Microstructure Variations in Paper Coating: Direct ... - ACS Publications

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Microstructure Variations in Paper Coating: Direct Observations Christina Dahlström* and Tetsu Uesaka Department of Natural Sciences, Engineering and Mathematics, Fibre Science and Communication Network, Mid Sweden University, Holmgatan 10, 851 70 Sundsvall ABSTRACT: Nonuniformities in the coating layer, such as porosity variations and binder distributions, are known to affect print uniformity and barrier properties. However, in the literature the results are rather scarce or sometimes conflicting. We acquired high-quality images of coated paper cross sections using field emission scanning electron microscopy in combination with a new argon-ion-beam milling technique to directly observe and analyze the coating microstructures in relation to underlying base sheet structures. The results showed that coating porosity varied with mass density of the underlying base sheet for the relatively bulky clay/GCC coating, whereas for the more compact clay coating, the effect was small. Areas with more fibers in the base sheet were more compressed by calendering, resulting in a decreased coating porosity. A unique binder enriched layer of less than 500 nm thickness was found at the coating surface as well as at the coating/base sheet interface. methods.6,13−17 In these studies, the coating nonuniformity in terms of coating thickness variations has been reasonably well clarified. However, the microstructure variations within the coating are still being debated and speculated because of the lack of appropriate experimental techniques. The coating uniformity plays an equally important role for packaging paper and board where barrier properties are necessary for oil, grease, oxygen, and water vapor.18−20 It is well-known that pinholes in the coating (uncovered area of coated surface) are detrimental to the barrier performance. Since the gas transmission resistance is directly proportional to barrier coating thickness, coating thickness variations is a major obstacle when developing barrier properties with minimum coating thickness. In addition, the coating microstructure variations are another level of coating nonuniformity that may affect the barrier resistance of the coating. In this paper we apply a previously developed technique, using argon ion beam milling and field emission scanning electron microscopy (FESEM), for direct observations of coating microstructures.21 The aim of this study is to try to answer some of the outstanding questions on coating microstructure variations in the area: (1) effects of compression (calendering) on coating porosity variations, (2) coating thickness vs coating porosity relations, (3) binder distributions within coating, and (4) surface structures related to the print mottle.

1. INTRODUCTION Paper is a porous fiber network and is often coated with various materials to enhance surface smoothness, optical properties, print performance, and also barrier properties. The coating is typically composed of mineral particles, mainly clay and calcium carbonate and binder. After coating, the paper is calendered, i.e., compressed, by smooth rolls to further enhance surface characteristics. Because of the original nonuniformities of the base sheet structure, coating uniformity is probably one of the most important characteristics that affects end-use performance. Coating uniformity may be divided into coating thickness uniformity and coating microstructure uniformity, such as pigment, pore, and binder distributions. In printing, the coating nonuniformity is manifested as print mottles, i.e., ink density variations in printed images. Extensive surface observations using scanning electron microscopy (SEM) were made by Xiang et al., Kim-Habermehl et al., and Chinga and Helle to distinguish less porous (closed) and porous (open) areas, and they all found certain correlations with the print mottle.1−3 Microprobes for measuring local liquid absorption have been used by Shen et al. and Xiang et al., and again the correlations with the print mottle were found.4,5 These studies clearly suggest the presence of some types of nonuniform coating structures underneath the coated paper surface. Coating thickness variations were first suspected as a cause of nonuniform ink setting and absorption by many researchers.6−11 Indeed general correlations with the print mottle were observed, and the results were discussed in terms of in-plane variations of the total pore volume in the coating as well as the pore size/porosity variations that may be coupled with coating thickness variations. In our earlier studies, we found that about a half of such coating thickness variations comes from surface height variations of the base sheet, and the rest is mostly of random nature.12 Another important source of variations is the distribution of binder within the coating, particularly binder migration toward the coated paper surface. Binder migration has been long suspected, and indirect experimental evidence have been accumulated by various © 2012 American Chemical Society

