Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Reexamination Certificate
1998-10-14
2001-04-24
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
Reexamination Certificate
active
06220083
ABSTRACT:
This invention pertains to rheometry, particular to a method and instrument for measuring elongational viscosity, and to apparatus and methods for using those measurements in on-line process control.
Rheology is the study of the deformations and flow of matter. A rheometer is a device for measuring the flow of a viscoelastic substance, usually a fluid, for example a polymer melt or solution. The viscosity of a complex substance may be characterized (depending on the geometry of the flow) by its shear viscosity, its elongational viscosity, or a combination of shear and elongational viscosities. Shear viscosity is the resistance to flow due to a force that is perpendicular to the normal of the plane on which the force acts (i.e., a force lying in the surface of flow). Elongational viscosity is the resistance to flow due to a force that is parallel to the normal of the plane on which the force acts. The shear viscosity may be thought of, for example, as the resistance to fluid flow between layers; and the elongational or extensional viscosity may be thought of as the resistance of the fluid to stretching. As a general rule, there is little correlation between shear viscosity and elongational viscosity for complex substances. Knowledge of one does not, in general, allow prediction of the other with any degree of confidence. (For simple or Newtonian fluids, the ratio of the elongational viscosity to the shear viscosity is determined by the geometry of the flow; e.g., in cylindrical flow this ratio is 3. However, polymer solutions and melts do not, in general, behave as Newtonian fluids, and there is no simple relation between shear and elongational viscosities.)
Shear rheometry, the measurement of shear viscosity, is a well-developed field. Most current rheometers measure shearing flow. By contrast, extensional or elongational rheometry is still in its formative stages. It is becoming increasingly apparent that extensional flow is important in many industrial applications, including fiber spinning; film casting; extrusion; and the fountain flow of the filling front during injection molding, where the primary flow field is extensional. There is a continuing need for improved methods for the accurate measurement of the extensional viscosity of fluids under operating conditions.
Compared to shear rheometry, the main difficulties in studying the extensional flow of viscoelastic fluids are that (1) it is difficult to generate a steady and controlled elongational flow field, and (2) it has been thought difficult to prevent, compensate, or even measure the shear effects that typically occur simultaneously during elongational flow.
Zahorski, “The Converging Flow Rheometer Reconsidered: An Example of Flow with Dominating Extension,”
J. Non-Newtonian Fluid Mech
., vol. 41, pp. 309-322 (1992) discusses theoretical predictions concerning two-dimensional planar extrusion in a lubricated converging flow rheometer; no experimental data are given. Zahorski states that the flow cannot be expected to be purely extensional—measurable shear effects were said to be certain.
Chatraei et al., “Lubricated Squeezing Flow: A New Biaxial Extensional Rheometer,”
J. Rheol
., vol. 25, no. 4, pp. 433-443 (1981) discloses a lubricated biaxial flow apparatus for measuring elongational viscosity in which a viscous material is compressed between two lubricated disks; no means are disclosed for achieving a constant elongational strain rate.
Williams et al., “On the Planar Extensional Viscosity of Mobile Liquids,”
J. Non-Newtonian Fluid Mechanics
, vol. 19, pp. 53-80 (1985) discloses an instrument for measuring planar extensional viscosity with lubricated converging flow in a hyperbolic planar nozzle. Such a device would primarily be useful for measuring the viscosity of solutions or other relatively low viscosity fluids. The apparatus would not be practical for measurements in high viscosity fluids such as many polymer melts, because it would be difficult to achieve a steady flow of such fluids in the planar nozzle. See also Binding et al., “On the interpretation of data from converging flow rheometers,”
Rheol. Acta
, vol. 28, pp. 215-222 (1989); Jones, “On the extensional viscosity of mobile polymer solutions,” Rheol. Acta, vol. 26, pp. 20-30 (1987); and James, “Flow in a Converging Channel at Moderate Reynolds Numbers,”
A.I. Ch.E.J
., vol. 37, no. 1, pp. 59-64 (1991).
