Proceeding Articles

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.

The porous nature of paper can be regarded as consisting of three components, namely, an external void system, an internal void system and discontinuities. The external void system can be associated with the concept of paper roughness and is totally open in one direction. The internal void system can be regarded as that which can be observed by simple optical microscopy (e.g. paper sections cut at an oblique angle). The discontinuities can be defined as all those phenomena that deprive the voids between fibres of the characteristics which would justify regarding them as capillaries with smooth surfaces. These would include the structural elements of the fibre surface, the fines in paper, the fibrils and the fibril bundles protruding into the interfibre voids, the pits in the fibre surface and pores in the cell walls.

The interactions between the components of the model system proposed above have been investigated using thickness, light transmission and water absorption measurements.

*Shortened version of the original article, prepared by R. W. Hoyland, UMIST, Manchester.

Fibres from a bleached kraft pulp grafted with polystyrene by means of radiation initiation were studied. The purpose of grafting was to incorporate a polymer with a defined rheological behaviour, thereby changing the composition of the cell wall. The grafts had a polystyrene content ranging from 6 to 35 per cent by weight.

In order to decrease the crystallinity of the cellulose, the fibres were treated with aqueous zinc solutions of 65 and 70 per cent concentration. In addition to decrystallisation effects these treatments caused chemical degradation and dissolution of the cellulose.

Mechanical spectroscopy of the grafted fibres revealed an intense interaction between the polystyrene and the carbohydrate macromolecules. Apparently the treatment with zinc chloride causes degradation and structural breakdown of the grafted fibre which results in a less intense interaction between the cellulose and polystyrene phases.

The results of creep measurements in water and toluene indicate that the polystyrene can control the properties of the matrix while the amorphous cellulose still governs the stress transfer between the reinforcing microfibrils.

The mechanism of the wet tensile performance of paper is described in terms of the hydrogen bond theory of paper. Distinctions are made between the action of wet strength agents and the stiffening action of crosslinks. Chemical crosslinks improve the wet stiffness of paper by reducing the moisture sensitivity of the cellulose network to the swelling action of water. Below the fibre saturation point, the effect of water in reducing the tensile modulus of crosslinked paper is quantitatively the same as in uncrosslinked paper. Wet stiffening arises only from the reduction in the fibre saturation point that the crosslinks create. The role of crosslinks as load-carrying elements is not important in wet stiffening. Rather, crosslinks function as swelling restraints to the network, so that a larger fraction of the pre-existing hydrogen bonds function to retain a larger fraction of the paper’s dry tensile modulus. In this respect, even crosslinked paper can be considered a hydrogen-bond-dominated solid.