Volume 27, Issue 2

A motive for fatiguing wood prior to defibration would be to reduce the energy consumption needed in mechanical pulping processes. Therefore, the effects of fatiguing pre-treatment were here studied on wood samples, on defibration and also on produced paper. The results indicate that pre-fatiguing changes the mechanic response of wood to be more favorable for harsh defibration which in turn is positive for the process efficiency.

The flow pattern in the grooves plays a major role for the homogeneity of refining as well as for the transfer and loading of fiber flocs in refining position on the bar edges. However, it is an area where very little information is available. In the present study, flow conditions in the grooves in a Low-Consistency (LC) - disc refiner were studied both experimentally and numerically. The experimental study involved high-speed imaging through a 3 cm peephole into a commercial refiner. The Computational Fluid Dynamics (CFD) simulation focused on the flow condition in a radial groove, considering both Newtonian and non-Newtonian flows. Flow conditions for stator and rotor grooves were modeled along the groove at different angular speeds and pressure differences over the refiner. Both the experimental and the modeling results show a dual flow pattern in the grooves; a rotational/spiral movement at the top of the groove and a flow in the direction of the groove at the bottom, which to the authors knowledge has not been reported in literature. The strong vortical motion at the top of the grooves observed both for the rotor and the stator are believed to be important for placing the fibers onto the bar edges and to induce shear forces in such a way that the fibers get treated. Moreover, a large sensitivity to suspension properties in terms of the development of flow pattern was detected.

Fiber suspension simulations are challenging since they involve transient fluid flow with immersed solid objects subject to large displacements and rotations. In the present work, a beam model in corotational formulation is coupled with a fluid solver using immersed boundary methods. The model is used to simulate a fiber in a shear flow and excellent agreement is found with Jeffery's equations. The shapes of fibers deforming in a shear flow are found to be in qualitative agreement with shapes observed in experiments. The flow of a fiber suspension is studied by simulating early paper forming with one-way and semi-two-way coupling. It is found that the flow through the fiber web needs to be resolved in order to predict the retention of fibers in the fiber web.

In this work, critical design and operational parameters for retention aids dosage are studied through a combination of computational fluid dynamics (CFD), experimentation and pilot-scale production trials. In the first part of this work, three different retention aids dosage strategies are investigated in conjunction with pilot scale production trials. In all dosage strategies, a maximum in the percentage filler retention was observed at a speed ratio of 1.1, while considerably lower retention levels were observed when the speed ratio was greater than 2.2. However, the different dosage strategies led to markedly different retention of filler material. In the second part of this work, two-phase computational fluid dynamics (CFD) was used to model the three different dosage strategies implemented in the pilot production trials. The location and magnitude of maximum strain in each nozzle was determined and for each dosage case this was found to occur just outside the dosage nozzle at the point of impingement between the dosage and outer flows. In the third part of this work, conditions leading to the onset of retention polymer degradation were determined using an experimental flow loop. The effect of dosage speed and elongational strain created inside the dosage nozzle were studied systematically. These experiments showed that the effect of relative dosage velocity on polymer degradation was minimal. However, large levels of polymer degradation were observed when the elongational strain in the dosage nozzle was increased, i.e. when the exit nozzle diameter was decreased. Together, the three sets of experiments suggest that elongational strain during dosage degrades retention aids polymers and therefore hinders filler retention during production.

An investigation of the impact of particle size on the mechanical retention of particles in a fibre network has been conducted. The particles used were five sets of quartz particle fractions having fairly narrow particle size distributions with average particle size ranging from a few μm to around 100 μm. The particles were used to model flocculated filler aggregates as part of a larger study of the effect of pre-flocculation on mechanical retention. Pre-flocculation of the filler is a possible strategy to increase the filler content of paper without deterioration of strength properties. A modified laboratory hand sheet former, known as the Rapid Drainage Device (RDD) was used. The major modification consisted of a long pipe that acted as a suction leg, which provides a dewatering vacuum at the same level as on a paper machine. The experimental results showed that mechanical filler retention increased linearly with particle size and grammage of the fibre layer above a critical grammage which depended on particle size. The linear relation was also seen in a pilot scale trial on the FEX pilot-paper machine at Innventia. During this trial fine paper was produced using pre-flocculated filler where the mean particle size of the flocs and fibres was measured in the flow to the headbox. The results from this pilot trial show that mechanical retention is an important part of the total filler retention. Drainage time and therefore drainage resistance increased with the grammage of the fibre layer and amount of quartz particle added. Drainage time, compared at total grammage (i.e. the sum of fibre and quartz particle grammage) was lowest for a fraction of medium-sized particles, with a median size of 35 mm. There was no obvious effect on retention or drainage resistance of a change in the dewatering pressure from 27.5 to 41.5 kPa.

