Sodium carboxymethyl cellulose is the most widely used water-soluble cellulose ether, and it is also the one most often specified by the wrong number. Buyers tend to lead with viscosity, but the parameter that decides whether the polymer dissolves, tolerates salt and survives a formulation is the degree of substitution. This guide covers both, the dissolution technique that causes most on-site failures, and how CMC compares with the other cellulose ethers. The specification itself is on the sodium CMC product page.
What the molecule is
CMC is cellulose that has been reacted with sodium hydroxide and sodium chloroacetate so that some of the hydroxyl groups on the anhydroglucose backbone carry a carboxymethyl group (-CH2COONa). The result is a linear, water-soluble, anionic polymer. Its properties are set by two independent variables: how many of those groups are attached (the degree of substitution) and how long the cellulose chain is (the degree of polymerisation, which is what shows up as viscosity).
| Feature | Value for this grade | What it controls |
|---|---|---|
| CAS | 9004-32-4 | The generic polymer; individual grades are distinguished by DS and viscosity, not by CAS |
| Appearance | Fine, white to cream powder | Handling, dispersion and colour of the final product |
| Purity | 99.5% min | Active content; the balance is moisture, sodium chloride and sodium glycolate from the reaction |
| Degree of substitution | 0.7 – 0.9 | Solubility, charge density, salt tolerance, compatibility |
| Viscosity (1%, 25 deg C, dry basis) | 2500 – 3000 mPa.s | Chain length; sets dose rate and pumpability |
| pH (1% solution) | 6.0 – 8.5 | Stability of the solution and residual alkali |
| Moisture | 8% max | Caking in storage, and the dry-basis calculation of viscosity |
| Ionic character | Anionic | Incompatibility with cationic materials; response to multivalent ions |
Degree of substitution: the parameter that decides everything else
DS is the average number of carboxymethyl groups per anhydroglucose unit, on a scale from 0 to 3. Each group contributes one negative charge and one point of water affinity, so DS sets the charge density and the solubility at the same time. It is not a quality ranking – it is a design choice, and the grade has to match the application.
| DS band | Behaviour in water | Salt and acid tolerance | Where it is used |
|---|---|---|---|
| Below about 0.4 | Not fully soluble; swells only | Very poor | Rarely sold as a soluble grade |
| 0.7 – 0.9 | Fully soluble in cold water; this is the general-purpose band | Moderate | This grade – food, detergent, paper, textile, general thickening |
| 0.9 – 1.2 | Soluble, higher charge | Better | Hard-water formulations, higher electrolyte loads |
| Above 1.2 | Soluble, strongly anionic | Good – tolerates hard water and brine | Detergents in hard water, oilfield fluids, textile printing |
Two consequences follow directly. First, a DS 0.7-0.9 grade is not the right choice for a brine-based drilling fluid or a hard-water detergent, however good its viscosity looks on the certificate – the polymer will salt out or lose its effectiveness. Second, DS and viscosity are independent: you can have a high-viscosity low-DS grade or a low-viscosity high-DS grade, and they are not interchangeable.
Viscosity: a number that only means something with its conditions
Viscosity is measured on a 1% solution at 25 deg C on a dry basis, and those three qualifiers are not decoration. Change any one of them and the reading moves – sometimes by a factor rather than a percentage.
| Variable | Effect on the measured viscosity | Practical implication |
|---|---|---|
| Concentration | Strongly non-linear; doubling the concentration can multiply the viscosity several times | A 2% solution cannot be compared with a 1% figure at all |
| Temperature | A few degrees moves the reading noticeably; solutions thin as they warm | The 25 deg C condition is part of the specification |
| Basis (dry vs as-supplied) | Moisture dilutes the active content, so an as-is figure reads lower | “On dry basis” has to be stated, or two lots cannot be compared |
| Shear rate and spindle | CMC is pseudoplastic, so the reading depends on the rotational speed | The spindle and speed belong on the certificate alongside the value |
| Dissolution completeness | Undissolved aggregates leave the reading low and unstable | A low reading on arrival may be a laboratory artefact, not a bad lot |
| Storage and age | Moisture uptake and slow chain degradation drift the value | Re-test after long storage rather than trusting the original figure |
| Viscosity band | Typical 1% value | Typical use |
|---|---|---|
| Low | Below about 100 mPa.s | Dispersing, suspension stabilising, tablet binding |
| Medium | About 300 – 1000 mPa.s | Detergent anti-redeposition, paper coating |
| High | About 1500 – 3000 mPa.s – this grade | Food thickening, textile sizing, general viscosity building |
| Ultra-high | Above about 5000 mPa.s | Where a very small addition is wanted, and pumpability has to be designed for |
Dissolution: cold water, and a technique that matters
CMC dissolves in cold water and does not dissolve in hot. Above roughly 55-60 deg C the particles swell and skin over instead of dissolving, and once a skin has formed it is very slow to hydrate. That single behaviour explains most “the CMC did not dissolve” complaints.
| Approach | How to do it | When it is the right choice |
|---|---|---|
| Direct addition to cold water | Sprinkle the powder slowly into the vortex of vigorously stirred cold water; never dump it in | Simple batches with good agitation |
| Dry blending | Pre-blend the powder with the other dry ingredients, then add the blend to water | Food and detergent formulations where other powders are present anyway |
| Pre-dispersion in a non-solvent | Wet the powder with ethanol, propylene glycol or a light oil, then add to water | Where agitation is weak, or where lumps cannot be tolerated |
| Hot-water dispersion | Disperse in hot water where it will not dissolve, then cool with agitation | Large tanks without high-shear mixing |
Once dispersed, a high-viscosity grade can take hours to reach full viscosity. The solution thickens progressively, so a batch that looks thin after ten minutes is not necessarily a failed batch – it may simply not be finished hydrating.
