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Influence of fibre modification on moisture sorption and the mechanical properties of paper
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fibre modification might be a way to improve the performance of paper, to increase its cost competiveness and enable new paper-based products to be developed. Therefore, the influence of fibre modification (with polyelectrolytes or by fibre cross-linking) on the mechanical properties of special importance for packaging paper grades was studied.

Creep deformation under varying humidity conditions (i.e. mechano-sorptive creep) is of outmost importance for the stacking life of paper-based boxes. The influence on creep behaviour of adsorbing polyallylamine (a cationic polyelectrolyte) to fibre surfaces or throughout the fibre walls was studied. Adsorption to fibre surfaces reduced the creep at constant humidity. The mechano-sorptive creep was not however influenced. The use of polyelectrolytes did not thus appear to be a feasible strategy for reducing mechano-sorptive creep.

Polyelectrolytes can however be efficient in improving other mechanical properties. The use of multilayers consisting of polyallylamine (PAH) and polyacrylic acid (PAA) was for example shown to significantly increase the strength of paper with much less densification and build-up of residual stress than is obtained by beating.

Cross-linking by oxidation with periodate radically decreased the mechano-sorptive creep of sheets made from the oxidised fibres. The basic mechanism behind the reduction in mechano-sorptive with cross-linking was found to be that the cross-linking slowed down the moisture sorption kinetics. A lower sorption rate led to smaller moisture content variations during the mechano-sorptive creep testing, and thus less sorption-induced swelling and stress concentrations at fibre/fibre joints. However, for cross-linking to be a practical way to reduce creep, the large problem of embrittlement must be solved.

The shear strength of couched sheets was measured to study the interaction between the sheets at different solids content. The shear strength was low until a solids content of approximately 60−70% was reached, which suggests that interactions important for the strength at complete dryness start to develop at this solids content. The effect of different fibre modifications and additives on how the fibres interact during the consolidation process is not always well understood. The method of shear strength determination could in the future be applied to modified fibres to hopefully increase the understanding of how different modifications influence the fibre/fibre interactions. A deeper understanding might reduce the time for the development of new and improved fibre modifications.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , 84 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:11
Identifiers
URN: urn:nbn:se:kth:diva-12318ISBN: 978-91-7415-606-5 (print)OAI: oai:DiVA.org:kth-12318DiVA: diva2:309618
Public defence
2010-04-23, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100616Available from: 2010-04-09 Created: 2010-04-07 Last updated: 2010-06-16Bibliographically 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: 2017-12-12Bibliographically approved
3. On The Mechanisms Of Mechano-Sorptive Creep Reduction By Chemical Cross-Linking
Open this publication in new window or tab >>On The Mechanisms Of Mechano-Sorptive Creep Reduction By Chemical Cross-Linking
2009 (English)In: Advances in Pulp and Paper Research, Oxford 2009: Transactions of the 14th fundamental research symposium / [ed] S.J. I'Anson, Lancashire, UK: The Pulp and Paper Fundamental Research Society , 2009, 1001-1017 p.Conference paper, Published paper (Refereed)
Abstract [en]

Despite the technical importance of mechano-sorptive creep in paper, the exact mechanism behind this phenomenon is still not fully understood. In this study it was shown that the mechano-sorptive creep of paper sheets can be significantly reduced by chemical cross-linking through periodate oxidation. The mechanism behind this reduction has been examined through creep measurements of both sheets and individual fibres. For sheets the creep acceleration due to varying humidity was significantly reduced by the chemical cross-linking. For single fibres, however, the creep acceleration was not affected by the chemical crosslinking. In fact the absolute creep rate for the periodate oxidised fibres were higher than that of the reference fibres. This clearly showed that the improvement in mechano-sorptive creep found on a sheet level does not originate from an improved creep resistance for individual fibres but rather from mechanisms operating at the fibre network level. Hygroexpansion and moisture sorption of the sheets during the humidity cycling used for creep testing have also been measured, and the results showed that both was reduced by the periodate oxidation. Reduced moisture sorptivity and hygroexpansion probably minimises stress concentrations at the fibre network level and thereby also the creep acceleration.

Place, publisher, year, edition, pages
Lancashire, UK: The Pulp and Paper Fundamental Research Society, 2009
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-12315 (URN)000281142700029 ()978-0-9545272-6-6 (ISBN)
Conference
14th Fundamental Research Symposium on Advances in Pulp and Paper Research Location: St Annes Coll, Oxford, ENGLAND Date: SEP 13-18, 2009
Note
QC 20100616Available from: 2010-04-07 Created: 2010-04-07 Last updated: 2012-03-22Bibliographically approved
4. The influence of periodate oxidation on the moisture sorptivity and dimensional stability of paper
Open this publication in new window or tab >>The influence of periodate oxidation on the moisture sorptivity and dimensional stability of paper
2008 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 15, no 6, 837-847 p.Article in journal (Refereed) Published
Abstract [en]

