Constant-stress rheometer

For uncompatibdized blends, the viscosity decreases with increasing number of layers in a multilayer constant stress rheometer. In the present case, the viscosity rather increases due to in situ reaction between nylon and maleated EPDM as illustrated in Figure 11.21. 

Measurements of the zero shear viscosity (20 °C) were made with a Bohlin VOR rheometer in the viscometry mode. If a Newtonian region was not found at the lowest measurable shear rates, the samples were characterized with a Bohlin-CS constant stress rheometer, with which it was possible to obtain extremely low shear rates. This approach was especially needed for highly viscous samples exhibiting pseudoplastic behavior on the VOR rheometer. Newtonian regions were found for each sample in this manner, yielding the zero shear viscosities reported. 

In a controlled-shear rate viscometer such as the Haake Rotovisco, T is determined as a function of Q, while in constant stress rheometers, such as the Carri-Med, is determined as a function of T. In order to determine surface slip in a co-axial cylinder viscometer, one must vary the ratio ri/r2 as well as T and (Mooney, 1931). Specifically, one can determine S2 at constant T using three combinations of cups and bobs of radii, r3/r2, and rj/r]. The coefficient of slip is given by ... 

Small deformation rheometry refers to testing procedures that do not cause structural damage to the sample. Constant stress rheometers, such as dynamic mechanical analyzers or oscillatory constant stress rheometers, are often used. 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

Thus, to predict creaming or sedimentation, it is necessary to measure the viscosity at very low stresses (or shear rates), and these measurements can be carried out using a constant stress rheometer (e.g., Carrimed, Bohlin, Rheometrics, Haake or Physica). 

There are several direct methods of measurement of yield stress. The constant stress rheometer is most frequently used to determine value in shear. Dzuy and Boger [1983, 1985] used a rotational vane viscometer. Yield stresses in compression can be calculated from the unrelaxed stress values in parallel plate geometry. Its value in elongation has been directly measured as the critical stress value below which no sample deformation was observed during 30 minutes of straining in an extensional rheometer. 

Rheological measurements on the slurry have been performed by Bohlin rheometer model VOR and constant stress rheometer C.. These two units combined are powerful enough to analyze slurries completely in viscometry, oscillatory and relaxation tests. Injection molding compound is fully characterized rheologically using capillary rheometer. 

The major advantage of a constant-stress rheometer over a constant-strain-rate rheometer is that, for a given polymer, a steady state is frequently achieved in the former mode but not in the latter one.(28) Even when a steady state is obtained with the use of both instruments, the total strain needed to achieve a steady state is lower for the constant stress viscometer. This extends the range of strain rates at which the extensional viscosity can be determined for an apparatus of a given size. Finally, it has been observed(4 20) that the stress tends to decrease slightly in a constant stretch rate experiment even after a plateau appears to have been reached. The physical significance of this last observation is not entirely clear. (29)... 

It is now important to calculate the stress exerted by the particles. This stress is equal to aApgfZ. For polystyrene latex particles with radius 1.55 pm and density 1.05 g cm , this stress is equal to 1.6 x 10 Pa. Such stress is lower than the critical stress for most EH EC solutions. In this case, one would expect a correlation between the settling velocity and the zero shear viscosity. This is illustrated in Chapter 7, whereby v/a is plotted versus 7(0). A linear relationship between log( /a ) and log 7(0) is obtained, with a slope of —1, over three decades of viscosity. This indicated that the settling rate is proportional to [7(0)] . Thus, the settling rate of isolated spheres in non-Newtonian (pseudo-plastic) polymer solutions is determined by the zero shear viscosity in which the particles are suspended. As discussed in Chapter 7, on rheological measurements, determination of the zero shear viscosity is not straightforward and requires the use of constant stress rheometers. 