This paper concerns the different refining loading conditions of a bleached softwood Kraft pulp on a Valley beater. The refining trials were performed in two stages; each stage was characterized by a constant normal force applied on the pulp in the gap clearance. In the first stage, the pulp was refined from an initial to a given pulp slowness (SR-45), with a force F n1 . Then, the refining was continued from the intermediate (SR-45) to the final pulp slowness with the force F n2 . By analogy with the pressing operation, we propose to introduce the concept of refining impulse defined as the product of the applied normal force by the duration time. This quantity is equal to the ratio of the refining energy to the peripheral speed. The refining intensity is proportional to the refining impulse divided by the number of crossing points. It was demonstrated that the pulp slowness evolution (SR) may be plotted as a master curve of the refining impulse. Consequently, the successive SR evolutions are only dependent on the refining impulse. It has to be noted that this quantity might be completed by the normal force itself to account for the prediction of the shortening of fibres.

A three dimensional numerical investigation of a commercial four-ply paperboard formed into a pear-shaped mould was presented. The numerical investigation included the effect of pressure, boundary conditions, material properties and different deformation and damage mechanisms such as delamination and plasticity. Simulations were done in both the MD and CD using different pressures. A paperboard model with a combination of anisotropic continuum model and a softening interface model had good deformation behavior during the forming simulations. Forming experiment that mimicked the simulations was performed. Numerical and experiment results were compared with good agreement.

Foam forming leads to sheet structures with exceptional volume of large pores. The link between fibre network structure and foam properties is investigated by comparing pore structure with measured bubble-size distribution. In foams produced by mechanical mixing, higher rotor speed leads to smaller average bubble size, whereas the effects coming from air content and surfactant are smaller and non-systematic. A significant drop in the average bubble size is seen when mixing fibres to foam. In sheets made with foam forming, there are more large pores compared to the water formed sheets. The size of these pores is affected by the sizes of the bubbles in the foam. Overall, pore size distribution is more strongly affected by the fibre type than by small changes in bubble size distribution.

The Scott bond test is the most commonly used test method for quantifying the delamination resistance of paper and board. The objective of this investigation was to validate the hypothesis that the Scott bond value would be dominated by the total energy under the force elongation curve in a z-directional tensile test. The investigation comprised three types of hand sheets with comparatively low strength values. Three test methods were used to obtain the energy for delamination: 1) Z-test, a z-directional tensile test, 2) Scott bond test, and 3) Simulated Scott bond test, a Scott bond type of test performed in a hydraulic tensile tester. The test data were expressed as a correlation between the failure energy obtained from the Z-test and the other two tests. The results showed that the Scott bond test gave slightly higher values than the Z-test for the weakest paper, but that the value tended to be much higher for the stronger papers. On the other hand the Simulated Scott bond test tended to give lower values than the Z-test. High speed photography was used to reveal several energy consuming mechanisms in the Scott bond test that can explain why this test gave higher values than the Ztest. The lower values from the Simulated Scott bond values are more difficult to explain. At this stage we can suggest that the failure mechanism is different if the paper is delaminated by pure tension or by a gradual delamination as in the Scott bond test.

In this paper a novel direct method for mechanical testing of single fibers and fiber joints is presented. Additional to the measurement of joint strength it is also possible to investigate loading geometry, deformation during testing, bending stiffness of fibers as well as bonding energy. Furthermore biaxial loading of fiber-fiber joints is possible. Initial experiments have shown that mode II and III shear strength of fiber-fiber joints can be measured with this setup. Initial results for fibers and fiber joints of unbleached softwood kraft pulp are presented. The resulting values are 1.1724·10 -9 to 3.8798·10 -11 Nm² for bending stiffness, 3.9180·10 -10 to 2.8793·10 -11 kJ/bond for bonding energy, 6.54 mN for mode II and 1.057 mN for mode III mean shear strength. Mode II mean shear strength under biaxial load with a preload of 20 mN perpendicular to the loading direction of the fiber-fiber joint was found to be 2.52 mN. These values are compared to results of other measurement techniques and discussed.

By using X-ray synchrotron microtomography imaging, this work aims at proposing a method to get 3D information on paper fibrous microstructures. Such technique is useful to better understand the links between the manufacturing conditions, the resulting microstructural and mechanical properties of the paper fibrous networks, together with the morphology of fibres and fibre-fibre bonds. Its usefulness is illustrated for the 3D analysis of model papers being produced by changing the wet pressing conditions. It is demonstrated that the image analysis allows the changes of parameters describing, for example, the fibre cross section shape and inclination, the bond area surfaces, the distance between bonds to be followed with respect to the processing conditions for a large set of fibres and bonds. The distributions of properties that can be drawn from this experimental analysis will allow mechanical or physical discrete modelling approaches for papers to be enriched.

