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New Insights into Coating Uniformity and Base Sheet Structures
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.
2009 (English)In: Industrial & Engineering Chemistry Research, ISSN 0888-5885, E-ISSN 1520-5045, Vol. 48, no 23, p. 10472-10478Article 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.

Place, publisher, year, edition, pages
2009. Vol. 48, no 23, p. 10472-10478
Keywords [en]
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: urn:nbn:se:miun:diva-10269DOI: 10.1021/ie900819cISI: 000272039200040Scopus ID: 2-s2.0-73249117590OAI: oai:DiVA.org:miun-10269DiVA, id: diva2:274852
Projects
Coating uniformityAvailable from: 2009-11-02 Created: 2009-11-02 Last updated: 2017-12-12Bibliographically 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. p. 61
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 129
Keywords
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, Tetsu

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