Rheometer

The rheometer is an instrument for determining the deformation and flow behavior of matter (see rheology). Colloquially, the term is often also viscometer is used; this term should be restricted to the devices that are mentioned in the appropriate article.

This article is about Scherrheometern, ie devices which apply a shear deformation. Devices that are used to measure the extensional viscosity is referred to as extensional rheometer, which are discussed separately.

The high-priced rheometer differ from the low -priced rotational next to the greater accuracy mainly due to the possibility in an oscillating mode, in which the sample to a sinusoidal load is subjected to be able to determine not only the "classical" rheological data, but also continuously even the parameters of viscoelastic samples. In most cases, the rheological practice, however, the rotating range of devices.

• 3.1 Experiment setup and procedure on the example of a bitumen sample

Design and operation

Measurement geometry

Exist As with rotational viscometers and rheometers at the two main methods of construction of the measuring system:

• Coaxial cylinder measuring system: The respective cylindrical measuring cups and measuring body have the same axis of rotation. Here, the measuring cup can stand still and rotate the measuring body ( Searle system) or vice versa ( Couette system).
• Plate / plate or plate / cone measuring system: On a stationary flat plate rotates at a certain distance, a second plate or a flat cone ( cone angle <3 °).

In the measurement of the sample between the rotating or oscillating and the static part of the assembly is sheared. From the geometry of the measuring arrangement, and the speed of the moving part there is the shear rate. What is necessary to maintain the movement of the torque is measured, from which can then be determined and thus the shear stress, the viscosity and other rheological characteristics.

Especially with the rheometers Platte/Platte- and plate / cone construction is the preferable method, as can be done very efficiently due to the small amount of sample temperature programs. The plate / cone design has over the plate / plate construction has the advantage that there is also the outwardly increasing velocity of circulation is balanced by the outward growing gap width, so that the entire measuring system ensure a homogeneous shear rate. For samples filled the plate / cone arrangement, however, less suitable.

The coaxial cylinder measuring systems have the advantage that the sample can not escape sideways and get a possible sedimentation of particles in the sample affects less on the measurement result. For this lie at the two ends of the measuring body still not clearly defined shear conditions, which lead to a measurement uncertainty. In addition, the cleaning effort is higher.

Tempering

Since the rheological properties are often significantly influenced by the temperature rheometer equipped with temperature control units to bring the measurement geometry to defined temperatures and to leave temperature profiles. Commonly used this liquid baths, convection and cooling systems and Peltier elements.

Actuator and sensor in the direction of rotation

The moving part of the measurement geometry, most of the upper portion is rotated by an electric motor with a rotating or oscillating motion. Either speed or torque, or the corresponding amplitude can be set. An encoder records the angular deflection, the moment can be measured by the input current of the motor.

The measurement accuracy depends mainly on the exact transmission of the measured torque in the drive unit to the measuring geometry. For low viscosity samples and / or low shear rates the apparent shear stress, and thus the torque is so small that it can be in the order of magnitude of the torque is caused by friction in the drive mechanism are located. That's why high-priced rheometer for a wide measuring range with air or magnetic bearings are fitted to minimize the friction.

Actuator and sensor in the axial direction

For handling processes such as sample introduction the upper measuring arrangement in the axial direction must be moved. In Platte/Platte- and plate / cone assemblies also the measuring gap must be set exactly. Modern devices have to contain a device for measuring the gap, so that the latter can be automatically adjusted and readjusted, if necessary, especially in case of change due to thermal expansion by temperature change.

Some devices also have a sensor for measuring the normal force. This allows a nachzuregelnde normal force on the sample pretend, so for example, a possible loss of the sample can be balanced and measured. Also train and compression tests are thus possible to a limited extent.

Discretionary parameters

• Viscosity ( as a function of different parameters such as temperature, time, shear rate etc.)
• Flow limit
• Complex viscosity
• Loss factor ()
• Complex shear modulus (G ', G ")
• With an existing normal force sensor and the measurement of shrinkage and expansion processes is possible.

Since the control of the rheometer and the recording of data on its own software is performed evaluations are diverse ( mathematical physics ) possible.

In oscillatory measurements, the plot of the values ​​of load and deformation results in a curve over time in general two time-shifted sine curves, ie the maximum deformation is compared with the maximum load of time. The time difference between the maxima is measured in relation to the duration of a full oscillation and δ as the phase angle referred ( unit degrees (°) or Gon, a full wave corresponds to 360 ° or 400 gon ). Is the phase angle is zero, i.e., the deformation appears in the moment when the load has been applied, it is a purely elastic material such as rubber. Does the tested material and viscous properties, the deformation appears later as the load. In the extreme case, a purely viscous material exhibits properties such as water. Then, the deformation in the moment in which the applied load is greatest, is equal to zero, but here is the deformation speed at the greatest. This corresponds to a phase angle of 90 degrees = 100 gon.

These relationships are valid only under oscillating load in the steady state, the value determined mainly describes but the material behavior during short-term loads.

Are partly mechanical extensions available which permit the clamping of solids or which convert the rotorische motion into linear motion, which also may be made to Dehnversuche solids ( Dynamic Mechanical Analysis ).

The cross-linking behavior of the polymers, such as rubber, can be examined with similar working Vulkametern.

Application

Rheometer are partially used in quality assurance, but primary. In product development and research Thus, for example, can be examined Aushärtevorgänge reactive adhesive systems. Even more detailed considerations to more complex effects such as thixotropy are possible by means of a rheometer. Furthermore, can be characterized with them the molecular architectures of materials with very low amounts of sample well.

Test preparation and drain the example of a bitumen sample

Rheometer can be used to determine the phase angle and the complex shear modulus of bitumen, which is used in road construction. A high complex shear modulus hardly takes deformations on roads which are traveled at high speeds, since during the short exposure duration of the roll-over due to the additional shear resistance to none as large deformation.

To create the bitumen sample, the material is pressed at 80 ° C to a thickness of 2 mm, then punched on test statistic and placed centrally on the lower measuring plate. After lowering the upper surface plate and the adjustment of the Prüfspaltes of one millimeter, the supernatants are cut off. To maintain a constant test temperature, a water bath is prepared by the sample (60 ° C).

To test the two plates are swinging shifted against each other or twisted, with the prescribed frequency of 1.59 Hz and a shear deformation of about 12% in the maximum points. Over a test period of several minutes, the applied load and the resulting deformation are τ γ separately recorded.