Measurements rheological

The above rheological techniques can be used to assess sedimentation and flocculation of suspensions. This will be discussed in detail below. 

Three different rheological measurements may be applied [36-39] (i) steady-state shear stress-shear rate measurements, using a controlled shear rate instrument  

Sedimentation of Suspensions and Prevention of Formation of Diiatant Sediments (Ciays) 

As discussed previously, most suspensions undergo separation on standing as a result of the density difference between the particles and the medium, unless the particles are small enough for Brownian motion to overcome gravity. 

For a very dilute suspension of rigid noninteracting particles, the rate of sedimentation Vq can be calculated by the application of Stokes law, whereby the hydrodynamic force is balanced by the gravitational force. 

When was calculated for three particle sizes (0.1,1, and 10 pm) for a suspension with density difference Ap = 0.2, the values of were 4.4x10 , 4.4x10 and 

Rheology is the science of the deformation and flow of matter. It is concerned with the response of materials to appHed stress. That response may be irreversible viscous flow, reversible elastic deformation, or a combination of the two. Control of rheology is essential for the manufacture and handling of numerous materials and products, eg, foods, cosmetics, mbber, plastics, paints, inks, and drilling muds. Before control can be achieved, there must be an understanding of rheology and an ability to measure rheological properties. 

Deformation is the relative displacement of points of a body. It can be divided into two types flow and elasticity. Flow is irreversible deformation when the stress is removed, the material does not revert to its original form. This means that work is converted to heat. Elasticity is reversible deformation the deformed body recovers its original shape, and the appHed work is largely recoverable. Viscoelastic materials show both flow and elasticity. A good example is SiEy Putty, which bounces like a mbber ball when dropped, but slowly flows when allowed to stand. Viscoelastic materials provide special challenges in terms of modeling behavior and devising measurement techniques. 

The study of flow and elasticity dates to antiquity. Practical rheology existed for centuries before Hooke and Newton proposed the basic laws of elastic response and simple viscous flow, respectively, in the seventeenth century. Further advances in understanding came in the mid-nineteenth century with models for viscous flow in round tubes. The introduction of the first practical rotational viscometer by Couette in 1890 (1,2) was another milestone. 

In the late twentieth century the science of rheology has grown rapidly. 

In addition to viscometers, optical devices such as microscopes and cameras can be used for defining and solving flow problems as weU as characterizing materials (3—5). Optical techniques allow the investigator to determine the physical stmcture of the material and visualize its flow processes. 

Perhaps the conceptually simplest type of rheometer can be constructed by sandwiching a material to be tested between two or three parallel plates that are separated by a distance H, and moving one plate parallel to the others at a velocity V (Fig. 12). The shear rate 7 is VIH, For normal liquids this is not practical, but for elastomers and compounds it is very much so. Apparatus of this type have been designed and used by Zakharenko et al, [Zl], 

Middleman [M24], Goldstein [G8], Furuta et al. [Fll], Lobe and White [LI4], Toki and White [T7], Montes et al. [M37], Osanaiye et al. [OlO], and K. J. Kim and White [K8a]. The apparatus (with constant-temperature chamber) may be placed in a tensile tester and operated in a mode with a fixed velocity V giving a constant shear rate. It may, on the other hand, be used in a creep mode with hanging weights. This provides constant stress experiments. At low stress levels one needs to compensate for the weight of the central member which exerts a gravitational stress [OlO]. At very low stresses one may accurately determine the yield value of rubber-carbon black compounds. Osanaiye et al. [OlO] have made measurements at shear stresses below the yield value. 

The shear rate 7 is the ratio of the velocity of the moving member V to the perpendicular distance between the plates, i.e.. 

The term VWt accounts for the decreasing active surface of the sandwich face. W is the width of the sandwich and t is experimental time. 

The concepts of the cone-plate and biconical rheometers developed in the 1940s (Fig. 13). The cone-plate instrument is due to Freeman and Weissenherg [FIO] and intended for modest-viscosity fluids. It has the basis of his rheogo-niometer which also measured normal stresses. The biconical rheometer was developed in the same period by Piper and Scott [P12] of the BRMRA and was from the beginning intended for rubber. Similar instruments are discussed by Turner and Moore [T12] and Montes et al. [M37, M38]. In the latter instruments, the pressure is controlled by charging the rubber into the rheometer by an attached pressure-driven device. 

A large number of different commercial viscometers, which provide a variety of geometries and a range of viscosity and shear rates, can be used to characterize a fluid. Specialized techniques have been developed to suit specific purposes [125-127], 

The determination of flow properties taken during the pigmentation and processing of thermoplastics are described in the literature on polymers [132,133], 

Since the rheology of many systems depends largely on the temperature, accurate and reproducible measurements require very careful temperature control. A 1°C temperature drop, for instance, increases the apparent viscosity / of an offset printing ink by approximately 15%. To demonstrate the correlation between thixotropy and temperature, Figs. 56 and 57 show the flow curves at different temperatures for two offset printing inks [134], Both materials clearly lose thixotropy-indicated by the area under the thixotropic loop-as the temperature increases. This effect is much more pronounced in the first case (Fig. 56), while the second ink exhibits a very slow decrease thixotropic behavior (Fig. 57). 

The flow behavior of a system may change significantly during the first few hours after the shearing action stops. This is particularly true for systems with a high pigment concentration. The plot in Fig. 58, for instance, represents the vis- 

DIN ISO 8780-1-1990 Dispersion characteristics of pigments and extenders - Methods of dispersion. 

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Raman measurements [INFRARED TECHNOLOGY AND RAMAN SPECTROSCOPY - RAMAN SPECTROSCOPY] (Vol 14) -rheological measurements [RHEOLOGICALMEASUREMENTS] (Vol21)... 

PVC emulsions for piNYL POLYMERS - VINYL Cm ORIDE POLYMERS] pol 24) -rheological measurements [RHEOLOGICAL MTEASUREMTENTS] pol 21)... 

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

Viscous Hquids are classified based on their rheological behavior characterized by the relationship of shear stress with shear rate. Eor Newtonian Hquids, the viscosity represented by the ratio of shear stress to shear rate is independent of shear rate, whereas non-Newtonian Hquid viscosity changes with shear rate. Non-Newtonian Hquids are further divided into three categories time-independent, time-dependent, and viscoelastic. A detailed discussion of these rheologically complex Hquids is given elsewhere (see Rheological measurements). 

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