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In slurries of pigment, carbon black or inorganic filler, more dispersant is not always better. Too little and the particles agglomerate; beyond the suitable range, free dispersant, resin interactions and changes in the rheology of the system can push the viscosity up again. A gradient test is therefore normally used to find the suitable range.
Too little
Particles tend to agglomerate and the slurry flows poorly.
Suitable range
Viscosity falls as particles are wetted and separated.
Too much
Free dispersant may change the rheology, and viscosity rises again.
Gradient test
Low, medium and high dosages compared under the same conditions.
1. Introduction
In coatings, inks, adhesives and conductive slurries, dispersants are often used to improve the wetting and dispersion of pigments and fillers. In practical formulation work a confusing phenomenon can appear: as the dispersant dosage is raised from a low level, the slurry viscosity first drops, and after a certain range it starts to rise again.
This does not necessarily mean the dispersant itself has a problem. The dosage may simply have exceeded the range that suits the current system.
2. How the viscosity changes with dosage
When pigment or filler enters the resin and solvent, the particles aggregate easily. Materials with a large specific surface area such as carbon black readily form structure between particles. If wetting and dispersion are insufficient, the agglomerated particles enlarge the spatial network inside the system and the slurry's flow is affected. At a low dispersant dosage, therefore, viscosity usually falls as the dosage increases and the particles are gradually wetted and separated.
Once the adsorption on the particle surface approaches a suitable state, adding more dispersant does not necessarily improve dispersion further. Part of it may be present in the continuous phase in a free state and interact with the resin, the solvent or other additives, changing the rheological state of the system.
In addition, when the pigment and filler concentration is high, the system itself may show marked shear dependence, and the measured viscosity is influenced by shear rate; the same slurry can give different results under different test conditions. It cannot simply be assumed that more dispersant always means better dispersion and lower viscosity.
| Low dosage | Wetting and dispersion are insufficient, agglomerates enlarge the spatial network and flow is affected. |
| Dosage increasing | Particles are gradually wetted and separated, and viscosity usually falls. |
| Beyond the suitable range | Free dispersant in the continuous phase interacts with resin, solvent or other additives and changes the rheology, so viscosity may rise again. |
3. Different materials need different amounts
The dispersant dosage depends on the specific surface area and surface properties of the pigment or filler and on the concentration used.
| Carbon black | Large specific surface area; particles form structure readily. |
| Titanium dioxide, inorganic fillers | Surface characteristics differ from carbon black, so the dispersant demand also differs. |
| Same dispersant, another pigment system | The suitable dosage may change. |
| A fixed percentage | Not suitable for adding directly in a real formulation. |
4. How to find the suitable dosage
The more practical method is a gradient test, for example low, medium and high dosages compared with all other raw materials and grinding conditions kept the same.
| What to observe | Besides initial viscosity: fineness, viscosity change after storage and the flow state of the slurry. |
| A down-then-up trend | If viscosity falls first and then gradually rises, the low-viscosity region in the middle deserves finer testing. |
| Test points | Do not judge dispersion from a single viscosity point. Rheology changes with shear conditions, and a result under one shear condition may not fully reflect real application. |
5. Choosing a dispersant is more than lowering viscosity
When screening dispersants, viscosity has to be judged together with storage stability, fineness and final application performance. Some schemes have a low initial viscosity but thicken or turn coarse after standing; others change little in initial viscosity but store well. What the dispersant must match is the whole slurry system, not a low viscosity at a single point in time.
Frequently Asked Questions
Why can the viscosity rise when more dispersant is added?
Usually because the dosage has exceeded the suitable range, free dispersant interacts with the system, and the slurry's own rheology changes.
Why is viscosity high when there is too little dispersant?
Particles aggregate, and with large-surface-area materials such as carbon black they form structure; insufficient wetting leaves agglomerates that enlarge the spatial network and hinder flow.
How can the suitable dosage be found?
Run a gradient test with low, medium and high dosages under identical conditions, and compare initial viscosity, fineness, viscosity after storage and flow state.
Is a single viscosity reading enough to judge dispersion?
No. Rheology changes with shear conditions, so a result at one shear condition may not fully reflect real application.
A slurry that gets thicker with more dispersant is usually linked to a dosage beyond the suitable range, interaction of free dispersant with the system, and the slurry's own rheological change. The reasonable approach is not to keep adding dispersant but to run gradient tests around the pigment and filler properties, the resin system, the dispersant type and its dosage, and to find the range that balances viscosity, dispersion state and storage stability.
