| 1751. |
- Larsson, Per A., et al.
(författare)
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Diffusion-induced dimensional changes in papers and fibrillar films : influence of hydrophobicity and fibre-wall cross-linking
- 2010
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 17:5, s. 891-901
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Tidskriftsartikel (refereegranskat)abstract
- The initial dimensional stability of paper measured as hydroexpansion, i.e. when paper is exposed to liquid water, has been considerably improved by combining a periodate-oxidation-induced cross-linking of the fibre wall with the subsequent adsorption of a hydrophobic polyelectrolyte multilayer consisting of three layers of poly(allylamine hydrochloride) and two layers of poly(acrylic acid). This reduced the rate of diffusion of water into the fibre wall at the same time as the diffusion distance was increased, i.e. the water has to diffuse all the way from the top of the sheet and not only from the individual fibre surfaces since capillary absorption was prevented. However, as a consequence, the hydrophobic sheets present a greater expansion maximum before contraction. It is suggested that this may be due to a higher moisture content in the top fibre layers of the hydrophobically modified papers than in the hydrophilic sheets, since all the water is concentrated to the top fibre layers of the hydrophobic papers. Sheets made from bleached kraft pulp or thermo-mechanical pulp as well as model sheets made from microfibrillated cellulose (MFC) were studied. The MFC-sheets were intended as a model of the fibre wall, i.e. a sheet without any fibre joints. The behaviour of the MFC-sheets was similar to that of ordinary sheets when subjected to water, which indicates that the properties of the fibre joints do not affect the hydroexpansion to any great content and that the expansion of the paper is directly linked to the expansion of the fibre wall.
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| 1752. |
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| 1753. |
- Larsson, Per A., 1980-
(författare)
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Hygro- and hydroexpansion of paper : Influence of fibre-joint formation and fibre sorptivity
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 1754. |
- Larsson, Per A., 1980-, et al.
(författare)
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Improved barrier films of cross-linked cellulose nanofibrils: a microscopy study
- 2014
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Ingår i: Green materials. - : Thomas Telford Ltd.. - 2049-1220 .- 2049-1239. ; 2:4, s. 163-168
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Tidskriftsartikel (refereegranskat)abstract
- It is highly desirable to replace gas barriers of aluminium and non-renewable plastics in order to lower our ecological footprint. One interesting candidate is films made from cellulose nanofibrils (CNFs), which after cross-linking have been shown to have competitive barrier properties even at a high relative humidity (80% RH). This work presents studies at even higher relative humidity (90% RH) and microscopic studies of what happens when unmodified and cross-linked CNF films are exposed to water. The microscopy techniques used were scanning electron microscopy of dry and wet cross-sections of films after freeze-drying and atomic force microscopy in the dry state and in the wet state shortly after wetting. Both techniques clearly revealed that the cross-links prevent the CNFs from separating from each other and hence prevent the films from swelling, so that the free-volume-sensitive gas permeability is maintained at a low level.
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| 1755. |
- Larsson, Per A., 1980-, et al.
(författare)
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Influence of fibre-fibre joint properties on the dimensional stability of paper
- 2008
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 15:4, s. 515-525
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Tidskriftsartikel (refereegranskat)abstract
- Measurements have been performed to clarify the connection between fibre-fibre joint properties and dimensional stability using laboratory sheets prepared from never-dried fibres, from heavily hornified fibres having a low molecular contact area between the fibres, and from both hornified and never-dried fibres treated with a polyelectrolyte multilayer (PEM) technique to increase the molecular contact area in the fibre-fibre joint. The influence of the drying mode, i.e. whether the sheets are dried freely or under restraint, was also evaluated. The results showed that neither paper strength nor fibre-fibre joint contact area had any significant influence on the dimensional stability of sheets dried under restraint. On the other hand, when the sheets were dried freely, the PEM-treated sheets expanded to the same extent as, or to an even greater extent than the non-PEM-treated sheets, even though they adsorbed less water for a given change in relative humidity. There was also a correlation between drying shrinkage and dimensional stability, where greater shrinkage was associated with a greater hygroexpansion in the freely dried sheets.
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| 1756. |
- Larsson, Per A., 1980-, et al.
