measurement and effects of paper coating structure

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Measurement and Effects of Paper Coating Structure R.W. Wygant, Ph.D. Coated Paper Analysis Work Group Leader ECC International Technology Center Sandersville, Georgia 31082 U.S.A. *Introduction The paper industry is continuously improving coating quality and printability to meet higher consumer standards. One key to improving coating quality is to empirically characterize coating structure to determine the factors influencing quality. The technologies used to characterize paper coating structure are evolving rapidly. These technologies are discussed here in the context of measurement of the physical and optical properties of coating surface and bulk structure and the influences of these properties on various end use performance attributes. The premise of this discussion is that techniques discussed are applicable to coated papers, and not necessarily to paper coating formulations or basestocks prior to the coating operation. This is a testing situation frequently faced by the paper industry and its suppliers. The goal of this chapter is to briefly familiarize the reader with the wide variety of testing techniques available for characterizing paper coating structures and the influences these structures may have on the performance of papers. An effort has been made to provide a sizable bibliography so that the interested reader may easily pursue more in-depth discussions of these techniques. Some of the references are primary references for particular testing techniques while others show interesting applications of those techniques. *Surface Structure **Topographical Mapping There are many ways to acquire topographical maps of paper surfaces. The most common involve scanning a surface with some type of sensing mechanism. Different types of sensors include diamond styli, a variety of optical sensors, confocal microscopes and atomic force microscopes. Each of these types of sensors has particular advantages and disadvantages. A different type of topographical measurement that has recently been applied to coated paper surfaces involves optical interferometry. Rather than scanning a probe over the surface, this technique images a relatively large area with coherent light. Through lightwave interference phenomena, this technique is able to map the topography of the entire area. Yet another technique is stereoscopy, employing optical or electron microscope images made at different viewing angles. /1--9/ ***Stylus profilometry. Stylus profilometry involves moving a stylus across a surface while measuring the vertical

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Page 1: Measurement and Effects of Paper Coating Structure

Measurement and Effects of Paper Coating Structure

R.W. Wygant, Ph.D.Coated Paper Analysis Work Group LeaderECC International Technology CenterSandersville, Georgia 31082U.S.A.

*Introduction

The paper industry is continuously improving coating quality and printability to meet higherconsumer standards. One key to improving coating quality is to empirically characterize coatingstructure to determine the factors influencing quality. The technologies used to characterizepaper coating structure are evolving rapidly. These technologies are discussed here in the contextof measurement of the physical and optical properties of coating surface and bulk structure andthe influences of these properties on various end use performance attributes. The premise of thisdiscussion is that techniques discussed are applicable to coated papers, and not necessarily topaper coating formulations or basestocks prior to the coating operation. This is a testing situationfrequently faced by the paper industry and its suppliers.

The goal of this chapter is to briefly familiarize the reader with the wide variety of testingtechniques available for characterizing paper coating structures and the influences thesestructures may have on the performance of papers. An effort has been made to provide a sizablebibliography so that the interested reader may easily pursue more in-depth discussions of thesetechniques. Some of the references are primary references for particular testing techniques whileothers show interesting applications of those techniques.

*Surface Structure

**Topographical Mapping

There are many ways to acquire topographical maps of paper surfaces. The most commoninvolve scanning a surface with some type of sensing mechanism. Different types of sensorsinclude diamond styli, a variety of optical sensors, confocal microscopes and atomic forcemicroscopes. Each of these types of sensors has particular advantages and disadvantages. Adifferent type of topographical measurement that has recently been applied to coated papersurfaces involves optical interferometry. Rather than scanning a probe over the surface, thistechnique images a relatively large area with coherent light. Through lightwave interferencephenomena, this technique is able to map the topography of the entire area. Yet anothertechnique is stereoscopy, employing optical or electron microscope images made at differentviewing angles. /1--9/

***Stylus profilometry.

Stylus profilometry involves moving a stylus across a surface while measuring the vertical

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deflection of the stylus. This technique can provide excellent vertical resolution, on the order of ananometer. However, the lateral resolution is typically on the order of 10 microns for paperapplications. This is because of the need to prevent damage to the paper surface. With a springloaded finite force on the stylus tip, as the stylus tip radius decreases, the pressure under the tipincreases. Most styli are manufactured with an included angle of 90 degrees, so that themaximum slope that can be measured is 45 degrees. There is also a finite physical inertia of thestylus mechanism. This results in the need to allow the stylus to come to an equilibrium afterlateral movement before a vertical deflection measurement can be reliably performed. If one canlive with relatively slow scanning speeds, this technique can provide very good topographicalmaps of fiber-sized and larger features on paper. Figure 1 shows a false color height map of a 1mm2 topographical image of a double coated SBS board acquired using an ECC Paperscapediamond stylus profilometer. Although this board has been double coated, fiber affects ontopography are clearly visible.

