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Quantitative microscopy of coating uniformity
Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Print quality demands for coated papers are steadily growing, and achieving coating uniformity is crucial for high image sharpness, colour fidelity, and print uniformity. Coating uniformity may be divided into two scales: coating thickness uniformity and coating microstructure uniformity, the latter of which includes pigment, pore and binder distributions within the coating layer. This thesis concerns the investigation of both types of coating uniformity by using an approach of quantitative microscopy.First, coating thickness uniformity was analysed by using scanning electron microscope (SEM) images of paper cross sections, and the relationships between local coating thickness variations and the variations of underlying base sheet structures were determined. Special attention was given to the effect of length scales on the coating thickness vs. base sheet structure relationships.The experimental results showed that coating thickness had a strong correlation with surface height (profile) of base sheet at a small length scale. However, at a large length scale, it was mass density of base sheet (formation) that had the strongest correlation with coating thickness. This result explains well the discrepancies found in the literature for the relationship between coating thickness variation and base sheet structure variations. The total variance of coating thickness, however, was dominated by the surface height variation in the small scale, which explained around 50% of the variation. Autocorrelation analyses were further performed for the same data set. The autocorrelation functions showed a close resemblance of the one for a random shot process with a correlation length in the order of fibre width. All these results suggest that coating thickness variations are the result of random deposition of particles with the correlation length determined by the base sheet surface textures, such as fibre width.In order to obtain fundamental understandings of the random deposition processes on a rough surface, such as in paper, a generic particle deposition model was developed, and systematic analyses were performed for the effects of particle size, coat weight (average number of particles), levelling, and system size on coating thickness variation. The results showed that coating thickness variation3grows with coat weight, but beyond a certain coat weight, it reaches a plateau value. A scaling analysis yielded a universal relationship between coating thickness variation and the above mentioned variables. The correlation length of coating thickness was found to be determined by average coat weight and the state of underlying surfaces. For a rough surface at relatively low coat weight, the correlation length was typically in the range of fibre width, as was also observed experimentally.Non-uniformities within the coating layer, such as porosity variations and binder distributions, are investigated by using a newly developed method: field emission scanning electron microscopy (FESEM) in combination with argon ion beam milling technique. The combination of these two techniques produced extremely high quality images with very few artefacts, which are particularly suited for quantitative analyses of coating structures. A new evaluation method was also developed by using marker-controlled watershed segmentation (MCWS) of the secondary electron images (SEI).The high resolution imaging revealed that binder enrichment, a long disputed subject in the area, is present in a thin layer of a 500 nm thickness both at the coating surface and at the base sheet/coating interface. It was also found that the binders almost exclusively fill up the small pores, whereas the larger pores are mainly empty or depleted of binder.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2012. , 61 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 129
Keyword [en]
Coating uniformity, coating microstructure uniformity, base sheet effects, argon ion beam milling, scanning electron microscopy, image analysis, binder distributions, autocorrelation analysis, random deposition process, simulation
National Category
Chemical Engineering Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:miun:diva-16454ISBN: 978-91-87103-21-6OAI: oai:DiVA.org:miun-16454DiVA: diva2:534006
Supervisors
Available from: 2012-06-15 Created: 2012-06-15 Last updated: 2012-10-31Bibliographically approved
List of papers
1. Base Sheet Structures that Control Coating Uniformity: Effects of Length Scale
Open this publication in new window or tab >>Base Sheet Structures that Control Coating Uniformity: Effects of Length Scale
2008 (English)In: TAPPI Advanced Coating Fundamentals Symposium Proceedings, TAPPI Press, 2008, 124-133 p.Conference paper (Refereed)
Abstract [en]

Characterization of the base sheet properties and coating layer properties was performed by using scanning electron microscope (SEM) images of paper cross-sections and image analysis. Frequency analysis was used to study how the base sheet properties affect coating thickness uniformity at different length scales. Samples analysed were Lightweight Coated (LWC) base sheets blade-coated on only one side with coat weights of 12 and 22 g/m2. A number of images were taken in sequence giving a total length of more than 6 mm. The results showed that the surface height variations of the base sheet control coating uniformity in the entire examined length scale, but with different mechanisms. At short wavelengths the coating mechanism was “level coating” where the coating suspension fills small pores (“levels”), whereas at longer wavelengths the coating suspension follows the surface profile and “contour coating” becomes more prevalent. In other words, the level- and contour-coatings represent the coating mechanisms in different length scales. Surface height variations can be explained by base sheet thickness only at short wavelengths, but at all other wavelengths the surface height variations were very much independent of the base sheet structure properties measured.

