| 1. |
- Gimåker, Magnus, et al.
(författare)
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Influence of beating and chemical additives on residual stresses in paper
- 2011
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Ingår i: Nordic Pulp & Paper Research Journal. - : Walter de Gruyter GmbH. - 0283-2631 .- 2000-0669. ; 26:4, s. 445-451
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Marin, Gustav, et al.
(författare)
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Stiffness and strength properties of five paperboards and their moisture dependency
- 2019
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Ingår i: Paper Conference and Trade Show, PaperCon 2019. - : TAPPI Press. - 9781510893948 ; , s. 1193-1208
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Konferensbidrag (refereegranskat)abstract
- Five folding box boards made on the same paperboard machine have been analyzed. The paperboards were from the same product series but had different grammage (235, 255, 270, 315, 340 g/m2) and different bending stiffness. The paperboards are normally used to make packages, and since the bending stiffness and grammage varies the packages performance will be different. Finite element simulations can be used to predict these differences. However, the stiffness and strength properties then need to be known. For efficient determination of the three-dimensional properties in MD, CD and ZD, it is proposed that the whole paperboard should be characterized with the following tests: in-plane tension, ZD tension, shear strength profiles and two-point bending. The stiffness and strength properties have with the proposed setups been determined at different relative humidity (20, 50, 70 and 90 % RH), and the mechanical properties have been evaluated as function of moisture ratio. The results showed a linear relation between mechanical properties and moisture ratio for each paperboard. The data was then normalized with data for the standard climate (50 % RH) and investigated as a function of moisture ratio. The results indicated that the normalized mechanical properties for all paperboards coincided along one single line and could therefore be expressed as a linear function of moisture ratio and two constants. Consequently, the study indicates that it is possible to obtain the mechanical properties of a paperboard, by knowing the structural properties for the preferred level of RH and the mechanical property for the standard climate (50 % RH and 23 °C).
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| 3. |
- Östlund, Magnus, et al.
(författare)
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Experimental determination of residual stresses in paperboard
- 2005
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Ingår i: Experimental mechanics. - 0014-4851 .- 1741-2765. ; 45:6, s. 493-497
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Tidskriftsartikel (refereegranskat)abstract
- A method for the experimental determination of the through-thickness residual stress distribution in paper-board is presented.The successive removal of thin layers from strips of board through surface grinding changes the stress-state and the bending stiffness resulting in a changed curvature, which is measurable. From tests of strips in both in-plane directions, stress distributions can then be evaluated using the Treuting-Read method. Geometrically nonlinear effects at the large deformations taking place are avoided through a proper choice of strip dimensions. Typical results are presented and factors influencing the accuracy of the determination are thoroughly discussed.
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| 4. |
- Östlund, Magnus, et al.
(författare)
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Influence of Paperboard Structure and Processing Conditions on Forming of Complex Paperboard Structures
- 2011
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Ingår i: Packaging technology & science. - : Wiley. - 0894-3214 .- 1099-1522. ; 24:6, s. 331-341
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Tidskriftsartikel (refereegranskat)abstract
- Experiments were made with a laboratory apparatus for forming paper sheets into double-curved structures. The purpose was to learn more about the design criteria for forming of paper materials into complex shapes. The influence of forming parameters such as moisture and temperature was studied, and the performance of some common pulps as raw materials for the paper sheets was evaluated. Papers made from short hardwood fibres performed worse than papers from other pulps, and the method of moisture application was more important than the amount of moisture that was applied.
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| 5. |
- Östlund, Magnus, 1974-
(författare)
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Residual stresses in paperboard and the influence of drying conditions
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The drying sequence in the manufacturing process for paperboard involves evaporation of water, primarily from within the fibres. The vapour is then transported out of the web by pressure or concentration gradients. As the moisture transport from the paper web to the ambient is quicker than the moisture transport within the fibre network to the surfaces of the web, moisture gradients develop through the thickness of the web. This work concerns effects on the mechanics of paper drying from the variation in moisture through the relatively thin structures of paper and paperboard.Distributions of inplane residual stresses through paper materials in the unloaded state after drying are believed to be caused by the varying moisture through the thickness during drying. The distributions in general exhibit compressive stress near the board surfaces and tensile stress in the interior of the board. This may be modified after drying and is also affected by structural variation in the material between different plies of multi-ply paperboards.The stress development during drying is important because it influences the resulting material properties of the paper and because it can lead to curl, which is a quality problem. The residual stresses themselves are an error source in simulation or evaluation of the mechanical behaviour of paper.In this work, residual stress distributions in paperboard were determined experimentally, to clarify the mechanisms of residual stress build-up. An experimental method for such tests was also developed. Based on the experimental findings, the mechanics of paper drying was modelled and the stress build-up simulated. Simulation offers a way of studying how the properties of paper develop during drying of wet paper webs.
