Volume 56, Number 9, 2002 APPLIED SPECTROSCOPY 11150003-7028 / 02 / 5609-1115$2.00 / 0q 2002 Society for Applied Spectroscopy

submitted papers

Raman Spectroscopic Analysis of Paper Coatings

PEI HE, SHIVASHANKER BITLA, DOUG BOUSFIELD, and CARL P. TRIPP*Laboratory for Surface Science and Technology (LASST) (P.H., S.B., C.P.T.), and Department of Chemistry, (P.H., C.P.T.), andDepartment of Chemical Engineering (S.B., D.B.), University of Maine, Orono, Maine 04469

An overview of the use of Raman spectroscopy for compositionalmapping of paper coatings is presented. Raman spectroscopy is ableto give the spatial distribution of pigments and binders in coatedpapers containing kaolin, anatase, and styrene butadiene (SB).Moreover, there are subtle differences in the Raman spectra of twoforms of calcite, ground (GCC) and precipitated calcium carbonate(PCC), and these differences can be used to monitor the spatialdistribution of coatings containing mixtures of PCC and GCC. Sur-face compositional mapping is relatively straightforward and isdemonstrated with measurements of SB/CaCO3 ratios in mottledregions in printed coatings and the measurement of the same pig-ment/binder ratio in both thin and thick coated regions. For depthpro� ling, microtome methods are preferred, whereas the data ob-tained using confocal-based methods do not work.

Index Headings: Raman spectroscopy; Paper coatings; Depth pro-� le; Surface mapping.

INTRODUCTION

Paper coatings are used to improve or enhance the op-tical and printing properties of the paper surface. A papercoating is applied on the paper surface as an aqueoussuspension of pigments and binders, and after drainingor evaporation of the aqueous phase, a dry coating struc-ture is formed consisting of a nonuniform mixture of pig-ments, binders, and voids. An additional small percentage(,1% by weight) of the remaining constituents can beconsidered functional additives, whose purpose is to im-part a desired property characteristic to the wet or drystate of the coating. The coating structure has a verystrong in� uence on the paper’s optical, mechanical, and� uid absorption properties,1 and therefore, paper coatingsare often analyzed by their physical or mechanical prop-erties, such as coating color viscosity, coat weight, wateradsorption rate on coating, and coating porosity. How-ever, chemical analysis of paper coatings is equally im-portant because many print quality issues are determinedby ink interactions with the coating components, binder/pigment distribution, and pigment–binder interactions.Given this importance, there is much interest in devel-oping analytical tools that can spatially identify the chem-ical composition of paper coatings.

The earliest analytical techniques involved simple

Received 25 February 2002; accepted 22 May 2002.* Author to whom correspondence should be sent.

physical separation of individual layers of the coatingusing a sharp blade (0.5;0.8 mm) and these continue tobe effective methods. 2–6 Standard chemical methods arethen used to measure the binder concentration at differentdepth levels on the paper coating.4–6 The development orapplication of new analytical techniques in recent yearshas led to signi� cant advances in determining the chem-ical composition in coatings. These techniques can bebroadly divided into either optical or electron beam meth-ods.

High vacuum techniques, such as electron probe microanalysis (EPMA),7,8 X-ray photoelectron spectroscopy(XPS),3,8–10 secondary ion mass spectrometry (SIMS),1,8,9,11

and laser induced plasma spectrometry (LIPS),12–14 havealso been applied to paper coating analysis. The disad-vantages of vacuum-based electron techniques is thatthey require tedious sample preparation, chemical anal-ysis is over large areas, and differentiation between typesof organic binders is dif� cult.3

Optical spectroscopic techniques include methods suchas attenuated total re� ectance (ATR) infrared spectros-copy,15–17 photo acoustic FT-IR spectroscopy,18,19 and ul-traviolet absorption (UV).10,20,21 The optical spectroscopictechniques are typically fast, nondestructive, and easy touse. UV absorption has limited use to the analysis ofunsaturated binder material, such as styrene butadiene la-tex. Infrared techniques are rich in informational contentand can be used to identify binders and pigments. How-ever, the infrared light penetrates deep within the paper,leading to high opacity due to absorption by water andwood � bers. Furthermore, there is poor spatial and depthresolution using either UV or IR spectroscopic tech-niques, although this is changing with the advent of spa-tial imaging ATR methods.22

