The dry-to-wet transition of fiber networks-Return to mechanical stability

In this article, we provide a comprehensive experimental, numerical, and theoretical explanation of the dry-to-wet transition of nonbonded fiber networks made of natural fibers. Given that the main functionality of many common products consisting of fluff pulp fiber networks requires absorption of liquids, we focus on understanding the solid volume fraction transition from a dry to a wet state as a crucial component for controlling properties such as permeability and capillary pressure, on which product function eventually depends. From studying the wetting of fluff pulp fiber networks with a distribution of fiber lengths, we show that the change in the solid volume fraction going from dry to wet state is driven by the disappearance of fiber-fiber adhesion. The mechanically stable state to which the network transitions is independent of its prior dry solid volume fraction and predetermined primarily by the fiber aspect ratio

Uniaxial compression of fibre networks – the synergetic effect of adhesion and elastoplasticity on non-reversible deformation

In this paper we study numerically and experimentally non-reversible deformation of anisotropic, semi-flexible fibre networks. We formulate a Discrete Element Model (DEM) with bonded particles to simulate uniaxial compression of such networks and use this model to describe and quantify the effect of elasto-plastic fibre contacts and fibre-fibre adhesion on non-reversible deformation. Our results show that inter-fibre adhesion plays a role for compression in a low solid volume fraction range where adhesive forces can overcome fibre deformation forces and moments. Also, elasto-plastic contacts between fibres become important at higher solid volume fractions when the yield criterion is exceeded. The combined case of fibres having elasto-plastic contacts and adhesion shows a significant synergetic effect leading to a degree of non-reversible deformation of the network far beyond that of networks with only elasto-plastic fibre contacts or inter-fibre adhesion

Stochastic modelling of 3D fiber structures imaged with X-ray microtomography

Many products incorporate into their design fibrous material with particular levels of permeability as a way to control the retention and flow of liquid. The production and experimental testing of these materials can be expensive and time consuming, particularly if it needs to be optimised to a desired level of absorbency. We consider a parametric virtual fiber model as a replacement for the real material to facilitate studying the relationship between structure and properties in a cheaper and more convenient manner. 3D image data sets of a sample fibrous material are obtained using X-ray microtomography and the individual fibers isolated. The segmented fibers are used to estimate the parameters of a 3D stochastic model for generating softcore virtual fiber structures. We use several spatial measures to show the consistency between the real and virtual structures, and demonstrate with lattice Boltzmann simulations that our virtual structure has good agreement with respect to the permeability of the physical material

Lime Mud Reburning. Properties and Quality of the Lime Produced

It is of utmost importance that the white liquor used in the digester of a sulphate pulp mill is of high quality. The white liquor is prepared in the causticizing plant, through causticizing of the sodium carbonate in the green liquor. The causticizing is made with slaked lime, which, in turn, has been calcined in the lime mud reburning plant and slaked in the green liquor. To enable high quality white liquor to be produced the lime has to be of high quality. A high quality lime is reactive in the slaking and causticizing and the later-formed lime mud has to be easily separated from the white liquor. This study has shown that the quality of the lime is closely correlated to the specific surface area of the lime. Lime calcined in different types of equipment was slaked and causticized in laboratory scale equipment. The specific filtration resistance of the later-formed lime mud was then determined. Reburned lime mud from a rotary kiln, a pilot scale fluidised bed and from a laboratory oven were all more reactive when the specific surface area of the lime was large. However, the later-formed lime mud was more easily separated from the white liquor if the lime had a small surface area. Since the size of the specific surface area of the lime is extremely important, the effects of various reburning conditions on the surface area were more closely examined. Experiments in a laboratory oven showed that the specific surface area of the lime is decreased by an increased temperature or increased residence time in the oven. Limes that were not completely calcined had a specific surface area between that of the parent and the calcined material. Furthermore, experiments in an atmosphere of pure carbon dioxide, at non-calcining temperatures, showed considerable sintering of lime mud before calcination. This is important since the characteristics of the parent material is of importance to the final product. Calcium carbonate pro analysi used as a reference, had ten times larger specific surface area. This is likely the effect of less impurities and of the different structure of the parent material. Lime mud and calcium carbonate were calcined in pure nitrogen in a quartz glass reactor. These experiments showed the largest reported specific surface area of calcined lime mud, 9.2 m2/g, when treated for 300 s at 700 \ub0C. The specific surface area of calcium carbonate p. a.calcined in the quartz glass reactor was about ten times larger than that of the lime mud. The decrease in specific surface area with time was found to follow the German and Munir equation. The temperature dependence was empirically estimated and included in the German-Munir equation. The .gamma. value, 3, indicates solid phase diffusion as the mechanism of sintering. The lowest specific surface area was obtained for lime produced in a rotary kiln, 0.2 m2/g. The lime from the pilot scale fluidised bed was more soft-burned and had a specific surface area that was ten times larger, 2 m2/g. This is presumably due to the good mixing in the fluidised bed reactor, which enables high heat and mass transfer in the reactor. Accordingly, the temperature and the concentration of carbon dioxide is kept at a low level. The largest surface area obtained in this study was that of calcium carbonate p. a.calcined in pure nitrogen, 99 m2/g

