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Notes: The effect of deformation history on the elongational behavior and spinnability of polypropylene melt was investigated by carrying out isothermal melt-spinning experiments. For the study, spinnerettes of different die geometries were used to investigate the effect, if any, of the entrance angle, the capillary length-to-diameter (L/D) ratio, and the reservoir-to-capillary diameter (DR/D) ratio on the elongational behavior of molten threadlines. An experimental study was also carried out to investigate the phenomenon of draw resonance in the extrusion of polypropylene melts through spinnerettes of different die geometries. Draw resonance is the phenomenon which gives rise to pulsations in the threadline diameter when the stretch ratio is increased above a certain critical value. The results of our study show that the critical stretch ratio at which the onset of draw resonance starts to occur decreases as the L/D ratio is decreased, as the entrance angle is increased, as the DR/D ratio is increased, as the melt temperature is decreased, and as the shear rate in the die is increased. Of particular interest is the observation that, at 180°C, the severity of fiber nonuniformity increases as the stretch ratio is increased, whereas at 200°C and 220°C, the severity of fiber nonuniformity first increases and then decreases as the stretch ratio is increased considerably above the critical value. A rheological interpretation of the observed onset of draw resonance is presented with the aid of the independently determined rheological data.

Notes: Melt spinning experiments were carried out to investigate the elongational behavior and fiber morphology of multiphase polymer systems. Materials chosen for study were blends of calcium carbonate-filled polypropylene with general-purpose polystyrene and blends of calcium carbonate-filled polypropylene with high-impact polystyrene. The former is a three-phase system in which the CaCO3 particles are dispersed, and the latter is a four-phase system in which CaCO3 particles are dispersed, together with rubbery butadiene particles. Note that polypropylene is incompatible with the matrix of high-impact polystyrene. The experimental technique described in part I of this series was used to determine the elongational viscosity. With the four-phase system, the apparent elongational viscosity tends to increase as the stretch ratio is increased above a certain critical value which appears to be ca. 25. This transition does not occur with the three-phase system and is attributable to elongation of the suspended rubber particles. The addition of small amounts of HIPS to PP-CaCO3 increases spinnability in general, whereas larger amounts decrease spinnability.

Notes: Blown-film extrusion experiments were carried out to investigate the elongational flow behavior of viscoelastic polymer melts at different melt temperatures. Materials chosen for study were high-density polyethylene, lowdensity polyethylene, and polypropylene. In the study, isothermal blown-film extrusion experiments were carried out in which the molten blown film traveled upward through a heated chamber of about 13 in. in length maintained at the same temperature as the melt. Axial tension was measured at the take-up roller, the axial profiles of bubble diameter were determined by a photographic technique, and, from the samples collected, the variation in the film thickness along the axial direction was found. These measurements were used later to determine the elongational viscosity, using the force balance equations. It was found, in the experiment, that a careful control of the pressure difference across the thin film permitted one to maintain the bubble diameter constant, and, therefore, depending on the choice of the extrusion conditions, either a uniaxial or biaxial elongational flow was made possible. The experimental results show that, depending on the materials, elongation rate, and melt temperature tested, the elongational viscosity may decrease or increase with elongation rate, and may also stay constant independent of elongation rate. It was observed that the data of elongational viscosity obtained under uniaxial stretching in blown film extrusion is consistent with the data of elongational viscosity obtained earlier by use of the melt-spinning operation.

Notes: An experimental study has been carried out to investigate flow instabilities in blown film extrusion. Two types of flow instabilities were observed, depending on whether a bubble was under uniaxial or biaxial stretching. Under biaxial stretching, the phenomenon of a surface wave-type instability was observed, yielding wavy bubble shapes which very much resembled water waves at the free surface. Under uniaxial stretching, another type of instability, frequently referred to as draw resonance, was observed. It was also observed that, once draw resonance occurs, the amplitude and frequency of bubble diameter pulsing increased with stretch ratio. Quantitative information was obtained from a series of motion pictures taken of bubble diameter in both types of flow instability. It was observed further that an increase in extrusion melt temperature enhanced the severity of bubble instability.

Notes: Having investigated the elongational flow behavior of polymer melts (part I of this series), we have carried out both theoretical and experimental studies in order to better understand the deformation and heat transfer processes involved in blown film extrusion. For the experimental study, nonisothermal experiments were carried out, using high-density and low-density polyethylenes. Measurements were taken of the axial tension, bubble diameter, and film thickness at a series of extrusion conditions (i.e., flow rate, pressure difference across the film, and take-up speed). For the theoretical study, an analysis was carried out to simulate the blown-film extrusion process, by setting up the force- and energy-balance equations on the blown bubble moving upward. The approach taken in the theoretical study may be considered as an extension of the earlier work by Pearson and Petrie who considered the isothermal operation of Newtonian fluids. In the present study, However, we have considered the nonisothermal operation of power law fluids, whose rheological parameters were determined by an independent experimental study an described in part I of this series. Four highly nonlinear differential equations were solved numerically with the aid of the CDC 360 digital computer, using the fourth-order Runge-Kutta method. The mathematical model predicts the bubble shape, temperature profile, and film thickness as a function of the distance along the machine axis. Comparison is made of the experimentally observed bubble shapes with the theoretically predicted ones, showing a reasonable agreement.