The structure of paper influences its properties and simulations of it are necessary to understand the impact of fiber and papermaking conditions on the sheet properties. We show a method to develop a representative structure of paper by merging different simulation techniques for the forming section and the pressing operation. The simulation follows the bending and drape of fibers over one another in the final structure and allows estimation of sheet properties without recourse to arbitrary bending rules or experimental measurements of density and/or RBA. Fibers are first modeled as jointed beams following the fluid mechanics in the forming section. The sheet structure obtained from this is representative of the wet sheet from the couch. The pressing simulation discretizes fibers into a number of solid elements around the lumen. Bonding between fibers is simulated using spring elements. The resulting fiber network was analyzed to determine its elastic modulus and deformation under small strains. The influence of fiber dimensions, namely fiber lengths, widths and thicknesses as well as bond stiffnesses on the elasticity of the network are studied. A brief account of inclusion of fines, represented by individual cubical elements is also shown.

We present a method for 3D measurement of fiber cross sectional morphology from handsheets. An automated procedure is used to acquire 3D datasets of fiber cross sectional images using an automated microtome and light microscopy. The fiber cross section geometry is extracted using digital image analysis. Simple sample preparation and highly automated image acquisition and image analysis are providing an efficient tool to analyze large samples. It is demonstrated that if fibers are tilted towards the image plane the images of fiber cross sections are always larger than the true fiber cross section geometry. In our analysis the tilting angles of the fibers to the image plane are measured. The resulting fiber cross sectional images are distorted to compensate the error due to fiber tilt, restoring the true fiber cross sectional shape. We use an approximated correction, the paper provides error estimates of the approximation. Measurement results for fiber wall thickness, fiber coarseness and fiber collapse are presented for one hardwood and one softwood pulp.

We present a method of determining the number of contacts in a planar fibrous network by first determining its average fibre segment length. An analytical expression is derived for the mean fibre segment length in terms of the average shortest path along fibres through the sample in the out-of-plane direction. The model is validated by applying the method to generated fibre networks, for which the segment lengths are known. Both the theoretical and experimental results show that the contact density is essentially proportional to the square of the solid content with a prefactor that depends only on the geometry of the fibres. The method is also tested on physical samples of cardboard, which were imaged with high resolution micro X-ray computed tomography. The results of the cardboards show similar behaviour as the generated fibre networks.

The transient response of a Population Balance based colloidal rheology model is studied in the context of nanofiber suspension rheology research. The model is calibrated against experimental rheology data for cellulose nano-whisker suspension and then subjected to transient shears. The non-equilibrium aggregate size distributions are reported.

We have carried out atomistic molecular dynamics simulations to study the mechanical properties of cellulose nanofibrils in water and ethanol. The studied elementary fibrils consisted of regions having 34 or 36 cellulose chains whose cross-sectional diameter across the fibril was roughly 3.4 nm. The models used in simulations included both crystalline and non-crystalline regions, where the latter were designed to describe the essentials parts of amorphous cellulose nanofibrils. We examined different numbers of connecting chains between the crystallites, and found out that the elastic constants, inelastic deformations, and strength of the fibril depend on this number. For example, the elastic modulus for the whole fibril can be estimated to increase by 4 GPa for each additional connecting chain.

The performance of the paper in a number of converting operations such as creasing, bending, printing, and plastic coating put great demands on the mechanical properties in the thickness direction of the material. The knowledge of strength, elastic- and plastic behavior in tension and compression in the thickness direction is needed for a comprehensive description of the performance of the material in these operations. In spite of its importance, very few publications deal with the evaluation of the entire tensile stress-strain curve of paper in the thickness direction. A likely reason for this is the intrinsic difficulty of testing a thin, uneven, porous, fibrous and compressible material such as paper with sufficient precision and testing time efficiency. The z-directional strength test is usually performed by fastening the paper by means of double-adhesive tape to metal platens. The platens are fastened in a testing machine and strained to break. The adhesion of the tape is the limiting factors for how strong papers that can be tested. The tape-based method also is expected to have a lower limit in grammage due to the penetration of the adhesive. The aim of the present publication was to show a procedure how to evaluate the entire stress-elongation curve in the z-direction of papers, using a lamination method for fastening the paper to the metal platens. From this curve the z-strength, z-modulus, z-strain at break, zenergy at break and z-fracture energy could be extracted. Such information is, so far, non-existing in the literature.

The impact of sample size on in-plane strain behavior in paperboard was investigated, with the aim to explore the differences between local and global properties in paperboard, and try to pinpoint the mechanisms behind such differences. The local properties are of interest in converting as well as for future 3D forming of paperboard. It is important to identify differences in behavior between local and global properties since most paperboards are evaluated against the latter. The methods used for evaluation were tensile tests in controlled environment and speckle photography. The results show that there is a difference in strain behavior that is dependent of the length to width ratio of the sample, that this behavior cannot be predicted by standard tensile tests and that it depends on the board composition. The speckle analysis revealed that the behavior is a result of the activation of strain zones in the sample. These zones are relatively constant in size and therefore contribute differently to total strain in samples of different size.

