Rheology of Dispersions E-Book

Tharwat F. Tadros

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 293

Veröffentlichungsjahr: 2011

Beschreibung

A dispersion is a system of unmixable phases in which one phase is continuous and at least one is finely distributed. Examples are found in many industrial applications, including emulsions, suspensions, foams, and geld. The control of their flow characteristics - rheology - is essential in their preparation, long-term physical stability and application.
Filling the need for a practical, up-to-date book connecting the stability/instability of the dispersion to its rheological behavior, this title aids in understanding the principles of rheology and the techniques that can be applied.

From the contents:
* General Introduction
* Interparticle Interactions and Their Combination
* Principles of Viscoelastic Behavior
* Rheology of Suspensions
* Rheology of Emulsions
* Rheology of Modifiers, Thickeners, and Gels
* Use of Rheological Measurements for Assessment and Prediction of the Long-Term Physical Stability of Formulations (Creaming and Sedimentation)

Leseprobe

Contents

Preface

1 General Introduction

References

2 Interparticle Interactions and Their Combination

2.1 Hard-Sphere Interaction

2.2 “Soft” or Electrostatic Interaction

2.3 Steric Interaction

2.4 van der Waals Attractions

2.5 Combination of Interaction Forces

2.6 Flocculation of Dispersions, and Its Prevention

2.7 Distinction between “Dilute,” “Concentrated,” and “Solid” Dispersions

2.8 States of Suspension on Standing

2.9 States of the Emulsion on Standing

References

3 Principles of Steady-State Measurements

3.1 Strain Rate or Shear Rate

3.2 Types of Rheological Behavior in Simple Shear

3.3 Time Effects During Flow: Thixotropy and Negative (or Anti-) Thixotropy

3.4 Rheopexy

3.5 Turbulent Flow

3.6 Effect of Temperature

3.7 Measurement of Viscosity as a Function of Shear Rate: The Steady-State Regime

3.8 Non-Newtonians

3.9 Major Precautions with Concentric Cylinder Viscometers

References

4 Principles of Viscoelastic Behavior

4.1 Introduction

4.2 The Deborah Number

4.3 Strain Relaxation after the Sudden Application of Stress (Creep)

4.4 Analysis of Creep Curves

4.5 The Berger Model (Maxwell + Kelvin)

4.6 Creep Procedure

4.7 Stress Relaxation after Sudden Application of Strain

4.8 Dynamic (Oscillatory) Techniques

References

5 Rheology of Suspensions

5.1 Introduction

5.2 The Einstein Equation

5.3 The Bachelor Equation

5.4 Rheology of Concentrated Suspensions

5.5 Rheology of Hard-Sphere Suspensions

5.6 Rheology of Systems with “Soft” or Electrostatic Interaction

5.7 Rheology of Sterically Stabilized Dispersions

5.8 Rheology of Flocculated Suspensions

5.9 Models for the Interpretation of Rheological Results

References

6 Rheology of Emulsions

6.1 Introduction

6.2 Interfacial Rheology

6.3 Bulk Rheology of Emulsions

References

7 Rheology Modifiers, Thickeners, and Gels

7.1 Introduction

7.2 Classification of Thickeners and Gels

7.3 Definition of a “Gel”

7.4 Rheological Behavior of a “Gel”

7.5 Classification of Gels

7.6 Rheology Modifiers Based on Surfactant Systems

References

8 Use of Rheological Measurements for Assessment and Prediction of the Long-Term Physical Stability of Formulations (Creaming and Sedimentation)

