50 July 2005 JCT CoatingsTech50 July 2005 JCT CoatingsTech

In this JCT COATINGSTECH series of in-troductory tutorial articles, the empha-sis sought is on the applications ofmodern analytical, characterization, andtesting methods for coatings. And whileit is easy to cite many examples of re-cent applications of mid-infrared spec-troscopy, the most common samplingtechniques employed have been well es-tablished for a long time and, as a con-sequence, mid-infrared spectroscopy hasbeen at the forefront of analytical meth-ods for characterizing coatings and sur-faces for many years. In 1980, theFederation of Societies for CoatingsTechnology (FSCT) published anInfrared Spectroscopy Atlas for the CoatingsIndustry,1 which contained over 1400mid-infrared spectra, covering categoriessuch as polymers, monomers, solvents,inorganic pigments and extenders, or-ganic pigments, and various additives. Afourth edition of Infrared SpectroscopyAtlas for the Coatings Industry, whichcontains ca. 2500 mid-infrared spectra,was published in two volumes by theFSCT in 1991.2 In addition to its estab-lished position as a favored techniquefor characterizing and analyzing poly-mers,3-6 the long tradition for usingmid-infrared spectroscopy to study andcharacterize in situ coatings stems es-sentially from two sampling techniquedevelopments in the 1960s. The first,known as ATR (Attenuated TotalReflection) mid-infrared spectroscopy, isan internal reflection spectroscopy tech-nique.7-11 The second, more specializedtechnique is an external reflection ap-proach undertaken at grazing angle ofincidence, which is often referred to asRAIRS (Reflection-Absorption InfraredSpectroscopy),12 or some other similaracronym. Mid-infrared ATR spec-

*14 Croft Hills, Tame Bridge, Stokesley, TS9 5NW U.K. Email: johnmchalmers@aol.com.

by John M. Chalmers, VSConsulting*

Infrared Spectroscopy in theAnalysis, Characterization, and

Testing of Coatings

troscopy, which is discussed in more de-tail later, interrogates a surface layer of asample to a depth in the approximaterange of 0.3 µm to 3 µm. RAIRS is usedto study thin films, typically in therange 100 Å –100 nm, such asLangmuir-Blodgett and vacuum evapo-rated films and self-assembled mono-layers supported on a smooth metal re-flective surface such as that of gold,silver, and copper.13 RAIRS remainslargely a more specialized research toolwith one of its other main areas of ap-plication being in the study of speciesadsorbed on metal catalysts. However,since the 1980s, with the introductionof high sensitivity modern Fouriertransform infrared spectrometers (FTIR),and in particular FTIR microscope sys-tems, mid-infrared reflection-absorptionspectroscopy at near normal angles of in-cidence has become quite a commonlyused direct samplingtechnique for finger-printing coating lay-ers on reflective sub-strates. It iscommonly referredto as the transflec-tion sampling tech-nique. Other mid-in-frared samplingmethods that havebeen used to exam-ine coatings includetransmission spec-troscopy of materialabraded from a sur-face, emission spec-troscopy, and themore recently ap-plied methods ofphotoacoustic anddiffuse reflection

spectroscopy. Almeida et al.14 have dis-cussed and compared the relative meritsof a range of FTIR sampling techniquesused for the study of surfaces and coat-ings. Sampling methods will be consid-ered in more detail later.

The infrared portion of the electro-magnetic spectrum is usually conve-niently broken down into three separateregions. It is common practice today tolabel mid- and far-infrared bands withthe unit of wavenumber, while near-in-frared band positions are more usuallyquoted in units of wavelength. The mid-infrared region covers 4000 cm–1 to 400cm–1 (2.5 µm to 25 µm wavelength);the near-infrared region extends overthe range from ca. 750 nm to 2.5 µm;and the far-infrared or lower wavenum-ber (longer wavelength) region occursbelow 400 cm–1.

Figure 1—Schematic of internal reflection spectroscopy. The in-frared incident beam transmitted through the denser medium of re-fractive index n1 is internally reflected at the boundary with themedium of lower refractive index n2, when incident at an angle ofincidence i which is greater than the critical angle ic.

Analytical Series

July 2005 51www.coatingstech.org July 2005 51

The strength of the mid-infrared re-gion is that it is the range in whichmost of the fundamental vibrationalfrequencies of organic molecules occur.A mid-infrared absorption spectrum isgenerated when infrared radiation inter-acts in an appropriate manner with themolecular vibrations of a sample.Absorption bands that are characteristicof the organic molecular vibrations areseen throughout the spectrum of thesample, many occurring within rela-tively narrow wavenumber ranges thatare associated with particular moleculargroupings. For instance, hydrocarbonstretches (νCH) normally occur withinthe range 3200–2800 cm–1, with bandsfrom vibrations of saturated species(e.g. –CH3, –CH2) occurring below3000 cm–1, while those associated withunsaturated groups, such as aromaticCH and =CH2, usually occur above3000 cm–1; the carbonyl (νC=O) willtypically be found within the range1850–1550 cm–1, with narrower regionswithin this range being more specific totype, such as ester, ketone, acid, etc.

