Two Numbers, and Only One of Them Is on the Batch Card
Pigment volume concentration is the fraction of the dry film volume occupied by pigment and extender. It is easy to calculate and it normally appears on the formulation sheet. Critical pigment volume concentration is the concentration at which there is exactly enough binder to fill the voids between those particles and coat them, with nothing left over. It is harder to know and it usually does not appear anywhere.
The relationship between the two is what decides how the film behaves. If the formulated concentration sits below the critical value, the particles are separated by binder and the film is continuous. If it sits above, there is not enough binder to fill the gaps and the film contains air voids. Both are legitimate products; they just fail in different ways and they ask different things of the dispersion stage.
| Condition | What the film looks like | What fails first | What the mill is for |
|---|---|---|---|
| Well below CPVC | Binder-rich, continuous, particles fully separated. | Cost. Binder is the expensive component and hiding power may be lower than it could be. | Separating and wetting every particle, because scattering depends on it. |
| At CPVC | Binder exactly fills the voids. The film is at its tightest packing. | Little margin. Small process variation moves the product either side. | Even dispersion, because a local concentration difference is now a defect. |
| Above CPVC | Porous. Air voids inside the film. | Scrub resistance, stain resistance, film integrity. | Wetting the pigment out evenly. Hiding partly comes from the voids, not the milling. |
The reason this matters at the mill rather than only in the laboratory is that the two conditions reward different things. Below the critical value, a poorly dispersed batch loses hiding power that no amount of let-down will recover. Above it, the same batch may still hide acceptably, because the voids are doing part of the work — and that is exactly why a plant can drift into over-grinding without noticing.
Below CPVC, Dispersion Is What Produces Hiding Power
Below the critical concentration the film is a composite of particles in a binder matrix, and light is scattered at every particle surface. The amount of scattering depends on how many separate particle surfaces exist, which depends on the pigment being separated into individual particles rather than sitting in clusters.
A cluster of ten particles presents roughly the surface of one large particle, so it scatters far less than ten separate ones. This is the whole argument for dispersion in a binder-rich formulation: the mill is not making the pigment finer, it is making more scattering surfaces from the same mass of pigment. Two batches with identical formulation and identical pigment grade can differ measurably in contrast ratio purely on dispersion quality.
| What is in the mill base | Separate scattering surfaces | Contrast ratio | Cause |
|---|---|---|---|
| Pigment still in agglomerates | Few. Each cluster acts as one large scatterer. | Low, and variable across the drawdown. | Dispersion stage too short, or dispersant insufficient. |
| Agglomerates opened, particles separated | Many, close to one per primary particle. | High and even. | Dispersion run to the point where the gauge stops falling. |
| Particles broken below the optimum size | Many, but each too small to scatter efficiently. | Falls again despite better dispersion. | Milling taken past the point the pigment rewards. |
The third row is the one that surprises people, and it is covered properly in the next section. The point here is that below the critical concentration the dispersion stage is a value-adding step, and its quality is visible in a number the customer cares about: contrast ratio, gloss, and the evenness of both across the panel.
There Is an Optimum Particle Size, and Finer Is Not Better
Light scattering by a small particle is strongest when the particle diameter is roughly half the wavelength of the light being scattered. For visible light that puts the optimum for a white hiding pigment in the region of a few tenths of a micron. Take the same pigment meaningfully below that and scattering efficiency drops: the particles are better dispersed but each one scatters less.
This has a direct consequence for how long the mill runs. Beyond the point where agglomerates are opened, further milling spends machine time and generates heat while the optical performance of a white pigment gets worse rather than better. The gauge reading will keep improving while the product does not, which is a confusing situation to be in without knowing the mechanism.
| Milling level | Gauge reading | Contrast ratio | Verdict |
|---|---|---|---|
| Short of the plateau | Coarse, specks visible. | Low and patchy. | Under-dispersed. The batch will not meet hiding specification. |
| At the plateau | Clean for the grade. | At its best for this formulation. | The economic and technical optimum. This is where to stop. |
| Well past the plateau | Still clean, no further change. | Starts to fall on white pigments. | Over-dispersed. Machine time and heat spent to make the product worse. |
For colored pigments the picture is different again, because transparency and shade matter more than scattering. An organic pigment taken finer often gains transparency and can shift shade, which may be desirable or may be a defect depending on the product. The rule that survives across pigment types is the same one: the target is set by what the film has to do, and the mill is stopped there.
