Thickeners flow behaviour

Equation 3.29 is helpful in showing how the value of the correction factor in the Rabinowitsch-Mooney equation corresponds to different types of flow behaviour. For a Newtonian fluid, n = 1 and therefore the correction factor has the value unity. Shear thinning behaviour corresponds to < 1 and consequently the correction factor has values greater than unity, showing that the wall shear rate yw is of greater magnitude than the value for Newtonian flow. Similarly, for shear thickening behaviour, yw is of a... 

The flow behaviour of aqueous coating dispersions, because of their high pigment and binder content, is often complex. They have viscosities which are not independent of the shear rate and are therefore non-Newtonian. Shear thickening (when the viscosity of the dispersion increases with shear rate) and shear thinning or pseudoplastic behaviour (when the viscosity decreases with shear rate), may... 

The addition of thickening agents to paints, pharmaceutical products and adhesives has many functions and the most important is for flow behaviour, gap filling and wetting ability for paints and adhesives. They are added at variable concentrations to pharmaceutical... 

In these equations, m and n are two empirical curve-fitting parameters and are known as the fluid consistency coefficient and the flow behaviour index respectively. For a shear-thinning fluid, the index may have any value between 0 and 1. The smaller the value of n, the greater is the degree of shear-thinning. For a shear-thickening fluid, the index n will be greater than unity. When n = 1, equations (1.12) and (1.13) reduce to equation (1.1) which describes Newtonian fluid behaviour. 

By plotting these data on linear and logarithmic scales, ascertain the type of fluid behaviour, e.g. Newtonian, or shear-thinning, or shear-thickening, etc. Also, if the liquid is taken to have power-law rheology, calculate the consistency and flow-behaviour indices respectively for this liquid. 

A basic appreciation of slurry rheology, or flow behaviour, is in oitant in many solid-liquid separations, e.g. when feeding pressure filters, punq)ing thickener underflow, hydrocyclone feed and exit streams and during cross-flow filtration. This Appendix is designed to introduce some of the terminology and basic concepts. A more thorough text such as Wilkinson [1960] should be referred to for further details, if necessary. 

Before discussing specific applications it is worth noting the practical significance of the flow behaviour of natural thickeners. Figure 2.10 shows the idealised flow curve of a thickener when measured over a very broad shear rate range and superimposed on this are some typical... 

Figure 1.10 Illustrating non-Newtonian flow behaviours. For a Newtonian fluid, the shear stress is proportional to the shear rate. In contrast, there are non-linear dependencies of shear stress on shear rate if shear thickening or shear thinning occur. A Bingham fluid has a finite yield stress...

The flow behaviour of colloids is very important to many of their applications. To take an everyday example, margarine should be stiff in the tub but flow under the pressure of the knife as it is spread on bread. The structural and dynamical complexity of colloidal systems leads to a diversity of rheological phenomena. The essential features of many of these effects (shear thinning, shear thickening, viscoelasticity) are common to different soft matter systems. Thus, rheology is discussed in Chapter 1 and is not explicitly considered further here. 

HEUR thickeners are synthesised from diisocyanates, polyethylene glycols and long-chain alcohols. Thus, they are also capable of undergoing hydrophobic interactions. However, in contrast to HASH thickeners they do not carry a charge. Because of this, they proved to be particularly useftil for applications where water resistance or barrier properties are important. With HEUR thickeners it is possible to develop water-based paints that come close to the Newtonian flow behaviour of solvent-based alkyd-systems, only. 

Many microbial polysaccharides show pseudoplastic flow, also known as shear thinning. When solutions of these polysaccharides are sheared, the molecules align in the shear field and the effective viscosity is reduced. This reduction of viscosity is not a consequence of degradation (unless the shear rate exceeds 105 s 1) since the viscosity recovers immediately when die shear rate is decreased. This combination of viscous and elastic behaviour, known as viscoelasticity, distinguishes microbial viscosifiers from solutions of other thickeners. Examples of microbial viscosifiers are ... 

The most frequently quoted example to illustrate this behaviour is the children s toy Silly Putty , which is a poly(dimethyl siloxane) polymer. Pulled rapidly it shows brittle fracture like any solid but if pulled slowly it flows as a liquid. The relaxation time for this material is 1 s. After t = 5t the stress will have fallen to 0.7% of its initial value so the material will have effectively forgotten its original shape. That is, one could describe it as having a memory of around 5 s (about that of a mackerel ). Many other materials in common use have relaxation times within an order of magnitude or so of 1 s. Examples are thickened detergents, personal care products and latex paints. This is of course no coincidence, and this timescale is frequently deliberately chosen by formulation adjustments. The reason is that it is in the middle of our,... 

The craze thickening, associated with the craze growth, implies an increase of fibril length. This is achieved by pulling out polymer chains from the craze-bulk interface, according to a behaviour analogous to plastic flow within the active layer (5-10 nm thick), as shown in Fig. 5b [21]. 

One may try to avoid the problem by the use of the upper-convected derivative, which ensures the coincidence of the principal axes of stress and strain. But doing that, it appears that any kinetics based on the stress amplitude is improper, since materials which exhibits thickening behaviour in elongation are, to the contrary, shear-thinning. Consequently no unique dependence can be expected for these two kinematics. The determination of a single set of parameters in various flows in then bound to be a compromise. 

---