Controlled Stress

Schematic diagram of an open loop, controlled stress rheometer. Torque is provided by a drag cup motor, and motion is measured with an optical encoder. Designs similar to this are used by Bohlin, Carri-Med, Haake, and Rheometrics. 

Errors introduced in transient viscosity measurements (broken lines) as a result of instrument inertia can be eliminated by an active control loop (—). (a) Stress ramp 0-W seconds for a 5 mPa S Newtonian standard. Adapted from Franck (1992). (b) Sinusoidal oscillations on a 100 mPa-s standard. 

5 Pa in 90 seconds. The response time of the system with parallel disk geometry is 

With proper inertia correction controlled stress rheometers are very versatile and may replace controlled strain instruments for many applications. The fact that stress and strain are measured on the same shaft in controlled stress instruments allows lower cost and simpler temperature control but also is the source of the inertia limitations. Table 8.2.1 summarizes the advantages and disadvantages of each control mode. 

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TA Instmments CSD Controlled Stress Rheometer D-E 10-2-10 10-2 -5 X 10 good TA Instmments, New Castle, Del. 

Controlled Stress Viscometer. Most rotational viscometers operate by controlling the rotational speed and, therefore, the shear rate. The shear stress varies uncontrollably as the viscosity changes. Often, before the stmcture is determined by viscosity measurement, it is destroyed by the shearing action. Yield behavior is difficult to measure. In addition, many flow processes, such as flow under gravity, settling, and film leveling, are stress-driven rather than rate-driven. 

Controlled stress viscometers are useful for determining the presence and the value of a yield stress. The stmcture can be estabUshed from creep measurements, and the elasticity from the amount of recovery after creep. The viscosity can be determined at very low shear rates, often ia a Newtonian region. This 2ero-shear viscosity, T q, is related directly to the molecular weight of polymer melts and concentrated polymer solutions. 

Fig. 29. Measurements of yield stress with a vane device and Rheometrics controlled stress rheometer. The torque required to cause yielding is between 1.88...

Specific Commercial Rotational Viscometers. Information on selected commercial rotational viscometers can be found ia Table 7. The ATS RheoSystems Stresstech rheometer is an iastmment that combines controlled stress as well as controlled strain (shear rate) and oscillatory measurements. It has a torque range of 10 to 50 mN-m, an angular velocity range of 0 to 300 rad/s, and a frequency range of seven decades. Operation and temperature programming (—30 to 150°C higher temperatures optional) are computer controlled. 

Bohlin also markets a computer-driven controlled stress rheometer, which determines viscosity as a function of stress and measures creep and... 

Haake has introduced other viscometers, including the RheoStress RSlOO, which offers controlled stress as well as controlled shear rate and oscillatory modes over a temperature range of —50 to 350°C (ambient to 500°C is also possible). This versatile viscometer covers a shear rate range of 10 ... 

Viscoelasticity can also be determined by a controlled stress rheometer. The shape of a creep curve can show that a fluid is viscoelastic, and the amount of recovery after the stress is removed gives a measure of elasticity. 

Additional complications can occur if the mode of deformation of the material in the process differs from that of the measurement method. Most fluid rheology measurements are made under shear. If the material is extended, broken into droplets, or drawn into filaments, the extensional viscosity may be a more appropriate quantity for correlation with performance. This is the case in the parting nip of a roUer in which filamenting paint can cause roUer spatter if the extensional viscosity exceeds certain limits (109). In a number of cases shear stress is the key factor rather than shear rate, and controlled stress measurements are necessary. 

The tests are performed under carefully controlled stress (load), temperature, time, and creep (elongation) conditions. To save time, tests for different constant loads are performed simultaneously on different specimens of the same material. Creep tests may be rather extensively conducted, as for example when developing creep data prior to the design and fabrication of the first all-plastic airplane (41). The usual procedure is to plot the creep versus time curve, but other combinations are possible. 

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For best results, the measurements are carried out in the controlled stress mode. The samples are subjected to a preselected defined stress profile, leading to comparable results. 

Gel formation was monitored using a controlled-stress rheometer (Carri-Med CS 50, TA Instruments, Guyancourt, France) with cone-and-plate geometry (cone diameter 4 cm, angle 3°58 ). The bottom plate was fitted with a Peltier temperature controller that... 

Controlled release formulations, 7 551 pesticide formulations, 7 551-553 Controlled rheology, 14 275 Controlled stress viscometers, 21 736-737 Controlled structure polymeric betaines, 20 481... 

Element Type Controlling Stress n. Support Rotation, 0, (2) ... 

A wide variety of viscometers suitable for liquids are currently available, often with computer control. Many quality control laboratories use simple, cheap, robust instrumentation, which performs quite adequately in a day to day context. However these instruments can have a very narrow range and do not always give well-defined shear rates. This makes them less suitable for research and development work and we will not consider them further here. Figure 3.3 shows schematically the two main instrument types in common use controlled stress, where the stress is applied electrically via a motor leaving us to measure the strain and... 

Figure 3.5 depicts the curves that would result from a controlled stress experiment, showing that there is significant curvature if the shear rate is simply calculated from cylinder sizes. This illustrates the importance of using the appropriate equation of state when analysing the viscometer response. 

TA Instruments CSL2 Controlled Stress Rheometer D-E 10 2 -109 10 6-5 x 103 good TA Instruments, New Casde, Del. 

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