wecandividereflectionmeasurementsintotwosubgroups.Themoreeasilyobtainableonesarebasedonthe monitoring of the intensity of the radiation reflected fromasurface. The simplest measurement is based on the integralmeasurement of the reflected diffusive radiation from the surfaceimplemented by the FTIR spectroscopy also known as DRIFTSspectroscopy. For more technical information and descriptions,see [2]. The second technique is based on the Attenuated TotalReflection (ATR) method [8]. Diffuse reflectance measurementsare carried out to determine the bidirectional reflection functionandthedirectional-hemisphericalandthehemispherical-directionalreflectance [1]. From these values we can derive other required

parameters,suchasemissivityorabsorbance,whichresultfromthelawof theconservationofenergyandKirchhoff’slaws,wherewesupposethat:

,ormoreprecisely ,[-] (1)

TwosimilarmethodsareusuallyusedforthemostcommonreflectionDRIFTtechniques.Themaindifferencebetweenthosetwomethodsis in the type of reflectometers. The first one employs integratingsphere reflectometers, while the second one employs integratingmirrorreflectometers.Adetaileddescriptioncanalsobefoundin[1].

3. experimental ir SpectroScopy meaSurementS of tHe coatingS StuDieD

There are several kinds of coatings on themarket, which consistofhollowceramicmicrospheres.Inthebuildingpractice,acoatingcompositionwhichconsistsofafiller(TiO2,CaCO3approximately32.3%ofthetotalamount),afillerofhollowceramicmicrospheresfrom5to60micrometers(approximately8.3%ofthetotalamount),abinding material (PVAc variance), other helpful ingredients(approximately12.6%ofthetotalamount)andwater(approximately46.9% of total amount) [3] is considered. Tomake acomparisonbetween them and the standard facing coating, almost all of thecoatings available in the Slovak and Czech Republics for thispurpose were selected. The aim was to compare their spectralemissivitypropertieswithstandardfacingcoatingsbymeansofthetworeflectivemeasurementmethods(DRIFT[2]andATR[8]).Thiscomparisonisalsolinkedtoaprevious,partlyreferenced,work[9].

3.1 Brief Description of the method and measuring instruments

Thebasic principle of the experimental verification is focused onaspectralanalysiswith theapplicationof infraredspectroscopyinaregionof longwave radiation. Inorder to compare the reflectivemethods available, three measurement methods were used,dependingonthemeasurementdeviceapplied:• DRIFTmethod– the NicoletMagna750 infraredspectroscopymeasurement device (Figure 1 on the left) was used. Thismeasurementdevicehandlesawidespectralrangefromthemidwavelengthinfrareddomain(MWIR)tothefarinfraredone(FIR),orinpractice(forourpurpose),thewavelengthsfromroughly2.0to 50.0 μm. For more technical details, see [7].Although thedevicebelongsamongolderinstruments,itstillprovidesaprecisemethod. Because the main experimental principle consists of

2012/2 PAGES 1 — 7

3Spectral emiSSivity propertieS of reflective coatingS

the DRIFT method [2], we need to use aspecial measuringattachment for such apurpose. The shape of the measuringattachmentandthetypeofdetectorappliedaresomeofthemostimportant parts of this experimental method. This experimentwasconductedonanavailablemeasuringinstrumentconsistingof an integratingmirrorwith ahemispherical shape (Fig. 1 on

theright).ThemeasurementswerecarriedoutattheInstituteofChemistryattheSlovakAcademyofSciencesinBratislava.

• ATRI.method– theNicolet5700FT-IRinfraredspectroscopymeasurement device with aNicolet Smart Orbit attachment(Fig.2ontheleft)wasused.ThemeasurementswerecarriedoutattheLaboratoryofSpectralMethods(DepartmentofInorganic

Fig. 1 The Nicolet Magna 750 infrared spectroscopy measurement device (on the left) and a view of the integrating mirror used with a hemispherical shape (on the right).

Fig. 3 The Nicolet 6700 FT-IR infrared spectroscopy measurement device (on the left) and a view of the measurement attachment (on the right).

Fig. 2 The Nicolet 5700 FT-IR infrared spectroscopy measurement device (on the left) and a view of the measurement attachment (on the right).


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Chemistry, Technology and Materials, Faculty of Chemicaland Food Technology, Slovak University of TechnologyinBratislava).

• ATRII.method– theNicolet6700FT-IRinfraredspectroscopymeasurement device with aNicolet Smart SpecularATRattachment (Fig. 3 on the left)was used. It provides aspectraloptical analysis of the reflective materials of amirror. Themeasurementswere carriedout at the Institute ofChemistry attheSlovakAcademyofSciencesinBratislava.

3.2 Description of the Samples prepared

The measurement devices which were used require specimensof asizeof roughly10× 10mm.Therefore,weprepared severalsamplesfromfourwhitecommercialcoatingswithvariouscoatingtechniques and some other materials for the comparison. All ofthese samples were prepared with acoating on adrawing (A3format)andwerethencuttotherequiredsize.ThedescriptionsandcharacteristicsarebrieflygiveninTab.1.

