Proceeding Articles

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We present a method which provides detailed insights to the dynamics of the water absorption process and water- paper interaction, based on transmittance measurements of ultrasonic beams. We found that the water absorption process of an uncoated paper- sheet comprises two consecutive time regimes. The underlying mechanism that governs the regimes’ shift is the combination of fibre surface modification by water and the recreation of the fibres lumen after wetting. In the first regime, water advances along the dry pore surface, which is hydro neutral, and the water forms a solid column inside the capillary (pore), while in the second regime, moving along the primed (wetted) surface of the capillary is a more favourable path as the surface becomes hydrophilic when wetted. Consequently, the water may not necessarily ¿ ll the entire capillary when the capillary expands in volume due to hydro- expansion and hence forms a hollow water column. We propose a model that enables us to determine/predict the depth of water absorption by the dry pore structure of the paper which is often the case for ink- paper interaction during printing. The results of our studies suggest that the depth of water penetration along the dry pore surfaces can very well be described by the Bosanquet model.

In the present paper we show that polystyrene–based copolymers, which carry a defined amount of photo–reactive benzophenone moities can be transferred and immobilized to paper substrates via a simple dip coating approach and subsequent illumination of the paper substrates with UV-light. Non-bound macromolecules can be removed from the cellulose fibers by solvent extraction. Thereby, the amount of immobilized polymer can be adjusted over a wide range by changing the polymer concentration in the dip coating solution. The resulting polymer-modified paper substrates were characterized using IR spectroscopy, scanning electron microscopy (SEM), fluorescence microscopy and static contact angle measurements. The polymers are attached to cellulose fibers using a photo–chemical approach and stable chemical micro patterns, including paper-defined microchannels, can be designed inside model paper substrates by using conventional UV-lithography. These channels are capable to control the fluid penetration by capillary actions. An engineering of the paper substrate itself allows to modulate the speed of the fluid transport of an aqueous solution inside paper-defined microchannels. The latter will become important for a number of applications.

We study the imbibition of picoliters (pL) sized inkjet droplets on controlled pore glass membranes (CPG). We do so using a variety of liquids, i.e., water, formamide and diiodomethane, as well as the CPG substrates, and measure the evolution of the imbibition process using high speed digital imaging. Here, experiments were conducted with a wide range of initial drop volume (100–600 pL) on 2–280 nm CPG membranes. We derive scaling laws through dimensional analysis of the equations of motion, and consider experimental parameters and liquid properties.

Linear elastic fracture mechanics modified to account for an effective fracture process zone is sufficient to characterize and predict fracture resistance for a wide range of papers. The simplicity of the method, which only requires the tensile strength and a measure of the effective fracture process zone length, gives it great advantage over other existing approaches. The results presented here show that for a wide range of commercial papers, samples widths as narrow as 50 mm are sufficient to determine the effective process zone length, and that scaling holds well enough to allow prediction for fracture of wide webs. The results indicate that the tensile strength of paper is a result of a fracture process where the defect is most typically induced from cutting the network structure along the edges. As a consequence, the inherent tensile strength of the network can be significantly larger than the measured tensile strength. The effective fracture process zone length parameter is taken as a measure of the inability for the paper to concentrate load near the crack tip. This ability for network structures to concentrate load has significant impact on the fracture resistance of the sheet relative to its tensile strength.

To design new and optimize existing paper and board material an understanding of how the paper making process affects the final paper properties and how we can control them is neccesary. There is a link missing between pulp properties and machine made paper properties. The aim with this work is to close this gap by proposing an engineering model which, based on furnish properties, makes it possible to predict tensile property profiles in MD and CD.

Two series of hand sheet trials were made to validate and formulate the model. The purpose with the first trial was to validate that the geometric mean of the studied properties in MD and CD is constant and equal to the isotropic value. The second trial was made to find relations between anisotropies of the studied properties and the fibre anisotropy.

The model was applied on a press draw trial made on a production machine. The strain and tensile property profiles were measured and predicted based on laboratory measurements on the furnish. The predictive capability of the model was regarded as fairly good, especially since the general behaviour of the paper properties was correctly captured. The deviation of predictions compared to measurements were around 10% or less for most of the evaluated positions and properties, except for MD tensile energy absorption index that was poorly predicted.

This paper concerns the question of how to predict mechanical performance of box and paperboard subjected to fluctuating load/ environmental conditions encountered in end-use. Particularly such performance is notoriously variable (stochastic), and is known to be very difficult to predict.

We have developed a theoretical framework for treating time- dependent, statistical failure based on the recent progresses in statistical physics of disordered materials. The main objective of this study is to experimentally determine the three key parameters that fully characterise the failure of component board subjected to general loading histories, namely the parameter c related to static strength and its uniformity, the load sensitivity/durability parameter W, and the uniformity parameter G of creep lifetime. Results showed that creep lifetime distribution is highly skewed with extreme scatters, but the distribution is still a class of Weibull distribution and can be handled without any problem. The durability parameter W also showed high values comparable with those for fibre-composites. These two results explained very well the variability and load sensitivity of box creep performance observed in the literature.

This proposed approach offers a new set of material property parameters, other than traditional strength, that can be fully exploited in both materials and structural design to enhance end- use performance in the most resource- efficient manner.

Nanofibrillar celluloses are promising new bio-based nanomaterials that can be prepared from paper- grade chemical pulps and other plant celluloses by mechanical shearing in water, usually after pretreatments. For example, enzymatic hydrolysis, carboxymethylation, addition of cationic polymers, TEMPO-mediated oxidation and others have been applied as wood cellulose pretreatments to reduce the energy consumption of the mechanical shearing process and to improve nanofibrillation level. Nanofibrillated celluloses (NFCs) prepared from wood cellulose by either enzymatic hydrolysis or partial carboxymethylation and subsequent mechanical shearing in water are convertible to nanopaper films and aerogels using a filtration process like that used in papermaking, which is advantageous for efficient removal of water from the strongly swollen NFC/water dispersions. NFCs have high molecular weights and long fibrils and form fibril network structures both in aqueous dispersions and dried nanopaper films/aerogels. This makes them preferable for use as base materials for nanocomposites. Thus, various nanopaper/matrix composites have been prepared, some of which show remarkably high mechanical strength including high ductility. When TEMPO- mediated oxidation is used as the pretreatment, almost completely individualized TEMPO-o xidized cellulose nanofibrils (TOCNs) with homogeneous widths of ~3 nm dispersed in water can be prepared from oxidized wood celluloses with carboxylate contents >1.2 mmol/g by gentle mechanical disintegration treatment. Because TOCN elements form nematic-ordered structures due to their self- assembling behavior in water, TOCNs are able to be converted to dense films with plywood- like layered structures, stiff hydrogels by acid treatment, aerogels with extremely high specific surface areas, and other unique bulk materials. When TOCNs are used to make nanocomposite materials, high mechanical strengths and gas- barrier properties can be achieved even with low TOCN-loading ratios.