8.1 Introduction

8.2 Sedimentation of Suspensions

8.3 Assessment and Prediction of Flocculation Using Rheological Techniques

8.4 Assessment and Prediction of Emulsion Coalescence Using Rheological Techniques

References

Index

The Author

Prof. Dr. Tharwat F. Tadros

89 Nash Grove Lane

Wokingham, Berkshire RG40 4HE

United Kingdom

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ISBN: 978-3-527-32003-5

Dedicated to our Grand-Children

Nadia

Dominic

Theodore

Bruno

Viola

Preface

The rheology (flow characteristics) of dispersions of both solid/liquid (suspensions) and liquid/liquid (emulsions) types is applicable to many industrial situations, including paints, printing inks, paper coatings, ceramics, cosmetics, food systems, pharmaceutical and agrochemical formulations, and liquid detergents. In all of these complex multiphase systems it is necessary to control the rheology of a formulation during its preparation, to maintain its long-term physical stability, and during its application. The requirements for a good paint formulation illustrate these points very well: a paint may consist of polymer particles (latexes) and pigments that must be maintained in a colloidally stable state not only in the formulation but also on coating, to produce a uniform film. In storage, the paint formulation should not show any sedimentation of the pigments, there should be no separation (syneresis), and it should produce a weak gel that is thixotropic in nature. This means that, on application, the gel structure must be broken under shear so as to produce a uniform film, but recovered within a controlled time scale so as to prevent the paint from dripping. But, if the gel structure is recovered too quickly after application, the paint film will show brush marks! In order to achieve such behavior on both storage and application, rheology modifiers–which sometimes are referred to as “thickeners” or “gels”–must be incorporated into the formulation. Today’s research chemist, when formulating a chemical product, must understand the basic principles of rheology and how to control the various parameters of the system so as to achieve a desired effect. In addition, the formulation chemist must design accelerated tests to predict any changes that might occur in the system during storage. Rheological measurements represent the most powerful tools for such predictions, as the formulation can be investigated without dilution or disturbing its structure.

This book is targeted at research scientists, both in industry and in academic institutes. Following a brief introduction (Chapter 1) which highlights the scope of the book, Chapter 2 is dedicated to understanding the colloidal properties of dispersions, where the theories of colloid stability–both for electrostatically and sterically stabilized dispersions-are briefly summarized. This is followed by a description of the conditions required for the stability/instability of a dispersion, with particular attention being paid to the states of suspensions and emulsions on standing. The various breakdown processes are described and analyzed in terms

of the interaction forces between the particles or droplets. Chapters 3 and 4 describe the basic principles of rheology, and the experimental techniques that can be applied to investigate the rheological properties of a system. In this case, it was convenient to separate the basic principles into two chapters. First, the steady-state principles (high deformation measurements) are outlined in Chapter 3, whereby a constant and increasing shear rate is applied to a system (which may be placed in the gap between concentric cylinders, parallel plates, or cone and plate geometry), after which the stress and viscosity are obtained as a function of shear rate so as to distinguish between Newtonian and non-Newtonian systems. The flow curves of shear stress versus shear rate can be fitted to various models, and particular attention is given to the reversible time-dependence of viscosity (thixotropy and negative thixotropy). Chapter 4 provides details of viscoelastic behavior (low-deformation measurements), with descriptions of three methods (that are equivalent, but not identical) which can be applied: (i) strain relaxation after the sudden application of stress (constant-stress or creep measurements); (ii) stress relaxation after the sudden application of strain; and (iii) dynamic (oscillatory) techniques. The models that can be applied to describe each of these measurements are briefly outlined. In this way, various rheological parameters can be obtained, such as creep compliance, residual (zero) shear viscosity, relaxation time, storage (elastic) and loss (viscous) modulus. Each of these parameters is important for controlling the physical stability of the dispersion, as well as predicting its long-term stability. Chapter 5 describes the rheology of suspensions, with four basic systems being outlined. The first of these is hard-sphere dispersions, where both repulsion and attraction are screened (neutral stability). In this case, the rheological behavior is simple as it depends only on the balance between Brownian diffusion and hydrodynamic interaction. These hard-sphere dispersions form the basis of the development of theoretical treatments of the rheology of suspensions. The second system is that of “ soft ” (electrostatic) interaction, where rheology is determined by double-layer repulsion. For this, models are available to relate some of the rheological parameters such as the high-frequency modulus to the double-layer repulsion energy of interaction. The third category is that of sterically stabilized systems which contain adsorbed or grafted nonionic polymers. In this case, the rheology is determined by the steric repulsion of the adsorbed or grafted chains. It is also possible to relate the high-frequency modulus to the steric interaction energy. Finally, flocculated and coagulated systems, where the rheology is determined by the van der Waals attractions, can be used to distinguish between weak (reversible) and strong (irreversible) systems. The rheology of these flocculated systems is complicated, as they are under nonequilibrium, and only semi-empirical models are available for this purpose. The use of a fractal dimension concepts allows information to be obtained on the structure of flocculated systems. Chapter 6 deals with the rheology of emulsions. It starts with a discussion of interfacial rheology and how this can be correlated with emulsion stability. The bulk rheology of emulsions is described at a fundamental level with special reference to the analysis of the rheology of concentrated emulsions and the effect of droplet deformability. A section is devoted to the rheology of high internal phase

