Fibre/fibre joints : their characterization and influence on different paper strength properties
Sammanfattning: The main goal of this work was to gain knowledge of how to process the fibres for a certain enduse purpose. To reach this knowledge it must be clarified how different fibre properties, mechanical and chemical as well as surface and bulk properties, will influence the physical properties of the formed paper. The influence of adsorbed polyelectrolytes onto the fibres, and the conditions of the surrounding media were also included in the investigation, as those parameters will influence the interactions between the fibres during paper formation. The contact points between the fibres, the fibre/fibre joints, that are a key parameter of this work, were studied with respect to strength and appearance of the contact zone. The reason to the new nomenclature is the fact that the strength of the fibre/fibre joints is dependent on numerous other factors than just chemical bonds. In the present work the fibre/fibre joint has been investigated from an adhesion point of view where the physical appearance of the contact zone is one important parameter for the measured fibre/fibre joint strength. A new technique was developed for characterization of physical appearance of the contact zone, to create a link between contact zone properties, joint strength and the physical properties of paper.. Single fibre/fibre joint strength and the appearance of the contact zone were used to create a finite element model that described the stress-strain behaviour of fibre/fibre crosses under tensile loading. Electrostatics interactions between the fibres, polyelectrolytes, and the surrounding media in the stock will together with the mechanical properties of the fibres, such as fibre flexibility, control not only the number of contacts formed in the sheet during formation but also the fibre/fibre joint strength. It was found that the surface properties of the fibres and especially the surface charges, natural or introduced by chemical treatment, were important for the strength properties of the paper. Creating a fibre with a low surface elastic modulus will favour the formation of a large and homogeneous contact zone between the fibres and hence the strength of the fibre/fibre joint. The fibre/fibre joint strength is naturally also controlled by the molecular work of adhesion in the area of contact. Fibre surface modifications have a large potential in the design of paper with specific paper properties. By adsorbing polyelectrolytes at the surface of dried fibres the paper strength was enhanced beyond that of never dried fibres but with the density of paper made from dried fibres and a dewatering capacity as dried fibers.
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