<Figure 1. Diamond Stylus Profilometer 1 mm2 Topographical Image of Double Coated SBSBoard.>

***Optical profilometry.

Optical profilometry is similar to stylus profilometry in that a sensing head is scanned across asurface. While vertical resolution is similar to that of stylus profilometry, lateral resolution,limited by the wave nature of light, can be on the order of one micron. There is no physicalcontact between the sensor and the surface, so there is no possibility of damage to the specimen.This allows more rapid scanning of surfaces. There are several different types of opticalprofilometer sensors. Most of these do have limitations on the maximum slope angle that can bemeasured because of design geometries. At too high a slope angle, the light reflected from thesurface will fall outside of the acceptance aperture of the sensing head. In general, opticalprofilometers provide faster scanning and higher resolution than stylus profilometers. However,they still lack sufficient lateral resolution to image paper coating pigments.

***Confocal microscopy.

Confocal microscopy provides a special case of optical profilometry wherein the theoreticallateral resolution limit is approximately the wavelength over square root two. This is achievedby imaging a pinhole onto the surface of interest, and re-imaging that spot onto a detectoraperture. While this technique allows the resolution of coarser paper coating pigments, manypigments are too fine to be resolved or imaged. A significant drawback of these devices is cost.The imaging technique involved results in low optical throughput, so that high intensity lightsources are required. These are typically argon or xenon ion lasers, which alone can be moreexpensive than other profilometers. In addition, the optical systems required are of highcomplexity and expense. Two particular advantages are the ability to do both fluorescenceimaging and volume imaging. If a fluorescent ink is used, for example, high resolution maps ofthe lay of the ink can be obtained. The ability to do volume imaging is unique and can oftremendous benefit in some applications. However, the high optical scatter designed into mostpaper coatings makes this technique less than ideal for volume imaging of paper coatings.

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***Scanning probe microscopy.

Scanning probe microscopy offers the highest resolution available for studying paper coatingsurfaces, on the order of a nanometer. This technique can provide excellent imagery of even thefinest paper coating pigments, inks, binders and polymeric additives. Scanning probemicroscopy is a relatively new field in which technical advances and new probe designs areappearing on a frequent basis. Scanning tunneling and atomic force microscopy (AFM) are thebest known examples of this instrument type. Newer modes of operation of atomic forcemicroscopes can provide lateral force (friction) and mechanical property (elasticity) maps. Nearfield scanning optical microscopy is becoming more widely known. Other probe types in variousstages of development include magnetic field sensors and chemical sensors for a variety ofdifferent species. The common theme among all of these instruments is exceptionally highspatial resolution obtained with piezoelectric scan transducers. These devices provide spatialmovements accurate to the order of an angstrom. The current technological drive is for thedevelopment of micro-sensors to complement this high resolution spatial positioning ability.Figure 2 shows an illuminated view of a 10 mm square (100 mm2) AFM topographical imageacquired from the same double coated SBS board sample shown in Figure 1. However, in thisfigure the visible detail is related entirely to the coating formulation. Kaolin platelets, needleshaped PCC and in some places amorphous blobs of latex binder are the dominant features.Figures 1 and 2 illustrate the importance of understanding surface topography over a broad rangeof scales.

<Figure 2. Atomic Force Microscope 100 mm2 Topographical Image of Double Coated SBSBoard.>

***Optical Interferometry

Optical interferometry for topographical mapping is a technique that has been applied to opticallysmooth surfaces for many years. Only since the late 1980's has this technique been applied tooptically rough surfaces, including coated paper. This technique is noncontact, can capture datafor an entire area at one time and can be used for dynamic deformation analysis. Resolution canbe easily varied, from the lower limit of around the wavelength of light to an upper limit definedby optical system apertures, which can be ten centimeters or larger.

***Stereoscopy

Stereoscopy implies the use of stereo, or binocular, images to derive height information. Eitheroptical or electron microscope images can be used. Thus, the range of resolution is quite wide,from perhaps several tens of nanometers up to centimeters. This technique does requiresignificant computational resources, however these are now commonly available. Modernanalysis algorithms have significantly improved both the speed and accuracy of this technique,making it a much more viable option then it was in past decades.

