Evaluation of Furnishes for Tissue Manufacturing

Water removal on a tissue machine becomes progressively more difficult and expensive in each successive zone. Since a big part of the cost is allocated to the drying section, improved water removal in the wet end may lead to huge savings in the manufacturing process. This can be accomplished by selecting proper raw materials and optimizing the treatment of the fibres in the furnish. The aim of the work described in this thesis was to investigate the influence of three particular furnish properties on dewatering of low grammage papers in the forming and press section; fibre species, beating and additives. The focus was to evaluate how the solids content varies as these furnish properties are changed, but also how the quality of the end product is affected. Water removal during suction is affected by the choice of pulp which can be explained by structural differences in the networks caused by differences in the morphology of the fibres. The total area of straight pores between the fibres is much higher for softwood pulps compared to hardwood pulps which will facilitate transport of both water and air through the sheet. Beating has a negative effect on the solids content reached in vacuum dewatering which can be coupled to internal and external fibrillation of the fibres. Water removal during pressing is affected by the choice of pulp controlled by the pore structure of the fibres and the ability to sorb water. More available water before pressing lead to that more water can be removed. Beating mainly delaminates macropores with small effects on micropores. Both water between the fibres and water in macropores is removed during wet pressing. The dryness after wet pressing is increased by addition of a wet strength agent (PAE) to the stock, probably due to crosslinking in the fibre wall. PAE-resins decrease the volume of both micro- and macropores which will leave less water deposited in the fibre wall. Tensile strength is increased with a wet strength agent and further increased by addition of a flocculant and a micropolymer to the stock. A lower absorption capacity is achieved with addition of PAE-resins due to formation of covalent bonds in the fibre wall