Until recently, Raman spectroscopy has had limiteduse for the analysis of industrial-based paper coatings be-cause of the presence of a strong � uorescence back-ground with most samples masking the much weaker Ra-man peaks. Fluorescence backgrounds have been sup-pressed with the advent of red diode laser based systemsand Raman spectra with most ‘‘real’’ coatings can nowbe obtained with little, if any, sample preparation. More-over, spatial mapping of the various components is rela-tively straightforward as most pigments and binders inpaper coatings have unique Raman signatures.

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FIG. 1. Raman spectra of coatings from two commercial coated papers(Note: the sloping backgrounds in both spectra are due to � uorescencefrom impurities in the kaolin of the coatings.).

In this paper, we describe several general trends in ma-terial latitude and mapping, both surface and depth, thatare observed when using Raman spectroscopy for com-positional analysis of paper coatings. These generaltrends are derived from recording and examining Ramanspectra collected on hundreds of different coated papersprovided to us by a consortium of companies with inter-est in paper coatings.

EXPERIMENTAL

Raman spectra were collected using a Renishaw Ra-man imaging microscope system 1000 equipped with anautomatic stage enabling software-controlled mapping.The excitation source is an SDL-XC30 diode laser fromSDL Inc, operating at a wavelength of 785 nm and 35mW power output at the sample position. The spot sizewas approximately 2 mm in diameter using a 503/0.75LEICA N PLAN objective, and the typical scan time was30 s.

The coatings used in this study were provided by anumber of different paper companies. The salient featuresneeded for interpretation of the data are provided in the� gure captions or the accompanying text.

For surface mapping, sample preparation consisted ofsimply taping a 1 cm 3 1 cm piece of coated paper to astandard microscope slide. Confocal and microtomemethods were compared for generating depth composi-tional pro� les. Confocal measurements were performedby a software-controlled adjustment of the z-axis of themicroscope stage. For microtomed samples, a simplemethod of immersion in liquid N2, followed by cuttingwith a sharp scalpel knife, was used. The depth pro� lesrecorded using a more laborious microtome procedure of� rst imbedding the paper in a polycarbonate resin fol-lowed by dicing was not signi� cantly different from thoseusing the simpler microtome method.

RESULTS AND DISCUSSION

Material Latitude. One of the challenges for paperanalysis by Raman spectroscopy or any other analyticalmethod is that there is a need to identify the spatial com-position of each component in the coating. Raman is par-ticularly useful in this respect because most pigments andbinders produce unique bands that appear at separate fre-quencies. Kaolin and calcium carbonate are the mostcommonly used pigments but coating may also includesmaller amounts of titanium dioxide (rutile and anatase),talc, and calcium or barium sulfate.23 While each of thesepigments produce sharp and uniquely identi� able Ramanbands, ink-jet-based coatings often contain amorphoussilica, and identi� cation of silica in these coatings re-mains dif� cult because silica is a weak Raman scatterer.

The primary purpose of binder is to hold the pigmentparticles together and to anchor the coating to the surfaceof the paper. The binders fall into two classes, naturaland synthetic products. The natural binders includestarch, such as cornstarch and potato starch, and proteins,such as casein. Synthetic binders include styrene buta-diene latex, acrylic latex, poly(vinyl alcohol), poly(vinylacetate), photographic gelatins, and vinylidene chloridepolymers.23 Typically, the binder level in the paper coat-ing is about 10;20 parts per 100 parts of pigment, and

this varies depending on the binder type and desired � nalpaper coating properties. For the most part, the interestlies in determining the pigment/binder ratio. While eachof the organic binders have identi� able Raman bands thatdo not overlap with pigment bands, our work has focusedon styrene butadiene for the simple reason that 95% ofthe industrial samples we received for analysis used sty-rene butadiene (SB) latex as the binder. This is somewhatfortuitous as this binder produces a strong Raman bandat 1001 cm21 arising from the styrenic component.

