Study On The Rheological Behavior And Characterization Of The Polymeric Melt In Pressure-driven Flows

Abstract:.The flow and deformation of polymeric melt are necessary for polymer processing. Rheological experiments help us understand the flow behavior and grasp the characterization of polymeric melt. It is the foundation for the process selection, the optimization of processing devices and the computer simulation of process.The pressure-driven flow is the major type of melt transportation during polymer processing. As the classic rheometric tool, a capillary rheometer can’t perform the dynamic test for the viscoelasticity characterization of polymeric melt, and provide none information about the pressure dependence of the shear viscosity. A rotational rheometer has the test mode of dynamic experiments, but their flow pattern is different from the flow pattern of the processing flow. In order to analysis the rheological properties of polymeric melt in processing, the following studies have been conducted.Improvement of the characterization methods and theories for rheological properties of polymeric melts in a pressure-driven flow has been made in the present paper. A new characterizing method of dynamic rheological behavior for melts in pressure-driven flows is proposed, and the mathematic model of dynamic rheological functions is built. Moreover, the test method for steady rheological properties of polymeric melts in pressure-driven flows is improved. This is achieved by adjusting the exit pressure of the capillary die within the capillary rheometer. The characterization for the pressure coefficient of the shear viscosity is realized on a basis of the traditional capillary rheometer. And the mathematic model of pressure coefficient is presented.A universal experimental platform for the pressure-driven flow was built up based on the principle of the classic capillary rheometer. The developing steps have been accomplished including the mechanical design, electric design, signal acquisition, and the software development for the measurement and control of experiments. It provides the basis for the multi test equipments introduced in the following of this paper.A slope structure of mold clamping was proposed based on the self-locking principle of slope, and a set of small mold for the injection molding experiments was developed. Based on the universal test platform for pressure-driven flow, the small mold with the slope clamping structure and the Kistler system for the mold pressure and temperature measurement, a piston type of injection molding equipment was built up. The data of the injection molding experiment show that the maximum melt pressure in the injection flow is up to70MPa. And the melt pressure decreases gradually to zero along the flow direction. Furthermore, the experiments of process simulation show the melt during the processing flow is usually under the unsteady state and the rheological behavior is often time dependent.Based on the partitioning feature of the capillary flow, an axially partitioned and rod shape transducer was developed for the shear force measurement of the polymeric melt. With the novel transducer the direct measurement of shear stress can be realized. The validating experiments show the transducer could solve the problem of the pressure correction by eliminating the influence of the entrance flow and exit flow on the shear force measurement of the middle segment. With the melt shear force transducer and the universal platform of the pressure-driven flow, an axially movable annular slit rheometer was built up. The annular slit rheometer could perform the dynamic test for the rheological properties of polymeric melt in the pressure-driven flow. Its experimental range is the vibration frequency of5to30Hz, the vibration amplitude of0.05to12.5mm and the volume rate of pressure-driven flow of zero to160ml/min. The validation experiment shows the test result of the steady rheological behavior on the annular slit rheometer is in accordance with that on the traditional capillary rheometer. And the annular slit rheometer could substitute a rotational rheometer to conduct dynamic experiments by its vibration shear mode. These validation experiments indicate the self developed rheometer is reliable.The dynamic experiments with Propylene (PP) sample and low density polyethylene (LDPE) sample denote that the influence of the type of the superimposed flow on the dynamic properties of the rheological behavior is significant. With the parallel superimposed pressure-driven flow, the amplitude of the complex modulus of the melt in the vibration shear flow becomes bigger than in the pure vibration shear flow. Under the vibration with the frequency of20Hz and with the amplitude of0.1mm, the complex modulus of melt superimposed a pressure-driven flow of the apparent shear rate3s-1is nearly double that of melt without superimposed flow, at the same time the loss angle decreases from46°to22.7°as superimposing a pressure-driven flow on the vibration shear flow. On the other hand, the test data with the rotational rheometer show that the complex modulus decrease and the loss angle increase as superimposing a drag flow on the oscillation shear flow. The experimental results indicate that the superimposed flow of pressure-driven type has the opposite effect on the dynamic properties of melt rheological behavior with respect to the superimposed flow of drag type.Based on the principle of traditional capillary rheometer with constant flow rate, using the self developed booster module a dual capillary dies method for the pressure dependence of the shear viscosity is proposed, and the steady test equipment with adjustable exit pressure is built up. The experimental study was conducted on the developed equipment with the low density polyethylene (LDPE) sample for its pressure dependence of the shear viscosity. Within the test range the pressure coefficient β of LDPE is11.7GPa-1. This result has no significant difference from the data of the lectures, which indicates the novel method of dual capillary dies for pressure coefficient test is feasible. By the new test method, the experimental data of the Polystyrene (PS) resin show that the shear viscosity of PS is more sensitive to pressure with respect to LDPE.The discovery of the difference between the pressure-driven flow and the drag flow in influence on the dynamic rheological properties of the polymer melt provides the new idea for polymer processing performance study. Meanwhile, a series of characterization equipments proposed in this paper provides the effective tools for the rheometric research on the rheological properties of polymeric melts.