Rheological Characterization of Materials

The fundamental rheological characterization of a material requires the experimental determination of a constitutive equation (a rheological equation of state) that mathematically relates stress and strain, or stress and strain rate. The constants in the constitutive equation are the rheological properties of the material. 

The necessary experimental data must be obtained using rheometers, instruments in which measurements can be made that provide values of 

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. 

Rheometrical data may be fundamental or empirical in nature depending on the conditions of measurement relative to the sample s rheological behavior conditions must be carefully controlled when fundamental data are required. 

A viscometer is an instrument in which shear stress and shear rate (the strain rate in shear) can be measured at the same location in the instrument independently of the properties of the material being tested. 

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In the story of numerical flow simulation, the ability to predict observed and significant viscoelastic flow phenomena of polymer melts and solutions in an abrupt contraction has been unsuccessful for many years, in relation to the incomplete rheological characterization of materials, especially in elongation. The numerical treatments have often been confined to flow of elastic fluids with constant viscosity, described by differential constitutive equations as the Upper Convected Maxwell and Oldroyd-B models. Fortunately, the recent possibility to use real elastic fluids with constant viscosity, the so-called Boger fluids [10], has narrowed the gap between experimental observation and numerical prediction [11]. 

Viscosity determinations are the simplest means of the rheological characterization of materials Viscosity measurements lack sufficient precision for the generalization purposes because filled materials are most Irequently non-Newtonian liquids and a singular numerical parameter cannot adequately describe complex properties. In spite of this deficiency, many important conclusions can be drawn Ifom viscosity data. 

Online monitoring of viscosity is currently used in industry. Despite the fact that useful rheological characterization of materials requires a wide range of shear rate measurements. 

Macroscopically, all rheological features of a material are described by the relationship between the stress and strain tensors. This relationship, referred to as the constitutive relationship, may differ from material to material because of a possible difference(s) in the molecular/stmctural origin of the stress in various materials. The origin of the stress is essential for understanding the macroscopic constitutive relationship(s) as well as for the rheological characterization of materials, which is the aim of this chapter. For some materials such as homoploymer systems and blends, the origin(s) of the stress is summarized below. 

To characterize rheological behavior of materials, some basic terms need to be defined. Consider a liquid material that is subjected to a shearing force as illustrated in Fig. 2. The liquid is assumed to consist of a series of parallel layers with the surface area A, the bottom layer being fixed. When a force is applied on the top layer, the top plane moves at a constant velocity, whereas each lower layer moves with a velocity directly proportional to its distance from the stationary bottom layer. The velocity gradient (dv/dr, the difference in velocity, dv, between the top and bottom planes of liquid separated by the distance, dr) is also called the rate of shear, G ... 

Specializes in rheological properties of materials but has recently expanded its product line to include surface characterization mechanisms. Provides dynamic contact angle fiber tensiometer, dynamic contact angle tensiometer, and contact angle tensiometer. 

A complete rheological characterization of a material is very time consuming and expensive and much of the data will be irrelevant to any particular process or problem. 

Aichholzer W, Fritz H.-G. Rheological characterization of thermoplastic starch materials. Starch 50(1998)77. 

We shall deal here with rheometiy, which concerns the techniques for characterizing the rheological properties of materials. The physical principles underlying the design of rheometers are presented. Stating them here is justified for several reasons ... 

Biaxial squeeze-flow techniques are relatively new methods for the rheological characterization of polymer melts. With the method employed in this study, a plastic specimen is uniaxially compressed between two flat steel compression plates. Uniaxial compression is theologically equivalent to biaxial extension for incompressible materials. To minimize the effects of friction and adhesion at the metal-polymer interfaces, a lubricated surface was interposed between the polymer and the compression plates. Tliis technique was developed some years ago as a tool to help understand pressure-forming fabrication processes being researched by The Dow Chemical Company. 

Chemically different polymers are mostly immiscible and form multiphase stmetures in their blends. Rheological properties of such multiphase materials reflect responses from respective component chains as well as from the phase stmeture. The component chain response, which could be rheologically characterized with the methods explained in the previous section, is usually much faster than the response from the phase stmeture. This fact offers a possible route for rheological characterization of the multiphase stmeture through viscoelastic data of the materials at long times/low frequencies. This section focuses on this characterization. 

The behavior of colloidal suspensions is controlled by iaterparticle forces, the range of which rarely extends more than a particle diameter (see Colloids). Consequentiy suspensions tend to behave like viscous Hquids except at very high particle concentrations when the particles are forced iato close proximity. Because many coating solutions consist of complex mixtures of polymer and coUoidal material, a thorough characterization of the bulk rheology requires a number of different measurements. 

There are three different approaches to a thermodynamic theory of continuum that can be distinguished. These approaches differ from each other by the fundamental postulates on which the theory is based. All of them are characterized by the same fundamental requirement that the results should be obtained without having recourse to statistical or kinetic theories. None of these approaches is concerned with the atomic structure of the material. Therefore, they represent a pure phenomenological approach. The principal postulates of the first approach, usually called the classical thermodynamics of irreversible processes, are documented. The principle of local state is assumed to be valid. The equation of entropy balance is assumed to involve a term expressing the entropy production which can be represented as a sum of products of fluxes and forces. This term is zero for a state of equilibrium and positive for an irreversible process. The fluxes are function of forces, not necessarily linear. However, the reciprocity relations concern only coefficients of the linear terms of the series expansions. Using methods of this approach, a thermodynamic description of elastic, rheologic and plastic materials was obtained. 

Flow and self-leveling characteristics of these products are governed by the rheological behavior of the slurrylike materials. At the low water-cement ratios required to ensure proper suspension of the solids, most selfleveling compositions are characterized by a yield stress and thixotropic behavior [75]. To obtain self-leveling properties, the yield stress has to be reduced and this is achieved by the selection and combination of suitable mix ingredients at... 

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