Proceeding Articles

The paper concerns itself with the effect of pre-curl (induced curvature during the wet state) on cockling when paper is dried. Laboratory experiments are carried out on copy paper to identify this interplay between induced curvature and cockling. Using the uniform wavelike deformation that occurs during the intermediate stage of drying in narrow paper strips as a proxy for the extent of cockling, it is demonstrated that increased pre-curl in MD, alleviates cockling. In addition, under two contrasting drying boundary conditions of free drying on a flat table and hang drying without weights, it is seen that free drying induces more cockles than hang drying. As a corroboration of the experimental observations, numerical experiments are carried out on flat (varying tensile loads) and curved specimens (varying radii), using moisture-induced deformations together with an orthotropic elastic material model. Using the Hausdorff distance metric to compare the deformed and undeformed geometries, it is shown that, both pre-curl in MD and tensile loading in CD, alleviate cockling. The results of these experiments could be useful for aftermarket applications of paper, like printing, where pre-curl could be induced mechanically to reduce or prevent cockling.

The characterization of pore sizes in paper is an important parameter, as numerous modifications of paper fibers influence or even aim to change them. yet, most methods for determining pore size only work in the absence of water (e.g. mercury porosimetry or computed tomography). However, the influence of swelling on pore size caused by water is of great interest, especially, but not only, for the porous material paper. Here we present a new method for determining the characteristic pore radius of paper sheets, being in direct contact with water. We call our device “porosity centrifuge”, in which the capillary forces that develop during the wetting and swelling process within a paper sheet are counter-balanced by a matching centrifugal force. While the capillary pressure is determined by the pore radius of the porous structure, some paper intrinsic parameters lead to a reduction from the predicted imbibition distance calculated from the force balance between centrifugal and capillary forces. Since we are able to modulate the degree of this reduction by changing the fiber type or by applying various fiber pretreatments, such as beating, reduction of fines content or calendering, we refer to it as “substrate coefficient”. Our method enables a simple and fast determination of characteristic pore radii in paper sheets using water as liquid.

The picture of the smallest structural units of wood fibres, that is, cellulose microfibrils and their bundles, has become more accurate during the last couple of decades, when information gained from several experimental characterisations has been drawn together. This work has been supported by computational methods that allow one to test the behaviour of postulated structures on the nanometre scale, and thus help in interpreting the experimental data. Bound water is an essential component in these models, as it affects both the structural swelling and the mechanical properties of the fibre wall nanostructure. Moreover, mechanisms on this scale can be expected to drive similar properties of macroscopic fibres. We suggest that several large-scale problems in papermaking and converting could be approached with atomistic molecular dynamics simulations for varied chemical compositions and external conditions. We demonstrate this by first showing that simulated moisture diffusion rates agree with measured ones at room temperature, and then determine diffusion rates at elevated temperatures that lack reliable experimental data. These predictions provide key knowledge for further development of high-temperature drying and pressing processes. The results are important also when linking material performance at varied external conditions to the composition of the fibres.

The imbibition of paper with water is a complex interplay between the transport of liquid water in the fiber network and a swelling of the fibers. While the swelling can be directly associated with the incorporation of water into the fibers, little is known how swelling affects the pore space in paper and the subsequent transport of liquids therein. In this work, the propagation of a water drop supplied to a sheet of paper is monitored in-situ using X-ray microcomputed tomography. This method provides a 3D image sequence that traces the time-dependent progression of swelling and liquid transport over a period between fifteen and forty-five minutes after the application of water. The associated water vapor appears to profoundly swell the fibers even before the liquid water front arrives, i.e., the water invades paper both in its liquid and its vapor state. Incorporation of water into fibers leads to a marked increase in sheet thickness that originates from an increase in fiber volume and, interestingly, from an effective increase of the volume of the pores. Whether the latter change in pore volume has an adverse or boosting effect on liquid transport cannot be established from the data because liquid water does not reside in interfiber pores. Instead, liquid water is found in the lumen of the fibers.

This paper describes the investigation of the water absorption behavior of low density, fibrous kitchen towels at dimensions from 10^-3 mm to 10² mm. The investigation involved both the experimental observation of radial wetting from a point source and the numerical simulation of the wetting using X-ray 3D microscopic data sets obtained from the same set of towel papers that were representative of conventional and premium products. The overarching aim was to examine the validity of the simulation, based on fundamental surface energetics of the condensed phases, in predicting the local flow patterns that are dependent on various structural features found in retail kitchen towel products. This study explored the relationships between local structural properties, including thickness, grammage, apparent density and out of plane deformation. Experimental results examined the local flow velocity in and around the various structural features of conventional and structured towels. Analysis of both experimental and simulated liquid regions included calculation of water absorbency capacity, aspect ratio, density analysis and through holes.

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