Proceeding Articles

The mechanism of the shrinkage of paper has been investigated microscopically and this has resulted in a better understanding of the structure of paper. It is shown that the shrinkage of paper gives rise to a `microstructure’, in terms of which Theological and other properties of paper, as well as the influence of papermaking variables upon them, can be explained. Although the theory is considered to be comprehensive, it has not been possible to develop it completely in this initial publication .

At the suggestion of Page and Tydeman, we have examined the structure of freely dried handsheets, using the scanning electron microscope . This was done because of the unique advantages of this instrument for this type of examination and in the hope that an independent test of their theory could be made.

In the present work, unbleached kraft pulp was beaten to 450 CSF and 60 g/m2 handsheets were formed in a British sheet machine under, standard conditions. The sheets were pressed at 50 lb/in2 between plates and blotters. After pressing, one sheet was dried on its plate with the usual restraining ring in place. A second sheet was peeled from the plate, floated on mercury so that it was free to shrink and air dried at room temperature.

The surfaces of these two sheets were then examined in the scanning electron microscope, using the technique described elsewhere at this symposium (pp . 101-108). The method of examination was to choose several fields at random and take low magnification pictures of these. In each of these low magnification fields, several right angle fibre crossings were selected and photographed at higher magnification. These were then used to estimate the extent of compression in fibres over bonded areas.

Visual examination of the pictures gave the following results. Three independent observers agreed that 12 out of 16 fibre crossings in the freely dried handsheet exhibited microcompressions compared with only 3 out of 12 in the plate-dried handsheet. In addition, the severity of the compressions was obviously greater in the former. For both handsheets, doubtful cases were listed as having no compressions.

In this short note, it is impossible to illustrate completely the extent and variety of the microcompressions observed. Only eight pictures of the freely dried handsheet have been chosen to illustrate some of the results (Fig. 1-8).

Firstly, the fundamental laws for the permeation, diffusion and penetration of fluids into porous material are explained and the available information on the porosity of paper is discussed. Using the classification in which neutral fluids like air, oil and grease are distinguished from those that interact with the pore walls like water and water vapour, the permeability to air and liquids like oil and grease is treated in greater detail. In addition to the description of the process of penetration, which is of special interest for printing inks, the main part of the paper deals with the behaviour of paper in relation to water and water vapour. Although the effect of swelling on porous structure has not yet been investigated sufficiently, some definite statements can be made on water and water vapour permeability . Furthermore, it is known how the moisture and the water content affect many properties of the paper, especially the strength.

Results of recent studies have been incorporated into the information already available. These investigations dealt with the penetration of water vapour into paper and the associated humidity and temperature effects. The paper closes with an account of some little-known studies on the effect of water vapour on paper and a general survey of dimensional stability and the tendency to curl.

Optical properties of paper are those that govern its visual appearance-mainly, colour, opacity and gloss. These depend on the fact that paper consists of a network of doubly refractive transparent fibres immersed in air as medium and, in some cases, the optical properties can be expressed in terms of the absorption and scattering coefficients of the fibres and loading materials.

Opacity is the property most thoroughly understood and there is a British Standard for its measurement. The colorimetry of near-white papers still gives rise to difficulties, particularly with those containing optical bleaches and recent work on the assessment of sheets containing appreciable amounts of fluorescent materials is reviewed. The measurement of gloss is the most difficult to perform and our knowledge here is still far from complete. Little-known research on gloss done during the past fifteen years at PATRA and by a group of Japanese workers is summarised.

The difficulties involved in interrelating paper properties are examined. Qualitative relationships through trends and experience are noted and the special case of quantitative interrelationship among burst, tensile and stretch is described .

More basic considerations such as fibre length, fibre strength, sheet density, fibre orientation and interfibre bonding are individually related to common strength properties and special consideration is given to tearing strength . These more basic considerations are themselves interrelated and an attempt is made at a unification of paper properties through such basic interrelationships . It is suggested that fibre strength, in a broad sense, is of particular importance in such unification .

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.