1965 Volume 1

NA

NA

A classification of the wide variety of solids according to physical, structural and chemical characteristics shows the category of heterophase solids to be the largest group of natural and artificial substances. Most manufacturing and manufactured materials fall in this category.

The structure of such materials can be described by the shape, size, concentration, orientation and topology of the constituent phases. Statistical methods in particular, geometric probability sometimes supplemented by certain physical ortopological restrictions, provide adequate tools for the description of naturally grown or artificially mixed systems.

Mechanical modulus properties as well as thermal and electrical properties can be treated in a fairly general manner,based either on field distortion of one phase by the presence of another phase or by means of phase models that give upper and lower limits for the property.

The problem of strength properties can be seen in terms of fracture mechanisms or as a statistical one, in which case extreme value theory provides a suitable method.

Organic polymers have always been useful for the making of fibres, films, rubbers, plastics, adhesives and coatings and cover a wide range of properties. For several years, systematic efforts have been made to explore the use of fully synthetic fibres on a paper machine either alone or together with cellulose materials. Several sheet properties can be substantially improved by this approach. More recently, the preparation of polymer systems has been studied that occupy an intermediate position between a uniform fibre and a homogeneous film. They have been called `fibrids’ and permit one to prepare two-dimensional sheet-like entities without the use of a liquid vehicle (such as water on a papermachine) that range in their properties from paper to textiles.

This paper is a condensed survey of the interactions between water and cellulose materials. It is introduced by some general remarks about the interaction of water with solids, with special attention to hydrogen bonds, solutions and gels, then about the chemical and physical properties of cellulose fibres. The main discussion is devoted to the interaction of water below and above the saturation point, dimensional changes and restricted swelling, drying and hysteresis and, finally, the quantitative measurements of the absorbed water.

Equilibrium water retention isotherms have been determined for papermaking fibres through the range of moisture contents that is of greatest papermaking interest.

The experimental method provides not only a measure of the swelling of the fibres and the equilibrium water retention at any force of water removal, but, conversely, the method measures the force with which the water is held and the force with which fibres or fibre elements are held together by residual water.

Factors affecting the mechanism of water retention and the amount of water held by fibres are demonstrated by the isotherms of model and papermaking fibres of widely varying properties and treatments. The part played by the mechanism and extent of water retention on the development of wet web properties during drying is discussed and the need is demonstrated for more information than is provided by the isotherm.

Finally, consideration is given to the role of swelling and water retention in pulp evaluation and some general principles are suggested by which the running and papermaking properties of a pulp might be predicted.

Micro-radiography has been used to determine the width and moisture content of individual pulp fibres during their drying from the swollen state. The method, which employs soft X-rays to produce contact radiographs of the fibres, is described and results are given for a spruce sulphite wood pulp. An assessment of changes in the thickness of the fibres while drying is also given.

Using a recently proposed concept of cell wall structure based on a multiplicity of lamellae coaxial with the cell, the effect of drying on the structure of the cell wall of bleached spruce sulphite tracheids has been examined by means of nitrogen adsorption. The data suggest that in the fully water-swollen wall there are up to several hundred lamellae, each of the order of 100 Å thick, with a median separation of about 35 Å. During drying, the lamellae draw together progressively into thicker and thicker aggregates, decreasing the total pore volume, but leaving approximately the same median separation in the spaces that remain. At dryness, the pore volume remaining in the wall is negligible.

During drying,the first pores to close  do not reopen when the fibres are treated with water,whereas the pores that close during the later stages of drying do so. The lamellae separation after drying at 25°C and reswelling remains at a median value of about 35 Å, but it drops after drying at 105°C and reswelling to 25 Å, owing to a greater permanency of pore closure in the pores of larger size. It is tentatively suggested that a fibre dries radially inwards towards the lumen and that the pores that tend to remain closed after drying are located towards the outside of the fibre, whereas the pores that reopen easily upon rewetting after drying are located towards the lumen.

Fibres of very different composition, chemical treatment and morphology, as well as native and regenerated cellulose fibres, possess a wide range of pore volumes, yet are shown to have very similar pore size distributions and it is suggested that this distribution is not therefore of biological origin, but is based on a property of the cellulose molecule. No obvious correlation was found between the fractional extent of irreversible pore closure upon drying and the composition of the fibres.

In the swollen cell wall of spruce sulphite pulp fibres, which contain almost 1 cm³ water per gram of dry material, about 20 percent of the water is present in macro reticular pores (spaces between lamellae) and 80 per cent in micro reticular pores(spaces within lamellae). This ratio persists throughout the drying cycle.

The relationship between cell wall porosity and the papermaking properties of fibres is discussed briefly in terms of the loss of porosity during drying and its regain during beating.

When elongated particles such as fibres are dispersed in water, they form a continuous network, provided the fibre concentration is above a certain level. A measuring technique utilising a concentric cylinder elasto-viscometer has been developed and used for studies of the mechanical properties of such networks.

Networks generally exhibited the same characteristic properties as solid visco-elastic bodies, hence should be characterised by methods used for such materials rather than by hydrodynamic methods. Nevertheless, a close connection was found between the mechanical properties of fibre networks as measured by quasi-static methods and the hydrodynamic behaviour of the same material, then considered as a fibre suspension.

A mechanism for the formation of fibre networks is proposed, in which the network is considered to derive its strength from the energy stored in the fibres when, after being bent in a turbulent shear field-such as is produced during agitation of the fibre suspension-they are prevented from straightening out by their interaction.

A mathematical model of random three-dimensional fibre networks of low concentration has been evolved. The fibre concentration, the length-to-radius ratio and the modulus of elasticity on bending are shown to be the most significant of the parameters determining the rigidity of the network. Experimental study of the shear modulus of model fibre networks substantiated the qualitative validity of the fibre network model.

A method is described of determining the distribution of fibres in the thickness of a sheet of paper. It relies on observing the disposition of a small proportion of dyed fibres in a transparentised sheet. All the samples examined show a highly layered structure.

Density profiles of the boundary of a sheet being formed in a drainage apparatus have been measured and the movement of single fibres was observed in approaching the forming zone in a model of the wire part of a paper machine. Both experiments show that a diffuse zone exists at the boundary of the forming mat, in which thickening of the stock takes place. The length of this zone is only a few millimetres and most of the formation takes place by filtration rather than by thickening. This finding is confirmed by computations based on a theory of formation, which includes both thickening and filtration. An argument is advanced that the layered structure of paper is the inevitable result of this mechanism of formation, which takes place at the usual papermaking consistencies. Much higher consistencies would be needed in order to produce a more felted structure of the sheet.