A calculation method has been derived to predict the void volume loss of dynamically loaded structured composites commonly used as press belts in paper manufacturing. The method is based on a viscoelastic model that uses two serial generalised three-parameter Maxwell models and allows for predicting the void volume loss as a function of the applied external load and load rate. Optical verification of the void volume losses revealed that the method accurately calculates these volume losses that appear in the structure due to viscoelastic compression. Applying it to different composite specimen types makes it possible to quantify the influence of matrix material formulation, geometrical structure, temperature and saturation conditions on the void volume loss of dynamically loaded composites. As a result, the matrix material formulation of the polyurethane matrix is identified as the key parameter influencing the void volume loss.

Paper, as a bio-based product, is a key material in advancing a sustainable circular economy. In papermaking, energy-intensive drying is required to remove residual water from the cellulose fibre network. Wet pressing is therefore a crucial step, reducing the water that must be evaporated in the dryer section and significantly lowering overall energy demand.

In wet pressing, the paper web enters a nip formed by two loaded rolls while supported by a press felt. The applied load drives water from the web through the felt into the voids of roll covers. Industrial observations suggest that the compression behaviour and saturation of both components strongly affect dewatering efficiency, yet their response under realistic press nip conditions remains insufficiently understood. This thesis aims to investigate the compressibility and liquid distribution of fibre systems under such conditions.

The work combines laboratory-scale experimental rigs, X-ray imaging techniques and calculation models derived from physical laws. This enables the quantification of dynamic compressibility and void volume loss in grooved polyurethane roll covers, a key factor in roll cover design. Studies of stress variations at the press felt–roll cover interface show that dewatering improves when high-permeability felt regions are created by the groove structure. The liquid distribution in press felts is characterised as a function of load and saturation, showing out-of-plane redistribution during compression due to the nonwoven morphology. Higher felt saturation enhances dynamic liquid transport, linking relative permeability to improved dewatering once nip saturation is reached. Finally, X-ray multi-projection imaging (XMPI) is shown to resolve pore-scale liquid transport mechanisms, enabling future studies of rewetting between the press felt and paper web.

Papper, som en biobaserad produkt, är ett nyckelmaterial för en hållbar cirkulär ekonomi. Vid papperstillverkning krävs energikrävande torkning för att avlägsna kvarvarande vatten ur cellulosafibernätverket. Våtpressning är därför ett avgörande steg som minskar mängden vatten som måste avdunstas i torksektionen och därmed sänker energiförbrukningen.

I våtpressning passerar pappersbanan en presszon mellan två belastade valsar, stödd av en pressfilt. Belastningen driver vatten från banan genom filten och in i valsöverdragets hålrum. Observationer visar att kompressionsbeteende och mättnad i båda komponenterna starkt påverkar avvattningseffektiviteten, men deras respons under realistiska förhållanden är ännu inte fullt förstådd. Denna avhandling undersöker därför kompressibilitet och vätskefördelning i fibersystem under pressförhållanden.

Arbetet kombinerar laboratorieexperiment, röntgenavbildning och fysikbaserade modeller. Detta möjliggör kvantifiering av dynamisk kompressibilitet och hålvolymförlust i spårade polyuretanvalsöverdrag, en nyckelfaktor för valsdesign. Studier av belastningsvariationer i gränsskiktet filt–valsöverdrag visar att avvattningen förbättras när högpermeabla filteregioner skapas av spårstrukturen. Vätskefördelningen i pressfiltar karakteriseras som funktion av belastning och mättnad och visar omfördelning i z-led under kompression till följd av den icke-vävda morfologin. Högre filt­mättnad ökar den dynamiska vätsketransporten och kopplar relativ permeabilitet till observerade förbättringar i avvattning när nipmättnad uppnås. Slutligen visas att röntgenmultiprojektionsavbildning~(XMPI) kan upplösa vätsketransport på pornivå och möjliggör framtida studier av återvätning mellan pressfilt och pappersbana.

Efficient mechanical dewatering in paper manufacturing is essential for reducing energy consumption and enhancing operational efficiency. Practical observations indicate that press felt and roll cover structures significantly influence dewatering performance. While previous studies have focused on micro-scale stress variations at the paper web-press felt interface, this study extends the analysis to the press felt-roll cover interface. Using a custom dynamic compression setup, we investigate how different groove patterns impact press felt dewatering. The results show that macro-scale stress variations play a crucial role, with controlled mechanical inhomogeneities enhancing felt permeability. Through multivariate regression analysis, an optimized groove pattern is identified that improves dewatering by approximately 7 % under highly dynamic pressing conditions. These findings offer valuable insights into optimizing press felt and roll cover interactions, providing a methodology to enhance nip dewatering efficiency. The study highlights the need to tailor groove patterns to specific press felts to ensure optimal water flow under saturated conditions. This research contributes to improving paper machine performance by maximizing water removal while reducing energy consumption, supporting both economic and environmental sustainability in the industry.

