Proceeding Articles

Developments in research on the beating process since the Cambridge symposium are reviewed with respect to changes in the individual fibres and the manner in which these primary effects influence pulp and paper properties . The following are regarded as primary effects breaking of intrafibre bonds, external fibrillation and foliation, formation of fines and fibre shortening .

The breaking of intrafibre bonds can take place at various dimensional levels, but can be referred to changes in the hydrogen bond structure. Properties discussed include specific volume, specific surface, flexibility, flow resistance, drainage resistance, wet web strength, drying tensions, bonded area, density, tenacity, extensibility, Young’s modulus, rupture energy, creep, tear ,factor and folding endurance.

Experiments on the influence of beating on the flow resistance and drainage resistance are described in some detail .

An effective and simple method for sectioning paper was developed that enabled determination of the distribution of filler in a large number of papers. Attention was concentrated on papers made on Fourdrinier machines, but, to obtain a complete picture of filler distribution, handsheets and papers made on cylinder machines were also investigated. From the resulting filler distribution curves, it appeared that handsheets and cylinder-made papers have a comparable filler distribution, which is the reverse of that of Fourdrinier-made and handmade papers.

It is generally supposed that the filler in Fourdrinier-made papers is concentrated in the top layer, whilst much less filler is present on the wire side. This was confirmed in our investigations. It even turned out that the filler content of a 10 per cent wire side layer is almost constant and depends only in the degree of beating and to some extent on the total amount of filler in the paper. This suggests the filler capacity of the extreme wire side layer to be the determining factor .

Furthermore, the influence was investigated of filler retention, type of filler (particle size), fibre composition, filler content, machine speed, dandy roll and open table rolls on filler distribution . From this, some general rules about the distribution of filler in paper were derived: it is determined by machine speed, total filler content and filler capacity of the extreme wire side layer. The filler distribution in the top layers of a Fourdrinier-made paper is affected also by the dandy roll and probably by the retention of the filler .

This general picture of filler distribution, together with the results of laboratory and mill experiments, gave rise to a theory of the causes of the observed phenomena. This theory states that self-filtration (drainage) of the three-phase system water|fibre |filler results in a heterogeneous distribution of filler particles in the fibre mat, having the same character as in Fourdrinier-made paper. It is concluded, therefore, that the uneven distribution is already present in the first stage of drainage and is strongly intensified by the extreme drainage conditions.

A papermaking furnish may be regarded as potential paper. The extent to which this potential is exploited depends upon the design and operation of the papermachine . The degree of useful utilisation of the furnish is largely determined by the orientation and distribution of the fibres and fines or filler particles in the finished sheet, both across the sheet and through its thickness . However, the structure of paper is the result of an extremely complex interplay between the intrinsic properties of a pulp and the manner in which the formation process is conducted. Consequently, the way individual components of the furnish are put together in the finished sheet is only indirectly and by no means completely under the papermaker’s control. In such a situation, analysis of the structure of paper, both on a microscopic and on a bulk level, provides the most direct link between the formation process and the characteristics of the finished paper.

Recently, the study of paper structure has taken a new turn. Theoretical approaches are being explored in an effort to relate a geometrically defined arrangement of fibres to the properties of the resultant sheet.^{(1, 2)} This approach is as yet in its infancy, but already promises to be a powerful one. The theoretical formulation of paper structure requires the use of an idealized model, however, in order that the problem can be set up in a manageable mathematical form. The choice of a model must depend on a knowledge of the structure of real paper. The analysis of the structure of commercially produced paper is therefore indispensable not only in its own right as an evaluation tool, but also as a complement to the theoretical studies.

The present investigation deals with the structure of newsprint. It was undertaken to obtain information on the manner and sequence in which the components of the newsprint furnish are deposited on the papermachine wire, also to establish the fibrous compositions of the top and wire side surfaces of newsprint . The work forms part of a general investigation of groundwood and newsprint evaluation, aimed at predicting the behaviour of a pulp on the papermachine wire in terms of parameters of the pulp that can be measured in the laboratory.

Sheet structure can be fully described in terms of(i) the distribution of fine material, (ii) the degree of fibre orientation and (iii) the degree of fibre flocculation throughout the sheet thickness.

The physical characteristics of common forming processes are discussed-those of the Technical Section sheetmachine, the Fourdrinier machine, uniflow and contraflow vats . The major characteristics of these processes are relative movement between stock and wire, drainage forces, stock concentration, recirculation of fines, manner of metering stock in relation to area of the formed sheet, conditions under which the forming process ends.

The effects of these physical process characteristics on the structure of the sheet are discussed and available evidence is presented.

It is concluded that all common forming processes can be described by means of component unit processes, which are orienting, continuous draining, intermittent draining, fractionating, emerging and flocculating . The manner in which these unit processes affect the sheet structure is described. Internal sheet structures typical of the common forming processes are presented schematically.

