Pseudoplastic solution

As substituent uniformity is increased, either by choosing appropriate reaction conditions or by reaction to high degrees of substitution, thixotropic behavior decreases. CMCs of DS >1.0 generally exhibit pseudoplastic rather than thixotropic rheology. Pseudoplastic solutions also decrease in viscosity under shear but recover instantaneously after the shear stress is removed. A plot of shear rate versus shear stress does not show a hysteresis loop. 

Xanthan Made primarily of linear glucose chain with side chains based on mannose and glucuronic acid residues A microbial gum made by glucose fermentation, forms viscous pseudoplastic solutions Emulsifying or suspending agent, particularly in toothpaste and ointments... 

Gives high viscosity, pseudoplastic solutions that show reversible... 

Use of pseudoplastic solution flow curve data and the Graessley model may enable prediction of polymer viscosity average molecular weight. Confirmation of this method for accurate molecular weight determination is presently underway. 

Zaidi A, Deckwer WD, Mrani A, Benchekchou B. Hydrodynamics and heat transfer in three-phase fluidized beds with highly viscous pseudoplastic solutions. Chem Eng Sci 45 2235-2238, 1990. 

The normal pH of a gum karaya dispersion is 4.3 to 4.8. The viscosity is maximum at about pH 8.5 (pH 7.0 to 11.0) but at alkaline pH values, the acetyl groups (content 8%) are removed. As acetyl groups are lost, the characteristic shortbodied (pseudoplastic) solutions become irreversibly transformed into ropy, stringy mucilages. The viscosity of gum karaya dispersions also decreases when salts are added. As with gum arabic, this lowering of viscosity and the concomitant lowering of surface tension improves its ability to stabilize emulsions. 

S. are easily soluble in water to yield pseudoplastic solutions of high stability against heating and changes of pH. They may be derivatized to filmforming products. 

Pseudoplastic fluids are the most commonly encountered non-Newtonian fluids. Examples are polymeric solutions, some polymer melts, and suspensions of paper pulps. In simple shear flow, the constitutive relation for such fluids is... 

The apparent viscosity, defined as du/dj) drops with increased rate of strain. Dilatant fluids foUow a constitutive relation similar to that for pseudoplastics except that the viscosities increase with increased rate of strain, ie, n > 1 in equation 22. Dilatancy is observed in highly concentrated suspensions of very small particles such as titanium oxide in a sucrose solution. Bingham fluids display a linear stress—strain curve similar to Newtonian fluids, but have a nonzero intercept termed the yield stress (eq. 23) ... 

The flow properties of sodium alginate solutions depend on concentration. A 2.5% medium viscosity sodium alginate solution is pseudoplastic, especially at the higher shear rates in the range of 10—10,000/s. 

Properties. Xanthan gum is a cream-colored powder that dissolves in either hot or cold water to produce solutions with high viscosity at low concentration. These solutions exhibit pseudoplasticity, ie, the viscosity decreases as the shear rate increases. This decrease is instantaneous and reversible. Solutions, particularly in the presence of small amounts of electrolyte, have exceUent thermal stabiHty, and their viscosity is essentially constant over the range 0 to 80°C. They are not affected by changes in pH ranging from 2 to 10. 

Solutions of welan are very viscous and pseudoplastic, ie, shear results in a dramatic reduction in viscosity that immediately returns when shearing is stopped, even at low polymer concentrations (230). They maintain viscosity at elevated temperatures better than xanthan gum at 135°C the viscosity half-life of a 0.4% xanthan gum solution is essentially zero, whereas a welan gum solution has a viscosity half-life of 900 minutes (230). The addition of salt to welan solutions slightly reduces viscosity, but not significantly. It has excellent stabiUty and theological properties in seawater, brine, or 3% KCl solutions... 

Effect of Shear. Concentrated aqueous solutions of poly(ethylene oxide) are pseudoplastic. The degree of pseudoplasticity increases as the molecular weight increases. Therefore, the viscosity of a given aqueous solution is a function of the shear rate used for the measurement. This relationship between viscosity and shear rate for solutions of various molecular weight poly(ethylene oxide) resins is presented in Figure 8. 

CMC hydrates rapidly and forms clear solutions. Viscosity buUding is the single most important property of CMC. DUute solutions of CMC exhibit stable viscosity because each polymer chain is hydrated, extended, and independent. The sodium carboxylate groups are highly hydrated, and the ceUulose molecule itself is hydrated. The ceUulose molecule is linear, and conversion of it into a polyanion (polycarboxylate) tends to keep it in an extended form by reason of coulombic repulsion. This same coulombic repulsion between the carboxylate anions prevents aggregation of the polymer chains. Solutions of CMC are either pseudoplastic or thixotropic, depending on the type. 

Solutions of HEC are pseudoplastic. Newtonian rheology is approached by very dilute solutions as well as by lower molecular-weight products. Viscosities change Httie between pH 2 and 12, but are affected by acid hydrolysis or alkaline oxidation under pH and temperature extremes. Viscosities of HEC solutions change reversibly with temperature, increasing when cooled and decreasing when warmed. 

Solutions of methylceUuloses are pseudoplastic below the gel point and approach Newtonian flow behavior at low shear rates. Above the gel point, solutions are very thixotropic because of the formation of three-dimensional gel stmcture. Solutions are stable between pH 3 and 11 pH extremes wiU cause irreversible degradation. The high substitution levels of most methylceUuloses result in relatively good resistance to enzymatic degradation (16). 

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). 

At low shear rates, aqueous solutions of polyacrylamide are pseudoplastic. With increasing shear rates and temperature the viscosity of the solutions decrease. At high shear rates during violent mixing and pumping operations the molecular weight of polyacrylamide decreases by destruction of macromolecules. 

Many microbial polysaccharides show pseudoplastic flow, also known as shear thinning. When solutions of these polysaccharides are sheared, the molecules align in the shear field and the effective viscosity is reduced. This reduction of viscosity is not a consequence of degradation (unless the shear rate exceeds 105 s 1) since the viscosity recovers immediately when die shear rate is decreased. This combination of viscous and elastic behaviour, known as viscoelasticity, distinguishes microbial viscosifiers from solutions of other thickeners. Examples of microbial viscosifiers are ... 

This equation is based on the assumption that pseudoplastic (shear-thinning) behaviour is associated with the formation and rupture of structural linkages. It is based on an experimental study of a wide range of fluids-including aqueous suspensions of flocculated inorganic particles, aqueous polymer solutions and non-aqueous suspensions and solutions-over a wide range of shear rates (y) ( 10 to 104 s 1). 

Pectins in aqueous solutions show pseudoplastic non-thixotropic behaviour, independent of their degree of methoxylation. Figure 1 shows the viscosity curve of a 2,5 % pectin solution, sheared the preselected shear rate-time function. The viscosity curves for the increasing and decreasing shear rate are superimposed. The pseudoplasticity of pectin solutions decreases with decreasing concentration. 

---