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The effects of cellulosic fiber charges on polyelectrolyte adsorption and fiber-fiber interactions
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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 [en]
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: urn:nbn:se:kth:diva-4158OAI: oai:DiVA.org:kth-4158DiVA: diva2:10965
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
List of papers
1. Polyelectrolyte adsorption on cellulosic fibers: I. High charge density polyelectrolytes
Open this publication in new window or tab >>Polyelectrolyte adsorption on cellulosic fibers: I. High charge density polyelectrolytes
(English)Manuscript (Other academic)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6285 (URN)
Note
QC 20100831Available from: 2006-10-25 Created: 2006-10-25 Last updated: 2010-08-31Bibliographically approved
2. On the indirect polyelectrolyte titration of cellulosic fibers. Conditions for charge stoichiometry and comparison with ESCA
Open this publication in new window or tab >>On the indirect polyelectrolyte titration of cellulosic fibers. Conditions for charge stoichiometry and comparison with ESCA
2006 (English)In: Langmuir, ISSN 0743-7463, Vol. 22, no 2, 824-830 p.Article in journal (Refereed) Published
Abstract [en]

The effect of electrolyte (NaHCO3) concentration on the adsorption of poly-DADMAC (poly-diallyldimethylammonium chloride) onto cellulosic fibers with different charge profiles was investigated. Surface carboxymethylated fibers were obtained by grafting carboxymethyl cellulose (CMC) onto the fiber surface and bulk carboxymethylated fibers were obtained by reacting the fibers with monochloroacetic acid. It was shown that nonionic interactions do not exist between cellulose and poly-DADMAC, rather electrostatic interactions govern the adsorption. Charge stoichiometry prevails under electrolyte-free conditions, whereas surface charge overcompensation occurs at higher electrolyte concentrations. It was shown that charge stoichiometry prevails if the thickness of the electric double layer kappa(-1) was larger than the mean distance between the charges on the fiber surface, as predicted by polyelectrolyte adsorption theories, taking lateral correlation effects into account. In a second set of experiments the ESCA technique served to independently calibrate the polyelectrolyte titrations for determining the surface charge of cellulosic fibers. Various molecular masses of poly-DADMAC were adsorbed to carboxymethylated fibers having different charge profiles. The adsorption of low M-w poly-DADMAC (7.0 x 10(3)), analyzed by polyelectrolyte titration, was about 10 times higher than that of the high M-w poly-DADMAC (9.2 x 10(5)). Despite the difference in accessibility of these two polyelectrolytes to the fiber cell wall, ESCA surface analysis showed, as expected, only slight differences between the two polyelectrolytes, This gives strong credibility to the idea that surface charge content of cellulosic fibers can be analyzed by means of adsorption of a high-molecular-mass cationic polymer, i.e., by polyelectrolyte titration.

Keyword
ray photoelectron-spectroscopy, cationic polyelectrolyte, kraft pulps, adsorption, surfaces, silica, polyacrylamide, electrolytes, multilayers; chloride
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6286 (URN)10.1021/la052217i (DOI)000234647200044 ()
Note
QC 20100831Available from: 2006-10-25 Created: 2006-10-25 Last updated: 2010-12-06Bibliographically approved
3. Indirect polyelectrolyte titration of cellulosic fibers. Surface and bulk charges of cellulosic fibers
Open this publication in new window or tab >>Indirect polyelectrolyte titration of cellulosic fibers. Surface and bulk charges of cellulosic fibers
2007 (English)In: Nordic Pulp and Paper Research Journal, ISSN 0283-2631, Vol. 22, no 1, 87-92 p.Article in journal (Refereed) Published
Abstract [en]

The polyelectrolyte titration technique was used to investigate the relationship between bulk and surface charges for various pulps and treatments. The total and surface charge content is different for hardwood and softwood pulps; however, the charge ratio is the same. Mechanical pulping gives a higher charge ratio than chemical pulping. Even though the amount of charges are changed during different bleaching sequences for chemical pulp, the charge ratio is not affected because the bleaching process is non-surface selective. fn order to change the charge ratio for chemical pulp, beating or surface carboxymethylation can be performed.

Keyword
polyelectrolyte titration, poly-DADMAC, charge ratio, carboxymethylation, carboxymethyl cellulose, PFI-beating
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6287 (URN)000248528700013 ()
Note
QC 20100831Available from: 2006-10-25 Created: 2006-10-25 Last updated: 2010-12-06Bibliographically approved
4. THe influence of colloidal interactions on fiber network strength
Open this publication in new window or tab >>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.

Keyword
CPM; Rheometer; Colloidal interactions, Charge density, Ionic strength, Counterion, Yield stress
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6288 (URN)10.1016/j.jcis.2006.08.066 (DOI)000245967700040 ()
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
5. The influence of mechanical surface linking and elastic fiber bending on fiber network strength
Open this publication in new window or tab >>The influence of mechanical surface linking and elastic fiber bending on fiber network strength
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 22, no 3, 371-375 p.Article in journal (Refereed) Published
Abstract [en]

The influence of mechanical surface linking and elastic fiber bending on the fiber network strength and elasticity (i.e. storage modulus and critical strain) was investigated for pulps with different lengths and stiffness, using a parallel plate rheometer. Results show that the storage modulus increases and critical strain decreases with increasing fiber length due to increased number of fiber contacts. Laboratory-cooked pulp has a higher storage modulus and a lower critical strain than commercial pulp because of straighter and stiffer fibers, which are more influenced by both mechanical surface linking and elastic fiber bending.

Keyword
mechanical surface linking, elastic fiber bending, floc, network strength, critical strain, rheometer
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6289 (URN)
Note
Uppdaterad från manuskript till artikel: 20100831 QC 20100831Available from: 2006-10-25 Created: 2006-10-25 Last updated: 2010-08-31Bibliographically approved

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