DD238271B1 - METHOD FOR DETERMINING VISCOELASTIC PROPERTIES OF FLOWABLE SUBSTANCES - Google Patents

Description

An uncomplicated measuring method with a corresponding device is described in the WP G 01 N 236779. By measuring pressure differences between two concentrically arranged chambers, which are connected by a gap bounded by a rotating disc and a fixed horizontal plate, a simple and easily manageable possibility for the detection of viscoelastic performance properties is given. Although the measuring method is suitable for a company Prodiiktprüfung, but does not allow to determine physically exact material functions of viscoelasticity.

Object of the invention

The object of the invention is to investigate viscoelastic properties of flowable substances by means of a simple measuring method which is suitable for use under conditions close to production in the substance-converting industry.

Explanation of the essence of the invention

The invention has for its object to determine in a disc rheometer viscoelastic properties of flowable substances, in particular the 1st normal voltage difference, physically accurate without dis dis technically complex and complicated to handle principles of measurement of VertikaSkräften to flow around the plates or disks.

According to the invention, the object is achieved by a method for determining the 1st normal stress difference in the flow in the gap between a fixed plate and a rotating disk of a disk rheometer, wherein the angular velocity of the rotating disk and the gap between the fixed disk and the rotating disk so setting the values of the deformation speed at the outer periphery of the disk to be in a range from 10 's' to 10 ³ s ~', then determining the radial pressure differences between the center and the outer edge of the gap between the fixed disk and the rotating disk Measures that one compares the measured radial pressure differences with a resulting from inertial influences pressure, which is dependent on a device constants, the density of the flowable substance and the angular velocity of the rotating disk, for determining the viscoelastic pressure difference Λρ and da ß one from the viscoelastic pressure difference, the first normal voltage difference S, on the relationship S, = η / Δρ / Π-a ")

where a is a further device constant and η is the exponent of the dependence of the viscoelastic pressure difference Ap on the rate of deformation at the edge of the rotating disk.

The advantage of the method according to the invention is that a possibility is created to allow an accurate determination of the first normal voltage difference in the flow in the measuring gap of a disc rheometer by simple and uncomplicated pressure measurements to be realized. The method thus allows a rheological characterization of viscoelastic liquids under practical conditions.

embodiment

The invention will be explained in more detail with reference to an embodiment. Fig. 1: Schematic representation of the measuring device Fig.2: Representation of the function Δρ _Μ = f ("TT ^ Fig.3: Representation of the overpressure values

WR

Fig.4: Representation of the function s, = f (-fp)

A polyurethane solution in dichloroethane having a density of 1.2 g cm ^-3 in the apparatus shown in FIG. 1 is investigated. It consists of a container which is divided by internals 1 in a cylindrical chamber 2 and a concentrically arranged annular gap-shaped chamber 3. The two chambers are connected by a channel in which a rotating disc 4 is located. The liquid to be examined is subjected to shear deformation in the actual measuring gap between the rotating disk 4 and a stationary plate 5 located above it.

By varying the angular velocity VV in a range of 0.06S ^ ¹ to 25.45S " ¹ and a change in the gap width h of 0.5 to 5.5mm at a disk radius R of 40mm, the dependence of the negative pressure values Ap _M in the outer annular gap-shaped Chamber is determined from the rate of deformation at the edge of the disc (see Fig. 2) by comparison with the overpressure values shown in Fig. 3, which are to be expected for the measuring instrument used and taking into account the density of the substance under consideration due to inertial effects; the course of the viscoelastic pressure difference Λρ, which is likewise shown in FIG. 2, is determined

 Using the relationship

S ₁ = n / Ap / (1-a ⁿ )

4, the dependence of the 1st normal stress difference Si on the deformation velocity at the edge of the disk is obtained, where η is determined to be 1.7 from the rise of the pressure curve Ap in FIG. 2 and the constant a for the measuring instrument used 0.25.

Claims (2)

Method for determining viscoelastic properties of flowable substances by determining the 1st normal stress difference in the flow in the gap between a fixed plate and a rotating disc of a disc rheometer, characterized in that the angular velocity of the rotating Sc! and adjusting the gap between the fixed plate and the rotating disc so that the values of deformation speed at the outer edge of the disc are within a range of 1 (T ¹ to 10 ³ S ^-1) By measuring the radial differences in pressure between the center and the outer edge of the gap between fixed plate and rotating disk, one obtains the measured radial pressure differences with a pressure resulting from inertial influences, which is dependent on a device constant, the fluidity of the fluid and the angular velocity of the comparing the rotating disc for determining the viscous pressure difference Δρ and from the viscoelastic pressure difference the first normal stress difference S _{1 is obtained} via the relationship S ₁ η / Δρ / (1-a ⁿ ) where a is a further device constant and η is the exponent of the dependence of the viscoelastic pressure difference Δρ on the rate of deformation at the edge of the rotating disk s ^: vid. For this 2 pages drawings Field of application of the invention The invention relates to the fields of technology in which liquids must be characterized in terms of their Fla properties. This applies above all to fluids which are used as starting materials and intermediates in processing in the spinning, coating and other shaping processes of the substance-converting industry and in which the occurrence of viscoelastic rheological behavior has consequences on the processability and mechanical properties of these substances after shaping and solidification or allows conclusions on their material composition. Characteristic of the known technical solutions A variety of methods for determining the rheological properties of fluids are known. An exact survey and systematization is given in K.Walter's "Rheometry", Capman & Hai, London, 1975. It is stated that a large group of measurement techniques relies on the analysis of flows caused by the movement of a body within the body to be examined Substance is induced. In turn, a distinction is made between processes which are based either on the measurement of the rate of rise or fall or the penetration depth of specimens in the fluid or on the measurement of the flow around rotating bodies. The last-mentioned measuring methods are of particular importance, since they permit both the determination of viscous properties of fluids on the basis of the measurement of the torque acting on the rotating body as a function of its rotational speed and also permit a determination of viscoelastic flow properties by detecting the additional normal stresses occurring in the flow , In K.Walter's "Rheometry", Chapman & Hai, London, 1975 an overview of all known methods is given, invariably on the realization of a heavy flow between fixed and rotating plates or cones and on the measurement of these bodies perpendicular to the direction of the The realization of a force measurement which is as clear as possible is the main problem of the measuring method and can only be solved with great technical effort. Plate viscometers described that allows accurate determination and adjustment of the distance between the bodies of revolution and at the same time guarantees its constancy during the measuring process. ¹ I DE-PS 2314670 is given the technical solution for a Rotationsrheometer, in which the measurement of the rotating Disc acting Norma lkraft is made possible by the power consumption of a servo motor, which resets the resiliently arranged rotational body in a position corresponding to the state before the normal force measurement. Both measuring methods have the disadvantage that they require extremely complex measuring techniques that can not be used as routine measuring instruments for reasons of complicated handling and maintenance. Apart from methods for normal stress measurements, which are based on the determination of normal forces on rotating or fixed disks and cones, the methods described in DE-AS 1 598408 are also known for the determination of viscoelastic flow properties with Totationsrheometern. This is a method using rheometers for viscoelastic substances, which consist of two rotating bodies which are rotatable about non-aligned axes in the same direction and at the same angular velocity. The mutually facing surfaces are connected by the measuring substance. The measured variable is the reaction moment occurring on a rotary body. The measuring method has the disadvantage that it is based on the realization of a not clearly describable flow situation and for this reason no physically exact evaluation can be provided.