Foam-formed fibre materials: Tailoring material performance by foam-fibre interaction

The demand for environmentally friendly alternatives to oil-based materials in consumables is growing fast. Paper and board, made of natural wood fibres, provide a good starting point but the properties of fibre-based materials need be expanded to compete with plastics in different sectors. Foam forming technology provides several advantages compared to traditional water forming processes. Firstly, this next-generation manufacturing platform enables wider use of raw materials including e.g. very long fibres or particles of varied densities. Secondly, a broad range of different types of fibre-based materials can be formed, extending their end-use applications from packaging to textiles and construction materials.In order to produce foam-formed fibre materials with desired performance, one has to understand the interaction between wet foam and fibres. The stability and structure of the foam carrier medium provides a tool to tailor the material density and pore size distribution (A. M. Al-Qararah et al. “A unique microstructure of the fiber networks deposited from foam-fiber suspensions”, Colloids and Surfaces A: Physicochem. Eng. Aspects, 482, 544-553, 2015). However, structure and rheology of wet fibre foams do not depend only on the physical features, such as air content or bubble size distribution, but also on the surfactant chemistry and fibre surface properties such as smoothness and surface energy. Such features are still largely unexplored and their effects on the final material performance are not clear. We have approached this problem by using simple model systems with gradually increased complexity. The interaction between an air bubble and a model surface in varied surfactant solutions was studied for both cellulose and silica surfaces of different hydrophobicity. The adsorption of SDS surfactant on a surface was measured separately by quartz crystal microbalance. Air bubbles were found to have repulsive interactions with hydrophilic surfaces and attractive ones with hydrophobic surfaces. Surfactant concentration played a significant role in the transition from attraction to repulsion for semi-hydrophobic surfaces. These results are in line with earlier literature (D. Kosior et al. “Influence of non-ionic and ionic surfactants on kinetics of the bubble attachment to hydrophilic and hydrophobic solids”, Colloids Surf. A, 470, 333-341, 2015) and support the conception that hydrophobic interactions are necessary to explain the attractive forces between bubbles and fibres. The findings can be used to design new foam-formed structures and materials with desired performance.