The presented results suggest that the concept of efficiency factor previously used to demonstrate that changes in inter-fiber bonding in paper do not change the shape of the stress-strain curve can be extended to describe the changes that are observed in the tensile response of paper subjected to previous straining. It is found that the pre-yielding response for samples that have fully recovered from previous straining scales with changes in maximum tangent modulus. This deformation is mainly recoverable. When the scaling holds, one can extract a reasonable approximation of the initial recoverable deformation, which is separate from the plastic deformation. In essence, the efficiency factor acts as a stress magnification factor that easily can be incorporated into a constitutive equation. Tracking the change in efficiency factor with straining allows one to account for the loss of observed compliance for the entire range of recoverable deformation.

A test method for simultaneous contactless measurement of lateral contraction and change in thickness of paper in tensile testing is presented. All the investigated paper samples showed transverse shrinkage as well as an increase in thickness during loading. Consequently, in-plane Poisson’s ratios for different tested papers were positive, whereas Poisson’s ratios in thickness direction had negative values. In general, the transverse shrinkage proved to be detectable while the resolution and accuracy of the thickness determination in some cases still requires optimization.

We have investigated a relation between micromechanical processes and the stress-strain curve of a dry fiber network during tensile loading. By using a detailed particle-level simulation tool we investigate, among other things, the impact of “non-traditional” bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds. This is probably the first three-dimensional model which is capable of simulating the fracture process of paper accounting for nonlinearities at the fiber level and bond failures. The failure behavior of the network considered in the study could be changed significantly by relatively small changes in bond strength, as compared to the scatter in bonding data found in the literature. We have identified that compliance of the bonding regions has a significant impact on network strength. By comparing networks with weak and strong bonds, we concluded that large local strains are the precursors of bond failures and not the other way around.

Thermography has been used to quantitatively analyze the plastic and elastic energy during deformation of paper. Sack paper samples were subjected to uniaxial tensile testing until rupture occurred. The temperature of the sample was simultaneously recorded with an infrared camera. The mechanical energy invested in the deformation was determined based on the force and deformation data. The thermal energy that accumulated in the sample during testing was estimated using the temperature measurements. Here, special attention was put on using the correct emittance values for the sack paper by measuring it with a new method. When comparing exerted mechanical energy with released thermal energy up to the time of sample rupture, about 40% to 60% of the mechanical energy could be detected as thermal energy. The lacking share of heat was most likely lost due to cooling of the sample during the experiments, as a lower share of detected mechanical energy was obtained for longer experiments. When comparing the increase in thermal energy during rupture to the elastic energy stored in the sample, an agreement of better than 90% was found.

Fibre deformations have a significant effect on fibre strength and sheet properties. There is little information, however, on the kinds of deformations different types of treatments induce and how they affect the fibre strength. In the present study, first-thinning bleached pine kraft pulp was treated with three mechanical devices: a wing defibrator (high consistency treatment), an E-compactor (high consistency treatment) and a conventional Valley beater (low consistency treatment). The fibre properties were determined with a fibre analyser. The fibre cutting induced by the hydrochloric acid (HCl) treatment (‘cleavage index’) was used for the quantification of the fibre defects. The zero-span tensile strength of dry and wet paper was used to estimate the fibre strength. Each mechanical treatment induced fibre deformations in its own characteristic way. The wing defibrator induced fibre kinks and curl whereas the E-compactor, in addition to fibre cutting, favoured kinks. Low consistency Valley beating straightened the fibres and released fibre deformations. The fibre deformations, especially the number of kinks, correlated well with the wet zero-span tensile strength. The cleavage index had some correlation with the zero-span tensile strength, but the results indicated that the cleavage index may not be directly related to the mechanical defects in fibres but depend more on the chemical conditions on the fibre surface and the wall structure.

This paper investigates the use of zerospan testing to measure the tensile strength of cellulose nanofibre sheets. The mechanical strength of cellulose nanofibre paper is a key property but tensile strength measurement requires a substantial amount test material, whereas a zero/short span test needs much less material. Sheets made from cellulose nanofibres, microfibrillated cellulose (MFC) and northern bleached softwood fibres were tested at spans ranging from 0 to 0.6 mm and tensile strength spans of 50 and 100 mm. For the cellulose nanofibres or the MFC sheets, strength was constant with span from 0 to 0.6 mm when tested dry and negligible when tested wet, except at zero span. The sheets made from the softwood fibres showed significant strength when tested wet at all spans from 0 to 0.6 mm. The results showed that for nanofibre materials, the zero or short span strength is measuring sheet tensile strength at a smaller sample length. The strength of the nanofibre sheets at 50 or 100 mm was smaller than at zero/short span due to sample size effects and Poisson contraction. The effects of starch and grammage on strength were also studied.

