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Modelling of Brightness Decrease of Coated Cartonboard as an Effect on Calendering: Microroughness and Effective Refractive Index Aspects
Karlstad University, Faculty of Technology and Science, Department of Chemical Engineering.
Innventia AB, Stockholm, Sweden.
Karlstad University, Faculty of Technology and Science, Department of Chemical Engineering.ORCID iD: 0000-0001-7368-7227
Mid Sweden University.
2010 (English)In: TAPPI 11th Advanced Coating Fundamentals Symposium, Munich, Germany, 2010: Symposium Proceedings, Norcross, GA, USA: TAPPI Press, 2010, 51-65 p.Conference paper, Published paper (Refereed)
Abstract [en]

Calendering of coated paper leads to a brightness decrease. The mechanism for this is not clear, although it has been discussed in the past. One common explanation is that the porosity of the coating layer decreases and hence scattering. By comparing simulated and measured results this paper shows that modifications of the surface properties account for the brightness decrease of GCC coated substrates with calendering. The effect of a deformable cartonboard substrate is investigated here and compared to a less deformable plastic film substrate. From simulations based on a two-layer Kubelka-Munk model, it is shown that the brightness decrease of the cartonboard due to calendering has a negligible contribution to the brightness decrease of the coated cartonboard. The brightness decrease was similar for coated plastic film and coated cartonboard. The thickness of GCC coated plastic films was not affected by calendering irrespectively of the pigment and latex size distribution. Monte Carlo light scattering simulations, taking into account the measured decrease of surface microroughness and increased effective refractive index, showed that surface modifications accounted for most of the observed brightness decrease of the GCC coated substrate, whereas the bulk scattering and absorption coefficients were not affected by calendering. It is also shown that the scattering coefficient is significantly dependent on the coat weight whereas the physical absorption coefficient is not.

Place, publisher, year, edition, pages
Norcross, GA, USA: TAPPI Press, 2010. 51-65 p.
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-6830ISBN: 1-59510-203-5 (print)OAI: oai:DiVA.org:kau-6830DiVA: diva2:392545
Conference
TAPPI 11th Advanced Coating Fundamentals Symposium, Munich, Germany, 11-13 October 2010
Available from: 2011-01-27 Created: 2011-01-27 Last updated: 2014-10-27Bibliographically approved
In thesis
1. Optics of coated paperboard: Aspects of surface treatment on porous structures
Open this publication in new window or tab >>Optics of coated paperboard: Aspects of surface treatment on porous structures
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Calendering of coated and uncoated paper is widely used to enhance optical properties such as gloss and print quality. The aim of this thesis is to characterize coatings and prints, and to validate models using experimental results from optical measurements of physical samples.

Calendering of coated paper often leads to a brightness decrease. The mechanism for this is not altogether clear. One common explanation is that the porosity of the coating layer decreases and hence the scattering decreases. By comparing simulated and measured results it was shown that modifications of the surface properties account for the brightness decrease of ground calcium carbonate coated substrates with calendering. Monte Carlo light scattering simulations, taking into account the measured decrease of surface microroughness and increased effective refractive index, showed that surface modifications accounted for most of the observed brightness decrease of the ground calcium carbonate coated substrate, whereas the bulk scattering and absorption coefficients were not affected by calendering. It was also shown that the scattering coefficient is significantly dependent on the coat weight whereas the physical absorption coefficient is not.

The penetration of ink in the z-direction of a substrate influences the quality of the print. The ink penetration affects print density, mottling and dot gain, common print effects that influence the preference of consumers. The pressure in the printing nip and the porosity of the substrate both affect the amount of ink that is pressed into the porous structure of a coating layer during printing. By printing pilot coated paperboard with different coating porosity and measuring the resulting optical properties of the prints, a basis for simulations of the different layers, that is to say the coating, the print and the mixed layer in between, was created. Results show that ink distribution is strongly affected by the roughness of the substrate. Fibres and fibre flocks underneath the two coating layers created an unevenly distributed coating thickness that affected the print quality. Differences in pore size and pore size distribution also affected the behaviour of the ink. A coating layer of broad pigment particle size distribution resulted in a relatively low print density, in comparison to coatings of narrowly distributed particle sizes. Comparison of dot gain showed that the coating layer of a narrow particle size distribution had a relatively low dot gain compared to others. In this work, these results are explained by the differences in ink distributions on and in the coating layers.

