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A novel approach to study the hydroexpansion mechanisms of paper using spray technique
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.ORCID iD: 0000-0002-7410-0333
Innventia AB, Stockholm.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology.ORCID iD: 0000-0001-8622-0386
2009 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 24, no 4, 371-380 p.Article in journal (Refereed) Published
Abstract [en]

A new method has been developed to measure the dimensional stability of printing paper by measuring the impact of liquid water on the in-plane dimensional change, i.e. the hydroexpansion, without any simultaneous mechanical interference that can occur when water is pressed into the sheet. This was achieved by using a specially developed spray technique and using electronic speckle photography to continuously measure the dimensional change as water is applied.

The in-plane expansion for a given change in moisture content was found to be lower in the case of hydroexpansion than for earlier reported hygroexpansion. After the initial expansion following the water application, it was found that sheets rapidly start to contract again already 10-20 seconds after being wetted, i.e. despite still having a fairly constant and significantly higher moisture content than the initial moisture content before water application. These effects suggest that there are different mechanisms behind hydroexpansion than hygroexpansion of paper, and that hygroexpansion measurements should be extrapolated with caution when evaluating papers with respect to printability.

Place, publisher, year, edition, pages
2009. Vol. 24, no 4, 371-380 p.
Keyword [en]
Dimensional stability, Hydroexpansion, Electronic speckle photography, Controlled spraying, RELATIVE HUMIDITY CHANGES, CREEP
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-12170ISI: 000273724200001Scopus ID: 2-s2.0-76349113333OAI: oai:DiVA.org:kth-12170DiVA: diva2:305243
Note
QC20100709Available from: 2010-03-23 Created: 2010-03-23 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Hygro- and hydroexpansion of paper: Influence of fibre-joint formation and fibre sorptivity
Open this publication in new window or tab >>Hygro- and hydroexpansion of paper: Influence of fibre-joint formation and fibre sorptivity
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Paper is a versatile, cheap and environment-friendly material. Nevertheless, there are several factors limiting its usefulness, and one of the major issues is that cellulosic and ligno-cellulosic fibres spontaneously sorb water. At the same time, the water uptake changes the dimensions of the paper. This phenomenon is usually referred to as a lack of dimensional stability and is often evident as misregister during multicolour printing, or curl, cockle and wavy edges during printing, copying, and storage, or, in a widerperspective, as a shortened lifetime of boxes during storage due to mechano-sorptivecreep.

This thesis aims to improve the understanding of the mechanisms behind the dimensional(in)stability of paper. It looks beyond finding the best starting material and explores what can be done chemically to further improve the dimensional stability. Furthermore, it compares traditional hygroexpansion measurements, where the dimensional change is measured as a function of atmospheric relative humidity, and dimensional changes caused by liquid water, referred to here as hydroexpansion.

The main parameters which have been studied are the ability of the fibres to join together and their ability to sorb water. In other words, how the degree of molecular contact within the fibre joints, as well as how the fibres are dried, affect the dimensional stability of the final paper, and whether it is possible to reduce the sorptivity of the fibres, and thus their ability to expand, by chemically cross-linking the fibre-wall.

It was found that the degree of fibre-fibre contact, modified by drying or adsorption of polyelectrolyte multilayers, had little influence on the hygroexpansion or on the hydroexpansion if the sheets were dried under restraint, whereas freely dried sheets with a reduced degree of contact showed a slightly better dimensional stability, at least during hygroexpansion. What, however, had a positive effect on both hygro- and hydroexpansion was the fibre-wall cross-linking. In this work, cross-linking was achieved by oxidising the cellulose to dialdehydecellulose which can form cross-links with adjacent cellulose molecules, and thus reduce the rate of water diffusion into the fibre and hence the uptake of water. In the case of the most oxidised and cross-linked fibres, the diffusion coefficient was found to be 2–3 times lower than that of the non-oxidisedreference. The effect of the cross-linking was, however, the most prominent the first time the moisture content of the paper was increased since cycled samples no longer show this lower adsorption rate. It is suggested that this is due to the formation of a new pore system when the moisture content is increased, and the slow creation of this pore system reduces the moisture uptake of the sample.

