Rheology and flocculation of polymer-modified microfibrillated cellulose suspensions

Anni Karppinen

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    In this thesis, the rheology and flocculation of microfibrillated cellulose (MFC) suspensionswas modified using different cationic and anionic polymers and surface modification. For this purpose, MFC suspensions were studied simultaneously with a dynamic rotational rheometer and two imaging methods. The flocculation tendency of the suspensions was mainly evaluated using photographing through a transparent rheometer cup, and for some suspensions, optical coherence tomography (OCT), which gives information of the structure and flow of thesuspension inside the gap. Three cationic polymers were studied as flocculants: two small molecular weight, high charge density polymethacrylates and high molecular weight, low charge density cationic polyacrylamide (CPAM). Polymethacrylates changed the gel strength of the MFC suspensions but did not have a significant effect on the flocculation even at a high polymer concentration. Instead, CPAM affected the gel strength and flocculation drastically at low concentrations. CPAM formed bridges between the MFC fibers, thus making the flocs very strong. Carboxymethylcellulose (CMC), xanthan gum and three different anionic polyacrylamides (APAM) were tested as dispersants for the MFC suspensions. They all reduced the gel strength of the suspensions at optimized concentration and prevented shear-induced flocculation. The dispersion mechanism was probably a combination of increased viscosity of the suspending medium and the entrapment of the polymer chains between the approaching MFC fibers. CMC and xanthan gum were more effective at preventing shear-induced flocculation than APAMs. Oxidation of pulp with TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) before grindingit to MFC gives the fibers negative surface charge and enhances the fibrillation upon grinding. The dimensions of the resulting fibers were smaller than without any pretreatment. Its water suspension was transparent and therefore the flocculation of the fibers could not be directly studied with the imaging methods. The gel strength was close to that of native MFC, although TEMPO-oxidized fibers have a higher surface charge. The combination of rheometry and imaging proved to be a useful tool to study the effect of flocculants and deflocculants on the MFC suspensions. The direct observation of the suspension structure aided the interpretation of the rheological results. The measurement of the gel strength is an easy way to optimize the flocculant or deflocculant concentration for MFC suspensions in processing or different applications.
    Translated title of the contributionPolymeereillä muokattujen nanoselluloosasuspensioiden reologia ja flokkautuminen
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Seppälä, Jukka, Supervising Professor
    • Pietikäinen, Pirjo, Thesis Advisor
    Publisher
    Print ISBNs978-952-60-5834-4
    Electronic ISBNs978-952-60-5835-1
    Publication statusPublished - 2014
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • microfibrillated cellulose
    • rheology
    • image analysis
    • flocculation
    • polyelectrolyte
    • gel strength

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