SYNOPSISA friction measurement system was designed that made possible the measurement of the friction coefficient between a polymer melt and a metal die wall. The shear stresses developed in the capillary rheometer and the friction coefficient measured in the friction measurement system were compared in a n attempt to understand the mechanism of fluoroelastomer processing aids (FPA) in the extrusions of polypropylene (PP) and linear low-density polyethylene (LLDPE). The apparent viscosity drops of LLDPE treated with FPA were larger than those of PP treated with FPA. The friction coefficient drops of LLDPE treated with FPA were also larger than those of PP treated with FPA. High viscosity FPA showed a moderate friction coefficient drop in the actual extrusion of PP even though it showed only a poor effect in the capillary rheometer. The frictional forces were calculated from the friction coefficient measurement made during extrusions of FPA-treated and untreated samples.
Streak‐photographic and stress birefringence techniques were used to analyze the flow of poly(ethylene terephthalate) through potential chain‐ordering die geometries. The streak photographs were used to determine velocity distributions and streamlines in various convergent dies. The different contours were seen to have a significant effect on the polymer streamlines and velocity distributions. The measured velocities were used to develop empirical equations, specific for each geometry, which relate velocity to position within the die and to throughput rate. Flow birefringence was used to determine the extent of molecular ordering. The optimum chain‐ordering die geometry was determined to be one which included a rapid initial decrease in cross‐sectional area. Birefringence was also used to monitor polymer flow instability. An unusual mechanism for instability was observed at intermediate throughput rates.
Flow crystallization experiments which utilized the Instron rheometer in conjunction with convergent dies were conducted for the purpose of producing high-modulus poly-(ethylene terephthalate) filaments directly. from the melt. A temperature gradient was imposed on the lower extremity of the dies in an attempt to control the site of the fluid-solid phase transformation, and "freeze in" any orientation derived from the elongational flow regime. Com arative studies were made sion temperatures ranging from 255 to 270°C. Die angle influenced the pressure at which maximum die swell and the onset of extrudate distortion occurred; however, barrel temperature showed little effect on this pressure. The minimum temperature produced by the temperature gradient was the over-riding factor involved with cessation of flow. In each experiment, the fluid-solid phase transformation produced by the temperature was always accompanied by extrudate distortion. Thus, only minimal comparative studies of the extrudates could be performed. In view of the above, it appears that utilizing a temperature gradient, by itself, to "freeze in" preferred orientation within the confines of the die presents difficulties. A modification which combines a temperature gradient with external tension and a rapid after-quench outside the die, now holds appeal for continuing studies.using dies with included angles o P 20,30, and 40", and extru-324
A streak photographic technique was used to determine the velocity distribution in a capillary produced by the flow of poly(ethylene terephthalate) (PET). This uniquely designed quartz die assembly, which was fitted onto a melt extruder, permitted visualization of polymer flow behavior during melt spinning. Aluminum tracer particles were mixed with polymer chips prior to extrusion. A laser beam was directed through a lens system that illuminated the tracer particles in the melt only in a thin, vertical cross section of the transparent quartz die. A pressure‐drop analysis was also performed on PET, under the same experimental conditions, to determine rheological properties of the polymer. By using these rheological properties theoretical velocity distributions were calculated for PET and then compared with the profiles obtained by the streak photographic technique.
A unique capillary die was designed which made possible the measurement of extrusion pressure at various locations along the capillary length. Entrance pressure drops, exit pressures, and other rheological characteristics were determined for the flow of poly(ethylene terephthalate) through this extrusion apparatus. The effect of die entrance angle, extrusion temperature, throughput rate and polymer molecular weight were considered. Two samples differing in molecular weight exhibited power‐law behavior at shear rates below 1000s−1. The entrance pressure drops and exit pressures were observed to increase with increasing molecular weight; furthermore, at a specific temperature, both‐increased with increasing shear rate. The values for entrance pressure drop obtained using Bagley analysis were consistently higher than those obtained from direct measurements.
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