Thixotropy

Thixotropy comes about first because of the finite time taken for any shear-induced change in microstructure to take place. Microstructure is brought to a new equilibrium by competition between, on the one hand the processes of tearing apart by stress diuing shearing, and on the other hand build-up due to flow- and Brownian-motion-induced collision, over a time that can be minutes. Then, when the flow ceases, the Brownian motion (the only force left) is able to slowly move the elements of the microstructure around to more favourable positions and thus rebuild the structure this can take many hours to complete. The whole process is completely reversible. 

In fact, any shear-induced change in the microstructure of a suspension takes time to occur. This is true for the simplest changes such as the transition from the random-at-rest situation for simple suspensions to the breakdown of floes to primary particles when flocculated suspensions are sheared hard. The former change is often over before the relatively slow mechanics of most viscometers could detect it, but the latter kind of change can take hours or even days to complete  

Thixotropy is a function of time and shear rate, and therefore cannot be properly accounted for in experiments where both these variables are changed simultaneously. This is true in loop tests where either shear rate or shear stress is varied in a triangular fashion with time, increasing linearly from zero to a maximum and back again. The measured results for the up- and down-curves are quite different for the first few loops, and the area between these curves is sometimes used as a measure of thixotropy, but these produce numbers that are impossible to relate to quantitative description of the phenomenon. For this reason, loop tests are not recommended for the serious study of thixotropy. 

The best experiments to properly measure thixotropy are those where the sample to be tested is sheared at a given shear rate until equilibrium is obtained, then as quickly as possible the shear rate is changed to another value. The typical response to such a step-wise change from one steady-state condition to another is, in terms of the viscosity, often characterised by the so-called stretched exponential model  

Further details on the subject of thixotropy may be found in a review published by the author [11]. 

Time-dependent fluid behaviour may be further sub-divided into two categories thixotropy and rheopexy or negative thixotropy. 

The term false body has been introduced to describe the thixotropic behaviour of viscoplastic materials. Although the thixotropy is associated with the build-up of structure at rest and breakdown of structure under shear, viscoplastic materials do not lose their solid-like properties completely and can still exhibit a yield stress, though this is usually less than the original value of the virgin sample which is regained (if at all) only after a long recovery period. 

Other examples of materials exhibiting thixotropic behaviour include concentrated suspensions, emulsions, protein solutions and food stuffs, etc. [Barnes, 1997). 

Example. A colloidal system can exhibit several of these characteristics at once. For example, paint must be plastic and thixotropic so that it will flow when brushed on and (only) immediately after brushing (for a smooth finish) a further benefit is that vigorous mixing readily disperses the pigments which then stay dispersed for some time when standing (high yield stress). 

Finally, shortly after brushing-on, the paint should cease to flow so that it doesn t run . 

10-20 Thick, pours easily, like thin milkshake 

30-40 Thick, hard to pour, forms peaks, can write on surface 

40-100 Flows poorly, will cleave to walls under gravity 

In concrete technology an important field of interest is a self-compacting concrete, which flows and compacts itself due to gravity (20). Therefore, no external vibration or other compaction is needed. It is possible to produce a very high performance concrete as self-compacting concrete. Because compacting work is not needed, the noise level during construction is lowered remarkably. 

There have been attempts to solve these problems using viscosityenhancing agents, for example, with water soluble polysaccharides such as welan gum or cellulose derivates (21). Cellulose fibers have been used in concrete materials in order to improve the mechanical properties of materials (22). 

Microfibrillar cellulose refers to isolated cellulose microfibrils or microfibril bundles that are derived from a cellulose raw material. Microfibrils typically have a high aspect ratio. The niunber average diameter is typically below 200 nm. The smallest microfibrils are similar to so-called elementary fibrils, which are typically 2-12 nm in diameter. 

In addition, microfibrillar cellulose can also be directly isolated from certain fermentation processes. Cellulose producing micro- 

The addition of microfibrillar cellulose increases the paste thixotropy both with and without plasticizer. Microfibrillar cellulose helps to make a self-compacting concrete more robust. Water bleeding and aggregate settlement are diminished and thus concrete durability properties are increased. Water bleeding is effectively prevented with the finest fibril additives. The aggregate settlement is also drastically decreased with microfibrillar cellulose (20). 

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Resin Viscosity. The flow properties of uncured compounded plastics is affected by the particle loading, shape, and degree of dispersion. Flow decreases with increased sphericity and degree of dispersion, but increases with increased loading. Fillers with active surfaces can provide thixotropy to filled materials by forming internal network stmctures which hold the polymers at low stress. 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Another method for estimating thixotropy involves the hysteresis of the thixotropic loop. The area of the thixotropic loop is calculated or measured, which works well with printing inks (3). In a variation of this method, the up curve on an undisturbed sample is deterrnined. The sample is then sheared at high shear (>2000 ) for 30—60 s, followed by deterrnination of the down curve (22). The data ate plotted as Casson-Asbeck plots, vs 7 / (14), as... 

