Introduction: Fillers — the “most underestimated” strategic component in coating formulations
Extender Pigment in coating formulations typically accounts for 10-40% (by volume)
——but is often mistaken as a “cheap filler” used merely to reduce cost. In reality——the selection of extender pigments (type/particle size/morphology/surface treatment)——has a decisive impact on the coating’s rheology (thixotropy/anti-settling), mechanics (hardness/wear resistance/crack resistance), optics (gloss/hiding power), anti-corrosion (barrier/labyrinth effect) and chemical resistance
——that is no less than that of resins and pigments
. “Extender pigments are not supporting actors in the formulation——they are the silent pillars of the formulation”
. From natural minerals (calcium carbonate/talc/mica/wollastonite/kaolin/barite) to synthetic extenders (precipitated SiO₂/fumed SiO₂)——the “extender universe” of coatings is vast and profound——the scientific selection and combination of extenders——is the key leap for coating formulation engineers from “qualified” to “excellent”.
I. Physical Properties and Functional Matrix of Six Natural Mineral Fillers
| Filler | Hardness (Mohs) | Refractive Index | Aspect Ratio | Oil Absorption (g/100g) | Density (g/cm³) | Core Function |
|---|---|---|---|---|---|---|
| Calcium Carbonate (CaCO₃) | 3 | 1.58 | ≈1 (equiaxed) | 15-25 | 2.7 | Filling / cost reduction / TiO₂ spacing |
| Talc Powder | 1 | 1.57 | >10 (flake) | 30-50 | 2.8 | Thixotropy / anti-settling / water resistance / hydrophobicity |
| Mica Powder | 2.5-3 | 1.58 | >50 (high aspect ratio / flake) | 40-60 | 2.8 | Shielding / maze effect / anti-corrosion |
| Wollastonite | 4.5-5 | 1.63 | >10:1 (acicular / length-to-diameter ratio) | 20-30 | 2.9 | Reinforcement / wear resistance / anti-cracking |
| Calcined Kaolin | 4-5 | 1.56 | >20 (flake) | 50-70 | 2.6 | Hiding power / matting / whiteness >90 |
| Barite (BaSO₄) | 3 | 1.64 | ≈1 (equiaxed) | 8-15 | 4.5 | High solid content / chemical resistance / weighting |
FAQ
Q1: How does the “TiO₂ spacing effect” of calcium carbonate make titanium dioxide “whiter”?
Titanium dioxide (TiO₂/refractive index 2.7) is the most effective white pigment in coatings—but TiO₂ particles agglomerate with each other in the coating
(particle size aggregates from >250nm to >500nm)—the scattering efficiency of TiO₂ drops sharply (>50%)
“Less light is scattered—the coating appears darker (grayish)”. Calcium carbonate (CaCO₃/refractive index 1.58/close to the refractive index of resin at 1.5)—is “transparent” (no hiding power) in the coating—but CaCO₃ particles (<1-5μm) “space” the TiO₂ particles apart
—preventing TiO₂ agglomeration—restoring the scattering efficiency of TiO₂—with the same TiO₂ addition—the coating is whiter—this is the “TiO₂ spacing effect” filler does not increase hiding power—but helps TiO₂ exert its hiding power better
.
Q2: Why can the “maze effect” of mica powder improve salt spray >200h?
The high aspect ratio (>50:1/flake diameter >10-50μm/thickness <1μm) flakes of mica powder——are overlappingly arranged in the coating
(similar to fish scales)——the penetration path of water molecules and Cl⁻——(1) in coatings without flakes straight penetration——> tens of μm
; (2) in flake coatings “tortuous penetration” > several times the straight-line distance
——this is the “maze effect” (Tortuosity)——the maze effect of mica powder flakes extends the penetration time of Cl⁻ in salt spray (ASTM B117) by >200-500h——this is the physical mechanism by which mica powder enhances corrosion resistance.
Q3: Needle-like reinforcement of wollastonite—why can it resist cracking?
The needle-like crystals of wollastonite (aspect ratio >10:1 / diameter >5-50μm / length >100μm)—similar to miniature “steel bars” in the coating provide fiber reinforcement
—The internal stress generated in the coating during shrinkage (curing / thermal expansion and contraction) is dissipated by the “pull-out / bridging” mechanism of needle-like wollastonite
—Microcracks stop propagating at the wollastonite fibers—coating crack resistance is significantly improved (>2 times). The “toughening” effect of wollastonite on epoxy (brittle) coatings is the best among fillers.
Q4: Why can the “high density” of barite (BaSO₄) improve the chemical resistance of coatings?
Barite density >4.5g/cm³——is >3.8 times that of resin (>1.2g/cm³)——in the coating it “sinks” to the bottom of the coating——forming a dense BaSO₄ layer
——this layer against the penetration of acid/alkali/solvent——(1) physical——the high-density BaSO₄ layer——the penetration rate of chemicals is due to the medium density difference (chemical density < BaSO₄)
——and slowed down; (2) chemical——BaSO₄ is one of the least soluble sulfates (Ksp=1.1×10⁻¹⁰)
——extremely low dissolution rate——the BaSO₄ layer in the coating——chemicals cannot “dissolve through”. The application of barite in chemical-resistant coatings (tank inner walls)——is its unique function.
Related Reading
Summary
Six natural mineral fillers—calcium carbonate (filling/TiO₂ spacer), talc (thixotropy/anti-settling), mica powder (barrier/labyrinth—corrosion resistance), wollastonite (reinforcement/crack resistance), kaolin (hiding/matting) and barite (chemical resistance/high solids)—each make unique functional contributions—not “cheap filling.” The scientific selection of fillers—enhancing coating performance—reducing cost (less expensive resin)—is a core strategy in coating formulation. Kexin New Materials provides customers with a full range of functional filler products and formulation technical support.