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New Method for Characterizing Paper Coating Structures Using Argon Ion Beam Milling and Field Emission Scanning Electron Microscopy
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. (Fibre-based Material Technology, FSCN)
FPInnovations, Pointe Claire, PQ, Canada.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. (Fibre-based Material Technology)
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.

Place, publisher, year, edition, pages
2011. Vol. 241, no 2, 179-187 p.
Keyword [en]
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: urn:nbn:se:miun:diva-12466DOI: 10.1111/j.1365-2818.2010.03418.xISI: 000286110500009PubMedID: 21118214Scopus ID: 2-s2.0-78651232833OAI: oai:DiVA.org:miun-12466DiVA: diva2:375321
Available from: 2010-12-07 Created: 2010-12-07 Last updated: 2012-06-15Bibliographically 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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