“Viscosity 2500-3000 mPa.s” is the number buyers compare, and it is the number most often compared wrongly. CMC is a pseudoplastic polymer, so its viscosity depends on concentration, temperature, shear rate, measurement geometry and the moisture basis of the sample. Change any of those and the figure moves. This page is about reading the number properly, measuring it reproducibly, and using it to control a process rather than just to accept or reject a delivery.
The four conditions that must travel with the number
| Condition |
Why it changes the reading |
What a certificate must state |
| Concentration |
Viscosity rises steeply and non-linearly with concentration – a 2% solution is not twice a 1% solution |
The concentration used, normally 1% |
| Temperature |
Solutions thin as they warm; a few degrees is visible on the dial |
The temperature, normally 25 deg C |
| Basis |
Moisture counts as diluent, so an as-is solution is weaker than a dry-basis one |
Whether the weight was corrected for moisture |
| Shear rate |
Shear-thinning behaviour means the reading depends on spindle and speed |
The instrument, spindle and rotational speed, or a named method |
A certificate that says only “viscosity 2500-3000” has told you almost nothing. Two suppliers can meet that figure with materials that behave completely differently in your tank.
How the measurement is normally made
| Method |
What it gives |
Best used for |
| Brookfield rotational viscometer, 1% solution |
A single apparent viscosity at one shear rate |
The industry convention for release testing and supplier comparison |
| Spindle and speed sweep |
A flow curve – viscosity against shear rate |
Understanding how the product will pump and coat, not just whether it passes |
| Ubbelohde or capillary viscometer |
Intrinsic viscosity, from which molecular weight can be estimated |
Comparing chain length across suppliers, where DS is held constant |
| In-line or process viscometer |
Continuous reading in the plant |
Controlling a running batch rather than a laboratory sample |
For release testing, Brookfield at a stated spindle and speed is the practical standard. For problem solving – “why does this lot pump differently from the last one” – a short sweep tells you far more than a single point.
Viscosity, molecular weight and dose
Viscosity is a proxy for chain length. A higher-viscosity grade gives the same thickening effect at a lower dose, which is why the trade looks attractive – but the substitution is not linear and it is not free.
| If you switch to a higher-viscosity grade |
What improves |
What gets worse |
| Dose required for a target viscosity |
Less powder per batch |
Dissolution time lengthens; a high-viscosity grade can take hours to hydrate fully |
| Handling |
Smaller bags, less storage |
Solutions are harder to pump and to mix without creating fisheyes |
| Shear sensitivity |
– |
Longer chains degrade faster under high shear, so viscosity can drop during processing |
| Cost per batch |
Often lower |
Only if the plant can dissolve and handle it; otherwise the saving is consumed by lost batches |
Measuring it reproducibly in your own lab
| Step |
Why it matters |
| Correct the powder weight for moisture |
A lot at 8% moisture and one at 4% give different solutions from the same nominal weight |
| Use the same water every time |
Hardness and pH change both the viscosity and the clarity; deionised water is the only comparable baseline |
| Dissolve fully before measuring |
An under-hydrated solution reads low and drifts upward as it stands |
| Thermostat the sample |
A water bath at 25 deg C removes the largest single source of variability |
| Fix the spindle and speed |
Same instrument, same geometry, same speed – otherwise the numbers are not a series |
| Wait for a stable reading |
Pseudoplastic solutions take time to reach equilibrium after the spindle starts |
When the delivered viscosity does not match the certificate
| Symptom |
Likely cause |
What to check first |
| Reads low immediately after opening |
Incomplete dissolution in the test, or moisture picked up in store |
Dissolution time and the moisture of the sample actually tested |
| Reads low and the solution is hazy |
Undissolved aggregates or uneven substitution |
The insoluble fraction, and the dispersion technique used |
| Reads high |
Understated moisture on the certificate, or evaporation during storage in a dry store |
The moisture on arrival, measured yourself |
| Starts right and drops during processing |
Shear degradation of the polymer chain |
Pump type and the shear history between make-down and use |
| Varies batch to batch by more than the limit |
Supplier is blending to hit a wide band, or the DP varies |
Three consecutive certificates, plus an intrinsic viscosity if you need to see chain length |
Using viscosity as a process control
| Application |
What viscosity controls |
Practical target |
| Food thickening |
Mouthfeel and stability |
Set by sensory trial, then held as a specification on the finished product |
| Detergent anti-redeposition |
Soil suspension, and whether the product pours |
Low to medium, because high viscosity makes a liquid detergent unpourable |
| Paper coating |
Pick-up and runnability at the coater |
Narrow band, because coating rheology is sensitive to small changes |
| Textile sizing |
Add-on and penetration into the yarn |
Depends on the fibre and the sizing machine speed |
| Battery electrode slurry |
Coating uniformity and settling |
Tight, since slurry rheology directly sets electrode quality |
The useful discipline is to specify viscosity as a band tied to a method, measure it the same way on arrival, and trend it across deliveries. A drift that stays inside the limit is still telling you something about the supplier’s process or their storage – and it is cheaper to notice it on the certificate than in a failed batch.
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