Proceeding Articles

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A method is described that classifies water in a cellulosic fiber and water system. Thermogravitmetric analysis (TGA) is used to determine hard-to-remove (HR) water from an isothermal drying curve. The HR water content is defined as the moisture ratio (g of water / g of oven dried sample) of the fiber-water system at the transition between the constant rate zone and falling rate zone of an isothermal drying curve. The TGA instrument provides tightly controlled drying conditions that allow one to distinguish small differences in drying behavior. The exact value of the HR water content was found to be influenced by the initial moisture ratio, solid mass, and isothermal drying temperature. Experiments at optimal conditions showed that the HR water content is linearly correlated with the water retention value. This correlation was examined in terms of the similar states of the fibers (i.e. minimum saturation point) when the HR water content and water retention value are measured. The dependence of the HR water content on the solid mass of the sample revealed two constants, y-intercept and slope. The y-intercept is considered to be linked with instrumentation and the slope is to be associated with the fiber. Once the HR water content measurements are adjusted for these constants, experiments may be conducted at any solid mass down to a few milligrams. The dependence of the HR water content on the isothermal drying temperature was linked to differences in the drying rate of the constant rate zone. It is proposed that the higher drying rate for higher isothermal temperatures results in the transport of water internal to the fiber becoming an important factor at higher moisture content. This results in a higher HR water content being observed. It was also found that the pulp yield affected the HR water content. At the same water retention value, the highest HR water content was found for mechanical pulp followed by unbleached chemical pulp, and bleached chemical pulp. These differences may be attributed to differences in chemistry, pore size and volume, and the dynamics of pore collapse during drying. This method can be performed on extremely small samples and provides a convenient and insightful characterization technique for cellulosic fibers.

An adhesive coating formed on the surface of a Yankee dryer is critical for manufacturing creped tissue and towel grades. Due to the complexity and dynamic nature of the creping process, there has been very limited information available on the structure of Yankee coatings. This paper discusses laboratory methods for preparing Yankee coating films and imaging techniques, atomic force microscopy (AFM) and scanning electron microscopy (SEM) that were used to characterize these films. The effects of
various modifying agents are demonstrated on the structural and compositional uniformity of the Yankee coating films. The applicability of the SEM and AFM data from this study to the actual creping process and the practical aspects of the results are also discussed.

A material model for drying paper is presented. Moisture-dependent material parameters, hygroscopic shrinkage, the elastic and the time-dependent responses of the material to load, and the effect of unloading at a higher stiffness than the load was applied at are modelled. The model is used to determine the effects of a varying moisture ratio through the paper during drying on free shrinkage development and stiffness development at free drying. Simulation results for the stress development during drying and the state of residual stress immediately after drying are also presented. The model predicts a variation of in-plane elastic moduli through the paper, a prediction that is studied by experiments.

There is a link missing between pulp properties and machine-made paper properties. The aim of this paper is to close a part of this gap by proposing an engineering model which, based on pulp or stock properties, makes it possible to predict the resulting anisotropic material behaviour of a multiply paper or board based on any given fibre anisotropy and drying restraint.

An anisotropic model for the shrinkage and stiffness inter-action between the individual plies in a multiply structure is formulated. The input data to the model is the isotropic restrained dried and free dried stiffness, the free shrinkage strain and the density of each ply in the multiply structure. The basis weight, fibre anisotropy and total strain after drying are variables in the model. This means that besides the standard handsheet procedure only measurement of shrinkage and stiffness for one extra free dried handsheet is needed for the calibration of the model.

The proposed model is validated with a series of anisotropic handsheet trials. Various combinations of single ply anisotropic handsheets were couched together into seven different multiply
boards, which were dried freely and restrained. The isotropic input data of the individual plies were used to predict the free and restrained dried tensile stiffness index and bending stiffness of the multiply boards. The agreement between the experimental and predicted results showed good agreement. The model constitutes a useful tool in engineering predictions and parametric investigations of the mechanical behaviour of multiply boards. As a demonstration of the use of the model, the relation for tensile and bending stiffness versus total strain accumulated during drying was predicted for different board compositions. The basis weight and fibre anisotropy were varied in one of its plies.

As another example the MD and CD stiffness profiles of a multiply board were simulated based on a given MD stretch and CD shrinkage profile.

All conventional paper machines exhibit a profile of cross-machine direction (CD) shrinkage which is developed during the drying stage of production. This paper suggests a new approach
to the understanding of these profiles by suggesting that CD shrinkage at a point in the paper depends on the distance of that point from both edges of the sheet and also on the length of the unsupported draws in the dryer section. The nature of the function is unimportant as long as it can be made to fit data for one combination of machine width and effective draw length; substituting other values will then change the shape appropriately. In this paper, a simple exponential decay of shrinkage with distance from the edge is used successfully.

This approach is initially demonstrated for laboratory results from the literature, where it successfully predicts changes produced by varying the length, width and by reversing the MD/CD orientation of samples. It is then shown to be consistent for changes of paper machine furnish, press section draw and sheet grammage produced in a series of trials on M-real, New Thames
PM6. It is finally shown that theses ideas explain the effect on CD shrinkage results from the literature for splitting the sheet at the press section of a newsprint machine and for reduction of dryer section restraint by deactivation of the blow-boxes.

