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Variations Related to Print Mottle in Starch-Containing Paper Coatings
Karlstad University, Faculty of Technology and Science, Department of Chemical Engineering.
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 [en]
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: urn:nbn:se:kau:diva-14409ISBN: 978-91-7063-444-4 (print)OAI: oai:DiVA.org:kau-14409DiVA: diva2:542200
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
List of papers
1. Effects of the Partial Replacement of SB latex with Dextrin Starch on the Thickness Distribution of Coating Layers
Open this publication in new window or tab >>Effects of the Partial Replacement of SB latex with Dextrin Starch on the Thickness Distribution of Coating Layers
2008 (English)In: Tappi 2008 Advanced Coatings Fundementals Symposium Proceedings, Atlanta: Tappi press , 2008, 154-163 p.Conference paper, Published paper (Refereed)
Abstract [en]

A pilot coating trial has been carried out where the synthetic binder in the coating colour was partially replaced with dextrin starch. The coating thickness and the coating thickness distribution were investigated by means of burnout tests and mercury porosity measurements. The coating thickness distribution is governed by the surface profile of the base paper underneath the blade; this surface profile is controlled by the compressibility of the base paper and the blade pressure. Dewatering of the coating colours plasticises the paper and increases its compressibility; a linear relationship between dewatering and the standard deviation in the coating thickness was found for a given blade pressure. The characterization of the coating layers showed a small but measurable increase in the thickness variation for increasing starch content, which was caused by the higher water retention of the starch colours. The surface profile of the uncoated and coated paper was investigated with an optical profilometer. The data from those measurements and the coating thickness distribution investigation were used analytically to calculate the roughness change of the base paper during coating. The roughness change was found to be inversely proportional to the original base paper roughness, indicating that the rougher base paper was more compressible.

Place, publisher, year, edition, pages
Atlanta: Tappi press, 2008
Keyword
mottling, mass distribution, coating thickness, blade coating, starch, dewatering, surface profiling, compressibility
National Category
Other Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-14403 (URN)1-59510-174-8 (ISBN)
Conference
Tappi Advanced Coating Fundamentals, Montreal, Canada
Funder
Swedish Research Council
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
2. Shrinkage of Coating Layers
Open this publication in new window or tab >>Shrinkage of Coating Layers
(English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669Article in journal (Refereed) Submitted
Abstract [en]

The shrinkage of coating layers during the process of consolidation between the first critical concentration (FCC) and the second critical concentration (SCC) has been studied. Ground calcium carbonate (GCC) grades with different particle size distributions together with binder systems based on CMC/latex, dextrin and oxidized starch were used in the formulations of the coating colors. The aim was to investigate differences in shrinkage behavior between these systems. The porosity and the pore radius of the coatings at FCC (filter cake) and SCC (final dry coating) were measured. The coating colors were rheologically characterized. It was shown that the shrinkage was governed by the capillary forces developed in the filter cake during the consolidation process. These forces were greater for dextrin than for CMC/latex and oxidized starch. This was attributed to a weaker chemical interaction between this binder and the pigment. At a given capillary pressure, the dextrin-containing coating was found to shrink more than the other two coatings. This is suggested to be due to a softer or more plastic deformable filter cake.

Place, publisher, year, edition, pages
Stockholm: Nordic Pulp & Paper Research Journal
Keyword
Paper Coatings, Starch, Co-binders, FCC, SCC, Shrinkage, CMC, Calcium Carbonate, Porosity
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-14404 (URN)
Note

Part of the thesis:Variations Related to Print Mottle in Starch-Containing Paper Coatings

Available from: 2012-07-30 Created: 2012-07-30 Last updated: 2017-08-11Bibliographically approved
3. The effect of Moisture on Deformation of Starch Containing Coating Layers during Calendering, studied using Confocal Laser Scanning Microscopy
Open this publication in new window or tab >>The effect of Moisture on Deformation of Starch Containing Coating Layers during Calendering, studied using Confocal Laser Scanning Microscopy
(English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669Article in journal (Other academic) Submitted
Abstract [en]

The effect of moisture level on the deformation of coating layers during calendering was studied using Confocal Laser Scanning Microscopy (CLSM) and porosimetry. Model draw down coatings with starch/latex and carboxymethyl cellulose (CMC)/latex on polyester and on paper were dried at different relative humidities. The samples were calendered and despite their higher moisture content, the starch/latex coatings did not deform much more than the CMC/latex coatings.  For coatings dried between 20 and 50% ambient relative humidity (RH), no significant changes in deformation behavior or in moisture content were observed. When the RH was increased to 90%, the moisture content and the deformation during calendering was much higher with both systems indicating that the interaction between latex and water plays an important role for the compression during calendering. An inverse relationship between fluorescence intensity and porosity was observed on coated polyester sheets when measured by the CLSM technique, but the cause of the relationship is unclear and there may have been more than one mechanism. Studies of coated paper showed that Starch/latex coatings have a higher standard deviation in the fluorescence intensity than CMC/latex coatings, indicating a less uniform surface in the case of the starch/latex coatings. The cause of this non-uniformity remains unclear, but it has the potential to impact print properties.

Place, publisher, year, edition, pages
Stockholm: Nordic Pulp & Paper Research Journal
Keyword
starch, paper coating, print mottling, confocal laser scanning microscopy, structural variations, calendering, moisture effects
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-14405 (URN)
Note

Part of disseratation: Variations Related to Print Mottle in Starch-Containing Paper Coatings

Available from: 2012-07-30 Created: 2012-07-30 Last updated: 2017-12-07Bibliographically approved
4. Porosity Variations in Coating Layers - Impact on Back-trap Mottle
Open this publication in new window or tab >>Porosity Variations in Coating Layers - Impact on Back-trap Mottle
(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
coated papers, porosity variations, binders, back trap mottle
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-14407 (URN)
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

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