Thixotropic substances

Smith, T. L. Mechanical Properties of Gels and Thixotropic Substances, Inst, of Phys. Handbook, Margo Hill Book Co., New York, 1963. 

Thixotropic. The flow rate of a thixotropic substance increases with increasing duration of agitation, as well as with increased shear stress. When agitation is stopped, internal shear stress exhibits hysteresis. Upon reagitation. generally less force is required to create a given flow than is required for first agitation. 

Chemists also make the paint that goes on the walls of a house. Paint is a thixotropic substance. Thixotropic means that the paint liquefies when it is subjected to shearing forces. In other words, the paint is thin and easy to spread as long as the paintbrush is stroking the paint onto the wall. But when the brush strokes stop, the paint thickens. 

This behavior is typical of a thixotropic substance where the structure is a function of the shear strain and die shear strain rate. It is imperative therefore that cognizance be taken of any pre-shearing prior to an experimental observation. Furthermore evidence will be presented later confirming that shearing can alter the initial or equilibrium state. A technique of investigating the rate of recovery of the structure will also be discussed. 

Fig. 1. Behavior of non-Newtonian substances 0) true plastic (sometimes called a Bingham body (2) pseudo plastic (3) dllatant (4) thixotropic and (5) rheopectic...

Rheopectic. If certain thixotropic suspensions are rhythmically shaken or tapped, they will set or build up very rapidly, a phenomenon termed rheopexy. Apparent viscosity of a rheopectic substance increases with time (duration of agitation) at any constant shear rate. 

Thixotropic gels (TPGS) of mono- and diglycerides have been used as carriers forsolid dispersions. In the development of a hard gelatin capsule formulation of a poorly water-soluble drug, propantheline bromide, the drug substance was Lrst dissolved in Miglyol 829 and the solution was then converted... 

Most polymeric substances are lime dependent to some extent and) = r] y, t), where t here refers to the time under shear. If shearing causes a decrease in viscosity the material is said to thixotropic, the opposite behavior characterizes a rheopectic substance. These patterns are sketched in Fig. 11-27. After... 

For some substances their viscosities may show time dependent changes. It may decrease with time under shear but return to its original value when the shear stress is removed. Such material is termed thixotropic. Negative thixotropic refers to substances that display the opposite behavior. Other fluids are solid-like and will not flow till some critical yield stress is exceeded. Such fluids are known as viscoplastic. ... 

A nonaqueous dentifrice gel and/or paste composition is disclosed in U.S. Patent 6,331,291 Bl, cited as a thixotropic and smooth-flowing substance. Brookfield viscosity values are reported in the patent for illustrative examples of the invention using Spindle no. 6 at 10 r/min and 23.5°C. The preferred viscosity range is 75,000 to 150,000 cP. 

Substances that behave in this way are called thixotropic (thik suh TROH pik). Some clays are thixotropic suspensions, of particular significance in the construction of buildings in earthquake zones. These clays can form liquids in response to the agitation of an earthquake, which can result in the collapse of structures built on the clay. 

Viscosity enhancing substances not only increase the viscosity but also introduce rheological qualities such as (pseudo) plastic, dUatant and thixotropic behaviour (see Sect. 18.2.1). 

The type of viscosity of a (partially) neutralised carbomer hydrogel is plastic a (large) yield value and a decreasing viscosity with increasing shear stress a carbomer gel is not thixotropic (see Sect. 18.2). Carbomer hydrogel is not compatible with a number of active substances because of the pH (about 6.5) that is needed for the gel formation. For instance salicylic acid is ionised at this pH and hence not effective as a keratolytic. Weak base salts (e.g. alkaloid, chlorhexidine, aluminium and zinc salts) will precipitate at this pH. 

The suspension should meet the following demands it must be soluble in the dispersion medium or be able to swell the necessary concentration must be as low as possible the micropores of the adsorbent must not be clogged when the adsorbent is activated (200-500 C), decomposition products must not affect the surface, or there must be no decomposition at all and it should not have 8iny tendency to foam during evaporation. Suitable substances are colloids, which in addition should be thixotropic. They make the filling of very long capillaries feasible. 

Screen-printing involves the movement of a thixotropic fluid through a mesh screen, which defines the shape and size of the desired electrode, on to the desired substrate material beneath. The thixotropic fluid contains a variety of substances typically including graphite, carbon black, solvents, and polymeric binder with the exact constituents and ratios... 

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