(författare)
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Oxygen and water vapour barrier films with low moisture sensitivity fabricated from self-crosslinking fibrillated cellulose
- 2013
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Ingår i: Advances in pulp and paper research, Cambridge 2013. - Lancashire, UK : Bury, Lancashire : The Pulp Fundamental Research Society. - 9780992616304 ; , s. 851-866
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Konferensbidrag (refereegranskat)abstract
- To replace petroleum-based barriers used in, for example, packaging applications with a bio-based alternative, the sensitivity to moisture must be lowered. The present work describes the fabrication and characterisation of cellulose-based films with remarkably improved oxygen and water-vapour-barrier properties at 80% relative humidity. This was achieved by fabricating films of self-cross-linking fibrillated cellulose after partial periodate oxidation to dialdehyde cellulose. At a relative humidity of 80%, films made of 27% and 44% oxidised cellulose, respectively, showed less than half the permeability of the untreated reference; 3.8 g·mm/(m2·24 h·kPa) and 3.7 g·mm/(m2·24 h·kPa) compared to 8.0 g·mm/(m2·24 h·kPa). This was presumably due to a lower moisture uptake in the films, and consequently less swelling. In the absence of moisture, films from both unmodified and modified fibrillated cellulose were ideal oxygen barriers, but at a relative humidity of 80%, films based on 27% and 44% converted cellulose had an oxygen permeability of 2.2 ml·µm/(m2·24 h·kPa) and 1.8 ml·µm/(m2·24 h·kPa), respectively, compared to 9.2 ml·µm/(m2·24 h·kPa) for the non-oxidised material.The cross-linking resulted in an embrittlement of the films, but the 27% oxidised material still had a tensile strength of 148 MPa and a tensile strain at break of 2.0%, which is sufficient in, for example, many packaging applications.
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| 1757. |
- Larsson, Per A., et al.
(författare)
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The Influence Of Grammage, Moisture Content, Fibre Furnish And Chemical Modifications On The Hygro- And Hydro-Expansion Of Paper
- 2009
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Ingår i: ADVANCES IN PULP AND PAPER RESEARCH, OXFORD 2009, VOLS 1-3. - 9780954527266 ; , s. 355-388
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Konferensbidrag (refereegranskat)abstract
- The conventional way to evaluate dimensional stability, regardless of end-use purpose, is to measure the change in dimensions when the moisture content is changed by changing the relative humidity. Sorption of moisture from moist air is a relatively slow process and for the evaluation of printing papers this may not be the most appropriate method. In the present work, data from conventional hygroexpansion measurements has been compared with data from hydroexpansion measurements, i.e. expansions caused by the sorption of liquid water, sprayed onto papers printed with a random speckle pattern, the expansion being monitored by electronic speckle photography. Sheets made from different pulps, with different fines contents and different modifications were studied at different grammages and water-transfer levels. The effect of drying-mode, i.e. restrained drying or free drying, was also studied. It was concluded that sheets expand less with a given amount of adsorbed water when it is sorbed in liquid form rather than from moist air. Chemical treatments known to increase both the dry and the wet strength, e.g. polyelectrolyte multilayers and cross-linking through periodate oxidation, did not significantly improve the dimensional stability when the papers were exposed to liquid water.
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| 1758. |
- Larsson, Per A., et al.
(författare)
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The influence of periodate oxidation on the moisture sorptivity and dimensional stability of paper
- 2008
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 15:6, s. 837-847
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 1759. |
- Larsson, Per A., et al.
(författare)
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Towards natural-fibre-based thermoplastic films produced by conventional papermaking
- 2016
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Ingår i: Green Chemistry. - : Royal Society of Chemistry (RSC). - 1463-9262 .- 1463-9270. ; 18:11, s. 3324-3333
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Tidskriftsartikel (refereegranskat)abstract
- Materials based on cellulose are predicted to be of great importance in a sustainable society. However, for materials such as paper to replace materials with a higher ecological footprint, they need to be strong, ductile, provide a gas barrier, and, sometimes, also be transparent. Improved properties, or even novel properties, are also important for use outside the conventional markets. This paper describes how cellulose fibres partly derivatised to dialcohol cellulose can be used to fabricate high-density materials by conventional papermaking techniques that simultaneously display all the above-mentioned features. The materials produced were characterised with respect to X-ray diffraction, dynamic mechanical thermal behaviour, visual appearance, oxygen permeability and tensile properties. The highest degree of modification studied, resulted in a material with thermoplastic features, a tensile strength of 57 MPa, a strain-at break of 44% and an oxygen permeability at 80% RH of 23 ml mu m (m(2) kPa 24 h)(-1). At a thickness of 125 mu m, these films have a total light transmittance of 78% (87% haze). However, by hot pressing the film for 2 min at 150 degrees C under a pressure of 16 MPa, and thereby increasing the density, the total transmittance increases to 89% (23% haze). The hot pressing can also be used to fuse individual pieces together, which is useful in many modern packaging applications. Altogether, this work shows how chemical modification of cellulose fibres can be used to induce novel properties and improve the range of application, and consequently provide an interesting bio-based material with a good potential to replace less sustainable materials.
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| 1760. |
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