Consequently, in addition to pat-tern-recognition fingerprinting of coat-ings, mid-infrared spectroscopy is alsowidely used as both a qualitative andquantitative tool for identifying ormonitoring functional groups presenton surfaces and in coatings, such as in astudy of rate of cure or surface oxida-tion/degradation. Bands occurring inthe near-infrared region are overtoneand combination bands mostly of fun-damental bands involving X–H (X = C,O, N) vibrations. Their relative intensityis one to several orders of magnitudeless than those observed within themid-infrared region, and a near-infraredspectrum tends to lack both the sensi-tivity and the fine detail necessary for itto be used as a general tool for charac-terizing coatings. It may, however, insome circumstances, provide a goodmethod for gauging coat thickness/weight. The far-infrared region exploreslow-frequency motions in molecularsystems (for example, skeletal bendingmodes of entire individual molecules)and has been more the realm of the ac-ademic researcher than of prime inter-est or value to industrial coatings tech-nology. However, with the developmentrecently of new Terahertz (THz) instru-mentation there is renewed interest inthis region, particularly within the phar-maceutical industry.15 In a recent publi-cation, the potential for nondestructiveanalysis of tablet coating thickness was

demonstrated using THz pulsed imag-ing.16

Almost exclusively today, all mid-in-frared spectrometers found in applica-tions laboratories are Fourier transformspectrometers, because of their highsensitivity, excellent wavenumber repro-ducibility, and multiplex advantage.17,18

MID-INFRARED SAMPLINGTECHNIQUES

The ATR TechniqueInternal reflection spectroscopy (IRS)

is a consequence of the optical propertythat radiation passing through a denser(higher refractive index) medium at anangle of incidence greater than the criti-cal angle will be totally internally re-flected at a boundary in contact with amaterial of lower refractive index. Aschematic of the ATR sampling methodis shown in Figure 1. The evanescentwave is confined within the vicinity ofthe surface of the rarer medium.19 The

evanescent (exponentially decaying)wave decreases with intensity with dis-tance normal to the surface into thisrarer medium and can be envisaged aspenetrating the surface layer of the rarermedium. The depth of penetration, dp,has become a convenient comparativeterm for different experimental arrange-ments. It is actually the depth at whichthe amplitude of electric field ampli-tude falls to 37% (1/e) of its value atthe surface. The depth of penetrationdepends on the angle of incidence andthe ratio of the refractive indices of thedenser to the rarer medium. It decreaseswith increasing angle of incidence. It isalso wavelength dependent, increasingwith increasing wavelength (decreasingwavenumber).

In the laboratory today, the mostcommonly used internal reflection ele-ments are ZnSe, Type II diamond, andGe. These have refractive indices (at ca.1000 cm–1 and 25°) of 2.4, 2.4, and 4,respectively. Diamond, because of cost,is usually only configured for use in mi-cro-ATR set-ups. Figure 2a compares dp

Figure 2—(a) Plots of depth of penetration for two refractive indices (2.4and 4) and two angles of incidence (45° and 60°). (b) Plots of effectivesample path length using an internal reflection element of refractive index4 at 45° angle of incidence for a single reflection element and ×3, ×9, and×25 multiple internal reflection elements.

52 July 2005 JCT CoatingsTech

values calculated for a single internal re-flection from both ZnSe and Ge inter-nal reflection elements at two com-monly used angles of incidence. It canbe seen readily that the surface layer/coating thickness probed increases sig-nificantly towards lower wavenumber. Avariety of geometrical shapes and sizeshave been used for internal reflectionelements, including tetrahedron, hemi-spherical, parallelepiped, trapezoid, androd. One of the most commonly usedin the laboratory now is the trapezoid,which is incorporated into the so-calledhorizontal ATR (H-ATR) units. Thesemay be several centimeters long, allow-ing for multiple internal reflections,thus increasing the number of times theinfrared radiation interacts with a sam-ple surface (see Figures 2b and 3) andthereby increasing the effective samplelayer pathlength. Although with thegood signal-to-noise ratio spectraachieved routinely with a FTIR spec-trometer, and because of their ease ofuse, modern single-reflection ATR acces-sories are being increasingly used to fin-gerprint coatings on polymer substrates.This is acceptable if the small contactregion examined, typically of the orderof 1 mm or less, may be considered rep-resentative of the coating layer. A classicexample of the use of ATR spectroscopyis shown in Figure 4 in which the tech-nique is used to identify the two differ-ent layers, one a heat-seal layer, theother a barrier layer, on each side of afood packaging film.3

RAIRSReflection Absorption Infrared

Spectroscopy (RAIRS) at grazing anglesof incidence, undertaken in practice typ-ically at about 80° or greater, involvesmeasurement of the change of reflectiv-ity of a substrate brought about by athin infrared absorbing layer.13,20 In ad-dition to the absorption properties ofthe thin film, the absorption bands in agrazing incidence angle RAIRS measure-ment represent changes in reflectivity,∆R/R, which depend on the infraredstanding wave intensity at the surface.Since changes in reflectivity ∆R/R due tothe thin film are generally small (a few%), the experiment ideally needs to becarried out under conditions where R issubstantial. Consequently, RAIRS ismost successfully applied on metal sur-faces, since the high reflectivity of themetal surface may be combined with anintense standing wave at the metal sur-

IR beam

Internal reflection element

SampleSample(a)

(b)

Figure 3—(a) Schematic showing multiple internal reflection. (b) Photographof a horizontal multiple internal reflection accessory (Horizon™). Above thehorizontal ATR element is a clamp for securing a solid sample into positiononto the internal reflection element. A liquid sampling trough can be seen inthe bottom left of the picture. (Photograph reproduced by permission ofHarrick Scientific Corporation, Ossining, USA.)

Figure 4—Mid-infrared spectra recorded from a food packaging film. The film comprises apolypropylene base with an ethylene/vinyl acetate copolymer heat-seal layer on one side (sec-ond, inner surface) and a vinylidene chloride/acrylonitrile/ester terpolymer barrier layer on theother side (first, outer surface). (a) is a transmission spectrum of the multilayer film; (b) and(c) are multiple internal reflection spectra recorded from each surface. In (b) and (c) character-istic spectra of each of the surface layers have been clearly isolated from that of the polypropy-lene base. In this measurement, a KRS-5 prism with a refractive index of ca. 2.4 at a 60° angleof incidence was employed. ©2000. Copyright John Wiley & Sons Ltd. (Reproduced with permis-sion from reference 3.)

