1981 Volume 1

A study was undertaken to assess the suitability of wheat straw to be pulped by means of the alkali-oxygen cooking process. The effects of some cooking variables, such as oxygen presence, type and amount of alkali, cooking temperature and time, on yield and Kappa number were evaluated.

The alkaline cooking of wheat straw in the presence of oxygen results in increased delignification. This is especially marked when the alkaline agent has a poor delignification capacity (sodium carbonate or bicarbonate). Thus oxygen seems to be very well suited to use in cooking processes with sodium carbonate liquors.

The use of such liquors can bring about a very interesting simplification of the recovery process.

In the second part of the work the influence of oxygen on pulp properties was evaluated as a function of the alkali used and of some cooking variables.

It is shown that the use of oxygen in alkaline cooking results in fibre degradation which affects the strength properties of the unbleached pulps only in the case of caustic soda cooking. The presence of oxygen during carbonate (or bicarbonate) cooking has favourable effects on pulp strength properties. This behaviour can be attributed to the fact that, in the case of carbonate cooking, the increment of delignification due to oxygen is far superior to that obtained in caustic cooking.

At equal delignification levels, the oxygen-carbonate pulps show better strength properties than the oxygen-caustic pulps. Long cooking times usually result in negative effects on strength properties.

The use of magnesium ions as inhibitors of carbohydrate degradation, had no definite effect on yield, viscosity, or strength properties.

The theme of this paper is a comparison of the role, properties, and technology of fibrous paper-making raw materials other than wood, with those of wood. Aspects considered are the economic role in world pulp supplies, the nature and properties of the fibres, technological problems arising in pulping, the properties and economic status of the resulting pulps, and the present and likely future of fundamental research.

To the vast majority of those connected with paper manufacture “cellulose pulp” is instinctively synonymous with wood pulp, especially in North American and Scandinavia. The aim of this paper is to redress this tendency, in particular from the point of view of the title of this Symposium. This is attempted by a comparison from all relevant aspects of the role, properties and technology of fibrous cellulose paper-making raw materials other than wood with those of wood. Thus, non-cellulose fibres (as of asbestos or glass) and modified cellulose fibres (usually derived from wood) are excluded.

This paper presents the results of an experimental and theoretical investigation into the fundamental mechanisms that govern ultrasonic propagation in fibre slurries. An experimental apparatus which measures the attenuation and velocity of ultrasound in slurries is described. Measurements on wood fibre slurries show, in contrast to previous work, that the effect of the fibre on the velocity of ultrasound is negligible. This observation led to the development of an isolated segment model, which can predict attenuation as a function of fibre properties and ultrasonic frequency. The theory assumes the fibres are isolated, isotropic, infinite cylinders. It accounts for scattering, heat conduction, viscous losses in the fluid, and relaxation processes in the fibre. Experiment and theory are shown to be in good agreement for some synthetic fibre slurries. In the more complicated case of wood fibre suspensions, the theory predicts attenuation to the correct order of magnitude and permits speculation about the effects of fibre properties.

The relationship between the torsional and flexural rigidity of single fibres, and the flow resistance of pulp suspensions was investigated from the point of the flocculation process.

The torsional and flexural properties of wetted single pulp fibres were measured by the torsional pendulum method and by the bending method respectively, for chemical pulps. A new experimental apparatus to measure the flow resistance of dilute fibre suspensions to rotary motion was developed in our laboratory.

A good correlation between torsional rigidity and the flexural property of single pulp fibres was observed, and the flexibility of fibres was found to increase intensely during the early stages of beating.

It was found that wall-shear stress, motion decay times, and the shapes of the motion decay curves of fibre suspensions depended on the flexural properties of the fibres, their length, and the concentration and temperature of the suspension.

An apparatus for studying the dynamic compression of saturated paper sheets is described. The apparatus can generate various combinations of haversine and square wave press pulses. The dewatering during a full loading cycle is studied, that is in both the compression and expansion phases. The total applied load, the hydraulic pressure and the thickness of the sheet are simultaneously recorded during the pressing operation. A total pressure of 10 MPa can be applied in pulses of durations as low as 5 ms. The techniques for hydraulic pressure and thickness measurement are examined. The performance of the apparatus is demonstrated and results from press tests with sheets of different grammages and different degrees of beating are given.

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.