The aim of the present work was to develop an analytic procedure for determination of the fracture toughness of paper materials based on laboratory material test data. Isotropic deformation theory of plasticity was used to model the tensile material behaviour of six different commercial paper grades. Closed-form analytic expressions for calibrating the material model based on tensile test data were developed. The analytically calibrated material model was shown to predict the non-linear tensile stress-strain behaviour of the investigated paper grades excellently. A closed-form analytic expression for determination of fracture toughness was developed based on the used material model and J-integral theory. The fracture toughness of the investigated paper grades was determined analytically based on laboratory fracture toughness test data. The suggested analytic procedure for determination of the fracture toughness was shown to be in excellent agreement with determinations of fracture toughness based on finite element analysis.

The aim of the present work was to develop an analytic fracture mechanics procedure that enables accurate predictions of failure of paper materials. Analytic expressions for prediction of the critical force and critical elongation of edge-notched paper webs were developed based on isotropic deformation theory of plasticity and J-integral theory. The analytic expressions were applied to predict the critical force and elongation of paper webs with different edge-notch sizes for six different paper materials. The accuracy of the analytic failure predictions was verified by numerical predictions and experiments on edge-notched paper webs, showing that the developed engineering fracture mechanics analysis procedure predicted failure accurately.

Time-dependent, stochastic failure is most common in practical situations where paper and board are used, such as web breaks and box failure. A theoretical treatment has been presented, starting from Coleman’s stochastic failure model to the models with structural hierarchy. Creep failure data of box and board published in the literature were analysed from the light of the proposed theoretical model. The results showed that the distribution of life time (not only mean life time) of corrugated boxes changes very sensitively with loading and environmental conditions. Generally the coefficient of variation varied from 6% to 90%. Another important parameter, the load sensitivity of damage growth also varied from 1.7 to 27. Such enormous variation of the key parameters is not a mere fluctuation of experimental condition, but represents the very nature of timedependent, stochastic failure. The stochastic failure model presented provides a framework for analyzing experimental data as well as for predicting life time distributions under general fluctuating loading conditions other than creep.

The key operation in tissue production is the creping at the Yankee cylinder. The tissue sheet is adhered to a Yankee cylinder and then detached from the surface with a blade. As a result a strong microstructure - crepe folds - is generated on the web. In this work the structure of creping pattern in CD and MD of tissue paper was studied. The samples available in this work consisted of bath tissue and towel grades. The significant difference between the samples was in the number of crepe folds. The tissue sheets were analyzed with an imaging system. The variation of crepe folds in images was studied in spatial frequency space through 2D Welch spectrum. It was noticed that the difference in creping pattern structure between the samples can be seen clearly in spectra. The creping pattern was simulated based on the creping pattern variation found from the Welch spectra of tissue paper images. The simulation was based on a set of sinusoidal terms representing several orientations and amplitudes. The amplitudes of sinusoidal terms located in regular grid in spectra were approximated with 2D normal distribution. Thus, the structure of creping pattern can be represented with three parameters originating from the deviations and location of 2D normal distribution. The creping pattern reconstructed with simulations contains the relevant features in the creping pattern of true tissue images.

Pre-flocculation is a technique that aggregates filler particles by means of polyelectrolytes. The size of the filler flocs is a critical factor affecting the properties of the paper. Process variables including the concentration of the flocculants, the stirring speed, and the dilution of the suspension change the size of the flocculated fillers. Ground calcium carbonate and cationic polyacrylamide were used to investigate the influence of these variables on flocculation. The median particle size and size distribution were examined and the tensile and optical properties influenced by the floc size and ash levels were evaluated. The addition of high concentration C-PAM increased the median particle size. Stirring speeds in the range of 1000 ~ 3000 rpm were used to simulate various turbulent environments, and it was shown that high stirring speeds drastically decreased the floc size. Diluting and low speed stirring the flocculated GCC suspension was effective in keeping the particle size intact without disruption before sheet forming. Larger GCC flocs in the handsheets resulted in better mechanical properties.

Different qualities of microfibrillated cellulose (MFC) were blended with thermomechanical pulp (TMP) and ground calcium carbonate (GCC) filler. The addition of MFC reduced the drainage of the pulp suspension but improved strength properties. Wet pressing experiments showed that optimal use of MFC and filler could enhance the strength and optical properties without reducing the solids content after wet pressing. Field-emission scanning electron microscopy (FESEM) revealed that MFC adsorbed onto and contributed to the bonding of the filler particles and fibres. The MFC binds the filler-MFC-fines aggregates to the fibre network and partially filled the pore network. As a result, MFC addition increased the air resistance and internal bonding of the sheet.