Rheology and compatibility
| Property | What it means | Where it bites |
|---|---|---|
| Pseudoplastic (shear-thinning) | Viscosity falls as shear rises, then recovers at rest | Good for suspending solids; pump and mixer sizing must allow for it |
| Anionic charge | The polymer carries negative charges along the chain | Precipitates with cationic surfactants and cationic polymers – never mix the two concentrates |
| Multivalent ions | Calcium, magnesium, aluminium can crosslink and gel or precipitate the polymer, more so at low DS | Hard water, cement, and any formulation built on divalent salts |
| pH window | Below about pH 5 the free acid form becomes insoluble; above about pH 10 the polymer degrades | Acidic foods, acid cleaners, strongly alkaline systems |
| Film formation | Dries to a clear, tough film | Useful in sizing and coating; a nuisance where tackiness is unwanted |
Production route in outline
| Stage | What happens | What it controls |
|---|---|---|
| Alkalisation | Cellulose is treated with sodium hydroxide to swell it and activate the hydroxyl groups | Reactivity, and therefore how evenly the substituent is distributed |
| Etherification | Sodium chloroacetate reacts with the activated cellulose to attach carboxymethyl groups | DS, and the sodium chloride and sodium glycolate by-products |
| Neutralisation and washing | Residual alkali is neutralised and salts are washed out | Purity, pH, and ash content |
| Drying and milling | The product is dried and ground to a powder | Moisture, particle size and therefore dispersion behaviour |
Grades and how they differ
| Grade | Reference | What changes |
|---|---|---|
| Industrial / technical | Supplier specification | Purity and microbiology as agreed; viscosity and DS as specified |
| Food grade | E466 in the EU; FCC where applicable | Microbiological limits and heavy metals tightened; the specification on this page carries zero coliforms, E. coli and Salmonella, which is the food-grade pattern |
| Pharmaceutical grade | USP / EP monograph | Compendial identity and purity tests, and viscosity grades defined by the monograph rather than by the supplier |
CMC against the other cellulose ethers
| Product | Ionic type | Water solubility | Salt tolerance | Typical role |
|---|---|---|---|---|
| Sodium CMC | Anionic | Cold water; insoluble in hot | Moderate, improves with DS | Food, detergent, paper, textile, battery binder |
| HEC (hydroxyethyl cellulose) | Non-ionic | Cold and hot water | Good | Paints, personal care, where electrolyte tolerance is needed |
| HPMC (hydroxypropyl methyl cellulose) | Non-ionic | Cold water; gels on heating | Good | Pharmaceutical matrices, tile adhesives, food |
| MC (methyl cellulose) | Non-ionic | Cold water; gels on heating | Good | Construction, food, ceramics |
| EC (ethyl cellulose) | Non-ionic | Not water-soluble; soluble in organic solvents | Not applicable | Tablet coatings, controlled release |
| PAC (polyanionic cellulose) | Anionic | Cold water | Better than CMC at equivalent DS | Drilling fluids, where CMC is often specified but PAC performs |
The practical summary: CMC is the cheapest and most widely available of the water-soluble cellulose ethers and is the default choice unless the system is strongly electrolytic (choose PAC or a high-DS CMC), needs hot-water solubility (choose HEC), needs a thermal gel (choose HPMC or MC), or must not be water-soluble at all (choose EC).
Buying notes
| Item | What to fix in writing |
|---|---|
| DS and viscosity together | Both numbers, with the viscosity test conditions (1%, 25 deg C, dry basis) stated |
| Viscosity method | Spindle and speed, or a reference to the method used, so figures are comparable between suppliers |
| Grade | Industrial, food (E466) or pharmaceutical – the test set differs, not just the price |
| Microbiology | For food grade, the absence tests in the specification, not only a total count |
| Packaging | 25 kg sack with PE liner; confirm liner integrity on arrival |
| Storage | Agree remaining shelf life; CMC is hygroscopic and a damp store changes the viscosity |
| Documentation | Batch COA with measured values, plus SDS and a grade statement |
Frequently asked questions
Why does my CMC not dissolve?
Almost always hot water, or powder dumped into water instead of sprinkled into the vortex. Above roughly 55-60 deg C the particles skin over. Disperse in cold water, or dry-blend it with other powders first.
Can I use a higher-viscosity grade at a lower dose to save money?
Sometimes, but the substitution is not linear. Viscosity is non-linear in concentration, dissolution takes longer at high viscosity, and pumping becomes harder. Trial it against the target viscosity rather than assuming a proportional saving.
Why did the solution turn cloudy or precipitate?
Common causes are pH below about 5, contact with a cationic surfactant or polymer, or a multivalent ion such as calcium or aluminium. Check the formulation for all three before blaming the batch.
Is DS or viscosity the more important number?
DS decides whether the polymer works in the system at all – solubility, salt tolerance, charge compatibility. Viscosity decides how much you need. A high-viscosity grade with the wrong DS for your electrolyte load will still fail.
What is the difference between CMC and PAC?
Both are anionic cellulose ethers. PAC is refined for higher and more consistent DS and is the standard choice in drilling fluids; CMC is the lower-cost general-purpose material. Where a fluid is highly saline, PAC or a high-DS CMC is the right specification.