Notes: Using the Han slit/capillary rheometer, rheological measurements were taken of several commercially available low- and high-density polyethylene melts, namely, three low-density polyethylene samples of Chemplex Corp. (CX 1005, CX 1016, and CX 3020), three low-density polyethylene samples of U.S. Industrial Chemicals Co. (NA 205, NA 244, and NA 279), two high-density polyethylene samples of Union Carbide Corp. (DMDJ 5140 and DMDJ 4306), and two high-density polyethylene samples of Mitsui Petrochemical Industries, LTD. Molecular characterization of these samples was carried out by the resin suppliers. The rheological measurements included (1) entrance pressure drop, (2) exit pressure, (3) pressure gradient, (4) die swell ratio. These then permitted us to determine the shear viscosity and normal stress differences. The rheological measurements were interpreted to identify the effects of long-chain branching and molecular weight distribution on the rheological properties of polyethylenes in the light of the existing molecular viscoelasticity theories. It was found that fluid elasticity is greater for polymers having a broader molecular weight distribution and that, for polymers having more long-chain branching, viscosities are lower while elasticities are higher.

Notes: The effects of molecular structure and cooling conditions on the severity of draw resonance was investigated by carrying out carefully controlled melt spinning experiments. For the study, two types of polymeric materials were used: one which exhibits viscoelastic behavior (high-density polyethylene, polypropylene, and polystyrene), and the other which exhibits almost Newtonian behavior [nylon-6 and poly(ethylene terephthalate)]. In order to investigate the effect of cooling on the severity of draw resonance, different methods of cooling the molten threadline were employed. In one set of experiments, isothermal chambers of various lengths (3, 6, and 12 in.) were attached to the spinnerette face, so that the molten threadline, upon exiting from the spinnerette, began to cool in the ambient air only after it had passed through the isothermal chamber. This method of cooling is called “delayed cooling,” providing both an isothermal region (inside the isothermal chamber) where only stretching occurs, and a nonisothermal region (outside the isothermal chamber) where both stretching and cooling occur simultaneously. In other experiments, the temperature profile of the molten threadline was controlled by adjusting the temperature of the heated chamber. This method of cooling provides a gradual drop of the threadline temperature, compared to the more sudden drop when spinning into a cold environment provided at the spinnerette exit. The severity of draw resonance was recorded on movie film, and the thread tension was measured with a low-force load cell transducer and recorded on a chart recorder. The temperature of the threadline along the spin direction was measured using a fiber optical probe attached to a Vanzetti Infrared Thermal Monitoring System (Model TM-1). It was found that the severity of draw resonance depended on the molecular structure and the way the molten threadline was cooled. Of particular interest is the observation that, for the viscoelastic materials investigated, cooling destabilized the molten threadline outside the isothermal chamber. This gave rise to more severe resonant behavior, at and above the critical draw-down ratio, in contradiction to the theoretical prediction by Fisher and Denn. It was observed, also, that the elasticity of the materials tended to destabilize the molten threadline (i.e., it increased the severity of draw resonance), again in contradiction to the theoretical prediction of Fisher and Denn. It is believed that morphological changes of polymers may play an important role in the occurrence of draw resonance when a melt threadline is stretched under cooling. Our study indicated that a good understanding of draw resonance of viscoelastic fluids requires more careful study than the classical hydrodynamic stability analysis reported by Fisher and Denn. They based their analysis on several convenient and yet unjustified assumptions, and solely on phenomenological considerations. We suggest that future theoretical analysis of draw resonance be carried out by considering a fluid model with a nonlinear memory function in order to properly account for the deformation history of the fluid, and the relaxation and cooling processes in the die swell region and the region below it.

Notes: An experimental study has been carried out of concentric and eccentric two-phase flow of polymer melts in a circular die. For the study, a special die was constructed such that two separate streams of molten polymer could be supplied to the die inlet. Materials used for study were low-density polyethylene, high-density polyethylene, and polystyrene. In the experiment, two different capillary length-to-diameter (L/D) ratios were employed: 4 and 18. For a die having an L/D ratio of 18, wall normal stresses were measured, permitting the determination of the pressure gradient and hence the viscous property. For each set of extrusion conditions (L/D ratio, flow rate, component ratio), extrudate samples were collected. These were later carefully cross sectioned and photographed in order to examine the shape of the interface between the two components. It has been found that the lower-viscosity components tends to wrap around the higher-viscosity component, which is consistent with the previous observations reported in the literature.