Geometries used for rotational rheometry (a—cylinder and cup; b, c and d—cone and cup; e and f—parallel plates)
Rotational viscometers shear the test fluid between cylinders, cones or plates [1, 10–12, 18–20] rotating about a common axis. The drag of the fluid on the rotating parts provides the mechanical force used to determine the rheological information. This method of testing has advantages over capillary flow viscometers in that:
Samples can be sheared for as long as required, thus allowing the time dependent behaviour of the fluid to be determined.
By use of suitable test geometries, a uniform shear rate can exist through the sample.
Small sample volumes can be used.
The shear rate is proportional to the angular velocity, which can be varied over large ranges to provide information on adhesive properties under different flow rates. Thus, the flow curve can be derived more practicably than by tube viscometry. Rotational rheometers come in two main types: controlled strain (including the common Brookfield types) and controlled stress. These different types of instruments are capable of performing all the standard viscometry measurements but each has advantages for certain types of measurement (e.g. controlled stress is preferred for accurate creep or yield stress measurements). Different instruments within each type have different operating capabilities, measurement ranges and sensitivities. High specification rotational rheometers are capable of operating in an oscillatory mode for determining visco-elastic properties [13, 25, 26].
However even the simplest rotational viscometers are considerably more expensive than cheap capillary flow systems. High shear rates cannot be achieved without an increase in the sample temperature due to shear heating effects (although with temperature control a compensation can be made). Tight tolerances in the measurement geometries and precise alignment are required to avoid inaccuracies in the measurements. Rotational rheometers are generally calibrated using standard reference fluids (although these are of comparatively low viscosity), but it is possible in many instruments to directly calibrate the torque and position sensors.
Early designs of rotational rheometers, for example the Brookfield type instruments [10, 12] have several user-set motor speeds and give an output as a voltage proportional to torque. The operator then uses conversion factors pertaining to the measurement geometry (normally a bob-and-cup system) and motor speed setting to calculate shear rate and viscosity. Instruments with computerised control, data acquisition and analysis functions provide more data and offer powerful solutions for qualitative, quality control functions in many industries and quantitative laboratory investigations of fluid properties. There are a large number of manufacturers and suppliers of rotational rheometers.
For the determination of the shear flow properties of adhesives, pastes and other ‘stiff’ materials using rotational rheometers, parallel plate geometries are recommended. The measurement gap should be set to allow the free movement of any filler materials in the test material. ‘Clogging’ problems may occur when solid particles are present and the gap is too small. If the material has a very high viscosity (e.g. highly filled adhesive pastes) then a small diameter, large measurement gap system is required. This is so that torque limits for the instrument are not exceeded. Specialist testing instruments have been developed for ‘stiff’ materials. It should be noted that all rheometers will have a small but finite compliance and that at high torque settings a considerable proportion of the total deformation can be that of the rheometer shaft rather than the sample [1]. This will introduce a significant error into the measurement unless a compliance correction procedure is followed.
Next: Thixotropy, Creep, Sag and Slump
