Thixotropic Mixtures

We stated above that for most applications the slurry behaves as a fluid of modified density, and that viscosity effects can usually be neglected. However, there is a limit to the permissible solids concentration. Relatively coarse solids can simply not be carried in suspension above a certain solids concentration for the particular slurry, say 40 per cent by mass. In the case of very fine solids, a thixotropic mixture is reached at higher solids concentration, in which the solid particles stay in suspension but the slurry viscosity is markedly increased. In fact the viscosity no longer exhibits a Newtonian relationship of direct proportionality between shear stress and velocity gradient. This is an area to keep away from in normal plant design, but for special applications like mine backfill plants, when high solids concentration is critical, it may be desirable to commission a special design based on materials... 

Added filler will increase the viscosity and resistance to flow of a polymer. This is essential in adhesives, which would drain out of the joint by gravity or be sucked out of the glue line by capillary attraction. Viscosity increase becomes larger as the particle size is reduced. Fumed (aerosol) silica, a filler that introduces a structure to the liquid, can produce a thixotropic mixture (Anon, n.d. Byrne, 1984). Ease of handling of a liquid adhesive can be improved by additions to the polymer of 3-10% of its own weight in silica. The thickening effect is greatest in non-polar solvents when the powder is well dispersed. 

When the shear stress of a fluid decreases with the duration of shear strain, the fluid is called thixotropic. The change is then classified as reveisible and structural decay is ob served with time under constant shear rate. Certain thixotropic mixtures exhibit aspects of permanent deformation and are called false thixotropic. 

Fredrickson (1970) discussed the modeling of thixotropic mixtures of suspensions of solids in viscous liquids and proposed that rheological tests be conducted to measure four constants to understand the qualitative nature of the mixture. 

Rheopectic or negative thixotropic mixtures are not common in mining and will not be examined in this chapter. 

The alkoxy titanate compounds formed by reaction of one mole of tetraalkyl titanate with one mole of a dialkanolamine are excellent esterification catalysts for the manufacture of phthalate-based plasticizers (112). If a 1 1 molar mixture of alkanolamine and water is used ia place of the alkanolamine, oligomeric titanate complexes are formed, which have high catalyst activity and can be used as thixotropic additives to paints and other aqueous coating formulations (113). 

The peroxides and accelerator should not be brought into contact with each other as they form an explosive mixture. When the resin is to be used, first the accelerator and then the peroxide are carefully dispersed into the resin, which may also contain inert fillers and thixotropic agents. 

Mixtures of the monotriethanolamine titanate and polyols, such as fructose and sorbitol, imparted thixotropic properties to polymer-containing cement (107). A similar reaction of triethanolamine and tetraalkyl titanates previously treated with stearic acid gives the corresponding 1-acyloxytitatranes (108). Aqueous dispersions of these materials can be used to treat fabrics to impart a high degree of water repellency. 

Thixotropy provides a shear thinning effect that is, viscosity decreases as the shear rate increases, and vice versa. This not only allows easy pumping, dispensing, and mixing of the adhesive, but also provides sag resistance once the adhesive is applied. The thixotropic fillers work by forming a temporary structure in the mixture, which can be broken down at high rates of shear. This structure is generally the result of van der Waals forces between molecules. 

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. 

Comments mixtures of microcrystalline cellulose and carboxymethylcellulose sodium that are dispersible in water and produce thixotropic gels are suitable as suspending vehicles in pharmaceutical formulations. The amount of carboxymethylcellulose present can vary between 8.3% and 18.8% w/w depending upon the grade of material. 

Thixotropic and dilatant behavior of the suspension can be monitored by viscosi-metric measurements as a function of shear rate and the duration of the experiments. Studies with a 25 wt.% suspension of LiChrospher 100, 7 gm in a mixture of dioxane-cyclohexane-n-octanol (45/45/10, v/v/v) have shown that the suspension has rather low thixotropic and pseudo-elastic behavior (Hallmann, 1992). The packing pressure of the axial dynamic column packing technique has a significant influence on the column performance (Tab. 3.13). 

U.S. 4950416 (1990) [50] Baxter (Vista Chemical) Thixotropic liquid aqueous alumina dispersion long-chain fatty acid or salt and mixtures Nonpolymeric and nonclay compositions... 

The mixture of calcium sulfite hemihydrate and calcium sulfate dihydrate (gypsum) comes out of solution as a finely divided precipitate. The product is not saleable and has to be disposed to tip. A further disadvantage of the process is that, because the crystallisation is not controlled, the fine particles form a thixotropic sludge which is difficult to de-water for disposal [12.3]. 

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