The forces on pulp fibres during refining

The refining process is widely used in the pulp and paper industry for the production of woodpulps or to develop the papermaking properties of wood fibres through repeated application of strain. The mechanical action of refiners is commonly characterized using energy-based methods, but Page has suggested that characterizing the stress-strain history of fibres would lead to a better understanding of the refining process [Fundamentals of Papermaking, 9th Fundamental Res. Symp., Vol 1, p. 1-38, 1989]. Recent work by Martinez et al. [J. Pulp Paper Sci., 23(1):Jll-18, 1997] and Batchelor et al. [J. Pulp Paper Sci., 23(1):J40-45, 1997] has produced a model of the forces imposed on agglomerates of fibres, called floes, when they are caught between refiner bars. The model assumes a linear stress-strain relationship during floe compression. However, in this work, experiments performed in a laboratory single bar refiner have shown that the stress-strain relationship becomes non-linear at high strain. The present work proposes a new model to predict the normal stress resulting from floe compression between the bars of a refiner. The model accounts for two separate mechanisms to explain the development of this normal stress: fibre bending and radial compression of fibres. The relative importance of the two mechanisms depends on the fibre and floe properties and the level of strain. Experiments have shown that the model accurately predicts the stress-strain behaviour over a wide range of strain and floe properties. For mechanical pulp floes, fibre bending was shown to account for, on average, 65% of the total normal stress, while fibre compression explained the other 35%. For a previously dried kraft pulp, the normal stress could be modeled with good accuracy by accounting for fibre bending alone. The shear force between the refiner bars was also measured in these experiments, and it was found to be directly proportional to the normal force through a coefficient of friction. Values for the coefficient of friction varied between 0.26 and 0.32 depending on the type of pulp considered.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat