Thixotropy control

Some reinforcement is required for polysulfide sealants. For aircraft compounds, some carbon black is needed for higher physicals. For all other applications the chief fillers are the calcium carbonates, which are available as ground marble dust or a precipitated grade. The cheapest sealants use mostly marble dust, but some precipitated grade is needed for thixotropy control. 

Grantham, C.K. and McLaurine, H.C. "Thixotropy Without Viscosity A New Approach to Rheology Control of Oil Muds," SPE paper 15415, 1986 SPE Annual Technical Conference and Exhibition of AIME, New Orleans, October 5 8. 

Since the rheology of many systems depends largely on the temperature, accurate and reproducible measurements require very careful temperature control. A 1°C temperature drop, for instance, increases the apparent viscosity / of an offset printing ink by approximately 15%. To demonstrate the correlation between thixotropy and temperature, Figs. 56 and 57 show the flow curves at different temperatures for two offset printing inks [134], Both materials clearly lose thixotropy-indicated by the area under the thixotropic loop-as the temperature increases. This effect is much more pronounced in the first case (Fig. 56), while the second ink exhibits a very slow decrease thixotropic behavior (Fig. 57). 

Microcrystalline cellulose (MCC) is obtained by a controlled acid treatment intended to destroy the molecular bonding in the amorphous zones of cellulose. Usually HC1 or H2SO4 are used at 110°C for 15 min over native cellulose or regenerated cellulose. Colloidal gels are thus obtained showing thixotropy. MCC is used in the preparation of pharmaceutical compressed tablets due to its binding and disintegration properties. 

Thixotropic agents Thixotropic agents impart thixotropy to the adhesive to control flow and increase viscosity. 

Control of Thixotropy. Often the adhesive application will require that the product be fluid for mixing and application, but it must not flow or sag once applied. For example, ASTM C920 defines a nonsag sealant as one that permits application in joints on vertical surfaces without sagging or slumping. This property is called thixotropy. Thixotropic materials, such as tomato catsup, toothpaste, etc., undergo a decrease in viscosity when subject to shearing. 

Fumed Silica. Today colloidal silica (fumed silica) is the most common thixotropic agent in epoxy resins. Fumed silica, an amorphous silicon dioxide, is a versatile, efficient additive used for flow control and thixotropy. Fumed silica has long been the dominant thixotrope employed in the adhesive and sealant industry. 

Flow characteristics can be regulated by the incorporation of fillers, by the use of scrims or woven tapes as internal shims within the adhesive itself, or by the careful regulation of the cure cycle. All these options along with a few more are available to the adhesive for-mulator and end user. Generally, fillers are incorporated to control the viscosity of the adhesives as well as other properties such as thixotropy and sag resistance. 

For the evaluation of the rheology of the silica dispersions, different test methods were applied (a) a shear rate-controlled relaxation experiment at = 0.5 s (conditioning), 500 s (shear thinning), and 0.5 s (relaxation) to evaluate the apparent viscosity, the relaxation behavior, and thixotropy (b) shear yield-stress measurements using a vane technique introduced by Nguyen and Boger [5] (c) low deformation dynamic tests at a constant frequency of 1.6 s in a stress range of ca. 0.5 - 100 Pa. All samples contained 3 wt% of fumed silica. 

Steady state shear stress a-shear rate y measurements This requires the use of a shear rate-controlled instrument, and the results obtained can be fitted to models to obtain the yield value Up and the viscosity as a function of shear rate. Time effects (thixotropy) can also be investigated. 

Block polymers, owing to the tendency for formation of regular strucmres tailored by molecular design, are ideal models for compatibilized, two-phase polymer blends or alloys. Blends do show similar rheological behavior, e.g., yield, pseudoplasticity, thixotropy, structural rearrangements, but since the morphology is more difficult to control, the interpretation of data could present serious difficulties. 

The silica, however, serves a dual purpose in that in addition to reinforcement it also Imparts thixotropy (flow control) to prevent the sealant from flowing out of the joints before cure. 

Modifiers. A very large number of modifications in the cured and uncured properties of anaerobic formulations can be brought about by the addition of components that have little or no effect on the fundamental anaerobic cure chemistry. These modifiers can increase the viscosity, control thixotropy, add color or fluorescence, increase sealing effectiveness, reduce strength, increase toughness, increase heat resistance, provide lubrication, and reduce settling of fillers. 

There are commercial cellulose-derived compounds in which a combination of derivatives is used for instance, AviccI RC 591 (a registered product of FMC Corp see Ref. 66) includes microciystalline cellulose that provides thixotropy to the suspension, and sodium carboxymethylcellulose. with pseudoplaslic nature, needed to ease the flow of suspension for administration. Its usefulness for rheology control has been checked with many drugs for example. Cordoba et al. (67) have shown that sulfamedioxazole suspensions with 1% Avicel display excellent stability and flow behavior. 

Organophilic clays for controlling rheological properties, especially thixotropy. 

Features Org. vise, increase rapid development of thixotropy butteryness pigment susp. optimum balance between sag control and leveling non-penetration... 

Stability depends are dependent upon the thixotropic aspects of the soil, and this in turn depends upon the electrochemical double layers (the opposite layer of charges which occurs at interfaces between colloidal particles in the surrounding solutions), so that electrochemical considerations control thixotropy, and thus the flow of the soil under pressure. Shifting of the foundation of buildings may, therefore, be a phenomenon which requires some knowledge of electrochemistry for its understanding. 

Fumed silica is widely used in industry as an efficient thickening agent providing shear thinning and thixotropy to liquid media like adhesives, resins, paints, and inks. Various parameters control the rheological performance of fumed silica ... 

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