Shoe press belts contribute significantly to the overall dewatering performance in the press section of a paper machine. Within the shoe press nip, the press belt faces a dynamic and multidimensional load that mainly leads to a compression of the structure. As this will cause a loss in void volume, knowledge of the dynamic compression characteristics of shoe press belts is crucial for optimized dewatering. A novel method was developed to examine the dynamic compression characteristics of grooved polyurethane press belts. Therefore, an experimental setup allowing realistic boundary conditions to test specimens was placed in a servo-hydraulic testing machine. Press belt specimens with different matrix material formulations and groove patterns were tested under varying load rates equivalent to different paper machine operational speeds. The results showed an evident sensitivity of the dynamic compression stiffness to the operational speed of the paper machine. This behavior was seen to be more sensitive to changes in the matrix material formulation than to adaptions of the groove pattern. As a result, the compression of the press belt within a shoe press nip is not only influenced by the peak pressure within the shoe press nip but also depends on the operational speed of the paper machine.

An experimental method was developed to examine the dynamic compression properties of structured polyurethane composites used as press belts within a shoe press of a paper machine. The objective was to investigate the influences of the geometrical surface structure and the matrix material composition on the compression properties. Two polyurethane formulations were tested under varying specimen conditions. The results show that the dynamic compression modulus increases with the applied load rate and that temperature and water saturation reduce the influence of dynamic effects on the compression modulus. Furthermore, it was observed that modifications of the matrix material have a more significant impact on the dynamic compression modulus than adaptions in the geometrical structure. This is addressed to the relatively small variations in possible surface designs. Finally, a rate-sensitivity index is introduced to quantify the tested specimens’ rate-sensitive behaviour.

In many industrial applications, nonwoven fibre networks are facilitated to operate under partly saturated conditions, allowing for filtration, liquid absorption and liquid transport. Resolving the governing liquid distribution in loaded polyamide-6 (PA6) fibre networks using X-ray computed micro-tomography is a challenge due to the similar X-ray attenuation coefficients of water and PA6 and limitations in using background subtraction techniques if the network is deformed, which will be the case if subjected to compression. In this work, we developed a method using a potassium iodide solution in water to enhance the liquid’s attenuation coefficient without modifying the water’s rheological properties. Therefore, we studied the evolving liquid distribution in loaded and partly saturated PA6 fibre networks on the microscale. Increasing the external load applied to the network, we observed an exponential decrease in air content while the liquid content was constant, increasing the overall saturation with increasing network strain. Furthermore, the microstructural properties created by the punch-needle process in the manufacturing of the network significantly influenced the out-of-plane liquid distribution. The method has been proven helpful in understanding the results of adaptions in both the fibre network design and manufacturing process, allowing for investigating the resulting liquid distribution on a microscale.

Flow instabilities such as Haines jumps in porous media are common phenomena that occur on sub-second timescales. X-rays are particularly suitable for investigating these processes because they provide non-destructive three-dimensional insight into the network structure and the liquid distribution within porous media. Studying imbibition events in four dimensions (three spatial dimensions plus time) is inherently challenging with conventional tomography because the required rapid sample rotation imposes significant centrifugal forces that alter the flow. Here, we demonstrate synchrotron X-ray multi-projection imaging (XMPI) to capture four-dimensional flow in an additively manufactured, homogeneous spherical pore network at 1.3 µm spatial and 50 Hz temporal resolution without the need for high rotational speeds. This enables in situ visualization of non-repeatable pore-scale events in both space and time, a capability unachievable with classical X-ray tomographic approaches. We compare the results to Shan–Chen multiphase Lattice Boltzmann simulations performed on the same geometry, finding both qualitative agreements and systematic differences in filling sequences and timescales. These discrepancies expose key limitations of current simulation methods in representing contact-line dynamics and realistic boundary conditions limitations that XMPI can directly overcome. By enabling high-resolution, real-time imaging of flow instabilities in opaque porous media, synchrotron XMPI provides a unique platform that bridges the gap between pore-scale experiments and simulations.

Nonwoven fibre networks underpin filtration, insulation and geotextiles, where liquid uptake, redistribution and release govern performance. In needle-punched felts, barbed needles mechanically entangle fibres and partially reorient them toward the thickness direction~($z$), creating out-of-plane “pillars” and heterogeneity. While mechanical and structural consequences of needling are well documented, dynamic $z$-direction transport in partly saturated networks remains difficult to access due to opacity and sub-second timescales. Here we combine micro-CT~(µCT) of dry structure with time-resolved X-ray radiography during droplet addition to quantify through-thickness transport as a function of saturation and needling intensity, using a compact Washburn-type descriptor for dynamics. Results show an exponential dependence of $z$-directional liquid transport on saturation, consistent with previous models for in-plane relative permeability of nonwoven networks. Additionally, increased needle-punch intensity reorients fibres toward the $z$-direction, forming preferential flow pathways that enhance through-thickness transport, even as single-phase permeability decreases. These findings underscore needle-punch as a key design parameter for tuning liquid transport in nonwoven fibre networks. The approach provides an experimental and modelling framework for dynamic, capillarity-driven transport in opaque fibrous materials.