Rheological measurements dispersed systems

Rheology Fundamentals, Malkin A. Ya, (Institute of Physics), ChemTec Publishing, 1995, 135 Rheology of disperse systems (meeting). Mill C, Franklin, 1959, 108 Techniques in rheological measurement, Collyer A.A, Chapman HaU, 1993, 75 The structure and rheology of complex fluids, Larson R.G, Oxford, 1999, 79.95... 

Rheological Measurements Three types of rheological measurements have been carried out. In the first type, transient (creep) measurements were performed on a 20% w/w dispersion of latex A, as a function of coverage by PVA. These experiments were carried out using a "Deer" rheometer (PDR 81, Integrated Petronic Systems, London) fitted with a stainless steel concentric cylinder. The procedures used have been described in detail before (21,22). 

The estimation of degree of dispersion can be made indirectly by measurement of electrical methods or measurement of mechanical properties. Boonstra54 used a coaxial electrode system to estimate dispersion form electrical resistivity whilst Belokur et al55 investigated the possibility of assessing dispersion from rheological measurements. 

With nonadsorbing polymer, rheological effects of similar magnitude accompany the phase transitions described earlier (Patel and Russel, 1989a,b). Since macroscopic phase separation takes weeks or months, rheological measurements performed within a few days on samples formulated within the two-phase region, with — Q>miJkT 2 - 20, detect a metastable structure that changes little over time. The systems respond as flocculated dispersions, but the microstructure recovers relatively quickly to a reproducible rest state after shear. Hence these weakly flocculated dispersions are quite tractable materials. 

The low shear rheology measurements also show a rapid increase in the viscoelastic properties (modulus and zero shear viscosity) with increase of bentonite concentration above the gel point (> 30 g dm bentonite). Several models have been proposed to account for the elastic properties of concentrated dispersions, of which that originally proposed by van den Tempel (25) and later developed by Papenhuizen (26) seems to be the most appropriate for the present system. According to this model, if the interaction energy minimum between adjacent particles is sufficiently negative, a three-dimensional network structure may ensue, giving an elastic component. Various models can be used to represent the three dimensional structure, the simplest of which would be either an ideal network where all particles are... 

One very important point that must be considered in any rheological measurement is the possibility of slip during the measurements. This is particularly the case with highly concentrated dispersions, whereby the flocculated system may form a plug in the gap of the platens, leaving a thin liquid film at the walls of the concentric cylinder or cone-and-plate geometry. This behaviour is caused by some syneresis of the formulation in the gap of the concentric cylinder or cone and plate. In order to reduce sHp, roughened walls should be used for the platens an alternative method would be to use a vane rheometer. 

Stability in colloidal dispersions is defined as resistance to molecular or chemical disturbance, and the distance the system is removed from a reference condition may be used as a measure of stability. The stability can be analyzed from both energetic and kinetic standpoints. The kinetic approach uses the stability ratio, as a measure of the stability. W is defined as fhe ratio of the rate of flocculation in the absence of any energy barrier to that when there is an energy barrier due to adsorbed surfactant or polymer. These processes are referred to as rapid and slow flocculation with rate constants kj and kg, respectively, such that W = kjlk. The stability of colloidal suspensions can be evaluated using various techniques. In practice, two methods are mainly used sedimentation and rheology measurements. 

From R D to quality control, rheology measurements for each phase of the product development life cycle involve raw materials, premixes, solutions, dispersions, emulsions, and full formulations. Well-equipped laboratories with stress- and strain-controlled oscillatory/steady shear rheometers and viscometers can generally satisfy most characterization needs. When necessary, customized systems are designed to simulate specific user or process conditions. Rheology measurements are also coupled with optic, thermal, dielectric, and other analytical methods to further probe the internal microstucture of surfactant systems. New commercial and research developments are briefly discussed in the following sections. 

Shear-Sensitive Systems. In addition to hydrodynamic effects and simple viscous behavior, the act of pigmentation creates a certain amount of complex behavior (13). If the particles are fine. Brownian movement (14-17) and rotational diffusion (14. 18. 19) are among the phenomena that cause dispersed systems to display complex rheology. The role of van der Waals forces in inducing flocculation (20) and the countervailing role of two electroviscous effects (17. 21. 22) in imparting stability, particularly in aqueous systems, have been noted. Steric repulsions appear to be the responsible factor in nonaqueous systems (23. 24). The adsorbed layer can be quite large (25-28). as detected by diffusion and density measurements of filled systems or by viscometry and normal stress differences (29). 

