Pigments & Fillers

Open a can of industrial paint and most of what settles to the bottom is pigment and filler. These solids rarely get the attention that resins do, yet they decide whether a film hides the substrate, how long it keeps its color, how well it resists water and ions, and whether a primer actively fights rust. Understanding the solid phase is essential to reading a product data sheet intelligently.

In the classic description of a liquid coating, the binder forms the continuous film, solvents or water carry it, additives fine-tune behavior, and pigments and fillers make up the dispersed solid phase.

Pigments versus fillers

The line between the two is functional rather than chemical:

• Prime pigments deliver color and hiding power. They have a high refractive index or strong light absorption, and they are usually the most expensive solids in the formula.
• Functional pigments deliver a specific protective job: sacrificial zinc, inhibitive phosphates, lamellar barrier flakes, or conductive and heat-reflective particles.
• Extenders or fillers are minerals with a refractive index close to that of most binders, so they contribute little hiding. They are used to build volume solids, control gloss, improve film toughness and reduce cost.

All three are insoluble in the binder. Dyes, by contrast, dissolve and are rarely used in protective coatings.

Color and hiding pigments

Titanium dioxide

Rutile titanium dioxide is the dominant white pigment because its very high refractive index scatters visible light efficiently. Grades are surface-treated (commonly with alumina, silica or zirconia) to improve dispersion and to suppress the photocatalytic activity that otherwise accelerates binder breakdown and chalking. The less photoactive rutile form is preferred over anatase for exterior work.

Inorganic colored pigments

Synthetic iron oxides (red, yellow, black and brown) are inexpensive, opaque, chemically stable and highly lightfast, which is why so many primers are red-brown or grey. Chromium oxide green, mixed-metal oxide pigments and carbon black are other durable options. Historic lead and chromate pigments are now heavily restricted; see lead paint hazards for what that means during maintenance work.

Organic pigments

Phthalocyanine blues and greens, quinacridones, azo pigments and others give bright, clean colors with high tinting strength. Durability varies widely between pigment families, so safety colors and brand colors on exterior steel need pigments rated for the expected UV exposure.

Anticorrosive pigments

Primers for steel rely on pigments that do more than fill space. The three mechanisms are often combined in one system:

• Sacrificial. High loadings of metallic zinc dust in zinc-rich primers keep particles in electrical contact with each other and with the steel. Zinc is anodic to iron, so it corrodes preferentially and protects small breaks in the film.
• Inhibitive. Slightly soluble pigments such as zinc phosphate, modified phosphates, calcium-exchanged silica and molybdates release ions when moisture penetrates the film, helping to passivate the steel surface. They depend on some water uptake to work and are more common in primers for mild to moderate environments.
• Barrier. Plate-shaped particles — micaceous iron oxide, aluminum flake, mica and glass flake — align parallel to the surface as the film forms and create a longer, more tortuous path for water, oxygen and ions.

Common extenders and fillers

Extenders are chosen for particle shape, hardness, chemical resistance and price. The table summarizes the minerals encountered most often in protective and industrial coatings.

Pigment volume concentration and CPVC

Pigment volume concentration (PVC) is the volume of all pigments and extenders divided by the total volume of non-volatile material in the dry film. Volume matters more than weight because barytes and zinc are dense while talc and silica are light.

As PVC rises, there is progressively less binder to surround each particle. At the critical pigment volume concentration (CPVC) the binder just fills the spaces between closely packed particles. Above CPVC, voids appear and properties change sharply:

• Gloss drops and the film becomes porous and permeable.
• Barrier properties and blister resistance decline.
• Tensile strength and flexibility fall.
• Hiding can increase, because air voids scatter light.

Protective topcoats and barrier coats are formulated well below CPVC. Some flat architectural paints and certain primers deliberately operate near or above it. Zinc-rich primers are a special case: they need very high zinc loading for particle-to-particle contact, which is one reason they are porous and need appropriate topcoating. The ratio of PVC to CPVC, sometimes written as the reduced PVC, is a more useful predictor of performance than PVC alone. PVC also links directly to volume solids and coverage, because solids that are not binder still occupy film thickness.

Dispersion, particle size and settling

Pigment powders arrive as agglomerates. Manufacturing breaks them down by wetting the particle surfaces, mechanically separating them in high-speed dispersers or mills, and stabilizing them so they do not flocculate again. The fineness of grind, often checked with a Hegman-type gauge, indicates how well this was done.

Particle size matters in several ways. Titanium dioxide scatters light best at a particle size of roughly half the wavelength of visible light. Barrier flakes need a high aspect ratio but must be thin enough to orient within the specified film thickness. Coarse extenders lower gloss and add texture, while poorly dispersed pigment causes specks, low gloss, color drift and weak spots in the film.

What pigment choice means on the job

Specifiers rarely choose pigments directly, but pigment packages still shape field results:

• Light, bright colors and certain organic pigments may need more coats to hide, so check hiding guidance before bidding.
• Barrier flake systems perform best when applied to their specified thickness and in the recommended direction or technique so the flakes orient properly.
• Acid-sensitive extenders such as calcium carbonate are inappropriate in acid service even when the resin itself is resistant.
• Dry-blending or adding aftermarket pigment changes PVC and can void performance claims.

Frequently asked questions

Are fillers just a way to make paint cheaper?

Lower cost is one reason, but well-chosen extenders also improve barrier properties, sanding, sag resistance, hardness and gloss control. Problems arise when filler loading pushes the film toward or past CPVC in a product meant to be a barrier coating.

Why are so many primers red or grey?

Iron oxide and zinc-based pigments are inexpensive, durable and protective, and they naturally produce red-brown and grey colors. Those colors also contrast with topcoats, which helps applicators see coverage.

What is micaceous iron oxide used for?

Micaceous iron oxide is a platy, naturally occurring iron oxide used in intermediate coats and some topcoats. The overlapping flakes reinforce the film and slow moisture and UV penetration into the layers below.

Can I tint an industrial coating myself?

Only with tinting products and amounts approved by the manufacturer. Unapproved colorants can alter cure, chemical resistance and pigment loading, and may compromise the warranty.