1965 Volume 1
Cambridgepp 242-267Visco-elasticity and Consolidation of the Fibre Network during Free Water DrainageAbstractPDF
Measurements of specific permeability and compressibility of pads of synthetic and wood pulp fibres have been made in an apparatus in which rate of flow of water through the pad, pressure drop across the pad, external applied load and pad thickness can be controlled and measured. For non-swelling fibres at high porosity, with no applied load, the Emersleben-lberall drag treatment was found to provide estimates of permeability in reasonable agreement with observed values. The relationships between solids concentration C and applied stress P and between C and pressure drop p have been studied separately in terms of the empirical compressibility equations C= MPN with p=0 and C=m(p)n with P=O. The connections between mean compacting pressure during flow and the total pressure drop, between the exponents N, n and between the coefficients M, m, are discussed. Factors influencing the compressibility and consolidation of the fibre network include the flexibility and lateral conformability of the wet fibres, which apparently affect not only their capacity to deform elastically, but also the extent to which irreversible relative movement can take place. These effects are illustrated by the properties of pads from two series of pulps, each covering a range of lignin content-Pinus radiata sulphate pulps at various stages of bleaching and Eucalyptus regnans NSSC pulps cooked to different degrees. When compacting pressures that are due to flow and external loads are applied simultaneously, the compressibility equations in Pand Ophold, within limits, for constant values of p and P,respectively.
Experiments with synthetic fibres and a special flow apparatus, yielding data for the water permeation of fibre mats in the viscous-turbulent flow regime are briefly described. It is found that, within the range of the variables concerned,the results conform well to a recently established empirical equation relating the flow resistance of a pad to the flow speed, pad porosity and fibre specific surface. This empirical expression is then used, along with an equation representing wet mat compression characteristics, to construct a theoretical model of high-speed filtration. The result is a system of non-linear partial differential equations for the suspension kinematics and the flow rate/density distributions within the forming mat. Examples of numerical solutions are presented and discussed.
When a constant pressure is applied to a fibre slurry initially at rest, it under goes a continuously decreasing acceleration, reaching a maximum filtration speed, after which the speed decreases uniformly, corresponding to a constant pressure drop filtration process. The peak speed may be as much as eight times greater than the speed characterising the final constant pressure zone. Theoretical results for the density distribution in a forming mat illustrate the effect of relative compressibility, for which the more compressible material exhibits a rapidly changing density profile near the supporting septum. It is also found that the rat eat which the mat builds up after peak slurry speed decreases with increasing time to an extent depending on the mat compressibility. Filtration experiments with a bleached sulphite pulp yield results that agree satisfactorily with the calculations, confirming predicted formation times to within less than 10 percent. The experiments thus further corroborate the predicted inverse relationship of formation time with applied pressure, as well as an approximate proportionality of formation time and sheet substance.
A method is described of determining the distribution of fibres in the thickness of a sheet of paper. It relies on observing the disposition of a small proportion of dyed fibres in a transparentised sheet. All the samples examined show a highly layered structure.
Density profiles of the boundary of a sheet being formed in a drainage apparatus have been measured and the movement of single fibres was observed in approaching the forming zone in a model of the wire part of a paper machine. Both experiments show that a diffuse zone exists at the boundary of the forming mat, in which thickening of the stock takes place. The length of this zone is only a few millimetres and most of the formation takes place by filtration rather than by thickening. This finding is confirmed by computations based on a theory of formation, which includes both thickening and filtration. An argument is advanced that the layered structure of paper is the inevitable result of this mechanism of formation, which takes place at the usual papermaking consistencies. Much higher consistencies would be needed in order to produce a more felted structure of the sheet.
