Pigment Dispersion: Milling, Particle Size and Colour Strength

Two factories can buy the same iron oxide from the same supplier, dose it at the same percentage in the same base, and produce a foundation that is visibly different on a drawdown card. The difference is rarely the pigment. It is the dispersion, and dispersion is where the colour strength of a product is actually decided.

The commercial weight of this is easy to underestimate. Pigment is the most expensive part of a colour formula, and a poorly dispersed pigment delivers less opacity per unit, which means more pigment is dosed to reach the target. Improving dispersion is therefore one of the few changes in colour manufacturing that reduces cost and improves the product at the same time. This article covers what dispersion involves, how it is specified, how it is measured, and where a laboratory result stops predicting the production result.

What dispersion actually involves

Dispersion is often described as grinding, and grinding is only one of four steps.

The first step is wetting. The carrier has to replace the air on the pigment surface, which requires the liquid to have a lower surface energy than the pigment and time to reach the surface. A pigment added to a carrier too quickly simply floats. The second step is deagglomeration: breaking the clusters that formed during drying of the pigment, which is the step a mill performs. The third is distribution, getting the separated particles evenly through the carrier. The fourth is stabilisation, keeping them separated so that they do not recombine, which is done by the dispersing agent and by the viscosity of the system.

A process that does three of those four will produce a product that looks acceptable on the day of manufacture and changes over time. Deagglomerating without stabilising gives a dispersion that flocculates in storage, which appears as a viscosity rise, a loss of colour strength or a shade shift in the finished pack. It is the most common dispersion complaint and the least visible at the time it is caused.

Particle size and what it does to hiding and hue

Particle size controls two things that a brand cares about most: how much the product hides, and what colour it appears to be.

Titanium dioxide has a well-known relationship between particle size and opacity. Around the middle of the visible wavelength range it scatters most efficiently, which is why the pigment is engineered to a primary particle size of roughly a quarter of a micron. Milled below that range it loses hiding power and starts to become translucent; left above it, it scatters less and the film looks weaker. This is why a foundation can be made to look more transparent simply by dispersing the same titanium dioxide more aggressively, which is a useful tool and also an easy way to overshoot a coverage target.

Iron oxide behaves differently. Particle size affects the hue as well as the strength, because the smaller particles scatter differently and appear warmer or cooler. A red iron oxide milled further than the specification will read as a different red, which is one route to a shade that fails a colour standard while every raw material matches its own certificate.

Organic pigments add a third behaviour. Many are supplied as lakes on a substrate and are far softer than an inorganic oxide, so they reach their intended particle size quickly and can be over-milled, which changes the shade and can release dye. Pearls are the opposite extreme: they are platelets, and the objective is not to reduce them at all but to distribute them without breaking the platelets, because a broken pearl loses its effect entirely.

Milling equipment and where each one belongs

Four types of equipment cover most colour production, and the choice follows from the target particle size and the viscosity of the mill base.

A high-speed disperser with a saw-tooth blade applies shear to a low-viscosity system and is efficient at wetting and distribution, and it will not deagglomerate hard clusters. A bead mill uses small grinding media in a chamber and is the standard route to fine particle size: media in the 0.3 to 1.0 millimetre range, a chamber that keeps the media in motion and a cooling jacket to remove the energy that the process puts in. A three-roll mill passes the paste between rollers and is well suited to high-viscosity pastes such as a lipstick colour concentrate, where the shear is applied in a thin film. A rotor-stator is useful for combining and wetting and is not a deagglomeration step on its own.

The choice of bead size is the parameter most often assumed to be fixed and should not be. Smaller media produce a finer grind and more heat per unit of energy, larger media break harder clusters faster. Media material matters as well, since a zirconia bead that wears will contaminate the batch, and a formula with a high pigment load will wear media faster than a lightly loaded one.

Temperature is the constraint that decides how aggressive the process can be. A colour concentrate that reaches sixty degrees in the mill will behave differently from one held at thirty, both in the dispersion state and in any heat-sensitive component that has already been added. The rule that keeps results repeatable is to add heat-sensitive ingredients after milling rather than before.

Specifying a grind and measuring it honestly

The most common specification error in colour work is to write a particle size figure without writing the method that produces it.

A micron figure can mean a laser diffraction median, a microscope measurement of the largest particles, or a reading off a grind gauge, and the three do not agree. A grind gauge reading, often reported on the Hegman scale, measures the largest surviving agglomerates in a drawdown, which is the figure that correlates with visible defects. A laser diffraction result measures the distribution of what is in suspension, which is the figure that correlates with optical behaviour. A specification that quotes one and verifies with the other will generate disagreements.

For production control, the pragmatic approach is a grind gauge reading as the in-process check, taken from a sample removed from the mill at a defined point, with the acceptance limit written as a number on the scale rather than as a description. The instrument result belongs on the development record, where it can be compared against the optical measurements that matter to the customer.

The sample point has to be defined as precisely as the number. A reading taken after a fixed number of passes, at a fixed flow rate and a fixed temperature, is repeatable. A reading taken when the operator judges the batch to be ready is not, and it is a common reason why two batches with the same specification behave differently in the press.

Colour strength and how to set a tolerance

Colour strength is the property the customer actually experiences, and it can be measured objectively with a simple procedure.

