Volume 16, Issue 3, September 1972
Index of content:
16(1972); http://dx.doi.org/10.1122/1.549257View Description Hide Description
A necessary prerequisite of the scientific design of polymer processing operations is a fundamental understanding of the rheology of melts. In fiber spinning the dominant rheological response has simple extensional geometry and we require information about viscosity,elasticity, and fracture during such flows. Industry needs both absolute quantitative information and also inexpensive qualitative tests for multiple samples. Experimentation demands the greatest quantity of accurate information at minimum cost, and this philosophy underlies a critical revue of approaches to measuring the rheology of polymer melts under tension. The decisive techniques so far developed lack versatility and are not suitable for multiple testing. The continuous drawing of a monofilament, or laboratory spinning, is an excellent qualitative approach but sufficient experimental control has yet to be attained. Two indirect routes, through normal stresses to a “normal” viscosity and by steady shear/dynamic rheometry correlation to an “irrotational” viscosity, are superficially attractive but theoretical complexity is a major problem. The most promising industrial approach is the study of converging flows. Analysis of melt flowing from a reservoir into a die yields a simple method of obtaining apparent extensional rheology data which is compatible with, and supplementary to, the results offered by the direct methods of measurement. The analysis allows the ubiquitous capillary rheometer to become a simple extensional flow meter. It is the alliance of the definitive, but costly and temperamental, methods with inexpensive simple qualitative techniques that forges rheological tools for tackling industrial problems.
16(1972); http://dx.doi.org/10.1122/1.549258View Description Hide Description
A recent development in extensional rheometry of molten polymers is based on the use of a pair of gears as a ‘rotational clamp’. In a tensile tester for polymer melts, the rotational clamp provides the draw‐down of the melt being extruded out of the die of a melt indexer. The result is a quick characterization of extensibility and tensile strength. In a new extensional rheometer, two rotational clamps are used which extend homogeneously a molten polymer rod floating on a silicone oil bath. For constant rotational speeds of the clamps, the (Hencky) strain rate is constant. The creep test with constant tensile stress can be performed in the same rheometer making use of a servo motor of quick response with an appropriate servo control. A further modification of the servo control is described by means of which any uniaxial stress or strain history can be imposed on the melt under test, with stress the only constraint. In this rheometer, the extensional behavior of a LDPE melt was measured at 150°C under a variety of mechanical conditions. The data presented demonstrate the versatility of the apparatus.
A Bubble Inflation Technique for the Measurement of Viscoelastic Properties in Equal Biaxial Extensional Flow16(1972); http://dx.doi.org/10.1122/1.549259View Description Hide Description
A bubble inflation technique for establishing equal biaxial extensional flow in viscoelastic materials is presented. This technique was used to measure biaxial extensional viscosity and elastic properties of a polyisobutylene at room temperature (23°C). A theoretical experimental procedure, based on certain idealizations, was developed for establishing bubble growth under constant stress. Modifications of the experimental design were introduced to correct for the nonidealities encountered in practice. The accuracy and reliability of the measurements are tested, and sources of error, and possibilities for future work are discussed. Several constitutive equations are examined with respect to biaxial extensional flow. Predicted behavior patterns are compared with experimental results.
Studies on Melt Spinning. I. Effect of Molecular Structure and Molecular Weight Distribution on Elongational Viscosity16(1972); http://dx.doi.org/10.1122/1.549260View Description Hide Description
A melt spinning experiment has been carried out to correlate spinning variables with the molecular weight, molecular weight distribution, and long chain branching of the polymer being spun. Materials chosen for study were three high density polyethylenes having widely different molecular weight distributions, a low density polyethylene having much long chain branching, a polystyrene, and a polypropylene. Molecular characteristics and rheological properties of the materials in simple shearing flow had been independently determined. For the study, a bench scale, melt spinning apparatus was built, and measurements were taken of thread tensions at the take‐up device. The axial profiles of thread diameter were measured by a photographic technique. The molten fibers traveled through a heated chamber maintained at the same temperature as the melt. These measurements were then used to determine the elongational viscosity in the absence of any temperature effect.
Interpretation of Tensile and Melt Spinning Experiments on Low Density and High Density Polyethylene16(1972); http://dx.doi.org/10.1122/1.549261View Description Hide Description
16(1972); http://dx.doi.org/10.1122/1.549262View Description Hide Description
The behavior of a series of molten polyethylenes has been studied in the extensional flow field of a fiber spinning apparatus. Separate rheogoniometric measurements have been made of the physical properties in shearing flows and the experimental results of the fiber spinning experiment are compared with the predictions obtainable using constitutive relationships in which all of the parameters are determinable from appropriate shearing experiments. Stress levels more than one order of magnitude greater than those predicted on the assumption of isothermal Newtonian behavior were observed.
16(1972); http://dx.doi.org/10.1122/1.549281View Description Hide Description
The crystallization of bulk tensile samples of glassy, amorphous poly(ethylene terephthalate) was studied as a function of various deformation and annealing treatments. Per cent crystallinity, orientation function, crystallite size, and long period spacing were measured to characterize the samples. Crystallization, necking, and orientation occurred during straining when, at a given temperature, the strain rate was sufficient to generate a critical stress level within the material. During subsequent annealingcrystallization was nucleated by the crystallites formed during deformation. The observed behavior was consistent with a model proposed by Dismore and Statton for drawn nylon 66 yarns and which was previously discussed by Dumbleton for the case of poly(ethylene terephthalate).
16(1972); http://dx.doi.org/10.1122/1.549264View Description Hide Description
This paper describes the state of development for false‐twist texturing of thermoplastic continuous filament yarns. Supply yarn parameters are shown to significantly affect the textured product. Quantitative and qualitative information is given to describe these effects. The thermodynamic and crystallization rate parameters are presented for nylon 6, nylon 66, and polyethylene‐terepthalate. Equations are given for calculation of temperature‐time profiles in yarns during texturing. Both the fundamentals of twisting using spindle or friction twisters are summarized. The effect of process variables on twist distribution and crimp rigidity are presented. Finally, experiments are described which show how force‐elongation curves for textured nylon 6 can be made similar to nylon 66. Recommendations for future work are discussed. Ways are needed to improve the dye uniformity and reduce barré from textured yarns in piece‐dyed fabrics. More information should be reported on the effect of processing constants (tension and temperature gradients) on crystallization rate phenomena and structural changes during texturing. Basic information on the effect of yarn and process constants on friction twisting with the many commercial units needs reported. The final goal of this fundamental work should be a mathematical model, which rigorously describes the texturing process. With this model, yarn and machinery manufacturers could optimize their products more fully for the use of the texturing mills.