Place, publisher, year, edition, pages
TAPPI Press, 2008
Keyword
coating, base sheet, SEM, image analysis, frequency analysis
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-6848 (URN)2-s2.0-57649138861 (ScopusID)6047 (Local ID)1595101748 (ISBN)978-159510174-7 (ISBN)6047 (Archive number)6047 (OAI)
Conference
TAPPI Advanced Coating Fundamentals Symposium; Montreal, QC; Canada; 11 June 2008 through 13 June 2008; Code 74236
Projects
Coating uniformity
Available from: 2008-12-02 Created: 2008-11-02 Last updated: 2016-09-23Bibliographically approved
2. New Insights into Coating Uniformity and Base Sheet Structures
Open this publication in new window or tab >>New Insights into Coating Uniformity and Base Sheet Structures
2009 (English)In: Industrial & Engineering Chemistry Research, ISSN 0888-5885, E-ISSN 1520-5045, Vol. 48, no 23, 10472-10478 p.Article in journal (Refereed) Published
Abstract [en]

Base sheet structures, such as surface roughness and mass density distribution (formation), have been known to affect coating uniformity. However, the literature is not necessarily consistent in determining which structure controls coating uniformity. This study employed scanning electron microscopy (SEM) and image analysis, combined with autocorrelation and frequency analyses, to investigate the fundamental mechanisms of coating and to resolve some of the controversies in the literature regarding the base sheet effects. The results showed that coating thickness variation resembles a process of random deposition with leveling. At small length scales (in the size of fiber width), leveling causes a very strong dependence of coating thickness variations on the surface profile of the base sheet, whereas at larger length scales, coating thickness variation diminishes in its intensity by the same leveling effect, but still retains a significant correlation with base sheet structure, particularly formation. Frequency analyses clearly showed that the discrepancies in the results for the base sheet effects in the literature are due to the length scales used in the experiments, that is, the sampling area and the resolution of the measurements.

Keyword
Base sheet; Coating thickness; Coating uniformity; Frequency Analysis; Length scale; Mass densities; Random deposition; SEM; Structure control; Surface profiles
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-10269 (URN)10.1021/ie900819c (DOI)000272039200040 ()2-s2.0-73249117590 (ScopusID)
Projects
Coating uniformity
Available from: 2009-11-02 Created: 2009-11-02 Last updated: 2012-06-15Bibliographically approved
3. New Method for Characterizing Paper Coating Structures Using Argon Ion Beam Milling and Field Emission Scanning Electron Microscopy
Open this publication in new window or tab >>New Method for Characterizing Paper Coating Structures Using Argon Ion Beam Milling and Field Emission Scanning Electron Microscopy
2011 (English)In: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 241, no 2, 179-187 p.Article in journal (Refereed) Published
Abstract [en]

We have developed a new method for characterizing microstructures of paper coating using argon ion beam milling technique and field emission scanning electron microscopy. The combination of these two techniques produces extremely high-quality images with very few artefacts, which are particularly suited for quantitative analyses of coating structures. A new evaluation method has been developed by using marker-controlled watershed segmentation technique of the secondary electron images. The high-quality secondary electron images with well-defined pores makes it possible to use this semi-automatic segmentation method. One advantage of using secondary electron images instead of backscattered electron images is being able to avoid possible overestimation of the porosity because of the signal depth. A comparison was made between the new method and the conventional method using greyscale histogram thresholding of backscattered electron images. The results showed that the conventional method overestimated the pore area by 20% and detected around 5% more pores than the new method. As examples of the application of the new method, we have investigated the distributions of coating binders, and the relationship between local coating porosity and base sheet structures. The technique revealed, for the first time with direct evidence, the long-suspected coating non-uniformity, i.e. binder migration, and the correlation between coating porosity versus base sheet mass density, in a straightforward way.

