1981 Volume 1
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.
The need to make the best use of wood supplies, conserve energy, and reduce the costs of pollution abatement has stimulated research to improve the yield, accelerate the rate, and reduce the odoriferous nature of kraft pulping. Because the structure of lignin is imperfectly understood, the quest for reagents which can accelerate delignification has been handicapped. In contrast, because the structure and reactions of cellulose and other wood polysaccharides are known, deductive methods have been applied in the search for reagents which can protect these polymers from alkaline degradation and thus improve pulp yields. The discovery of the dramatic accelerating effect of anthraquinone on alkaline delignification can, in fact, be traced back to a series of investigations which began with a search for reagents capable of inhibiting the alkaline degradation of cellulose. Against this background, research on pulping additives in reviewed, with particular reference to the current status and potential of catalytic accelerators in alkaline pulping.
Cambridgepp 195-225Fundamental and Practical Aspects of Paper-Making with Recycled FibresAbstractPDF
The basic mechanism for the development of an irreversible swelling hindrance is discussed. It is demonstrated that changes in cell wall structure caused by pulping, chemical modification and straining during drying influence the ability of fibres to reswell after drying.
The results indicate that structural alterations take place during the drying of a cell wall. It is suggested that the basic phenomenon is the irreversible closing of cell wall pores. This essentially leaves a fibre wall which is more resistant to the mechanical treatment which promotes swelling and more prone to fragmentisation and the production of fines.
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 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.
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.
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.
Cambridgepp 527-559An Investigation of the Biaxial Failure Envelope of Paper: Experimental Study and Theoretical AnalysisAbstractPDF
In the theoretical part of this paper, the Tsai-Wu tensor theory is used to determine the equation for the biaxial strength envelope of a material.
One great advantage of using the Tsai-Wu theory for investigating biaxial strength envelopes is that it may be transformed to arbitrary co-ordinate axes by means of tensor transformation laws.
In the experimental part of the paper, the strengths in compression and shear are evaluated using thick-walled tubes consisting of two layers of paper glued together with a non-penetrating glue. In addition, the strength of paper under biaxial stress is determined by subjecting paper tubes to axial tension or compression combined with a radial tension caused by internal pressure.
In the final part of the paper, the application of the Tsai-Wu tensor theory to the prediction of failure of a corrugated container is discussed, as well as practical means of limiting the number of experiments necessary to establish the biaxial failure envelope of paper.
Cambridgepp 479-525Some New Concepts of the Relation between Fibre Orientation, Fibre Geometry, and Mechanical PropertiesAbstractPDF
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.