Shear-induced flow behavior of three polymers in different size dies

摘要

A capillary rheometer with self-developed circle dies, whose diameters were 1, 0.7, 0.5, and 0.2 mm, was used to investigate the influence of channel diameter on the rheological behavior and shear-induced mechanism of polymer melts flowing through micro and macro dies. The different shear levels were imposed on three kinds of polymers, isotactic polypropylene, polystyrene, and polyurethane, that have different properties. In contrast to the dies with diameters of 0.5, 0.7, and 1 mm, with the increasing shear rate the growth of the logarithmic shear stress increased in the 0.2 mm die, and the apparent viscosity was far lower than that in a traditional 1 mm diameter die, which we attribute to a wall slip effect. The percentage of the melt velocity attributed to wall slip strongly depended on the die size. In the die with diameter of 0.2 mm, the percentage of the velocity due to wall slip of the three polymers was so high that their flow patterns were close to plug flow, especially for the polyurethane melt. Moreover, in the 0.2 mm die, increased shear stress exerted on polymer melts resulted, with the increasing shear rate, in the index of non-Newtonian behavior tending to 1 gradually, which indicates that the flow behavior was close to a Newtonian fluid. Based on the experimental results and quantitative analysis, rheological mechanisms are proposed for the different channels.