Above CPVC the Job Narrows, Which Is Easy to Get Wrong
Above the critical concentration the film is porous, hiding power comes partly from air voids, and the dispersion requirement relaxes. The pigment still has to be wetted and evenly distributed, but the scattering argument no longer rewards extended milling. Plants that run one grinding recipe across a whole product range usually discover this the expensive way: the high-PVC products are being milled for longer than they need.
| Product type | Typical position | What dispersion has to deliver |
|---|---|---|
| High-PVC matt and ceiling paints | Above CPVC. | Even wetting and distribution. Long milling does not pay back in hiding power. |
| Mid-PVC silk and eggshell | Below CPVC, often close to it. | Full separation. This is the band where dispersion quality decides the selling properties. |
| Low-PVC gloss and exterior | Well below CPVC. | Full separation plus tight fineness, because gloss exposes any remaining agglomerate. |
| Primers and fillers | Often near or above CPVC. | Even distribution. Hiding usually comes from the pigment loading rather than the milling. |
The practical instruction that follows is not "mill everything less". It is that the dispersion target should be written per product rather than per plant. A plant with four products across the range has four different correct answers, and a single setting will be too little for the gloss and too much for the ceiling paint.
What to Measure Instead of Only the Gauge
The gauge measures the largest remaining agglomerate. That is a useful and necessary check, and it is a poor sole indicator of whether the dispersion is doing its job for the formulation.
| Measurement | What it catches | Where it is done | Cost of getting it wrong |
|---|---|---|---|
| Hegman gauge | Large agglomerates and contamination specks. | Mill outlet or tank sample, every batch. | Fast to take, and blind to everything below its resolution. |
| Contrast ratio | Whether hiding power is where the formulation expects. | Drawdown over black and white, cured. | Slow to run, decisive when a complaint arrives. |
| Gloss at fixed angle | Residual fine agglomerate and surface unevenness. | Cured drawdown or panel. | The property a customer notices first on a gloss product. |
| Scrub resistance | Whether the film is above CPVC when it should not be. | Cured film, standard test. | The failure that shows up months later in the field. |
| Color acceptance | Uneven dispersion showing as rub-up or float. | Tinted drawdown, rubbed area against unrubbed. | The most common reason a batch is rejected after tinting. |
Nothing here replaces the gauge, and the gauge does not replace any of them. The point is that a dispersion stage judged only on gauge reading is judged on one third of what the formulation actually asks for.
Where the Machine Fits Into This
None of the above argues for a particular machine type. It argues that the machine has to be able to hold a documented dispersion level repeatably, because the target differs by product and there is no margin for drift.
In an emulsion paint plant the practical arrangement is usually a basket mill dispersing and grinding the mill base in the vessel it will be let down in. The advantage is not only that a transfer is removed: it is that the dispersion level can be verified before let-down rather than after, when the batch is already committed. An immersion basket mill for emulsion paint suits that because the sample point is the batch itself.
Where the plant runs several products across the range, the ability to stop the grind at a verified point matters more than raw capability. A mill that can grind finer than the optimum is not an advantage if nobody stops it there. The layout questions around that — where the tank sits, how let-down is charged, how colors are changed — are covered in the coating production line overview.
Two related notes are worth reading alongside this one: the color strength note covers the colored pigment case in detail, and the sulphur grinding note covers what changes when the material has a thermal limit instead of an optical one.
Send Us the Formulation and the Film Targets
Tell us the pigment system, the pigment volume concentration, the batch size and the film properties you have to hit. Our engineers will work back to a dispersion level and a mill arrangement, and quote directly from the Wuxi factory.
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Where These Mills Are Built
Basket chambers, screens and lift columns are machined and assembled in the same plant in Wuxi. That matters to a dispersion target because reproducibility is a machining property before it is an operating one: a screen slot that varies between machines gives a different grinding result from the same recipe.
It is worth asking any supplier what they measure before dispatch, because the answer tells you what the dispersion target is really resting on.
