Browsing by Author "Paltakari, Jouni, Prof., Aalto University, Department of Forest Products Technology, Finland"

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    Brightness reduction of peroxide-bleached mechanical pulp and fillers in the wet end of a paper machine
    (Aalto University, 2013) Varhimo, Pekka; Konn, Jonas, Principal Research Scientist, Kemira, Finland; Puunjalostustekniikan laitos; Department of Forest Products Technology; Paper Converting and Packaging; Kemian tekniikan korkeakoulu; School of Chemical Technology; Paltakari, Jouni, Prof., Aalto University, Department of Forest Products Technology, Finland
    This study examined the brightness reduction of peroxide-bleached mechanical pulp and paper fillers in the wet end of a paper machine. The effect of different process variables was studied on a laboratory scale using a device that simulates the short circulation of a paper machine. The fines-containing wire water was circulated in the device and wire water samples were taken during this circulation. Fines sheets were prepared from the samples and their optical properties were measured to explore the darkening of the fines fraction during the circulation. In addition, the darkening of the fiber fraction and paper fillers was studied. The results indicated that the brightness reduction of the peroxide-bleached mechanical pulp in the wet end of a paper machine was partly due to the formation of light-absorbing chromophores in the fibers and fines and partly due to the adsorption of light-absorbing substances to the surfaces of fibers and fines from the water phase. Paper fillers do not contain lignin or other structures with a high darkening tendency and their brightness reduction was caused by the adsorption of light-absorbing dissolved and colloidal substances (DCS). Pulp washing was found to be an efficient way to reduce the tendency for darkening of the pulp itself and also the fillers, because it removes light-absorbing substances from the pulp suspension. In addition to the wood-based DCS, the most important factors affecting the brightness reduction of peroxide-bleached mechanical pulp were the residence time of the pulp in the wet end and the iron concentration of the white water. Other factors decreasing the brightness notably, but to a lesser extent, were high shear rate generated to the pulp, carbon dioxide gas, chloride ions, and calcium ions. No change was noted in the light absorption of the peroxide-bleached mechanical pulp between acidic and neutral test conditions. Thus, from brightness point of view, paper grades from peroxide bleached mechanical pulp can be manufactured equally well at neutral or at acidic process conditions. The Precipitated Calcium Carbonate (PCC) fillers were found to have somewhat higher tendency for darkening than the Precipitated Calcium Sulfate (PCS) filler.
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    Micro and nanofibrillated cellulose (MNFC) as additive in complex suspensions: influence on rheology and dewatering
    (Aalto University, 2014) Dimic-Misic, Katarina; Gane, Patrick, Prof., Aalto University, Department of Forest Products Technology, Finland; Puunjalostustekniikan laitos; Department of Forest Products Technology; Natural Fiber Products; Kemian tekniikan korkeakoulu; School of Chemical Technology; Paltakari, Jouni, Prof., Aalto University, Department of Forest Products Technology, Finland
    The traditional forest products, such as paper, packaging and viscose products, have their well-established place, but further efforts urgently need to be made to meet growing ecological demands, increasing economic pressure and to develop new technologies for utilisation of high performance materials. Nanocellulose applied in paper making suspensions and as water retention control aids may introduce additional strength properties in traditional products, and when considered for coatings may deliver oil and gas barrier properties as well as targeted liquid interactions based on surface energy criteria and designed pore and gel network structures. The properties increase the potential for their use in a broad range of novel products. The processes, firstly by which nanocellulose is both created, as part of cellulose structures in nature, and subsequently produced determine their aqueous suspension rheology and dewatering behaviour, and thus, by using knowledge of their rheological behaviour when utilised in complex suspensions, will support the creation of new controls in water-based production processes. The use of rheology for the characterisation of nanocellulose suspensions and their applications in a range of industries as diverse as the oil and papermaking industries has been the subject of numerous studies in recent years. Although many studies have been conducted relating to papermaking furnishes and their water suspensions while including nanocellulose containing materials, these rheological investigations were conducted independently from conditions of dynamic dewatering. For the first time, this work sets out to observe and try to overcome experimental difficulties related to rheometry and dynamic dewatering of high consistency viscoelastic gel-like nanocellulose suspensions using a variety of experimental techniques and methods. Once reliable rheological measurements were designed and the methodology established, the work goes on to analyse the possible structures attained by nanocellulose containing suspensions in combination with macroscopic fibres, pigments and fibril dispersing polymers, which traditionally are used as pigment flocculant water retention aids. These analyses are used to support and challenge the basic hypotheses of the thesis in relation to the impact these structures will have on properties such as process applicability, phase separation, substrate coverage, coating and material uniformity.