Studies On Extension Rheological Behaviors And Mechanisms Of Polyethylene Melts

In many processing operations, polymer melts experience extensional flows. The elongation behaviors of polymer melts have important effects on the processing procedures and the mechanical properties of final products. The thorough knowledge of the extensional properties of polymer can effectively optimize the production procedure and improve the products quality. In this article, we comprehensively studied the extensional flow behaviors and mechanisms of polyethylene melts from the modeling, simulation and measurements.Four extensional viscosity models were derived in this work The model based on White-Metzner equation is reasonable to describe the relation between shear viscosity and extensional viscosity; the model based on Moore dynamic equation that with some modifications of the original Moore equation can correctly describe both the shear and extensional rheology of polymer melts; the PTT model with temperature parameter can clearly demonstrate the relation between temperature and extensional viscosity; the model based on Cross model is simple and suitable to apply in finite element simulation. These four models were verified with experiment results and the effects of their parameters were analyzedNumerical simulation of Cogswell model, Binding model and Liang model were performed on MATLAB. The effects of model parameters on the natural convergent angle, boundary streamline and vortex length were discussed. Finite element simulations of 4:1 entrance flow were performed on the FLUENT with the extensional viscosity model based on Cross model. The effects of the change of Trouton ratio on the formation of vortex size were discussed.The extensional rheological properties of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and polypropylene (PP) were measured by melt spinning technique. The influences of temperature on the melt strength and draw ability were analyzed. Applied the Arrhenius equation to calculate the activation energy of melt strength for these four materials and compared the temperature sensitivity of them. A"Local match"was used to obtain the extensional stress and viscosity. The curves of extensional stress versus strain rate and extensional viscosity versus strain rate were plotted by the results. The effects of extensional strain rate, temperature and extrusion velocity on the extensional stress and viscosity were discussed. According to the measurement curves, the mastercurves of melt spinning were drawn. The properties of LDPE and LLDPE were compared by their mastercurves. The relations between the ratio b with temperature and with extrusion velocity were comprehensively investigated. According to result that each b had a unique relation with temperature and with extrusion velocity, the extensional viscosities of some setting conditions were calculated. The results of estimation were compared with experimental data. The entrance flow measurements that based on Cogswell model were performed to obtain the extensional viscosity of LDPE and LLDPE and the results were compared with those obtained by melt spinning technique.