Abstract
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.
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