The subject of this paper is a study of the properties of paper sheets made from suspensions of a known state of flocculation, the sheet formation being performed under controlled conditions. The delay time between end of agitation and start of drainage and the rate of drainage were varied.

The flocculation of the sheet increased with delay time and decreased with increasing drainage rate, indicating a strong qualitative relation between these two phenomena in their influence upon flocculation. The flocculation of sheets increased linearly with the flocculation of suspensions, a quantitative comparison showing that the state of flocculation represented in terms of mass distribution improved during drainage. Tensile strength increased with decreasing flocculation, tear decreased, porosity passed a minimum at intermediate flocculation.

The measurement of paper uniformity is discussed with special reference to those methods providing information that may be used to investigate the sources of variation of structure and properties. The flocculation of fibres in the stock and the hydrodynamic properties of the head box are shown to be the major causes of heterogeneity.

Fibre flocculation is both a direct source of heterogeneity and a factor affecting the fluid mechanical properties of the stock. The flow properties of fibre suspensions are described on the basis of pipe flow studies and the significance of these studies to head box flow is discussed.

The head box is considered as a source of non-uniformity with some reference to the significance of secondary flows. Experimental methods are described that may be used to assess head box performance to investigate conditions that give rise to poor uniformity.

Areas in which further research is desirable are indicated.

If we want to describe the flow of fluids through a porous medium, we generally use for the equation of motion an empirical relationship that is usually called Darcy’s law (see Scheidegger⁽¹⁾)

The course of the papermaking process, as it occurs on a Fourdrinier machine, is analysed to illustrate how each functional operation performed by the machine influences the final product. The analysis starts with the role played by the slice and its approaches and special emphasis is given to the many compromising factors that determine holey roll design and behaviour. The effect of slice design on orientation, flocculation and jet delivery is also considered. It is shown that the concept  of a fibre network structure for the stock with a strength that varies with fibre consistency, length and type explains many of the observations .

The distinction between macro- and microformation is defined and the relative effects of the head box and table suction on these properties are illustrated. It is concluded that gross relative motion of stock on the wire is detrimental to macroformation, but that short range relative motion is beneficial to microformation and results in a more uniform fibre distribution than is possible by random turbulent diffusion processes in the head box alone.

The variation of sheet properties across the sheet thickness are discussed and it is concluded that selective filtration that occurs during the forming operation is the principal cause, not the backwashing of the sheet by inflow of water at table rolls, as is ,frequently reported.

A brief speculation on the construction of the `idealised’ machinemade paper sheet is presented.

It is a privilege and an honour for me to have been asked by the organising committee to give the concluding summary to this symposium. Although I have been working on cellulose and cellulosic fibres off and on since graduate school, until now, I have not published a single paper in the field of paper structure and properties. This symposium has been a most informative and enlightening part of my education as a scientist . Because of my assignment, I have tried to understand and, if possible, assimilate the various contributions to the symposium. Needless to say, this has been a strenuous task. The hydrodynamic discussions have been difficult for me to follow . I have had to annoy both chairmen and speakers by asking questions in my attempts to get a more complete picture of the subject matters discussed: I am grateful for the answers . My situation has often reminded me of a story from the University of Uppsala, my Alma Mater. One of the physics professors was known to have an unusually detailed and comprehensive knowledge of both classical and modern physics . One of his colleagues asked him, out of curiosity, `How did you acquire all this fabulous knowledge? You are, after all, a highly specialised physicist in atomic spectra.’ ‘Well, you see, I have to take many oral examinations . By a systematic questioning of the students, then comparing the answers from the bright ones, I have learned a lot in all branches of physics, without reading all the books,’ was his reply. This is in fact what I have done here.

Plant fibres are elongated cells, that is, grown objects whose structure can only be fully understood from a development point of view. The young differentiating fibre cell has a very thin wall, consisting of an amorphous matrix of pectic and hemicellulosic material reinforced by only a few percent of cellulosic microfibrils. Curiously enough, this percentage (say, 5 percent by volume) corresponds approximately to the amount of iron rods in reinforced concrete! The microfibrils with a diameter of about 250 Å are arranged in a dispersed interwoven texture (Fig. 6). This so-called primary wall contains in the living cell some 90 percent. of water and, of the technically important dry matter, only less than half is cellulose, which alone is left over in the macerated preparations for the electron microscope. The growth of the primary wall consists in a widening of the existent texture combined with a continuous neoformation of new wall lamellae (multi-net growth) . The differentiation of pit fields and bordered pits occurs during this growth. The pit areas no longer increase their surface, though their distance may still considerably increase (mosaic growth, Fig . 5).