Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Reexamination Certificate
2001-03-30
2003-05-13
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
C073S054040, C073S054050, C073S054060
Reexamination Certificate
active
06561011
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for measuring the viscosity of plastic materials, particularly of polymer melts in accordance with the principle of determining the pressure drop of the material when flowing through a capillary and to a measuring method for determining the flow curve of such plastic material.
Such an apparatus and such a method are known for example from DE 42 20 157.
The quality control in the production of polymers comprises a number of examinations of different properties, such as the Theological properties, the determination of foreign particles in the melt, the determination of the chemical composition or, respectively, of the additives in a polymer.
In the past, the various properties were determined by separate measuring apparatus and methods. Today, however, the trend is to determine a multitude of properties in a single measuring apparatus or method.
Today, laboratory extrusion apparatus are in use in which at the same time or in successive steps:
a polymer is melted,
rheological properties of the polymer, such as the viscosity, the melt flow index (MFI), or the melt volume index (MVI) are continuously determined.
a foil is produced which is continuously tested optically for the presence of foreign matter, such as gels, specks, black specks, or
the foil is further utilized to continuously determine the content of certain additives by means of infrared spectroscopy.
The determination of the viscosity is therefore a partial task, which must be incorporated optimally into the total operating process of a laboratory extrusion apparatus.
With continuously operating so-called on-line rheometers, a distinction is to be made on one hand between so-called side stream rheometers which are fed by a side stream out of the extrusion apparatus and which discharge the material flow out of the capillary onto the ground and, on the other hand, the so-called in-line rheometers which are arranged in the mainstream of the melt between the worm kneader and the nozzle and wherein the melt, after passing through the rheometer, is returned to the main stream and discharged through the nozzle for further processing.
In the state of the art, the viscosity of a plastic material is determined, for example, by measuring the pressure loss upon flowing through a capillary of a predetermined dimension and length.
In that case, pre-conditions for the determination of the viscosity are:
the knowledge of the accurate amount of the material flowing through the capillary; for the examination of polymer melts, this amount is preferably defined by the use of a gear pump which is driven with a predetermined speed so as to provide a uniform flow volume of viscous material,
the knowledge of the geometry of the measuring capillary; if a slot capillary is used, the width, the height, and the length of the capillary are known; if a round capillary is used, the diameter and the length are known,
the adjustment of an accurate temperature of the melt and the area around the measuring capillary;
the knowledge of the specific weight of the substance to be measured;
the measurement of the pressure difference between inlet and outlet of a round capillary that is, respectively, the pressure difference between two points of a slot capillary. With this measurement value, a measuring point of the viscosity with a predetermined shear velocity can be obtained utilizing known mathematical equations. However, with polymer melts, the non-Newton or structure-viscous behavior is a big problem, that is, at different shear speeds, different viscosity values are obtained.
For determining the behavior of melts in the various, very different processes employed in the industrial plastic processing industry, very different shear speeds have to be covered, that is shear speeds in the range of about 1:10
4
.
With a capillary and a melt pump, normal ranges of 1:10 and maximal ranges of 1:100 of the shear speed can be covered. This, however, is possible only by varying the speed of the melt pump in a very wide range.
Therefore, for some time already, arrangements of multiple capillaries are used in order to increase the range of the shear speeds that can be measured.
In the known arrangement according to DE 42 20 157 A1, for example, slot capillaries with stepwise decreasing heights are utilized. In this way, the range of the shear speeds that can be measured is increased by the power of ten. In order to cover the wide range of shear speeds, additionally the speed of the melt pump must be changed substantially. When used as an in-line measuring apparatus, the travel time of the material in the relatively long nozzles is substantial whereby the material can be damaged.
DE 42 36 407 discloses an apparatus for measuring the viscosity of viscous materials particularly of polymer melts utilizing the principle of determining the pressure drop experienced by a mass flowing through a capillary with predetermined cross-section and predetermined length for use in a laboratory, particularly for the continuous measurement in a manufacturing plant especially with an integrated quality control. It includes a controllable melt pump for generating a predetermined melt flow and is installed in a heatable device and several capillaries are installed in the apparatus for accommodating a large range of flow speeds.
Ind. Lab. (1974) 40, pages 1467-1468 discloses a four channel viscosimeter, wherein melt can be supplied at the same time to four exchangeable capillaries. The mass flows in the capillaries act on a metal strip. The deformation of the metal strip is sensed and analyzed at the same time in all four capillaries.
In another known apparatus according to GB-A-2 271 856, several round capillaries are slideably so arranged that melt can be supplied to them from a melt pump in succession. It is a disadvantage in this arrangement that a uniform heating of the slideable capillaries is problematic. Especially, the use of a single mass pressure sensor for the complete pressure range to be measured is problematic, since, in this case, the measuring accuracy is insufficient in the very low pressure ranges with capillaries of large cross-section and very low shear velocities.
Furthermore, U.S. Pat. No. 4,677,844 discloses an apparatus for measuring the viscosity of viscous masses on the basis of the principle of determining the pressure drop of the mass when flowing through a capillary with a defined cross-section and defined length, wherein four capillaries are in communication with a cylinder and these capillaries are placed in communication with the surrounding atmosphere by operating a control mechanism. When a piston in the cylinder is operated, the mass contained in the cylinder is pressed through a selected capillary. That measurement step is repeated for each capillary.
In another arrangement, several capillaries are supplied with melt by several melt streams of a multiple gear pump. This is consequently a multiple arrangement of individual melt pumps each with an associated capillary.
Such an arrangement is disclosed for example in U.S. Pat. No. 4,425,790. Herein, three or four capillaries are arranged adjacent one another in series. From one capillary to the next, the size of the flow passages of the capillaries increases wherein the ratio of the capillary length to the capillary cross-section remains essentially the same for all capillaries. A heated polymer melt is pressed by means of a pressure pump through the capillaries with a constant volume flow rate. Sensors determine the pressure and temperature in each capillary. In this way, it is possible to determine the polymer viscosity in each capillary for different shear velocities.
In this way, large shear speed ranges can be measured with a single apparatus. However, the consumption of melt is doubled or tripled which results in increased new material expenses and causes re-granulation. In contrast, it is desirable, particularly in the continuous quality control, to minimize the amount of testing material being wasted.
In other known systems as they
Collin Heiner
Collin Heinrich H.
Bach Klaus J.
Dr. Collin GmbH
Larkin Daniel S.
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