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THe influence of colloidal interactions on fiber network strength
2007 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 309, no 2, 511-517 p.Article in journal (Refereed) Published
Abstract [en]

Various forces govern the fiber–fiber interaction in a flowing suspension, causing fibers to create flocs. The aim with this investigation was to examine the influence of colloidal interactions on the fiber network strength by varying surface charge density, electrolyte concentration, and type of counterion. This was accomplished by comparing surface force measurements, utilizing colloidal probe microscopy (CPM), with the apparent yield stress, using a parallel plate rheometer. Results show that by increasing the charge density by grafting carboxymethyl cellulose (CMC) to the surface, a large electrosteric repulsion is created, which gives weaker network strength. Increasing the electrolyte concentration decreases the repulsion. The network strength was, however, not affected by electrolyte concentration for untreated fibers whereas a high electrolyte concentration increased the yield stress for CMC-treated fibers. The change of counterions affect the repulsion, causing a change in network strength due to differences in the surface swelling of cellulose.

Place, publisher, year, edition, pages
2007. Vol. 309, no 2, 511-517 p.
Keyword [en]
CPM; Rheometer; Colloidal interactions, Charge density, Ionic strength, Counterion, Yield stress
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-6288DOI: 10.1016/j.jcis.2006.08.066ISI: 000245967700040OAI: oai:DiVA.org:kth-6288DiVA: diva2:10963
Note

Tidigare titel: Influence of colloidal interactions on fiber network strength QC 20100831

Available from: 2006-10-25 Created: 2006-10-25 Last updated: 2016-12-09Bibliographically approved
In thesis
1. The effects of cellulosic fiber charges on polyelectrolyte adsorption and fiber-fiber interactions
Open this publication in new window or tab >>The effects of cellulosic fiber charges on polyelectrolyte adsorption and fiber-fiber interactions
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The surface charges of cellulosic fibers contribute to several papermaking operations that influence the manufacture and final properties of paper. This thesis investigates the effect of the surface charges on wet-end chemistry, e.g. through the interaction of cationic polyelectrolytes with the fiber surface charges, and on the network strength of pulp suspensions. The polyelectrolyte titration method was used to investigate the interaction of the fiber charges with cationic polyelectrolytes. Techniques were developed to fluorescent label the adsorbing cationic polyelectrolyte in order to visualize the adsorption behavior. Fluorescent confocal laser scanning microscopy (CLSM) was used to determine the extent to which the cationic polyelectrolyte adsorbs into the porous fiber wall. It was shown that the polyelectrolyte charge density limits the adsorption to the surface under electrolyte-free conditions. Adsorption into the fiber wall only occurs for two conditions: 1) if the molecular mass is sufficiently low or 2) the electrolyte concentration is high enough to screen the charges along the polyelectrolyte backbone but not the interactions between the polyelectrolyte and the fiber charges.

Aside from the polyelectrolyte properties, the fiber charge density contributes to the adsorption behavior of cationic polyelectrolytes. The fiber charge profile was altered by bulk and surface carboxymethylation. The electrolyte concentration at which a deviation from 1:1 stoichiometry occurs was shown to be dependent on the amount of surface charges, such that the deviation in stoichiometry occurs at a higher electrolyte concentration for pulps having a higher surface charge. A hypothesis was developed to test the conditions at which the deviation in adsorption stoichiometry occurs, which was defined as the critical electrolyte concentration (CEC). It was found that the CEC corresponded to the electrolyte concentration at which the distance between the fiber charges was on the order of the Debye length. Electron spectroscopy for chemical analysis (ESCA) was used as an independent calibration procedure to validate for which a 1:1 stoichiometry occurs. The analysis with ESCA agreed well with the polyelectrolyte titration method for measurement of fiber surface charges. When measured under appropriate conditions, i.e. electrolyte concentration and molecular properties, the fiber surface charge can accurately be measured by the polyelectrolyte titration method.

The charge profiles of various pulp types and treatments were also examined. Having been established as a valid technique, the polyelectrolyte titration method was again used to measure the surface charge while conductometric titration was used to measure the total charge content. The amount of bulk and surface charges vary depending on the pulping method and type of wood, although the ratio between the bulk and surface charge (i.e. the charge ratio) is similar for chemical pulps. The mechanical pulp has a higher charge ratio because it contains more fines material than chemical pulp. Bleaching of the chemical pulp decreases the amount of bulk and surface charges, although the charge ratio remains essentially constant. However, methods such as beating or carboxymethyl cellulose (CMC) grafting are available to increase the charge ratio.

The effect of the charge profile on fiber-fiber interactions was studied on both a microscopic and macroscopic level. Colloidal probe microscopy (CPM) was used to investigate the microscopic interactions between two cellulose surfaces. Cellulose surfaces, prepared by spin-coating a dissolving pulp onto silica, were used to model the fiber surface, which is too rough for surface force measurements. The charge density of the model surface was increased by CMC grafting. Results showed that increasing the surface charge density created large electrosteric repulsions, due to CMC the chains protruding out from the surface. These interactions on the microscopic scale affect the fiber network strength, which was measured with a parallel plate rheometer. When the repulsion is increased between the fibers, caused by the increase in the surface charge, fiber flocs break apart more easily due to a reduced friction between the fiber surfaces.

The forces acting on the fiber network can also be mechanical in origin. The fiber length and flexibility were altered in order to study the influence of mechanical surface linking and elastic fiber bending on the fiber network strength. Using the storage modulus (G’0) as a measure of fiber network strength, longer fibers were found to create a stronger network due to an increased amount of fiber contacts. Flexible fibers have a lower network strength than stiff fibers because the fibers come to rest in a less strained position such that the the influence of elastic fiber bending on the fiber network strength is predominant.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 53 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:30
Keyword
adsorption, electrolyte, polyelectrolyte, polyelectrolyte titration, stoichiometry, fluorescence, CLSM, carboxymethylation, ESCA, charge ration, colloidal interactions, mechanical forces, network strength, floc, CPM, rehometer
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-4158 (URN)
Public defence
2006-11-10, STFI-salen, KTH, Drottning Kristinas väg 61, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100831Available from: 2006-10-25 Created: 2006-10-25 Last updated: 2010-08-31Bibliographically approved

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