2013 Volume 1
Cambridgepp 449-468On Travelling Web Stability Including Material Viscoelasticity and Surrounding AirAbstractPDF
The aim of this research is to gain more understanding of the physics of the transportation of materials having viscoelastic characteristics, high transport speeds, a small thickness and a large surface area. This study introduces new models that take into account both material viscoelasticity and the fluid-structure interaction between the travel- ling material and the surrounding owing fluid. A web (continuum) travelling between two fixed supports is considered, modelling the web as a Kelvin–Voigt type viscoelastic panel and the air ow as a potential ow. Stability of the system is studied with the help of its eigenfrequencies (eigenvalues) for two different types of ow geometries. First, a ow inside an enclosure with a rectangular cross-direction, through which the panel is travelling, is added to the equations of out-of-plane motion of the panel with the help of added mass coefficients. Secondly, a free stream potential ow obstructed by the travelling panel is analyzed using the analytical solution for the aerodynamic reaction pressure. Some numerical examples are given for both models.
Cambridgepp 469-492The Heat of Sorption in Paper Drying – An Investigation of Measurement Methods and Influence of Pulp ParametersAbstractPDF
When the dry content during pulp drying reaches a level above 75% to 80% the free water, i.e. unbound water, has been evaporated. The remaining water is bonded to the surface due to physisorption, additional energy is necessary to overcome these bonding effects. This additional energy is called heat of sorption. At 80oC and a dry content of 95% for unbleached softwood kraft pulp the evaporation energy increases up to 2800 kJ per kg water compared to the latent heat of water of about 2300 kJ/kg at the same temperature.
Different measuring methods to determine the heat of sorption HS are described in the literature, the reported values for HS of pulp show large differences. The first aim of this work is to compare the results of different measurement methods using the same sample pulp. We investigated calculation of HS from sorption isotherms collected with a conventional climate chamber (ESC) and differential vapor sorption (DVS) analysis. Furthermore we applied direct measurement of the heat flux generated by sorption using differential scanning calorimetry (DSC), differential scanning calorimetry combined with thermogravimetric analysis (DSC/TGA) and reaction calorimetry (RC). All but one measurement method delivered consistent results in the range of HS=40 kJ/kg to 70 kJ/kg additional energy due to surface sorption. The advantages and disadvantages of the different measurement techniques are discussed.
In the second part of this work the impact of pulping and pulp treatment on the heat of sorption has been investigated. Refining and bleaching seems to have no impact. Pre drying reduces the overall heat of sorption by 15 %. The addition of inorganic fillers to paper reduces its heat of sorption due to a negligible HS of the filler.
In conclusion the total additional energy caused by sorption effects is less than 2% of the overall energy necessary to fully dry the pulp. Additionally the heat of sorption is nearly unaffected by pulping, bleaching, refining or pre-drying of the pulp. Therefor the heat of sorption HS is playing only a minor role for industrial applications of paper drying.
Cambridgepp 495-509Rheological Characterization of Micro-Fibrillated Cellulose Fibre Suspensions Using Multi scale Velocity Profile MeasurementsAbstractPDF
A rheometric method based on velocity pro ling simultaneously by optical coherence tomography and the ultrasound velocity profilometry was introduced and used in a preliminary study of the rheological and boundary layer ow properties of micro fibrillated cellulose. The two velocity pro ling methods appear adequate and complementary for rheological characterization of opaque complex fluids. The ultrasound method is useful in measuring the velocity profile in the interior parts of the tube, while the optical technique is capable of high-resolution measurement of the boundary layer ow close to the tube wall.
The preliminary results obtained for a 0.4% micro-fibrillated cellulose suspension show typical shear thinning behaviour in the interior part of the tube while the near wall behaviour shows existence of a slip layer of thickness ~200 m. Both the velocity profile measurement and the imaging mode data obtained by the optical coherence tomographic method indicate that the slip layer is related to a concentration gradient appearing near the tube wall. In a sublayer of thick- ness ~100 m, the fluid appears nearly Newtonian, and the viscosity value approaches that of pure water with decreasing distance from the wall.
Cambridgepp 511-520Experimental Investigation of the Influence of Fibre Morphology on the Interrelation of Flocculation and Network StrengthAbstractPDF
It is widely accepted and discussed in the literature that fiber morphological parameters like fiber length, fiber curl and fiber flexibility affect flocculation phenomena and the properties of the fiber network in suspension via mechanical entanglement and forces arising at fiber to fiber contact points. The focus of this work is the interrelation of the parameters floc size distribution, network strength and fiber morphology. Pulp samples of different length distribution, fiber curl, kink index and fiber flexibility are evaluated concerning their flocculation tendency and the strength of the fiber network using methods already established in the literature. The simultaneous measurement allows the investigation of the interrelation of these parameters. It is shown that floc size distribution and network strength are highly correlated. Still, depending on the morphological properties fiber curl, kink index and fiber flexibility, samples of comparable floc size distribution and different network strength (and vice versa) are evident.
Cambridgepp 521-538Observation of the Turbulent Transition of A Fibre Suspension in Hagen-Poiseuille FlowAbstractPDF
The focus of the present work is an experimental study of the transition to turbulent ow of papermaking fibre suspensions in a cylindrical pipe. The suspensions used in this study possess yield stress. With this class of fluid the axial profile in fully developed slow ow is characterized by an unyielded or plug zone. With increasing ow rates the size of the plug diminishes. One of the remaining open questions with these suspensions is the role of the plug during transition.
In this work we characterize the size of the plug using ultrasound Doppler velocimetry (UDV) as a function of ow rate for dilute, i.e. less than 2% consistency, papermaking suspensions in a 50 mm diameter, 10 m long cylindrical pipe. The plug size was determined through analysis of local spatial and temporal variations of the velocity, strain- rate and the fluctuating component of velocity. With this, we were able to estimate the yield stress of the suspension through knowledge of the applied pressure gradient and find the yield stress to be in the range of 2–10 Pa, depending upon the consistency and Reynolds number Re. We observe complex behavior with the plug in which we see initially that with increasing velocity, the plug diminishes through a densification-type mechanism in a response to an increase frictional pressure drop. At higher Re, it diminishes through an erosion-type behavior. We estimate the critical Reynolds number Rec for the disappearance of the plug to be Rec ~ 105.
Cambridgepp 585-598Detailed Insights to Liquid Absorption and Liquid-Paper InteractionAbstractPDF
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.
Cambridgepp 599-618Designing Microfabricated Paper Devices through Tailored Polymer AttachmentAbstractPDF
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.
Cambridgepp 619-634Inﬁltration and Dimensional Scaling of Inkjet Droplets on Thick Isotropic Porous MaterialsAbstractPDF
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.
Cambridgepp 637-672Utilization of Modiﬁed Linear Elastic Fracture Mechanics to Characterize the Fracture Resistance of PaperAbstractPDF
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.