If hygroexpansion is compared with hydroexpansion, it is evident that a given change in moisture content does not correspond to the same absolute expansion, the maximum hydroexpansion being lower by a factor of 2–3 than the hygroexpansion of the same paper. This is probably because the applied liquid water is never equally distributed in the fibre network before it evaporates. Another effect of the more dynamic absorption of liquid water and the subsequent hydroexpansion is that at least in non-restrained samples there is first a rapid initial expansion which is followed 5 to 15 seconds later by a rapid in-plane contraction. It is suggested that this contraction is due to a combination of the release of dried-in strains, drying, and an increase in surface roughness.

Abstract [sv]

Papper är ett mångsidigt, billigt och miljövänlig material. Det finns dock ett antal faktorer som begränsar papperets användbarhet, och en av de mest problematiska är att cellulosa- och lignocellulosafibrer spontant tar upp vatten. Samtidigt som papperet tar upp vatten ändras dess dimensioner. Detta fenomen brukar kallas bristande dimensionsstabilitet och uppträder vanligen i form av registerfel vid flerfärgtryck eller som krökning ut ur planet, buckling eller vågiga kanter vid tryckning, kopiering och lagring eller, med en bredare definition, också som en förkortad livslängd hos lådor på grund av mekanosorptiv krypning.

Den här avhandlingen syftar till att öka förståelsen för mekanismerna bakom pappers dimensions(in)stabilitet och söker förbättringar bortom det bästa utgångsmaterialet genom att kemiskt försöka förbättra godtyckligt startmaterial. Vidare har syftet varit att jämföra resultaten från traditionella hygroexpansionsmätningar, där papperets dimensionsförändring mäts efterhand som fuktinnehållet ändras vid en given förändring i relativ luftfuktighet, med de dimensionsförändringar som uppstår då papperet utsätts för flytande vatten, som i den här avhandlingen benämns hydroexpansion.

De faktorer som huvudsakligen har studerats, är fibrernas förmåga att foga sig samman med varandra i pappersnätverket och deras förmåga att ta upp vatten. Med andra ord, hur påverkas dimensionsstabiliteten av en hög respektive låg grad av fiber-fiberkontakt samt fri respektive inspänd torkning och huruvida det går att minska fibrernas förmåga att ta upp vatten, och således deras förmåga att expandera, genom att kemiskt tvärbinda fiberväggen.

Det konstaterades att graden av fiber-fiberkontakt, förändrad genom torkning eller adsorption av polyelektrolytmultilager, hade ringa inflytande på såväl hygro- som hydroexpansionen om papperet hade torkats inspänt, medan en något ökad dimensionsstabilitet kunde uppnås för fritt torkade ark vid en minskad kontaktgrad. Det som däremot genomgående påverkade båda expansionssätten positivt var fiberväggstvärbindningen, som i det här arbetet åstadkoms genom att oxidera cellulosan till dialdehydcellulosa, vilken i sin tur kan bilda kovalenta bindningar med angränsande cellulosamolekyler. Detta minskade diffusionshastigheten, mätt som en minskning i effektiv diffusionsko efficient, med en faktor 2–3 jämfört med den obehandlade referensen. Det visade sig dock att tvärbindningen hade bäst effekt första gången fukthalten i papperet ökades, då papper som utsatts för fuktcykling inte längre uppvisade samma reducerade adsorptionshastighet. Detta misstänks bero på att det bildas ett nytt porsystem när fuktinne hålleti fibrerna ökar och att det är den långsamma bildningen av detta som är orsaken till det långsamma fuktupptaget. När expansion orsakad en av förändring i relativ luftfuktighet jämfördes med expansion orsakad av flytande vatten, det vill säga vid jämförelse mellan hygro- och hydroexpansion, visade det sig att vid en given förändring i ett givet pappers fuktinnehåll, orsakade flytande vatten en 2–3 gånger lägre expansion. Detta är troligen en effekt av att det pålagda vattnet inte hinner fördelas helt jämnt i fibernätverket innan det börjardunsta. En annan effekt av den snabbare och mer dynamiska absorptionen av flytande vatten är att efter en snabb initial expansion börjar papperet kontrahera efter 5 till 15 sekunder. Detta föreslås bero på en kombination av frigörelse av intorkad töjning, torkning och en ökad ytråhet.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xiii, 68 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:8
Identifiers
urn:nbn:se:kth:diva-12171 (URN)978-91-7415-587-7 (ISBN)
Public defence
2010-04-09, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100709Available from: 2010-03-23 Created: 2010-03-23 Last updated: 2013-10-29Bibliographically approved

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