Analytical Series

July 2005 53www.coatingstech.org

face. RAIRS is normally conducted usinga polarizer set for p-polarized infraredradiation, because at a grazing angle ofincidence for p-polarized radiationthere is a phase change of about 90°.This results in the incident and reflectedbeams combining to produce a strongelectric field which is effectively polar-ized perpendicular to the surface. For s-polarized radiation there is an 180°phase change on reflection and as aconsequence a node in the electric fieldoccurs at the metal surface. These prop-erties lead to the metal surface selectionrule, which states that only vibrationalmodes with dipole moment compo-nents perpendicular to the metal surfacemay be excited, and allow one to evalu-ate molecular orientation in ultra-thinfilms, such as self-assembled monolay-ers. Figure 5 is a schematic showing thesum of the incident and reflected vec-tors for s- and p-polarization.

Tolstoy et al.21 have recently pub-lished a handbook that covers in detailand reviews the theory, for both flat andpowdered substrates, the practical meth-ods of measurement and applications ofmid-infrared spectra recorded from ul-trathin films (films of ~1 nm thickness)covering submonolayer to severalmonolayers. Depending on the system,infrared spectroscopy may be sensitiveto 10–5–10% of a monolayer.21 Thesenanotechnology films, which find ap-plications in microelectronics and opto-electronics, may be used, for example,as passivating or protective coatings.

Photoacoustic SpectroscopyPhotoacoustic spectroscopy (PAS), as

a mid-infrared laboratory sampling tech-nique, came to prominence shortly afterthe advent of modern FTIR spectroscopy.The approach offered some unique ad-vantages. It is less sensitive to surfacecondition than ATR and can be used toprobe a greater range of samplingdepths, from several to 100 mm orgreater.20,22 In a photoacoustic FTIRmeasurement, the mid-infrared beam isincident on the sample, which is placedinside a cell filled with a coupling gas,usually He. The modulated FTIR beamthat is absorbed by the sample causes ab-sorption-induced heating in the sampleand consequent oscillations of the sam-ple temperature. The thermal wavespropagate to the sample surface, produc-ing pressure fluctuations in the couplinggas. The absorption characteristics ofthe sample are measured by sensing

with a microphone the thermal-expan-sion pressurization of the coupling gascaused by heat transfer. The magnitudeof a PAS signal varies linearly with in-creasing absorbance, which can be aconsequence of either concentration orsampling depth or both. However, thedependence is nonlinear at extremes oflow and high values of absorptivity, andsignal saturation occurs athigh signal levels. The phaseof the PAS signal, whichcorresponds to the time de-lay associated with heattransfer in the sample, maybe used to determine thedepth within the samplefrom which the PAS signaloriginated.

In a conventional rapidscan FTIR spectrometer,lowering the scan speed(mirror velocity), thus de-creasing the frequency ofmodulation of the mid-in-frared beam, will increasethe sampling depth.However, the modulationfrequency in this scanningmode is wavelength de-pendent and increases withincreasing wavelength, solike ATR spectra when com-pared with transmissionspectra, the relative intensi-ties of bands in a PAS spec-trum appear more enhancedwith decreasing wavenum-ber. Figure 6 shows a seriesof FTIR PA spectra recordedusing different interferome-ter mirror scan speeds froma polytetrafluoroethylene(PTFE)/polyimide layeredstructure. While at thefastest scan speed the PA

spectrum is almost entirely due to theuppermost PTFE layer, at decreasing in-terferometer mirror scan speeds there isincreasing intrusion into the PTFE spec-trum of bands characteristic of the un-derlying polyimide layer.

The variation in modulation fre-quency can be overcome by using a step-scan interferometer, whereby an external

90o Phase shift 180o Phase shift

p - Polarization s - Polarization

IR IR

Figure 5—Schematics of grazing-incidence reflection-absorptionspectroscopy showing how for p-polarization the electric field vectorsnear the sample surface sum, while for s-polarization they effectivelycancel.

Figure 6—Mid-infrared rapid scan FTIR spectra from aPTFE/polyimide/PTFE layered sample, with layer thicknessof 12.5 µm, 50 µm, and 12.5 µm, respectively, recorded atdifferent interferometer mirror scan speeds. Note how thereis increasing contribution to the recorded spectrum frombands characteristic of the polyimide with decreasing mir-ror scan speed, i.e. decreasing modulation frequency.

54 July 2005 JCT CoatingsTech

modulation is applied such that eachwavenumber within a spectrum is sub-jected to the same modulation fre-quency, thereby enabling mid-infraredspectra to be derived from particulardepths—a process that may be envis-aged as “optical sectioning.” A simpleexample is shown in Figure 7, whichshows the phase-rotated spectra derivedfrom a 400 Hz phase-modulated step-scan FTIR measurement from a 25 µm-thick commercial film based onpolypropylene.23 The PAS spectrashown correspond to those extracted at0°, 30°, and 150° phase rotation. Thebulk (150°) spectrum is clearly that ofpolypropylene homopolymer. While oflower signal-to-noise ratio, the surface(0°) spectrum clearly indicates that thecoating layer is polyethylene. This wasthought to be present at a coat layerthickness of between 0.4 and 0.5 µm.In this case we were not able to obtainsuch good discrimination by multipleinternal reflection ATR (Ge prism, 45°angle of incidence) spectroscopy, sincein the fingerprint region of the mid-in-frared spectrum the nominal depth ofpenetration was calculated at ca. 0.95 µm.

