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Optics of coated paperboard: Aspects of surface treatment on porous structures
Karlstad University, Faculty of Technology and Science, Department of Chemical Engineering.
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. , p. 65
Series
Karlstad University Studies, ISSN 1403-8099 ; 2011:1
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-6721ISBN: 978-91-7063-333-1 (print)OAI: oai:DiVA.org:kau-6721DiVA, id: diva2:380890
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
List of papers
1. Characterization and modelling of the effect of calendering on coated polyester film
Open this publication in new window or tab >>Characterization and modelling of the effect of calendering on coated polyester film
2009 (English)In: Advances in Printing and Media Technology: Vol XXXVI, 2009, p. 301-308Conference paper, Published paper (Refereed)
Abstract [en]

Polyester films were coated with a coating colour based on a ground calcium carbonate with narrow particle size distribution. Four different coat weights were produced. The coated sheets were calendered and the samples were exposed to up to twenty nips in the calender at a line load of 300 kN/m at 50°C. The brightness and the thickness were measured after each calendering nip. Reflectometry was used to measure the refractive index and the microroughness of the samples. Calendering lead to a decrease of the Kubelka-Munk scattering coefficient and an increase of the absorption coefficient of the coated transparent films. Taking into account non-uniform surface reflection at the boundaries between media of different refractive indices, a large part of the reflectance decrease due to calendering could be attributed to an increase of the effective refractive index at the coating surface and a decrease of the surface microroughness.

Keywords
Coating, Calendering, Gloss, Brightness, Refractive index
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-6828 (URN)978-3-9812704-1-0 (ISBN)
Conference
Proceedings of the 36th International Research Conference of airigai, Stockholm, Sweden, 2009
Available from: 2011-01-27 Created: 2011-01-27 Last updated: 2014-10-27Bibliographically approved
2. Modelling of Brightness Decrease of Coated Cartonboard as an Effect on Calendering: Microroughness and Effective Refractive Index Aspects
Open this publication in new window or tab >>Modelling of Brightness Decrease of Coated Cartonboard as an Effect on Calendering: Microroughness and Effective Refractive Index Aspects
2010 (English)In: TAPPI 11th Advanced Coating Fundamentals Symposium, Munich, Germany, 2010: Symposium Proceedings, Norcross, GA, USA: TAPPI Press, 2010, p. 51-65Conference 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
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-6830 (URN)1-59510-203-5 (ISBN)
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
3. Flexographic Ink-Coating Interactions, Effects of Porous Structure Variation of Coated Paperboard
Open this publication in new window or tab >>Flexographic Ink-Coating Interactions, Effects of Porous Structure Variation of Coated Paperboard
2012 (English)Conference paper, Published paper (Refereed)
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-6831 (URN)
Conference
8th International Paper and Coatings Chemistry Symposium 2012, Stockholm, Sweden, 10-14 June
Available from: 2011-01-27 Created: 2011-01-27 Last updated: 2017-12-06Bibliographically approved

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