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| 6. |
- Östlund, Magnus, et al.
(författare)
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Residual stresses in paperboard through the manufacturing process
- 2005
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Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 31:4, s. 197-201
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Tidskriftsartikel (refereegranskat)abstract
- The residual stresses in a machine-made multi-ply board with chemical pulp in all plies were monitored as the board progressed after the initial drying through calendering, moisture treatments and coating. After drying, the stresses were compressive near the surfaces. In the coated board at the end of the machine, the stress state was reversed to a large extent with tensile stresses in the outermost layers. The various moisture applications on the board in the post-processing steps are regarded to have caused the change in the residual stress state. In another machine-made board, with mechanical pulp in the middle plies, the stresses after drying were limited to the surface plies made from chemical pulp, but were again compressive at the surfaces. This stress state was largely unaffected by post-processing. The possibility for stress reversal and the implications on in-plane and out-of-plane dimensional stability of the boards are discussed.
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| 7. |
- Östlund, Magnus, et al.
(författare)
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The influence of drying conditions on residual stress build-up in paperboard
- 2004
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Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 30:11, s. 312-316
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Tidskriftsartikel (refereegranskat)abstract
- The through-thickness distributions of in-plane residual stress have been determined for some laboratory-made paperboards. The effects of drying variables such as temperature, moisture flow resistance at the surfaces and thickness of the boards on residual stresses were studied. The drying rate was shown to influence residual stresses only for relatively slow drying. Residual stresses correlate well with the moisture gradient during drying for various drying methods. Some effects of the drying conditions on paper tensile stiffness are discussed. One-sided heating for curl control is also discussed.
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| 8. |
- Östlund, Magnus, et al.
(författare)
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The influence of drying restraints and beating degree on residual stress build-up in paperboard
- 2004
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Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 30:11, s. 289-293
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Tidskriftsartikel (refereegranskat)abstract
- The through-thickness distribution of in-plane residual stress has been determined for laboratory-made paperboards, to study the causes of these stresses. The degree of restraint during drying is shown not to influence the residual stresses in a significant way. However, a higher level of beating of the pulp leads to higher residual stresses. The drying gradient through the sheet combined with the shrinkage of paper as moisture content decreases is suggested as the main reason for residual stress build-up. No effect an free shrinkage was seen when attempting to limit the drying gradients by decreasing the grammage.
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| 9. |
- Östlund, Magnus, et al.
(författare)
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The influence of through-thickness variation on the mechanics of paper drying
- 2005
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Ingår i: ADVANCES IN PAPER SCIENCE AND TECHNOLOGY. - 9780954527235 ; , s. 213-240
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Konferensbidrag (refereegranskat)abstract
- A material model for drying paper is presented. Moisture-dependent material parameters, hygroscopic shrinkage, the elastic and the time-dependent responses of the material to load, and the effect of unloading at a higher stiffness than the load was applied at are modelled. The model is used to determine the effects of a varying moisture ratio through the paper during drying on free shrinkage development and stiffness development at free drying. Simulation results for the stress development during drying and the state of residual stress immediately after drying are also presented. The model predicts a variation of in-plane elastic moduli through the paper, a prediction that is studied by experiments.
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| 10. |
- Alfthan, Johan, et al.
(författare)
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A micromechanical model for mechanosorptive creep in paper
- 2002
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Ingår i: Journal of Pulp and Paper Science (JPPS). - 0826-6220. ; 28:3, s. 98-104
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Tidskriftsartikel (refereegranskat)abstract
- The creep of paper is accelerated by moisture cycling, a phenomenon known as mechanosorptive creep or accelerated creep. In this paper stress created at bonds due to anisotropic swelling during absorption and desorption of moisture, in combination with nonlinear creep, are proposed to be the cause for mechanosorptive creep. Two simplifled models are first discussed in order to demonstrate the suggested mechanism. A three-dimensional fibre network model composed of elastic fibres and inelastic bonds is then studied by finite element calculations. The relative sliding in the bonds is described by a nonlinear creep model which, in combination with anisotropic hygroexpansion of the fibres results in accelerated creep of the network.
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