Although 99% of the coating is pigment and binder,the remaining 1% of additives in the paper coatings arevery important. The general functions of additives are tomake the basic coating color compatible with the rawstock and the coating equipment and to provide controlof foaming, lubricity, cross-linking, rheology, and waterretention.24 Some examples of additives include poly-phosphate, alkali silicates, and alkali polyacrylates as dis-persants; sodium alginate, carboxymethylcellulose, hy-droxyethylcellulose, and polyarylates for water retention,and rheology modi� ers.24 Our experience with recordingmany different commercial and in-house-formulatedcoatings has shown that identi� cation of the additiveswas below the detection limit of our technique. As a re-sult we focus our work on determining the spatial com-position of the major components in the paper coatings.

Figure 1 shows the Raman spectra of the surface oftwo typical commercial coated samples. In both spectra,there are clearly identi� able bands due to calcium car-bonate, anatase, and styrene butadiene latex. Calcium car-bonate has a characteristic band at 1086 cm21, styrenebutadiene produces a strong band at 1001 cm21, and an-atase has four strong bands at 637, 517, 398, and 144cm21. Typically, a direct correlation between band inten-sity (or integrated intensity) and quantity of material isdif� cult because the intensity of the band will vary dueto geometric effects arising from nonuniformity in papercoating morphology, unknown scattering cross-section,and other less de� ned effects. Rather than measure theabsolute intensity of any one band, the ratio of intensity(or integrated intensity) of bands due to different com-ponents is calculated. Computing a ratio is a more rele-

APPLIED SPECTROSCOPY 1117

FIG. 2. Raman spectra of (A) kaolin and (B) kaolin after photobleach-ing for 30 min.

vant measurement because it is the relative change inspatial composition that is the important unknown vari-able. For example, the coating used to generate Ramanspectrum B in Fig. 1 is of higher gloss than the coatingused to generate spectrum A, and a higher gloss is con-sistent with a coating containing a higher quantity of ti-tania relative to calcium carbonate or styrene butadiene.In spectrum B, the ratio of the intensity of the titaniaband at 144 cm21 to either the intensity of the CaCO3

band at 1086 cm21 or the styrene butadiene strong bandat 1001 cm21 is greater than in Fig. 1A.

If desired, the ratio in band intensity can be convertedto a % wt ratio. In this case, calibration curves are usedto determine the relative amount of each component inthe coating layer. To generate the calibration curves, dif-ferent amounts of pigments and binders are dry blended.After thorough mixing, the blend is deposited on glassslides and Raman spectra are recorded at a 20-mm spotsize. The accuracy of this measurement varies and fortypical CaCO3 /SB coatings, we � nd the accuracy in the% wt ratio is about 65% at the 95% con� dence level.

It is noted that the detection of TiO2, CaCO3, and SBRaman bands are possible despite the presence of a broad� uorescence background in Fig. 1A. This broad � uores-cence is due to adsorbed organics, impurities, or chemicaldispersants on the kaolin and not to kaolin by itself.While the � uorescence hampers the detection of Ramanbands due to kaolin, it is possible to quantitatively mea-sure CaCO3, TiO2, and styrene butadiene intensity ratiosin samples containing kaolin. We have routinely mea-sured these band ratios in coatings containing as muchas 60% kaolin by weight. Thus, the presence of � uores-cence with kaolin-based samples does not prevent quan-ti� cation of the other components of the coating.

Given the prevalence of kaolin in many coatings, therewas a clear need to develop methods to overcome thedif� culty in detecting Raman bands due to kaolin im-posed by the strong � uorescence. Switching to NIR ex-citation at 1.064 mm in FT-Raman based systems wouldaccomplish this task.25–27 With our 785-nm-based system,we � nd that the � uorescence in kaolin-based coatings canbe reduced by photobleaching the sample. This is shownin the inset curve in Fig. 2 where the underlying weakerRaman bands due to kaolin are detected after 30 min ofphotobleaching at maximum laser power. In the coatedpaper, this long-term exposure to maximum laser powerdid not result in a change in intensity of the Raman bandsdue to the SB binder or CaCO3 and TiO2 pigments. Whilethis procedure adds signi� cantly to the scan time (espe-cially when attempting mapping), another simple trickcan be used to detect kaolin in coatings that do not haveanatase as a co-pigment. All kaolin-based coatings con-tain bands due to an intrinsic amount of anatase (i.e.,band at 144 cm21 in Fig. 2) in kaolin pigments. Thisanatase component of the kaolin is not separated fromthe kaolin by the deposition or drying conditions used togenerate paper coatings. In these coatings, the distributionof kaolin can be indirectly monitored by the strong ana-tase band at 144 cm21 without the need for photobleach-ing protocols.

