1989 Volume 2

There is a large body of literature on wet pressing;almost all of it deals with water removal and much of it is empirical in nature. Though we have been forced to infer what happens inside a roll press nip by making observations from the outside, our qualitative and quantitative knowledge of the water removal process has improved greatly over the years. There have been some direct experimental measurements of several important variables inside the nip(applied pressure, fluid pressure, and mid nip roll separation), but only at the system boundaries (the roll surfaces). Direct data is still lacking on other important variables inside the nip such as localized pressure gradients, sheet thickness and sheet dryness, localized deformation, a good definition of the interfacial region, direct measurements of parameters in the thickness direction,and thermodynamic properties. So far, wet pressing models have had limited success in making a priori predictions of sheet water removal and have not begun to address paper properties, the next major thrust of wet pressing research. We have conjured up a mental picture of wet pressing which seems to fit well the observations made from the outside. It is quite possible that this picture is more inaccurate and incomplete than we imagine, but this state of affairs is actually exciting because it means much remains to be learned about the fundamental mechanisms of wet pressing. In this learning process, paper property development is expected to receive equal,if not greater,attention.

The dynamic compression behaviour of the wet fibre mat is the key factor to understanding the mechanisms of wet pressing. At present this behaviouris imperfectly understood. The main reason for this may be the lack of suitable measuring equipment. To fill this gap, laboratory devices (press simulators) were built at the Finnish Pulp and Paper Research Institute. The simulators permit controlled pressing to be performed within a wide speed range, the fastest pressure rise being equivalent to that encountered in the presses of the fastest paper machines. The web thickness and the hydraulic pressure created in the web during pressing can be measured acturately. Papers with grammages of 50 and 100 g/m2 and with various furnish compositions were tested with these devices.

The expansion in the Z-direction of individual layers in a wet sheet after leaving the nip of a double-felted press nip has been studied. In this investigation, pads of up to five wet sheets of CTMP, bleached kraft or mixtures of the two pulps, were used to simulate multilayer high-grammage sheets such as carton board. Using different lay-ups it was thus possible to study the expansion of both homogeneous and heterogeneous sheets.

The expansion of 350 g/m2 sheets is illustrated by the air fraction in the wet sheet immediately after the nip. The results show that a sheet of CTMP can expand up to ten times as much as a sheet of bleached kraft. It is also shown that CTMP has the ability to expand even when sandwiched between bleached kraft as middle layer in a stratified sheet.

Historically, paper has been consolidated by wet pressing followed by drying on steam cylinders or by other low intensity evaporative processes. Although these time-tested pro cesses will continue to be the mainstay of papermaking for some time, they are now being challenged by new systems that exploit the interactive effects of increased pressures and temperatures. We want to review our understanding of these hybrid processes and the unprecedented performance potential they offer. At the same time, we want to integrate .them into a common context with the more conventional web consolidation processes. To do this, we define four classes of systems mechanical, thermal, thermal with restraint, and thermomechanical. Each class uses mechanical, thermal, and interactive effects in a unique way to determine water removal rates, energy efficiency, property development potential, and machine size and complexity. In a parallel and unifying fashion, each class also occupies a distinct region on coordinates of specific energy consumption and working temperature. This diagram is offered as a starting point for integrating wet pressing, hot pressing, cylinder drying, press drying, Condebelt drying, impulse drying, and others into four distinct classes of web consolidation systems.

In order to better understand the physics of impulse drying, two numerical models have been developed to predict the transient heat transfer, vapor pressure development, and vapor liquid flow during impulse drying. The first model, MIPPS-I, examines impulse drying as a moving boundary problem in which a sharp front of steam displaces a saturated liquid phase. While several key insights were obtained with this approach, a comparison of predictions with experimental data suggested that the sharp-interface assumption should be abandoned in favor of a two-phase zone between the dry and saturated regions. A new model, MIPPS-II, was then developed which allows a two-phase zone to develop. Both models use finite-difference forms of the mass, momentum, and energy conservation equations adapted for porous media.

Analysis of the numerical results in light of experimental data helps clarify some of the transport processes in impulse drying. In particular, it appears that the impulse drying process depends on the continued boiling of liquid near the hot surface with condensation occurring in the cooler, more saturated regions. The process of boiling and condensation is tied to sheet permeability and pore structure. The liquid for sustained boiling is available in saturated dead-end pores or is supplied by capillary flow.

