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Influence of adsorbed polyelectrolytes and adsorption conditions on creep properties of paper sheets made from unbleached kraft pulp
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2007 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [sv]

Papper uppvisar betydande tidsberoende mekaniska egenskaper som krypning och spänningsrelaxation. Det är känt att krypningen hos pappret påverkar till exempel en wellpapplådas förmåga att bära last under lång tid. En möjlighet att tillverka papper som kryper långsammare är därför önskvärd. Polyelektrolyter används ofta för att öka styrkan hos papper, och skulle kanske också kunna användas till att minska papprets krypning. Inverkan av polymera additiv på pappers krypegenskaper är emellertid knapphändigt beskrivet i litteraturen. Existerande studier har visat att polymera additiv inte påverkar krypningen hos starka papper och att fiberväggarna och fiber/fiber fogarna har fundamentalt olika betydelse för krypegenskaperna.

Avsikten med denna avhandling var att undersöka hur adsorberade polyelektrolyter påverkar krypegenskaperna hos pappret. Ett av huvudsyftena var att studera om adsorptionen av en katjonisk polyelektrolyt – polyallylamin – endast till fiberytan eller tvärs hela fiberväggen ger olika effekt på krypningen hos papper tillverkade av dessa fibrer.

En ny teknik där polyelektrolyten märks med en fluorescerande markör gör det möjligt att visualisera var i fibern de adsorberade molekylerna befinner sig. Resultaten visar att adsorption vid låg jonstyrka under kort tid bara ger adsorption till de yttre delarna av fiberväggen. Hög jonstyrka och lång adsorptions tid resulterar å andra sidan i adsorption tvärs hela fiberväggen. Med hjälp av denna teknik blev det också möjligt att klarlägga vilken inverkan polyelektrolytens läge i fiberväggen har på de slutgiltiga arkens mekaniska egenskaper.

Krypprovning av de tillverkade arken visade tydligt att polyallylamin som endast adsorberat till fibrernas yttre delar minskade krypningen vid både 50 % och 90 % relativ luftfuktighet. Den uppnådda effekten visade sig dock bero på vilken typ av fibrer arken tillverkades av. Adsorption av katjoniserad stärkelse till fibrernas yta gav ingen nämnvärd effekt på arkens krypegenskaper, detta trots att stärkelse gav lika hög arkstyrka som polyallylamin.

När polyallylamin adsorberades tvärs igenom fibrerväggen ökade krypningen vid 90 % relativ luftfuktighet väsentligt. Detta föreslås bero på att den adsorberade polyelektrolyten avsväller fibrerna vilket ger färre fiber/fiber kontakter och därmed en sämre fördelning av mekanisk last i arken. Det var emellertid inte möjligt att dra några definitiva slutsatser angående mekanismerna bakom de observerade skillnaderna i krypegenskaper.

Abstract [en]

Paper materials exhibit a significant time-dependent mechanical behaviour, such as creep and stress-relaxation. It is known that the creep of the paper affects the performance of corrugated boxes. The production of a paper having a lower creep rate is therefore desirable. Polyelectrolytes commonly used to increase the strength of paper could be an alternative for improving the creep properties. The influence of polymeric additives on the creep properties of paper is, however, poorly described in the literature. Published studies have shown that polymeric additives do not affect the creep behaviour of fully efficiently loaded paper sheets and that the fibre cell walls and the fibre/fibre joints have fundamentally different effects on the creep behaviour.

The aim of the present thesis was to examine the influence of adsorbed polyelectrolytes on the creep behaviour of paper sheets made from the modified fibres. One of the main objectives was to establish whether there is a difference in effect on creep properties between adsorbing a cationic polyelectrolyte – polyallylamine – to the fibre surfaces or throughout the fibre cell walls.

A technique which includes the labelling of polyelectrolytes with a fluorescent dye and microscopy of single fibres provided a visual record of the localisation of the adsorbed polyelectrolyte. This method showed that a low ionic strength and a short adsorption time resulted in adsorption of the polyelectrolyte only to the external parts of the fibres. A high ionic strength and a long adsorption time on the other hand, resulted in adsorption throughout the fibre walls. This made it possible to study the relationship between the mechanical properties of the sheets and the localisation of the adsorbed polyelectrolyte.

Creep testing of the sheets showed that the adsorption of polyallylamine to the exterior parts of fibres decreased the creep at both 50% and 90% RH. The effect depended, however, on the type of fibre used. Adsorption of cationic starch to the fibres gave no significant reduction in creep rate, despite the fact that starch and polyallylamine had similar effect on the paper strength.