2. MATERIALS AND METHODS 2.1. Samples. The paper samples used in this study were coated on one side with 10 g/m2 of coating, using a cylindrical laboratory coater with a blade configuration (CLC-6000, SimuTech International Inc.) at FPInnovations at a speed of 1000 m/min. The coating was made from GCC (Hydrocarb 90, wide particle size distribution and the D50 was 0.70 μm), Received: Revised: Accepted: Published: 8246

December 8, 2011 May 18, 2012 May 29, 2012 June 6, 2012 dx.doi.org/10.1021/ie202874z | Ind. Eng. Chem. Res. 2012, 51, 8246−8252

Industrial & Engineering Chemistry Research

Article

Table 1. Coating Formulation and Calendering Conditions

a

formulation

clay/ground calcium carbonate ratio

latex/starch ratio

binder content (pph)

calendering pressure (kN/m)

calendering temperature (°C)

clay/GCCa GCC clay, mild cal.b clay, hard cal.b

70/30 0/100 100/0 100/0

75/25 100/0 75/25 75/25

9.3 17 19 19

300 300 65 300

130 130 50 130

Both calendered and uncalendered samples were analyzed of this formulation. bMild cal. and hard cal. refers to calendering conditions.

clay (Capim DG, 92 wt % < 2 μm and the D50 was 0.56 μm), starch (Penford Gum), and SBR latex (Dow CP-692 NA) in different formulations. The base paper was made from kraft and groundwood (GWD) pulps. The coated paper was calendered using one pass soft-nip calendering at 100 m/min. The coating formulation and calendering conditions are presented in Table 1. Printing was done at 23 °C and 50% RH with a Prüfbau lab printer (one nip), speed 3.0 m/s, and pressure 3.6 MPa. The ink was a commercial ink for coated paper from Sun Chemicals. The print mottle was evaluated on a solid black area at a 1.2 g/ m2 print density, by a mottle index, Paprimottle developed at FPInnovations. 2.2. Sample Preparation. The five different paper samples were cut in 2 × 2 cm2 pieces. Osmium tetraoxide (OsO4) was used to stain the latex binder. Caution must be taken when working with OsO4 because of its highly toxic nature. The starch is, however, not stained by this procedure. The stained paper samples were gold sputtered on both sides to create a thin gold layer, which makes the paper surfaces easy to detect. The sample preparation for argon ion beam milling was established in our previous work.21 A Hitachi E-3500 ion milling system (Hitachi High-Technologies Corporation, Japan) was used to prepare the cross sections. The milling process parameters are shown in Table 2. The ion beam current

generated using 7 kV accelerating voltage and around a 7−8 mm working distance. To minimize the signal depth, a low accelerating voltage was used. A BEI and a SEI of the same coating area are visualized in Figure 1. Two different magnifications were used: 5000× for porosity and binder measurements and 1000× for characterizing the underlying base sheet structures in the area where the porosity measurements were performed. The image size was 2560 × 1960 pixels and the pixel size was 9.9 nm (magnification 5000×) and 49.6 nm (magnification 1000×). BEI, SEI, and low-magnification images were taken at the position of interest of the paper cross sections. In the literature, the number of images needed at these magnifications to obtain a statistically significant value of porosity was found to be 30.22 This was also confirmed in our studies. Therefore, 30 of the above-mentioned types of images were taken on the obtained cross section, for all 5 different paper samples. The coating layer pore structure is clearly visualized in the SEI, obtained with this new etching technique in combination with FESEM. The porosity determination is based on markercontrolled watershed segmentation (MCWS) of the SEI.21 Before segmentation, image registration of the BEI and SEI were performed to make sure that no displacement occurred between the images. The coating layer surface and the coating/ base sheet interface were manually outlined using ImageJ software (National Institutes of Health). Also, some manual “cleaning” of the SEI was performed in Adobe Photoshop from, for example, striations, dust, and artifacts, if any. The Matlab function regionprops was used to obtain the pore properties, such as pore size, orientation, and aspect ratio. Determination of the binder distribution was done by segmenting the BEI according to greyscale histogram thresholding using Matlab. Before segmentation, some steps of contrast enhancement were necessary to obtain a better segmentation between the binder and the pigments. Also, the bright thin gold layer was subtracted from the image before measurements. One advantage of this cross sectioning technique in combination with image analysis is that point-to-point correlations can be done between the coating structures and the base sheet structures at the same position. For this purpose, the images of the base sheet were cropped to correspond to the same area analyzed in the coating layer, since they were

