2022 Volume 1

We discuss a new mean-field theory to describe the compression behaviour of thick low-density fibre networks. The theory is based on the idea that in very large systems, the statistics of free segment lengths causes the stress-deformation behaviour to be quantitatively predictable. The theoretical ideas are supported by several different experimental characterisations. Firstly, we have carried out single-fibre buckling tests using hemp fibres, which indicate a maximum level of axial stress before deformation localization, after which the load carrying ability of a fibre decays. Secondly, the stress-compression behaviour of over 130 different foam-formed lightweight fibre materials were measured. For kraft pulps with low fines content, the average stress compression behaviour closely follows the theoretical prediction as described in terms of a universal s-function. Moreover, the acoustic emission can be described by the same function until collective phenomena cause deviations from the predicted behaviour. Similar deviations at smaller compressive strains are seen with furnishes with high fines content or added nanocelluloses together with samples with large voids. The localized buckling deformations lead to rapid stress re-distributions and subsequent fibre displacements in a fibre network as shown with in-situ CCD imaging.

When coating lightweight grade paper with aqueous coating color, wrinkles could appear on roll to roll coating system. This study is conducted from fundamental and theoretical point of view to laboratory experiments. The impact of water content will be taken in count. It has been experimentally observed that the appearance of wrinkles during a coating process on a roll to roll pilot depends on different parameters: web tension; misalignment angle; the coefficient of friction between the roll and the side of the paper in contact; the water content in the paper that appears to favour the wrinkles formation. Moreover, this work highlights the impact of discretisation of the variable represented wrinkles number i on the prediction model. Initial tests of the use of DIC during a tensile test have been carried out. These initial tests are very promising and the development of this technique is currently being developed. The perspective of this work is the improvement of the DIC study on the formation of out-of-plane displacement during the coating process. It can reduce the cost of non-conformities and thus reduce the waste of raw materials.

The larger hygro-expansivity of freely compared to restrained dried handsheets has been intensively studied during the past decades. To investigate the role of the fibers forming the sheets on this complex phenomenon, in this work, the hygro-expansivity of fibers picked from freely and restrained dried handsheets is characterized. To do so, a versatile, highly accurate, fiber hygro-expansion methodology based on Global Digital Height Correlation is proposed, which enables identification of the transient full-field hygro expansivity of single paper fibers. It was found that the longitudinal, transverse and shear hygro-expansivity of fibers picked from freely dried handsheets is significantly larger than fibers picked from the restrained dried handsheet. Furthermore, a restrained dried fiber can yield the hygroexpansivity of a freely dried fiber after being subjected to a sufficiently long wetting period, implying that the moisture-induced release of dried-in strain drives the hygro-expansivity differences. Finally, the sheet-scale hygro-expansivity is comparable to longitudinal fiber hygro-expansivity for both handsheet types. The presented results are of key importance for understanding the paper hygromechanics and improve their applicability.

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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