Xanthan gum viscosity

M. E. Slodki and M. C. Cadmus. High-temperature, salt-tolerant enzymic breaker of xanthan gum viscosity. In E. C. Donaldson, editor. Microbial enhancement of oil recovery recent advances Proceedings of the 1990 International Conference on Microbial Enhancement of Oil Recovery, volume 31 of Developments in Petroleum Science, pages 247-255. Elsevier Science Ltd, 1991. 

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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. 

Xanthan gum dissolves in acids and bases, and under certain conditions, the viscosity remains stable for several months. Xanthan gum has exceUent StabiHty and compatibUity with high concentrations of many salts, eg, 15% solutions of sodium chloride and 25% solutions of calcium chloride (79). 

Welan has similar properties to xanthan gum except that it has increased viscosity at low shear rates and improved thermal stabiUty and compatibihty with calcium at alkaline pH (90). The increased thermal stabiUty has led to its use as a drilling mud viscosifter especially for high temperature weUs. The excellent compatibihty with calcium at high pH has resulted in its use in a variety of specialized cement and concrete appHcations. 

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... 

Low molecular weight (1000—5000) polyacrylates and copolymers of acryflc acid and AMPS are used as dispersants for weighted water-base muds (64). These materials, 40—50% of which is the active polymer, are usually provided in a Hquid form. They are particularly useful where high temperatures are encountered or in muds, which derive most of their viscosity from fine drill soHds, and polymers such as xanthan gum and polyacrylamide. Another high temperature polymer, a sulfonated styrene maleic—anhydride copolymer, is provided in powdered form (65,66). AH of these materials are used in relatively low (ca 0.2—0.7 kg/m (0.5—2 lb /bbl)) concentrations in the mud. 

For products intended to remain stable dispersions for an extended period, a particle size of 2 p.m or less is desirable. A thickening agent is usuaUy added after the reaction has been completed and the mixture is cooled in order to prevent settling and agglomeration. Examples of thickeners are guar gum, xanthan gum, and hydroxyethylceUulose. The final products are generaUy between 40 and 50% soUds, with a viscosity of 1500 5000 mPa-s(=cP). 

The alternative large scale recovery method to precipitation is ultrafiltration. For concentration of viscous exopolysaccharides, ultrafiltration is only effective for pseudoplastic polymers (shearing reduces effective viscosity see section 7.7). Thus, pseudoplastic xanthan gum can be concentrated to a viscosity of around 30,000 centipoise by ultrafiltration, whereas other polysaccharides which are less pseudoplastic, are concentrated only to a fraction of this viscosity and have proportionally lower flux rates. Xanthan gum is routinely concentrated 5 to 10-fold by ultrafiltration. 

It has a high viscosity (thickness) even when very little is used. When mixed with xanthan gum or locust bean gum, the viscosity is more than when either one is used alone, so less of each can be used. 

For modestly viscous oils—those having viscosities of approximately 20-100 centipoise (cP)-water-soluble polymers such as polyacrylamides or xanthan gum have been used to increase the viscosity of toe water injected to displace oil from toe formation. For example, polyacrylamide was added to water used to waterflood a 24 cP oil in toe Sleepy Hollow Field in Nebraska. Polyacrylamide was also used to viscosify water used to flood a 40 cP oil in the Chateaurenard Field, France. With this process, toe polymer is dissolved in toe water, increasing its viscosity. 

Most of the xanthan gum used in oil field applications is in the form of a fermentation broth containing 8% to 15% polymer. The viscosity is less dependent on the temperature in comparison with other polysaccharides. 

For suspensions primarily stabilized by a polymeric material, it is important to carefully consider the optimal pH value of the product since certain polymer properties, especially the rheological behavior, can strongly depend on the pH of the system. For example, the viscosity of hydrophilic colloids, such as xanthan gums and colloidal microcrystalline cellulose, is known to be somewhat pH- dependent. Most disperse systems are stable over a pH range of 4-10 but may flocculate under extreme pH conditions. Therefore, each dispersion should be examined for pH stability over an adequate storage period. Any... 

Certain mixtures of polymers have been shown to form complexes which exhibit substantially higher than expected solution viscosity under low shear conditions. Xanthan gum blends with guar gum (38, 39), sodium poly(styrene sulfonate) (40), polyacrylamide (41), sulfonated guar gum (38), sodium poly(vinylsulfonate) (40), hydrolyzed sodium poly(styrene sulfonate-co-maleic anhydride) (38), and poly(ethylene oxide) (41) and blends of xanthan gum and locust bean gum have exhibited substantially higher than expected solution viscosity (42, 43). 

Compared to partially hydrolyzed polyacrylamide, xanthan gum is more expensive, more susceptible to bacterial degradation, and less stable at elevated temperatures (1). However, xanthan gum is more soluble in saline waters, particularly those containing divalent metal ions generally adsorbs less on rock surfaces and is substantially more resistant to shear degradation (1,34). The extensional viscosity of the semi-rigid xanthan molecule is less that that of the flexible polyacrylamide (263). 

When dissolved in more saline waters, xanthan gum produces a higher apparent viscosity than the same concentration of polyacrylamide (292). Prehydration of xanthan in fresh water followed by dilution in the saline injection water has been reported to provide higher viscosity than direct polymer dissolution in the same injection water. Optical rotation and intrinsic viscosity dependence on temperature indicate xanthan exists in a more ordered conformation in brine than in fresh water (293). 

Xanthan gum shows a good solubility in water, giving a highly viscous solution with a pseudoplastic appearence and a temperature independent viscosity. Xanthan gum is used in pharmaceuticals for its excellent emulsifying and suspending properties. The pseudoplastic properties of this gum enables tooth pastes and ointments both to hold their shape and to spread readily. 

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