**Air Flow Roughness

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There are several instruments available that can provide average surface roughness measurementsusing air flow techniques. Examples of these include the Parker Print-Surf, Bekk, Bendtson andSheffield devices. Newer brands employ the same measurement geometries with significantlygreater ease of use achieved using modern electronics. These devices typically measure the airflow between the paper surface and a metal edge pressed against the surface. The geometry,applied pressures and backing material vary between devices and even between procedures onthe same device. While some of these devices can be calibrated to provide measurements inRMS micron roughness, for example, the size of features they measure is dependent upon thedevice geometry. There is also the possibility that this type of measurement can be influenced bythe porosity of the coating layer itself, with air flowing inside the coating layer as well asbetween the coating and the measurement head. The major advantage of these types ofmeasurements is that they can be performed very quickly, making them ideal for productionenvironments. /10--13/

**Coefficient of Friction

The coefficient of friction of a paper surface can be affected by both the topography andchemistry of the surface. Topography can affect friction on both microscopic and macroscopicscales. Stick-slip phenomena can occur in relation to features varying in size from formation(mm -- cm) to coating pigment organization (< micron). Traditional macroscopic techniques forfriction measurement include inclined plane for static friction and level plane for static anddynamic friction. New atomic force microscopy techniques are capable of measuring frictionalforces on a nanometer scale. For most paper surfaces, it might be safe to assume that an increaseor decrease in friction may be due to an increase or decrease in roughness. However, for verysmooth surfaces friction may increase as roughness decreases due to increasing contact area. /14--15/

**Optical Imagery

Optical imagery can be employed for studying paper surfaces on scales ranging from themacroscopic down to the order of a micron. Macrophotography can be employed to measure oflatex binder migration, for example. A sample must be illuminated with the appropriate ultra-violet wavelength to be absorbed by latex carbon double bonds. The subsequent decrease inlocal UV reflectivity can be photographed with a suitable panchromatic film. The uniformity ofthe latex concentration at the coating surface can then be found from such images. Quantitativedetermination of latex concentrations requires careful calibration with calibrants of knownconcentration.

Fluorescence microscopy can be performed using a suitably equipped microscope. Mercury arclamps and long-wavelength pass mirrors can be used to direct the excitation light to the samplewhile preventing that wavelength from reaching the detector. This technique is useful forgauging the depth of penetration of fluorescently labeled fluids into paper coating layers. Thiscan be done using physical cross sections of embedded paper samples. Embedding media mustbe carefully chosen to minimize dissolution of the fluorephore into the media. Examples of

Page 5: Measurement and Effects of Paper Coating Structure

fluids that can be studied in this way include inks, varnishes and fountain solutions. /16--19/

**Contact Angle Analysis

***Fluid droplet. /20,21/

Fluid droplets can be imaged as they impinge upon solid surfaces. Various recording techniquescan be employed, including high framing rate photography and video recording, directdigitization of a video signal or even realtime digital analysis. High speed movies can beanalyzed by hand or automatically analyzed by computer. Different types of analysis have beenemployed, from measuring the contact angle and volume of droplets deposited with near zeromomenta, to measurements of the maximum spreading of droplets impinging on surfaces withwell-controlled momenta. The particular advantage of this type of measurement is that thedynamic interactions (wetting, imbibition, elastic deformation) between fluids and surfaces canbe characterized on time scales relevant to industrial processes such as printing or paper coating.

***Immersion and extraction.

In this technique, a solid sample is quasi-statically immersed then extracted from a fluid ofinterest. The sample is typically hung from a transducer that records the forces acting on thesample. These forces include gravitational and buoyant forces, as well as the attractive orrepulsive forces between the solid sample and the fluid. For porous samples, the immersion andextraction cycle must be repeated until a stable hysteresis curve is obtained. This technique canprovide data on attractive or repulsive forces between fluids and solids, even for porous samples.

**Optical Properties

***TAPPI Gloss.

The TAPPI glossmeter is an instrument universally used in North America. Similarinstrumentation is used globally by the paper industry, both in laboratories and for online processcontrol. The measurement principle involves shining a near-collimated light beam on a surfaceand measuring the intensity of light reflected into a specific aperture.

Near grazing incidence angles (75° from the surface normal) are used for most types of paper.The 75° TAPPI geometry is used with an 11.4° degree acceptance aperture for detection of thegloss signal. It is common to use more normally incident light (20° from the normal) for printedpaper or very smooth paper. The TAPPI 20° uses a 5° acceptance aperture. A grazing incidencelight beam will spread less than a beam with incidence near the normal. This fact, combinedwith the different detector apertures for the two TAPPI instruments, makes the 75° instrumentsuitable for relatively rough surfaces and the 20° instrument suitable for much smoother surfaces.

The choice of incidence angle and detector aperture should be chosen so that the specularlyreflected beam is only partially captured by the measurement aperture. Surface roughnessvariations, leading to variations in the width of the specularly reflected beam, will then change

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the intensity of light falling into the sensing aperture. /22--25/

***Goniophotometry.

In goniophotometry, the distribution of light intensity reflected from a surface is measured withhigh angular resolution. The width of a specularly reflected light beam can thus be measuredwith high accuracy. The width of the reflected beam is calculated either as a full width at halfmaximum, for example, or as the standard deviation of the angular distribution of reflectedoptical intensity, which is approximately Gaussian. This angular distribution is caused by thedeviation of microscopic surface facets from the macroscopic plane of the surface. An exampleof such a facet is the face of a kaolin platelet at the surface of a paper coating layer. These facetsare too small to be resolved with the unaided eye, but are responsible for the apparent glossinessof a surface. The lower the width of the goniophotometer curves the glossier the appearance ofthe surface. Figure 3 illustrates the facet angle model, showing how the Gaussian reflection peakarises from the Gaussian distribution of facet angles. Figure 4 shows how this relates to thevisual perception of the sharpness of gloss.

