Tailoring the microporous structure of fibre materials with foam carrier

Use of foam as a material carrier in the manufacturing of novel paper-like structures has recently been studied intensively [1]. Tailoring the microporous structure requires control over various physical and chemical foam properties including air content, bubble size, interfacial rheology and foam stability. The relative importance of these factors has been studied in mixing experiments and in laboratory forming of sheet structures using foam-fibre mixtures. In axial mixing [2], the bubble size is affected by several physical factors such as rotational speed, air content and surface tension. At high shear rates, the bubble size becomes small and the importance of surface interactions increases affecting the rheological behaviour described by the Herschel-Bulkley equation. The inclusion of natural fibres (like wood fibres) to the mixing environment reduces the mean bubble size. However, the inclusion of the regenerated fibres (e.g. viscose fibres) does not affect the mean bubble size in the same way. The likely reason behind this behaviour is the rough surfaces of the natural fibres (and their fine particle fraction) that lead to high contact forces between the fibres and the foam bubbles [2,3]. The bubble size is inherited in the formed microstructure as a characteristic mean pore size after the foam carrier is removed from the foam-fibre system. With the same surfactant, bigger bubbles increase the mean pore size. Besides the above physical parameters, we have studied the effect of different surfactant types on the bubble size, interfacial rheology and the microstructure. The effects of the chemically different surfactants can exceed that of the varying bubble size leading to a decrease in the mean pore size even though the average bubble size increases. This provides an additional handle to the tailoring of the microstructure of the end product. 1) A. M. Al-Qararah, A. Ekman, T. Hjelt, J. A. Ketoja, H. Kiiskinen, A. Koponen, J. Timonen, A unique microstructure of the fiber networks deposited from foam-fiber suspensions. Colloids and Surfaces A: Physicochem. Eng. Aspects 482, 544-553 (2015). 2) A. M. Al-Qararah, T. Hjelt, A. Koponen, A. Harlin, J. A. Ketoja, Response of wet foam to fibre mixing. Colloids and Surfaces A: Physicochem. Eng. Aspects 467, 97-106 (2015). 3) A. Jäsberg, P. Selenius, A. Koponen, Flow rheology of fibre-laden aqueous foams. Proceedings of Papercon 2015, 19.4 - 22.5.2015, Atlanta, Georgia