Fig. 8. Casson plots of sheared and unsheared paints. The degree of divergence of the lines is used to estimate thixotropy.

P-Ghloroalkoxy Titanates. The reaction of TiCl with epoxides, such as ethylene or propylene oxide (qv), gives P-chloroalkyl titanates (8,9). One example is Ti(OCH2CH2Cl)4 [19600-95-5]. The P-chloroalkoxy titanates can be used to biad refractory powders and ia admixture with diethanolamine to impart thixotropy to emulsion paints (10). 

Metal complexes prepared by reacting less than one mole of an alkan olamine with an excess of a polyhydric alcohol, such as polyethylene glycol 200—400 or glycerol, reportedly impart a greater degree of thixotropy to systems containing protective organic coUoids (501). 

Asphalts develop an internal stmcture with age, steric hardening (3), in which viscosity can increase upon aging without any loss of volatile material (73,83). Those with a particularly high degree of gel stmcture exhibit thixotropy. 

HPC is available in a number of viscosity grades, ranging from about 3000 mPa-s(=cP) at 1% total soHds in water to 150 mPa-s(=cP) at 10% total sohds. HPC solutions are pseudoplastic and exceptionally smooth, exhibiting Htde or no stmcture or thixotropy. The viscosity of water solutions is not affected by changes in pH over the range of 2 to 11. Viscosities decrease as temperature is increased. HPC precipitates from water at temperatures between 40 and 45°C. Dissolved salts and other compounds can profoundly influence the precipitation temperature (50,81). 

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). 

Process Viscosity, mPa-s(=cP) Cure temperature, °C Thixotropy Filler, % Glass, %... 

To optimize the lesin system foi a given process and part, consideration should be given to fillers that can gready affect the cost and performance of the composite. Because of their low viscosity, fillers can often be added to polyesters. Fillers are often much cheaper than the resin they displace, and they can improve the heat resistance, stiffness, and hardness of the composite. Certain fillers, such as fumed siUca, impart thixotropy to the resin, increasing its resistance to drainage. 

The most important coating appHcation for the nonreactive polyamide resins is in producing thixotropy. Typical coating resins such as alkyds, modified alkyds, natural and synthetic ester oils, varnishes, and natural vegetable oils can be made thixotropic by the addition of dimer acid-based polyamide resins (see Alkyd resins). Specialty high performance coating appHcations often requite the properties imparted by dimer acid components. 

Mewtonian andMon-Mewtonian Materials. A Newtonian material s viscosity is shear-independent, whereas non-Newtonian materials are shear-dependent (Eig. 7). Eor most potting materials, a Newtonian material is preferred because the material is required to flow under all electronic components, but not be susceptible to shear. However, when flowable material is used for conformal coating appHcations, a non-Newtonian material with thixotropy agent added is desired since the material should flow on the electronic substrate but stop at the edge without creeping or mnover at the circuitry. 

Rheopectic behavior is the opposite of thixotropy. Shear stress increases with time at constant shear rate. Rheopeclic behavior has been obsei ved in bentonite sols, vanadium pentoxide sols, and gypsum suspensions in water (Bauer and Colhns, ibid.) as well as in some... 

Conventionally, wastes are eonsidered as being predominantly either solid, liquid or gaseous but as illustrated in Table 16.3, they may be multi-phase. Solid waste eompiises liquid slurries, sludges, thixotropie solids and solids of varying partiele sizes it may be heterogeneous. Typieal examples are given in Table 16.4. 

Because most latices have low viscosities by compounding, most of the waterborne rubber adhesives are sprayable. Thickeners such as fumed silicas can be added to increase viscosity and thixotropy. This means that even at relatively large viscosities (over 10 Pas) many water-based rubber adhesives can be sprayed. Dip and curtain applications require viscosities between 0.05 and 0.3 Pas, whereas brush application works with viscosities between 1 and 50 Pa s. 

Neoprene AG ( 961). It is a high-gel polymer which exhibits a high degree of thixotropy. It is specially designed for spraying applications. 

Fumed silicas (Si02). Fumed silicas are common fillers in polychloroprene [40], natural rubber and styrene-butadiene rubber base adhesives. Fumed silicas are widely used as filler in several polymeric systems to which it confers thixotropy, sag resistance, particle suspension, reinforcement, gloss reduction and flow enhancement. Fumed silica is obtained by gas reaction between metallic silicon and dry HCl to rend silica tetrachloride (SiCU). SiC is mixed with hydrogen and air in a burner (1800°C) where fumed silica is formed ... 

Thixotropy Describes those fluids whose apparent viscosity decreases... 

Appearance of rheological effects — yield stress, non-Newtonian viscosity, thixotropy... 

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