Transflection Transflection is the term used to de-

scribe a near-normal incidence reflection-absorption measurement. It derives frommeasuring a coating on a reflective sub-strate, in which the mid-infrared beamat a near-normal angle of incidence(typically ca. 0°–30°) passes throughthe coating, is reflected back from the

substrate, and passes through the coat-ing again before being detected. The re-sulting double-pass spectrum will there-fore resemble that recorded intransmission from a sample of abouttwice the coating thickness. However,superimposed on this reflection-absorp-tion spectrum will be a specular (front-surface) reflection spectrum. This specu-lar reflection spectrum exhibits thedispersion in the refractive index thatoccurs through an infrared absorptionband. It leads to apparent distortion ofthe infrared absorption spectrum, par-ticularly noticeable for strong bands,and its influence clearly increases withdecreasing coating thickness. In evaluat-ing FTIR transflectance spectra, it is im-portant that the influence of the specu-lar reflectance component isappreciated, otherwise erroneous resultsand correlations may likely beformed.18

The sampling technique can providea very quick and convenient way of fin-gerprinting coatings on applicationssuch as the insides of beverage cans.

FTIR MicroscopyTransflection is one of a range of

sampling techniques, along with trans-mission and micro-ATR spectroscopy,that are commonly used with FTIR mi-croscopy. The coupling of microscopywith FTIR spectroscopy has enabled thestudy and measurement at high lateralspatial resolution of a wide range ofsamples and problems related to thecoatings industry. It is widely used in

industry, alongside a range of other ana-lytical techniques, in a forensic capacityfor elucidating problems associatedwith surface/coating defects/imperfec-tions, contamination, and failure.Typically these examinations are limitedto a localized area of greater than ca. 10µm diameter, although using the mid-infrared radiation emanating from asynchrotron does offer significant ad-vantages at high lateral spatial resolu-tion.24

For characterizing thick coating/sur-face layers (ca. >10 mm thickness), it isoften relatively straightforward to mi-crotome a cross-section from a multi-layer structure and interrogate the coat-ing and other layers independently by atransmission measurement.

In addition, to single-point spectra,FTIR ATR-microscopy coupled with us-ing a Focal Plane Array (FPA) detectoroffers the potential to image surfaceheterogeneity and composition.20

Different species and phases of the or-der of the pixel size of the array, whichis usually about 6.25 µm × 6.25 µm,may be highlighted; a typical array de-tector today may comprise 64 × 64 or128 × 128 pixels.

Other Sampling TechniquesAs appropriate, a range of other mid-

infrared sampling techniques have beenapplied to studies and problems relat-ing to the coatings industry, although inrecent years not as widely as those con-sidered above. These include: transmis-sion, emission, and diffuse reflection.Historically, abrasion was often em-ployed, whereby surface material wasabraded and examined in transmissionas a KBr disc.25 Abraded material maynow be more conveniently examined bydiffuse reflection, where the abradedmaterial is simply dispersed in and ana-lyzed as a mixture with dry, powderedKCl.18 Collecting diffuse reflection spec-tra from surfaces that are overlaid withdry powdered KCl has also been em-ployed in particular circumstances, suchas that of recording mid-infrared spectrafrom coupling agents coated onto silicaglass fibers.26 Since a sample that absorbsmid-infrared radiation will also emitmid-infrared radiation, an analyticallyuseful emission spectrum may also berecorded from a thin coating layer on ametallic substrate by emission spec-troscopy.27 In the laboratory, emissionspectroscopy has been supplanted largelyas a sampling technique for fingerprint-

Figure 7—Phase-rotated spectra extracted from a 400 Hz phase modulation PASFTIR step-scan measurement on a 25-µm thickness commercial polypropylenefilm.23

Analytical Series

July 2005 55www.coatingstech.org

ing thin coatings on reflective metal sub-strates by the more convenient transflec-tion approach.

APPLICATIONS ANDEXAMPLES

Since this article is not intended as areview, but as a fairly concise tutorial,the references included have been cho-sen simply to illustrate a particular typeof application. Many are from more re-cent published work; they are, however,very far from being exhaustive and fullyrepresentative of the wide range ofanalyses undertaken and materials stud-ied. For the majority of practical pur-poses, the intensity of a mid-infraredabsorption band is proportional to thenumber density of vibrating species giv-ing rise to that band. Hence, monitor-ing of the relative intensity of a mid-in-frared functional group band or theratio of two bands can be an extremelyuseful method, and is widely used inthe study of coating cure processes andcoating and surface layer weatheringand degradation. In addition to beingvery characteristic of the functionalgroups present and their concentration,mid-infrared spectra of organic poly-mers may also be very sensitive to stateof order. They are characteristic also ofstereoregularity, molecular conforma-tion, and molecular orientation and or-dering.3,4,20

Coating Identification, ChemicalStructure, and Thickness

DeterminationA straightforward coatings applica-

tion of mid-infrared ATR spectroscopy isillustrated in Figure 4 and representsprobably one of the more common usesof mid-infrared spectroscopy in indus-try—that of fingerprinting organic poly-mer coatings on multilayer samples thatare thicker than about 1 µm. For coat-ings that are thinner than this and onan organic polymer substrate, there willbe intrusion of absorption bands of thesubstrate within the recorded ATR spec-trum. However, while a “pure” spectrumof the coating may not necessarily beisolated in this manner, if a suitablewindow is available within the sub-strate’s mid-infrared spectrum in whichan absorption band of the coating ispresent, a coating layer thickness maybe determined to a much lower thick-ness value. Providing a method can be

calibrated by reference to values deter-mined by some other suitable means,then precise quantitative practical meth-ods suitable for at-line monitoring of acoating thickness may be developed. Inmy time in the plastics industry, we de-veloped mid-infrared ATR-spectroscopyquality assurance methods that were ap-plied routinely for determining coatthickness (or coat weight) levels downto the order of 500 Å or less.

