1977 Volume 1

The mechanisms involved in the transport of water in high polymers are reviewed. The discussion is mainly based on intact capillary-free models where the solution-diffusion mechanism is operative. The different patterns of behaviour which have been found for the sorption isotherms and the concentration dependence of the diffusion constants will be presented. Examples of the behaviour found with cellulose itself, two cellulose derivatives and with grafted cellulose and celullose acetate will be discussed in detail. In particular, experiments concerned with the sorption ofwater into wood pulp and grafted wood pulp will be presented and some theoretical and practical aspects of the results obtained considered.

A simple gravimetric technique for determination of the rate of capillary imbibition of wetting liquids by porous solids is described, and illustrated with results obtained with a wide variety of liquids on a range of glass-fibre filter papers.

Application of the familiar Washburn equation to the results yields an ‘imbibition equivalent’ pore size. Although, in the case of fibrous materials, it may be difficult to relate this to any real geometrical property, it nevertheless provides a practically useful description of the materials’ behaviour. The effects of prewetting on subsequent imbibition are explored.

Capillary rise is one of the most complicated transport phenomena in porous media. So far there has been no theory which can claim to describe it adequately. As for fine powders, adequate experimental data in cases of imbibition do not even exist.

Both measuring and evaluation methods have, therefore, been developed by which the most essential parameters can be obtained. Tests on fine powders show that the capillary rise is decisively influenced by dynamic effects. Static capillary pressure-so far regarded as the driving force-can only be decisive at very low transport velocities. The penetration rate of liquid into fine powders is characterised by a dynamic capillary pressure, at least in the initial stages of high velocities.

The newly developed theory was tested with fine powders of different substances.

The results confirmed the original assumptions. Whether, or to which extent, this theory can also be applied to fibrous materials, remains to be seen from tests still
to be carried out.

The present state of knowledge concerning the transudation of aqueous liquids into paper is reviewed. Theories for the swelling of cellulose and cellulose fibres are examined and related to the swelling of a porous web of paper. A method of measuring, simultaneously, the penetration and resultant swelling of the sheet by aqueous solutions is described. The results of the above measurements for water, water containing a wetting agent, starch solution, cuprammonium hydroxide solution and a glycerine/water mixture are reported. It is shown that the swelling is an integral part of the penetration process. Deviations from the Lucas-Washburn equation are due to the neglect in the penetration equation of the swelling of the sheet. Apparent diffusion coefficients are calculated from the swelling data and it is shown that swelling is controlled by a diffusion mechanism.

Paper is a porous structure containing two broad classes of pore: inter and intra-fibre, and penetration of liquids such as water is determined by the geometries of the two classes. In this brief note we wish to describe a technique and present preliminary results on surface wetting and absorption into single
fibres.

The objective of this study was to obtain a better understanding of the way in which the response of individual cellulose fibres to changes in relative humidity is relevant to the dimensional behaviour of paper and to the role of the fibres in composite materials, where dimensional instability is the main impediment to their wider use. The torsional response of individual pine tracheids to changes in relative humidity has been measured, and it is proposed that the observed angle of twist on drying a collapsed fibre is a function of the microfibril angle, the wall thickness, the fibre length and the fractional linear shrinkage across the microfibrils. A link between fibre twist and paper shrinkage is suggested. In thermomechanical pulps the temperature of defibration apparently affects the fibre twist in a way which is inversely related to the degree of fibrillation and fibre damage. The mechanical properties of wet fibre webs have been studied as a function of moisture content. The wet web strength and other properties of thermomechanical pulps depend on the fibrillation induced during refining which in turn depends on the relationship of the refining temperature to the lignin glass transition point.

Paper is treated as a member of the class of hydrogen-bond dominated solids for which the author derived a set of equations connecting E to the effective number of H-bonds per cm³,  N, and to the parameters of this bond. In this paper it is shown that the effect of water on such solids is to reduce E in one of two modes or regimes. In regime (1), a unimolecular reaction leads to a simple breakdown of H bonds on addition of water. In regime (2), which begins when the regain exceeds a critical value equal to the B.E.T. monomolecular layer, the reaction is still unimolecular in N, but is complicated by a co-operative breakdown phenomenon as envisaged by Frank and Wen’s ‘flickering clusters’ concept. Equations derived using these concepts and a co-operative index, C.I ., based on Starkweather’s thermodynamic  calculations, are checked against all data available to the author on paper, other cellulosics, Nylon 66 and wool with satisfactory results.

Machine calendering at conventional temperatures and line pressures has been compared with calendering at elevated temperatures, and calendering with an induced z-directional gradient in web moisture content . Print uniformity and web strengths are considerably improved by the latter calendering method, while rub off, set off, and print through remain at normal levels.

A moisture gradient is established by calendering the paper web before applied moisture has time to wet the paper’s surface. Study of the wetting time shows that newsprint from thermomechanical pulp wets faster than stone groundwood newsprint, that wetting time is decreased when web moisture content is increased, and that pulp drying history is particularly important-papers from bale pulps exhibiting wetting times orders of magnitude greater than those from slush pulps.

The absorption of mineral oil has been measured for papers calendered under different conditions . The studies show that calendering, and moisture gradient calendering in particular, slows the rate of oil absorption-while increasing web moisture content increases the absorption rate.

There are a number of studies on the relationship between the mechanical properties of paper and adsorbed water. In other previous papers-Anderson⁽¹⁾, Yoshino⁽²⁾, and Kadoya⁽³⁾ which concerned moisture dependence of mechanical properties of paper under impact conditions, it was found that the maximum values in the impulse or rupture energy existed at 15-18 per cent moisture content. The reason why such a maximum value exists is, however, left unsolved, because of the complexity of failure mechanism under impact conditions.

The glass transition temperature is the temperature at which an amorphous polymer changes from a hard glassy form into a rubberlike elastic form or to a viscous fluid. At this glass transition, which is a secondary transition, the temperature derivatives of both physical and mechanical properties of the polymer change.