| Abstract:Most of the polymer processing operations are completed at high shear rate, for example, the shear rate during injection molding is at a range of from about10s-1to104s-1. Therefore, it is useful to study on dynamic rheological behavior of polymer melts at high shear rate. It can be used to check the validity of proposed viscoelastic models or characterize the micro-structure of polymer melts at high shear rate. It can also be used to guide the improvement and optimization of processing equipment and technology. However, the research in this field is little, mainly because the maximum shear rate of the commercial rheometer can only be achieved less than10s-1.Based on the self-established measurement theory, and the self-built experimental platform, the study was realized on the dynamic rheological behavior of polymer melts at high shear rate. Depending on the functional analysis of experimental platform, it was decided that the flow is a parallel superposition of oscillatory upon steady Poiseuille flow. Shear stress, shear strain and shear rate are the most important qualities for rheological measurement. However, they cannot be measured directly. Therefore, theories were established to obtain them calculated from some physical qualities which can be measured directly such as piston velocity, melt pressure and size of barrel and capillary. According to careful calculation, some key parts were designed or determined. In order to ensure the credibility of test, the dynamic displacement control error of experimental platform was studied. The results show that both the temperature and displacement control errors of MAR are small enough for the test requirement. The errors of temperature are smaller thaną0.4?. The absolute and relative errors of vibration frequency don't exceed0.1mm and0.2%. The relative errors of vibration amplitude are at the range of0.5%?5%.Based on the self-built experimental platform, material functions were derived to characterize the nonlinear viscoelasticity of polymer melts at high shear rate. The experimental studies on shear stress show that the shear stress can be decomposed to steady state component and dynamic component. The dynamic component of shear stress is the nonlinear responsibility of shear rate, so a series of new material functions should be defined since the traditional material functions are invalid in this situation. The experimental results also show that there is couple phenomenon between steady state and dynamic flow fields, and is increased with the increase of amplitude. New material function ?? is defined as the apparent shear viscosity of steady-state component of the superposition flow. Based on Ft-Rheology and Stress Decomposed method, the material function was derived for superposition of oscillatory deformation upon steady shear flow.PP and its composite system with excellent performance were selected to do experimental study. The results show that, ?? is much lower in dynamic test with proper vibration frequency and amplitude, comparing with it in corresponding steady-state test. Therefore, it is a good way to introduce proper vibration to decrease the flow resistance and save energy in polymer processing. This phenomenon can be explained by reptation model and free-volume theory. The rheological experiment results show that there is obvious change of microstructure of PP/CaCO3composites under vibration while there isn't of PP/PS composites.The forecast values of metrical function based on Wagner model were calculated. According to the comparison between the calculation results and the measured values, it can be forecasted well for both the trend of ?? changes with amplitude and the fundamental amplitude ?1of shear stress calculated by Ft-Rheology with small amplitude. But, Wagner model was not very good at forecast the value of ????1when the vibration amplitude is large. Therefore a new damping function which is more universal should be proposed to make Wagner model better. |
PDF Full Text Request