Uses of High-Viscosity Polymer Solutions

The fact that very low concentrations of polymer give highly viscous solutions is exploited commercially in a number of applications. The thickening action of polymers is often necessary for water-based substances, such as foods, toothpastes, or emulsion paints, but examples also occur of the use of polymers to thicken solvent-based products, such as paint stripper. 

The main polymers used as thickeners are modified celluloses and poly(acrylic acid). Several different modified celluloses are available, including methyl-, hydroxypropyl methyl-, and sodium carboxymethyl-cellulose and their properties vary according to the number and distribution of the substituents and according to relative molar mass of the parent cellulose. Hence a range of materials is available, some of which dissolve more readily than others, and which provide a wide spread of possible solution viscosities. Poly(acrylic acid) is also used as a thickener, and is also available in a range of relative molar masses which give rise to give solutions of different viscosities. 

There are numerous applications where the development of high viscosity is necessary in a finished product. For example, thickeners, mainly based on poly(acrylic acid), are used to give body to so-called emulsion paints. Emulsion paints are not formulated from true emulsions (Le. stable dispersions of organic liquids in water), but are prepared from latexes, that is, dispersions of polymer in water. Since latexes do not contain soluble polymers, they have a viscosity almost the same as pure water. As such, they would not sustain a pigment dispersion, but would allow it to settle they would also fail to flow out adequately when painted on to a surface. Inclusion of a thickener in the formulation gives a paint in which the pigment does not settle out and which can readily be applied by brush to a surface. 

Paint strippers are also formulated to have high viscosity, otherwise they run off vertical surfaces and thereby fail to penetrate or solubilise the paint to which they have been applied. Hydroxypropyl methylcellulose is the main thickener for paint strippers, which use methylene chloride (dicldoromethane) as the principal component. Hydroxypropyl methylcellulose is useful for this purpose because it is soluble in the orgaiuc component but is not sensitive to the presence of any water that may also be present in the paint stripper. 

Sodium carboxymethylcellulose is acceptable for use in food, and is employed in a variety of foodstuffs. It is used to prevent formation of ice crystals in ice creams to control the consistency of cheese spreads to stabilise the emulsions needed in salad creams and to thicken toothpaste. 

There are numerous applications where the development of high viscosity is necessary in a finished product. For example, thickeners, mainly based on poly(acrylic acid), are used to give body to scxalled emulsion paints. Emulsion paints 

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The usefulness of xanthan in polymer flooding for enhanced oil recovery is based on its ability to yield large increase in viscosity at low polymer concentrations under high-temperature and high salinity conditions. This important property of xanthan is determined both by its molecular weight and by the conformation adopted in solution (1). 

The technique involving an evacuated capillary viscometer is limited to the measurement of polymer solutions having viscosities less than ca. 103 poise a point which has been made previously by Hadjichristidis and Roovers 132). This is a consequence of the viscometer type (Ubbelohde) which has been used in these determinations. Despite this practical limitation, it has been stated 78) that there is no inherent limit for viscosity measurements, even in conventional types of viscometers provided that the tubes are sufficiently wide. However, the limitations of operating with an evacuated viscometer, and the flow behavior of high viscosity (> 103 poise) polymer solutions, clearly reveals that the foregoing claim is unrelated to reality. 

There is, however, a mass transfer problem of demixing at lower temperatures caused by high viscosities. Concentrated polymer solutions tend to take hours to form two distinct liquid phases. A solution to this problem is the use of the lower critical solution temperature. Because of their thermodynamic nature, all polymer-solvent mixtures tend to form two liquid phases ( LL ) with low viscosities, at higher temperatures (LCST) as depicted in Figure 3. 

When the injection volume was up to 0.1 PV, the water-cut reduction and oil rate increase slowed down, and polymer stopped breaking through high-permeability channels (Xie et al., 2001). The weak gel was made of 400 mg/L HPAM and 60 mg/L Cr. Its viscosity was 130 mPa-s after aging 180 days at 50°C. Compared with 1000 mg/L polymer solution that was used before gel injection, the weak gel cost was reduced by 21% (Fan et al., 2004). Produced water was used to make the polymer solution. It was observed that if the produced water from producers was used inunediately, polymer solution viscosity loss was up to 60%. However, if the produced water was used some time after it was produced, the viscosity loss was significantly reduced (Xie et al., 2001). 

Several wholly aromatic polyamides have been prepared by use of triphenyl phosphite in NMP-pyridine solution containing 4 wt-% LiCl (Table 3). The combination of isophthalic acid with diamines gives a polymer of high viscosity whereas tere-phthalic acid (TPA) with pKa values similar to those of isophthalic acid does not yield high-viscosity polymers. 

A completely fresh and different approach to polymer solutions is being studied in at least two places under the auspices of the Paint Research Institute. Some years ago, Prausnitz (25) reported on polymer segment-interaction theory to predict phase separation. This was based on a lead by Wilson (24) who proposed a local volume fraction concept or domain theory for explaining deviations from ideality. More recently, Prausnitz (27) reported on further work involving anomalous viscosities in high solid polymer solutions. High viscosities may occur when solvent mixtures are used which have values near the extreme ends of the solu-... 

To create an initial porous structure before thermal binder removal, it is also possible to extract parts of the binder system by solvent extraction. In this technique the ceramic green bodies are immersed in a liquid which solvates one of the components in the binder used. A prerequisite of this process is that the binder component to be removed first has a high solubility in the solvent to ensure rapid dissolution. Furthermore the viscosity of the resulting polymer solution should be low, and diffusion of the liquid inside the body should be fast. Swelling of the binder components during solvent extraction can cause defects to be formed inside the ceramic body because of the internal stresses generated. Solvent extraction is a relatively fast and the cost of the equipment relatively low. 

Carboxymethyl cellulose (CMC) plays an important role in food industry. Because of high viscosity, nontoxicity, and nonallergic nature, it is used as a viscosity modifier or thickener and to stabilize emulsions in various products including ice cream. It is also a constituent of many nonfood products, such as jellies, toothpaste, laxatives, diet pills, water-based paints, detergents, textile, and various paper products. In laundry detergents, CMC is used as a soil suspension polymer which creates a negatively charged barrier to soils in the wash solution. CMC is also used as a lubricant in nonvolatile eye drops. 

High-viscosity gels (e.g., the high-Mw LPA) require either in situ polymerization or very high pressure to replace them in the capillary. In contrast, many of the low-viscosity polymer solutions do not require polymerization by the user. It is necessary only to dissolve a known amount of the polymer in basic buffers, such as tris-borate-EDTA (TBE) or 3-[[tris(hydroxymethyl)methyl]amino] propane-sulfonic acid (TAPS) (pH 8-9), or isoelectric buffers, such as His or Lys. Because the low conductivity of the isoelectric buffers minimizes Joule heating, high electric fields can be used for rapid separation. For oligonucleo-... 

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