Proceeding Articles

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.

The molecular mechanisms behind the interactions between fibres in fibrous networks, and their link to paper/network strength, have long been under intense scientific investigations and scientific debate (Wågberg and Annergren 1997, Lindström et al. 2005; Hirn and Schennach 2017) but still there is no unified view on how the strength of fibre/fibre joints and network strength can be linked to different molecular mechanisms (Wohlert et al. 2022). Historically the interaction between cellulose-rich fibre surfaces was ascribed to hydrogen bonding and elaborate models were developed for linking mechanical properties of fibrous networks to the H-bonding between the surfaces (Nissan and Batten Jr 1997). This is however an oversimplification for several reasons. First of all, the H-bonds are very specific, which means that they are short-ranged and not implicitly additive over the material volumes engaged in the contact region between the fibres. Secondly the molecular interactions in the fibre/fibre contact zones are actively participating in the formation of the fibre joints, i.e. in the making of the fibre/fibre joints. The interactions in the wet state will hence affect the dimensions of the wet and dry contact zones and they will also create built-in stresses in the zones of contact in the dried fibre/fibre joint. The dry mechanical properties of the fibrous networks, i.e. the breaking of the joints and the fibres, will then be controlled by the molecular contact zone, the interactions in the contact zone, and also outside the molecular contact zone but within the molecular interactions range, the number of molecular contacts/network volume and the individual fibre strength.

In this work we visualize the evolution of the 3D microstructure of a mono-disperse nylon fibre floc undergoing uniaxial compression. In total seven stages of compression were visualized using X-ray microcomputed tomography. We observe that at the early stages of compression, densification occurred through sliding and rotation; at later stages, densification occurred though individual fibre deformation. We quantified these images and estimated the density of interparticle contacts ρ_c as a function of the compressive strain ε_c and found that ρ_c ∝ ε³_c

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 chemithermomechanical pulps (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.

Apart from its wide application in the paper, textile and biomedical industry, cellulose is now an emerging alternative reinforcement to improve the properties of polymers. Numerous research focuses on the development of renewable nanocomposites. In this context, nanocellulose liberated from plant cell walls or produced by bacterial serves as excellent candidate due to its inherently nano-sized nature, high crystallinity and high Young’s modulus. However, numerous cellulose-reinforced polymer nanocomposites reported in literature often failed to fully exploit the fibril tensile stiffness and strength, estimated to be 114 GPa and >2000 MPa, respectively. Nanofibrils can be compounded directly into polymers as reinforcement or used in paper form to produce laminated paperbased composites.

In recent years the application of paper for packaging has been growing steadily. Food packaging underlies special scrutiny, as the package has to prevent the escape of aroma molecules at the one hand and prevent the contamination of the packaged good with unwanted substances on the other hand. For the transfer of organic molecules from the packaging to the food and vice versa one has to look at volatile organic molecules. For transport of such molecules through the porous structure of paper the interaction of the molecules with the surface of the paper fibers plays an important role.

Salt hydrate phase change materials (PCMs) have been intensively used for thermal energy storage (TES) due to their sharp melting points, high energy storage density, small volume change and low cost. However, the problems of phase separation, supercooling and relatively low thermal conductivity of salt hydrate PCMs need to be addressed for high-efficiency TES. In this research, cellulose nanofibrils (CNFs) and CNFs-based composites were used to improve the TES performance of sodium acetate trihydrate (SAT). The effect of CNFs on the phase stability of SAT was investigated and the involved mechanism was explored by the rheological study. CNFs/graphene nanoplatelets (GNPs) composites and CNFs/silver nanoparticles (AgNPs) composites were prepared and used to improve the TES efficiency of SAT. Results indicate that adding 0.8% of CNFs to SAT increased the viscosity, enhanced solid-like rheological behaviors by entangled nanofiber network, and successfully eliminated phase separation of SAT. Owing to the excellent dispersing capability of CNFs, the aggregations of GNPs and AgNPs were avoided in the prepared CNFs/GNPs and CNFs/AgNPs composites. The resulting SAT-based composite PCMs were phase-stable and exhibited improved thermal conductivities over pure SAT due to the thermal conductivity enhancers, GNPs and AgNPs. Besides, with the combined use of sodium phosphate dibasic dodecahydrate and CNFs/AgNPs0.02 composite, the supercooling degree of SAT decreased to 1.2 °C. The prepared composite PCMs exhibited reasonable phase change temperature and enthalpy, and improved thermal stability. In summary, green and versatile CNFs based composites were prepared, and they successfully overcame the drawbacks of salt hydrate PCMs for TES applications.

