Motivated by sustainability arguments there is a recent interest in forming of advanced structures in paper and paperboard. Therefore, in this paper, hydro-forming of papers and the effect of different fibre raw materials, beating, strength additives (PVAm), grammage and wet and dry papers have been investigated experimentally and numerically.
The experiments were carried out in laboratory hydro-forming device. Softwood sheets performed better than hardwood sheets, since they had higher strain at break. The ability of paper to withstand hydro-forming successfully was primarily dependent of the strain at break of the paper in relation to the straining required to fill the mould. Forming of wet sheets were also investigated; overall the wet sheets formed better than the dry sheets, which was due to higher strain at break and lower elastic energy. Since the forming was displacement controlled, there was no significant difference in the effects of beating, amount of PVAm or grammage.
Finite element modelling was performed to identify local strains and predict problematic regions. Simulations were also performed to determine how anisotropic sheets would behave, as well as to compare the process of hydro-forming with press-forming. The papers could be strained to higher strain levels than the measured strain at break because the paper is supported by the membrane and mould during the forming operation. The maximum strain a paper can withstand can be increased if the paper can slide into the mould, i.e. by having a lower coefficient of friction between the steel mould and the paperboard.
During hydro-forming the paper is supported by a rubber membrane, which gives lower strain levels than the corresponding press-forming operation due to the difference in how the paper is deformed. Press-forming therefore required paper with higher strain at break. Higher friction results in more paper being pulled into the mould, which contributes to wrinkling of the paper. Simulation of tray forming of a creased sample was performed, which showed that high friction or compliant creases decreased the circumferential compression.
Oxfordpp 27-41Enhanced Test Devices for the Development of Novel Paper-like Materials for Sandwich-StructuresAbstractPDF
High performance sandwich components have a great significance in aerospace applications. Particularly, lightweight sandwich structures made of honeycomb or foldcores show excellent load carrying capabilities. Both types of cores are usually made of aramid paper coated with phenolic resin. Therefore, the development of improved paper-like materials seems to be a promising approach to improve the mechanical performance of this kind of cores. An essential part of this development process is the evaluation of the new materials by the complete characterisation of the mechanical properties. This is still a challenging task, since the resulting papers are orthotropic and most of the existing testing procedures and devices are not suitable for very thin sheet materials. This is particularly true for investigating stiffness and strength properties under compressive and shear loading.
The paper presents a novel single-curved compression test device as well as an adapted shear-frame for the in-plane characterisation of very thin specimens. These devices have been applied in the development process of a new paper-like material that consists of three layers in order to increase the stiffness and strength of honeycomb- and foldcores. The performance of this material was evaluated by comparing relevant mechanical properties to that of state of the art paper materials. Based on the experimental results the benefits of the new paper-like material could be shown.
Curl is unwanted dynamic behaviour that appears already in the manufacturing process and evolves through the end-use of paper or board due to moisture content changes and mechanical treatments. In this paper, the analytical and numerical approaches are used to reveal the sensitivity of the curl tendency to fundamental variables affecting deformation behaviour of paper. Good agreement between the measured curvatures induced into the sheet by the photocopying process and the simulated curvatures is achieved. Paper is treated as a multi-layered material, and finite element simulations are performed by using hygro-elasto-plastic and hygro-viscoelastic material models. Also analytical calculations are carried out to support the conclusions. The results show that the prediction and control of curl is not straightforward; curl depends not only on the fibre orientation structure but interacts in a complex way with the through-thickness dry solids content profiles during moisture content changes and external mechanical forces acting on paper. Despite the complexity of the phenomenon, the simplified computational approaches presented in this paper can be used for analysing and optimising the paper structure and process parameters to prevent detrimental curl.
Cambridgepp 3-39Local Structural Orientation of Towel and Tissue Grades in Two and Three DimensionsAbstractPDF
This paper describes the study of new methods for characterizing the orientation of fiber segments in low density paper towel from two- and three-dimensional X-radiographic data sets. The end use properties of the absorbent hygiene grades such as paper towels and tissues stem from an open porous structure where stochastically distributed fibers are contorted by post forming processes to increase bulk, stretch, flexibility and softness, while maintaining adequate strength. The orientation of free fiber segments that form the network are kinked and curved in three dimensions by processes including creping, through air drying and embossing. Providing a linkage between process conditions and the end use properties through the characterization of the network structure is the overarching goal of this investigation. A method is presented for mapping the 2D, in-plane orientation of fiber segments using soft (6kV) X-radiographs and an algorithm for calculating the image moments for circular sub-regions that surround each point. The eigenvectors form the major and minor axes of the inertial ellipse from which the principal orientation may be extracted. Colorized maps representing the local orientation are used to examine the effects of embossing and creping, as well as comparing different forming processes. A method for characterizing fiber segment orientation in three dimensions uses a similar approach applied to binarized X-ray micro-computed tomographic data sets. The inertial ellipsoid is determined by performing principal component analysis on the covariance matrix of the voxels contained within a spherical region surrounding each solid voxel within the structure. The eigenvectors are used to extract the shape and principal orientation of the ellipsoids which are plotted as colorized representations in 3D space. The 2D and 3D plots demonstrate the sensitivity of the method to orientation of fiber segment mass, while mean fiber orientation plots reveal differences between samples.