Curing and Film FormationStudies

The particular sensitivity of mid-in-frared spectroscopy to oxygenatedspecies, such as hydroxyl and carbonylgroups, and unsaturated species, such asmany double and triple bonds, makes ita particularly valuable analytical tech-nique and investigative tool for thecoatings industry.

There are many published studies us-ing mid-infrared spectroscopy that re-late to polymer cure and crosslinkingmechanisms and rate; many are citedwithin references 3–5. Kinetic studiesundertaken with a modern rapid scanFTIR spectrometer allow for sequentialspectra to be recorded at time-intervalsmuch lower than 1 sec if necessary.Many studies of thermal cure and UV-curable mechanisms are made in trans-mission or using internal reflectionspectroscopy from thin films cast fromresin solutions or in situ onto a poly-mer base film or metallic substrate.Typical of theseare thermal cureof epoxy resinbased coatingsystems, forwhich the rate ofcure may be fol-lowed simply byobserving the re-duction in ab-sorption inten-sity of a band at915 cm–1 attrib-uted to the epoxyring. Anothercommon exam-ple is that of fol-lowing the curereaction of apolyurethane lac-quer by monitor-ing the decreaseof a band due tothe isocyanategroup. Dual-cure

formulations have been developed toovercome cure deficiencies in UV non-illuminated areas of protective coatings,such as those in shadowed areas ofthree-dimensional structures or deepwithin thick, pigmented coatings. In arecent study, Studer et al.28 monitoredin real-time the decay in intensity of thecharacteristic isocyanate band occurringat 2271 cm–1 in a thermal and photo-chemical curing study of a “dual-cure”system, in which the depletion of theacrylate bonds was monitored at 1410cm–1. The spectra shown in Figure 8 aretaken from a real-time mid-infrared spec-troscopic monitoring of a UV-curableresin.29 In this instance, the reflection-ab-sorption spectra were recorded using arapid scan FTIR spectrometer from aresin sample on an Al substrate con-tained within a heatable reflection-ab-sorption cell. The UV illumination wasswitched on and off with a high-speedshutter synchronized with the scan sys-tem of the FTIR spectrometer. In Figure 8the series of spectra were recorded at 45min time intervals during the photoin-duced polymerization reaction. Theyrepresent the difference between therecorded spectrum and that of the un-cured resin. The negative doublet at1615 cm–1 and 1636 cm–1 results from adecrease in the acrylic C=C concentra-tion; the negative C=O peak at 1680cm–1 results from a decrease in the pho-toinitiator concentration; the peaks at1725 cm–1 and 1740 cm–1 are a conse-quence of frequency shifts of the acrylicC=O band arising from a loss of conju-

Figure 8—Series of mid-infrared difference spectra derived from spec-tra recorded at 45 min time intervals during the UV-curing reaction ofan acrylate resin. (Reproduced from reference 29, ©2000, with permis-sion from Elsevier.)

56 July 2005 JCT CoatingsTech

gation. Figure 9 compares the UV-curingprofile determined from a reflection-ab-sorption study under nitrogen of a 5-µm thick film of lauryl acrylate contain-ing a high concentration (10%) of thephotoinitiator Irgacure 184 with the de-crease in intensity of the carbonyl banddue to the photoinitiator.29 A band at810 cm–1, attributed to the acrylateC=C–H group, was used to monitor theconsumption of C=C groups in terms ofa percentage reduction in intensity. Acarbonyl band at 1680 cm–1 was used tomonitor the decrease in photoinitiator.The rapid scan FTIR system is clearlywell-capable of following the UV-curingprocess, and shows that in the first 0.5sec of UV illumination that ca. 70%conversion has taken place and thatconversion of the C=C is completed af-ter ca. 1.2 sec.29 The kinetics of the car-bonyl intensity loss apparently does notmirror the resin curing reaction. Afterinitiating the curing reaction, in a sec-ondary reaction the photoinitiator con-centration is depleted in a bimolecularreduction step that results in a decreasein the number of initiator carbonylgroups.29 Thermal curing of a polyimideprepolymer and cyclization to imideformation may also be observed readilyby an increase in intensity of an imidecarbonyl band near 1780 cm–1.

Surface TreatmentsSurface oxidation, corona discharge,

plasma treatment, and solvent etching

have all been employed to enhance ad-hesion of polymer surfaces and eachimparts a modification to the surface,either chemically by introducing spe-cific functional groups, such as –OH,–NH, C=O, –ONO2, or fluorination, orby a change in surface crystallinity.5,25

These surface property modificationsare often observable readily in a mid-in-frared ATR spectrum recorded from themodified surface layer. In a recently re-ported study,30 FTIR spectroscopy wasused to complement X-ray photoemis-sion spectroscopy (XPS) to characterizeacrylic acid thin films deposited by RFplasma assisted chemical vapor deposi-tion, as part of an investigation into sur-face engineering of polymeric film coat-ings and its influence on proteinattachment kinetics for biomedical ap-plications.

