The rheological properties of mixed pulp fiber suspensions were measured with a parallel-plates type rheometer. The effects of shear rate and mixture ratios of softwood and hardwood kraft pulp fibers on flocculation of the fibers were evaluated using a high speed CCD camera. The flow curves of the suspensions showed individual three zones derived from different flocculation conditions of the systems. The effect of the long fibers on the flocculation is more remarkable than the short ones. The two-dimensional fast Fourier transform technique was used to analyze the fiber orientation in polar coordinate. The fiber orientation in flow direction is rather irregular, even under high shear rate conditions. The present method is useful for evaluating the flow behavior of the fiber suspensions and the fiber orientation.
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