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Porosity Variations in Coating Layers - Impact on Back-trap Mottle
Karlstad University, Faculty of Technology and Science, Department of Chemical Engineering.
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
Karlstad University, Faculty of Technology and Science, Department of Chemical Engineering.
(English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669Article in journal (Other academic) Submitted
Abstract [en]

A pilot coating trial was performed to study 1) whether the drying strategy introduces porosity variations in the coating layer and 2) whether porosity variations caused by the subsequent supercalendering are linked to back-trap mottle in offset prints. The porosity variations and the mean porosity were indirectly measured using a burn-out test. Coating colours were compared with three different binder systems 1) carboxy methyl cellulose (CMC)/SB-latex, 2) dextrin/SB-latex and 3) oxidized starch/SB-latex. The results showed that neither the mean porosity nor the porosity variations were affected by the drying strategies studied. During calendering the coating with the CMC/SB-latex binder system was compressed the most, but the porosity variations were nevertheless small. Both the compression and the porosity variations were influenced by the drying strategy, which shows that the strategy had an impact on the mechanical properties of the dry coating layer. With CMC/SB-latex and dextrin/SB-latex, the mottle increased when the porosity variations introduced by the calendering increased. In the case of the oxidized starch/SB-latex, the opposite was true. It was concluded that with CMC/SB-latex and dextrin/SB-latex, print mottle is caused by calendering and with oxidized starch/SB-latex by non-uniformities introduced during dewatering and drying, possibly binder migration.

Place, publisher, year, edition, pages
Stockholm.
Keyword [en]
coated papers, porosity variations, binders, back trap mottle
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-14407OAI: oai:DiVA.org:kau-14407DiVA: diva2:542195
Note

Part of the thesis: Variations Related to Print Mottle in Starch-Containing Paper Coatings, http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-14409

Available from: 2012-07-30 Created: 2012-07-30 Last updated: 2017-08-14Bibliographically approved
In thesis
1. Variations Related to Print Mottle in Starch-Containing Paper Coatings
Open this publication in new window or tab >>Variations Related to Print Mottle in Starch-Containing Paper Coatings
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Starch in paper coatings is known to increase the risk of print mottle in lithographic offset printing. The objective of this study was to increase the understanding of this behaviour. Four phenomena that could lead to print mottle, where the presence of starch might be important, have been identified: uneven binder migration, uneven coating mass distribution, uneven deformation during calendering and differential shrinkage. The latter three were investigated in this project.

Starch-containing coating colours often have high water retention. A relationship between the water retention of the coating colours and the distribution of coating thickness was found in a pilot trial. A theory is proposed, where the surface profile of the base paper beneath the blade, that governs the coat weight distribution in blade coating, is affected by moisture from the dewatering coating colours and the compressive force exerted by the blade.

Drying strategies were studied to see whether they would induce porosity variations in the coating layers. There is a strong connection between the rate of evaporation and the shrinkage of the coating layer, but no porosity variations due to the choice of drying strategy were found. Shrinkage is governed by the capillary forces. At the same capillary pressure, the coating shrinks more for some binder systems, which is suggested to be due to a weaker chemical interaction between the binder and the pigment.

Oxidized starch/latex coatings, stained with a fluorescent marker, had a greater standard deviation in fluorescence intensity than CMC/latex coatings caused by a difference in either porosity or latex distribution. It was shown that calendering introduces porosity variations into the coating layer that are larger for starch-containing coatings. The drying strategies appeared to have a significant effect on these porosity variations and they correlated positively with print mottle in some cases and in another case negatively. In the case of the negatively correlated, the mottle was probably caused by variations in surface porosity existing prior to the calendering.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2012. 64 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2012:37
Keyword
pigment coating, porosity variations, coating structure, coat weight variations, starch, dextrin, CMC, latex, print mottle, calendering, pilot coating, offset printing
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-14409 (URN)978-91-7063-444-4 (ISBN)
Public defence
2012-09-28, 9C203, Nyquist salen, Karlstad Universitet, Karlstad, 12:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Available from: 2012-09-11 Created: 2012-07-30 Last updated: 2017-08-14Bibliographically approved

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