The spreading and adhesion behavior of latex is a crucial factor in determining the structure and properties of the low-latex-content pigment coating layer. In spite of its importance, there has been a lack of analytical techniques to describe these characteristics in a coating layer. This study presents novel parameters and techniques for quantitative analysis of the spreading and adhesion behavior of latex binder in a coating layer, which is based on cross-section SEM image analysis. A comprehensive series of model coating layers were prepared to achieve varying extents of latex spreading. This was prepared by using latices with different glass transition temperatures and various drying or dry-sintering conditions. Parameters were developed to determine and quantify the different morphological characteristics of latices in the coating layers, which had advantages over qualitative observation from the SEM images. Developed analytical techniques were applied to reveal correlations between structural properties and the coating end-use properties, which contributed to a better understanding of coating structure development. A transitional change in coating structure was observed in the low latex spreading domain and this largely influenced the end-use properties of coating layers such as the mechanical properties and light-scattering efficiency. Micro-roughness of the coating surface and gloss were positively correlated to the spreading extent of the latex. It was notable that despite low levels of coating shrinkage due to the low latex content, the micro-roughness still changed with latex spreading variations. Quantitative analysis of latex spreading and mechanical properties of the coating layer also enabled us to separate the adhesion behavior of latex binder into physical and chemical aspects. This would propose a strategy for paper manufacturers to optimize their coated paper products.

There is an ever-increasing interest towards utilizing nanocellulose as barrier coatings and films, with many companies moving towards pilot scale production of nanocellulose to be used primarily for barrier coatings. However, high suspension viscosity and yield stress, poor adhesion to substrates, poor moisture sensitivity, and additional drying infrastructure needed for large-scale processing of nanocelluloses are some of the challenges that need to be addressed before commercialization. The current work aims at understanding and addressing the above challenges and to develop high-throughput continuous processes required to convert nanocellulose suspensions into barrier coatings and films. Rheology of different types of nanocelluloses across a wide range of shear rates is evaluated with special attention on the influence of dispersants (carboxymethyl cellulose (CMC) and Sodium polyacrylate (NaPA)) on the suspension processability and coating quality. A slot-die applicator is used to apply nanocellulose suspensions as a thin layer on a paper substrate in a continuous process. For moisture protection, biodegradable polymers and dispersions are applied onto the nanocellulose-coated paper via extrusion or dispersion coating. The resulting multilayer structure is then evaluated for its barrier properties viz., oxygen, water vapor, mineral oils, and grease at different test conditions. CMC addition reduces the yield stress, increases water retention, and slows down structure recovery (post high-shear) for nanocellulose suspensions, and thus has positive influence on coating quality and barrier properties. A new Casson-power-cross model was proposed to explain the viscosity behavior of cellulose nanofibrils (CNFs) across a wide shear-rate region, and Herchel-Bulkley model explains the viscosity behavior of cellulose nanocrystals (CNCs). Water vapor permeance for multilayer coatings remained below the control single-layer moisture-barrier materials, and oxygen permeance values were similar or lower than that of pure nanocellulose films. Glycerol and sorbitol plasticizers further improve oxygen barrier and kaolin addition improves the adhesion at nanocellulose/thermoplastic interface. The results provide insight into understanding the various factors that influence the continuous processing of a wide variety of nanocellulose suspensions into biodegradable barrier coatings and will pave the way for industrial production of sustainable packaging.