Unique compression behavior of foam-formed sheets in wet pressing and calendering

We compare dynamic compression behavior of foam and water formed sheets for different furnishes. The main qualitative difference between the two forming methods is the strong thickness recovery of the foam-formed sheet after the first wet pressing pulse. Moreover, significantly higher solids content is achieved with foam forming. There are important differences among the studied furnishes depending mainly on the type and content of fines. The measured changes in sample thickness and solids content, together with simple mathematical formulas, enable analyzing the development of volume fractions of fibers, water and air during processing. Curiously, the advantage of foam-formed sheets coming from the improved compression recovery holds not only in wet pressing but also after calendering. This suggests that the unique recovery properties obtained with foam forming are related mainly to the fiber network structure rather than the lower water surface tension or improved dewatering. The work shows the most potential furnishes for high bulk and discusses how the studied process operations can be done effectively in the case of foam forming enabling new wet pressing strategies.