Table 2. Process Parameters accelerating voltage discharging voltage stage control time

5 kV 3.5 kV 3.75 rpm 8h

was optimized by adjusting the gas flow. The obtained cross sections were around 2 mm in length. To avoid striations, it is important to rock the sample during milling and to choose the milling time long enough. 2.3. Image Acquisition, Processing, and Analysis. Digital images were obtained using a field emission scanning electron microscope, FESEM Hitachi SU-70 (Hitachi HighTechnologies Corporation, Japan). Before image acquisition, the samples were carbon coated (Gatan Inc., CA) to obtain an electrically conducting surface. The backscattered electron image (BEI) and secondary electron image (SEI) were

Figure 1. The left image is a BEI and the right image is a SEI of the same coating cross section area (clay coated sample with hard calendering conditions, see Table 1). The white bar is 5 μm. 8247

dx.doi.org/10.1021/ie202874z | Ind. Eng. Chem. Res. 2012, 51, 8246−8252


Article

obtained with 5× lower magnification than the images of the coating layer. The average “mass length”, as a measure of areal density of the fiber mass, was obtained by measuring the total area of fibers in the binary image divided by the image width. The area density of fiber mass is obtained by multiplying the mass length by fiber density (∼1500 kg/m3). 2.3.1. Surface Characterization by Rolling Ball Method. The coating layer surface was manually outlined and the images were made binary. Before surface measurements, all the binary images were aligned using linear regression lines of the coating layer surface. To avoid the effects of a finite sample length at the edges of the images, a reflection image of the aligned binary image was added at both ends of the aligned binary image, giving a total image length of 75 μm. The rolling ball plugin with a radius 20 μm in ImageJ was used to evaluate the surface in terms of surface pores available for ink transfer in the length scale similar to a fiber width, Figure 2. Routines for rolling ball

Figure 3. Coefficients of variation (CV) for coating porosity for different pigment systems and calendering conditions.

Table 3. Porosity and Roughness Data sample a

clay, mild cal. clay, hard cal.a clay/GCC uncalendered clay/GCC calendered a

has previously been used to define the coating surface profile.23 Finally, the image was cropped to its original size and segmented using greyscale histogram thresholding. 2.3.2. Surface Roughness. The roughness parameter, arithmetic average (Ra), was measured on the aligned binary images. In eq 1, n is the number of measured data points in the surface profile, yi the distance at xi, and ym the average distance from the regression line. 1 n

n

∑ i=1

yi − ym

Ra (μm)

16 16 13 9

0.27 0.17 0.29 0.13

Refers to calendering conditions.

density areas of the base sheet is more compacted than lower mass density areas, resulting in lower coating porosity. Figure 4 indeed suggests a correlation between coating porosity and mass length for the calendered sheets from the clay/GCC formulation. For the uncalendered sheets, there was virtually no correlation. However, this effect was not found for the pure clay system, Figure 5. The reason might be that clay already has a compacted structure, and therefore, the effect of calendering pressure is minimal, as also seen in Figure 5, showing that the calendering conditions did not make a significant change in the average coating porosity. Another potential source of porosity variations is coating thickness variations. In the literature it has been speculated that the regions of thin coating layer take up most of the load during calendering, thus having lower porosity.3,6 On the other hand, it was suggested that the coating consolidation process causes porosity variations: lower coat weight has higher porosity.24 However, the results did not confirm, clearly, either of the trends. Figure 6 show the relationships between coating porosity, as determined for each image of 25 μm length, and the corresponding coating thickness. Although there is an overall trend of the higher porosity in the higher coat thickness areas, the correlations are not strong. One reason might be that the local compression of the coating layer in the calender nip will be affected by the fiber mass distribution in the neighborhood as well. Some interesting features were seen from this direct observation. Figure 7 shows a large horizontal crack for claycoated, particularly hard-calendered sheets. This may be an inherent problem of layered structures, which tend to develop interlayer cracks when nonuniform compression is applied. Another interesting feature is the orientation of clay particles in the thickness-direction, especially in the higher coating thickness areas (see Figure 8). Both features may have significant impacts on barrier properties. 3.2. Binder Distributions within Coating. The spatial distribution of binder, particularly the migration of binder toward the coating surface has been debated over the years. Pöhler et al. and Kenttä et al. used FESEM cross sections to determine binder distribution through the coating layer. The