Oxfordpp 139-158Development of Cellulose Nanofibre Quality with Mechanical Energy: Effect of Starting MaterialAbstractPDF
Energy efficient production of nanocellulose fibres is key to establishing this highly-promoted materials in an industrial scale. In this work, we attempt to explain how the mechanical energy input and the chemical composition of the raw materials affect the quality of nanofibres. Bleached eucalyptus Kraft (BEK) pulp, a commercially availble microfibrillated nanocellulose from cotton, and whitewater fines collected from a radiata pine thermomechanical pulping (TMP) mill were used to produce cellulose nanofibres. BEK was the most responsive to mechanical fibrillation due to low crystallinity and it produced high aspect ratio nanofibres, while TMP whitewater fines were the most difficult to process and resulted in low aspect ratio nanofibres. Nanofibres were then added to TMP newsprint to evaluate the effect on tnesile strength. Nanofibres produced from BEK were able to increase the tensile strength the most, while nanofibres from TMP whitewater fines had the least effect. The results showed that a high aspect ratio and a surface chemical composition favouring more hydrogen bonds i.e. pure cellulose, are the key criteria when selecting nanofibre for strength improvement in paper.
We studied fully developed pipe flow of fibre-laden aqueous foams and decoupled their bulk rheological properties boundary effects like slippage at the pipe wall. The air volume fraction of the foams varied between 70% and 75%. The addition of hardwood fibres at the consistency 20 g/kg to plain aqueous foam increased viscosity more than 100%, while with microfibrillated cellulose at a consistency of 25 g/kg the increase was about 30%. The effect of synthetic (cellulosic)rayon fibres was negligible at the consistency of 20 g/kg. All the studied foams could be described as shear-thinning power-law fluids with significant slippage at the pipe wall by particles size and interactions between particles and bubbles.
Considering the analogy between the pressing of a paper sheet and the refining of a pulp suspension, the refining impulse is introduced. For beaters, disc or conical refiners, whatever the running mode (continuous or batch), the refining impulse is found to be a controlling variable for the pulp properties, and consequently for the paper properties. In a Valley beater, different normal forces were applied. The SR evolution versus the refining impulse exhibits a unique curve whatever the experimental conditions. For disc and conical refiners, the refining impulse depends on the net power, the rotation speed, the bar width, or the average bar angle. A unique parameter is used to fit each set of trials to obtain a single curve of the SR evolution. This parameter corresponds to the global friction coefficient f. he fiber length and the swelling (WRV) depend also on the refining impulse. However, as in pressing theories, the applied pressure has also to be introduced as a complementary parameter. Consequently, the paper properties are shown to depend also on both the refining impulse and the applied pressure.
The standard method of representing refining data is to plot fibre or sheet properties as a function of refiner Specific Energy Consumption (SEC), for separate refining trials done at different Specifics EDG Loads (SEL). This approach does not allow for refining outcomes to be predicted when refining at other values of SEL and does not allow for refining conditions to be optimized to satisfy multiple constraints. In addition, the change in fibre properties is determined by the number of impacts on a fibre and the energy used in each impact, while SEC is the product of number and energy used in each impact. The paper describes a new representation of refining data where the two axes of the plot are SEC/SEL, which is proportional to the number of impacts, and 1/SEL, which is proportional to the inverse of the energy used in each impact. Data from refining trials are then plotted as lines of equal value. The paper shows how flow and power limited for a low consistency refiner are represented on such a plot. The utility of the approach is demonstrated with refining data of a CTMP pulp with three different refining plates and three different speeds.
Oxfordpp 209-228Fractionation of Bifer Pulp in a Hydrodynamic Fractionation Device: Influence of Reynolds Number and Accept Flow RateAbstractPDF
Fibre fractionation in the Hydrodynamic Fractionation Device (HDF) was studied for changing suspension flow parameters, i.e. different channel Reynolds numbers Re and accept flow rates up to 20% of the feed flow rate. The suspension flow behaviour was described using images recored with a high-speed camera system. Fractionation performance was determined based on mass balances for a variety of length fractions of the pulp. Low Reynolds number flow characterised by Re = 1300 led to the formation of a fluid gap between the wall and the fibres located at the chaneel centre. Best fractionation performance was achieved for flow at this Reynolds number: no fiber removal was observed at 10% accept flow rate, and only 1% of the fibres were removed at 20% accept flow rate. A design space was established that highlights the optimum settings for fractionation in an HDF, which at low Re and high accept flow rate. Surprisingly, we found a significant increase of fines mass flow rate in the accept upon an increase of the Reynolds number. We speculate that a flow regime-dependent interaction of fines with the fibres exists in the HDF that critically affects the amount of fines in the fluid gap near the wall.
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