When liquid packaging board is made aseptic in the filling machine the unsealed edges of the board are exposed to hydrogen peroxide. A high level of liquid penetration may lead to aesthetic as well as functional defects. To be able to make a priori predictions of the edge wicking properties of a certain paperboard material is therefore of great interest to paper industry as well as to packaging manufacturers. The aim of this paper is to present a new analytical theory for prediction of the edge wicking properties of paperboard. The theory is based on Darcy’s law and the ideal gas law to describe the physical behavior of water flow in paperboard. The theory is compared to a recently published multi-scale framework and with pressurized edge wick experiments. The agreement is very good for paperboard samples of different sizes. The conclusion from the work is that both analytical theory and detailed simulations are useful to predict edge wicking properties of paperboard material.

We have performed free-energy-based two-dimensional lattice Boltzmann simulations of the penetration of liquid into the edge of a porous material. The purpose was to gain further insight into possible mechanisms involved in the penetration of liquid into the unsealed edges of paper and paper board. In order to identify the fundamental mechanisms we have focused on a model structure that consists of a network of interconnected capillaries. Two different mechanisms were identified: pinning at corners of solid surfaces and increased pressure in dead-end pores. These mechanisms significantly decelerate or even stop the liquid penetration into the porous structures.

Mass transfer properties of fibre network and coated paper are essential for understanding the barrier properties of the products and further advance in their application. In this study, different unmodified and coated papers, e.g., (Poly lactic acid (PLA), zein grafted paper) were prepared and characterized with regard to mass transfer properties. Water vapor, carbon dioxide (CO 2 ) and oxygen (O 2 ) transmission rates through the cellulose paper films were measured and the results discussed. The effects of sample film thickness and samples were found to be strongly dependent on the temperature and the relative humidity difference (mass transfer driving force). On the other hand water vapor permeabilities relative humidity. Water vapor diffusivities of the samples were also measured from the uptake rate measurements using Fickian diffusion slab model for a wide range of relative modified samples were found to be generally low compared to unmodified (reference) paper sample. Among the investigated samples, PLA/polyhedral oligomeric silsesquioxane POSS-bentonite modified paper sample showed higher mass transfer resistance to water vapour and the gases investigated in this study (CO 2 and O 2 ). It showed lower water transmission rate (104 g/m 2 .day) compared to PLA-coated paper (130 g/m 2 .day), zein coated paper (179 g/m 2 .day) and control sample (359 g/m 2 .day) at the relative humidity gradient RH=74% and temperature of 25 o C. The oxygen transmission rate for PLA/(POSS-Bentonite) coated paper was found to be lower than for the other modified papers. Zein grafted paper showed better barrier property for water vapor than oxygen. Water vapor permeation through paper films shows an Arrhenius type of dependency with temperature, indicating activated process. The activation energies reveal diffusion dominated process for all paper samples investigated in this study, according to the solution-diffusion mechanism used to describe the permeation processes.

We present an extension of a Kubelka- Munk based fluorescence model in which we introduce an apparent scattering (S UV ) and absorption (K UV ) coefficient for all wavelengths below 400 nm. We describe a method for modelling the total radiance factor of multi-layer papers and for estimating the optical parameters (S, K and Q) of each layer. Assuming that the fluorescent whitening agent only absorbs below 400 nm, we are able to determine S UV , K UV and the apparent quantum efficiency, Q(UV,λ) for 400 nm<λ<700 nm, from spectral radiance measurements in the visual part of the electromagnetic spectrum. We test the proposed method on different layered constructions made of three individual pilot paper layers. The proposed method allows the papermaker to determine the illumination independent fluorescence characteristics of single- and multilayer paper layers using a conventional singlemonochromator spectrophotometer operating in the visible part of the electromagnetic spectrum, and also to predict the radiance factor of fluorescing layered papers.

The Kubelka-Munk scattering and absorption coefficients of a set of paper samples are assessed using reflectance measurements in d/0 geometry. The coefficients display the widely studied dependence between light scattering and light absorption, since the light scattering coefficient decreases in regions of high absorption. It is shown using general radiative transfer theory that part of this dependence can be explained and eliminated by taking into account the geometry of the d/0 instrument and the single scattering anisotropy, thus capturing the angular variations of the light reflected from the samples. These findings allow the papermaker to better predict the reflectance from mixtures of pulps, fillers, dye, and FWA, and to better evaluate bleaching efforts.

A custom-made imaging reflectometer was used to study the microgloss properties of coated papers. To quantify the relative contributions of surface and bulk scattering to the total intensity of reflected light, a pair of polarizer lenses was employed. The mean value and the standard deviation of local grey level of captured images at micro-scale (with a spatial resolution of 16 μm x 16 μm) were determined to quantify the average microgloss and microgloss nonuniformity of samples, respectively. It was found that surface microgloss; i.e. local light reflection in the specular direction due to surface light scattering, increased linearly with increasing the total microgloss of paper. However, bulk microgloss; i.e. local light reflection in the specular direction due to the bulk scattering, remained nearly constant for a wide range of coated papers with varying coating weight, formulation and calendering condition. The relative contribution of bulk scattering to the total microgloss increased as the sheet progressively became less glossy. On the other hand, microgloss nonuniformity was governed by the local variation in surface light scattering and was affected by pigment particle size, pigment morphology and calendering conditions. Finally, both surface microgloss and surface microgloss nonuniformity were power-law functions of the surface texture parameter (L c /λ)/(σ h /λ) 2 where L c , σ h and λ are correlation length, RMS surface roughness, and wavelength of incident light, respectively.

