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The mechanical strengths of neat low‐density polyethylene (LDPE), a blend of LDPE with linear low‐density polyethylene (LLDPE), and a composite of LDPE with wood flour (wood/LDPE) were investigated in molten and solidified states under tensile deformation. The results are discussed in terms of the effects of LLDPE and wood contents, roller speed, and volumetric flow rate. In LLDPE/LDPE blends, incorporating LLDPE from 0 to 30 wt% into LDPE caused a slight increase in drawdown force, a larger fluctuation in drawdown force, and a reduction of maximum roller speed to failure. The mechanical properties of the solidified LLDPE/LDPE corresponded to those of the molten LLDPE/LDPE with regard to the effect of LLDPE content. For wood/LDPE composites, increasing the wood flour content in molten LDPE caused considerable reductions in drawdown time and maximum roller speed to failure. The drawdown force increased with increasing wood flour up to 10 wt% before it decreased at the wood loading of 20 wt%. A number of voids and pores on the extrudate surfaces became obvious for the composites with 20 wt% of wood content. Increasing wood content enhanced the tensile modulus for the solidified LDPE but decreased its tensile strength. Unlike those of LLDPE/LDPE blends, the changes in tensile modulus and strength of solidified wood/LDPE composites with wood content did not correspond to those of the molten composites. In all cases, the drawdown force increased with increasing roller speed. The effect of volumetric flow rate from the extruder on the mechanical strengths of the solidified blends was more pronounced than on those of the molten ones. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers

An experimental rig was designed and constructed for melt strength measurement and was assembled at the end of a single screw extruder used for the production of low-density polyethylene (LDPE) melt. The experimental rig was coupled with a high speed data logging system and a personal computer for the real-time measurement of melt strength. The molten LDPE was extruded through a capillary die, forming a continuous filament before being pulled down by speed-adjustable mechanical rollers until the filament failed. A digital camera was used for measuring the actual extrudate size at failure point. The drawdown forces as a function of volumetric flow rate from the extruder, roller speed, die temperature and take-up style were of interest in this study. It was found that the experimental rig could be used for accurate measurement of the mechanical strength for the LDPE melt. The experimental results suggested that the melt strength of LDPE was dependent upon the volumetric flow rate through the die from the screw extruder, roller speed, and the take-up style. For ladder-step take-up, increasing roller speed resulted in non-linear increases in the drawdown forces, the drawdown force changes being associated with the molecular disentanglement and elastic resistances of the branched LDPE melt. The drawdown forces of the LDPE melt measured under the rapid speed take-up method were 40–60% greater than those tested under the ladder-step speed take-up method, depending on the volumetric flow rate used in the screw extruder. The tensile viscosity of the LDPE melt was found to decrease slightly with strain rate and die temperature in the testing conditions used in this work.

This study used a newly developed rotating die system for purposes of reducing entrance pressure drop and sharkskin fracture for molten polypropylene (PP) and wood/polypropylene (WPP) composites in a single‐screw extruder. The sharkskin fracture characteristics of the PP and WPP composite surfaces were examined quantitatively via roughness profiles and relaxation time evaluations, and qualitatively through scanning electron microscopy under the effects of wood content, shear rate, die temperature, and die rotation speed. The experimental results suggested that the entrance pressure drop of PP increased with increasing wood content and shear rate. The die entrance pressure drop for WPP composite melt with 30 wt % wood content could be minimized by 20–50% by using a die rotation speed of 70 rpm. The roughness level (sharkskin) and relaxation time were found to increase with increasing wood content, but could be minimized by rotating the die—the die rotating effect being more meaningful for WPP when compared with neat PP extrudate. The rotating die system was found to be an effective technique for minimizing the extrusion load and fracture level of extrudate skins for high‐viscosity materials such as the WPP composites used in this work. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

An experimental arrangement to simultaneously measure the melt strength, velocity profiles, and elongational viscosity profiles across the cross section of a molten filament that emerged from either a circular or slit die for low-density polyethylene (LDPE) under nonisothermal and isothermal conditions is proposed. The proposed experimental rig was based on a parallel coextrusion technique of colored LDPE melt layers into an uncolored melt flowing from the barrel into and out of a die to form a continuous filament before they were pulled down by mechanical rollers until the filament failed. The experimental rig was also equipped with a high-speed data-logging system and a personal computer for real-time measurements. The results suggest that the draw-down forces changed continuously with changing roller speed, and the velocity profiles of the melt were not uniform across the LDPE filament during the stretching of the melt. Greater drawdown forces and local melt velocities were obtained in the slit die or under the nonisothermal condition. The drawdown forces and velocity profiles in both dies were affected by the volumetric flow rates from the extruder and the roller speeds used, with the effect being more pronounced for the circular die. The elongational viscosity profiles of the LDPE filament were not uniform across the filament cross section and corresponded well to the obtained velocity profiles. The elongational viscosities of the LDPE filament were relatively higher when the filament was extruded and stretched in the circular die and under the nonisothermal condition. The changes in the elongational viscosity profiles were more sensitive to changes in the volumetric flow rate and roller speed in the circular die.