<Figure 3. Facet Angle Model Explanation of Goniophotometer Peak Formation.>

<Figure 4. Visual Perception of Goniophotometer Peak Width.>

This measurement differs from TAPPI standard gloss measurements in that the angles resolvedare closer to those resolved by the human eye (< 1°) and the spot size at a 75° incidence angle isapproximately 1 x 3 mm (dependent upon the light source used). This type of device may also beused to measure the effective index of refraction of a surface. This is done by finding the ratio ofintensities for light polarized parallel and perpendicular to the plane of incidence. The index ofrefraction is then found using the Fresnel equations. For coatings with equal solid phase indicesof refraction, differences in the effective indices of refraction are indicative of differences in thevoid volume at the coating surface.

Although goniophotometry can be used to measure the effective index of refraction of a surface,peak width measurements are independent of the refractive index. This is in contrast to glossmeasurements. Gloss measurements of printed surfaces are strongly influenced by the high indexof refraction of inks and are thus almost always greater in value than measurements done ofunprinted surfaces. However, goniophotometric measurements show that printed surfaces arefrequently rougher than unprinted surfaces, and this increased roughness can be seen./26--31/

***Specular reflectance uniformity.

Several different strategies have been employed for the measurement of specular reflectance, orgloss, uniformity. The Tobias Mottle Tester was originally designed to measure print densitymottle using a drum scanner and densitometer sensor head. Gloss mottle can be measured withthis device by replacing the densitometer head with a specular reflectance measurement head.

Another technique for imaging gloss mottle involves illuminating a sample with a large area

Page 7: Measurement and Effects of Paper Coating Structure

collimated light beam and placing an imaging device in the specular reflection path. Thistechnique can yield high spatial resolution. However, there can be spatial variation in the glossangles measured.

Other proprietary devices are based upon x-y sample scanning and tightly focused light beams.These devices yield high spatial and intensity resolution with optical geometries that are positioninvariant. However, this type of measurement involves complex optical systems and is relativelyslow.

Measurements of visual sensitivity to print density mottle have indicated that the smallest featuresizes that can be perceived are on the order of half a millimeter. However, due to the muchhigher contrast, it is probable that gloss mottle features an order of magnitude smaller can bevisually perceived, on the order of 50 microns. /32--39/

*Bulk Structure

**Mercury Porosimetry

Mercury porosimetry is one of the most widely used techniques for measuring pore sizedistributions of paper coatings. The measurement principle in mercury porosimetry involvesimmersing a sample in mercury and measuring the volume of mercury that intrudes the sample asthe pressure is increased. Mercury does not wet the surfaces of paper components, so all voidintrusion is forced by the applied pressure. At any particular pressure, there is a minimum voiddimension that will be intruded. Thus, it is possible to determine the distribution of void sizes ina sample by increasing the pressure in discrete steps.

Particular problems with mercury porosimetry are system and sample elasticity and thepossibility of narrow necked, or bottle shaped, voids. System elasticity can, in principle, bedetermined and corrected. However, this correction is difficult and rare in practice. Sampleelasticity will vary, but can be partially corrected for on theoretical bases. It is generally notpossible to identify narrow necked voids. Such voids will not be intruded until the appliedpressure forces the mercury through the neck. The analysis will then erroneously indicate thatthere is a void volume that has a minimum dimension equal to the neck minimum dimension.

When examining mercury porosimeter data derived from coated paper samples, there is atendency to assume that a calculated mean pore radius for a sample is indicative of acharacteristic pore size within the coating. Unfortunately, the mean pore radius for a coatedpaper sample is typically influenced more by the larger basestock pores and the system andsample elasticities than by the small pores within the coating layer. However, there are typicallyidentifiable peaks in the pore size spectra of coated papers in the neighborhood of 50 to 100nanometer pore radius. The peak position on the pore radius axis and the volume represented bythe peak both appear to vary logically with alterations to samples. /40--42/

Figure 5 two sets of mercury porosimeter pore size spectra. These are only partial spectra,showing only coating pores rather than the entire spectra including basestock porosity. Each set

Page 8: Measurement and Effects of Paper Coating Structure

consists of two separate scans performed on identical paper samples, showing that themeasurement has good reproducibility. The two scan sets are from paper coated with differentcoatings on the same basestock. Although the Fine #1 Kaolin coating has greater total porevolume, the larger pores produced by the Engineered Kaolin create greater optical scatter andthus better opacity and brightness.

