Study Of Rheological Behavior And Mechanism In Extrusion And Extensional Processes For Polyethylene And Its Blend Melt

Currently, constitutive equations based on the molecular theories are far from perfect, andcan not predict shear and extensional flows of polymer fluids accurately in very wide range ofdeformation rates. As to the studies involving entry convergent flow for polymer fluids, it isdifficult to determine too many parameters experimentally in these studies, so these studiesare not easy to apply in engineering practice. The instruments measuring extensional viscosityare expensive and complicated to operate, measuring range is narrow. In-depth studyrheological behavior and mechanism of polymer materials has important theoretical andpractical significance for optimization process, optimization molding design and improvementof product quality. Therefore, the methods of theoretical analysis, experimental study andfinite element simulation are used to study the extrusion and extensional flows ofpolyethylene and its blends deep and systematically in this paper. The main research contentsof this paper are as follows:Based on network theory of polymer fluids and tensor analysis, the creation anddestruction rates of network junctions are analysed, and the molecular motion is discussed, asingle-mode finitely extensible constitutive equation of polymer fluids is deduced. Steady andtransient expressions of shear viscosity, first normal stress coefficient, second normal stresscoefficient and extensional viscosity are obtained furhter. Moreover, a single-mode slidingconstitutive equation of polymer fluids is deduced by discussing the creation and destructionrates of network junctions and introducing the sliding phenomenon. Steady and transientexpressions of shear viscosity, first normal stress coefficient and second normal stresscoefficient are obtained. These two constitutive equations are evaluated by parameter analysis,model comparison and experimental verification. Based on minimum energy and variationprinciple, some equations including entry pressure drop, entry nature convergent half angle,vortex length and extensional viscosity are deduced for polymer fluids, these equations haveless parameters and are convenience in engineering applications. These equations areevaluated by parameter analysis, model comparison and experimental verification.The rheological properties are measured by using capillary rheometers for low densitypolyethylene (LDPE)/poly(butylene succinate)(PBS) blend melts and high densitypolyethylene (HDPE)/PBS blend melts. The effects of processing conditions (e.g. shear rate,extensional strain rate and temperature) and material ratio on the rheological behaviors (e.g.entry pressure drop, shear and extensional flow) are analysed. The extensional master curvesare used in the LDPE/PBS and HDPE/PBS blend melts. The predictive abilities of extensional viscosity using four approaches (e.g. extensional master curve, BP neural network, entrypressure drop and constitutive equation) are discussed for polymer fluids.By introducing this single-mode sliding constitutive equation and choosing the suitablemethods and parameters the finite element simulations about entry convergent flow areconducted, the effects of mesh numbers, volume flow rate, runner transition arc, runnercontraction ratio and runner convergent angle on the entry convergent flow are analysed. Thefinite element simulations are conducted based on the sliding constitutive equation andexperiment data of shear viscosity of5wt%polyisobutylene of tetradecane solution, and thesimulations are compared with experimental data about flow rate, shear stress and first normalstress difference in entry convergent flow.