3.3 results of the experiment and Discussion

The results evaluated were obtained by the DRIFT and ATRmeasurementmethods. Themeasurements were performed on anadequateamountofsamples(aboutfivesamplesfromeach type).Eachtypeofsamplewasmeasuredseveraltimes.Theprecisionofeachmethodandinstrumentwasverifiedusinganadequateamountofmeasurements.Thesameresultswereobtainedforeachtypeofcoating,soonlytheoneselectedwaspresented.The results seen inFigs.4and5weremeasuredbymeansof theDRIFT method. The spectral reflectance curves of the samplesmeasuredasafunctionofthewavelengthareshown(inaspectral

range from 2.5 to 25.0 μm – the range of the long-wavelengthradiation).Equation (1) can be used for determining the measurementresults in terms of the emissivity values.Themeasured data arerepresented in (Tab. 2) as integral emissivity values. The mostimportantregionissomewherebetween8.0and14.0μm,whereallthestandardbodiesradiatethemaximumoftheirenergy(roughlyonthe10.0μm).Ascanbe seen, therearenodifferencesbetween theS1 standardbuilding surfaces (reference standard facing coating) and thosewhichconsistofhollowceramicmicrospheres(S2-5).Thestandardreference coating has even higher reflectance values than theotherscompared.SampleS5hadtheworstresultsandthebiggestdifference from the others.The rest of the sampleswere roughlyat the same level.The aluminum foil (S7)wasmeasuredonbothsides.Thedifferencebetweenthemisinthe20%reductionoftheirreflective properties as is shown in Fig. 4.Also evident is athinvisible foil (S7_a), whichmarkedly filters the spectral propertiesofthesidemeasured.SampleS8withitsmetallicparticlescanbeconsidered based on its results as acoating with low emissivityproperties (spectral reflectance of more than 70%). The spectralpropertiesofsampleS9aremorethan10%higherinsomeregionsthan thoseconsistingofhollowceramicmicrospheres.Thiseffectcauses ahigher gloss and smoother surface of the coated samplethan the others. It is also important tomeasure and compare thesamedegreeof roughness of all themeasured surfaces using thismethod.The results from theATRmeasurementmethod (Fig. 6) indicatesignificantdifferencesintheinfraredspectrumsincomparisonwiththeDRIFTmeasurementmethod.However,thereareconsiderabledifferences in some areas; the results indicate non-standard andunrealvalues.Theseresultsonlyhaveanexploratorynature; they

Tab. 1 Description of samples measured.Sample Sample description Application ColourS1 Disperseacrylatefacingcoating,reference Internalandexternal WhiteS2 Facingcoatingofhollowceramicmicrospherebase External WhiteS3 Flexiblecoatingconsistingofhollowborosilicatemicrospheres Internalandexternal WhiteS4 Coatingofhollowceramicmicrospherebase Internalandexternal WhiteS5 Coatingconsistedofhollowceramicandsiliconemicrospheres Internalandexternal WhiteS6 Thermographyetalon-thermaspot Lab.knownemissivity0.95 WhiteS7 Aluminumfoil,bothsidestested,S7_a–mat,S7_b-glossy Reflectivematerial SilvergreyS8 Aluminumpigmentedcoating Internal SilvergreyS9 Coatingbasedonaerogels Automobileindustry White

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donotrepresenttheopticalpropertiesofthecoatingsmeasured,butaffecttheirinternalstructureandmaterialcomposition.The results on Fig. 7 were measured by means of theATR II.method,bywhichanalmostperfectmatchcanbeobtainedwiththe aluminum foil sample (S7) in comparison with the DRIFTmethod. Some samples were partly recorded only with theirglossysurface.Othersampleswerenotrecordedbecauseoftheirincreasingabilitytoreflectthediffuseradiationontheirsurface.

This method is not appropriate for diffuse reflectance surfaces,becauseitemploysmirrorreflectancetechniquesfromthesurfacemeasured.These methods also result in some defects and disadvantages,whichaccrue fromusing the techniqueandmeasurementmethod.Measuring amirror´sreflected surfaces is more precise thanmeasuringmatt,roughanddiffusereflectedsurfaces.Moredetailedinformationcanbefoundin[4].

Fig. 4 Progress of spectral reflectance of coatings measured (the DRIFT method).

Fig. 5 Progress of spectral reflectance of coatings selected from Fig. 4 (the DRIFT method).

Tab. 2 Final spectral emissivity as integral values in the selected infrared spectrums (DRIFT method).sample: emissivity ελ S1 S2 S3 S5 S6 S7_a S7_b S8 S9

2.5–25.0μm 0.87 0.88 0.90 0.90 0.87 0.35 0.12 0.32 0.878.0–15.0μm 0.93 0.92 0.94 0.96 0.90 0.35 0.11 0.31 0.88


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4. concluSion

The infrared spectroscopymethod applied is useful and availablefordeterminingspectralradiativeproperties.Theapplicationofthismethodisnotcommonlyimplementedinthebuildingphysicsarea.The results obtained from the infrared spectroscopy (the DRIFTmethod) indicate that there are no differences in the spectralreflectance(emissivity)valuesbetweenstandardbuildingsurfaces(referencestandardfacingcoating)andthoseconsistingofhollowceramicmicrospheres.The DRIFT method provides awide range of possibilities ofspectralmeasurements for all the tested samples,whether diffuseormirrorreflective,sotheseresultscanbeconsideredasastarting

pointforfuturework.Furthermore,theresultsindicatethatthefinalvaluesaremorecredible(Tab.2)thantheothermethodsused.On the other hand, theATRmethods are not appropriate for thespectral analysis of diffuse reflective materials, because theyemploymirror reflectance techniques from the surfacemeasured.WhiletheresultsfromtheATRI.methodonlyhaveanexploratorynature, which do not represent the optical properties of thesamplesmeasured,butinfluencetheirinternalstructureandmaterialcomposition,theresultsfromtheATRII.methodareonlyappropriateforidealmirrorreflectivesurfacesandmaterials.

This research is supported by the VEGA No. 1/0647/09 research project.

Fig. 6 Progress of spectral reflectance of the coatings selected (the ATR I. method).

Fig. 7 Progress of spectral reflectance of the coatings selected (the ATR II. method).

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spectral emissivity properties of reflective coatings · m. Čekon spectral emissivity properties...

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