Effects of length distribution on the steady shear viscosity of semiconcentrated polymer-fiber suspensions

The steady state shear viscosity of glass fibers in a polyethylene oxide polymer solution was determined. The experiments were conducted for milled fibers that show a wide distribution of aspect ratios as well as for fiber samples that have a uniform and well-defined aspect ratio. In particular, results for one-modal, bi-modal, and tri-modal samples obtained by mixing fibers of uniform lengths were examined. These results allowed us to analyze the effect of the aspect ratio distribution on the viscosity and to determine the validity of different conventional averaging techniques that have been used so far to characterize the aspect ratio of milled fibers. It was found that all these conventional averaging methods do not give a good representation of the effect of the fibers length and that the widely used number-averaging technique fails to account for the important contribution of the long fibers vs. the short ones. A new model-based averaging method is proposed and its performance compared to that of other conventional averaging techniques is discussed.