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The mechanisms of edge wicking in retortable paperboard
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis reports on an investigation of the mechanisms of edge wicking in retortable paperboard. Retortable board is used for packaging preserved food, a process which requires that the package and its contents be sterilised by exposure to high temperature steam for up to three hours. The board used must thus have higher water repellence than traditional liquid packaging. Water vapour that condenses on the cut edges on the outside of the board causes particular concern.

The paperboards studied were made from refined and unrefined bleached softwood kraft pulp and from refined unbleached softwood kraft pulp in one or two plies with different sizing levels and structures. Samples of each of the boards were immersed in a water bath at 95 °C. Other samples from the same boards were exposed to a combination of saturated steam at 130 °C and condensed steam in a special apparatus developed for this purpose. The board samples were placed on a cooling plate to simulate the temperature gradient when a container of cold food is sterilised with heated steam. Still other samples were exposed to heated steam in the absence of condensation by hanging them freely in the apparatus.

The extent of edge wicking in the boards was determined by gravimetric measurements and also by near-infrared (NIR) spectroscopy, a technique that yields more information about the moisture content profiles and the mechanisms of liquid sorption. The moisture content profiles of boards exposed to saturated steam in the absence of condensation show more uniform water uptake. This phenomenon can be explained by rapid vapour phase transport throughout the pore structure, followed by slower water uptake in the fibres. By contrast, in samples exposed to both heated steam and condensed steam, there was both liquid sorption in the fibre network and rapid vapour phase transport of the steam. The moisture content in those samples was much higher close to the edge and lower behind the liquid frontier.

The edge wicking of high temperature water was greatest in board that was not fully sized, in low density board, and in board made from unrefined pulp. The greater edge wicking in board made from unbleached pulp can be accounted for in terms of its greater swelling potential. In the low density board and the board made from unrefined pulp, the lumens at the unsized edge and the weaker bonding strength are suggested to affect the results. Capillary sorption takes place in lumens and delaminated bonds at the edge and then any further propagation takes place entirely by fibre diffusion. Capillary sorption may also be important in damaged areas where broken fibre-fibre bonds are exposed to liquid. The weaker bonded area in low density board therefore also contributes to water sorption.

Edge wicking in the boards placed on the cooling plate and exposed to a combination of saturated and condensed steam was of a different nature. For these samples, density was not an important factor. The difference is due to the different mechanism at work in this case, where the relative area of the edge exposed to condensed steam was larger in the high density boards than in the low density boards. In the high density boards, condensed steam was sorbed in the structure by diffusion in the fibre structure, whereas in low density boards, the dominant effect was vapour phase transport in the void spaces followed by water uptake in the fibres.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , 21 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:28
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-4155OAI: oai:DiVA.org:kth-4155DiVA: diva2:10947
Presentation
2006-10-20, STFI-salen, KTH, Drottning Kristinas väg 61, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101125Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2010-11-25Bibliographically approved
List of papers
1. Determination of the in-plane wetting of a board structure using NIR spectroscopy
Open this publication in new window or tab >>Determination of the in-plane wetting of a board structure using NIR spectroscopy
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 22, no 1, 111-116 p.Article in journal (Refereed) Published
Abstract [en]

Edge wicking is one of the most critical parameters of liquid packaging board. In this study near infrared (NIR) spectroscopy in combination with multivariate analysis was used to measure the in-plane wetting profile of hard sized board structures after exposure to water. This was done using a moving NIR probe and registering the spectra in series. Two grades of commercial board and one laboratory-made board were tested. The NIR spectra of samples with moisture contents of between 5% and 50% (completely soaked), measured gravimetrically, were collected. Two partial least squares (PLS) models were created using the NIR spectra as the x matrix and the moisture content measured gravimetrically as the y matrix, one containing observations of the two commercial board grades and the other one containing observations of the laboratory-made board. The correlation coefficients, R-2, of the calibration sets were 0.99 for each of the two models respectively, and the root mean square errors of prediction (RMSEP) for the test sets containing new observations were 2.1% and 1.3% respectively. Two different trials were set up to verify the NIR method. In the first trial, the average moisture contents of the samples, measured gravimetrically, were compared with the moisture content profiles measured using the NIR technique. In the second trial, the edge wicking values for samples immersed in water for different durations and at different temperatures were measured using NIR spectroscopy, and the results were compared to those of a reference method for measuring edge wicking. The method gives information about the mechanisms of liquid penetration and more details about the moisture content profiles of sized boards and is useful in studying edge-wicking mechanisms in paper boards.