Durability StudiesThere have been many reported stud-

ies of the effects of weathering, degrada-tion, and oxidation on a wide variety ofpolymer surfaces. For instance, Johnsonand McIntyre31 reviewed the potential ofseveral analytical test methods for UVdurability predictions of polymer coat-ings. Of those evaluated, electron spinresonance (ESR) and FTIR-ATR spec-troscopy were deduced as preferable be-cause of the short exposure times neces-sary to obtain significant results underUVA that predict, quantitatively, a coat-ing’s lifetime. Decker et al.32,33 used

mid-infrared FTIR spectroscopy recentlyto study the drying and UV-radiationcuring32 and weathering resistance33 ofwater-based UV-cured polyurethane-acrylate (PUA) coatings. Initially in theweathering resistance study, after drying,the UV-curing reaction was monitoredby real-time mid-infrared spectroscopyby following the decrease in intensity ofa band associated with the acrylate dou-ble bond. The extent of photodegrada-tion to UV exposure was then evaluatedby observing relative intensity changesin characteristic bands. For instance,Figure 10a compares the mid-infrared spectra of a UV-cured water-based PUA coating before and after2000 hr accelerated UVA-weathering,while Figure 10b shows an example ofthe decay of CH and urethane linkagesand build-up of oxidative productsfrom a PUA coating with variation inexposure time. Yang et al.34 used bothmid-infrared step-scan PAS and ATR-mi-croscopy in an accelerated weathering(QUV) degradation study of a highgloss polyurethane topcoat over a chro-mate pigmented epoxy primer. Theirobservations supported increased for-mation of polyurea at the film-air inter-face with increasing exposure time.Keene et al.35,36 have very recently pub-lished FTIR microscopy and topographi-cal detailed investigations into failureanalysis of Navy coating systems. A dia-mond-element micro-ATR system wasused to collect spectra from the samplesurfaces. In addition, cross-sections ofthe two different high-solids poly(ester-urethane) military coating systems fromdifferent aging protocols were micro-tomed to a thickness of 3 µm, and ex-amined by transmission FTIR mi-croscopy with a sampling aperture of 10µm × 70 µm.35 The depth-resolved ex-aminations allowed for a vertical sam-pling resolution within the topcoat ofapproximately 5–10 µm.

A wide variety of mid-infrared spec-troscopic photodegradation studies ofpaint films have been reported, andmany are cited in a recent review articleof vibrational spectroscopy in the paintindustry.37 In one of the reported stud-ies, micro-ATR was used to evaluate andcompare the degradation of coatings af-ter one-year exposure in Florida. While,for a white water-based acrylic systemthe observed changes in its mid-infraredspectrum were small, being confined es-sentially to an increase in hydroxylgroups (3430 cm–1), a shift in the car-bonyl band, and possibly an increasedintensity between 1650 and 1600 cm–1,

Figure 9—UV-curing profiles of lauryl acrylate with 10% photoinitiator.The relative decrease of the acrylate was determined from the intensityof a mid-infrared peak at 810 cm–1; the photoinitiator relative concen-tration decrease was determined from the carbonyl band at 1680 cm–1.(Reproduced from reference 29, ©2000, with permission from Elsevier.)

Analytical Series

July 2005 57www.coatingstech.org

a white water-based alkyd paint showedmuch greater changes throughout itsspectrum. Reference 37 also contains atable listing changes observed in themid-infrared spectra on degradation ofa range of binder materials.

Stratification and MigrationFor mixed systems such as paints, it

can be important not just to character-ize a bulk layer, but to also to investi-gate whether any stratification of thecomponents has occurred, either by de-sign or failure. For very thick films,clearly cross-sectioning and measuringsuccessive layer thickness elements bytransmission mid-infrared microspec-troscopy is a possibility. For thin films,both step-scan PAS and ATR may beused for depth profiling. A simple ex-ample from reference 37 of a self-strati-fying coating system is shown in Figure11. The paint was cast onto a metal foil.An ATR spectrum was recorded directlyfrom the film’s top surface, showing itto be almost entirely composed of a flu-orinated resin; the spectrum of theother surface was obtained by directlycasting the paint onto an ATR internalreflection element. This surface was pre-dominantly that of the epoxy resin.

Since enrichment of a surfactant atan interface may adversely affect apaint’s properties, such as water sensi-tivity or adhesion, it is important to beable to depth profile the compositionof a coating layer.37 While using differ-ent angles of incidence and different re-

fractive index ATR elements allows forsome mid-infrared spectroscopy depthprofiling, its range is limited and it isinfrared wavelength dependent. Agreater depth range is achievable withstep-scan PAS and the depth probed isindependent of the radiation wave-length. Both ATR and step-scan PAShave been employed in analyzing possi-ble migration of surfactants, which arepresent in most latex and emulsionpaints.37 As an example, both tech-niques have been used in a detailedquantitative study of films formed frommixtures of polystyrene and poly(n-butyl acrylate) latexes with the anionicsurfactant sodium dioctyl sulfosuccinate(SDOSS).38 The location of nonassoci-ated surfactant can be ascertained froma characteristic SO3

– group absorptionband at 1050 cm–1 arising from freeSO3

–Na+ on the SDOSS; bands due toassociated SDOSS occur at 1046 cm–1

and 1056 cm–1 for water associationand acid association, respectively. TheATR measurements were undertaken us-ing polarized mid-infrared radiation togain insight into the orientation ofSDOSS molecules, particularly near in-terfaces, while step-scan PAS was usedto probe depths up to 22 µm from ei-ther the film-air or the film-substrate in-terface. Some observations that were de-duced from the data were: thatsurfactant molecules accumulate nearboth interfacial regions; that there is ahigh concentration of surfactant groupsnear the film-air interface; and that atshallow depths the nonassociated

SDOSS groups are aligned preferentiallyperpendicular to this surface, while theassociated groups are oriented with aparallel preference. (Full details of theexperimental procedure and results di-agnosis may be found in reference 38;other migration studies may be foundin references 37 and 38 and referencestherein.)

