Thixotropic clays

Pignon F, Magnin A, Piau J-M, Cabane B, Lindner P, Diat O (1997) Yield stress thixotropic clay suspension Investigations of structure by light, neutron, and X-ray scattering. Phys Rev E 56 3281-3289... 

The coefficient Tj is termed the modulus of rigidity. The viscosities of thixotropic fluids fall with time when subjected to a constant rate of strain, but recover upon standing. This behavior is associated with the reversible breakdown of stmctures within the fluid which are gradually reestabflshed upon cessation of shear. The smooth sprea ding of paint following the intense shear of a bmsh or spray is an example of thixotropic behavior. When viscosity rises with time at constant rate of strain, the fluid is termed rheopectic. This behavior is much less common but is found in some clay suspensions, gypsum suspensions, and certain sols. 

The main use of these clays is to control, or adjust, viscosity in nonaqueous systems. Organoclays can be dispersed in nonaqueous fluids to modify the viscosity of the fluid so that the fluid exhibits non-Newtonian thixotropic behavior. Important segments of this area are drilling fluids, greases (79,80), lubricants, and oil-based paints. The most used commercial products in this area are dimethyl di (hydrogen a ted tallow) alkylammonium chloride [61789-80-8] dimethyl (hydrogen a ted tallow)aLkylbenzylammonium chloride [61789-72-8] and methyldi(hydrogenated tallow)aLkylbenzylammonium chloride [68391-01-5]. 

The property of thixotropy ia various degrees of complexity (3). Thixotropic properties may lead to loss of stabiUty as shown by the sometimes catastrophic flow of quick clays, especially ia Norway, Sweden, and Canada (15). 

To prepare stable emulsions ia this way gelation of the continuous medium is necessary. The appearance of a Hquid emulsion may be retained by choosing a polymer for the continuous phase, giving a thixotropic solution with short breakdown and buildup times. The polymers used for this purpose are natural gums (qv) or synthetic polymers. Clay particles also act as viscosity enhancers. The members of the bentonite family derived from... 

Time-dependent fluids are those for which structural rearrangements occur during deformation at a rate too slow to maintain equilibrium configurations. As a result, shear stress changes with duration of shear. Thixotropic fluids, such as mayonnaise, clay suspensions used as drilling muds, and some paints and inks, show decreasing shear stress with time at constant shear rate. A detailed description of thixotropic behavior and a list of thixotropic systems is found in Bauer and Colhns (ibid.). 

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

Clay materials show a different behavior. They are either cation-poor or cation-rich sheet silicates. They can swell by taking up varying amounts of water between the sheets. If the intercalated cations are hydrated as in montmorillonite, they act as cation exchangers. Montmorillonite, especially when it has intercalated Ca2+ ions, has thixotropic properties and is used to seal up drill holes. The effect is due to the charge distribution on... 

Several conditions are necessary for satisfactory surface coatings, e.g., nondrip applications. There is a balance between the ease of use and sagging of the paint during and after administration. Thixotropic additives, e.g., organo-montmorillonite clays are added. These form 3-dimensional networks, which are easily broken down by shear. 

Rheopexy is time-dependent shear-thickening, and is sometimes observed as an acceleration of thixotropic recovery - for example, bentonite clay suspensions often set only slowly on standing but quite rapidly when gently disturbed. 

Thixotropic Time-dependent pseudoplastic flow. At constant applied shear rate, viscosity decreases. In a flow curve, hysteresis occurs. Paint, quicksand. In bentonite clay gels which liquefy on shaking and solidify on standing, there is a time-dependent aligning to match the induced flow. After the shear rate is reduced it takes some time for the original alignments to be restored. 

Rheopexy refers to dilatant flow which is time dependent. At a constant applied shear rate viscosity increases, as shown in Figure 6.15. In a flow curve, hysteresis occurs (but opposite to the thixotropic case). An example of a rheopectic system is a bentonite clay gel system which sets slowly on standing, but sets quickly when gently agitated. 

Flow and fusion promoters Thixotropic agents Antiaging Antioxidants PMMA and acrylate ester copolymers, MBS Fumed silica, clays Sterically hindered phenols, sec-aromatic amines, phosphates, thioethers... 

The addition of asbestos fibers at one time provided excellent thixotropic adhesive formulations, especially at elevated temperatures. However, health and environmental regulations have severely limited the use of this material. Today, fumed silica, precipitated calcium carbonate, certain clays, and cellulose and other fibers offer thixotropic properties at relatively low levels of loading. 

Other mineral additives have been used for many years as functional extenders and fillers in adhesives and sealants. These include kaolin (hydrated aluminum silicate), bentonite (hectorite clay), talc (magnesium silicate), and attapulgite (hydrated magnesium aluminum silicate) additives.1213 Kaolin and talc are considered to be viscosity thickeners whereas attapulgite is more of a conventional thixotrope. They are considered to be very cost-effective rheological additives. 

Particularly attractive method for preparation of synthetic zeolite is recrystallization of natural aluminosilicates, such as kaolinite (halloysite), previously formed for elimination of plastic flow of highly thixotropic, pulverized zeolite. Some additional components of initial mixtures, such as texture modifiers (hard coal, lignite, cellulose, silica, aluminum oxide) are also introduced. They enrich the structure of zeolite adsorbent in transport pores and prevent an excessive compression of the clay material during the formation process. This results in an increase in product efficiency during the crystallization of zeolite phase. 

There are more complicated structures nuermediate between pyrophyllite and talc with variable substitution of Al and Mg. ElectroneulraKty is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual clays lorming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip paints. They are derived from the formulation AUCOHliSi Oin-xH, with variable amounts of water, Mg " " (in place of some Al ). and compensating cations. M" (M = Ca in fuller s earth, which is converted to bentonite. M = Na). Vermiculile likewise has variable amounts of water and cations. It dehydrates (O a talc-like structure with much expansion when heated (see page 750). 

Calcium Carbonate, Calcium Silicate, Powdered Aluminium, Copper Alumina, Flint Powder, Carborundum, Silica, Molybdenum Disulphide Chopped Glass Mica, Silica, Powdered or flaked Glass Metallic Filler or Alumina Colloidal Silica, Bentonite Clay Improved Thermal Conductivity Improved Machinability Improved Abrasion Resistance Improved Impact Strength Improved Electrical Conductivity Improved Thixotropic Response... 

Bheopectic Fluids. Rheopectic fluids are characterized by an increase in their viscosity with time at a constant shear rate and fixed temperature. As for a thixotropic fluid, a hysteresis loop is also formed with a rheopectic fluid if it is sheared from a low to a high shear rate and back to a low shear rate. However, a different rate is usually followed upon lowering the shear rate, as is shown in Figure 3. Bentonite clay suspensions and sols are typical examples of rheopectic fluids (3). 

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