Rheological properties of the gels were performed with a constant stress rheometer (Rheolyst ARIOOO N, TA Instruments) with a cone and plate geometry (cone diameter 40 mm and 2° angle). Rheological experiments were cmied out in steady flow, creep and dynamic oscillation modes over a wide range of shear rates, frequencies, temperatures and time. To prevent evaporation of the hydrocarbon a solvent trap (as supplied by TA Instruments) was installed for all experimrats. 

Experimentally, the dynamic shear moduli are usually measured by applying sinusoidal oscillatory shear in constant stress or constant strain rheometers. This can be in parallel plate, cone-and-plate or concentric cylinder (Couette) geometries. An excellent monograph on rheology, including its application to polymers, is provided by Macosko (1994). 

While the dynamic experiments described above are to be conducted in the linear viscoelastic range, another experiment can be conducted in which the results obtained in the non-linear range are useful. With a controlled-stress rheometer, one can conduct an experiment in which the stress is increased continuously at a constant oscillatory frequency, say 1 Hz. Results obtained in such an experiment are shown schematically in Figure 3-40. As the stress is increased continuously, initially, G and G" remain relatively constant until at a critical value of stress, Oc, the magnitude of G decreases sharply and that of G" also decreases not as sharply after a slight inerease. One may also use the value of the applied stress at which the curves of G and G" intersect... 

Neat polymers and their blends were studied In dynamic shear field (using RMS) and In constant shear stress field using Rheometrlc Stress Rheometer, RSR. The molecular parameters of polymers and blends were determined by Size Exclusion Chromatography in trichlorobenzene at 14O C. The morphology of freeze-fractured specimens was characterized In Scanning Electron Microscope, SEM, Jeol JSM-35CF. 

Many oscillatory rheometers can also perform creep experiments. Essentially, these protocols will place the sample under a constant stress for a fixed amount... 

Rheological measurements were performed in a stress rheometer fixture with a 2-cm cone and plate having a 1° cone angle and gap of 27 pm. Dynamic shear moduli were measured at 0.5% strain between 0.1 and 100 rad/s. Creep compliance was measured with a constant applied stress in the range of 0.1 to 5 kPa. Both measurements were performed over a series of temperatures to obtain data for time-temperature superposition. 

A new universal extensional rheometer for polymer melts has been described by Miinstedt [91]. It was specifically designed with the idea of making measurements on small samples possible in research laboratories under a variety of physical conditions, e.g. at constant stress or constant stretching rate, as well as relaxation and recoil experiments. 

Extensiometer n A rheometer for measuring the exten-sional flow properties of molten polymers. In one early form, the Cogswell rheometer, useful at tensile viscosities over 10 Pa s, unidirectional tensile force was exerted on a polymer rod by a dead-weight acting through a cam and pulley. As the cam rotated, the moment arm exerted by the weight on the rod decreased in proportion to the rod cross section so as to maintain constant stress. 

Extensional viscosity based on constant stress measurements [73] have also been reported [74,75], In one case [74], the filament is extended vertically on top of a bath, whereas in the other case [75], the vertical sample is immersed in the bath. The commercial equipment (Rheometrics Extensional Rheometer) available for the measurements of extensional viscosity from Rheometrics is based on the latter [75]. 

Measuring yield stress of concentrated suspensions can be carried out using various rheological techniques that can be broadly classified under two categories the controlled rate rheometry and the controlled stress rheometry. A controlled rate rheometer deforms a specimen at a constant shear rate and measures the shear stress. On the other hand, a controlled stress rheometer imposes a constant shear stress on a specimen and then measures the corresponding strain. The latter approach involves a more sophisticated control system and is only introduced in the last ten years. These techniques can be further classified as direct (or static) or indirect methods (or dynamic). The indirect determination of yield stress involves the extrapolation of experimental shear stress - shear rate data to obtain a yield stress, which is the shear stress at zero shear rate. This is illustrated in Figure 9. It is evident that the choice of the model or methods yield differing values of yield stress. 

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