Cambridgepp 299-304Dependence of Sheet Properties on Formation and Forming Variables – Prepared ContributionAbstractPDF
Previous work on the sectioning of paper has not fully utilised the power of the light microscope. This paper describes techniques developed to enable the structure of paper to be seen in considerable detail in cross-section. The techniques are illustrated by sections of a wide range of types of paper. The consolidation of the structure of paper during manufacture is revealed by micrographs of the effects of beating, pressing, drying with and without restraint, super calendering and creping.
The scanning electron microscope has been used to observe changes in the structure of paper at different stages of pressing and drying.
In the first experiments, beaten kraft pulp handsheets were subjected to various pressing and drying treatments. The structure at the solids content achieved was stabilised by freezing and drying by sublimation under vacuum. Photo micrographs show the collapse of the fibres and consolidation of the paper structure during processing.
Samples of the web were obtained at positions from the wet end to the reel of operating kraft, bond and newsprint paper machines. As soon as the specimens were sampled, they were quickly frozen and later dried under vacuum in the laboratory.
The influence of water removal on the web and fibre structure by pressing and drying is illustrated. The relative importance of fibre conformity and fine material differs for the three paper grades. Under pressure, fibres are deformed plastically, particularly at crossing points and asperities. Collapse of fibres on removal of water from the lumen and the fibre walls by drying can usually be distinguished from that produced by mechanical pressure.
A description is given of experiments designed to substantiate some facets of the authors’ comprehensive theory of paper shrinkage and structure that was presented at the Oxford symposium. In particular, considerable evidence is presented in support of the basic concept in the theory-that is, the hypothesis of `adhesion before shrinkage’ of the constituent fibres. Examples are shown of a phenomenon that is the direct result of the latter process, termed necking of the fibres. Other factors important in the drying and shrinkage process are discussed.
The studies upon which this contribution is based were made to investigate the structure and properties of high stretch papers. Webs produced by compaction and by creping, the two main commercial processes, were examined by light microscopy and physical testing.
The micro photography shows a variety of web configurations found in crepe papers, including examples of wave formations, internal delamination and two sidedness. The characteristic fibre orientation and densification are illustrated by photo micrographs of webs taken before and after the compacting process.
The mechanical behaviour of high stretch papers is illustrated by typical stress/strain curves and a discussion of their behaviour during the process of straining.
After a review of the development of extensible papers, a description of the double-roll compacting process and its variables is given. Its principal feature is the venturi section formed in the nip between a rubber and a steel roll, between which the paper web passes in a semi-dry state. On running the rubber roll more slowly than the steel roll, the web will shrink in the machine-direction. Experiments on a pilot machine showed an increase in the compacting effect with increasing nip pressure and speed difference, though with certain limitations. When considering nip width and peripheral speed difference as primary variables, however, linear relationships with the paper properties were found. The nip width will vary with the nip pressure and rubber thickness and hardness.
The mechanism of double-roll compacting is considered to involve tangential forces, which move the rubber towards the back side of the nip, where it contracts, thereby shrinking the web. The structure of the resulting extensible paper was examined by photo micrographs of surface and cross-sections, by measuring the thickness changes on stretching and by load elongation measurements. The fibres appear curved after the compacting operation. This will result in the breaking of bonds when stretching the paper and in an ultimate breaking load lower than for flat kraft. The total rupture energy, however, is considerably higher.
An apparent increase in the rubber roll diameter on increasing nip pressure was observed. This will cause a decrease in the mean speed difference at the nip. At a limited set speed difference, the rubber roll was found to change from being driven to be driving on increasing the nip pressure. In an appendix, the nip width and the slip have been treated theoretically as well as experimentally.
Cambridgepp 445-472Ultrasonic Impedometric Studies in the Cellulose Pulp/Water SystemAbstractPDF
The high frequency shear mechanical behaviour of cellulose pulp/ watersystems during theprocess of drying from 3 percent solids to total dryness has been non-destructively and continuously monitored by the technique of ultrasonic impedometry. Unusual fibre/water interactions have been detected at both extremes of the concentration range studied. These interactions are given interpretation in molecular terms.