Of the two main pigments, calcium carbonate is by farthe easiest to detect and monitor in coatings. Calciumcarbonate is easily identi� ed by its strong sharp band at

1086 cm21 irrespective of whether it is aragonite or cal-cite in structure. Coatings containing aragonite, calcite,or mixtures of the two can easily be individually identi-� ed by their characteristic rotary modes below 300 cm21.This band appears at 282 cm21 in calcite and at 203 cm21

in aragonite. There are two main types of calcite pig-ments used in paper coatings: ground calcium carbonate(GCC), which is mined from natural deposits and groundto � ne sizes, and precipitated calcium carbonate (PCC),chemically synthesized to desired particle size and mor-phology.23 Coatings may contain mixtures of PCC andGCC, and there are differences in the properties impartedto paper coatings for GCC vs. PCC. Natural GCC pro-duces easier drying of the paper web and shows a lowersize demand, a more closed sheet structure, and higherpaper strength, while PCC-based coatings provide higherlight scattering.23

Our experience in recording spectra of various calciteshas shown that there is a small but notable differencebetween the two in that the Raman bands of PCC aregenerally wider and shift to lower frequencies comparedto those of GCC. We attribute the broadening to the pres-ence of hydrogen bonded water in the PCC samples. Fig-ure 3 shows the Raman spectra for the 282 cm21 bandfor both GCC and PCC, and the peak maxima andFWHM (full width at half-maximum) of various calcitemodes are listed in Table I. A comparison of the peakmaxima and FWHM of the four major Raman bands ofa representative sample of PCC (ALBAGLOS, SpecialtyMinerals) and GCC (OMYACARB 5, OMYA) showsthat all Raman bands of PCC have larger FWHM valuesthan those of GCC. Furthermore, the PCC bands are lo-cated at lower frequencies than the corresponding GCCbands (i.e., peak maxima of PCC and GCC in Fig. 3appear at 279.7 cm21 and 281.9 cm21, respectively). Al-though this represents a subtle difference in the spectraof PCC and GCC we have been able to use this to quan-tify the composition of coatings to 65 % wt PCC/GCCratio prepared using mixtures of GCC and PCC.28

In order to measure PCC/GCC ratio, a calibrationcurve was generated from coatings containing known

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FIG. 3. Raman spectra of the lattice rotary bands of ground calciumcarbonate (GCC) and precipitated calcium carbonate (PCC).

TABLE I. Raman peak maxima and FWHM of GCC and PCC.

GCC

Peak maxima(cm21) FWHM

PCC

Peak maxima(cm21) FWHM

155.9281.9712.7

1086.4

8.112.24.94.6

155.1279.7712.0

1085.7

12.718.88.86.9

FIG. 4. (A) Raman spectra of coatings containing different GCC/PCCmixtures. (B) Calibration curve used to determine the % amount of PCCin a calcite coating mixture. The ordinate is computed by dividing theintensity measured at 269.5 cm21 by the intensity at the peak maxima.

mixtures of GCC and PCC. The spectra recorded and thecalibration curve are shown in Fig. 4. As the percent ratioof PCC to GCC increases, there is a broadening of the282 cm21 band to lower frequency. A calibration curveis generated by normalizing the intensity measured at apoint on the low frequency side (269.5 cm21) to the in-tensity at the peak maxima. These calibration curves havebeen used to show that � ne pigment migration occursduring the drying process.28 In coatings consisting oflarge (;2.3 mm diameter) PCC and small (;0.3 mm di-ameter) GCC, the Raman data showed that there was ahigher-than-expected concentration of PCC at the surface.Because two different sizes of calcite pigments were usedin the study, the data showed that pigment migration isthe result of size differences and not surface chemistrydifferences. This information would be dif� cult to extractby other methods.

