Flow Of Molten Polymers Research Articles

Influence of elastic effects on capillary flow of molten polymers

AbstractCapillary viscometer measurements for a number of polyethylenes and polystyrenes reveal deviations from linearity in plots of applied pressure versus capillary length to diameter ratio, at constant rates of shear, resulting in apparently negative end corrections. This is contrary to previous physical considerations. Some polymers obey Hooke's law in shear permitting the separation of the end correction into two components. A Couette correction, dependent upon the capillary dimensions, is constant for any given polymer, shear rate, and temperature; an elastic shear strain correction, dependent upon the transit time through the capillary, may be determined by post‐extrusion measurements. Determination of these two correction leads to linear plots of pressure versus capillary length to diameter ratio and enables flow curves to be calculated independent of the capillary dimensions and the swell properties of the polymer.

Flow of molten polymers through porous media

AbstractThe flow of molten polymer through porous media has been investigated at constant temperatures and flow rates. Temperatures of 375°, 400°, and 425°F., flow rates from 12 to 60 g./min. and particle sizes from 0.054 in. to 6 mm. in diameter were studied. A modified Darcy's law was developed using the Metzner‐Reed modification of the Mooney‐Rabinowitsch relation. This form was then used to develop a modified friction factor plot which correlated the data with an average error of 5.02%. Calculated superficial velocities from the Darcy's law equation checked experimental values generally to within 10%. A technique for estimating shear rates when curved rheological flow curves occurred was also developed.

On the Pipe Flow and the Ejection of Molten Plastics

Many experiments on the flow of molten polymers have hitherto been performed by means of relatively short capillary tubes. The static pressure of the flow is measured indirectly by the load of plunger. In this paper the results of experiments made with the use of comparatively long tubes-not capillary-are described. As for the pressure, the elastic deformation of the tube according to the static pressure of the flow was measured directly by the wire strain gage. Polyethylen and polypropyrene were used as testing materials.The shearing stress vs. shear rate curve can be approximated by two straight lines as shown in Fig. 3. The shear rate at this crooked point of the flow curve corresponds to that of the maximum swelling ratio, and over the region of this shear rate appears the vibration of pressure.The swelling mechanism by Barus effect was investigated by introducing the Voigt model after McIntosh, on the other hand, and it was assured that the experimental tendency had good agreement with the theoretical ones as shown in Fig. 5. So it seems that Barus effect can be explained by a modified Voigt model of Fig. 6. The flow rate at which the smooth figure of ejection is obtained is calculated from Eq. (10). The facts thus obtained by this study will serve as the design for dies of injection machine.

Stress relaxation in molten polymers. II

AbstractA new analytical method is suggested for the evalution of the stress relaxation phenomena arising when a viscous flow of molten polymers is suddenly stopped. This method is based on the consideration of the relaxation isochronism and of other most typical features of the experimental stress–time relationships. Such features can be deduced by means of an analysis in elementary non‐Hookean bodies in series having a stress–time low of a particular analytical type. Such an analysis is applied to four samples of high and low pressure polyethylenes and natural rubber. The new parameters difined in this way are discussed in order to state some correlations with the structural features. Attention is drawn on the importance of an elasticity number defined as the ratio of the relaxation area to the corresponding Newtonian viscosity.

Measurement of the flow of molten polymers through short capillaries

A method of treating experimental flow measurements on high polymers to obtain the basic shear diagram is described. Shear stress data are corrected to eliminate capillary end effects, and shear rate data are modified for the non-Newtonian behavior of the materials. Differences in flow behavior of commercial low-density polyethylenes are illustrated, and the effects of the two corrections are demonstrated. The method is applicable to other polymer melts. Although the data could be expressed by the power law, the fact that logarithmic plots of shear stress versus shear rate are not linear suggests that such relationships would be approximate. The data are expressed better by the threeconstant Powell-Eyring equation, although the significance of the three parameters is uncertain. The need to obtain the rheological constants from the basic shear diagram rather than from an uncorrected or partially corrected capillary flow diagram is emphasized. In support of previous investigators, irregularities in the extrudate were observed at high shear stresses. However, it was not possible to determine at what pressure the roughness was initiated. The shear diagrams calculated from the experimental data do not exhibit any point of inflection as reported in some studies.

Unstable flow of molten polymers: A second site of melt fracture

Instability in melt flow initiates within capillaries for some samples of the two types of polymers linear polyethylene and copolymers of tetrafluoroethylene and hexafluoropropylene. This behavior contrasts with a previously described instability which initiated at capillary inlets. The instability initiating within capillaries results, initially, in fine-scale surface roughness and, ultimately, in slip or plug flow. Instabilities initiating at both sites appear to result from elastic failure or melt fracture at critical elastic strains of about 5 units. Data supporting these conclusions were obtained from capillary flow and flow birefringence, and rotational viscometry.

Theories for the flow of dilute solutions of polymers and of nondiluted liquid polymers

AbstractThe formulation of two theories for the flow of polymeric systems and some of the principal results are presented in this paper. The theory for the flow properties of dilute solutions of polymers is related to the work of Kirkwood and Riseman, and Rouse, Bueche, and Zimm. The combined effects of long‐range excluded volume, hydrodynamic shielding and branching are considered in computations for intrinsic viscosity. The dependencies of solution viscosity on velocity gradient and frequency of oscillation are derived. The continuum theory is adequate for describing the combined effects of finite deformation and viscoelastic flow of molten polymers.

Unstable Flow of Molten Polymers

Unstable Flow of Molten Polymers

An instability in the flow of molten polymers

An instability in the flow of molten polymers

Injection Molding - Flow of a Molten Polymer into a Cold Cavity

Injection Molding - <i>Flow of a Molten Polymer into a Cold Cavity</i>

An instability in the flow of molten polymers

In the flow of molten high polymers through capillaries, an instability occurs at shear stresses near 106 dynes/cm2. The instability results in emerging streams of irregular shape. Several explanations have been proposed:Reynolds' turbulence, buckling during elastic recovery of the emerging stream, and rupture or fracture of the molten polymer at the entrance to the capillary. The first two are found not to apply. The latter appears correct on the bases of experimental observations and rheological considerations.