<Figure 5. Mercury Porosimeter Coating Pore Spectra Produced Using Different CoatingPigments.>

**Fluid Imbibition

Dynamic fluid imbibition measurements have been made in a few different ways, with the hopeof predicting or explaining print density, print mottle, fountain solution interference andglueability. Capillary suction testing involves filling a capillary tube with a fluid of interest andbringing the end of the capillary into contact with a paper surface. Fluid drawn from the tubeinto the paper is measured against time, and the paper area wetted is also easily measured. Anenhancement of this is to scan the capillary across the paper surface to detect nonuniformity inthe absorption. Care must be taken to insure that the end of the capillary tube has a diameternarrow enough to resolve absorptivity features of interest, for example, on the order of half amillimeter for print mottle. This is because the fluid will tend to form a film between the end ofthe tube and the paper surface, so that lateral resolution is defined by the outer tube diameterrather than the inner diameter. Absorption of impinging fluid droplets can also be measuredusing image sequences from dynamic fluid droplet contact angle analysis equipment. /43--45/

**Air Permeability

There are several different devices used to measure the flow of air through a paper sheet.Examples of these include the Parker Print-Surf, Gurley porometers and the Sheffield device.Low pressure Gurley instruments are generally not suitable for coated papers due to extremelylong measurement times. Air permeability measurements can provide information about therelative porosities of different sheets. If two different coatings on the same basestock havedifferent air permeabilities, it is possibly due to structural differences within the coating layers. Itshould be noted that, due to the low viscosity of air, these measurements are related more to totalpore volume and pore connectivity than to pore size. /46--49/

**Staining

There is a wide variety of staining techniques that can be used to help characterize the structureof paper coatings. Osmium and iodine can be use to stain latex and starch binders, respectively.Microscopic electron or optical imagery can then show the spatial distribution of these coatingcomponents. Other common techniques include Croda red wipe and K&N staining. These arethought to help show the structure and porosity of coatings through the strength of the staining.Coatings that are more open will imbibe greater amounts of the stain, resulting in a greater coloror brightness change. Local variation in the stain density may be predictive of some types ofprint mottle. Varying the length of time that coatings undergo staining can show qualitative

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differences in absorptivity between papers. A major advantage of these types of techniques isthat they can be done with simple equipment, yet yield data in good agreement with morecomplex techniques such as mercury porosimetry. /50,51/

**Brightness, Color and Opacity

The brightness, color and opacity of papers are all affected both globally and locally by coatingstructure. Addition of structuring pigments or chemicals can enhance coating optical scatter,providing higher brightness as well as greater whiteness and opacity. Local coatweight variationscan greatly influence the uniformity of these attributes. There are a number of standardizedtechniques available for measuring these optical properties. Preferences between types ofinstruments vary with both application and geography. /52--60/

**Transmitted Light Imaging

Transmitted light imaging can be performed in a variety of ways. Pinhole source and detectorscanners can be used, as can light tables or suitably equipped microscopes. Both the basestockand the coating contribute significantly to optical absorption and scatter. It is difficult at best toseparate the two influences. Even if an uncoated basestock sample is available forcharacterization, it is probable that the coating operation itself will alter the properties of thebasestock. It is possible to compare different coatings on the same basestock, as long ascoatweight effects are taken into account. /61/

**Coating Burnout Imaging

A technique that is in some ways similar to transmitted light imaging is coating burnout imaging.This technique allows measurement of the lateral variation of coating properties. Using thistechnique with uncalendered sheets gives an indication of coating mass distribution. Comparisonof the burnout brightness distribution of calendered to uncalendered sheets provides an indicationof the porosity distribution of the calendered sheet. /62/

**Transmitted Electron Imaging

An advantage of using electrons (or beta rays) rather than photons in transmission imaging is thatelectrons are more strongly influenced by the coating than the basestock. This is due to thehigher average atomic weight of inorganic pigments in comparison to the predominately carbonbasestock. Heavier atoms have denser electron clouds, and thus interact more strongly withbeamed electrons. Thus, a transmitted electron image will give a more accurate picture ofcoatweight distribution than a transmitted light image. /63/

**Kubelka-Munk Theory

Kubelka-Munk theory can be used to deduce information about relative pigment packingefficiency within coatings. It is sometimes naively assumed that pigment scatter is an intrinsicproperty, and that the scatter of pigment mixtures can be calculated as linear combinations of the

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component pigments. However, this is not so, as scatter depends intricately upon theenvironment in which a pigment particle is immersed. It is not difficult to produce pigmentmixtures that pack either better or worse than the individual components. These will yieldscatter coefficients below or above those predicted by linear combination calculations. Thus, if acoating formulation produces a scatter higher or lower than expected, it may be deduced that thecoating has a different porous structure than its components would have alone. Differences couldbe either in the total pore volume or in the average pore radius. /64/

**Physical Cross Sectioning

Physical cross sectioning provides some of the best information obtainable about the internalstructure of paper coatings. However, it can be highly labor intensive. Typically, a paper sampleis embedded in a hard, penetrating media such as epoxy or acrylic to minimize sampledeformation during microtoming or grinding. The embedding medium should be chosen basedupon the sample type. Thick samples such as boards with multiple heavy coating layers requirethe use of a low viscosity, slow curing epoxy. Samples with components that may diffuse intothe medium require a rapid curing medium such as an acrylic.