Figure 2. Image illustrates how the large rolling ball would “touch” the outermost coated surface, image width 25 μm.

Ra =

porosity (%)

(1)

3. RESULTS AND DISCUSSION 3.1. Coating Porosity Variations. It is common to measure porosity of the coating layer, but it is rather rare to determine the variations of coating porosity, for example, in the in-plane direction. The porosity variations are important both for print quality and barrier properties of coated papers. Figure 3 shows the coefficient of variation for coating porosity in different pigment systems and calendering conditions. The variations were determined for the 30 measured images. The coefficient of variation typically varied in the range of 20−30% among the 30 image areas. The clay coating formulation exhibited less porosity variations compared to the mixed pigment system. In this respect it is instructive that, recently, high aspect ratio clays are used for barrier coating.19 Table 3 shows the corresponding porosity and roughness data for these samples. It has long been speculated in the paper coating area that porosity variations can be created by the mass density distribution of the underlying base sheet. When calendering pressure is applied on coated paper, the coating in higher mass 8248



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Figure 4. Relationship between coating porosity and mass length for the calendered (left) and uncalendered (right) clay/GCC sample.

Figure 7. BEI of the clay coating illustrating a large crack, ∼20 μm in length, near the base sheet (white bar is 5 μm).

Figure 5. Relationship between coating porosity and mass length for the clay coated sample, calendered under mild and hard conditions.

coating layer was divided into five layers from the coated surface, each 1 μm thick.22,25 They found no binder migration toward the coating surface. Arai et al. used electron spectroscopy for chemical analysis (ESCA) in combination with razor blade scraping for depth profiling, where every scraping removed 0.5−0.8 μm.26 They found a higher binder content near the coating surface. Laser-induced plasma spectroscopy (LIPS) was used by Häkkänen et al., where each laser pulse ablated approximately 0.5 μm of the coating layer.27 They found that the C/Ca intensity ratio, as regarded as the binder content, was higher in the topmost layer than the second layer. Ozaki et al. used confocal laser scanning microscopy (CLSM) to study binder distribution,6 with the typical resolution of around 0.2−0.7 μm.28 Vyörykkä et al. used confocal Raman microscopy to investigate the binder distribution with a lateral resolution of 2.5 μm and a depth resolution of 4 μm.17 In both cases, no clear evidence of binder migration was found. Atomic

Figure 8. BEI of a thick region in the clay coating that illustrates how the pigment orientation varies.

force microscopy (AFM) was used for detecting binder distributions in coatings applied on a plastic film (copier grade transparency).29 Because of the limited sample area (10

Figure 6. Coating porosity is shown as a function of coating thickness (left image corresponds to the clay coating and the right image to the clay/ GCC coating). 8249



