Martin Zatloukal

On PVT and Rheological Measurements of Polymer Melts

Abstract The relation between PVT and rheological measurements of several polymer melts including polyethylenes, polypropylene, polystyrene, poly(methyl methacrylate), and polycarbonate has been taken into investigation. Pressure-temperature dependent viscosities, determined on rotational and backpressure-modified capillary rheometers, were fitted through the Carreau-Yasuda model. PVT data was analyzed by the help of the Simha-Somcynsky equation of state (SS EOS). The thermodynamical parameters of the SS EOS were connected to constant-stress viscosity (experimental) and zero-shear viscosity (extrapolated). The Doolittle relationship was modified into the form of η = exp(C1 ln(h′h)). The relation was employed and tested for the data evaluation. It proved to be a good tool for linearization of PVT and rheological data.

Numerical Simulation of Polymer Coextrusion Flows

Abstract Non-isothermal calculation of flow history dependent visco-elastic stresses in a coextrusion die is performed using the 8-mode modified Leonov constitutive equation and the deformation rate field from finite element simulations. It is shown that a heuristic criterion based on the difference of normal stress differences across the layer interfaces in the merging area may be used to potentially detect the onset of interfacial instabilities. The developed technique can be a useful tool for coextrusion die design as well as for proper selection of materials and process conditions for coextrusion.

Increase of long-chain branching by thermo-oxidative treatment of LDPE: Chromatographic, spectroscopic, and rheological evidence

Low-density polyethylene was thermo-oxidatively degraded at 170 °C, i.e., degraded in the presence of air, by a one thermal cycle (1C) treatment during times between 30 and 90 min, and by a two thermal cycles (2C) treatment, i.e., after storage at room temperature, an already previously degraded sample was further degraded during times between 15 and 45 min. Characterization methods include gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, as well as linear and nonlinear rheology. A reduction of molar mass was detected for all degraded samples by GPC, as well as an increase of the high molar mass fraction of the 1C sample degraded for the longest time. Intrinsic viscosity measurements indicate also a reduction of molar mass with increasing degradation times for both 1C and 2C samples. Thermo-oxidation is confirmed for 1C and 2C samples by analyzing specific indices in FTIR. Linear viscoelasticity seems to be in general only marginally affected by thermo-oxidative exposur...

Visualisation and analysis of interfacial instability in coextrusion of LDPE melt

Abstract Details presented cover the study of interfacial instability of a low density polyethylene melt flow in a coextrusion flow visualisation cell. The cell design splits a single melt feed into two meltstreams using a flow divider. The meltstreams are made to converge at an angle of 30° and flow into a common die land of 1.0 mm height. The flows in the confluent region and die land to the die exit were observed through the side windows of the cell. The relative stream velocities, hence layer thickness ratio, of the two streams were varied using an adjustable restriction plate in the bottom channel. Stress and velocity fields were quantified using stress birefringence and particle image velocimetry techniques. Wave type interfacial instability was observed in the extrudate when the major–minor stream layer thickness ratio exceeded 2 : 1. Flow at immediate entry to the die land appeared stable when the extrudate exhibited instability. However, a disturbance was observed in the flow towards the exit of the die land. The frequency of the disturbance was quantified and found to be the same as that of the wave disturbance in the extrudate. A modified Leonov model and Flow 2000 software were used to simulate the melt flow through the geometry. A total normal stress difference (TNSD) sign criterion has been used to predict the presence of the wave interfacial instabilities in the coextrusion geometry.

