A study on the structure of paper : the links between paper and fibre properties

The work shown in this thesis, focuses on the effects of fibre geometry (fibre length, width and coarseness) to the paper strength and bulk. It is believed that fibre networks, for a given furnish, can be optimized through fractionation to create a stronger and bulkier paper. A first set of experiments was performed to determine the correlations among fibre properties of a softwood kraft pulp furnish. The fibre properties were measured using a Fibre Quality Analyzer (FQA). Coarseness (ω) and width (D) were found to increase linearly with length (Lf ). These correlations are thought to be influenced by the tree species and the pulping process. A second set of experiments was aimed to determine empirical expressions of bulk and tensile index (TI) in terms of the fibre geometry distribution (ω, Lf and D) and the press-drying pressure (P). Bulk and TI was measured for handsheets made at different pressures from different size distributions. These distributions were created by a combination of fractionation on a Bauer McNett Classifier (BMC) and fibre cutting. The determined relations show agreement with experimental work from other researchers. In order to provide insight into the causes of such behavior, simulations of two-dimensional (2D) and three-dimensional (3D) random networks were performed. Matlab was used to write the simulation codes. Mechanical response of fibres (affected by the fibre geometry) to the forces induced by drying or pressure is not considered directly in the simulations. However, 2D simulation models are a good representation of high press-drying conditions and high fibre flexibility, whereas 3D models are better predicting air-dried (low pressure) paper made from fibres of high rigidity. Geometric statistics of random networks explain some of the experimental observations. The statistical outputs of the 2D simulations were; network coverage (c), network thickness (τN), number of fibre crossings (NC) and the relative bonded area (RBA). In the case of 3D simulations, only τN was determined. ...