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Base Sheet Structures that Control Coating Uniformity: Effects of Length Scale
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.ORCID iD: 0000-0003-3407-7973
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. 124-133 p.
Keyword [en]
coating, base sheet, SEM, image analysis, frequency analysis
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-6848Scopus ID: 2-s2.0-57649138861Local ID: 6047ISBN: 1595101748 (print)ISBN: 978-159510174-7 OAI: oai:DiVA.org:miun-6848DiVA: diva2:113940
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
In thesis
1. Quantitative microscopy of coating uniformity
Open this publication in new window or tab >>Quantitative microscopy of coating uniformity
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
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:nbn:se:miun:diva-16454 (URN)978-91-87103-21-6 (ISBN)
Supervisors
Available from: 2012-06-15 Created: 2012-06-15 Last updated: 2012-10-31Bibliographically approved

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Dahlström, ChristinaUesaka, TetsuNorgren, Magnus
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