On the mechanics of pulp fibre networks - a combined numerical, theoretical and experimental study.

Doctoral thesis, 2023

This work explores mechanics of fibre networks, especially for pulp fibre networks commonly found in hygiene products, such as baby diapers, incontinence and feminine care products, bathroom tissue and kitchen towels. Given that the main functionalities of these products are directly dependent on network configuration, designing these complex network structures for better performance, better utilisation of materials and less resources used requires in-depth knowledge and understanding of the structure and properties on the fibre level. The current work combines experimental, theoretical, and numerical results using the Discrete Element Method (DEM) to further this in-depth understanding of network deformation. The investigation starts with focusing on the effect of compression on network deformation, showing how deformation of the constituent fibres are dominated by bending of fibres at low solid volume fraction while at higher solid volume fraction transitioning to being dominated by fibre-fibre contact deformation. This is followed by showing the relationship between network solid volume fraction and tensile strength and stiffness. We continue by shifting the focus to permanent network deformation. The effect of fibre-fibre adhesion and fibre plastic contact deformation on permanent network deformation due to compression is investigated. Results show a synergetic effect of combining the two phenomena resulting in a significant increase in permanent deformation. Finally, we explain the dominating physics behind deformation due to the network transitioning from a dry to a wet state showing how the network reverts to a state of maximum unforced packing. The novel findings in this work further elucidates the mechanisms behind deformation of fibre networks, in particular for pulp fibre networks, and in consequence the functionality of products consisting of such networks. This knowledge provides insights that can be translated into improved performance, more sustainable resource use as well as optimization of the manufacturing process for a wide range of hygiene products.