Rheological flow terms

Rheology. Flow properties of latices are important during processing and in many latex appHcations such as dipped goods, paint, inks (qv), and fabric coatings. For dilute, nonionic latices, the relative latex viscosity is a power—law expansion of the particle volume fraction. The terms in the expansion account for flow around the particles and particle—particle interactions. For ionic latices, electrostatic contributions to the flow around the diffuse double layer and enhanced particle—particle interactions must be considered (92). A relative viscosity relationship for concentrated latices was first presented in 1972 (93). A review of empirical relative viscosity models is available (92). In practice, latex viscosity measurements are carried out with rotational viscometers (see Rpleologicalmeasurement). 

The formulation of coatings must be matched to the required property profile in terms of the resin/hardener system as well as the additives which influence the rheology, flow, deaeration, mechanical and chemical properties, resistance to yellowing and film colour. 

Viscosity Consistency—Related but quite different rheological (pertaining to flow) terms. A metal ball will penetrate a viscom substance (asphaltum) no matter how long it takes, but may stay on top of a weak but highly consistent semisolid (starch paste) for an indefinite period. 

Rheology, a term first proposed by Prof. Bingham (Lafayette College, Indiana, USA), is defined as the science that studies the deformation and flow of matter. When a body of material is subjected to a force that causes it to deform, the resulting behaviour of the material will be dependent on the magnitude and nature of force to which it is subjected, as well as the strength and degree of permanence of the structure or forces that exist within the material. 

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. 

Rheology concerns the study of the deformation and flow of soft materials when they respond to external stress or strain. If the ratio of its shear stress and shear rate is a straight line, the material is termed Newtonian otherwise, it is termed non-Newtonian (Figure 4.3.2(a)). As the slope of the curve is the viscosity rj, a shear-thinning fluid exhibits a reduced viscosity as the shear stress increases, whereas a shear-... 

Newtonian flow, and their viscosity is not constant but changes as a function of shear rate and/or time. The rheological properties of such systems cannot be defined simply in terms of one value. These non-Newtonian phenomena are either time-independent or time-dependent. In the first case, the systems can be classified as pseudoplastic, plastic, or dilatant, in the second case as thixotropic or rheopective. 

The two viscous rheological properties are m, the consistency coefficient, and n, the flow index. The apparent viscosity function for the power law model in terms of shear rate is... 

A convenient term for the rheological properties of an unvulcanised elastomer (see Rheology). It has been defined as the susceptibility to, and retentivity of deformation , and also the degree of flow which takes place under given conditions of temperature and pressure . The use of the term viscosity is a more appropriate description. Plasticity Retention Index... 

As with spherical particles the Peclet number is of great importance in describing the transitions in rheological behaviour. In order for the applied flow field to overcome the diffusive motion and shear thinning to be observed a Peclet number exceeding unity is required. However, we can define both rotational and translational Peclet numbers, depending upon which of the diffusive modes we consider most important to the flow we initiate. The most rapid diffusion is the rotational component and it is this that must be overcome in order to initiate flow. We can define this in terms of a diffusive timescale relative to the applied shear rate. The characteristic Maxwell time for rotary diffusion is... 

The last term in equation 4.28 is not a simple geometric characterisation of the flow passages, as it also depends on the rheology of the fluid (n). The constant b is a function of the shape of the particles constituting the bed, having a value of about 15 for particles of spherical, or near-spherical, shapes there are insufficient reliable data available to permit values of b to be quoted for other shapes. Substitution of n = 1 and of /x for k in equation 4.28 reduces it to equation 4.13, obtained earlier for Newtonian fluids. 

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