Proceeding Articles

In the first part of this paper (by E. Back) the fundamentals of press-drying are reviewed. Effects of process variables in single stage press-drying promoting the flow of wood components (especially lignin) under heat, moisture, and pressure are illustrated. For press-drying of hardwood pulps the role of residual lignin in parenchyma cells, with delayed removal in pulping, is analysed. The possibilities of short, multiple-stage press nips as useful for continuous paper production are exemplified.

In the second part (by R. Swenson) the application of the press-drying process is discussed for different paper grades. Various pilot plant approaches to dynamic, i.e. continuous, press drying are shown. The variables which affect the product and process are presented, with the results obtained when going from static press-drying to a dynamic slow speed press-drying machine. The problems of high speed press-drying to a paper machine is shown.

Interest in the application of press-drying to paper-making has arisen primarily because of the possible improvements to the physical properties of paper using high yield pulp furnishes and increasing amounts of hardwood fibres. A major proportion of the development work on press-drying has centred around the production of heavy paper grades, specifically liner-board.

The need for the increased use of hardwood in the pulp and paper industry has been recognised for a long time. At the present time, only 42% of the forested land in the United States is occupied by softwood timber which in turn supplies wood for 75% of the forest products. On the other hand, the lesser used hardwood species occupy 55% of the forested land area, but supply wood for only 25% of the forest products⁽⁴¹⁾. Satellite photography land surveys have shown that when softwood forests are cut, the tendency is for the land to grow back with hardwood species rather than the original softwood species. As a result, the amount of forested land being occupied by hardwood species trees has been increasing each year. The lesser demand for hardwood has resulted in a price differential where hardwoods are now $10-20 U.S. per cord cheaper than softwoods.

The effects of different drying strategies on the development of the strain to failure of paper and its relationship with drying shrinkage is studied. It is postulated that the linear superposition principle can be applied to drying strategies. The general mathematical expressions to describe the strain to failure due to drying strategies are given.

The linear relationship between the strain to failure and sheet shrinkage is not found. The relationship is entirely dependent upon the drying strategies.

Using the linear superposition principle, a graph which can predict the strain to failure by different drying strategies is constructed.

The explanation of the in-plane tensile stress-strain curve of paper has long been a matter for debate. In an earlier study it was shown that the elastic modulus of paper is given by an equation Ep = aφEf, where a is a function of the orientation distribution of the fibres in the sheet, φ describes the efficiency of stress transfer between them, and Ef is the elastic modulus of the fibres. As a result of extensive work on the effect of various paper-making treatments on the stress-strain response of paper, we have now shown that the plastic regime can be described in a similar manner, that is to say, in terms of the visco-elastic properties of the fibres, the orientation factor, and the efficiency factor. It is concluded that the non-linear behaviour of the stress-strain curve of paper originates primarily from the properties of the component fibres and not from the sheet structure.

A wave theory describing paper as a three-dimensional homogeneous orthotropic plate is discussed, and the theory compared with experiment. The results indicate that as long as the wavelength is large compared to typical fibre dimensions, paper may be considered to behave as a homogeneous orthotropic plate. This allows determination of all nine orthotropic elastic constants.

Measurement of the three Young’s moduli, the in-plane shear modulus, and the in-plane Poisson’s ratios were made as functions of fibre orientation, wet straining, and density. Qualitatively, the results show that a variable producing a change in properties in one direction, alters the properties in the other two directions in a predictable way. The in-plane shear modulus and the Poisson’s ratios, expressed as (vₓᵧvᵧₓ)¹/² were found to be relatively insensitive to fibre orientation and wet straining, except at the highest levels of each.

Following the initial work of Cox (1), a number of studies has been carried out with the objective of predicting the in-plane mechanical behaviour of paper in terms of the geometrical structure of the fibre network and the mechanical properties of the fibres. Recently Perkin s(2) developed a self-consistent model based on the straight segments of the fibres.

The relationship between ‘handle’ and the bending properties of paper and paper-board is investigated statistically. The handle of Paperboard was recognised to be explained by bending stiffness. In contrast,  the handle of paper is complicated by bending stiffness alone. The highest correlation coefficient is obtained for paper by applying the formula Sm²/d where Sm is the maximum bending moment at the curvature of 2.5cm⁻¹ and d is the thickness of the paper.

A simple method of observing the liveliness of paper is also proposed.

I would like to draw your attention to the fact that the fibres of which paper is made retain their individuality in the sheet, even in dense papers.

In many experiments, conditions are such that the discontinuous structure of paper has little chance to show. This is why the modulus of elasticity, for example, comes out the same whether it is determined by bending, by a sonic method, or by stretching in a tensile tester of standardized inertia. Such experiments assure us that the laws of continuum physics are consistent and that it is still worth reading and studying them.

The aim of the present investigation is to find out those variables of the paper-making process that cause variation in the light transmission of paper, independently of basis weight variation. Furthermore, is intended to discover under what conditions and to what accuracy the distribution of areal mass (formation) can be characterised by the areal distribution of light transmittance. The study is carried out by measuring values of beta-ray transmission and light transmission at exactly the same points of paper samples using an aperture of 1 mm diameter and analysing the correlation of the results.

At this stage of the study the effects of furnish composition, beating, wet pressing, and calendering on the correlation between mass distribution and distribution of transmittance have been analysed. It can be seen that at least prolonged beating and heavy calendering change the distribution of transmittance in such a way that the optical formation measurement does not give a true picture of the distribution of mass.

The factors which are responsible for the surface reflection of coated paper at specular angles have been explored by means of models of specular reflection and of coating roughness. The model of specular reflection used is based upon the addition of the roughnesses which arise from independent sources, i.e. the roughness caused by the base paper on the one hand, and by the coating pigments on the other. Coating surface roughness is approached theoretically by the incorporation in a model of the influences of particle shape and size, size distribution and the hypothetical statistics of particle position at the surface.

The theoretical predictions have been tested with the aid of experimental data, on specular reflection, Hunter gloss, profilometric roughness, and particle size. The samples studied were different types of coated paper, blade-coated on a pilot scale.