Keyword
AKD-Sizing, Edge penetration, Liquid packaging board, Moisture content, Near infrared spectroscopy, Wicking
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6272 (URN)10.3183/NPPRJ-2007-22-01-p111-116 (DOI)000248528700016 ()
Note
QC 20101125. Uppdaterad från Manuskript till Artikel (20101125).Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2017-12-14Bibliographically approved
2. A test apparatus for evaluating edge wicking in sized paperboard suitable for retortable packaging
Open this publication in new window or tab >>A test apparatus for evaluating edge wicking in sized paperboard suitable for retortable packaging
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 22, no 2, 194-199 p.Article in journal (Refereed) Published
Abstract [en]

The development of packaging board for liquids has been followed by the development of packaging laminate based on paperboard for canned food. This packaging will be filled with a food product and then exposed to high-temperature steam. A test apparatus is needed to simulate the preservation process and facilitate study of critical factors affecting such boards that must be highly water repellent. This paper presents an example of one such apparatus that can be used to study edge wicking in boards. The test apparatus consists of a large mixing chamber, where saturated steam and pressurised air can be mixed and a smaller test chamber in which the test sample is placed on a cooling plate to simulate the temperature gradient occurring when a package of cold food is preserved by heat sterilisation. Test samples can also hang freely in the test chamber. In this study, one commercial board and one laboratory-made board were exposed to saturated steam at temperatures between 112 degrees C and 130 degrees C, corresponding to a pressure of between 1.53 and 2.70 bar. The temperature of the cooling plate was regulated by varying the temperature of the inlet water between 24 degrees C and 72 degrees C. The results show that the temperature of the saturated steam has a large effect on edge wicking - the higher the temperature, the greater the edge wicking. The study also examined the effect of the magnitude of the temperature gradient that occurs when the board is placed on the cooling plate and exposed to saturated steam. It was found that differences in the temperature of the cooling plate do not affect edge wicking at a constant steam temperature.

When samples on the cooling plate are exposed to high-temperature steam, water condensation takes place at the board edge, resulting in water migration into the structure and a clear liquid front in the board. In freely hanging samples, which were not exposed to condensation, the water uptake was uniform throughout the structure. Vapour phase transport took place rapidly throughout the structure and was followed by much slower water uptake into the fibres.

Keyword
edge penetration, AKD-sizing, wicking, moisture content, near-infrared spectroscopy, liquid packaging board, autoclaving
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6273 (URN)10.3183/NPPRJ-2007-22-02-p194-199 (DOI)000248057800008 ()
Note
QC 20101125. Uppdaterad från Manuskript till Artikel (20101125). Tidigare Titel: "A test apparatus for evaluatin edge wicking in sized boards used for preserved fool". Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2017-12-14Bibliographically approved
3. The effect of sizing and paper structure on paperboard for retortable packaging
Open this publication in new window or tab >>The effect of sizing and paper structure on paperboard for retortable packaging
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 22, no 2, 200-209 p.Article in journal (Refereed) Published
Abstract [en]

 Food packaged in a retortable paperboard-based carton is sterilised by exposing the container to high-temperature steam. The board used must have greater water repellence than common liquid packaging board, since heated steam will condense on the uncovered edges. The research reported in this paper studied critical factors affecting edge wicking in sized paperboards immersed in water at 95 degrees C or placed on a cooling plate and exposed to saturated steam at 130 degrees C. The paperboards were made from refined and unrefined bleached pulp as well as from refined unbleached pulp, in one or two plies, with different sizing levels and densities. The swelling of the boards was also examined before and after treatment at 150 degrees C for 6 hours.

For samples immersed in water, the sizing level, paper structure and fibre swelling of the board affected the edge wicking. The unsized edges had a marked effect on the results, for this was where edge wicking was initiated. Boards with lower sizing levels, lower density, made from unrefined pulp and made from unbleached pulp showed more edge wicking than other boards.

All the samples exposed to saturated steam on a cooling plate showed similar patterns of edge wicking, except that boards with a low level of AKD showed far more edge wicking than others. However, the moisture content profiles in the samples measured with NIR spectroscopy were more uniform in the sample made from unrefined pulp and the low-density samples than in samples with higher density, which had much higher moisture content at the cut edge. These results can be explained by the fact that the saturated steam condenses at the edge of the high density boards resulting in liquid edge wicking by diffusion into the fibre network while the steam is also able to be transported into the pore structure of the low density board followed by condensation or diffusion into the fibre network.

Heat-treated boards immersed in water showed a lower moisture content and a lower expansion in the z-direction than untreated references. The reduction was larger in boards made from unbleached pulp, which may explain the different findings relating to edge wicking when boards made from bleached and unbleached pulp were immersed in water or exposed to steam.

Keyword
edge penetration, moisture content, fibre swelling, AKD sizing, paper structure, liquid packaging board, autoclaving, density
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-6274 (URN)10.3183/NPPRJ-2007-22-02-p200-209 (DOI)000248057800009 ()
Note
QC 20101125. Uppdaterad från Manuskript till Artikel (20101125). Tidigare Titel: "The effect of sizing and paper structure on board for canned food".Available from: 2006-10-23 Created: 2006-10-23 Last updated: 2017-12-14Bibliographically approved

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