1993 Volume 3

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Three-dimensional topographical maps of paper surfaces under load have been quantified using the confocal laser scanning microscopy. Distributions of the paper surface pores of the same area under different loads were evaluated by the Equivalent Surface Pore (ESP). The ESP roughness of the uncompressed and compressed surfaces of TMP and bleached kraft papers, calendered to the same Print-Surf roughness with different calendering processes, were used to evaluate the local static compressibility of these paper surfaces. Assuming an exponential decay of roughness with pressure, the local static compressibility is defined as the slope of the roughness as a function of the logarithm of the applied pressure. Upon calendering, the local compressibility of the paper surface decreases. The compressibility after calendering depends both on the calendering process and on the furnish. The stiffer TMP fibres present more residual compressibility than the kraft fibres, already pre-collapsed in the uncalendered sheet. The surface compressibility increases with the internal pore volume. The calendered papers were gravure printed at different printing pressures and the number of missing dots counted. A theory is developed which links roughness to ink coverage. It is proposed that roughness is linearly related to the logarithm of the number of missing dots where the slope represents the surface compressibility. Theoretical derivations have been experimentally verified. It was also found that the static roughness is linearly related to the dynamic roughness.

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Fracture toughness is a material’s ability to resist propagation of a preexisting crack. In most end uses where fracture,toughness can be an important performance parameter for paper, stresses are applied in the plane of the sheet. Therefore, like tensile strength and elastic modulus, the fracture toughness of paper should be measured under in ‘ -plane loading. Our current industry practice of measuring the out-of plane tearing resistance by the Elmendor for Brecht-Imset tests seems inappropriate. Several techniques of fracture mechanics have been applied in recent years to characterize the plane stress fracture toughness of paper. An important consideration is whether a material property was measured particularly for tough papers.This contribution provides a background on these techniques.

We study the strength of paper using computer simulations. Our model is a stressed mechanical 2D random fibre network (RFN), the equilibrium of which is computed using a commercial FEM (Finite Element Method) solver Abaqus. The novelty in our approach lies in more detailed introduction of mechanical properties of fibres and bonds into the mechanical model, enabling the study of effects of microscopic (fibre/bond – level) properties.on relations between macroprocessor properties.

The mechanical properties of fibres and bonds are defined by Young’s modulus (or moduli in anisotropic case) and stress displacement curve. In our video presentation, fibres are purely elastic whereas bonds are elastic-plastic. The video shows the behaviour of network being stressed by forcing increasing displacement on one boundary

The paper converting process forms part of that whole area of engineering known as conversion technology, which applies the rules and procedures acquired in converting to the manufacture of finished products from paper and board.

The resulting conversion techniques consist of a sequence of sub processes in which certain changes in the state of the materials are brought about. The sum of these changes leads from the raw materials paper and board up to the final products.

For the reshaping, separating and connecting processes covered by this report, the specific characteristics and physical principles will be described and the main options of technical realisation discussed. In a number of applied examples, details of the real physical mechanisms encountered in the individual processes will be gone into.

The state of knowledge just set out is required for optimised designing of paper and board articles as well as of the necessary procedures and machinery. It is furthermore an absolute necessity with a view to realising the best possible manufacturing processes whilst taking ecological and work hygiene considerations into account and putting the means of production to economical use

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