1961 Volume 1

Using the techniques described in a previous paper, a study has been made of the breakage of fibre-to-fibre bonds in paper sheets under tensile strain . The frequent occurrence of partial separation in the areas of bonding and the relative rarity of total breakages are of particular interest. The way in which the bonded area loss is distributed between the bonds has been examined with particular reference to the effect of drying tension and beating . Both partial and total breakages are more common in freely dried sheets than in sheets dried under tension. Possible reasons for the occurrence of partial bond breakages are discussed.

Microscopic examination of the nature of paper failure, when tensile stresses are imposed upon the sheet, has shown that two important factors governing paper strength are the strength of individual fibres and the strength of bonding between fibres . A satisfactory method of determining individual fibre tensile strength has been published by this laboratory.⁽¹⁾ Using this method, the changes in strength of individual fibres of Loblolly pine springwood and summerwood holocellulose were followed during extraction with alkali of increasing concentration . The results of this work are reproduced in part in Table 1.⁽¹⁾

Following considerations of previous work, the authors’ investigations indicate that the first interfibre bonds to rupture under tensile strain do so by a peeling action with shear deformation of the paper in locally weakened regions. Progressive breakdown occurs along specific narrow bands within which strain is largely concentrated. The direction of these `strain lines’ has been predicted. Only for weak papers does final fracture occur along a single strain line, when considerable shear also results. For strong papers, only short lengths of strain lines form in a widely scattered cross-cross formation. In all cases, ultimate failure depends on the relationship between the tensile strength of fibres and the shear strength of bonds.

The mechanism of strain line formation results in paper thickness increases. Permanent deformation, frozen-in stresses and the effects of drying stresses are largely associated with strain line formation and frictional forces between fibre ‘mats’. Changes in moisture content modify the frictional effects and stress distribution and some aspects of dimensional stability can be explained. The finer details of fibre orientation distribution in machine-made papers can be attributed to shear in strain lines.

A stress/strain law is derived for a hydrogen-bonded network that is isotropic for all strains. The calculus of variations is used in conjunction with the Principle of Least Work to obtain the distribution of strain among the hydrogen bonds of the system as a function of their orientation . In addition to unidirectional applied stress, the theory can be used to predict the effects of two- and three-dimensional tension. The solution for unidirectional strain is compared with a modified form of the original treatment of the theory by Nissan and experimental evidence is used to corroborate the postulate that these two solutions `bracket’ the stress/strain behaviour for anisotropic (oriented) systems .

The physical properties of paper have been interpreted on the basis of the theory of the statistical geometry of fibrous networks. One example of each of the three main groups of properties is discussed: The maximum pore size and the optically bonded area on the basis of the theory of multiplanar structures and of the number of bond failures in the fracture of random two-dimensional networks . The experimental results support the statistical geometric approach to structure and physical properties.

A theory of the elasticity of handsheets has been formulated, based on the hypothesis that elastic deformation is caused by bending and, to a small extent, shearing and stretching of the unbonded parts of the fibres within the plane of the sheet. Experiments have shown that the theory holds for dense, well-bonded sheets, but deviates for light, poorly bonded sheets by a factor directly related to a geometric property the mean free fibre length . It is concluded that the arrangement of the fibres in handsheets is the connecting link between the fibre properties and sheet elasticity.

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.