Alternatively, it is possible to obtain relatively high quality cross sections using freeze fracturetechniques. This can be done by freezing the specimen with liquid nitrogen, for example, thenfracturing with a knife edge. Such cross sections are typically suitable for electron microscopy.However, the unpolished roughness of such sections may be great enough to exceed the limiteddepth of field of most optical microscopes.

When scanning electron microscopy is used to image a sample, the solid block is usually used. Atransmission electron microscope requires the use of thin sections. An optical microscope can beused with either thin sections or a block. However, if optical fluorescence imaging is employed,a thin section must be used to avoid fluorescence from within the bulk of a sample. Opticalmicroscopes have sufficient resolution to measure coating thickness or the roughness at the topof the coating or at the coating/basestock interface. Optical fluorescence imaging techniques canbe used to gauge the penetration of fluorescent penetrant fluids, and stains can be used tomeasure the distribution of starch, for example. Electron microscopy offers higher resolution sothat multiple coating layers can sometimes be distinguished by differences in pigment particlesize. Elemental mapping may also be carried out, for example to identify the location of osmiumlabeled latex.

Cross sectional examination can reveal many different structural features of paper coatings, forexample, the coating thickness distribution. The shape of this distribution can show differencesin the tendencies of coatings to penetrate the basestock. These can be seen by comparing themaximum depth of penetration and the skew of the distribution. The mean coating thickness,combined with knowledge of the coatweight, yields the bulk of the coating. Coating bulk relatesto optical scatter, and can be influenced by coating formulation, application and finishing. Theroughness of the top of the coating layer can be compared to that of the bottom of the coating(the top of the basestock) to find a smoothing index, which is dependent on the coatingformulation and application as well as the basestock itself. On multiply coated grades, the

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tendency towards delamination between layers can sometimes be compared. /65--68/

Figure 6 shows an optical micrograph of a double coated board cross section. This image wasacquired using reflected light. Note that the two coating layers are easily differentiated due to agross difference in the brightness of the pigments in the two layers. A significant amount ofcoating pigment can also be seen to have penetrated below the top fibers of the basestock.

<Figure 6. Optical Micrograph of a Double Coated Board Cross Section.>

**Neutron Scattering

Neutron scattering can be used to deduce information about the structure within paper coatings.With proper choices of neutron energy and grazing incidence angle, it is possible to determinethe average alignment of clay platelets in a coating layer, for example, as well as the variation inthe alignment. The great disadvantage of this technique is that there are very few facilitiesworldwide in which this type of experiment can be conducted. /69/

**X-ray Diffraction

X-ray diffraction measurements can be used to measure the degree of alignment of kaolinparticles. By slow and careful measurement of the shape of the primary kaolin diffraction peak,an alignment index can be found. However, this technique requires that the detector be heldmotionless for lengthy periods as various angles in order to measure the peak shape withsufficient accuracy. Some X-ray diffractometers insist on continuously scanning the detector andthus are not suitable for this technique. An advantage of this technique over neutron scattering isthat suitable X-ray diffractometers are readily available. /70, 71/

*Effects of Coating Structure

**Brightness, Opacity and Color

Coating structure can affect measurements of sheet brightness, color and opacity in several ways.Brightness meters that employ directional lighting (GE geometry) have greater sensitivity tosurface finish than diffuse (ISO geometry) instruments. Thus, changes in sheet gloss will affectGE brightness readings more than ISO readings.

The primary influence of coating structure on these measurements is through optical scatter.Optical scatter has a linear dependence on coating void volume, that is, if you double the voidvolume (without affecting void size) you can expect the scatter to double. The dependence ofscatter on void size is highly nonlinear. If you double the size of voids, the scatter couldquadruple, for example. This is due to the wave nature of light. There will be an optimum voidsize, on the order of half a wavelength, that will give greater scatter than larger or smaller voids.

**Sheet Gloss

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Sheet gloss is primarily dependent upon the surface structure of coating. This type ofmeasurement can be responsive to a broad bandwidth of feature sizes. This bandwidth caninclude features from the microscopic, on the order of 100 nm or less, all the way up tomacroscopic features approaching the size of the area illuminated on the sheet. Of course, thebandwidth affecting different glossmeter geometries will vary somewhat.