Place, publisher, year, edition, pages
Karlstad: Karlstad University, 2010. 65 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2011:1
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-6721 (URN)978-91-7063-333-1 (ISBN)
Presentation
2011-02-11, Rejmersalen, 9C 204, Karlstads universitet, Karlstad, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2011-02-01 Created: 2010-12-22 Last updated: 2011-10-14Bibliographically approved
2. Surface and porous structure of pigment coatings: Interactions with flexographic ink and effects on print quality
Open this publication in new window or tab >>Surface and porous structure of pigment coatings: Interactions with flexographic ink and effects on print quality
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Each day, we are confronted with a large amount of more or less important information that we have to consider, and even in our digital society we need paper for communication, documentation and education. Much of the paper we use or are confronted by in our daily life, such as newspapers, books and packages, contains printed images or texts, and the appearance of both the print and the supporting surface is important. A good contrast between a printed text and the paper makes it easier to read, a detailed print of an illustration makes it more informative, and clear and evenly distributed colours on a package or on a poster make it more appealing. All of these qualities depend on the optical properties of the paper product and the the behavior of light illuminating the different materials.

The aim of the work described in this thesis is to characterize the structure of coatings and prints, and to validate models for the optical response and interaction of ink and coating based on optical measurements of physical samples. It is the interactions between the printing ink and the porous structure of the coating layers that are subject to investigation. Experiments have been employed to relate the physical conditions in a flexographic printing nip to the ink setting, affected by the physical and chemical properties of the coating, to the resulting optical response of the printed paperboard.

Abstract [en]

The aim of the work described in this thesis is to characterize the structure of coatings and prints, and to validate models for the optical response and interaction of ink and coating based on optical measurements of physical samples. It is the interactions between the printing ink and the porous structure of the coating layers that are subject to investigation. Experiments have been employed to relate the physical conditions in a flexographic printing nip to the ink setting and the resulting optical response.

By comparing simulated and measured results, it was shown that modifications of the surface properties account for the brightness decrease when substrates are calendered. Light scattering simulations, taking into account the surface micro-roughness and the increase in the effective refractive index, showed that surface modifications accounted for most of the observed brightness decrease, whereas the bulk light scattering and light absorption coefficients were not affected by calendering.

Ink penetration affects the print density, mottling and dot gain. Results show that ink distribution is strongly affected by surface roughness, differences in pore size and pore size distribution. For samples having different latex amounts and different latex particle sizes, a higher print force did not increase the depth of penetrated ink to any great extent, but rather allowed the wetting to act more efficiently with a more evenly distributed ink film, a higher print density and fewer uncovered areas as a result. Uncovered areas could be linked both to local roughness variations and to local wettability variations on the surface. Samples with different ratios of calcium carbonate/kaolin clay pigment showed an increased porosity and an increase in print density with increasing amount of kaolin in the coating layer.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2013. 81 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2013:49
Keyword
Coating, Pigment, Calendering, Porosity, Flexographic Printing, Print quality
National Category
Natural Sciences
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-30030 (URN)978-91-7063-528-1 (ISBN)
Public defence
2013-12-20, Rejmersalen 9C 204, Karlstads universitet, Karlstad, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2013-11-28 Created: 2013-11-15 Last updated: 2013-11-28Bibliographically approved

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http://www.tappi.org/Bookstore/Technical-Papers/Conference-Papers/2010/2010-TAPPI-Advanced-Coating-Fundamentals/Modelling-of-brightness-decrease-of-coated-cartonboard-as-an.aspx

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Bohlin, ErikJohansson, Caisa

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