Polymer and Composite Rheology, Second Edition

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Introduction

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Shear

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Instruments

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Shear

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Polymer

Chapter 4

84

HASE-associative thickeners are modifications of traditional alkali-soluble thickeners that are carboxyl functional copolymers produced by free-radical polymerization of monomers such as methacrylic acid and ethyl acrylate [44]. The modification consists of the attachment of hydrophobic groups along the polymer backbone. These thickeners are essentially insoluble in water at low pH but dissolve at pH values above 7. Solution viscosity increases because of chain expansion due to repulsion between the carboxylate anions along the polymer backbone and also due to the formation of an intermolecular network resulting from hydrophobic interactions. The variation in zero-shear viscosity with pH and polymer concentration is shown in Figure 4.13 for aqueous solutions of an HASE-model polymer prepared by the Union Carbide Corporation [44]. The solution viscosity is independent of polymer concentration and comparable to the solvent viscosity at low pH. However, as the pH increases beyond 5, the viscosity increases suddenly, by as much as six orders of magnitude for the most concentrated solution. Thereafter, the change in viscosity with pH is quite gradual. A possible explanation for this behavior is provided is Figure 4.14. At low pH, the polymer exists as a dispersion rather than as a solution. When an alkali such as ammonium hydroxide is added to raise the pH, acid groups on the polymer chain are neutralized and the polymer dissolves. With increasing pH, the viscosity increases because (1) the polymer chain expands due to charge repulsion, and (2) intermolecu-

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4.13 Zero-shear viscosity of HASE 5142 at different pHs. (From Ref. 44.) FIGURE

Shear Viscosity of Polymer Solutions

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.

85

Polymers are insoluble, they exist as dispersion

COOH groups are not hydrolyzed

pH < 6.0 Intra-molecularinteraction

Solubilizing of particles commences Carboxylate anions form by acid base reaction Polymer coils begin to unfold due to replusions between negative charges Intra-molecular interactions dominate 6.0 e pH c6.5 Inter-molecularinteraction

Optimum repulsions between negative charges occur Polymer coils form a network structure due to inter-molecular interactions

Intra-molecular interaction

DH> 8.0

FIGURE 4.14 Pictorial representation of HASE polymer behavior with pH. (Unpublished results of Dr. K. C. Tam.)

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Extensional

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Solid-in-Liquid

Solid-in-Liquid

Solid-in-Liquid

Chapter 10

240

observed with unimodal suspensions having a solids volume fraction in the neighborhood of 0.5. Shear thickening is seen in Fig. 10.10 for PVC suspensions in dioctyl phthalate [311. At volume fractions exceeding 0.5, the shear thickening is so severe that one actually observes a discontinuity in the flow curve; the behavior, however, is entirely reversible on lowering the shear rate. Note also that even suspensions of noncolloidal particles exhibit shear thickening. Indeed, according to Barnes [32], shear thickening is so common that all suspensions of solid particles can be made to show shear thickening given the right circumstances. In general, though, shear thickening rarely lasts for more than one decade of shear rate, and the viscosity subsequently levels off or even decreases with further increases in shear rate. The shear rate at which shear thickening is first observed is a strong function of W D n Zthe ; critical shear rate decreases as @KOrn increases toward unity. One way to ameliorate the viscosity increase is to use a It is also found particle size distribution, for this results in an increase in that the critical shear rate is lower for the same volume fraction of large spheres than it is for small spheres. In addition, the severity of shear thickening goes down as the particle diameter decreases. It is for this reason that the particle size distribution should be broadened with the help of small spheres rather than large spheres.

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FIGURE 10.10 Effect of volume fraction and shear rate on the viscosity of 1.25micrometer PVC suspensions in dioctyl phthalate. (From Ref. 31.)

Solid-in-Liquid

Solid-in-Liquid

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Short-Fiber

Short-Fiber

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Gas-Containing Melts

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Chemorheology

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Chemorheology

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Chemorheology

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Chemorheology

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Chemorheology

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Chemorheology

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Chemorheology

Melt

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