Rheometrics data sheet, “RME—Rheometrics elongational rheometer for melts” (1993) describes a system for measuring the elongational viscosity of a fluid in which the fluid is supported by a gas stream, and the ends are pulled apart by traction at an exponentially increasing speed; the disclosures of the following two references are similar to that of the Rheometrics data sheet in many respects, except that the fluid was floated on oil rather than supported by a gas stream: Meissner, “Rheometer zur Untersuchung der deformationsmechanischen Eigenschaften von Kunstoff-Schmelzen unter definierter Zugbeanspruchung,”
Rheol. Acta
, vol. 8, no. 1, pp. 78-88 (1969); and Meissner, “Dehnungsverhalten von Polyäthylen-Schmelzen,” Rheol. Acta, vol. 10, no. 2, pp. 230-242 (1971).
Crevecoeur et al., “Fibril Formation in InStu Composites of a Thermotropic Liquid Crystalline Polymer in a Thermoplastic Matrix,”
J. App. Pol. Sci
., vol. 49, pp. 839-849 (1993) discloses a trumpet-shaped die for measuring elongational viscosity in polymer melts containing fibers.
F. Cogswell, “Converging Flow of Polymer Melts in Extrusion Dies,”
Polym. Eng. Sci
., vol. 12, pp. 64-73 (1972); and F. Cogswell, “Measuring the Extensional Rheology of Polymer Melts,”
Trans. Soc. Rheol
. vol. 16, pp. 383-403 (1972) provide estimates for elongational and shearing viscosity derived from measurements with a capillary rheometer, based on assumptions that may not adequately model actual conditions of polymer flow, for example, the assumption that there is only a low pressure loss when a fluid flows from a large section to a small one.
See also J. Meissner, “Development of a Universal Extensional Rheometer for the Uniaxial Extension of Polymer Melts,”
Trans. Soc. Rheol
., vol. 16, pp. 405-420 (1972); C. Kwag, “An Assessment of Cogswell's Method for Measurement of Extensional Viscosity,”
Polym. Eng. and Sci
., vol. 31, pp. 1015-1021 (1991); W. Andrew,
Applied Instrumentation in the Process Industries
, vol. 1, pp. 234-236 (1974); B. Bersted, “Refinement of the Converging Flow Method of Measuring Extensional Viscosity in Polymers,”
Polym. Eng. and Sci
., vol. 33, pp. 1079-1083 (1993); P. Revenu et al., “Validation of Cogswell's Convergent Flow Analysis,”
J. Appl. Polym. Sci
., vol. 62, pp. 1783-1792 (1996); and D. Cousidine (ed.),
Process Instruments and Controls Handbook
, pp. 11.3-11.25 (1985).
The present inventor's prior U.S. Pat. No. 5,357,784 discloses a method and apparatus for measuring the elongational viscosity of a fluid, in which a low viscosity fluid “skin” encapsulates and lubricates a viscous core of the fluid being characterized. A high viscosity material, such as a polymer melt, is caused to flow at a controlled constant strain rate in an essentially pure elongational flow regime, for example inside a die, by using lubricated flow and a semi-hyperbolic surface design. Lubricated flow results from skin/core flow, in which the viscosity of the skin is sufficiently lower than that of the core to cause the core to experience essentially pure elongational flow. With a semi-hyperbolic surface, essentially pure elongational flow can be maintained at a steady-state, controlled, constant elongational strain rate. The apparatus and method of prior U.S. Pat. No. 5,357,784 are well suited for the uses taught in that patent. However, prior U.S. Pat. No. 5,357,784 taught that the flow must be lubricated in order to measure elongational viscosity accurately; i.e., that a lubricating skin layer was needed due to the effect of the shearing gradients associated with a rigid boundary. Although the technique of the prior patent has been successful, in many applications it is i
Board of Supervisors of Louisiana State University and Agricultu
Larkin Daniel S.
Runnels John H.
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