Historical Review of Die Drool Phenomenon in Plastics Extrusion

Die drool phenomenon is defined as unwanted spontaneous accumulation of extruded plastics on die exit face(s) of extrusion die during plastics extrusion process. Such accumulated material builds up on die exit face(s) and frequently or continually sticks onto the extruded product and by this way reduces the quality of the product. When the die drool appears, the extrusion process must be shut down and the die exit face(s) must be manually cleaned, which is time consuming as well as money consuming. Die drool has been observed from the beginning of plastics extrusion and the first published remark about it was made in 1946. For a long time it was considered as only an engineering problem and even if a wide range of suppressing ideas based on modification of extrusion dies, plastics materials, and processing conditions have already been patented, its source remained unclear. During the last few years, a number of experimental as well as theoretical research papers focusing on its fundamental nature have been published. Nowadays, die drool is considered as a fundamental rheological phenomenon. The force which drives the building up of extruded plastics on die exit face(s) is negative pressure (suction) occurring in the die exit edge(s) region where the free surface of the extruded plastics is created. Moreover, two different die drool types (external and internal) exist. The formation mechanisms of external/internal die drool are based on negative pressure occurring in the die exit region together with deformation of free extrudate surface/material separation wherever inside the extrusion equipment, respectively. From the processing point of view, the internal die drool is much more problematic than the external one, primarily due to a higher build up rate and compact shapes of the accumulated drool mass. Two theories of internal die drool formation mechanism based on wall slip theories (flow-induced molecular weight fractionation and stress-induced cohesive chain disentanglement) have been recently proposed and tested. In this historically ordered review, breakthrough works in the field of die drool research are presented, many ways to suppress it are shown, techniques for its quantitative evaluation and experimental methods for its analytical investigation are introduced, external and internal die drool types are explained, and theories of external as well as internal die drool formation mechanism are presented and discussed in depth, which can be especially helpful for plastics extrusion experts as well as for the rheological community.

Theoretical and experimental investigation of interfacial instability phenomena occurring during viscoelastic coextrusion

Abstract The fully viscoelastic finite element method (FEM) together with the flow visualisation technique were employed to quantify the effect of die design on wave type interfacial instabilities in coextrusion. It has been shown experimentally that minor channel geometry has a strong impact on wave type interfacial instabilities and the results can be correlated through novel criteria called Total Stress Difference (TSN) which takes into account the bulk change in total stress in the normal as well as the tangential direction with respect to the interface. It has also been shown that a specific type of high stress area occurs around the interface at the end of the converging section. This total stress has been found to be non-monotonic along the interface and is related to the onset of zigzag type interfacial instabilities when the total stress achieved 200 kPa.

Pressure and Temperature Dependence of LDPE Viscosity and Free Volume: The Effect of Molecular Structure

Abstract Temperature and pressure dependencies of shear and elongational viscosities were examined using rotational and capillary rheometers. Two different batches of the same polymer grade have shown that the molecular structure diversion significantly influences the magnitudes of pressure coefficients, which vary more than the temperature coefficients. The pressure effect on viscosity notably depends on the amount of long-chain branching in polymer. Further, the paper shows that pVT data analysis via the Simha-Somcynsky equation of state can be employed to reveal differences in temperature and pressure viscosity dependencies through the free volume fraction.

Modelling of viscoelastic coextrusion flows in multi-manifold flat dies

Abstract A recently proposed modification of the viscoelastic Leonov model is employed as a stress calculator in FEM analysis with a full u-v-p-t numerical scheme for coextrusion flow in multimanifold flat dies with 30° and 90° entrance angles. It is shown that the predicted stresses, interface location and streamline fields are in good agreement with the measurements. It is also shown that extensional viscosity has to be used in the modelling of the coextrusion flow to confirm experimental data.

Characterization of Carbon Nanotube Based Polymer Composites Through Rheology

Poly(methyl methacrylate)/Multi‐wall carbon nanotubes (MWCNT) nanocomposites were prepared by solution method. Here the dispersions of MWCNT in PMMA solutions were sonicated for appropriate time followed by dispersions coagulation. Both types of CNT materials were used such as pure MWCNT and the same MWCNT after their adequate surface treatment. The aim of treatment was to covalently attach organic material onto surface of CNT to process their better dispersion in polymeric matrix leading to more effective CNT reinforcement effect. The state of CNT dispersion was characterized thought rheology measurements with help of parameters like elasticity and viscosity of the melt. Also the effect of sonication onto pure PMMA matrix was determined.

Evaluation of Thermally Induced Degradation of Branched Polypropylene by Using Rheology and Different Constitutive Equations

In this work, virgin as well as thermally degraded branched polypropylenes were investigated by using rotational and Sentmanat extensional rheometers, gel permeation chromatography and different constitutive equations. Based on the obtained experimental data and theoretical analysis, it has been found that even if both chain scission and branching takes place during thermal degradation of the tested polypropylene, the melt strength (quantified via the level of extensional strain hardening) can increase at short degradation times. It was found that constitutive equations such as Generalized Newtonian law, modified White-Metzner model, Yao and Extended Yao models have the capability to describe and interpret the measured steady-state rheological data of the virgin as well as thermally degraded branched polypropylenes. Specific attention has been paid to understanding molecular changes during thermal degradation of branched polypropylene by using physical parameters of utilized constitutive equations.