The hygroexpansion of paper was significantly reduced, up to 28% lower amplitude of change when the paper was subjected to a change in relative humidity from 20 to 85% RH, by oxidation of the fibre wall. Never-dried bleached kraft fibres were oxidised with sodium periodate, which specifically oxidises the C2-C3 bond of 1,4-glucans so that the cellulose is partly converted into dialdehyde cellulose. Since both the dry and wet strength of laboratory sheets were significantly improved, the dry tensile strength increased from 24 kNm/kg up to 66 kNm/kg and the relative wet tensile strength increased from 1.5% up to 40%, it is suggested that the aldehydes form hemiacetal linkages within the fibre wall during the consolidation and drying of the sheets. The mechanical, hygroexpansive and moisture sorptive properties of the sheets made from the oxidised fibres were studied. The results showed that the main reason for the reduced hygroexpansion was a decrease in moisture sorptivity, i.e. when the sheets made of fibres with different degrees of cross-linking were subjected to the same change in relative humidity, the more cross-linked fibres showed a smaller change in moisture content. It was also shown that the hygroexpansion coefficient, i.e. the moisture-normalised dimensional change, was not significantly changed by the periodate oxidation, i.e. indicating that there are no improvement in dimensional stability if the paper is subjected to a specific amount of water.

Keyword
Cross-linking, Dimensional stability, Hygroexpansion, Moisture adsorption, Periodate oxidation, RELATIVE HUMIDITY CHANGES, POROUS STRUCTURE, FIBER JOINT, CELLULOSE, STRENGTH, WATER, WOOD, MECHANISM, CREEP
Identifiers
urn:nbn:se:kth:diva-12169 (URN)10.1007/s10570-008-9243-3 (DOI)000260307000009 ()2-s2.0-54949090722 (Scopus ID)
Note
QC20100616Available from: 2010-03-23 Created: 2010-03-23 Last updated: 2017-12-12Bibliographically approved
5. Influence of beating and chemical additives on residual stresses in paper
Open this publication in new window or tab >>Influence of beating and chemical additives on residual stresses in paper
2011 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 26, no 4, 445-451 p.Article in journal (Refereed) Published
Abstract [en]

Residual stresses are the stresses remaining in a material when all external forces are removed. Residual stresses in paper can influence the converting and end-use performance. There are well-established methods for determining residual stresses in paper, and some knowledge exists of how to control and tailor the residual stresses. However, there is an increasing demand to be able to tailor paper grades with respect to their mechanical properties. Pulp fibres are commonly beaten to improve the mechanical performance, but beating also increases the sheet density, de-watering resistance, and residual stresses of the paper produced. This work examines whether beating and the addition of chemical additives, i.e., a single layer of poly(allylamine) or a multilayer of poly(allylamine) and poly(acrylic acid), exert different effects on the build-up of residual stresses in paper. Both beating the fibres and adding polyelectrolytes increased the in-plane strength, stiffness, and residual stresses of the paper sheets prepared. The fact that the residual stresses did not scale linearly with the stiffness of the prepared sheets suggests that both beating and polyelectrolyte addition made the fibre/fibre joints transfer load at a lower solids content, such that stresses were transferred between fibre layers in the sheet earlier in the drying process, thus increasing the residual stresses. The fact that the strength gain when building polyelectrolyte multilayers induced less residual stresses than when the strength was increased by beating indicates the possibilities for producing paper with high strength but less residual stress.

Keyword
Residual stress, Beating, Additives, Polyelectrolyte multilayers
National Category
Chemical Engineering Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-12316 (URN)000298868000012 ()2-s2.0-84855525449 (Scopus ID)
Note
Updated from manuscript to article in journal. QC 20120412Available from: 2010-04-07 Created: 2010-04-07 Last updated: 2017-12-12Bibliographically approved
6. Shear strength development between couched papers during drying
Open this publication in new window or tab >>Shear strength development between couched papers during drying
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

The out-of-plane properties (e.g., out-of-plane shear strength) of paper materials are very important for their performance during converting and end use. There is, however, a lack of published data on how shear strength develops throughout the stages of paper manufacturing. The present study investigates how the shear strength developed between couched sheets during drying in a Rapid-Köthen laboratory sheet drier. The shear strength of sheets was measured, starting from sheets with a solids content of approximately 35% all the way to fully dry sheets. Shear strength development was examined between both never-dried and rewetted sheets made of unbeaten and beaten pulp. The results indicate that the shear strength increased with increasing solids content at all solids contents investigated. The shear strength was low (<120 kPa) up to a solids content of approximately 60–70%, after which it increased rapidly with increasing solids content, suggesting that interactions important for the shear strength of dry paper start to develop at this particular dry content.

National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-12317 (URN)
Note

QC20100616

Available from: 2010-04-07 Created: 2010-04-07 Last updated: 2016-06-08Bibliographically approved

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