| 1. |
- Edvardsson, Sverker, et al.
(författare)
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System stability of the open draw section and paper machine runnability
- 2009
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Ingår i: ADVANCES IN PULP AND PAPER RESEARCH, OXFORD 2009, VOLS 1-3. - 9780954527266 ; , s. 557-575
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Konferensbidrag (refereegranskat)abstract
- The present work is concerned with the system dynamics and stability of the open draw sections of paper machines where web breaks occur most frequently. We have applied a novel particle-based system dynamics model that allows the investigation of complex interactions between web property fluctuations and system parameters, without any constraints of a particular geometrical web shape or boundary conditions assumed a priori. The result shows that, at a given machine draw and web property parameters, the open draw section maintains its steady-state until it reaches a certain machine speed limit. At this speed the system looses its stability and the web strain starts growing without any limit, and thus leading to a web break. A similar instability can also be triggered when web properties suddenly fluctuate during steady-state operation. The parametric sensitivity studies indicate that, among the web property parameters studied, the elastic modulus of the wet web has the largest impact on the critical machine speed as well as on the detachment point where the web is released from the first roll. Further analysis shows that the decrease in dryness has a (negative) synergistic effect causing an increased risk of system instability. It is, therefore, most important to control, not only average dryness, but also its variations in order to enhance paper machine runnability.
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| 2. |
- Gimåker, Magnus, et al.
(författare)
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On The Mechanisms Of Mechano-Sorptive Creep Reduction By Chemical Cross-Linking
- 2009
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Ingår i: Advances in Pulp and Paper Research, Oxford 2009. - Lancashire, UK : The Pulp and Paper Fundamental Research Society. - 9780954527266 ; , s. 1001-1017
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Konferensbidrag (refereegranskat)abstract
- Despite the technical importance of mechano-sorptive creep in paper, the exact mechanism behind this phenomenon is still not fully understood. In this study it was shown that the mechano-sorptive creep of paper sheets can be significantly reduced by chemical cross-linking through periodate oxidation. The mechanism behind this reduction has been examined through creep measurements of both sheets and individual fibres. For sheets the creep acceleration due to varying humidity was significantly reduced by the chemical cross-linking. For single fibres, however, the creep acceleration was not affected by the chemical crosslinking. In fact the absolute creep rate for the periodate oxidised fibres were higher than that of the reference fibres. This clearly showed that the improvement in mechano-sorptive creep found on a sheet level does not originate from an improved creep resistance for individual fibres but rather from mechanisms operating at the fibre network level. Hygroexpansion and moisture sorption of the sheets during the humidity cycling used for creep testing have also been measured, and the results showed that both was reduced by the periodate oxidation. Reduced moisture sorptivity and hygroexpansion probably minimises stress concentrations at the fibre network level and thereby also the creep acceleration.
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| 3. |
- 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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| 4. |
- Lindström, Stefan B, 1974-, et al.
(författare)
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Evolution of the paper structure along the length of a twin-wire former
- 2009
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Ingår i: ADVANCES IN PULP AND PAPER RESEARCH, OXFORD 2009, VOLS 1-3. - Bury, UK : Pulp Paper Fund. Res. Soc.. - 9780954527266 ; , s. 207-245, s. 207-245
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Konferensbidrag (refereegranskat)abstract
- A particle-level numerical model is used to simulate forming with a twin-wire former configuration. The development of the paper structure along the length of the former is observed to explain the effects ofthe dewatering elements on the paper structure at different jet-to-wire speed ratios, consistencies, and target basis weights. The simulations indicate that most of the structure development takes place in the initial part of forming (forming roll) and, in some instances, at the drop to atmospheric pressure after the forming roll. Dramatic effects onthe through-thickness fibre orientation anisotropy are observed when the consistency is varied by changing the jet thickness, while changes in basis weight had less impact. The through-thickness concentration gradient was almost uniform throughout the forming process, except in the lower range of typical papermaking consistencies. This indicates that the dewatering mechanism is normally thickening, rather than filtration.
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| 5. |
- Nygårds, Mikael, et al.
(författare)
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DEVELOPMENT OF THE NOTCHED SHEAR TEST
- 2009
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Ingår i: ADVANCES IN PULP AND PAPER RESEARCH, OXFORD 2009, VOLS 1-3. - BURY : PULP & PAPER FUNDAMENTAL RESEARCH SOCIETY. - 9780954527266 ; , s. 877-897
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Konferensbidrag (refereegranskat)abstract
- The notched shear test (NST) will be proposed to measure shear strength of paperboard by utilizing standard in-plane tensile testing equipment. The test is a further development of the double notch shear test specimen; where plastic lamination has been utilized. With the new test setup it becomes possible to measure shear strength profile in the thickness direction of paperboard. As a spin-off of the NST also the strip shear test (SST) was suggested. The SST test can be used as a quick measure of shear strength. It is shown that the SST measurement correlates well with the traditional rigid shear test (RST). In order to verify that the NST specimen failed due to shear stresses, finite element simulations were performed. The simulations showed that the shear zone had a uniform shear stress field at the time of failure. Moreover, with the finite element simulations it was also possible to predict the force-displacement curve.
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