1977 Volume 1

Today, several important parameters in sheet forming on the paper machine are changing considerably.

The twin wire concept is about to become the standard for new installations. Here, the drainage zone is drastically reduced compared to the fourdrinier, with corresponding reduction in drainage time. Besides, there is a general increase in paper machine speed. The drainage elements are changed, reducing the degree of pulsed drainage.

Also the papermaking furnish is changing.

New fibre raw materials are introduced by a change from mature to juvenile trees on the one hand and the increasing use of tropical hardwoods on the other. This implies a significant change in the fibre compositions of the furnishes.

This tendency is enhanced by another technical trend, partly caused by scarce wood supplies, towards pulps of higher yields. The trend is significant both within the conventional chemical pulping processes, and for the development of new processes like the thermomechanical pulping methods.

The change in paper forming speed, and in the fibre composition may cause problems for the production process. It therefore seems useful to try to produce new information on the influence of fibre form on the pulp drainage characteristics.

The importance of wet-web strength in the manufacture of newsprint is considered. A number of case histories are presented, which illustrate how the assessment of wet-web strength can be effectively used to clarify practical observations on paper machines. These observations were concerned with the influence on paper-machine operation of normal fluctuations in the properties of the mechanical and chemical pulp components of a newsprint furnish. Wet-web strength measurements were carried out at moisture contents of the web which reflect its ability to release water under standard drainage and pressing procedures. A rapid wet-web burst testing technique was developed that is suitable for evaluating wet-web strength in a mill environment.

The availability of accurate draw measuring equipment makes it possible to measure these two properties of the wet web running on the paper machine. An example has occurred recently, which illustrates the principle of such measurements.

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.