Surface Mapping. Inks give rise to strong Ramanbands associated with the chromophore backbone; thus,an obvious application of Raman mapping capabilities isin the measurement of ink density variations (i.e., back-trap mottle) with nonuniformity in pigment/binder distri-bution. There is minimal � uorescence with magenta inksand no � uorescence with yellow inks using our 785-nm-based system. Detection of Raman bands due to yellowand magenta inks on coated papers is relatively easy asboth inks produce strong Raman bands in the 1700–1200cm21 region and these ink bands do not overlap or inter-fere with the detection of Raman bands due to pigmentsor binders. Furthermore, these measurements are rela-tively straightforward because it is easy to visually in-spect print defect regions to be scanned by the Ramanspectrometer. An example of this process is shown in Fig.5 for a printed mottle of pigmented yellow ink on a Sizedpaper with a simpli� ed coating containing only 10 pphSB/CaCO3. In this speci� c example, the coating containsno other pigments or binders. The three indicated areason the coating represent three different levels of ink den-sity, and the accompanying bar graph is a 10-point av-erage of the SB/CaCO3 ratio of each area, showing adirect correlation with the ink density and the SB/CaCO3

ratio.Measurement of the spatial distribution of pigment/

binders at the surface of coated papers can also provide

insight in to the coating process. While it is possible toobtain spatial maps of coatings containing multiple pig-ments, we demonstrate the usefulness of mapping withthe same coating containing only SB/CaCO3, whichshowed a connection between differences in the binder/pigment ratio and mottle (see Fig. 5). The drying rate ofthe coating is thickness dependent, and it has been pos-tulated that this can lead to differences in surface pig-ment/binder ratio between thick- and thinner-coated re-gions.29 The picture of the SB/CaCO3 coated paper in Fig.6 shows a dark central region representative of a thinner-coated region. The question is does this thinner-coatedarea have a different SB/CaCO3 ratio compared to thesurrounding thicker-coated regions.

To investigate this possibility, a total of 99 spectrawere acquired at 11 equally spaced points in a horizontal

APPLIED SPECTROSCOPY 1119

FIG. 5. Bar graph showing the SB/CaCO 3 ratio for print mottle (mottle is created by applying pigmented yellow ink, with Bristow wheel at a rateof 0.3 cm/s, on coated paper where coating contains 10 pph SB with CaCO3). The three regions scanned are indicated in the optical image of thecoated paper.

line 39 equally spaced vertical lines across a 50 3 40mm area. The second column in Fig. 6 shows the 3D and2D contour maps generated from using the CaCO3 1086cm21 Raman peak intensity. There is a ‘‘well’’ in the 3Dmap and a dark central region in the 2D plot indicatinga smaller amount of CaCO3, which is consistent with athinner coating in this region. A similar result is obtainedwhen using the SB band at 1001 cm21 to compute thecontour maps. This is shown in the second column ofplots in Fig. 6. In contrast, the plot of the cellulose Ra-man band at 1120 cm21 shows the opposite trend. There

is a ‘‘hill’’ representing an increase in cellulose contri-bution to the spectrum in the thinner (dark) regions ofthe paper. While the above trends are expected, the moreinteresting data comes from plotting the SB/CaCO3 ratio.This is shown in the last column of Fig. 6. In this case,we do not obtain a ‘‘well’’ or a ‘‘hill’’ at the center ofthe map. A relatively � at pro� le is obtained showing asmall point-by-point nonuniformity in the pigment/binderratio. Thus the Raman data clearly shows that there islittle difference in the surface pigment/binder ratio be-tween thick and thinly coated regions.

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FIG. 6. Surface mapping of SB/CaCO 3 coated paper.

FIG. 7. (A) Microtomed depth pro� le of double-layered coating. (B)Confocal depth pro� le of double-layered coating.

