Extrusion Of Linear Low Density Polyethylene Research Articles

Influence of die surface on the efficiency of fluoropolymer processing aids during the extrusion of linear-low density polyethylene

Nowadays, fluoropolymer based Polymer Processing Aids (PPA) are currently used to eliminate surface defects and to reduce die pressure in linear polyolefins extrusion. For 10 years, the main origins of the PPA actions are known: PPA discontinuously coat the die surface and create wall slip that delays the sharkskin defect. However, the microscopic parameters that govern PPA efficiency are not really understood. Based on several experimental measurements (Laser Doppler Velocimetry, Scanning Electron Microscopy, surface analysis), we propose an analysis of the mechanisms of PPA actions in various extrusion conditions. Different kinds of die surfaces have been studied and the results show that PPA efficiency can be greatly improved by controlling die topology and chemistry.

Multi-scale phenomena induced by fluoropolymer processing aids during the extrusion of linear-low density polyethylene

Using polymer processing aids (PPA) based on fluoropolymers not only reduces die pressure, but also postpones extrusion defects for linear polyolefins. The purpose of this work is to investigate the mechanisms explaining PPA efficiency. An experimental set-up was developed to study the PPA actions at different scales, based on a slit die with transparent walls and removable steel inserts, in line with a laboratory extruder. Original results are presented, correlating the evolution of die pressure, surface defects, velocity profiles measured by Laser Doppler Velocimetry and PPA die coating analyzed by Scanning Electronic Microscopy. During extrusion, PPA droplets and stretched aggregates appear first randomly on the whole die surface. Then, coating morphology progressively changes, leading to a network of interconnected and heterogeneous PPA streaks. Quantification of surface deposition shows that the PPA concentration grows towards the die exit, in concordance with spatially evolutionary wall slip. Surface defects elimination is correlated to the presence of discontinuous deposits at the die exit.

Fluorine-containing arborescent polystyrene-graft-polyisoprene copolymers as polymer processing additives

Linear low density polyethylene (LLDPE) can suffer from melt extrusion defects including sharkskin, cyclic melt fracture, and gross melt fracture during processing. Arborescent polymers are dendritic macromolecules with characteristics, such as a compact structure and a rigid spherical topology, making them potentially useful as polymer processing additives (PPA) to alleviate melt extrusion defects. Arborescent polystyrene-graft-polyisoprene copolymer samples were synthesized from polystyrene substrates of linear and branched architectures functionalized with acetyl groups, and coupled with polyisoprene macroanions. A linear polyisoprene sample was also investigated for comparison. The polymers were hydrosilylated with (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylsilane on 17–52% of the isoprene units and blended with LLDPE at 0.1 and 0.5% w/w to evaluate their performance as PPA by extrusion at different shear rates. All the samples led to some degree of improvement in the extrusion of LLDPE, albeit the performance of the branched additives was inferior to a commercial fluoroelastomer PPA. The lower molecular weight and more compact (G0 or comb-branched) PPA generally performed better than those with a high molecular weight. Several PPA samples induced the early onset of cyclic melt fracture but glossy, defect-free surfaces were obtained at higher shear rates. This suggests that a minimum shear rate is required for these additives to coat the extrusion die under the experimental conditions used.

Silanols cured by borates as lubricants in extrusion of LLDPE. Impact of elasticity of the lubricant on sliding friction

Addition of a viscoelastic material based on silanols cured by boron oxide was used to delay sharkskin and stick–slip instabilities in extrusion of linear low-density polyethylene (LLDPE). Delay of flow instabilities to rates of extrusion 25–35 times higher than without additive and about 40% less extrusion pressure at the same throughput are achieved by the use of this material as an additive (∼0.1%) to LLDPE or as a coating of the extrusion die. Mechanical properties of the lubricant were changed by small variations of composition to investigate the impact of elasticity on lubrication and sharkskin delay. Both lubrication and sharkskin delay were considerably improved when more elastic lubricants were used while the chemical composition of the lubricants was nearly the same. Filling the lubricants with powders of metal oxides or especially particulates having plate-like particles (kaolin, mica, BN) helped to delay the flow instabilities further to even higher throughputs. Together with experimental results, we present a tentative explanation for the importance of elasticity of polymer processing aids in the delay of sharkskin and the stabilization of slip.

Melt Flow Properties During Capillary Extrusion of Linear Low-Density Polyethylene

The melt flow properties of a linear low-density polyethylene (LLDPE) were measured by means of a capillary rheometer under the experimental conditions of temperatures from 220° to 260°C and apparent shear rates varying from 12 to 120 s−1. The end pressure drop (ΔPend) was determined by employing the Bagley's plotting method. The results showed that ΔPend increased nonlinearly with increasing shear stress. The end pressure fluctuation phenomenon was observed at lower shear stress level, and several plateau regions were generated in the end pressure drop-shear stress curves, suggesting onset of the wall-slip phenomenon during die extrusion of the resin melt. The critical shear stress with onset end pressure fluctuation phenomenon increased with a rise of temperature. Furthermore, the melt shear flow did not strictly obey the power law. The melt shear viscosity decreased nonlinearly with increasing shear stress and with a rise of temperature, whereas the dependence of the melt shear viscosity on the test temperature accorded with a formula similar to the Arrhenius expression.

