Viscoelasticity thickeners

The cellulose ethers constitute another important group of cellulose derivatives prepared from alkali cellulose by standard etherification reactions between the hydroxyl groups and an alkyl halide. The properties of the ethers depend on the extent of the reaction that is, the degree of etherification. In general, the ethyl celluloses are water-insoluble thermoplastic materials, whereas methyl ether, ethyl hydroxyethyl cellulose, and carboxymethyl cellulose are soluble in cold water and are used as viscoelastic thickeners and adhesives. 

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 ... 

Some examples where the viscoelastic behaviour of polymer solutions are exploited are their use as thickeners in ... 

The main function of the foam stabilising agent is to reinforce the intercellular film wall by contributing rheological characteristics of viscoelasticity. The increased viscosity may also assist handling. The aim, as so often with auxiliaries, is to achieve an optimum balance. If the bubbles are too thin and wet too quickly they will collapse prematurely, whilst too stable a film could hinder uniform application. Examples of products used as foam stabilisers include thickening agents such as the polysaccharides, hydroxyethylcellulose, methylcellulose,... 

In the case of fluids without yield stress, viscous and viscoelastic fluids can be distinguished. The properties of viscoelastic fluids lie between those of elastic solids and those of Newtonian fluids. There are some viscous fluids whose viscosity does not change in relation to the stress (Newtonian fluids) and some whose shear viscosity T] depends on the shear rate y (non-Newtonian fluids). If the viscosity increases when a deformation is imposed, we define the material as a shear-thickening (dilatant) fluid. If viscosity decreases, we define it as a shear-thinning fluid. 

As a consequence of their tendency to form lamellar phases or rod shaped micelles at low concentration, cationic surfactants are frequently employed as the primary surfactants to thicken high salt formulas [72,73]. The viscoelastic nature of certain cationic surfactant solutions has been employed in a novel way to allow for a solution of sodium hypochlorite not to be easily diluted and therefore to remain at a higher concentration for the purpose of oxidizing clogs of human hair which form in drains [73 ]. Low concentrations of cetyl trimethyl ammonium chloride in combination with two hydro tropes form viscoelastic solutions with the values of viscosity and Tau/Go shown in Table 6.6. 

Shear-Thinning and Shear-Thickening Fluids Viscoelastic Fluids... 

Boersma, W. H., Laven, J., and Stein, H. N. 1992. Viscoelastic properties of concentrated shear-thickening dispersions. 7. Colloid and Interface Sci. 149 10-22. 

The shearing characteristics of non-Newtonian fluids are illustrated in Fig. 7. Curves A and B represent viscoelastic behavior. Curve C illustrates the behavior if the fluid thins with increasing shear, generally referred to as shear thinning or pseudoplasticity. The opposite effect of shear thickening or dilatancy is shown as curve D. 

The use of thickeners such as H EC and xanthan gum this will increase the low shear rate viscosity of the medium and hence slow down the diffusion of the small particles, preventing their entry into the oil droplets. These thickeners can produce gels in the continuous phase that is viscoelastic, and this can prevent particle diffusion. 

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