Theoretically, sheet gloss should also be responsive to the effective index of refraction of asurface. It is the index of refraction of a surface that determines the reflectivity of that surface.The index of refraction depends on both void volume and the material present in the coating.However, most pigments and binders have very similar indices of refraction, between 1.5 and1.6. Titanium dioxides do have much higher indices of refraction, 2.5 for anatase and 2.9 forrutile. Increasing levels of TiO2 should increase the index of refraction (if roughness is constant)and reflectivity and thus the gloss of a surface. In the same way, increasing the void volume(with air's n ÷ 1) should decrease the index of refraction and thus the gloss./72/

**Print Gloss

Print gloss and delta gloss are strongly affected by both the surface and the bulk structure ofcoatings. Of course, the strongest influence on print gloss is the sheet gloss. Print gloss is alsoaffected by pore radius and by microscopic surface topography. Interestingly, the fact that sheetgloss and microscopic topography both influence print gloss independently suggests that themicro-topography influences print gloss in a different way than it influences sheet gloss.

The effect of pore radius on print gloss is possibly through both capillary pressure and phaseseparation. Smaller pores have higher capillary pressure and thus greater suction pulling the inkfrom the paper surface. Inks are typically multiphase systems composed of pigments, resins andsolvents. It has been hypothesized that fine pores could pull the solvent from the full ink andthus set ink more rapidly than large pores. It has also been theorized that surface pores are ofgreater importance than bulk pores. However, neither of these ideas seems to have been provenquantitatively.

Sheet gloss is influenced by a broad bandwidth of roughness from the microscopic to themacroscopic. Yet, print and delta gloss are influenced by microscopic topography independentlyof sheet gloss. The reason for this is that the ink layer is capable of hiding a significant amountof microscopic roughness. This mechanism is independent of the influence of microtopographyon sheet gloss./73--77/

**Print Mottle

Print mottle is a general term that describes an uneven appearance of color density or gloss of ahalftone or a solid print area. It is most visible in uniform color areas where the surface fails toabsorb ink evenly due to basestock formation or coat weight variations. Coated boardssometimes show mottling because of rough surface topography. Poor basestock formation canlead to print mottle through nonuniform compression during calendering, resulting in variation incoating layer densification. Sheets that contain binder migration, coat weight variations, inferior

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coating profile and heterogeneous pigment blending can all cause mottle formation on theprinted sheet. Several ink stain test methods work well in predicting print mottle. K&N ink andCroda Red Wipe are frequently used in the mills and paper testing laboratories.

Print mottle can be measured many ways. Popular techniques include programmable scanningdensitometry and digital image analysis. Densitometer heads typically have sensing heads withlateral resolution on the order of a millimeter, which is too large to resolve the finest features towhich the visual system is responsive. These systems can usually be programmed to measure thevariability in many different print areas. Image analysis systems can have pixel sizes on the orderof tens of microns, easily exceeding human visual resolution. General mottle analysis systemstypically assume that input images are supposed to represent single tones, and that any variationis representative of error in the print process. Either solid tones should be used, or halftone dotsshould be unresolved or digitally filtered out. The mean intensity and standard deviation yieldthe contrast of the mottle, which can be calculated as the ratio of the standard deviation to themean. The contrast is probably the single strongest influence on the visual perception of mottle.However, the lateral size of features, and lateral periodic variations in the density also contributeto visual perception. The lateral size of random features can be found by calculating theautocorrelation length along the intensity surface. The worst (most strongly perceived) featuresize is about 1 or 2 millimeters, with the strength of perception dropping off for very small orvery large features. Many organic visual systems (specifically including mammalian vision) arehardwired to respond to periodic features, which can be measured digitally using frequencytransforms, such as the Fourier or wavelet transforms. Frequency domain power spectra shouldbe weighted against a visual response function to determine the visibility of mottle features. Ahalftone print, for example, takes advantage of the averaging functions in the visual system toproduce apparently continuous color variations. Other types of texture analysis, co-occurrencematrices for example, can also be used to predict the strength of perception of print mottle./78--88/

**Offset Halftone Dot Fidelity.

The fidelity of halftone dot reproduction can be influenced by coatweight as well as coatingstructure. LWC coatings, for example, frequently leave partially exposed wood fibers that cancause ink to wick. It should be expected that variation in coating structure will influence dotsize, with more closed, impermeable areas allowing dots to spread more. Variability in dot sizeand shape can make process print colors appear muddy and washed out. Measurement ofhalftone dot size and shape can be done routinely with microscope-based image analysis systems.Full color systems can sometimes analyze overprinted dots using color thresholding techniques.Many dots should be analyzed from multiple sheets of particular paper samples to account forboth large and small scale variations in the papers. There will usually be strong overlaps in thepopulations of dot sizes and shapes among different paper samples. However, if sufficientnumbers of dots are measured the means of these populations can usually be statisticallydifferentiated. /89/

Figure 7 shows an optical micrograph of offset halftone dots from a fleshtone area of a print.Note that with a full color image analysis system it was possible to identify a dot of one color

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even though it overlaid a dot of another color.