The numerical results show that the development of an internal vapor zone is critical to several features of the impulse drying process. The pressurized vapor zone enhances water removal through direct displacement and also possibly by reducing or eliminating rewet. Relationships between sheet properties and internal vapor pressure and water removal can now be better understood with the aid of the models.

Several new pieces of experimental information are also presented which have guided recent model developments and, at the same time, can be interpreted in terms of results from the models. The new experimental data include flash x-ray visualization of interface motion in impulse drying and several measurements of thermal processes in both paper and model fibrous porous media.

Pressure drop for air flow through dry and moist paper has previously been expressed in terms of permeability as determined by Darcy’s law, an approximation now demonstrated to be substantially in error at through flow rates relevant to through drying. The more fundamental, non dimensional treatment using, the Reynolds number-friction factor model has never been applied for paper because of the need for characteristic dimension in Reynolds number. The use of various assumptions for this characteristic dimension in terms of permeability, specific surface, and llagen-Poiseuille equivalent capillary diameter are now shown to be in substantial disagreement with the pore structure of paper as examined by scanning electron microscopy.

A new characteristic dimension for flow through paper has been determined by application of fundamental principles of momentum transport. This characteristic dimension was determined for kraft paper over a wide range of basis weight, 25-250 g/m2 , and over the full range of moisture content from wet to dry. With this characteristic dimension, Reynolds number is rigorously the ratio of the inertial to the viscous contribution to momentum transport. With variation in .moisture content, the value of this characteristic dimension changes between two asymtotic limits which differ by a factor of about 2.5. The limits of these asymptotic regions correspond to known water-fiber relations. The values of the characteristic dimension agree with measurements by scanning electron microscopy.

A theoretical relationship between Reynolds number and friction factor is shown to fit a set of about 3000 measurements of pressure drop taken with about 150 sheets of kraft paper over a wide range of air through flow rate, paper moisture content and basis weight. This successful treatment, based on momentum transport theory, not only eliminates the need for the Darcy law permeability approximation, which leads to errors up to 600% for through flow rates used industrially in through drying, but also provides the basis for theoretical analysis’of heat and mass transport phenomena during through drying.

Machine-made papers often have an unsatisfactory cross direction profile with regard to their mechanical properties. The edges often have a lower strength and stiffness than the middle of the web.

Laboratory studies of the behaviour of paper during drying have shown the potential of improving CD properties without any deterioration in MD properties. This is achieved by preventing paper shrinkage in both directions during drying.

After extensive trials on a full-size paper machine equipped with a new cross-direction restraining technique, it has been demonstrated that the laboratory results can be repeated in a continuous process. This invention has the potential of solving some old problems in papermaking such as evening out the cross-direction profile and improving strength and stiffness properties, dimensional stability and surface properties.

The structure of interfibre bonding in hand sheets made of a hardwood BKP was investigated throughout the sheet formation stages using a transmission electron microscope (TEM), a scanning electron microscope (SEM) and a scanning laser microscope (SLM). The sample preparation techniques for TEM and SLM were newly developed for observation of the bond formation and bond structure.

Observations were made on couched wet webs, pressed wet webs, drying stages of the wet webs and dry sheets . Structures of the bonded zone and bonded fibres were characterized based on these observations. In this paper we discussed how the structures are affected by beating, couching, pressing and drying. We also discussed the roles of these structures in the physical properties of the fibre bonding.

Surface application has gained more and more popularity as a means of increasing the quality of paper and board. In its broadest sense, surface application covers everything from sur face-sizing to extrusion coating, but this review will concentrate on surface sizing and pigment coating, i.e. processes taking place as an integrated part of the paper manufacture. Because the goals of the surface treatmant are different and depending on the quality of the paper desired, the review will cover our present fundamental understanding of the processes themselves. The presentation will start with a description of the water or liquid penetration in paper. Then the mechanics of various blade coating processes and the liquid transport it blade coaters will be explained. Last will come a presentation of the size press and new equipment for surface sizing.

Using techniques of coating thickness analysis and frequency transform procedures developed in the EM laboratories it has been possible to describe the basesheet surface profile changes caused by the coating application. Relaxation of the fibre network has been observed and found to depend not only on the base stock furnish, formation etc., but to a large extent on the coating pigment type, size distribution, coating colour rheology and most importantly the dewatering interactions between the coating colour and the base. Factors determining the dewatering characteristics of a coating colour will include the base absorbency, pigment particle packing and suspension fluid viscosity.