Dry hiding power of calcium carbonate (GCC/PCC) in matte paints mainly comes from diffuse light scattering via micro-pore voids, rational particle packing, refractive index difference, and uniform dispersion. Since CaCO₃ is an extender pigment without intrinsic tinting strength, its dry hiding performance can be significantly upgraded by particle optimization, gradation control, surface treatment, formulation matching and processing adjustment.
1. Optimize Particle Size & Bimodal Gradation
- Optimum particle size: Control CaCO₃ at d₅₀ = 1.5–3.0 μm, d₉₇ ≤ 8 μm.Too fine particles cause agglomeration and high oil absorption; too coarse ones produce uneven film surface and weak light scattering.
- Bimodal particle distribution: Match fine powder (0.5–1.5 μm) and medium powder (2.0–5.0 μm).Reasonable stacking forms uniform micro-air voids inside the matte film, strongly enhancing diffuse reflection and dry hiding.
- Particle morphology: Prefer rhombohedral/cubic calcite CaCO₃; avoid needle-shaped aragonite or irregular fragments. Regular morphology creates stable void structure and consistent matte scattering effect.
2. Control Oil Absorption of CaCO₃
- Maintain oil absorption at 18–24 g/100g for matte paint grade CaCO₃.
- Excessively high oil absorption makes resin fully fill the inter-particle voids, eliminating light scattering pores and reducing dry hiding.
- Reduce oil absorption by mechanical particle shaping and mild surface coating to minimize internal micro-pores of CaCO₃.
3. Mild Surface Modification
- Adopt low-dosage fatty acid / stearic acid modification (0.3–0.8%), avoid over-coating.
- Function: Improve dispersion, prevent CaCO₃ agglomeration, and retain proper inter-particle voids for light scattering.
- Over-modification will make particles arrange densely, fill voids, and greatly weaken dry hiding power.
4. Reasonable Extender Blending & Refractive Index Synergy
- CaCO₃ refractive index ≈1.56, matched with acrylic/alkyd resin (n=1.48–1.52); the tiny RI difference produces effective light scattering.
- Compound with small amount of talc, kaolin or precipitated barium sulfate to build multi-layer light scattering interfaces. Synergistically improve dry hiding while maintaining low matte gloss.
- Optimize filler proportion to partially assist TiO₂ hiding and reduce formula cost.
5. Adjust Paint PVC & Formula System
- Control PVC (Pigment Volume Concentration) at 65–75%, slightly above CPVC.Moderate excessive filler forms dense micro-pore network, which is the core source of dry hiding for matte paints.
- Properly reduce binder dosage: Avoid excessive resin infiltrating and filling the voids between CaCO₃ particles. Keep the three-phase interface of air–filler–resin for light diffuse reflection.
- Synergize with matting agents: Match micro-wax powder / silica matting powder with graded CaCO₃ to stabilize matte degree and further boost dry hiding.
- Select low-foam polymer dispersant: Realize full dispersion of CaCO₃ without agglomeration, ensuring independent light scattering of each particle.
6. Optimize Milling & Dispersion Process
- Control fineness ≤25 μm after sand milling; eliminate coarse agglomerates that cause uneven hiding.
- Adopt step feeding: Disperse CaCO₃ and inorganic extenders first, then add binder and matting agent, to prevent resin from wrapping particle pores in advance.
- Low-shear post-stirring: Avoid destroying the formed particle stacking void structure and keep stable dry hiding performance.
7. Improve Whiteness & Reduce Impurities
- Require CaCO₃ whiteness ≥95%; strictly control iron oxide and clay impurities.Impurities absorb visible light, cause yellowing and local light loss, lowering both whiteness and dry hiding.
- Remove ultra-fine clay impurities to prevent micro-agglomeration and uneven film hiding.
Key Summary
- Use 1.5–3.0 μm bimodal graded cubic CaCO₃;
- Keep oil absorption 18–24 g/100g with mild stearic acid modification;
- Set paint PVC 65–75% and reserve micro-air voids;
- Compound with talc/kaolin and silica matting agent for synergistic scattering;
- Control dispersion fineness and avoid particle agglomeration.
These measures can obviously improve dry hiding power of CaCO₃, maintain uniform matte gloss, and reduce TiO₂ dosage in matte interior/exterior paints.