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The thesis studies the relationship between inter-fiber forces present within a cellulose fiber web under varying external conditions. It particularly concentrates on the degree of fiber to fiber bonding and fiber entanglement as a function of moisture content. Finite element analysis of the fiber bonding is used in conjunction with the experimental results to analyze and explain the inter-fiber behaviors taking place within a sheet.

The paper industry utilizes fillers either to reduce the cost or to provide desired functional or end-use properties of paper products. However, there are disadvantages associated with higher filler loadings beyond a certain level, which reduces paper strength. The present study focused on improving the physical property of filled papers. Three methods of structuring fillers were designed; precipitation with starch, complexation with starch and fatty acid, and regeneration with cellulose. Because cellulose and starch have hydroxyl groups on the chemical structure, the hydrogen bonding between fillers and wood fibers is assumed to be occurred by structuring fillers. For starch application, we used two different approaches; salt precipitation and fatty acid complexation. The cooked starch can be precipitated by certain salt solutions such as (NH4)2SO4. Also, the cooked starch can be complexed with fatty acid to produce an insoluble crystalline structure. When starch composites with clay made by both methods were put into the furnish as fillers, dramatic strength improvement was achieved such as 100-200% gains in tensile strength. This is due to the strong bonding between clay fillers and wood fibers, which is determined by Z-directional tensile strength. One of advantages is that using the starch-fatty acid complex has an inherent water repellent property, sizing effect. For cellulose as a bonding material, N-methylmorpholine-N-oxide was used as a solvent to dissolve the cellulose. The advantage of using this method is that we can use the low grade cellulose. The physical properties of the cellulose coated clay handsheets were significantly improved, but optical properties such as brightness and opacity were inferior to the hadnsheets filled with starch-clay composites due to relatively large particle size. In order to model the strength improvement by the composite filler, BDT theory, which is a modified Pages Equation, was used. After calculating the factors such as surface area and specific bond strength, the model matched well with the experimental results. Using this model, the tensile strength improvement could be predicted in terms of the change of bond strength and composite size.