Abstract
The influence of strain during drying is known to have a large effect on its mechanical properties like tensile strength, Young’s modulus and hygroexpansion. In this study we investigate free and restrained dried paper and investigate the relation between the paper microstructure and its mechanical properties. The first part of the work investigates the development of the mechanical properties as well as paper internal stresses and strains (elastic-, inelastic- and hygrostrain) at different moisture contents during the drying process. Emphasis is put on the changes of hygrostrains and the paper hygroexpansion coefficient during drying. One main finding was that in constrained drying the drying stresses are considerably below the yield stress and, as a consequence, the deformations are mainly inelastic (creep) and only marginally elastic. In the second part we are analyzing the microstructure of free and restrained dried sheets by X-ray microtomography (mu-CT). From the mu-CT analysis relevant network parameters such as number of contact points, fiber bond area, free fiber length and fiber curl were extracted. Also a novel method to investigate the interface angle of the fibers in the vicinity of the bond sites was established. The results showed that the major difference from the drying procedure manifests itself in the out-of-plane curl of the fibers. Statistically number of contact points, bond area and in-plane curl were not different whereas in-plane curl and free fiber length were different. The interface angle appears to be slightly affected by the drying procedure but is overall very low.
Highlights
Depending on whether the paper is dried freely or under constraint, the resulting product shows very dissimilar mechanical properties, even at the same density
We examine the changes in the microstructure and the mechanical properties paper sheets undergo during different drying regimes
The results show that the constrained drying increased the elastic modulus by a factor of 3–4 compared to the free drying at the same level of anisotropy and with the
Summary
Depending on whether the paper is dried freely or under constraint, the resulting product shows very dissimilar mechanical properties, even at the same density. The largest contribution to the hygroexpansivity has long been attributed to changes in the individual fiber dimensions in the transverse direction during drying. One of the pioneering works with direct observation of the transverse hygroexpansivity of softwood pulp fibers with the help of micro-radiography was presented by Tydeman et al (1966). Despite the essentially twodimensional observation allowing the visual detection of the fiber width change only, the authors used the optical density of the image on the radiograph to correlate it with the fiber thickness change. By doing this, they could observe both the width and thickness change. The fiber wall thickness was later reported to be an important parameter with thinner wall thickness resulting in larger hygroexpansion. Pulkkinen et al (2009), Uesaka and Moss (1997)
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