Rheological property of solids

The effect of aging and of process variables on the rheological properties of solid proplnts has been the subject of mechanical shear relaxation spectroscopy (Ref 4). The technique is of interest to such filled polymer systems generally in that anisotropy in the viscoelastic properties can be readily observed... 

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of solids, liquids, and gases, and their mechanical response to external forces can he very complex. 

PLASTICITY. A rheological property of solid or semisolid materials expressed as the degree to which they will flow or deform under applied stress and retain the shape so induced, either permanently of for a definite tune interval. It may be considered the reverse of elasticity. Application of heat and/or special additives is usually required for optimum results. 

Shama, F., Sherman, P. 1968. An automated parallel-plate viscoelastometer for studying the rheological properties of solid food materials. In Rheology and Texture of Foodstuffs. S.C.I. Monograph No. 27, pp. 77-90, Society of Chemical Industry, London. 

Rheometers can be divided into two broad types viscometers, used to measure the rheological properties of liquids, and solids rheometers, used to measure the rheological properties of solids. Viscometers and solids rheometers are not mutually exclusive in application some viscometer geometries can be used for testing solids, while some solids rheometer geometries can be used for testing (viscous) liquids. 

Amorphous solids are considered as glasses, which have rheological properties of solids and molecular properties of liquids (Kittel 1986). The behavior of amorphous glasses can be explained by heat content or molar volume changes with the changes of temperature. When the heat content or molar volume of a sample is plotted against... 

Organotitanates form regular adsorbed layers on the filler surfaces. This assures a high degree of dispersibility of the solid particles of the filler, removal of moisture and air from the surfaces, a material improvement of the rheological properties of filled compositions. Also, it is possible to use much greater percentages of cheap... 

The existence of yield stress Y at shear strains seems to be the most typical feature of rheological properties of highly filled polymers. A formal meaing of this term is quite obvious. It means that at stresses lower than Y the material behaves like a solid, i.e. it deforms only elastically, while at stresses higher than Y, like a liquid, i.e. it can flow. At a first approximation it may be assumed that the material is not deformed at all, if stresses are lower than Y. In this sense, filled polymers behave as visco-plastic media with a low-molecular and low-viscosity dispersion medium. This analogy is not random as will be stressed below when the values of the yield stress are compared for the systems with different dispersion media. The existence of yield stress in its physical meaning must be correlated with the strength of a structure formed by the interaction between the particles of a filler. 

Actually, some fluids and solids have both elastic (solid) properties and viscous (fluid) properties. These are said to be viscoelastic and are most notably materials composed of high polymers. The complete description of the rheological properties of these materials may involve a function relating the stress and strain as well as derivatives or integrals of these with respect to time. Because the elastic properties of these materials (both fluids and solids) impart memory to the material (as described previously), which results in a tendency to recover to a preferred state upon the removal of the force (stress), they are often termed memory materials and exhibit time-dependent properties. 

The use of a heterogeneous fuel, in which the metal compound is suspended in a liquid fuel, avoids a third storage vessel because it is used with the oxidizer as in a conventional bipropellant system. The technical problems are then associated only with the stabilization of the suspension, with the rheological properties of the stabilized fuel, and with the reactivity of the suspended solid with its carrier. 

In the definition of rheology there are two processes, deformation and flow. Deformation suggests the presence of solid-like behavior and flow suggests the presence of fluid-like behavior. Many foods have both solid and fluid properties. The objective of this section is to provide methods for the evaluation of the rheological properties of foods. In Chapter HI, flow properties of foods are the focus. In Chapter H2, deformation properties are the focus. 

In contrast to the mechanical and rheological properties of materials, which have defined physical meanings, no such definitions exist for the psychophysical assessment of equivalent textural properties of foods. To identify material properties, or combinations of these, which are able to model sensory assessments requires a mixture of theory and experimentation. Scientific studies of food texture began during the twentieth century by the analysis of the rheological properties of liquid or semi-solid foods. In particular Kokini14 combined theoretical and experimental approaches in order to identify appropriate rheological parameters from which to derive mathematical models for textural attributes of liquid and semi-solid foods, namely, thickness, smoothness and creaminess. 

The rheological properties of Slurries are important in many ways. They determine the pumpability of Slurries whether there will be segregation of the solid and liq phases (the solid components of Slurries are almost invariably more dense than the liqs) and, probably most important of all, they control the sensitivity and performance of Slurries. Control of Slurry rheology is a highly specialized subject and the reader is referred to Chapter 7 of Ref 9... 

Corn stover, a well-known example of lignocellulosic biomass, is a potential renewable feed for bioethanol production. Dilute sulfuric acid pretreatment removes hemicellulose and makes the cellulose more susceptible to bacterial digestion. The rheologic properties of corn stover pretreated in such a manner were studied. The Power Law parameters were sensitive to corn stover suspension concentration becoming more non-Newtonian with slope n, ranging from 0.92 to 0.05 between 5 and 30% solids. The Casson and the Power Law models described the experimental data with correlation coefficients ranging from 0.90 to 0.99 and 0.85 to 0.99, respectively. The yield stress predicted by direct data extrapolation and by the Herschel-Bulkley model was similar for each concentration of corn stover tested. 

Stirred tanks are typically used for the thermochemical pretreatment. To simulate flow of corn stover slurries in stirred tanks, the rheologic properties of these suspensions must be known. The corn stover slurries in stirred tank reactors typically range from 10 to 40% solids (3). 

Figure 3.2 illustrates a classification of the rheological behavior of solids and fluids. Examples of different flow behaviors are shown in the lowest boxes. Figure 3.2 also illustrates the resulting shear stress as a function of the (shear-)deformation y or, for fluids, the shear rate y. The two most important material properties for our discussion in this chapter are the viscoelastic and the Newtonian fluid circled in the figure. 

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