Other ApplicationsThe published literature on the use

of mid-infrared spectroscopy to iden-tify, characterize, and elucidate on acoating’s performance is extensive andin the space available here to discussits contribution to the coating’s indus-try the number of application exam-ples and fields has had to be very lim-ited. There are many specialized anddiverse areas such as surface engineer-ing, inorganics, paper coating, elec-tronic devices, and biocompatibilitywhere mid-infrared spectra can give in-sight and understanding into a coatingproduct’s performance. For instance,conducting polymers have been shownto have excellent corrosion resistance,39

and in a study of the corrosion protec-tion properties of a polyaniline-polypyrrole composite coating on Alpanels, reflection-absorption infraredspectroscopy was used to compare thechemical structure of the various inor-ganic-filled formulations of coatingsthat were produced by electrochemicalpolymerization. Manso et al.40 usedFTIR (alongside XRD and SEM-EDX) in

Figure 10—(a) Mid-infrared spectra of a UV-cured water-based PUA coating before (continuous line) and after (dashed line) 2000 hr QUV-A weath-ering. (b) Plots derived from mid-infrared data showing the decay of CH and urethane linkages and build-up of oxidative products from a QUVaging of a PUA coating with variation in exposure time. (Reproduced from reference 33, ©2004, with permission from Elsevier.)

58 July 2005 JCT CoatingsTech

testing sol-gel CaTiO3 coatings for bio-compatible applications.

The applications emphasized in thisarticle have been towards those investi-gations that are undertaken typicallywithin a research, technical, or produc-tion support laboratory. The sensitivityof mid-infrared spectroscopy to func-tional group type and concentrationclearly makes it also a favored methodfor simple monitoring of a product syn-thesis, both in the laboratory and at-line, and for on-/in-line measurementsduring product manufacture. Near-in-frared gauges for cross-web scanning offilms for monitoring web-based coat-weight and determining moisture levelsduring product manufacture are alsowidely used.41 Both mid- and near-in-frared spectroscopy also find uses incompositional monitoring, productquality assurance, and raw material vali-dation. Mid-infrared spectroscopy mayalso be used to sense and quantifyvented vapor and off-gas emissions,both within the working environmentand remotely.

CLOSING REMARKS,ADVANCES, AND PROSPECT

The sensitivity, specificity, and diver-sity of applications of mid-infraredspectroscopy will ensure that it remainsa popular and prominent analytical

technique within the coatings industry.Undoubtedly, there will be increaseduse of both near- and mid-infrared tech-nology in process analytical measure-ments, and one has yet to see the poten-tial of imaging applied to problems ofcoating uniformity and heterogeneity.Also, novel techniques, such as mid-in-frared photothermal microspectroscopy(PTMS), are being developed and evalu-ated.42 PTMS, when combined withother scanning probe microscopy tech-niques, should allow for the nonde-structive localized mapping of thetopology and chemical and thermalcharacteristics of a coating at high lat-eral spatial resolution.

References(1) Infrared Spectroscopy Atlas for the Coatings

Industry, Federation of Societies forCoatings Technology, Philadelphia,1980.

(2) Infrared Spectroscopy Atlas for the CoatingsIndustry, 4th Ed., Federation of Societiesfor Coatings Technology, Philadelphia,1991.

(3) Chalmers, J.M., “Infrared Spectroscopyin Analysis of Polymers and Rubbers,”in Encyclopedia of Analytical Chemistry,Vol. 9, pp. 7702-7759, Meyers, R.A.(Ed.), John Wiley & Sons Ltd.,Chichester, 2000.

(4) Chalmers, J.M. and Everall, N.J.,“Qualitative and Quantitative Analysisof Polymers and Rubbers by VibrationalSpectroscopy,” in Handbook ofVibrational Spectroscopy, Vol. 4, pp. 2390-

2418, Chalmers, J.M. and Griffiths, P.R.(Eds.), John Wiley & Sons Ltd.,Chichester, 2002.

(5) Pennington, B.D., “InfraredSpectroscopy in Analysis of PolymerStructure – Property Relationships,” inEncyclopedia of Analytical Chemistry, Vol.9, pp. 7675-7702, Meyers, R.A. (Ed.),John Wiley & Sons Ltd., Chichester,2000.

(6) Koenig, J.L., Spectroscopy of Polymers, 2ndEd., American Chemical Society,Washington, D.C., 1999.

(7) Fahrenfort, J., Spectrochim. Acta, 17, 698(1961).

(8) Fahrenfort, J. and Visser, W.M.,Spectrochim. Acta, 18, 1103 (1962).

(9) Harrick, N.J., J. Phys. Chem., 64, 1110(1960).

(10) Harrick, N.J., Phys. Rev., 125, 1165(1962).

(11) Harrick, N.J., Ann. New York Acad. Sci.,101, 928 (1963).

(12) Greenler, R.G., J. Chem. Phys., 44, 310(1965).

(13) Umemura, J., “Reflection-AbsorptionSpectroscopy of Thin Films on MetallicSubstrates,” in Handbook of VibrationalSpectroscopy, Vol. 2, pp. 982-998,Chalmers, J.M. and Griffiths, P.R. (Eds.),John Wiley & Sons Ltd., Chichester,2002.

(14) Almeida, E., Balmayore, M., and Santos,T., Prog. Org. Coat., 44, 233-242 (2002).

(15) Tady, P.F. and Newnham, D.A.,Spectroscopy Europe, 16 (5), 22-25(2004).