Depth Pro� les. While knowledge of the spatial pig-ment binder ratio at the surface is more important thanthe corresponding depth spatial distribution, there are cas-es (i.e., ink penetration, binder migration) where thisdepth pro� le is desirable. Paper coatings range fromabout 10 to 40 mm in thickness. By far, the most easyand convenient approach to constructing compositionaldepth pro� les would be to use confocal methodologies.In this way, the surface and depth mapping of the papercoating could be accomplished in one task by controlledx-y-z movement of the microscope stage. However, thecoating is nonuniform, containing many scattering com-ponents that even with the use of an oil immersion lensrender confocal measurements dif� cult at best.30,31 In con-trast, microtome methods are practical and useful becausemeasurement across the edge of the paper is recorded atthe same 2-mm spatial resolution over the entire thicknessof the coating.

To illustrate the above points, we have recorded depthpro� les using confocal and microtome methods on a two-layered coating. Both samples consist of a SB latex with90% CaCO3, 10% clay precoat, followed by a SB latex,with 100% CaCO3 topcoat. The sample labeled X had alower solids precoat followed by a standard topcoat.Sample Y used extra latex in a higher coating solid pre-coat followed by a standard topcoat. Figure 7A showsthe results obtained for the microtomed depth pro� le ofthe layered samples. In both coatings, the decrease/in-crease in SB/CaCO3 ratio occurs at the same location,about 4 mm from the surface. This is consistent with theoptical microscope image of the microtomed sample,which shows a clear delineation between the two coatings

APPLIED SPECTROSCOPY 1121

at a point 4 mm below the surface. Moreover, the trendin composition change is also consistent with the startingcompositions. Sample X has a higher latex concentrationin the precoat and thus, the SB/CaCO3 ratio increasesbelow 4 mm. In contrast, sample Y contains lower latexlevels in the precoat, and this results in a decrease in theSB/CaCO3 ratio below the 4 mm point. Furthermore, thedata shows that the undercoating composition has no ef-fect on the composition of the surface coating, as thereis no difference in the surface SB/CaCO3 ratio for bothsamples.

The corresponding depth pro� les measured by the con-focal method are shown in Fig. 7b. In contrast to themicrotomed data, we do not detect any difference occur-ring at the 4 mm depth. The SB/CaCO3 ratio remainsrelatively constant (albeit, at a lower value for sample Ythan sample X) at all depths. The depth values in Fig. 7bare de� ned as the distance moved by the sample stagecloser to the lens. The initial point (depth 5 0 mm) occurswhen the lens is focused on the surface. Clearly, thesedepth values are not valid as the actual slice within thesample from which the Raman scattered light is collectedis unknown. Recent work by Everall31 has showed thatfor a uniform refractive medium, the confocal depth res-olution is severely degraded even at moderate depths andthe apparent position is much shallower than the truepoint in the sample. For complex refractive media asfound in paper coatings, this situation is further compli-cated by the presence of a multitude of refractive anddispersive events. Therefore, the use of the confocalmethod for depth pro� le measurements does not work.Physical sectioning and lateral mapping through a cross-section is the preferred method.

CONCLUSION

Raman spectroscopy can easily detect the CaCO3, an-atase, rutile, styrene butadiene, and other coating com-ponents in paper coatings and can be used to distinguishthe different morphology of CaCO3 pigment, GCC, andPCC. Kaolin-based coatings have a large � uorescencebackground in their Raman spectra that can be minimizedby photobleaching or by monitoring bands due to anataseimpurities in the kaolin.

Surface mapping of the compositional distribution isrelatively straightforward and provides vital informationfor determining print quality/compositional relationships.Raman spectra recorded on microtomed samples can beused to monitor changes in the depth composition, where-as confocal methods are suspect because the depth res-

olution is ill de� ned for the highly dispersive coating.The microtomed method is recommended over the alter-native confocal method as there is a high con� dence inthe position of the laser beam and the spatial resolutionremains constant throughout the pro� le.

ACKNOWLEDGMENTS

This work was supported by the industrial sponsors of the Universityof Maine Paper Surface Science Program.

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