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μm of the coating layer), it was difficult to take statistics and thus to detect any trend. In the literature, surface measurement techniques, such as UV-absorption, ESCA, or X-ray photoelectron spectroscopy (XPS), and energy-dispersive spectroscopy (EDS), were used to detect the amount of binder at the surface.13−16,26 These surface measurements seem to be pointing toward some migration of binders, but the details of the internal distributions are obviously not known. The binders typically have a diameter around 100 nm, which requires ultrahigh resolution for the measurements and thus special demands on sample preparation. The argon ion beam milling technique produces smooth cross sections free from artifacts and, in combination with FESEM, it is adequate for this type of measurement.21 In Figure 9 we can see clear binder enrichment at the coating surface. This was also confirmed by image analysis. Figure 10

nm thick. It is interesting to note that binder enrichment was found both at the surface of the coating and also at the interface with the base sheet. However, the clay coating calendered under mild conditions does not have an enriched layer of binder at the coated surface. It should also be noted that the binder enrichment is only in the outermost areas, corresponding to less than 500 nm. This may explain why some of the measurements were unable to detect this binder distribution. Although the exact mechanism of the binder distribution is still being debated,30 this study has confirmed a special pattern of binder distribution. Figure 11 reveals another interesting feature of the binder distribution. The small pores are almost exclusively filled with

Figure 11. BEI of the coating layer containing the mixed pigment layer (uncalendered). The white bar is 5 μm.

the binders, whereas the large pores are mostly empty, depleted of the binder. One explanation can be that the binder is attracted to smaller pores during the consolidation process by capillary forces. 3.3. Surface Structures Related to Print Mottle. Porosity variations in the coating layer and coating surface are often considered to cause print defects, such as print mottle.1−3,6,24 Therefore, we first measured porosity in the coating layer for each image of 25 μm width, determined its variation in different locations of the sample, and tried to relate it to print mottle. However, the result showed no correlation with mottle. Obviously, the porosity of the entire coating layer may not represent the surface porosity which controls ink transfer and ink penetration. This is supported by the recent hydrodynamic simulations done by Holmvall and co-workers and Dubé and coworkers.31,32 They showed that ink-substrate interactions in the printing nip are limited to only the very surface of the substrate (a few micrometers), and the ink transfer is more controlled by surface pores smaller than typical fiber width. This means that the structure parameter relevant to print mottle may be the surface profile that contains only the smaller surface pores. For this purpose, we measured two kinds of surface roughness parameters for each image, one is the standard Raroughness and the other is the rolling-ball roughness, as previously described, and determined their variations among different images (different locations). Because of the limited size of the SEM images (25 μm), the rolling ball radius was chosen to 20 μm, similar to a fiber radius. The results are

Figure 9. BEI of the clay coated sample (hard calendered), where the black/white line visualizes a 500 nm thin layer of binder enrichment near the surface. White bar is 1 μm.

shows the binder distribution through the coating layer, from the coated surface down to the base sheet. The coating layer is divided into 20 layers of equal thickness in the thicknessdirection. The coating layer thickness is in average about 6 μm, which means that every measured layer is approximately 300

Figure 10. Figure shows how the binder amount is distributed from the coated surface down to the base sheet, divided into 20 layers. 8250



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shown in Figures 12 and 13. Interestingly there was no strong correlation with variance of Ra-roughness, whereas the variance of rolling ball roughness showed a very strong correlation.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +46(0)60 14 88 13. Fax: +46(0)60 14 88 20. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors would like to thank Rafik Allem of FPInnovations for valuable discussions, his assistance in using the FESEM and the cross section polisher, and interpretation of the results. We would also like to thank Xuejun Zou, David Vidal, Sylvie Sauriol, and Sylvie St.-Amour of FPInnovations for providing paper samples prepared under controlled conditions. We also appreciate financial support provided by IMERYS, European Regional Foundation, County Administrative Board of Västernorrland, and Mid Sweden University−FSCN.

Figure 12. Print mottle as a function of the variance of Ra.

