Improvement of wet and dry web properties in papermaking by controlling water and fiber quality

Hanna Lindqvist

    Research output: Types of ThesisDoctoral Thesis

    Abstract

    Today's need for faster and more efficient processes in papermaking alsogive rise to greater demands on the mechanical properties of the paper web.Web breaks occur not only due to a low tensile strength of the wet paperweb; the reason can also be an insufficient web tension. Most problems withthe runnability of the paper machine occur during the wet pressing and/orin the beginning of dryer section when the dry content of the paper is in therange 30-70%. Web tension is created by a speed difference in the webtransfer between the press and dryer section. The web tension is notconstant after the straining in the open draw, but drops 20-60% due torelaxation. Most of this relaxation occurs during the first 0.5 s afterstraining.In this work, the effects of different parameters on dewatering, wet webstrength and relaxation, as well as on the mechanical and surface propertiesof the final product were investigated. A long term aim is to improve therunnability of the paper machine and paper quality by controlling andunderstanding the properties of the process water, wood fiber, and fibernetwork.By using a non-ionic surfactant at concentrations below the critical micelleconcentration (cmc), the dewatering was more efficient thereby increasingthe dry content after dewatering and wet pressing and thus increasing thetensile strength of wet paper sheets. For dry papers, the density and thetensile strength increased. At concentrations above cmc, the mechanicalproperties for both the wet and the dry sheets deteriorated, one reasoncould be that the surfactant hindered the formation of hydrogen bonding. Alower pH during papermaking, resulted in a lower dry content, which mightbe due to swelling of fibers. By using Na+ instead of Ca2+, the wet tensilestrength decreased, perhaps because the coefficient of friction wasdecreased allowing the fibers to slide over each other.The properties of the fibers were changed by using either gentle or harshrefining. The effect of fines was studied by adding fines to the original pulpor by removing the fines from refined pulp. The gently refined pulpgenerated fines without a decrease in the fiber length. The fibers were morestretched out and flexible after refining, which resulted in increased wet Today's need for faster and more efficient processes in papermaking alsogive rise to greater demands on the mechanical properties of the paper web.Web breaks occur not only due to a low tensile strength of the wet paperweb; the reason can also be an insufficient web tension. Most problems withthe runnability of the paper machine occur during the wet pressing and/orin the beginning of dryer section when the dry content of the paper is in therange 30-70%. Web tension is created by a speed difference in the webtransfer between the press and dryer section. The web tension is notconstant after the straining in the open draw, but drops 20-60% due torelaxation. Most of this relaxation occurs during the first 0.5 s afterstraining.In this work, the effects of different parameters on dewatering, wet webstrength and relaxation, as well as on the mechanical and surface propertiesof the final product were investigated. A long term aim is to improve therunnability of the paper machine and paper quality by controlling andunderstanding the properties of the process water, wood fiber, and fibernetwork.By using a non-ionic surfactant at concentrations below the critical micelleconcentration (cmc), the dewatering was more efficient thereby increasingthe dry content after dewatering and wet pressing and thus increasing thetensile strength of wet paper sheets. For dry papers, the density and thetensile strength increased. At concentrations above cmc, the mechanicalproperties for both the wet and the dry sheets deteriorated, one reasoncould be that the surfactant hindered the formation of hydrogen bonding. Alower pH during papermaking, resulted in a lower dry content, which mightbe due to swelling of fibers. By using Na+ instead of Ca2+, the wet tensilestrength decreased, perhaps because the coefficient of friction wasdecreased allowing the fibers to slide over each other.The properties of the fibers were changed by using either gentle or harshrefining. The effect of fines was studied by adding fines to the original pulpor by removing the fines from refined pulp. The gently refined pulpgenerated fines without a decrease in the fiber length. The fibers were morestretched out and flexible after refining, which resulted in increased wet tensile strength and residual tension. The fines of fibril-type promoted theformation of fiber-fiber bonds and increased the bonded area. The fineswere important for the wet tensile strength while the fiber properties, i.e.the internal/external fibrillation, were critical for the residual tension.Harsh refining resulted in shortening of the fiber length. The formed wetweb composed of damaged fibers contained more water, which resulted in adecreased dry content after wet pressing. When the non-ionic surfactantwas used together with the harshly refined pulp, addition of surfactant gavea shorter dewatering time. When the surfactant was added together withfines to original pulp, the wet tensile strength decreased slightly. The mostlikely reason was adsorption of the surfactant to the surfaces of the finesand fibers which resulted in the particles and fibers sliding over each othermore easily.Native galactoglucomannans (GGMs) were cationized (C-GGM),carboxymethylated (CM-GGM), and iminated (A-GGM). Native GGMsdispersed the fibrils on the fiber surface when added to the pulp. Thesemore outstretched fibrils were able to interact with fibrils on other fibers,thereby increasing the wet tensile strength and the elastic modulus. C-GGMacted most likely through an electrostatic mechanism, where the cationicchain was adsorbed to negatively charged surfaces. CM-GGM was assumedto form an ionic bond to metal ions, i.e. magnesium and calcium, present inwood. A-GGM acted probably through the hydrophobic hydrocarbon chainattached to the reducing end of the polysaccharide. The tail was orientedtowards hydrophobic particles; the paper containing A-GGM had the mosthydrophilic surface.When native GGM, C-GGM and A-GGM were sprayed on freshly formedpapers and the surface was analyzed, characteristic signals were found foreach of the derivatives using Time-of-Flight Secondary Ion MassSpectrometry (ToF-SIMS). C-GGM penetrated though the paper, as signalswere also detected on the unsprayed side of the paper. A-GGM formed afilm on top of the fibers, and by using Fourier Transform InfraredSpectroscopy (FTIR) it was concluded that the hydrophobic tail wasoriented close to the paper surface but not at the outmost surface.The results obtained in this thesis form a valuable knowledge base, whichcan be used for controlling runnability of the paper machine and the paperquality. By using a readily available hemicelluloses and modifying themusing simple methods, additives with different properties and effects onpapermaking can be obtained.
    Original languageUndefined/Unknown
    Publisher
    Print ISBNs978-952-12-2948-0
    Publication statusPublished - 2013
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • Papermaking
    • initial wet web strength
    • Dewatering
    • Non-ionic surfactant
    • Refining
    • Spruce galactoglucomannan

    Cite this