The standard approach is a tint reduction: a known quantity of the mill base is let down with a white base at a fixed ratio, drawn down on a card with a defined applicator, and measured on a spectrophotometer. Comparing the reduced sample against a retained standard gives a strength figure, usually expressed as a percentage, and a colour difference figure that separates lightness from hue. A reduction also makes small differences visible, which a full-strength drawdown hides.

The tolerance should be set from the process capability rather than from an aspiration. If the mill holds the strength within five per cent across ten batches, a three per cent tolerance will fail regularly and a seven per cent tolerance will let a genuine shift through. Measuring the first ten batches and then setting the limit is cheaper than arguing about a marginal batch at the end of a quarter.

Strength is also the link to the business case. A dispersion that delivers five per cent more strength at the same pigment dose allows the dose to be reduced, and on a formula where pigment is the dominant raw material cost, that reduction pays for the extra milling time many times over. Presenting the change that way makes it a cost project rather than a technical preference.

Flocculation, settling and the storage side of dispersion

A dispersion that passes the day it is made can fail in storage, and the failure usually appears as one of three symptoms.

Flocculation is the recombining of separated particles into loose clusters, which raises viscosity, reduces strength and can shift the shade. It is caused by insufficient dispersing agent, by an incompatible carrier, or by a pigment with a surface treatment that does not match the carrier. Hard settling is the opposite problem: particles that stay dispersed but drop out of suspension and form a cake that cannot be stirred back in. It is more common in low-viscosity systems and is controlled by the rheology rather than by the mill.

Seeding, where fine aggregates become visible in a film, is a third symptom and usually points at the wetting step rather than at the milling step, since particles that were never fully wetted will not be separated by shear. Each of the three has a different fix, and the check that separates them is a comparison of the same batch after storage against the retained drawdown, measured instrumentally rather than judged by eye.

The practical implication for a brand is that the dispersion specification has to include a storage check. A batch released on the strength measured on the day of manufacture has not been assessed against the complaint that will arrive in six months.

Pigment dispersion variables at a glance

Where the laboratory result stops predicting production

A dispersion developed in a laboratory mill and transferred to a production mill will differ unless the transfer is planned, and the differences are predictable.

Scale affects the energy per unit of volume, the residence time in the milling chamber and the rate at which heat is removed. A production mill usually delivers more total energy and removes heat less efficiently per litre, so a formulation that stabilises well in a small mill may flocculate in a large one. The standard remedy is to hold the grind gauge reading and the strength target as the transfer criteria and to re-establish the dispersing agent level on the production mill rather than copying the laboratory figure.

Batch size also changes the order of addition and the mixing times, which is why a process description written as a sequence of times rather than as a sequence of observations travels better between sites. A note that says mix for twenty minutes at sixty degrees is transferable; a note that says mix until dispersed is not, and it is the type of instruction that produces the batch that fails a strength check a month later.

The last transfer point is the press or the fill line. A dispersion that meets the grind and the strength specification can still press badly if the viscosity is out of range, which is why the release criteria for a colour concentrate usually include viscosity and solids as well as colour.

What to put in the brief

The brief that produces a repeatable colour specifies the pigment, the process and the acceptance test together, because any two of the three without the third leaves a gap.

Name the pigments and their supply form. State the coverage target for the finished film, since it determines how hard the titanium dioxide should be dispersed. State the shade standard and how it is measured, so that a colour difference figure has a meaning. And state the storage conditions the concentrate has to survive, because the dispersion has to hold in a drum as well as in the finished pack.

• The pigment list, with supply form and the supplier's surface treatment.
• The coverage target for the finished film, stated as a test method.
• The shade standard and the instrument method used to compare against it.
• The grinding equipment available, and whether the formula must run on more than one site.
• The grind gauge limit and the sample point where it is taken.
• The colour strength tolerance, with the basis for the figure.
• The storage period the concentrate must survive before use.
• The heat-sensitive ingredients that must be added after milling.

Answering those lines gives a formulator the information to choose a dispersing agent and a milling route that hold on the production line, rather than a laboratory result that has to be renegotiated at scale-up.

Where our capability fits

We develop colour cosmetics with the dispersion specified as a process rather than as an instruction, so the wetting step, the media, the temperature window, the sample point and the strength tolerance are all written down and can be reproduced batch to batch. Colour strength is measured instrumentally against a retained standard, which turns a shade argument into a number and lets pigment dose be reduced when the dispersion supports it.

We work with brands on private label (your brand on a proven base), OEM/ODM contract manufacturing, and full custom formula development — whichever matches where your brand is today. Our lines include a long-lasting waterproof foundation, a vitamin C brightening face cream and a solid essence stick format, all of which rest on the same milling, wetting and strength control disciplines that a colour concentrate needs.

Tell us the coverage and shade you are targeting and the equipment the formula will run on, and we will map the pigment route, the dispersing system and the release criteria against your specification. Start at our inquiry page, email capsulebiotech@163.com, or message the right team on WhatsApp: Capsule Series (capsules only) +86 158-7655-5319, Cosmetics Series (serum, hair care, body care, fragrance) +86 158-7655-5319. Shade architecture is covered in our article on shade ranges and pigments, and pressed formats in our guide to compact and pressed powder production.

General manufacturing information, not regulatory or legal advice. Permitted colourants, purity limits and labelling requirements vary by market and by product.