(1) Xiang, Y.; Bousfield, D. W.; Coleman, P. S.; Osgood, A. The Cause of Backtrap Mottle: Chemical or Physical? Proceedings of the TAPPI Coating Conference, Atlanta, GA, 2000; pp 45−58. (2) Kim-Habermehl, L.; Pollock, M.; Wittbrodt, E.; Roper, J.; McCoy, J.; Stolarz, J.; Langolf, B.; Rolf, M. Characterization of Coated Paper Surface Morphology and Its Correlation to Print Mottle. Proceedings of the Pan-Pacific and International Printing and Graphic Arts Conference, Québec City, Canada, 1998; pp 71−76. (3) Chinga, G.; Helle, T. Relationship between the Coating Surface Structural Variation and Print Quality. J. Pulp Pap. Sci. 2003, 29, 179− 184. (4) Shen, Y.; Bousfield, D. W.; Van Heiningen, A.; Donigian, D. Linkage between Coating Absorption Uniformity and Print Mottle. J. Pulp Pap. Sci. 2005, 31, 105−108. (5) Xiang, Y.; Bousfield, D. W.; Hassler, J.; Coleman, P.; Osgood, A. Measurement of Local Variation of Ink Tack Dynamics. J. Pulp Pap. Sci. 1999, 25, 326−330. (6) Ozaki, Y.; Bousfield, D. W.; Shaler, S. M. Characterization of Coating Layer Structural and Chemical Uniformity for Samples with Backtrap Mottle. Nord. Pulp Pap. Res. J. 2008, 23, 8−13. (7) Inoue, T.; Matsubayashi, H.; Kline, J. E.; Janes, R. L. Effect of Coating Structure on Ink Absorption Mottle. Proceedings of the International Printing & Graphic Arts Conference, Pittsburgh, PA, 1992; pp 33−40. (8) Allem, R. Characterization of Paper Coatings by Scanning Electron Microscopy and Image Analysis. J. Pulp Pap. Sci. 1998, 24, 329−336. (9) Whalen-Shaw, M.; Eby, T. A Study of Back-Trap Mottle in Coated Papers Using Electron Probe Microanalysis. TAPPI J. 1991, 74, 188−194. (10) Kent, H. J.; Climpson, N. A.; Coggon, L.; Hooper, J. J.; Gane, P. A. C. Novel Techniques for Quantitative Characterization of Coating Structure. TAPPI J. 1986, 69, 78−83. (11) Xiang, Y.; Bousfield, D. W. The Influence of Ink-Coating Interaction on Final Print Density in Multicolor Offset Printing. Proceedings of the TAPPI International Printing & Graphic Arts Conference, Savannah, GA, 2000; pp 299−309. (12) Dahlströ m, C.; Uesaka, T. New Insights into Coating Uniformity and Base Sheet Structures. Ind. Eng. Chem. Res. 2009, 48, 10472−10478. (13) Zang, Y.-H.; Aspler, J. S. The Effect of Surface Binder Content on Print Density and Ink Receptivity of Coated Paper. J. Pulp Pap. Sci. 1998, 24, 141−145. (14) Al-Turaif, H. A.; Bousfield, D. W. The Influence of Pigment Size Distribution and Morphology on Coating Binder Migration. Nord. Pulp Pap. Res. J. 2005, 20, 335−344.

Figure 13. Print mottle as a function of the variance of surface pore length.

Of course, in this study, the number of samples is limited and the rolling ball diameter was not optimized, and therefore, it is difficult to draw any conclusions at this stage. However, it would be worth further studies to re-examine the applicability of this surface parameter.

4. CONCLUSIONS We used the cross section polisher via the argon ion beam milling technique and FESEM for direct observations of coating microstructures. One advantage of this technique is that correlations can be obtained between the coating structures and the base sheet structures point-to-point. The often speculated effect of calendering and the mass density distribution of the underlying base sheet on porosity variations was confirmed for the mixed pigment system in the length scale of 25 μm. Areas with more fibers in the base sheet are more compressed, resulting in a decreased coating porosity. However, this was not detected for the pure clay coatings, probably because the structures are already compacted. Coating thickness variations are another suspected source for porosity variations. However, there were no consistent correlations observed for different coating formulations or calendering conditions. Binder migration has been debated for many years in the literature. The high-resolution imaging revealed that a 500 nm thin layer of binder enrichment is present both at the coating surface and at the base sheet/coating interface. 8251



