Thixotropic systems

Thermoplastic polyamides are used in coatings to modify alkyd resins (qv) in thixotropic systems (238) and to plasticize nitroceUulose lacquers (239). DETA-taH oil fatty acid-based polyamides are suggested for use as corrosion inhibitors in alkyd paints (240). Printing inks for fiexo-gravure appHcation on certain paper, film and foil webs rely on EDA- and PDA-based polyamides for their specific performance (241). 

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.). 

A study was made of the comparative stabilities at various exposures of an upset stomach remedy suspension. This product consisted of a dispersion of bismuth subsalicylate and phenylsalicylate in an aqueous system. Methyl-cellulose and magnesium aluminum silicate were selected as the suspending agents, because the presence of polyvalent metallic ions precluded the use of hydrocolloids affected by these ions. In addition, it was found that methyl-cellulose contributed a demulcent effect. The viscosity, as well as the suspension characteristics of the combination of protective colloids used, was of a synergistic nature. These colloids formed a thixotropic system. The thixotropy undoubtedly aids in stabilizing this system. In order to make this product palatable and impart elegance, color and flavor were added. Sample 1 (with protective colloids) showed no separation, while sample 2 (without... 

Mixed thermal/thixotropic systems improved resistance to leakage may be realized for low or moderate melting point systems. Above its melting point, the liquid can still be immobile because of thixotropy. 

Newton s law states that for a liquid under shear, the shear stress T is proportional to the shear rate. In this sense, most of the unpigmented vehicles used in the paint and printing ink industries are considered ideal or Newtonian liquids. The ratio of the shear stress t to the shear rate D is thus a constant t), dependent only on temperature and pressure. This is not true for specialized gel varnishes and thixotropic systems, which are designed to have special rheological properties. 

An increasing pigment concentration is associated with more or less pronounced thixotropy (Figs. 53 and 54). Thixotropic systems have a gel structure. 

Thixotropy is the time-dependent analogue of shear-thinning and plastic behaviour, and arises from somewhat similar causes. If a thixotropic system is allowed to stand and is then sheared at a constant rate, the apparent viscosity decreases with time until a balance between structural breakdown and structure re-formation is reached. If the sheared system is then allowed to stand, it eventually regains its original structure. A thixotropic hysteresis loop (Figure... 

Goodeve and Whitfield (1837-39) studied the equilibrium between the spontaneous building of an internal structure and its breakdown. The apparent viscosity of a thixotropic system was measured at different rates of shear. If r)r represent the apparent viscosity and extrapolated residual viscosity, respectively, then we have,... 

Say, for example, it is not possible to suspend the necessary amount of particles in a thixotropic system, or that similarly in the hybrid it was not possible to burn the metal efficiently because the large quantities of metal particles must be projected into the flame from a decomposing surface creating only a small amount of gaseous components. Then it is possible to place small quantities of metal in both the liquid and solid to obtain higher combustion efficiency and thus performance. Figure IV.B. 1. represents this approach as the gelled hybrid (F). 

Brush marks. Lines of unevenness that remain in the dried paint film after brush application. Brush marks and ropiness are associated with poor flow and sticky application. These defects are more often encoimtered in highly pigmented products and in certain latex paint formulations. Tbo rapid recovery of consistency in a thixotropic system will also cause these defects. The remedy may be the addition of a flow promoter, reduction in consistency, or modification of the rheological properties. 

Flooding. An extreme form of floating in which pigment floats to produce a uniform color over the whole surface which is markedly different from that of a newly applied wet film. Again, thixotropic systems or specialist additives will provide a remedy. 

Thixotropy is a rheological property that results in yield stress on standing. Thixotropic flow is defined as a reversible, time-dependent, isothermal gel-sol transition. Thixotropic systems exhibit easy flow at relatively high shear rates. However, when the shear stress is removed, the system is slowly reformed into a structured vehicle. The usual property of thixotropy results from the breakdown and buildup of floccules under stress. A small amount of particle settling takes place until the system develops a sufficiently high yield value. The primary advantage of thixotropic flow is that it confers pourability under shear stress and viscosity and sufficiently high yield stress when the shear stress is removed at rest. 

FIGURE 8.8 Flow behavior of a symmetrically thixotropic system. (Adapted from Harris, J., Rheology and Non-Newtonian Flow, Longman Group, Ltd., London, 1977.)... 

Some systems exhibit flow behavior opposite of thixotropic systems, that is, viscosity increases with increasing shear rate. Such fluids are referred to as dilatant or rheopectic. This type of behavior is not common for liquid products containing a low concentration of the dispersed phase. 

The solution system, polyisoprene in benzene does not show the Ostwald type of flow curves that is, it is not a thixotropic system. The flow curves in this case are a general type of non-Newtonian or pseudoplastic flow. We applied (1) to this system the results are summarized in Table III. As shown in Table III, the values of are about constant irrespective of... 

Coagulation structures are formed by pigments and fillers of paints, varnishes and polymers. Spacial networks that form during coagulation of clay suspensions by electrolytes represent a characteristic example of thixotropic systems. Due to their ability to undergo structuring in aqueous media, finely dispersed bentonite and montmorillonite clays are widely used as main components of drilling liquids (see Chapter IX, 3) [32]. 

The full rheological curve of such thixotropic system may be presented as a graph showing the effective viscosity, ry- = x/y, as a function of the shear stress, x, (Fig. IX-25). In this Figure r min corresponds to viscosity of the system with completely disintegrated structure. For the above mentioned bentonite... 

Usually, when one deals with thixotropic systems, instead of equilibrium (true) values of rheological characteristics some reference (imaginary) values are measured, such as those recorded after a particular period of time had elapsed since a complete disintegration of thixotropically reversible structure. In some cases, in order to reach an equilibrium between... 

Curve 2 of Figure 2 shows a typical rheopexic hysteresis where the shear rate is increased slowly at 7 < yx, held constant for some time at 7 = 7 and is finally decreased gradually with a constant speed. This behavior is a reverse phenomenon of the thixotropy and is sometimes called antithixotropy. It occurs much less frequently than thixotropy and appears to be restricted to suspensions of very anistropic particles such as needles. In general, the models used for thixotropic systems can be adopted to simulate the behavior of the rheopexic systems. 

Smaller equilibrium strains than those for thixotropic systems Larger equilibrium strains than those for viscoelastic systems... 

Chem. Descrip. Dipropylene glycol dimethyl ether CAS 111109-77-4 EINECS/ELINCS 404-640-5 Uses Solvent, coupling agent, coalescent, water remover for coatings (PU coatings, latexes, solv.-based), agric., resins, cleaning prods., electronics cleaners, paint removers diluent for thixotropic systems in extraction processes formulation stabilizer aprotic reaction medium for polymerization reactions... 

Like plastic systems, a thixotropic system is transferred from a gel to a flowing system by exerting a stress o > o, the yield stress. This may be achieved by simply shaking or stirring. In contrast to plastic behavior, after releasing the stress, thixotropic systems need a finite time to recover the gel state. 

Hydrogel materials that show thixotropic behavior are very interesting for biomedical applications and are potentially useful candidates for the direct injection of delivery matrices. Under stress (shaking or agitation, for example), a thixotropic system thins, becomes less viscous and flows, but after the period of stress the viscosity increases and thickening is restored over time. For the encapsulation and delivery of delicate or thermally non-robust goods (cells or proteins), this appears to be a particularly useful concept they can be injected through a thin needle under pressure into a tissue because of their thixotropic character. 

---