Dewatering of foam-laid and water-laid structures and the formed web properties

Jani Lehmonen, E Retulainen, Jouni Paltakari, Karita Kinnunen-Raudaskoski, Antti Koponen

Research output: Contribution to journalReview ArticleScientificpeer-review

Abstract

The use of aqueous foams as a carrier fluid for pulp fibers instead of water has re-emerged in the paper and board industry in recent years. In foam forming, a surfactant is needed to reduce the surface tension of the carrier liquid and to create foam as a process fluid and flowing medium. This presents the following questions: (1) How do the water forming and foam forming processes differ? (2) How do the obtained wet/dry fibre sheets differ after forming and after wet pressing? (3) Which differences in the process behavior and sheet properties are due to the surfactant, and which are due to the presence of air bubbles in the flowing medium? The answers to these questions were sought by using an experimental academic approach and by applying a special dynamic vacuum assisted sheet former. Although foams are much more viscous than water, dewatering times were found to be approximately equal in water and foam forming at higher vacuum levels. The hydrodynamic resistance of sheet was approximately constant during water forming, while in foam forming resistance was initially even smaller than in water forming but it increased with time, being substantially higher at the end of the forming process. In certain cases, surfactant alone was found to have a similar, albeit often lower, effect on the sheet properties of foam. Surfactant improved sheet dryness (both after forming and wet pressing), lowered density, and lowered strength properties also in water forming. Foam, on the other hand, had a crucial effect particularly on certain structural properties such as formation and porosity. The difference between water and foam-laid sheets typically reduced in line with higher wet pressing pressure. This suggests that the role of surface tension and foam bubbles in controlling interfiber contact is overridden by wet pressing pressure. Thus applying foam as a carrier fluid has characteristic effects both on the papermaking process and the end product properties. The main features of foam forming can be explained by the chemical effects caused by the surfactant, and the structural effects caused by the foam bubbles.
Original languageEnglish
Number of pages20
JournalCellulose
Early online date16 Nov 2019
DOIs
Publication statusE-pub ahead of print - 16 Nov 2019
MoE publication typeA2 Review article in a scientific journal

Fingerprint

Dewatering
Foams
Water
Surface-Active Agents
Surface active agents
Fluids
Surface tension
Vacuum
Papermaking
Fibers
Pulp
Structural properties
Hydrodynamics

Keywords

  • foam forming
  • water forming
  • dewatering
  • structural properties
  • in-plane strength properties
  • interfiber contact
  • z-strength

Cite this

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title = "Dewatering of foam-laid and water-laid structures and the formed web properties",
abstract = "The use of aqueous foams as a carrier fluid for pulp fibers instead of water has re-emerged in the paper and board industry in recent years. In foam forming, a surfactant is needed to reduce the surface tension of the carrier liquid and to create foam as a process fluid and flowing medium. This presents the following questions: (1) How do the water forming and foam forming processes differ? (2) How do the obtained wet/dry fibre sheets differ after forming and after wet pressing? (3) Which differences in the process behavior and sheet properties are due to the surfactant, and which are due to the presence of air bubbles in the flowing medium? The answers to these questions were sought by using an experimental academic approach and by applying a special dynamic vacuum assisted sheet former. Although foams are much more viscous than water, dewatering times were found to be approximately equal in water and foam forming at higher vacuum levels. The hydrodynamic resistance of sheet was approximately constant during water forming, while in foam forming resistance was initially even smaller than in water forming but it increased with time, being substantially higher at the end of the forming process. In certain cases, surfactant alone was found to have a similar, albeit often lower, effect on the sheet properties of foam. Surfactant improved sheet dryness (both after forming and wet pressing), lowered density, and lowered strength properties also in water forming. Foam, on the other hand, had a crucial effect particularly on certain structural properties such as formation and porosity. The difference between water and foam-laid sheets typically reduced in line with higher wet pressing pressure. This suggests that the role of surface tension and foam bubbles in controlling interfiber contact is overridden by wet pressing pressure. Thus applying foam as a carrier fluid has characteristic effects both on the papermaking process and the end product properties. The main features of foam forming can be explained by the chemical effects caused by the surfactant, and the structural effects caused by the foam bubbles.",
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Dewatering of foam-laid and water-laid structures and the formed web properties. / Lehmonen, Jani; Retulainen, E; Paltakari, Jouni; Kinnunen-Raudaskoski, Karita; Koponen, Antti.

In: Cellulose, 16.11.2019.

Research output: Contribution to journalReview ArticleScientificpeer-review

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AU - Retulainen, E

AU - Paltakari, Jouni

AU - Kinnunen-Raudaskoski, Karita

AU - Koponen, Antti

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