emulsions (HIPE). Chapter 7 provides information on the rheology of modifi ers, thickeners and gels, with various systems being described ranging from physical gels (obtained simply by an overlap of the polymer coils) to associative thickeners (hydrophobically modified polymers), crosslinked polymers, and particulate gels (including swellable clays and silica gels). One important rheology modifi er that is used in many cosmetic creams and liquid detergents is based on surfactant liquid crystalline phases. In Chapter 8, the various rheological methods that can be applied to assess and predict the long-term physical stability of dispersions are described, with practical examples being given to illustrate the validity of these methods.

Although this book is by no means comprehensive, its aim is to provide both the fundamentals and applications of the rheology of dispersions. It should serve as a valuable text for those scientists in industry who deal regularly with the formulation of chemicals, and also as an introduction to those research workers investigating the rheology of dispersions.

February 2010

Tharwat Tadros

1

General Introduction

Several types of disperse system may be defined, depending on the nature of the disperse phase and medium; these are summarized in Table 1.1.

The present book deals with the rheology of three main disperse systems, namely solid in liquid (suspensions), liquid in liquid (emulsions), and liquid in solid (gels). It is essential to define the dimensions of the particles or droplets of the internal phase. Systems where such dimensions fall within 1 nm to 1000 nm (1 μm) are classified as “colloidal systems,” whereas those which contain particles or droplets larger than 1 μm are outside the colloid range. However, in both cases, the property of the system is determined by the nature of the interface which separates the internal phase from the medium in which it is dispersed. Clearly, with colloidal systems the interfacial region presents a significant proportion of the whole system. The structure of the interfacial region determines the properties of the system, and in particular the tendency of the particles to form aggregate units or to remain as individual particles.

Two main types of interfacial structure may be distinguished. The first type occurs with charged interfaces, whereby a double layer develops as a result of the presence of a surface charge which is compensated near the interface by unequal distribution of counter-ions and co-ions. At the interface, there will be an excess of counter-ions and a deficiency of co-ions. This picture of the double layer has been introduced by Gouy and Chapman [1, 2], and is referred to as “the diffuse double layer.” Later, Stern [3] introduced a modified picture whereby the first layer of the counter-ions is regarded to be fixed (due to specific adsorption) and the rest of the double is diffuse in nature. In this way, Stern defined a potential ψd at the center of the first fixed layer; this potential may be close to the measurable elec-trokinetic or zeta potential. A modification of the Stern picture was later introduced by Grahame [4, 5], who considered two planes: an inner Helmholtz plane (IHP) at the center of the counter-ions that lose their hydration shell (chemisorbed counter ions); and an outer Helmholtz plane (OHP) at the center of the physically adsorbed counter-ions with their hydration shell. As will be seen in Chapter 2, the extension of the double layer depends on the concentration of counter-ions and co-ions, as well as their valency. At low electrolyte concentrations (<10 moldm for 1:1 electrolyte and (<10 moldm for 2:2 electrolyte), the double layer is sufficiently extended.