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- Gimåker, Magnus, 1980-
(författare)
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Influence of fibre modification on moisture sorption and the mechanical properties of paper
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fibre modification might be a way to improve the performance of paper, to increase its cost competiveness and enable new paper-based products to be developed. Therefore, the influence of fibre modification (with polyelectrolytes or by fibre cross-linking) on the mechanical properties of special importance for packaging paper grades was studied. Creep deformation under varying humidity conditions (i.e. mechano-sorptive creep) is of outmost importance for the stacking life of paper-based boxes. The influence on creep behaviour of adsorbing polyallylamine (a cationic polyelectrolyte) to fibre surfaces or throughout the fibre walls was studied. Adsorption to fibre surfaces reduced the creep at constant humidity. The mechano-sorptive creep was not however influenced. The use of polyelectrolytes did not thus appear to be a feasible strategy for reducing mechano-sorptive creep. Polyelectrolytes can however be efficient in improving other mechanical properties. The use of multilayers consisting of polyallylamine (PAH) and polyacrylic acid (PAA) was for example shown to significantly increase the strength of paper with much less densification and build-up of residual stress than is obtained by beating. Cross-linking by oxidation with periodate radically decreased the mechano-sorptive creep of sheets made from the oxidised fibres. The basic mechanism behind the reduction in mechano-sorptive with cross-linking was found to be that the cross-linking slowed down the moisture sorption kinetics. A lower sorption rate led to smaller moisture content variations during the mechano-sorptive creep testing, and thus less sorption-induced swelling and stress concentrations at fibre/fibre joints. However, for cross-linking to be a practical way to reduce creep, the large problem of embrittlement must be solved. The shear strength of couched sheets was measured to study the interaction between the sheets at different solids content. The shear strength was low until a solids content of approximately 60−70% was reached, which suggests that interactions important for the strength at complete dryness start to develop at this solids content. The effect of different fibre modifications and additives on how the fibres interact during the consolidation process is not always well understood. The method of shear strength determination could in the future be applied to modified fibres to hopefully increase the understanding of how different modifications influence the fibre/fibre interactions. A deeper understanding might reduce the time for the development of new and improved fibre modifications.
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| 2. |
- Magnusson, Mikael S.
(författare)
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Interfibre Joint Strength under Mixed Modes of Loading
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The load carrying capacity of interbre joints are one of the key entities for build-up of strength inpaper materials. In order to gain insight in how to tailor the macroscopic properties of such materialsby chemical and/or mechanical treatments at a microscopic level, direct measurement of individualbre{bre crosses are typically performed. However, the state of loading in the interbre joint, intesting of individual bre{bre crosses, is in general very complex and an increased understandingfor how to evaluate the mechanical properties of interbre joints is desirable. In Paper A, amethod for manufacturing and measuring the strength of isolated interbre joints is presented. Themethod is applied to investigate the strength of bre{bre crosses at two dierent modes of loading.Also, an investigation on the manufacturing conditions is presented. The strength distribution ofindividually prepared bre{bre crosses is characterized and it was found that the median strengthin a peeling type of loading was about 20% compared to samples tested in the conventional shearingtype of loading. In Paper B, a procedure for evaluating interbre joint strength measurementsin terms of resultant forces and moments in the interbre joint region is presented. The methodis applied to investigate the state of loading in bre{bre crosses tested in peeling and shearing,respectively. It is shown that for a typical interbre joint strength test, the load components otherthan shear, cannot in general be neglected and is strongly dependent on the structural geometry ofthe bre{bre crosses. In Paper C, four distinctly dierent load cases; peeling, shearing, tearingand a biaxial type of loading was tested mechanically and evaluated numerically in order to gainmore information on how interbre joints behave in dierent modes of loading. In Paper D, thein uence of a chemical additive on the interbre joint strength is investigated on the microscopic(joint) scale and correlated to the eect previously observed on the macroscopic (sheet) scale. Xraymicrotomography and image analysis was used to understand structural changes in the brousnetwork in terms of the number of interbre joints as well as the average interbre joint contact area.The results showed that the median interbre joint strength increased by 18% upon adsorption, andthat the polyelectrolyte increased the number of contacts between the bres as well as an increasedarea of contact. In Paper E, the damage behaviour of individual interbre joints is analyzed. Froman extensive number of mechanical tests, the typical damage behaviour is identied and a failurecriterion is used to study the in uence of failure properties to give indications on how to tailor thematerial to optimize the joint strength.
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| 3. |
- Magnusson, Mikael S., 1984-
(författare)
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Testing and Evaluation of Interfibre Joint Strength under Mixed-Mode Loading
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The failure properties of interfibre bonds are the key for the build-up of strength in fibrous materials such as paper and paperboard. In order to tailor the properties of such materials by chemical or mechanical treatments and to learn how such modifications influence the properties at a microscopic level, direct measurement of individual fibre--fibre crosses are typically performed. However, the state of loading in the interfibre joint, in testing of individual fibre--fibre crosses, is in general very complex and a greater understanding for how to evaluate the mechanical properties of interfibre joints is desirable.In Paper A, a method for manufacturing multiple fibre--fibre cross specimens and a procedure for testing interfibre joints at different modes of loading is presented. The method is applied to investigate the strength of fibre-fibre crosses with different geometry and at two principally different modes of loading. Also, an investigation on the influence of drying pressure, the drying method as well as a comparison of pulp fibres from two different degrees of refining is presented. The force at rupture is scaled in terms of different geometric parameters; nominal overlap area, length and width of the joint region. It is shown that neither of the methods of scaling unambiguously reduced the coefficient of variation of the mean strength and that the force at rupture in a peeling type of loading was about 20% of the ones tested in the conventional shearing type of loading.In Paper B, a procedure for evaluating interfibre joint strength measurements in terms of resultant forces and moments at rupture is presented. The method is applied to investigate the state of loading in fibre-fibre crosses tested in two principally different modes of loading. It is shown that for a typical interfibre joint test, the modes of loading other than pure shear, cannot in general be neglected and is strongly dependent on the structural geometry of the fibre-fibre crosses. Also, the stress state in the interface centroid was estimated in order to quantify how the mode of loading influence the amount of normal stresses that develop in relation to the amount of shear stresses in the interfibre joint.
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