The continuous extrusion of a metallocene linear low-density polyethylene through a transparent capillary die with and without slip was analyzed in this work by rheometrical measurements and particle image velocimetry (PIV) For this reason, a comparison was made between the rheological behaviors of the pure polymer and blended with a small amount of fluoropolymer polymer processing additive Very good agreement was found between rheometrical and PIV measurements The pure polymer exhibited stick-slip instabilities with nonhomogeneous slip at the die wall, whereas the blend showed stable flow The slip velocity was measured directly from the velocity profiles and was negligible for the pure polymer before the stick-slip but increased monotonously as a function of the shear stress for the blend The flow curves and the slip velocity as a function of the shear stress deviated from a power law and were well fitted by continuous “kink” functions Comparison of PIV data with rheometrical ones permitted a direct proof of the basic assumption of the Mooney theory Finally, the analysis of the velocity profiles showed that there is a maximum in the contribution of slip to the average fluid velocity, which is interpreted as the impossibility for the velocity profile to become plug like in the presence of shear thinning

Rheo-PIV analysis of the slip flow of a metallocene linear low-density polyethylene melt

This review is devoted to the comprehensive description of instabilities of different types that appear at the interface due to various physical reasons. The conditions and mechanisms of the disintegration of jets and droplets under the action of surface forces, beginning with the classical Rayleigh disintegration and ending with the current notions of the dependence of the critical value of Capillary Number on the ratio between phase viscosities in single droplets and concentrated emulsions, are considered. Physical mechanisms and criteria of the development of convective instabilities of different types, including the formation of the dissipative structures of Taylor-Benard types as consequences of thermal convection and the formation of Marangoni-Benard cells due to the nonuniformity of surface tension, as well as the instability of mass transfer through the interface in multicomponent systems resulted in the formation of regular cellular structure are compared. The attention was focused on the effect of the elasticity of a flowing liquid, which is inherent to polymer solutions and melts, on the character of interaction between the liquid and the solid wall. The set of surface phenomena observed in this case is described and relevant physical mechanisms of instability resulted in the formation of regular structures (“periodic defects”) on the jet surface are discussed. The problem of the development of edge instability on a free surface upon rotational flows is touched upon. An ample set of versatile phenomena that occur at the interface in stratified flows is briefly illustrated by the Kelvin-Helmholtz, Rayleigh-Taylor, and Richtmyer-Meshkov instabilities.

Surface instabilities

Triboelectrification of molten linear low-density polyethylene under continuous extrusion

Extrusion through brass dies has been found to be useful to eliminate sharkskin and stick-slip instabilities in linear low-density polyethylene. However, the physicochemical interaction of polyethylene melts with brass dies is not well understood. In the present work, such an interaction is analyzed by using electrical measurements. Experiments were performed in a single screw extruder at a temperature of 190°C with a metallocene linear low-density polyethylene and a brass die. The interaction between the polymer melt and the die changed with time. Different flow curves were obtained as the extrusion time was increased to reach an invariant flow curve. An irreversible transition from slip to no-slip at the die wall under stable flow was observed, consistent with a change in the sign of the measured electric charge. The critical shear stress for the onset of the stick-slip also increased with the extrusion time. Electrical measurements reproduced the stick-slip dynamics with fidelity. Pressure oscillations...

Study of the stick-slip phenomenon of linear low-density polyethylene in a brass die by using electrical measurements

An experimental rig coupled with a high speed data-logging and recording system and a personal computer was specially designed and constructed for the real-time measurement of mechanical strength (in terms of drawdown force) as a function of volumetric flow rate and roller speed for virgin low-density polyethylene (LDPE) and reprocessed LDPE during a filament stretching process. The effect of the number of extrusion passes for the reprocessed LDPE was our main interest. The experimental rig was connected to the end of a single-screw extruder, which was used to melt and extrude the polymers. The LDPE filaments were then solidified and collected for studying the mechanical properties. The mechanical strength of the virgin LDPE and reprocessed LDPE were investigated in both molten and solidified states. The mechanical strengths of the virgin and reprocessed LDPEs under these two states are discussed and compared in terms of change in magnitude under a wide range of processing conditions (volumetric flow rate...

Effects of Roller Speed, Die Temperature, Volumetric Flow Rate, and Multiple Extrusions on Mechanical Strength of Molten and Solidified LDPE under Tensile Deformation

The influence of extrusion under strong slip conditions on the extensional properties of linear low-density polyethylene was studied in this work. The material was extruded at two different temperatures under strong slip and no slip conditions, and was subsequently subjected to uniaxial elongational flow by means of a Rheotens device. Strong slip was evident through the elimination of sharkskin distortions and the stick-slip instability, as well as by the electrification of the extrudates. The extrudate swell was smaller in the presence of slip when comparing with no slip conditions at constant apparent shear rate, but it was found to be a unique function of the shear stress if comparison was performed at constant stress. The draw ratio and melt strength of the filaments obtained under slip conditions were larger compared to those without slip. In addition, draw resonance was postponed to higher draw ratios during the extrusion with strong slip at constant apparent shear rate. It is suggested that slip of the polymer at the die wall decreases the shear stress in the bulk, and therefore, restricts the disentanglement and orientation of macromolecules during flow, which subsequently produces the increase in draw ratio and melt strength during stretching.

Teresita de Jesús Guadarrama-Medina

Papers

Enhanced melt strength and stretching of linear low-density polyethylene extruded under strong slip conditions