A web is a material whose length is much in excess of the width and the width is much larger than the thickness. Various grades of paper, plastic films, metal foils and laminates can all be broadly categorized as webs. Web media are often stored for periods of time in the form of a wound roll. This form of storage is chosen because it is simply the most convenient and often the only form for storing vast lengths of web with minimal damage and loss. Winding can damage the web and has become a topic which has received considerable attention in the literature. Amongst operations in a web process line, the winder and the unwinder are often sites at which web defects appear that may result in a loss of quality or may lead to a break or burst of the web resulting in lost productivity. The purpose of this paper is to review the state of the science of winding and unwinding rolls. The threads of pertinent information in the literature focus on: the stresses that are wound into rolls, how those stresses are affected by the type of winder employed, how those stresses vary during roll storage, how the stresses predicted by models can be used to predict wound roll defects, and finally the measurements that are available to verify modeling efforts or help solve production problems. Each thread will be examined in this review from the perspective of what references exist but also giving enough detail that an appreciation for the significance of a topic can be developed.

This paper will consider the low consistency refining kinetics of a pulp suspension at a given level of applied specific energy. The mechanical treatment on fibres caught in the confined zones of gap clearance (bar crossings) reveals some heterogeneity. It can be experimentally verified that the physical effects on fibres are not the same whether the confined zones are close to the internal radius (input side) or close to the external radius (output side). One must also account for the dynamic sliding motion of the confined zones through the rotation of the rotor plate (or cone) in front of the stator. Hence, each bar crossing has its own sliding velocity.

In order to predict the radial variability of the cutting effect on fibres, through engineering parameters that can be easily determined (radial coordinate; angular velocity; width of bars, width of grooves, average bar angle, both for rotor and stator patterns), a theoretical understanding of the radial distribution of the pressure must be undertaken. The pressure is locally applied on
the pulp pads in confined zones of the gap clearance.

The best homogeneous results are obtained either with a cylindrical or a conical refiner where the bars are parallel to the rotation axis. The degree of variability on the cutting effect on
fibres is increased with an increase of the bar inclination versus the radial direction and a decrease of the ratio between the internal and external radius.

The role of refining intensity and specific energy in refining of softwood kraft fibre fractions was studied. Several paper properties can be improved by selective refining of fractions. The tensile strength-dewatering resistance relationship benefits from low-intensity refining of the long-fibre fraction. The specific energy input determines the increase in fibre swelling which contributes to a higher sheet density and improved tensile strength. The apparent density-roughness relationship benefits from mild refining of the short-fibre fraction. Refining intensity has a strong effect on the magnitude of the gap between bar surfaces, on fibre shortening, and on the coarseness of fibres with high cell wall thickness. For the short-fibre fraction, which appeared to floccu-
late less, the maximum intensity causing “pad collapse” and more severe fibre shortening was lower than for the long-fibre fraction and feed pulp. The fraction-specific intensity and gap behaviour are believed to relate to the compressibility of flocs under the stress applied by bar surfaces – a phenomenon discussed in recent studies concerning the forces acting on fibre flocs.

The properties of paper are largely dependent on the bonds between the fibres. This is, of course, primarily true of those strength properties that are directly related to the number of bonds in the paper. Other properties are also dependent on such bonds, properties such as the opacity of the paper, its smoothness, porosity, dimensional stability, pore size distribution, linting
propensity, density, stiffness, formation, and compressibility to mention a few.

The normal way of affecting the number of bonds in a paper is through the choice of fibre material and through a correct beating of the pulp. It is true that properties of paper may be manipulated through the choice of beater type, its specific edge load etc to expand the property or process space in paper manufacture. There are still many limitations as to what can be achieved by beating and other process tools, so the practical paper-maker is continuously looking for ways to expand property and process space to be able to manufacture new products or boost paper machine productivity.

In this review the terms “bonding” and “joint strength” will be used interchangeably. “Joint strength” includes both the adhesion zone (2D zone of bonding) and the cohesion zone (3D zone of bonding).

Despite massive efforts over the years, our understanding of the molecular mechanisms of bonding is still in its infancy. There is still the fundamental argument as to the relative contribution of hydrogen bonds, ionic bonds, dipolar interactions, induced polar interactions, long-range van der Waals forces, and covalent forces (for wet strength resins) in various situations. Taken to the extreme, it was once believed that lignin contributed little to bonding in lignin-rich pulps, because they were assumed to be poor hydrogen bonding agents. Not anymore, as it has been realised that strong bonding can be created between mechanically liberated pulp fibres. Though critical experiments still need to be formulated to examine such matters, this review will not focus on them.

It is acknowledged, that hydrogen bond theories have been formulated by Corte and Shashek (1955), Nissan and Sternstein (1964) and others, but it has not been possible to further expand our knowledge from the initial formulations.

This review will instead focus on the use of various dry and wet strength additives to improve bond strength. The authors have made efforts to relate the discussion to the historical and current context of dry and wet strength resins, and to discuss more recent developments in understanding adhesive and cohesive failure.

Hence, after some general considerations and introduction to the concepts of process and property space in paper manufacture, a brief discussion of current paper strength theories will be made. A more detailed account of adhesive and cohesive failure mechanisms will follow, after which dry and wet strength resins will be reviewed. As far as wet strength agents are concerned,
traditional wet strengthening will be given less emphasis; the focus of this later part will instead be on potential chemistries to alleviate tensile creep and compression creep under moist conditions.