Sodium carboxymethylcellulose, a water-dispersible, colorless, odorless, nontoxic powder, does not ferment under normal conditions of use. Consequently, it is preferred to starch for applications in other than high-pH and salt-saturated muds. CMC costs more than starch, and less is needed to reduce filtration rate and cake thickness.
Composition. The water-dispersible cellulosic polymers are made by chemical modification of water-insoluble cellulose, which furnishes the polymeric backbone. Although the basic unmodified cellulose chain is composed of repeating anhydroglucose rings, each of which has three hydroxyl groups capable of substitution (see Figure 11-11) fibrous cellulose is a complex structural mixture of crystallites and amorphous material.1061' 07a-,08c Consequently, in the preparation of cellulose derivatives, a single cellulose chain will show differences in availability to reaction depending upon the structure, and the substitution will not be uniform.
Figure 11-12 shows two anhydroglucose units, illustrating sodium carboxymethylcellulose formation by reaction with a primary alcohol (left) and a secondary alcohol (right). Such regularity of substitution is solely for illustration. If such a replacement of hydroxyls continues throughout the cellulose molecule, the resultant CMC has a degree of substitution (DS) of one. Such regularity, however, is not to be expected, because of structural heterogenerity. DS. therefore, simply means that on the average, a certain degree of substitution has been made. DS of commercial CMC ranges from 0.5 to 1.5. Solubility of CMC in water increases with DS.
Cellulose chain length or degree of polymerization (DP) (the number of anhydroglucose units in the molecule) determines the molecular weight and the viscosity of the aqueous suspension. The higher the molecular weight, the higher the viscosity of the suspension. Viscosity measurements in dilute suspensions serve to characterize polymer molecules. DP ranges from 500 to 2,000.
A variety of products can be made by controlling the molecular weight of the cellulose chain and the degree of substitution.
Figure 11-11. Simplified structural formula for cellulose chain composed of repeating anhydroglucose units, each having three hydroxyl groups capable of substitution.
Production. Chemically purified cellulose is treated with caustic soda solution to form alkali cellulose.144 During this step in the process, some reduction in molecular weight may occur. Selection of the cellulose source affords some control of DP. For example, cotton linters form high-viscosity CMC, wood pulp forms medium viscosity, and alkali cellulose forms low viscosity.
The addition of monochloracetic acid or sodium monochloracetate to form sodium carboxymethylcellulose is the next step in the process. Sodium chloride is formed as a by product, and some of the sodium monochloracetate is converted to sodium glycolate. The excess sodium hydroxide, if any, is neutralized. The impurities are removed by an alcohol-water wash before the pure-grade product is dried and ground. The technical-grade product may contain around 30% sodium chloride.
CMC in Drilling Fluids. Sodium carboxymethylcellulose is an anionic polymer (see Chapter 4) and is adsorbed on clays. Filtration is sharply reduced by low concentrations of CMC, and especially by the higher molecular weight (and higher viscosity) products. The low-viscosity grade CMC is used for filtration reduction of heavily weighted muds. The medium-viscosity grade is used in muds having the common range of solids contents.
CMC suspensions are shear-thinning; they have high apparent viscosities at very low shear rates. The apparent viscosity decreases with rise in temperature (as is the case with other polymers). The viscosity at 300 F (150 C) is about one-tenth that at 80 F (27 C).135 Thermal degradation of CMC is accelerated as temperature approaches 300°F (150°C).
The effectiveness of CMC in reducing filtration and raising viscosity decreases as salt concentration increases. Some calcium-containing muds are thinned by small additions of CMC. As is observed with starch. CMC is coprecipitated along with calcium and magnesium by raising the pH.