1965 Volume 2

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The relationships between bonding strength and web consolidation as affected by beating and wet pressing have been investigated. The effect of both the forces of external pressure and surface tension on the structure and on single fibre elements and crossings is discussed in the light of relevant publications. Wet pressing can exercise both direct effects and effects attributable to the induced liquid flow in the sheet. Various types of bonding strength measurement have been covered and attention has been devoted to the question whether data on the magnitude of the bonded area can be derived from scattering measurements. On introducing variations in the bonded area by means of variation in the wet pressure, investigations have shown that, for a given increase in bonded area, beating is more effective than wet pressing in increasing the rupture strength.

In the present investigation, use has been made of a method developed previously to relate irreversibly expended work to bond breakage with a view to assessing the effect of beating and wet pressing on the bonding strength value. These tests indicate that, for some pulp qualities with a high hemicellulose content, the bonding strength rises with increases in the degree of beating and falls on the application of higher wet pressure. For bleached and unbleached pulp with a low hemicellulose content, the bonding strength is (as has been found earlier) virtually independent of both beating and wet pressing. The possible reasons for the beating and wet pressing effects are discussed and some alternatives proposed. Although no satisfactory explanation can be found, it is believed that beating increases the possibilities of intimate contact between the surfaces and gives rise to a structure that has larger radii of curvature between the building elements. On the other hand, wet pressing may permanently damage the cell walls and result in less entangled contact zones and a structure of more angular configuration.

The Kubelka-Munk equations are presented and some consequences of these regarding the influence of brightness, formation and optical bleaching agents on opacity are discussed. The main part of the paper deals with the lights cattering properties of paper and the influence on the opacity of such different variables as fibre dimensions, pulping and bleaching processes, pulp drying,beating, fibre orientation, pressing and drying on the paper machine. It is shown how intimately opacity is affected by the degree of fibre-to-fibre bonding within the sheet.

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Uncoated paper webs containing ground wood were pressed between surfaces of various hardnesses, using different combinations of pressure and moisture content. The physical and printing properties of the pressed webs were measured. For a given web, the properties that affect print-through such as opacity, scattering power and bulk were determined by the combination of moisture content and pressure that was used. In addition, roughness was influenced by the hardness of the surface used to press the paper. Some treatments gave results similar to super calendered paper in all respects but for gloss, even though no shear was used during pressing. It was concluded that an optimum degree of compacting existed for paper intended to be printed on both sides.

A theory is presented to describe the relationship between the structure ofasheetofpaperandtheflowoffluidsthroughit.Basedonthemultiplanarconcept of paper, it defines a pore through the sheet in terms of structural and hydrodynamic variables. The effective pore size distribution thus depends on the type of flow,as well as on the structure of the sheet. It is in all cases approximately lognormal, with a standard deviation proportional to the mean.

The theory is applied to the problem of the maximum pore size and to laminar flow, for which the connection with the Kozeny-Carman equation is established.

The theory correctly predicts relationships between physical and structural variables and fibre/sheet properties. Numerical agreement with experiments is still limited by the lack of an appropriate definition of a layer in the multi-planar model.

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