When polyallylamine was adsorbed into the fibre cell walls, the creep at 90% RH increased. It is suggested that this was due to a deswelling of the fibres by the adsorbed polyelectrolyte, which resulted in fewer fibre/fibre contact points and hence a less efficient distribution of stresses in the sheet. It was not, however, possible to draw any definitive conclusions about the mechanisms behind the observed differences in creep behaviour.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , 38 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:70
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-4553OAI: oai:DiVA.org:kth-4553DiVA: diva2:12827
Presentation
2007-11-23, STFI-salen, KTH, Drottning Kristinas väg 61, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101105Available from: 2007-11-25 Created: 2007-11-25 Last updated: 2010-11-05Bibliographically approved
List of papers
1. Influence of polymeric additives on short-time creep of paper
Open this publication in new window or tab >>Influence of polymeric additives on short-time creep of paper
2007 (English)In: Nordic Pulp and Paper Research Journal, ISSN 0283-2631, Vol. 22, no 2, 217-227 p.Article in journal (Refereed) Published
Abstract [en]

Cationic polyallylamine (PAH) and cationic starch were adsorbed to unbleached softwood kraft pulp fibres in order to study the effect of these additives on the creep properties of the resulting paper sheets. For the fibres treated with PAH, the effect of molecular mass, adsorbed quantity and heat treatment of the prepared sheets at 160 degrees C was also studied. PAH was found to not influence the sheet density, whereas starch addition induced moderate densification. Both additives produced major improvements in dry strength measurements as stress and strain-at-break. PAH-treated sheets also showed a relatively small but statistically significant improvement in tensile stiffness, unlike starch-treated sheets in which the improvement was too small to be statistically significant. As expected. there was an inverse relationship between the adsorbed amount and the molecular mass of the PAH. However, for comparable adsorbed amounts, the molecular mass made no detectable difference to the mechanical properties. PAH was also labelled with fluorescein isothiocyanate and adsorbed to the pulp fibres. Examination of the fibres with a confocal scanning laser microscope revealed that the different molecular mass fractions adsorbed only to the exterior parts of the fibres (i.e. the molecules did not penetrate throughout the fibre wall). The creep behaviour of the paper sheets was evaluated under tensile loading using a specially designed testing apparatus, in both 50% RH and 90% RH at 23 degrees C. Strain was monitored as function of time and applied load. PAH was found to lower both instantaneous and time-dependent deformation during the relatively short measuring time of 100 seconds used in this work. Cationic starch had little or no effect on creep. Thus it is clearly demonstrated that PAH treatment of fibres results in sheets with a lower creeping tendency over short periods compared to sheets made with non-treated and starch-treated fibres.

Keyword
creep; mechanical properties; polyallylamine; cationic starch; confocal fluorescence microscopy
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7717 (URN)10.3183/NPPRJ-2007-22-02-p217-227 (DOI)000248057800011 ()2-s2.0-34547345846 (Scopus ID)
Note
QC 20100616Available from: 2007-11-25 Created: 2007-11-25 Last updated: 2010-08-11Bibliographically approved
2. Adsorption of polyallylamine to lignocellulosic fibres: effect of adsorption conditions on localisation of adsorbed polyelectrolyte and mechanical properties of resulting paper sheets
Open this publication in new window or tab >>Adsorption of polyallylamine to lignocellulosic fibres: effect of adsorption conditions on localisation of adsorbed polyelectrolyte and mechanical properties of resulting paper sheets
2009 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 16, no 1, 87-101 p.Article in journal (Refereed) Published
Abstract [en]

Cationic polyallylamine (PAH), was adsorbed onto lignocellulosic fibres, and a fluorescent label on the polyelectrolyte enabled its location to be shown by confocal fluorescence microscopy. The adsorption time and ionic strength were varied to study their effect on the localisation of the adsorbed PAH. The microscopy showed that a long adsorption time, 24 h, and a high ionic strength, 10(-1) M NaCl + 5 x 10(-3) M NaHCO3 or higher, resulted in the adsorption of polyallylamine throughout the fibre walls. Shorter adsorption times and/or lower ionic strength resulted in adsorption only to the fibre exterior. By preparing sheets from fibres with polyelectrolyte adsorbed either to the exterior parts or into the fibre cell wall and testing their mechanical behaviour, a link was established between the localisation of adsorbed polyelectrolyte and the mechanical properties. Adsorption to the fibre exterior led to an increase in tensile strength and strain at break. The creep deformation at 90%RH was also slightly reduced by the adsorption of low molecular weight PAH (15 kDa). When polyallylamine was adsorbed throughout the wall of the lignocellulosic fibres, the mechanical properties were not however improved and the creep deformation at 90%RH actually increased somewhat.

Place, publisher, year, edition, pages
Springer Netherlands, 2009
Keyword
Adsorption, Polyallylamine, Confocal fluorescence microscopy, Creep, Mechanical properties, RELATIVE HUMIDITY CHANGES, MOISTURE-CONTENT, CHARGE-DENSITY, CREEP, MODULUS
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12311 (URN)10.1007/s10570-008-9240-6 (DOI)000262086200009 ()2-s2.0-58149289529 (Scopus ID)
Note
QC20100616Available from: 2010-04-07 Created: 2010-04-07 Last updated: 2010-08-26Bibliographically approved

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