In the case of concentrated (structured) disperse systems the essential features that are usually of interest are their mechanical and rheological properties and behavior. The main parameter describing these features is the cohesive force, F, px in Chapter IX), or the strength of immediate contacts between particles. Stability is manifested as a correlation between the applied mechanical stress, P, and the sum of strengths of individual contacts, i.e. as the %FX product, where x is the number of contacts per unit area [35].In this case one only needs to evaluate the force in the immediate contact, without distance measurements. The experimental devices for such measurements may be extremely soft (pliable),which makes them very sensitive towards the measured forces. Corresponding methods and highly sensitive instruments for direct measurements of cohesive forces between individual particles of any nature in any media were developed by the authors and their co-workers [38-41], and are described in Chapter IX. 

The turbidity methods are unsatisfactory for lar particles and/or high particle number concentrations when multiple scattering effects intrude. As shown by Hunter et al. (1975), rheological measurements can then be used to detect flcK ulation. Stable dispersions exhibit an Ostwald-type flow curve whereas flocculated systems behave in a pseudoplastic fashion. Bocculation is thus accompanied by a large increase in the Bingham yield value, t, of the dispersion (see Fig. 5.4). 

The rheological characterisation of non-Newtonian fluids is widely acknowledged to be far from straightforward. In some non-Newtonian systems, such as concentrated suspensions, rheological measurements may be complicated by non-linear, dispersive, dissipative and thixotropic mechanical properties and the rheometrical challenges posed by these features may be compoimded by an apparent yield stress. 

In many cases, a comprehensive characterization of the rheological properties of systems, such as concentrated colloidal dispersions, can require measurements of dynamic mechanical behaviour at frequencies outside the range of conventional, commercially available, rheometers (typically 10 Hz to 10 Hz). In particular, consideration of the relative time scales of particle-fluid displacement and interfacial polarization mechanisms in such systems reveals the need for enhanced high frequency ranges (above ca. 10 Hz). 

Clays compatibilized and evenly dispersed in a polymer matrix tend to build networks at low concentration. Rheological measurements, performed on RCN based on various types of rubbers, revealed the pronounced rubber-clay interaction, when measurements were taken at zero shear. The filler networking phenomenon was observed as well in matrices based on various other rubber systems. At zero shear, the viscosity of RCN is thus higher than the one of... 

A range of methods are available for making rheological measurements (qv) (39-42). A frequently encountered problem involves knowing the parti-cle/droplet/bubble size and concentration in a dispersion and the need to predict the suspension, emulsion, or foam viscosity. Many equations have been advanced for this purpose. In the simplest case, a colloidal system can be considered Einsteinian. Here, the viscosity of the colloidal system depends on that of the continuous phase, r]o, and the volume fraction of colloid, 0, according to the Einstein equation, which was derived for a dilute suspension of noninteracting spheres ... 

To fully assess the properties of suspension concentrates, three main types of measurements are required. Firstly some information is needed on the structure of the solid/solution interface at a molecular level. This requires investigation of the double layer properties (for systems stabilised by ionic surfactants and polyelectrolytes), adsorption of the surfactant or polymer as well as the extension of the layer from the interface (adsorbed layer thickness). Secondly, one needs to obtain information on the state of dispersion on standing, such as its flocculation and crystal growth. This requires measurement of the particle size distribution as a function of time and microscopic investigation of flocculation. The spontaneity of dispersion on dilution, i.e. reversibility of flocculation needs also to be assessed. Finally, information on the bulk properties of the suspension on standing is required, which can be obtained using rheological measurements. The methods that may be applied for suspension concentrates are described briefly below. 

It is well known that filler-filled systems have very complex rheological properties and structures which vary in different flow fields and show special phenomena accompanied by structural changes. The processability of these dispersed systems is determined not only by their viscosities but also by their elasticities. In order to clarify the nature and the reformation process of their internal structure, dynamic viscoelasticity measurements were carried out extensively [6-8,47-51,53]. For systems filled with solid particles, the viscoelastic properties especially the dynamic ones as well as the steady flow properties have been studied extensively, and it has been found that the rheological properties of the dispersed systems differ from those of polymer solutions and melts in several ways. 

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