This paper presents a method to describe, with good accuracy, the relation between variations in paper properties and variations of the spectral optical response of an uncoated paper. The dependence between density, filler content, grammage, and the spectral optical response is characterized by a multivariate model. The model is based on large-scale measurements data on a set of paper samples that have been produced with different values of grammage, density and filler content, representing the variations within a normal 80 g/m² uncoated paper. From the optical measurements the light scattering (s) and light absorption (k) coefficients have been estimated according to the Kubelka-Munk theory. The results from this study will give valuable input to optical modeling activities, where the optical variations are predicted from measured small-scale variations in underlying paper properties. The variations in the paper properties can be used to model the light scattering coefficient, s, but there were too small variations in the light absorption coefficient, k, to find any significant dependence to the paper properties for the samples studied in this work. Furthermore, linear models give sufficient accuracy in the intervals studied. Additional findings from this study are the different effects of wet-pressing and calendering on the light scattering coefficient.

Fold cracking of coated papers has become important because of the increase in the coat weight and the content of low cost co-binder. The effects of base paper characteristics of coated paper were investigated to reduce fold cracking. We evaluated the fold cracking of coated paper prepared with different mixing ratios and beating degrees of pulp fibers. Hardwood bleached kraft pulp (Hw-BKP) and softwood bleached kraft pulp (Sw- BKP) were used to prepare base papers for coating. The mechanical and structural properties of base paper such as tensile strength, strain at break, short span compressive strength, and formation were measured. The fold cracking was affected by mixing ratio of pulp fibers. Base paper consisting of Hw-BKP and Sw-BKP at a 90:10 mixing ratio showed the least fold cracking. When the amount of Sw-BKP was greater than 10% or if only Hw- BKP was used, the fold cracking increased on coated paper. When the pulp was highly beaten, the mechanical strength of paper was improved, but the fold cracking was severe. An increase in the tensile strength did not reduce the fold cracking.

The effects of coating formulation on thermal characteristics of coating layers (namely thermal diffusivity, specific heat capacity and heat conductivity) were systematically studied and their impact on xerography print quality was evaluated. Model coatings were prepared using ground calcium carbonate or kaolin pigment mixed with styrene butadiene latex binder in various proportions (from 6 to 25 pph). As expected, porosity was shown to be a key parameter for thermal conductivity of the coating layers, and is mainly determined by the latex concentration. Particle size distribution (PSD) and pigment morphology also affected the thermal characteristics of the coating layers. It was found that the bulk thermal conductivity of the coating layers can be accurately predicted by a geometric mean model based on the pigment, latex and air contents. Print quality on model coated papers was evaluated in terms of print gloss, toner adhesion and pairwise visual ranking. It was demonstrated that print gloss is improved by decreasing the bulk thermal conductivity of the coatings. The coating formulated with the pigments with the steepest PSD and 10 pph of latex had a relatively low thermal conductivity and the best print quality.

The interaction between a liquid and a paper surface is important for a number of paper treatment processes, where absorption is of special significance during printing. Many absorption measurement techniques use a large available volume of liquid to characterise absorption, when compared to the volume of the coating. The uniformity of the absorption is also seldom characterised. We have developed a new technique, which is presented in this article, to study the uniformity of absorption of a small amount of liquid. This technique is based on the short-time absorption (tenth of a second) of a coloured liquid, the blotting of excess liquid and a characterisation of the pattern of the stain. This method made it possible to detect differences among coating layers with different compositions. In many cases, the absorption non-uniformity could be linked to variations in the coating thickness and/or wettability. The thinner and thicker areas of the coating layers were interpreted as having different pore structures. Neither the coating thickness nor the wettability could provide a full explanation, which showed the need to develop a method to characterise absorption uniformity instead of only relying on measuring the total absorption potential.

Production of lightweight coated paper (LWC) utilises more and more deinked pulp (DIP), for both economical and environmental reasons. A critical requirement on DIP quality for such grades is the cleanliness of the pulp. Indeed, the coating layer may not fully cover specks in the base paper. The objective of this work was to determine the required specifications of the coating layer to produce LWC with DIP base paper. The specks masking phenomena by the coating layer was studied from both optical measurements on coated handsheets made from DIP, and computer simulations of specks contrast reduction. The impact of coating layer parameters (coat weight and optical properties), base paper parameters (grammage and optical properties) and specks nature (grammage and optical properties) on specks masking is studied.

