In the coatings industry, there's a saying: "A large bucket of coatings, half a bucket of minerals!" While seemingly exaggerated, this saying reveals the crucial role of non-metallic minerals in coatings.
Stepping into a coatings production workshop, you'll find that the key factors determining coating performance are often hidden in unassuming white powder. They are the "skeleton" and "muscle" of the coating—providing support for the film-forming substance, giving the coating its function, and allowing expensive resins to "spend less and do better."
I. Why Can't Coatings Do Without Non-metallic Minerals?
The basic formulation of coatings consists of four main components: film-forming substance (resin), solvent, pigment, and filler. Among them, non-metallic minerals primarily play the role of functional fillers.
Their value is reflected in two dimensions:
1. Economic Dimension: Cost Reduction and Efficiency Improvement
Titanium dioxide costs tens of thousands of yuan per ton, while calcium carbonate only costs a thousand yuan. The rational use of mineral fillers can significantly reduce formulation costs while ensuring performance. In some primers, the mass fraction of fillers even exceeds that of resin, becoming the highest-weighted component.
2. Functional Dimension: Performance Enhancers
Different mineral crystal structures endow coatings with different "superpowers"—corrosion shielding, increased hardness, rheology control, gloss adjustment… These functions are often beyond the capabilities of resins alone, or rather, are more cost-effectively achieved by minerals.
II. Analysis of the Application of Non-metallic Minerals in Coatings
1. Calcium Carbonate (CaCO₃)—The "First Filler" in Coatings
Identity Profile: Main component is calcium carbonate, divided into heavy calcium carbonate (ground ore) and light calcium carbonate (chemical precipitation). It has high whiteness and stable chemical properties, making it the most widely used filler in the coatings industry.
Functions in Coatings:
Volume Increase and Cost Reduction: Fills volume, replacing some resin, and is the main filler in primers and low-to-mid-range latex paints, with an addition amount of 20%-40%.
Application Friendly: Improves the build-up and sanding properties of coatings, preventing film sagging.
Optical Adjustment: Light calcium carbonate has high oil absorption and can be used for matting; ultrafine heavy calcium carbonate provides a certain hiding power and, when used in conjunction with titanium dioxide, can improve the utilization rate of titanium dioxide.
Application Scenarios: Interior wall latex paint, primer, putty
Technical Points: Particle size distribution significantly affects performance—too coarse results in a rough film, while too fine increases oil absorption and viscosity. Generally, finer calcium carbonate is used for topcoats, while the finer particle size can be used more leniently for primers.
2. Kaolin (Al₂O₃·2SiO₂·2H₂O) – The “Shielding Guardian” of Coatings
Identity: Hydrated aluminum silicate, with a lamellar crystalline structure, available in washed and calcined forms. Calcination results in higher whiteness and increased porosity.
Functions in Coatings:
Shielding Enhancement: The lamellar structure layers within the coating, extending the penetration path of moisture and corrosive media. This “maze effect” is key to improving coating durability.
Suspension and Anti-settling: Improves coating storage stability and prevents pigment sedimentation and clumping.
Dry Hiding Power: The microporous structure of calcined kaolin creates an “air-mineral” interface, effectively scattering light and partially replacing titanium dioxide – a crucial method for cost reduction in formulations.
Application Scenarios: Architectural latex paints, primers, industrial paints
Technical Considerations: Calcined kaolin has a much higher oil absorption rate than washed kaolin. Adjustments to the emulsion and additive amounts in the formulation are necessary to avoid excessive viscosity.
3. Talc (3MgO·4SiO₂·H₂O) – The "Flexible Champion" of Weather Resistance and Corrosion Protection
Identity: Hydrated magnesium silicate, with a flake or fibrous structure, soft texture, and smooth feel, it is a common multi-functional filler in coatings.
Functions in Coatings:
Corrosion Barrier: The parallel arrangement of the flake structure effectively blocks water and oxygen penetration, significantly improving the coating's corrosion resistance.
Improved Feel: Gives the paint film a unique smooth touch, improving sandability, which is especially important in automotive putty.
Weather Resistance and Crack Resistance: Reduces internal stress caused by temperature changes in the coating film, lowers the risk of cracking, and extends the coating's service life.
Application Scenarios: Anti-corrosion primers, automotive putty, exterior wall coatings
Technical Points: The flake structure of talc is a double-edged sword – flakes that are too large may affect the coating's gloss, while flakes that are too small weaken the shielding effect. The appropriate mesh size must be selected based on performance requirements.
4. Bentonite – A Stabilizer for Coating Storage
Identity: A layered clay mineral primarily composed of montmorillonite, possessing excellent water absorption, ion exchange, and thixotropic properties, making it a commonly used rheology modifier in coatings.
Functions in Coatings:
Thixotropic Thickening: Forms a gel network in aqueous or solvent-based systems, preventing pigment sedimentation and sagging, thus improving storage stability.
Application-Friendly: Gives coatings the characteristic of being "thick when still, thin when stirred," facilitating spraying and brushing and improving the application experience.
System Compatibility: Sodium-based bentonite is used in water-based coatings, while organically modified bentonite is required for solvent-based coatings—incorrect selection can lead to thickening failure or even demulsification.
Application Scenarios: Water-based paints, solvent-based paints, inks
Technical Points: Activation of bentonite (e.g., adding a polar activator) is a crucial step in maximizing its performance. Insufficiently activated bentonite significantly reduces its thickening effect.
III. Four Golden Rules for Selection and Application
Faced with a dazzling array of mineral fillers, how can formulators make the right choice?
Rule 1: Performance Matching First
Determine the core performance requirements based on the final application of the coating—anti-corrosion coatings prioritize barrier properties, choosing mica and talc; wear-resistant coatings prioritize hardness, choosing quartz powder and wollastonite; high-gloss topcoats prioritize gloss, choosing precipitated barium sulfate.
Rule 2: Controllable Particle Size Distribution
Different particle sizes of the same mineral have drastically different functions. Ultrafine powders improve performance but increase costs, while coarse powders reduce costs but may sacrifice surface quality. A balance must be found between performance and cost.
Rule 3: Oil Absorption Cannot Be Ignored
Minerals with high oil absorption (such as calcined kaolin and precipitated silica) will increase resin usage, thus driving up costs. When using such fillers, the total cost of the formulation needs to be recalculated.
Rule 4: The Value of Surface Treatment
Mineral fillers treated with coupling agents have better compatibility with resins and a more significant reinforcing effect. Although the unit price increases, the value brought by the overall performance improvement often exceeds the increase in cost.
