Browsing by Author "Kinnunen-Raudaskoski, Karita"

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  • Dewatering of foam-laid and water-laid structures and the formed web properties
    (2020-02-01) Lehmonen, Jani; Retulainen, Elias; Paltakari, Jouni; Kinnunen-Raudaskoski, Karita; Koponen, Antti
     A2 Katsausartikkeli tieteellisessä aikakauslehdessä
    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. Graphic abstract
  • Foam as a carrier phase – a multipurpose technology for industrial applications
    (2017) Kinnunen-Raudaskoski, Karita
     School of Chemical Engineering | Doctoral dissertation (article-based)
    The applicability of foam in paper/board manufacturing processes was studied. The pulp and paper production in Finland has been forecast to decrease by up to a third from 2007 to 2020. The declining trend is mainly due to the weakening of main export markets. (Hetemäki et al., 2009). For the renewal of paper industry new solutions for savings in energy and raw materials are needed. Newly developed materials like nano materials may also challenge the conventional pro-cesses. New technologies are needed and the interest towards foam technology is growing. The main focus in this thesis was on foam forming and coating and on nano/microfibrils. The suitability of foam in biosludge treatment was also studied. The work showed that the strength loss due to the bulky structure of the foam formed paper, can be regained by using cellulose microfibrils (CMF) as strength additives, instead of wet pressing or beating. The strength enhancement was received with a lower bulk lost. The studied CMF grades had different responses to the strength properties of the pulps used; bleached soft wood pulp and chemi-thermo mechanical spruce pulp. This indicates that CMF used has to be chosen depending of paper properties needed. Dewatering was more effective due to the porous structure of the foam formed sheets and the formation of the papers better than in water forming, even with 12 mm synthetic fibers. Foam coating applications were done both to dry and wet fiber webs. Two different coating methods were used in the dry web ap-plications. The work showed that using foam it is possible to create thin functional surfaces on paper/board. Titanium dioxide and zinc oxide functionalized cellulose nanofibrils (CNF) created antimicrobial and photocatalytic properties on the paper and grease and water vapour barriers were achieved with polyvinyl or ethylene vinyl alcohol foam. In wet web application done at the former section polymers known to enhance the strength properties of paper were used. Due to foam destruction using vacuums, the whole web structure was treated with the chemical. The enhance-ment both on wet and dry web strength properties was detected with an increase in the dry matter content of the paper and without bulk lost. In addition to 'a carrier phase' function, the ability of foam to displace liquid water from a porous medium, was investigated. The foam enhanced dewa-tering in foam forming as well as in foam coating of the wet web. Based on these observations the foam assisted dewatering was studied on biosludge treatment. According to the results, the filtration time was shortened, the filtrate was cleaner and the sludge cake 10% drier. The change in dry solids content was quite small due to the insufficient vacuum level in filtration, 2 kPa. However, the result indicates that with a more efficient filtration vacuum it is possible to use foam to assist filtration. This thesis shows that foam technology offers several benefits for the paper/board industry, and combining it with nanomaterials enables the development of novel paper products.
  • Foam forming under dynamic conditions
    (2020-08) Lehmonen, Jani; Retulainen, Elias; Kraft, Marko; Paltakari, Jouni; Kinnunen-Raudaskoski, Karita
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Using foam as a carrier fluid in papermaking gives interesting new opportunities. Foam as a more viscous fluid than water is expected to behave differently in a dynamic process. This study presents results obtained under dynamic forming conditions in a semi-pilot scale research environment. Effects of process configurations and running conditions on increased forming speed, web properties, and difference between waterlaid and foam-laid processes are shown. The studies were carried out using a water-laid former and the same environment modified for foam forming. In order to achieve increased forming speed, the open headbox was replaced with a closed headbox, and the former geometry was updated. The process foam was boosted with an additional foam pulper. The foam pulper was used as a machine chest for improving the dispersion of fibers into the foam. A much broader tensile strength ratio range (~3 to 8) was achieved with foam forming than with water-laid forming. Foam-laid paper had a broader pore size distribution and higher mean pore size. Formation and the formation spectra of foam-laid sheets were more uniform, leading to improvements in the properties of the fiber network.