Papermaking techniques were used for the production of low-cost and easily recyclables Li-ion paper-cells with an average grammage of 314 g m -2 , a thickness of 726 μm and a specific capacity of about 90 mAh g -1 . Paper electrodes, anode and cathode, were prepared using micrometric-sized graphite and LiFePO 4 as inorganic filler and sheets of paper were used as separator between them.

Hybrid printed electronics, in which printed structures and silicon-based components co-exist will likely be among the first commercial solutions. In this case the paper substrate acts much in the same way as circuit boards, containing conductive tracks and acting as a carrier for the electrical components. It is important to consider the contacting of the components to be able to produce low resistance electrical contacts to the conductive tracks. Supercapacitors are able to deliver a large amount of current in a short time and are a good option for short term energy storage and if the printed product is to be used only one, or a few times, it can be the only power source needed. When manufacturing printed electronics, the overall resistance of the printed tracks as well as the contact resistance of the mounted components will add up to the total resistance of the system. A high resistance will cause a voltage drop from the power source to the component. This will waste power that goes to Joule heating and also the voltage and current available to components may be too low to drive them. If the intention is to use a power supply such as batteries or solar cells this becomes a limitation. In this article have been tested several conductive adhesives used to contact paper based supercapacitors to ink jet printed silver tracks on paper. The best adhesive gives about 0.3 Ω per contact, a factor 17 better compared to the worst which gave 5 Ω. The peak power that is possible to take out from a printed system with a flexible battery and super capacitors is about 10 times higher than compared with the same system with only the battery.

Paper based supercapacitors are prepared by stacking a paper between two graphene electrodes and soaking these in an aqueous electrolyte. We demonstrate that supercapacitors can easily be manufactured by using proven paper technologies. Several different electrode materials were compared and two types of contacting material, silver and graphite foil were tested. The influence of the paper used as separator was also investigated. The supercapacitors with a graphene-gold nanoparticle composite as electrodes showed a specific capacitance of up to 100 F/g and an energy density of 1.27 Wh/kg. The energy density can further be increased by using other electrolytes. The silver contacts showed a pseudo capacitance, which the graphite contacts did not. The papers tested had a minor effect on the capacitance, but they have an influence on the weight and the volume of the supercapacitor.

Poly(3,4-ethylenedioxythiopene): poly (styrene-sulfonate), or PEDOT:PSS, as well as singlewalled carbon nanotubes, were incorporated into an inkjet ink. Handsheets were prepared which contained varying amounts of TiO 2 filler, internal sizing agent, fixation agent, and either softwood or hardwood kraft pulp. The ink was jetted onto the handsheets to form conductive layers with apparent conductivity as high as 0.018 S/cm on internally alkyketene dimer-sized softwood kraft handsheets with no other additives. Internal sizing increased conductivity at low filler loadings by preventing PEDOT:PSS from penetrating into the substrate, resulting in a conductive ink film on the surface of the sample. Unsized handsheets allowed more rapid absorption, and therefore deeper penetration, of the PEDOT:PSS ink, which resulted in a more diffuse conductive layer. The inclusion of a polyethyleneimine retention aid for TiO 2 filler decreased conductivity significantly even in unfilled sheets by interaction with PSS - counterions. A positively charged fixation agent, poly(diallyldimethylammonium) chloride, reduced PEDOT conductivity through the retention of nonconductive PSS - anions.

Recent findings have shown that it is possible to use the Layer-by-Layer technique to create nanofibrillated cellulose / polyethyleneimine interference films whose colour change with relative humidity. This study uses different optical models to describe spectral ellipsometry measurements data of interference films and how the film properties alter in dry and humid environments. The results indicate that water condensation initially is filling the surface pores within seconds whereas relaxation of the film to adjust to the added water is a slower process that reaches a steady state after »20 min. The maximum swelling ratio of the LbL films is almost independent of the number of layers within the film, but decreases considerably by crosslinking via heat treatment. The films show a distinct birefringence with optical axis perpendicular to the surface. Analysis of the moisture response with different optical models indicates that the films swell uniformly in the thickness direction with no separate water film on top. The results provide important understanding for the design of NFC based LbL films for visual moisture sensors and interactive security paper.

We investigate anisotropic light propagation in paper using both a theoretical model and experiments. The theoretical model utilizes the Monte Carlo method to solve the photon transport equation numerically. It is assumed that wood fibres are represented by infinitely long, homogeneous and straight cylinders. The layer-like microstructure and anisotropic orientation of the fibres is considered in the model. The conical scattering by cylindrical objects, the wood fibres, is argued as the main source of anisotropic scattering. Simulations revealed that laterally resolved transmittance exhibits directional dependence. Experiments on light transmitted through a standard kraft liner product confirmed that light in fact do propagate more in the machine direction than in the cross direction. Reasonably good agreement was obtained between experimentally and numerically obtained iso-intensity patterns.