How to Read a CMC Spec Sheet: Degree of Substitution vs Viscosity vs Purity

Three numbers define most sodium carboxymethyl cellulose (CMC) grades. The degree of substitution (DS), the average number of carboxymethyl groups per anhydroglucose unit, governs solubility, salt and acid tolerance and solution clarity. Viscosity, measured on a dilute solution with a Brookfield viscometer, is a proxy for molecular weight and tells you how strongly the grade thickens, while purity tells you how much of the powder is actually sodium CMC rather than the salts left over from manufacture.

Where each number comes from

CMC is produced in two main steps. Cellulose from cotton linters or wood pulp is first treated with sodium hydroxide to form alkali cellulose, which opens up the structure and activates the hydroxyl groups. It is then etherified with monochloroacetic acid or its sodium salt, which attaches carboxymethyl groups to the cellulose chain.

• Main reaction: cellulose–ONa + ClCH₂COONa → cellulose–O–CH₂COONa + NaCl.
• Side reaction: NaOH + ClCH₂COONa → HOCH₂COONa (sodium glycolate) + NaCl.
• Purification: the salts are washed out with aqueous alcohol for refined grades, or left in for technical grades.

The ratio of reagent to cellulose and the reaction efficiency set the DS. The chain length of the starting cellulose, and how much it is deliberately or accidentally degraded during alkalisation, set the molecular weight and therefore the viscosity. The washing step sets the purity.

Degree of substitution

Each anhydroglucose unit in cellulose carries three hydroxyl groups, at the C2, C3 and C6 positions, so the theoretical maximum DS is 3. Commercial CMC typically lies between about 0.4 and 1.5; below roughly 0.4 the polymer swells but does not fully dissolve in water. The food-additive specifications of JECFA and GB 1886.232-2016 both allow DS 0.20–1.50.

DS is usually calculated from sodium content. In the JECFA method, an alcohol-washed, salt-free sample is ashed to sodium sulfate, its sodium content is calculated, and DS = 162 × %Na ÷ (2300 − 80 × %Na). On this basis a sodium content of about 8.85 % corresponds to DS 0.90, and 9.50 % corresponds to DS 1.00. ASTM D1439 also describes acid-wash and non-aqueous titration methods.

Why DS drives salt and acid tolerance

Every carboxymethyl group carries a negative charge in solution. The charges repel one another, keep the chain extended and keep it surrounded by water. Salt screens these charges: the chains contract, viscosity falls and, in low-DS grades, the long unsubstituted stretches of chain associate through hydrogen bonding into haze, gels or precipitate. A higher and more uniform DS leaves fewer such stretches, so the polymer stays dissolved in brine.

Acid works in a similar way. The carboxyl groups have an apparent pKa of roughly 4 to 4.5, so as the pH falls they become protonated and the chain loses charge. A high-DS CMC keeps enough ionised groups to stay soluble and protective, which is why acidified milk drinks use Type 9 food grades with DS ≥ 0.90. Multivalent cations such as aluminium, iron(III) or copper can cross-link CMC and precipitate it outright; the JECFA identification test uses exactly this reaction.

Uniformity and clarity

Two grades with the same average DS can behave differently if one is substituted more evenly. Uniform substitution, the defining feature of polyanionic cellulose (PAC), gives clearer solutions with fewer fibres and gels and better salt tolerance. A simple check is to compare viscosity in deionised water and in a salt solution, which is why GB/T 5005 tests drilling grades in fresh water, 4 % brine and saturated brine.

Viscosity: what the number really means

Viscosity depends on molecular weight (degree of polymerisation), concentration, temperature, shear rate, dissolution time and the ions present in the water. CMC solutions are shear-thinning, so the same solution reads lower at a higher spindle speed, and viscosity falls as temperature rises, so a reading taken at 20 °C will be noticeably higher than one taken at 25 °C.

Most specifications use a Brookfield LV rotational viscometer at 25 °C on a solution prepared on a dry basis, with the spindle and speed chosen for the range. GB 1886.232-2016 measures food CMC at 2 % and switches to a 1 % solution when the 2 % value would reach 2,000 mPa·s or more, with the sample conditioned in a 25 °C water bath. Sentai measures viscosity at 25 ± 0.1 °C.

1 % versus 2 % solutions

Viscosity rises steeply and non-linearly with concentration. Doubling the concentration typically raises viscosity several-fold, and by an order of magnitude or more for high-molecular-weight grades, so a 2 % figure cannot be converted to a 1 % figure with a fixed factor. This is why Sentai's food range quotes its FL and FM grades at 2 % and its FH and FVH grades at 1 %, following the standard's rule. Always compare grades at the same concentration and by the same method.

Purity: refined, high-purity, semi-refined and technical grades

Purity is the sodium CMC content on a dry basis, calculated as 100 % minus sodium chloride and sodium glycolate. It decides how much active polymer you are buying and how much salt you add to your product. Moisture is reported separately, so correct for both when you calculate a dose.

Mapping DS to applications

Reading a spec sheet in practice

1. Identify the chemistry and the standard the grade is tested against, such as GB 1886.232-2016 for food or GB/T 5005 for drilling.
2. Check DS against your pH and electrolyte load: neutral, low-salt systems tolerate DS 0.7–0.9; acidic, salty or hard-water systems need DS ≥ 0.9.
3. Read the viscosity window with its concentration, method and temperature, and check its width as well as its midpoint.
4. Check purity, chloride and moisture, and calculate the dose on a dry, active basis.
5. Look for application-specific lines: gel count for batteries, filtrate loss for drilling, heavy metals and microbiology for food.
6. Ask for the certificate of analysis of a recent lot, not only typical values.

How Sentai can help

Sentai specifies every grade across its 13 CMC, PAC and CMS product families with a stated DS, viscosity window, solution concentration and test method, so grades can be compared like for like. If you are comparing a current supplier's material, send the technical data sheet or a sample and the applications team will map it to the closest Sentai grade and explain any differences in test conditions. Free 100–300 g samples are available for laboratory evaluation.