Tham khảo tài liệu 'fundamentals of polymer engineering part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14 Flow Behavior of Polymeric Fluids INTRODUCTION In order to use polymers whether available in the form of pellets or as melt from a polymerization reactor the material has to be converted into useful shapes such as fibers films or molded articles. This is done using unit operations such as fiber spinning and injection molding which are analyzed in detail in Chapter 15. Here we simply mention that flow is an integral part of any shaping operation and very frequently it is useful to know quantities such as the pressure drop needed to pump a polymeric fluid at a specified flow rate through a channel of a given geometry. The answer is easy to obtain if we are working with low-molecular-weight liquids that behave in a Newtonian manner all we need by way of material properties is information about the temperature dependence of the shear viscosity. If the process is isothermal the shear viscosity is a constant and it can be measured in any one of several ways. Polymer melts and solutions however have a steady shear viscosity that depends on the shear rate. Therefore it is a material function rather than a material constant. For polymeric fluids the typical shape of the steady shear viscosity curve as a function of shear rate is shown in Figure . At steady state the viscosity is constant at low shear rates. It usually decreases with increasing shear rate and often becomes constant again at high shear rates. There is therefore a lower Newtonian region characterized by the zero shear rate viscosity Z0 and an upper Newtonian region characterized by an 573 Copyright 2003 Marcel Dekker Inc. 574 Chapter 14 Figure Qualitative behavior of the steady shear viscosity of polymeric fluids. infinite shear viscosity Z1. Between these two regions the viscosity versus shear rate behavior can usually be represented as a straight line on logarithmic coordinates this is the power-law region. In the foregoing we have been careful to append the word steady to the shear .