Flow induced coating of fluoropolymer additives: Development of frustrated total internal reflection imaging

In the extrusion of linear low-density polyethylene (PE), fluoropolymer processing additives (PPAs) are used to eliminate the surface defect known as “sharkskin” by coating the die wall and inducing slip at the PPA/PE interface. We describe an in situ optical method for measurement of the coating thickness by exploiting the phenomenon of frustrated total internal reflection. By correlating the optical and pressure measurements, extrudate appearance and auxiliary experiments, we can elucidate the kinetics of the coating process. The PPA droplets first adsorb in the entrance region of the die and migrate under shear stress towards the capillary exit where they act to suppress sharkskin. We find that a uniform coating in the range of 25–60 nm is sufficient for sharkskin elimination. The steady state coating thickness near the exit ranges from 200 to 400 nm depending on the shear rate and concentration. We develop a mass balance model for calculation of the steady state coating thickness which compares well with our experimental data.

Flow Enhancement in the Continuous Extrusion of Linear Low-Density Polyethylene

Enhanced throughput and the elimination of sharkskin were observed over an extended temperature range when linear low-density polyethylenes having the same molecular weight, polydispersity, and melt index, one with and one without commercial additives, were extruded through α-brass dies. The additives appear to play a significant role in the detailed flow behavior, however, most notably in an irreversible transition to a “normal” flow curve at stresses on the order of 0.3 MPa. There are indications of a slip-like phenomenon, including the occurrence of electrostatic charge on the extrudate. A high-throughput, low-extrusion-pressure processing window like that associated by Keller with a transition to a mesophase was observed for the polymer containing no additives, but not for the polymer containing additives.

Extrusion characteristics of thermoplastic melts containing fluoropolymers

AbstractThis paper investigates the influence of fluoropolymers on the extrusion characteristics of three different thermoplastic melts. The presence of a small amount of fluoropolymer reduced the pressure drops, extrudate swell, and retarded the onset of surface defects in linear low density polyethylene (LLDPE), while little or no effect was found in polystyrene (PS) and polyamide 12 (PA 12). Among the fluoropolymers investigated in this study, vinylidene fluoride‐hexafluoropropylene (VDF‐HFP) copolymer was the most effective additive in reducing the pressure drop and extrudate distortion of LLDPE, polyvinylidene fluoride (PVDF) intermediate, and tetrafluoroethylene‐hexafluoropropylene (TFE‐HFP) copolymer exhibited the least effectiveness. Contact angle measurements on the die surface indicated that all fluoropolymers preferentially deposit on the die surfaces during the extrusion of LLDPE and PS blends with small amounts of fluoropolymers.

The effect of die materials and pressure-dependent slip on the extrusion of linear low-density polyethylene

The flow of linear low-density polyethylene through stainless-steel slit dies occurred at shear rates approximately 12% higher than in identical α-brass dies at the same wall shear stresses, indicating near-wall slip. The flow curves were independent of gap spacing. We show through the slip theory of Hill and co-workers [J. Rheol. 34, 891–918 (1990)] that a measurable gap dependence of the flow curve is not a necessary consequence of wall slip; the flow curves for both stainless steel and α-brass dies can be fit with the same rheological parameters, with a difference in the work of adhesion accounting for the differences in the flow curves. X-ray photoelectron spectroscopy revealed differences in the chemistry of brass surfaces with different pretreating, corresponding to small differences in flow curves. Fluorocarbon-coated die surfaces showed no more slip than stainless steel, while the flow curve with gold-coated surfaces followed stainless steel at intermediate stress and brass at high stress.

Polymer melt flow in very thin slits

Extrusion of linear low-density polyethylene and polystyrene through slit dies with spacings as small as 34 μm indicates qualitative differences between the polymers in the region of unstable flow. Polyethylene appears to exhibit wall slip, while slip is not evident with polystyrene. An apparent wall-induced reduction in polystyrene viscosity reported by Burton and coworkers was not observed.

Mechanism of Fluoroelastomer Processing Aid in Extrusion of LLDPE**

Abstract Fluoroelastomers are being used as processing aids for extruding thermoplastics. The mechanism of the function is studied by using a Capillary Rheometer and x-ray photoelectron spectroscopy. Four different die diameters with a die aspect ratio of 40 were used in extrusion of linear low density polyethylene (LLDPE). Slip velocities at the same pressure drop increase with the addition of a fluoroelastomer at 250 to 3000 ppm. This helps delay the onset of melt fracture to higher shear rates and produces smoother extrudates. This study shows that fluoroelastomer wets the die surface and migrates to the surface of extrudate.