<Figure 7. Offset Halftone Dots, with One Dot Digitally Identified.>

**Blister Resistance

Blistering occurs in the drying ovens of heatset offset printing presses. As the paper is heated toset the ink, moisture in the basestock will evaporate. If the vapor can not easily escape throughthe coating layer, internal pressure can build up to the point that the paper literally explodes. It isintuitively obvious that more open coating structures should have greater resistance to blistering.Measurements of air permeability should correlate well to blister resistance./90/

**Glueability and Surface Strength

There are several different glues used in the packaging industry. However, many boardproducers rely upon K&N ink brightness loss as a measure of glueability. Absorption of K&Nink can be related to coating pore size and volume. Use of engineered pigments to open up thecoating structure can enhance K&N ink absorption as well as surface strength as measured bylaboratory picking. However, these types of pigments also have lower surface area than moretraditional pigments. Thus, it is debatable whether the added pick strength and glueability aredue to the more porous structure or the lower binder demand, and thus greater strength, ofcoatings produced with engineered pigments./77/

**Rotogravure Printability

The most common rotogravure printability measure is that of the prevalence of missing dots. Acommon measure is the distance to the twentieth missing dot on an IGT Heliotest print. Thismeasurement is typically performed by a technician, hunched over a workbench and rapidlybecoming nearsighted. More modern methodologies employ digital image analysis withautomatic sample and camera positioning. Dot identification algorithms can be based onthresholding techniques, or can discern the spatial distribution pattern of dots using localintensity minima hunting operators. Advantages of digital systems include machine objectivity,the ability to measure larger sample sets more quickly and ability to find the percentage ofmissing dots as well as the adjacency number. The adjacency number is the average number ofmissing dots in a group, and has been claimed to be more closely related to visual perception ofquality than the percentage of missing dots. Regardless of how missing dots are measured, theyare generally due to poor contact between the paper and the printing roll. Poor contact may beblamed on surface roughness. However, the dynamic, compressed roughness of the paper in theprinting nip may not be similar to the uncompressed roughness. Thus, it is common to measurethe compressibility of rotogravure papers.

Measurements of dot fidelity and print density uniformity can be applied to rotogravure prints aseasily as to offset prints. However, a characteristic of rotogravure printing is the desirability ofthe ink forming a continuous film of varying thickness, rather than offset printing's varyinghalftone dot sizes. Thus a measure of ink film continuity is also useful. A typical measure is the

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ink density at which a film looses its continuity, lower being better./91,92/

Figure 8 shows an IGT laboratory rotogravure print of well separated dots suitable for missingdot counting. Figure 9 shows a Diamond International laboratory roto print of a specific tonaldensity at which the continuous ink film has begun to break up. In some areas of this image thereappears to be a continuous ink film while in other areas discrete dots are visible.

<Figure 8. Laboratory Rotogravure Print of Dots Suitable for Missing Dot Counting.>

<Figure 9. Laboratory Rotogravure Print of Tone Where Continuous Ink Film Breaks Up.>

**Inkjet Printability

The wide variety of physical processes employed in different types of nonimpact printing requirean equally broad range of coating structures. Highly fluid ink jet inks will tend to wick alongexposed fibers. Thus, measures of dot fidelity and edge sharpness along solidly printed areasbecome important. These can quantify the ability of the coating structure to localize the largevolumes of ink typically used in this printing method. Thermal prints, from facsimile machinesfor example, are dependent upon the thermal diffusion properties of coatings, as well as thesmoothness to insure good thermal contact. Color laser printability is also dependent uponthermal properties of coated sheets that may have to withstand multiple toner fusion processes./93,94/

Figure 10 shows an image of inkjet print dots printed on a premium coated inkjet paper. Notethe poor shape of these dots. This is due in part to the lateral motion of the inkjet printhead as itis ejecting dots toward the surface. However, the paper coating can have a significant influenceon the shape and size of inkjet dots.

<Figure 10. Inkjet Print Dots.>

*Conclusions

There are many techniques available for studying paper coating surfaces, structures andprintability. Some have been in use for decades, others are in various stages of development ordeployment. Perhaps the greatest challenge lies in finding the right technique to employ inattacking a particular problem. A primary goal of this paper was to provide a resource forresearchers to be able to identify the appropriate techniques for their problems. Interested readersshould consult the references before making any decisions about the applicability of particularmethods.

*References

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**Bulk Structure

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**Effects of Coating Structure

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