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The picture of the smallest structural units of wood fibres, that is, cellulose microfibrils and their bundles, has become more accurate during the last couple of decades, when information gained from several experimental characterisations has been drawn together. This work has been supported by computational methods that allow one to test the behaviour of postulated structures on the nanometre scale, and thus help in interpreting the experimental data. Bound water is an essential component in these models, as it affects both the structural swelling and the mechanical properties of the fibre wall nanostructure. Moreover, mechanisms on this scale can be expected to drive similar properties of macroscopic fibres. We suggest that several large-scale problems in papermaking and converting could be approached with atomistic molecular dynamics simulations for varied chemical compositions and external conditions. We demonstrate this by first showing that simulated moisture diffusion rates agree with measured ones at room temperature, and then determine diffusion rates at elevated temperatures that lack reliable experimental data. These predictions provide key knowledge for further development of high-temperature drying and pressing processes. The results are important also when linking material performance at varied external conditions to the composition of the fibres.

Foam forming of cellulose fibre materials is based on an interaction between fibres and bubbles, which can take several material properties to new levels. To control the formed structure, the mechanisms of this interaction have been systematically investigated. This started with captive bubble studies where we analysed the interaction of a single bubble with various smooth cellulose and silica model surfaces. The bubbles adhered only to hydrophobic surfaces, and this attraction was sensitive to the surface tension. From this simplest case, the studied system gradually became more complex. We found that a bubble adheres weakly also to a submerged cellulose nanofibre (CNF) film, which could be explained by nanoscale surface roughness capturing nanobubbles. The interaction with real fibres was studied by pressing a single bubble against a fibre bed in water and sodium dodecyl sulphate (SDS) solution. Fibre type and surface tension had all apparent effects on the attachment. In the case of natural fibres, the presence of hydrophobic lignin clearly increased the fibre attachment on a bubble, while added SDS decreased the attachment with all fibre types. These findings agreed with the mechanisms found earlier using the model surfaces. Finally, when forming thick nonwoven materials using hydrophilic and hydrophobic viscose fibres, differences in fibre network structure and strength properties depended on the fibre hydrophobicity and surfactant type, as suggested by the results obtained in simpler systems.

We have studied ways of improving strength properties of paper made from high yield pulps and lignin-rich chemical pulps by utilizing the thermoplastic properties of the lignin present in the fibre walls. Both dry and wet strength can be improved by hot pressing of sheets made from lignin-rich pulps. In this paper, we focus on aspects of the wet-strength development as a function of lignin content and temperature. Here we apply an activation energy evaluation approach to study lignin intermixing or interdiffusion. By means of hot pressing, it is possible to reach wet strength levels up to 50% of the dry strength level, provided that we use pulps with high enough lignin content. Our study included hot pressing of high yield pulps such as thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), high-temperature chemithermomechanicalpulps (HTCTMP), unbleached northern softwood kraft (NSK) and northern bleached softwood kraft (NBSK). The sheet pressing trials were performed for varied temperatures from room temperature up to 270°C. As the activation energy for the high yield pulps and the lignin-rich NSK were all in the range of 20-32 kJ/mol, we suggest that the wet strength development as function of temperature has a similar mechanism as long as the pulp fibres contain enough lignin. We also suggest that the phenomenon involves intermixing and/or interdiffusion of wood polymers between adjacent fibres when they are in a close contact. Most probably both the amorphous wood polymers, i.e. the linear hemicelluloses and the cross-linked lignin, mix with each other across the fibre-fibre or even more probable over the fibril-fibril contact surface. While the hemicellulose can intermix already at room temperature under moist conditions, the lignin intermixes more easily at the higher temperature we use. We do not know how far the hemicellulose or lignin could move within the fibre walls, but it seems that the amount of lignin present on the fibre surfaces plays an important role.

The physico-chemical characteristics of wet fibre surfaces and their role in fibre bonding and paper properties have been under debate and research for decades. The gel-likeness of the fibre surfaces has been addressed in many studies but has not been explicitly demonstrated. In this study, the structure of wet beaten kraft pulp fibre surface and its similarity with microfibrillated cellulose (CMF) was shown using helium ion microscopy (HIM) imaging. Beaten kraft fibres and CMF were dried using two mild drying methods to preserve the delicate fibrillated structures. The fibre surface had a strong resemblance with gel-like CMF material. The amount of external fibrillation varied along the fibre length and was often shown to extend tens of micrometers from the fibre surface. The gel-like behaviour of wet fibrillated material was demonstrated using rheological tests. The examined CMF and CNF samples with solids contents of 1.97% and 1.06%, respectively, showed gel-like behaviour. A low gelling point suggests that fibrillated fibre surfaces have the ability to transfer forces at very low consistencies, and that the ability increases as a power function of the solids content. This is assumed to play a remarkable role in increasing inter-fibre adhesion and in transmitting inter-fibre forces within consolidating webs, whether arising from external tensions or internal drying stresses.

We discuss a new mean-field theory to describe the compression behaviour of thick low-density fibre networks. The theory is based on the idea that in very large systems, the statistics of free segment lengths causes the stress-deformation behaviour to be quantitatively predictable. The theoretical ideas are supported by several different experimental characterisations. Firstly, we have carried out single-fibre buckling tests using hemp fibres, which indicate a maximum level of axial stress before deformation localization, after which the load-carrying ability of a fibre decays. Secondly, the stress-compression behaviour of over 130 different foam-formed lightweight fibre materials were measured. For kraft pulps with low fines content, the average stress compression behaviour closely follows the theoretical prediction as described in terms of a universal s-function. Moreover, the acoustic emission can be described by the same function until collective phenomena cause deviations from the predicted behaviour. Similar deviations at smaller compressive strains are seen with furnishes with high fines content or added nanocelluloses together with samples with large voids. The localized buckling deformations lead to rapid stress re-distributions and subsequent fibre displacements in a fibre network as shown with in-situ CCD imaging.