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(15) Zang, Y.-H.; Du, J.; Du, Y.; Wu, Z.; Shaoling, C.; Yuping, L. The Migration of Styrene Butadien Latex During the Drying of Coating Suspensions: When and How Does Migration of Colloidal Particles Occur? Langmuir 2010, 26, 18331−18339. (16) Engström, G.; Rigdahl, M.; Kline, J.; Ahlroos, J. Binder Distribution and Mass Distribution of the Coating Layer - Cause and Consequence. TAPPI J. 1991, 74, 171−179. (17) Vyörykkä, J.; Vourinen, T.; Bousfield, D. W. Confocal Raman Microscopy: A Non Destructive Method to Analyze Depth Profiles of Coated and Printed Papers. Nord. Pulp Pap. Res. J. 2004, 19, 218−223. (18) Stannett, V. T. Fundamentals of Barrier Properties. Proceedings of the Fundamental Properties of Paper Relating to Its Uses, Cambridge, U.K., 1973; pp 412−427. (19) Meizanis, P. Hyper-Platy Kaolins for Improved Barrier Performance. Proceedings of the PaperCon '09 Conference, St. Louis, MO, 2009. (20) Andersson, C. New Ways to Enhance the Functionality of Paperboard by Surface Treatment - a Review. Packag. Tecknol. Sci. 2008, 21, 339−373. (21) Dahlström, C.; Allem, R.; Uesaka, T. New Method for Characterising Paper Coating Structures Using Argon Ion Beam Milling and Field Emission Scanning Electron Microscopy. J. Microsc. 2011, 241, 179−187. (22) Pöhler, T.; Juvonen, K.; Sneck, A. Coating Layer Microstructure and Location of Binder: Results from SEM Analysis. Proceedings of the TAPPI Advanced Coating Fundamentals Symposium, Turku, Finland, 2006; pp 89−99. (23) Chinga, G.; Helle, T. Structure Characterisation of Pigment Coating Layer on Paper by Scanning Electron Microscopy and Image Analysis. Nord. Pulp Pap. Res. J. 2002, 17, 307−312. (24) Xiang, Y.; Bousfield, D. W. Effect of Coat Weight and Drying Condition on Coating Structure and Ink Setting. Proceedings of the Advanced Coating Fundamentals Symposium, San Diego, CA, 2001; pp 51−61. (25) Kenttä, E.; Pöhler, T.; Juvonen, K. Latex Uniformity in the Coating Layer of Paper. Nord. Pulp Pap. Res. J. 2006, 21, 665−669. (26) Arai, T.; Yamasaki, T.; Suzuki, K.; Ogura, T.; Sakai, Y. The Relationship between Print Mottle and Coating Structure. TAPPI J. 1988, 71, 47−52. (27) Häkkänen, H.; Houni, J.; Kaski, S.; Korppi-Tommola, J. E. I. Analysis of Paper by Laser-Induced Plasma Spectroscopy. Spectrochim. Acta B 2001, 56, 737−742. (28) Preston, J. S. The Surface Analysis of Paper. Proceedings of the 14th Fundamental Research Symposium, Oxford, U.K., 2009; pp 749− 838. (29) Di Risio, S.; Yan, N. Characterizing Coating Layer Z-Directional Binder Distribution in Paper Using Atomic Force Microscopy. Colloids Surf. A 2006, 289, 65−74. (30) Ranger, A. E. Binder Migration During Drying of Pigment Coatings. Paper Technol. 1994, 10, 40−46. (31) Holmvall, M.; Uesaka, T.; Drolet, F.; Lindström, S. B. Transfer of a Microfluid to a Stochastic Fibre Network. J. Fluids Struct. 2011, 27, 937−946. (32) Dubé, M.; Drolet, F.; Daneault, C.; Mangin, P. J. Hydrodynamics of Fluid Transfer. J. Pulp Pap. Sci. 2008, 34, 174−181.

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Microstructure Variations in Paper Coating: Direct ... - ACS Publications

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