| Sodium Carboxymethylcellulose Chemistry |
| CMC is a cellulose ether that is produced by reacting
alkali cellulose with sodium monochloroacetate under rigidly controlled
conditions. Figure 1 shows the structure of the cellulose molecule. It is visualized as a polymer chain composed of repeating cellobiose units (in brackets). These, in turn, are composed of two anhydroglucose units (ß-glucopyranose residues). In this structure, n is the number of anhydroglucose units (which are joined through 1,4 glucosidic linkages), or the degree of polymerization, of cellulose. |
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Figure 1 Structure of Cellulose  |
| Each anhydroglucose unit contains three hydroxyl groups, shown in white. By substituting carboxymethyl groups for some of the hydrogens of these hydroxyls, as shown in Figure 2, sodium carboxymethylcellulose is produced. The average number of hydroxyl groups substituted per anhydroglucose unit is known as the “degree of substitution,” or DS. If all three hydroxyls are replaced, the maximum theoretical DS of 3.0 (impossible in practice) results. |
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9004-32-4 Cellulose, carboxymethyl ether, sodium salt |
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Figure 2 Idealized Unit Structure of CMC, with a DS of 1.0  | |
| Optimum water solubility and other desirable physical
properties of CMC are obtained at a much lower degree of substitution than
3. The most widely used types of Aqualon CMC have a DS of 0.7, or an
average of 7 carboxymethyl groups per 10 anhydroglucose units. Higher
degrees of substitution give products having improved compatibility with
other soluble components. Cellulose ethers, such as CMC, are long-chain polymers. Their solution characteristics depend on the average chain length or degree of polymerization (DP) as well as the degree of substitution. Average chain length and degree of substitution determine molecular weight of the polymer. As molecular weight increases, the viscosity of CMC solutions increases rapidly. Approximate values (weight averages) for the degree of polymerization and molecular weight of several viscosity types of Aqualon CMC are given in Table I. |
| Table I Typical Molecular Weights for Representative Viscosity Types of Aqualon CMC (DS = 0.7 in All Cases) | ||
| Viscosity Type | Degree of Polymerization | Molecular Weight |
High |
3,200 |
700,000 |
| The degree of neutralization of carboxymethyl groups
also impacts viscosity. In solution, the degree of neutralization is
controlled by the pH. At the end of the carboxymethylation, the reaction mixture contains a slight excess of sodium hydroxide, which is usu-ally neutralized. Although the neutral point of CMC is pH 8.25, the pH is generally adjusted to about 7-7.5. If the pH to which the CMC is neutralized is 6.0 or less, the dried product does not have good solubility in water; solutions are hazy and contain insoluble gel particles. If the pH is 4 or below, the dried product is insoluble in water. Back |
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