Harri Kiiskinen

Drying of foam-formed mats from virgin pine fibers

ABSTRACT The purpose of this work was to select an efficient drying technique for drying of highly porous thick fiber foam mats with minimum impact on their structure after forming and drainage. Thick fiber mats were produced from wood fibers using foam-forming technology and dried using several different drying methods. The mixture of pine fibers and surfactant (foaming agent) in water was blended using a high-speed blender. After fiber foam generation, a sample mold was filled with wet fiber foam, and after drainage, drying experiments were performed. For comparison, experiments were carried out in an oven, an impingement dryer assisted with a vacuum, and a combined impingement-infrared dryer. At low moisture contents, through-air drying experiments were also carried out. Drying curves, temperature profiles, and shrinkage were measured from the produced mat structures. The most promising drying technique in this study was the combined impingement-infrared drying, used until the fiber mat became permeable, followed by through-air drying until the desired final moisture content was achieved.

Porous structure of fibre networks formed by a foaming process: a comparative study of different characterization techniques

Recent developments in making fibre materials using the foam‐forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure‐characterization methods, both two‐dimensional and three‐dimensional techniques were used to explore the porous structure of foam‐formed samples made with two different types of cellulose fibre. These techniques included X‐ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X‐ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two‐dimensional imaging techniques. The two‐dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam‐fibre interaction.

The effect of in-line foam generation on foam quality and sheet formation in foam forming

Abstract Foam forming has recently attracted increasing interest due to the paper industry’s continual efforts to find new possibilities to minimize raw material consumption, and to improve energy and water efficiency. Foam forming is also thought to be a possible solution to the industry’s need to widen its product portfolio with novel and more valuable products. In foam forming, foam properties (air content, bubble size and half-life) are obviously key process variables, but there are only a few studies in which their effect on the sheet properties have been studied in pilot conditions. Moreover, all previous studies have used foam generated in stirring tanks, and there are hitherto no studies in which in-line foam generation has been considered. In this paper both these gaps are filled with experiments performed in VTT’s pilot foam forming environment. The combination of tank and in-line generation was found to work well in foam forming, providing extra flexibility for foam generation and decreasing surfactant needs. The results show that foam forming generally improves formation, but the foam quality can have a significant effect on sheet properties.