Lime Mud Reburning. Properties and Quality of the Lime Produced

It is of utmost importance that the white liquor used in the digester of a sulphate pulp mill is of high quality. The white liquor is prepared in the causticizing plant, through causticizing of the sodium carbonate in the green liquor. The causticizing is made with slaked lime, which, in turn, has been calcined in the lime mud reburning plant and slaked in the green liquor. To enable high quality white liquor to be produced the lime has to be of high quality. A high quality lime is reactive in the slaking and causticizing and the later-formed lime mud has to be easily separated from the white liquor. This study has shown that the quality of the lime is closely correlated to the specific surface area of the lime. Lime calcined in different types of equipment was slaked and causticized in laboratory scale equipment. The specific filtration resistance of the later-formed lime mud was then determined. Reburned lime mud from a rotary kiln, a pilot scale fluidised bed and from a laboratory oven were all more reactive when the specific surface area of the lime was large. However, the later-formed lime mud was more easily separated from the white liquor if the lime had a small surface area. Since the size of the specific surface area of the lime is extremely important, the effects of various reburning conditions on the surface area were more closely examined. Experiments in a laboratory oven showed that the specific surface area of the lime is decreased by an increased temperature or increased residence time in the oven. Limes that were not completely calcined had a specific surface area between that of the parent and the calcined material. Furthermore, experiments in an atmosphere of pure carbon dioxide, at non-calcining temperatures, showed considerable sintering of lime mud before calcination. This is important since the characteristics of the parent material is of importance to the final product. Calcium carbonate pro analysi used as a reference, had ten times larger specific surface area. This is likely the effect of less impurities and of the different structure of the parent material. Lime mud and calcium carbonate were calcined in pure nitrogen in a quartz glass reactor. These experiments showed the largest reported specific surface area of calcined lime mud, 9.2 m2/g, when treated for 300 s at 700 \ub0C. The specific surface area of calcium carbonate p. a.calcined in the quartz glass reactor was about ten times larger than that of the lime mud. The decrease in specific surface area with time was found to follow the German and Munir equation. The temperature dependence was empirically estimated and included in the German-Munir equation. The .gamma. value, 3, indicates solid phase diffusion as the mechanism of sintering. The lowest specific surface area was obtained for lime produced in a rotary kiln, 0.2 m2/g. The lime from the pilot scale fluidised bed was more soft-burned and had a specific surface area that was ten times larger, 2 m2/g. This is presumably due to the good mixing in the fluidised bed reactor, which enables high heat and mass transfer in the reactor. Accordingly, the temperature and the concentration of carbon dioxide is kept at a low level. The largest surface area obtained in this study was that of calcium carbonate p. a.calcined in pure nitrogen, 99 m2/g

Diepoxide treatment of softwood kraft pulp: influence on absorption properties of fibre networks

Many different approaches to the introduction of intra-fibre cross-links in fibres for use in absorption products are described in the patent literature, but relatively little has been done in terms of academic research. In this study, the long fibre fraction of a Scandinavian softwood kraft pulp has been cross-linked with the diepoxide 1,4-butanediol diglycidyl ether (BDDE). The fibre properties and the performance of the pulp in low density fibre networks were analyzed. In experiments, the cross-linking agent (BDDE) was dissolved in acetone and then mixed with the pulp. It was shown that a prior wash with NaOH (0.1 M) activates the polysaccharides and increases the extent of cross-linking, detected as a decrease in the water retention value. Chemical analysis of modified fibres also gave a clear indication of that cross-linking reactions actually occurred. The pulp properties could furthermore be influenced by varying time, temperature and the amount of BDDE in the reaction. It was also shown that it is possible to use water as a solvent for the cross-linking agent, even though the reactivity of water towards diepoxide must be considered to be high. The cross-linked pulps showed wet bulk under load comparable to that of the mechanical pulps and enhanced properties than unmodified Scandinavian softwood kraft pulp. It was also observed that fibre network test pads of the cross-linked pulp fibres, after being tested in the wet state, regained a substantially large part of their low-density structure when air-dried

Computational high-throughput screening of fluid permeability in heterogeneous fiber materials

We explore computational high-throughput screening as a design strategy for heterogeneous, isotropic fiber materials. Fluid permeability, a key property in the design of soft porous materials, is systematically studied using a multi-scale lattice Boltzmann framework. After characterizing microscopic permeability as a function of solid volume fraction in the microstructure, we perform high-throughput computational screening of in excess of 35 000 macrostructures consisting of a continuous bulk interrupted by spherical/elliptical domains with either lower or higher microscopic permeability (hence with two distinct microscopic solid volume fractions and therefore two distinct microscopic permeabilities) to assess which parameters determine macroscopic permeability for a fixed average solid volume fraction. We conclude that the fractions of bulk and domains and the distribution of solid volume fraction between them are the primary determinants of macroscopic permeability, and that a substantial increase in permeability compared to the corresponding homogenous material is attainable