| 1. |
- Brandberg, August, 1990-, et al.
(författare)
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Characterization and impact of fiber size variability on the mechanical properties of fiber networks with an application to paper materials
- 2022
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 239-240
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Tidskriftsartikel (refereegranskat)abstract
- Cellulose fibers come in a wide range of shapes and sizes. The heterogeneity of the fiber length, width, wall thickness, curl and external fibrillation is detrimental to the mechanical performance of products such as paper and paperboard. Although micro-mechanical models of these materials sometimes incorporate features of this heterogeneity, so far there is no standardized method of fully incorporating this. We examine a large number of industrial mechanical fiber pulps to determine what information such a standardized method would have to have. We find that the method must allow for both non-Gaussian distributions and dependence between the variables. We present a method of characterizing mechanical pulp under these conditions that views the individual fiber as outcome of a sampling process from a multivariate distribution function. The method is generally applicable to any dataset, even a non-Gaussian one with dependencies. Using a micro-mechanical model of a paper sheet the proposed method is compared with previously presented methods to study whether incorporating both a varying fiber size and dependencies is necessary to match the response of a sheet modeled with measured characterization data. The results demonstrate that micro-mechanical models of paper and paperboard should not neglect the influence of the dependence between the characteristic shape features of the fibers if the model is meant to match physical experiments. © 2022 The Authors
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| 2. |
- Brandberg, August, et al.
(författare)
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The role of the fiber and bond in hygroexpansion and curl of paper
- 2019
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Ingår i: Paper Conference and Trade Show, PaperCon 2019. - : TAPPI Press. ; , s. 1256-1271
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Konferensbidrag (refereegranskat)abstract
- The underlying mechanism of hygroexpansion of paper products' deformation resides on the microscale and is a product of the coupling between hydrophilic fibers and micromechanical details of the fiber network, including the geometry and alignment of fibers and bonds. We present a micromechanical framework suitable for studying hygroexpansion from the fiber- and bond-level. Using the developed model, we show that due to the threedimensionality of the bond, the transverse deformations of the fibers are not transferred effectively to the in-plane deformation of the sheet. At the same time, the longitudinal deformation of the fiber accounts for a large portion of the hygroexpansion even in highly oriented sheets. On the other hand, the out-plane deformation of paper is predominantly controlled by the strain gradient in the bonds which stems from transverse shrinkage or expansion of the fibers in the bond region. Therefore, considering the bonds as three-dimensional entities is vital for the analysis since a two-dimensional approximation overestimates the impact of the transverse deformation of the bonds to the in-plane properties and underestimates moisture-induced out-of-plane deformations of the fiber network.
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| 3. |
- Brandberg, August, et al.
(författare)
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The role of the fiber and the bond in the hygroexpansion and curl of thin freely dried paper sheets
- 2020
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Ingår i: International Journal of Solids and Structures. - : Elsevier. - 0020-7683 .- 1879-2146. ; 193-194, s. 302-313
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Tidskriftsartikel (refereegranskat)abstract
- A computationally efficient method to study the in-plane and out-of-plane dimensional instability of thin paper sheets under the influence of moisture changes is presented. The method explicitly resolves the bonded and the free segments of fibers in the sheet, capturing the effect of anisotropic hygroexpansion at the fiber level. The method is verified against a volumetric model. The importance of longitudinal fiber hygroexpansion is demonstrated in spite of the absolute value of longitudinal hygroexpansion being an order of magnitude lower than the transverse hygroexpansion component. Finally, the method is used to demonstrate the formation of macroscopic sheet curl due to a moisture gradient in structurally uniform sheets in the absence of viscoelastic or plastic constitutive behavior and through-thickness residual stress profiles.
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| 4. |
- Mandlez, D., et al.
(författare)
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Quantifying the contribution of fines production during refining to the resulting paper strength
- 2022
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Ingår i: Cellulose. - : Springer Nature. - 0969-0239 .- 1572-882X. ; 29:16, s. 8811-8826
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Tidskriftsartikel (refereegranskat)abstract
- Pulp refining is an essential process step prior to paper production. The contribution of fines production during refining to the resulting paper strength so far has mostly been considered qualitatively. A quantitative and experimental evaluation regarding their effect has not yet been published. Unbleached softwood kraft pulp was refined using a PFI mill and a disc refiner at different refining intensities. Prior to handsheet forming, fines were removed in a lab scale pressure screen from one part of the refined and unrefined samples in order to investigate the difference in tensile strength between sheets with and without fines, which were furthermore produced with and without additional wet pressing. It was found, that fines formed in a disc refiner at 250 kWh/t are responsible for up to 25% of the breaking length increase, while the PFI mill at 10,000 revolutions fines only contribute to a maximum of 12%. In terms of fines efficiency, the disc refiner was able to achieve higher results compared to the PFI mill, which however might be attributed to the higher fibre flexibilization in the PFI mill. Thus fines formed in the refining process are of high importance for strength development especially for the disc refiner.
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| 5. |
- Urstöger, G., et al.
(författare)
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Microstructure and mechanical properties of free and restrained dried paper : a comprehensive investigation
- 2020
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Ingår i: Cellulose. - : Springer Science and Business Media B.V.. - 0969-0239 .- 1572-882X. ; 27:15, s. 8567-8583
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Tidskriftsartikel (refereegranskat)abstract
- The influence of strain during drying is known to have a large effect on its mechanical properties like tensile strength, Young’s modulus and hygroexpansion. In this study we investigate free and restrained dried paper and investigate the relation between the paper microstructure and its mechanical properties. The first part of the work investigates the development of the mechanical properties as well as paper internal stresses and strains (elastic-, inelastic- and hygrostrain) at different moisture contents during the drying process. Emphasis is put on the changes of hygrostrains and the paper hygroexpansion coefficient during drying. One main finding was that in constrained drying the drying stresses are considerably below the yield stress and, as a consequence, the deformations are mainly inelastic (creep) and only marginally elastic. In the second part we are analyzing the microstructure of free and restrained dried sheets by X-ray microtomography (μ-CT). From the μ-CT analysis relevant network parameters such as number of contact points, fiber bond area, free fiber length and fiber curl were extracted. Also a novel method to investigate the interface angle of the fibers in the vicinity of the bond sites was established. The results showed that the major difference from the drying procedure manifests itself in the out-of-plane curl of the fibers. Statistically number of contact points, bond area and in-plane curl were not different whereas in-plane curl and free fiber length were different. The interface angle appears to be slightly affected by the drying procedure but is overall very low.
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| 6. |
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