Keyword
Argon ion beam milling; coating uniformity; field emission scanning electron microscopy; image analysis; marker-controlled watershed segmentation; paper
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:miun:diva-12466 (URN)10.1111/j.1365-2818.2010.03418.x (DOI)000286110500009 ()21118214 (PubMedID)2-s2.0-78651232833 (ScopusID)
Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2012-06-15Bibliographically approved
4. Microstructure Variations in Paper Coating: Direct Observations
Open this publication in new window or tab >>Microstructure Variations in Paper Coating: Direct Observations
2012 (English)In: Industrial & Engineering Chemistry Research, ISSN 0888-5885, E-ISSN 1520-5045, Vol. 51, no 24, 8246-8252 p.Article in journal (Refereed) Published
Abstract [en]

Non-uniformities 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 analyse 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 fibres 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.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2012
Keyword
Porosity variation; Binder distribution; Coating uniformity; Calendering effect; Field emission scanning electron microscopy; Image analysis
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-15988 (URN)10.1021/ie202874z (DOI)000305358600008 ()2-s2.0-84862513976 (ScopusID)
Projects
Coating Uniformity
Available from: 2012-03-08 Created: 2012-03-08 Last updated: 2015-06-23Bibliographically approved
5. Surface Evolution of Pigment Coating
Open this publication in new window or tab >>Surface Evolution of Pigment Coating
(English)Manuscript (preprint) (Other academic)
Abstract [en]

    We studied the surface evolution of coating by using a random deposition model of particles. In order to capture the real coating structure development, we included a volume exclusion effect to represent particle-particle interaction, and a levelling effect to represent surface tension effect. In this study we investigated three cases: (1) random deposition on a flat surface, (2) random deposition on a flat surface with levelling, and (3) random deposition on a rough surface with levelling.

When plotting in logarithmic scale, the roughness initially increased linearly with average number of particles deposited for all three cases but reached saturation after a certain amount of deposited particles. The result resembles a ballistic deposition process where agglomerates are developed over the surface due to lateral growth. Even a flat, uniform surface creates roughness during random deposition of particles.

Autocorrelation analysis showed that the correlation length continues to increase with the number of particles deposited. The aggregated structures were easily seen in the autocorrelation function.

Experimental and simulated data on the rough surface were compared and they were in agreement, confirming that the coating process is essentially a random process with some local correlation in the length scale of a typical fibre width. 

Keyword
Microstructure, Films, Particle Processing, Simulation
National Category
Other Natural Sciences
Identifiers
urn:nbn:se:miun:diva-16255 (URN)
Available from: 2012-05-23 Created: 2012-05-21 Last updated: 2012-12-20Bibliographically approved
6. Coating Microstructures: Binder Distributions
Open this publication in new window or tab >>Coating Microstructures: Binder Distributions
2012 (English)In: 12th TAPPI Advanced Coating Fundamentals Symposium Proceedings, Co-located with the 16th International Coating Science and Technology Symposium, ISCST 2012, Tappi , 2012, 250-257 p.Conference paper, Presentation (Other academic)
Abstract [en]

 Non-uniformities within 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 on coating microstructures are rather limited or sometimes conflicting.We obtained 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 analyse the binder and pore distribution. This technique produces high quality images that allow microstructure characterisation of the coating layer.The binder distribution measurements showed that the binder is almost exclusively filling up the small pores, whereas the larger pores are mainly empty and depleted of binder.

Place, publisher, year, edition, pages
Tappi, 2012
Keyword
binder distribution, image analysis, field emission scanning electron microscopy, cross section polisher, microstructure, coating
National Category
Other Natural Sciences
Identifiers
urn:nbn:se:miun:diva-16257 (URN)2-s2.0-84876057054 (ScopusID)978-159510220-1 (ISBN)
Conference
12th TAPPI Advanced Coating Fundamentals Symposium, Co-located with the 16th International Coating Science and Technology Symposium, ISCST 2012; Atlanta, GA; United States; 10 September 2012 through 12 September 2012; Code 96506
Available from: 2012-05-23 Created: 2012-05-21 Last updated: 2013-05-07Bibliographically approved

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