Rheological profiles

Both of these models show contributions from the viscosity and the elasticity, and so both these models show viscoelastic behaviour. You can visualise a more complex combination of models possessing more complex constitutive equations and thus able to describe more complex rheological profiles. 

Dispersion is a term for systems containing various phases of at least one continuous and one finely dispersed. Referring to mineral slurries, this is typically a suspension of a mineral in water. This suspension normally contains some more additives for improved stability. One important additive in these systems is the dispersant. Interparticle forces hold the particles together and these interactions are reduced by the use of dispersants. This can be indicated by improved rheology profiles. 

PVC plastisols or pastes should have a viscosity suitable for handling and storage, as well as the manufacturing process. This is necessary for thinning under the appropriate shear conditions. Paste PVC resins (or blends of resins) are designed to satisfy a particular rheology profile, but sometimes the addition of viscosity modifiers is necessary. Fumed silicas are typically used. Calcium sulfonate gels have also been promoted (458). 

Cellulose ethers swell or are colloidally soluble. They all raise viscosity and have specific rheology profiles (seeO Fig. 18). Many cellulose ethers are surface-active, and they reduce the surface activity of water by about 20 to 25% depending on type. Cellulose ethers are generally compatible with other hydrocolloids and many other substances. The field has recently been reviewed by Majewicz et al. [95]. The primary manufacturers of industrial cellulose ethers are listed in O Table 14. 

Simple classifications of fluids can be made on the basis of their rheological profiles. Figure 3.78 shows the (a) shear stress and (b) viscosity profiles for various systems. From Figure 3.78 one may define the following systems. Newtonian systems have a constant viscosity with respect to shear rate. Dilatant (or shear-thickening) systems have a viscosity that increases with respect to shear rate. Pseudo-plastic (or shear-thinning) systems have a viscosity that decreases with respect to shear rate. Yield-stress materials are materials that have an initial structure that requires a finite stress before deformation can occur. The stress that initiates deformation is defined as the yield stress. 

Viscoelastic dentifrice compositions are disclosed in patents SK279419B and PL169998B. Hair care compositions comprising at least one associative polymer are disclosed in patent EP1088545. They are said to be easy to dispense and apply to the hair, having an excellent rheology profile. 

In this chapter we survey the most common types of rheology modifiers that are used today in liquid detergents. This covers both natural and synthetic modifiers, with numerous subclasses in each, as illustrated in Table 5.1. Guidelines for the types of rheological profiles each modifier can provide, as well as general formulation issues, are presented. 

In reality, few systems are Newtonian, and some of the other principal rheological profiles are also shown in Figure 5.1. In many cases a Newtonian behavior is not desirable for a formulated product. This can be illustrated by the case of a spray cleaner. A certain minimum viscosity is often required such that the material appears to be concentrated in the bottle. The visual appearance is referred to in this chapter as the apparent viscosity and is generally considered to correspond to a shear rate of the order of 10 sec-1 (reciprocal seconds). If the formulation is Newtonian, then the viscosity will remain the same even at the relatively high shear rates corresponding to spraying (Figure 5.2). This is not desirable, as the spray pattern obtained varies considerably with the viscosity of the fluid in the spray nozzle, and better atomization is observed when the viscosity is low. Consequently, an ideal profile for such a formulation is one in which the viscosity decreases as... 

In terms of the overall rheology profile of acrylic polymers when used in finished formulations, the behavior of the nonassociative thickeners is relatively easy to predict, as there is little interaction from a rheological point of view between the thickener and the matrix. Significantly higher polymer levels will be required if electrolytes are present, but the overall formulation rheology (e.g., pseudoplasticity, yield development) will remain similar. In most circumstances, though,... 

In the following examples some of the matrix effects observed between rheology modifiers and different surfactants are illustrated. The case of a crosslinked ASE polymer in the presence of a nonionic alkyl polyglucoside (APG) surfactant is shown in Figure 5.6. The rheological profiles of the polymer in different concentrations of APG are very similar to those of the aqueous polymer results, indicating that the surfactant has very little effect on the rheological behavior of the ASE polymer. 

As stated earlier, both shampoos and conditioners must have sufficiently high viscosities to remain easily in the hand. In addition, however, the product must also have the correct rheological profile. Thus, it must have good stand-up in the hand, but must be sufficiently shear thinning that it is dispensed easily from the package. 

For more complex shampoos, including emulsions, a variety of thickeners is available besides salt, including gums, associative thickeners, synthetic polymers, and long-chain alcohols (Section II.C.2). These materials are used to attain a desired viscosity, to stabilize a product, and to attain a desirable rheological profile. Since they affect product structure, they can also affect lather. Viscosity modifiers, therefore, should be chosen to give the best mix of lather and rheological properties. 

Experience has shown structured fluids to be more difficult to manufacture, due to the complexity of their rheological profiles. In addition to elasticity, dilatancy, and rheopexy, certain structured fluid compositions may exhibit solid-like properties in the quiescent state and other flow anomalies under specific flow conditions. For emulsions and solid particulate dispersions, near the maximum packing volume fraction of the dispersed phase, for example, yield stresses may be excessive, severely limiting or prohibiting flow under gravity, demanding special consideration in nearly all unit operations. Such fluids pose problems in... 

The use of rheology additives such as clays, plant exudates and natural polymers to formulate paints dates back to ancient times. These materials are used to thicken the fluid, suspend dispersions of additives in the fluid and improve the stability of the ensuing dispersion as a function of temperature and shear history. This paper classifies cellulosic rheology modifiers with respect to their influence on paint properties related to rheological profiles. 

Coatings formulations based on water borne polymers usually depend on specific ingredients for rheological profile adjustment and overall properties optimization. There is a large variety of additives which can be used to modify the rheology of a system. Natural organic derivatives (e.g. cellulose ethers) have been traditionally used in the paint industry and are well established. Synthetic thickeners, on the other hand, have appeared later but quickly became very popular, mostly because of their ease of use (liquid versus solid)... 

The goal is to offer to paint formulators an easy way to convert from cellulosic thickeners to designed rheology modifiers maintaining rheology profile and overall properties. 

The rheological profile obtained from the viscometer is fitted with Ostwald-de Waele Power law model. This mathematical model has been reported to have lowest standard error. The power law mathematical model is given by ... 

Fig. 2. Rheological profiles for two commercial PVDF grades. The melt rheology converges at high shear rates above 100 rad/s.

Using a Rheometrics mechanical spectrometer and powdered polymer samples, the authors compared the rheological behaviour of two polymers with similar chemical compositions but different structures. The rheological profiles of polymers 21 and 22 were determined between 140 and 400°C by increasing the temperature at 10°C min from 140 to 190°C and from 300 to 400°C. In the predominant region of isoimide-imide conversion (190-300°C), the temperatme was raised by 2 or 5°C increments, the dynamic viscosity rj being measured at each temperature step. At 190°C, the viscosity of poly(isoimide) 21 was approximately 5 X 10 Pas and decreased to a minimum value of 10 Pas at 243°C as the polymer softened and melted. Thermal conversion to polyimide 22 concurrently... 

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