Proceeding Articles

Recent literature relating to the structure of paper and its load/ elongation behaviour is reviewed.

Following an outline of the more important load/elongation features and the effects on them of testing conditions and sheet variables, paper structure is considered in terms of the fibre (its strength, conformability and response to drying tension), the interfibre bond (its structure, area, frequency, strength and energy) and the sheet geometry. The structural changes that occur in the sheet as a whole and in its various elements during elongation and rupture are also described.

Theories relating mechanical properties to sheet structure are summarised in three categories-general theories, quantitative theories for the prediction of sheet elasticity and the application of the Griffith crack theory to paper. Some concluding remarks are offered about the significance, particularly in sheet elasticity theories, of the ability of a paper to distribute load evenly over all its structural elements.

The theory of anisotropic elasticity for paper is reviewed. Sonic pulse velocity measurements were used to evaluate Young’s modulus, shear modulus and Poisson’s ratio of paper. The effects of fibre orientation and drying stresses on paper elasticity are readily measured by sonic velocity. A previously proposed relationship between the fourin-plane elastic constants is approximately true for well-bonded paper. Local modulus variations in a single specimen can be detected by sonic velocity measurements. Any dependence of sonic velocity on substance is due to real differences in mechanical properties of the sheet.

The treatment of pulps at low consistencies in conventional beaters or refiners is partially a destructive process that severely damages and shortens the fibres. These disadvantages are over come in high consistency refining (HCR), a technique developed commercially in the USA, which treats a pulp at consistencies of 20-40 per cent by transmitting mechanical energy to a semi-solid fibre pad between the plates of a discrefiner. The refining action is brought about by strong inter fibres hearing actions, by rubbing and sliding of fibres on each other and by the intensive internal friction forces that result. This mechanism gives the fibres a unique appearance that makes it possible readily to detect the HCR pulp in a furnish.

The preservation of the average fibre length while developing extensive fibrillation is the most important feature of the HCR process. The properties of webs formed from an HCR pulp and a jordan refined stock differ noticeably in every phase of sheet consolidation. HCR fibres lurries frequently have a poorer formation, but seem to drain better-at least, in the higher freeness range. The tensile strength of wet HCR fibre mats is not much different from that of conventionally refined pulps, but they have a higher stretch. This was confirmed incommercial newsprint runs. HCR results also in a lower water retention after wet pressing and a greater web shrinkage in the dryer section.

Papers produced from HCR pulps show a very high tearing strength, a higher stretch and a somewhat lower tensile strength than papers obtained from a jordan refined stock. This is largely a result of the much lower fines content of HCR pulps.

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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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