Proceeding Articles

Experiments have been performed in order to demonstrate that water is expressed from small cavities in the fibre matrix during the pressing operation.

The gel water* in the fibre was tagged by mixing a fibre suspension with a solution of a polysaccharide (dextran) with a known relative molecular mass, capable of entering only cavities greater than its molecular diameter. When a fibre mat is subjected to an external pressure under these conditions, the water expressed from smaller cavities dilutes the polysaccharide solution. By using solutions of polysaccharides of different relative molecular masses, it was possible to demonstrate from which cavities the water was expressed.

Pressing experiments were performed with different kinds of fibres. Bleached and unbleached fibres, a mechanical pulp and viscose fibres were used.

The effects of beating, drying and de-crilling were investigated.

As a means of access to the more complicated phenomena of wet pressing, we have observed and interpreted compression dewatering in an extensively simplified case. It appeared best to begin with a system characterised by a single spatial co-ordinate; a system containing synthetic fibres which would be non-swelling and nearly identical, filtration-formed into a uniform bed, and mechanically conditioned; a system for which one could expect to find stable and reproducible compressibility and permeability properties. It also appeared that we could construct a theory of the behaviour of such a system, and develop effective numerical procedures by which to obtain the predicted response to rapidly varying applied stress. Experiments and theory were to include conditions in which the increasing stress on the solid portion of the system would induce significant non-uniformity.

Several widely recognised operational problems in papermaking have been attributed to the felt structure. All of these result from phenomena occurring at the macroscopic level, e.g. shadow and yarn-marking.⁽¹⁾ However, phenomena occurring at the microscopic level, i.e. at the scale of the individual fibres of the felt and paper, have received less attention.

Recent experimental studies⁽²⁻³⁾ on press felts show that their surface properties strongly affect water removal in wet pressing. Two mechanisms have been proposed to account for these effects : rewetting, and load uniformity. While a first approach to a quantitative discussion of the effect of felt properties on rewetting has been made,⁽³⁾ account of the structural influence on load uniformity has hitherto been considered only from a qualitative standpoint.⁽²⁻⁴⁾ In this briefnote we present some findings of a more quantitative study⁽⁵⁾ to investigate the latter problem. In this study we have evaluated felt roughness and assessed its effect on water removal.

Wet-web saturation, a process developed for the addition of polymer
to fibrous networks, involves three distinct stages:

(1) Web consolidation and water removal by wet pressing.
(2) Latex saturation of the wet-web by capillary and hydrostatic forces.
(3) Redistribution and removal of excess latex by squeeze rolls.

The results presented in this paper are part of an ongoing programme to gain a
better understanding of the saturation process and its influence on the properties
of polymer impregnated networks. This experimental investigation using a laboratory press is concerned with the saturation process (primarily pressing) behaviour of wood pulps in the basis weight range of 250-1 500 g/m².

Special laboratory equipment has been developed to measure drying rates as a function of the product moisture content for Through-Drying of textiles, porous paper and other permeable products. This equipment allows very precise measurements even for very fast drying processes (1 second and less), which are typical in Through-Drying. Experimental results for the drying rate as well as for the permeability (pressure drop) as a function of the product moisture content for various drying conditions for porous paper (tissue) are presented. The analysis of these results reveals, that the drying rate during the ‘constant rate period'(surface evaporation period) can be predicted from standard heat and mass transfer equations only if the pore size distribution is taken into account. For the purpose of extrapolation and scale-up a special overall mass transfer equation has been introduced. For Through-Drying of tissue material, and of textiles, this equation represents the experimental results with fairly good accuracy.

The dry-creping process is dependent on a balanced adhesion of the paper web to the MG-cylinder at the moment of removal of the web by the doctor blade. The adhesion of the web usually is on a layer of organic origin attached to the metal surface. The physical appearance of the intermediate layer has been studied on production machines and on various model surfaces and its chemical composition analysed. Laboratory techniques have been developed for studies of the peeling of fibrous webs from metal surfaces. Variables associated with the building-up of the intermediate layer and their effect on the peeling resistance have been investigated. Different pulps show widely different adhesion and react to different degrees to drying variables.

The purpose of this work is to study the relationship between the drying stresses and internal stresses and the mechanical properties of paper.

An apparatus was designed to measure the drying stresses, moisture content and surface temperature during the drying of paper.

The internal stresses in paper were measured using the technique described by Kubdt et al.⁽¹⁻²⁾ The internal stress was found to be equal to the drying stress, independent of the structure of the sheet.

The results also show that the drying stress developed during restrained drying correlates strongly with the mechanical properties of the paper when different types of fibre are compared under the same drying conditions.

The mechanisms involved in the transport of water in high polymers are reviewed. The discussion is mainly based on intact capillary-free models where the solution-diffusion mechanism is operative. The different patterns of behaviour which have been found for the sorption isotherms and the concentration dependence of the diffusion constants will be presented. Examples of the behaviour found with cellulose itself, two cellulose derivatives and with grafted cellulose and celullose acetate will be discussed in detail. In particular, experiments concerned with the sorption ofwater into wood pulp and grafted wood pulp will be presented and some theoretical and practical aspects of the results obtained considered.

A simple gravimetric technique for determination of the rate of capillary imbibition of wetting liquids by porous solids is described, and illustrated with results obtained with a wide variety of liquids on a range of glass-fibre filter papers.

Application of the familiar Washburn equation to the results yields an ‘imbibition equivalent’ pore size. Although, in the case of fibrous materials, it may be difficult to relate this to any real geometrical property, it nevertheless provides a practically useful description of the materials’ behaviour. The effects of prewetting on subsequent imbibition are explored.

Capillary rise is one of the most complicated transport phenomena in porous media. So far there has been no theory which can claim to describe it adequately. As for fine powders, adequate experimental data in cases of imbibition do not even exist.

Both measuring and evaluation methods have, therefore, been developed by which the most essential parameters can be obtained. Tests on fine powders show that the capillary rise is decisively influenced by dynamic effects. Static capillary pressure-so far regarded as the driving force-can only be decisive at very low transport velocities. The penetration rate of liquid into fine powders is characterised by a dynamic capillary pressure, at least in the initial stages of high velocities.

The newly developed theory was tested with fine powders of different substances.

The results confirmed the original assumptions. Whether, or to which extent, this theory can also be applied to fibrous materials, remains to be seen from tests still
to be carried out.