(16) Fitzgerald, A.J., Cole, B.E., and Taday,P.F., J. Pharm. Sci., 94 (1), 177-183(2005).

(17) Griffiths, P.R. and de Haseth, J.A.,Fourier Transform Infrared Spectrometry,John Wiley & Sons, Inc., New York,1986.

(18) Chalmers, J.M. and Dent, G., IndustrialAnalysis with Vibrational Spectroscopy,Royal Society of Chemistry, Cambridge(1997).

(19) Mirabella, F.M., “Principles, Theory andPractice of Internal ReflectionSpectroscopy,” in Handbook ofVibrational Spectroscopy, Vol. 2, pp. 1091-1102, Chalmers, J.M. and Griffiths, P.R.(Eds.), John Wiley & Sons Ltd.,Chichester, 2002.

(20) Urban, M.W., “Infrared and RamanSpectroscopy and Imaging in CoatingsAnalysis,” in Encyclopedia of AnalyticalChemistry, Vol. 2, pp. 1756-1773,Meyers, R.A. (Ed.), John Wiley & SonsLtd., Chichester, 2000.

(21) Tolstoy, V.P., Chernyshova, I.V., andSkryshevsky, V.A., Handbook of InfraredSpectroscopy of Ultrathin Films, JohnWiley & Sons, Inc., New York, 2003.

(22) McClelland, J.F., Jones, R.W., and Bajic,S.J., “Photoacoustic Spectroscopy,” inHandbook of Vibrational Spectroscopy, Vol.2, pp. 1231-1251, Chalmers, J.M. andGriffiths, P.R. (Eds.), John Wiley & SonsLtd., Chichester, 2002.

(23) Chalmers, J.M., Everall N.J., Hewitson,K., and Grady, A., Polym. Mater. Sci.Eng., 82, 402-403 (2000).

Figure 11—Mid-infrared spectra recorded from the surfaces of a self-stratifying epoxy resin-fluorinated resin coating system. Also shownare reference spectra of the pure resins. (©2002. Copyright John Wiley& Sons Ltd. Reproduced with permission from reference 37.)

CT

Analytical Series

July 2005 59www.coatingstech.org

(24) Chalmers, J.M., Everall, N.J., Hewitson,K., Chesters, M.A., Pearson, M., Grady,A., and Ruzicka, B., Analyst, 123, 579-586 (1998).

(25) Willis H.A. and Zichy V.J.I., “TheExamination of Polymer Surfaces byInfrared Spectroscopy,” in PolymerSurfaces, pp. 287-307, Chapter 15,Clark, D.T. and Feast, W.J. (Eds.), JohnWiley & Sons, Chichester, 1978.

(26) Cole, K.C., “Vibrational Spectroscopy ofPolymer Composites,” in Handbook ofVibrational Spectroscopy, Vol. 2, pp. 2456-2482, Chalmers, J.M. and Griffiths, P.R.(Eds.), John Wiley & Sons Ltd.,Chichester, 2002.

(27) Mink, J., “Infrared Emission Spectro-scopy,” in Handbook of VibrationalSpectroscopy, Vol. 2, pp. 1193-1214,Chalmers, J.M. and Griffiths, P.R. (Eds.),John Wiley & Sons Ltd., Chichester,2002.

(28) Studer, K., Decker, C., Beck, E., andSchalwm, R., Eur. Polym. J., 41, 147-167(2005).

(29) Kip, B.J., Berghmans, T., Palmen, P., vander Pol, A., Huys, M., Hartwig, H.,Scheepers, M., and Wienke, D., Vib.Spectrosc., 24, 75-92 (2000).

(30) Rossini, P., Colpo, P., Ceccone, G.,Jandt, K.D., and Rossi, F., Mater. Sci.Eng. C, 23, 353-358 (2003).

(31) Johnson B.W. and McIntyre, R., Prog.Org. Coat., 27, 95-106 (1996).

(32) Masson, F., Decker, C., Jaworek, T., andSchwalm R., Prog. Org. Coat., 39, 115-126 (2000).

(33) Decker, C., Masson, F., and Schwalm,R., Polym. Degrad. Stabil., 83, 309-320(2004).

(34) Yang, X.F., Vang, C., Tallman, D.E.,Bierwagen, G.P., Croll, S.G., and Rohlik,S., Polym. Degrad. Stabil., 74, 341-351(2001).

(35) Keene, L.T., Halada, G.P., and Clayton,C.R., Prog. Org. Coat., in press (2005).

(36) Keene, L.T., Vasquez, M.J., Clayton, C.R.,and Halada, H., Prog. Org. Coat., inpress (2005).

(37) Carr, C., “Vibrational Spectroscopy inthe Paint Industry,” in Handbook ofVibrational Spectroscopy, Vol. 4, pp. 2935-2951, Chalmers, J.M. and Griffiths, P.R.(Eds.), John Wiley & Sons Ltd.,Chichester, 2002.

(38) Urban, M.W., Prog. Org. Coat., 32, 215-229 (1997).

(39) Shah, K., Zhu, Y., Iroh, O., andPopoola, O., Surf. Eng., 17(5), 405-412(2001).

(40) Manso, M., Langlet, M., and Martínez-Duart, J.M., Mat. Sci. Eng., C, 23, 447-450 (2003).

(41) NDC Infrared Engineering, Maldon,Essex, U.K.

(42) Hammiche, A., Bozec, L., German, M.J.,Chalmers, J.M., Everall, N.J., Poulter, G.,Reading, M., Grandy, D.B., Martin, F.L.,and Pollock, H.M., Spectroscopy, 19 (2),20-41 (2004).