1961 Volume 2

The present knowledge of the manner in which pulp strength properties and papermaking are influenced by the fibre morphology of the original wood is discussed.

Fibre length has been shown to be particularly important for tearing resistance; it is of less importance for properties more related to fibre bonding. The thickness of the cell wall has an important bearing on most paper properties. Fibres with a thick cell wall give bulky, coarse surfaced sheets, whereas those with a thin wall give dense, wellformed sheets. The thick-walled fibres adversely influence bursting strength, tensile strength and particularly folding endurance, but they enhance tearing resistance, particularly when they are long. Basic density of the wood, which is indicative of cell wall thickness, may be used for assessing the value for papermaking of wood from within any one tree, from within a species or from the many species of one genus, the lower the basic density the better the general papermaking properties. In hardwoods, the vessel elements contribute little towards strength and cause trouble in printing . There is little evidence that cell diameter or the cell length/diameter ratio, in themselves, have any significant influence . The organisation of the cell wall in the individual fibres can influence paper properties. This is shown quite clearly when reaction wood fibres, which possess a markedly different organisation from that of normal wood fibres, are considered.

Paper formation phenomena are more closely related to fibre structure and fibre morphology in the case of non-wood fibres than with chemical woodpulp fibres . Nevertheless, the non-wood fibres commonly used for papermaking cannot be considered as a class on their own, but must be studied separately in these respects . This method is applied in particular to cotton, linen, cereal straws, esparto grass, bagasse and bamboo, with special reference to the dimensional structure of the fibres and their associated structures . Further research work on these fibres on the same lines and scale as that accorded to woodpulp fibres should give valuable information on the factors determining paper formation on the machine.

Based on the assumption that the most basic property of a paper is its apparent density, pulping effects such as yield, number of fibres per unit weight, lignin and hemicellulose contents and fibre strength are discussed with reference to the formation of bonds and paper structure . Special attention is devoted to the influence of different hinds of sulphite cooking such as conventional acid sulphite, bisulphite and two-stage cooking, also kraft cooking and bleaching. .Finally, non-chemical operations in pulping such as chipping, fibre fractionation and drying are briefly surveyed.

In Giertz’ paper, it has been pointed out that the papermaking properties of fibres depend in part on the pulping processes used in their production. The purpose of this contribution is to outline some recent investigations on how morphological factors, especially those that affect the path of penetration of the pulping medium into wood, are of influence on the composition and structure of the fibres ultimately used in sheet formation. The composition of the fibres is especially influenced by the manner of their separation during the pulping process . As background to this discussion, it will be recalled that the basic organisation of a typical fibre or tracheid is as shown in Fig. 1 . The layers corresponding to those in Fig. 1 can be seen in the electron micrographs Fig. 2 and 3. Furthermore, as shown by Lange,⁽¹⁾ most of the lignin of the cell wall is concentrated in the region external to the layer S2 and, for Eucalyptus regnans and Pinus radiata (the species here under discussion), similar values were obtained by Wardrop, Dadswell and Davies.⁽²⁾ It was shown further in this investigation that the lignin concentration in the middle lamella is greatest at the cell corners and somewhat greater between radial walls than that between tangential walls.

The role of fibre collapse in determining the extent of bonding and the density and strength of paper has been discussed in several of the papers presented at this symposium.^{(1-5 )} The effect appears to be twofold: firstly, fibre flattening permits larger contact areas between fibres and, secondly, fibre collapse results in a greater flexibility of the fibres.

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.