Why silica matting agent causes white spots in coatings (and how to fix it)

Short answer

White spots in matte coatings are usually not caused by the silica itself, but by how it is dispersed and matched with the system. In most cases, the issue comes from poor dispersion, particle size mismatch, or incompatible surface treatment.

1. Poor dispersion is the most common reason

Silica matting agents have high surface area and tend to form agglomerates. If dispersion is not sufficient, these agglomerates remain in the coating film and appear as white spots after drying.

Typical signs:

• Visible white dots or specks
• Uneven gloss across the surface
• Slight roughness when touched

What usually goes wrong:

• Shear force is not high enough
• Silica is added at the wrong stage
• No suitable dispersant is used

In practice, for precipitated silica with typical surface area in the range of 300–700 m²/g, proper dispersion often requires relatively high shear conditions. If dispersion is not fully achieved, defects are very likely to appear.

👉 In real cases, most white spot issues are related to dispersion rather than the silica quality itself.

2. Particle size does not match the film thickness

Another common issue is improper particle size selection.

• If particles are too large → they protrude from the film → light scattering → white spots
• If particles are too small → matting efficiency drops → gloss becomes unstable

A practical guideline:

In many cases, problems occur when the upper particle size (D90) is not controlled properly and exceeds the effective film thickness.

3. Surface treatment incompatibility

Surface treatment plays a major role in how silica behaves in different systems.

Hydrophobic silica (for example, silane-treated or wax-treated grades):

• Works well in solvent-based or UV systems
• May cause wetting and dispersion issues in waterborne systems

If the treatment is too hydrophobic for the system:

• Particles may not wet properly
• Micro-aggregation can occur
• White spots or haze may appear

This is often misinterpreted as a “dispersion issue”, while in fact it is a compatibility problem.

4. Excessive loading

Increasing dosage does not always improve matting performance.

When loading is too high:

• Particles start to overlap
• Dispersion becomes less stable
• Surface defects become more visible

For most coating systems, a typical working range is around 3–8%.
Once the level goes significantly higher, the risk of defects increases noticeably.

How to fix white spots

Based on practical experience, the following adjustments are usually effective:

✔ Improve dispersion

• Increase shear force if possible
• Use a suitable dispersant
• Optimize the addition sequence

✔ Adjust particle size

• Use smaller particle size for thinner films
• Pay attention to D90, not only D50

✔ Match surface treatment

• Waterborne systems → avoid overly hydrophobic grades
• Solvent/UV systems → hydrophobic silica is usually more stable

✔ Optimize dosage

• Start from a lower level and increase gradually
• Avoid overloading just to reduce gloss

If the issue still remains

If white spots persist even after improving dispersion:

Check the overall formulation:

• Binder compatibility
• Drying speed (too fast can trap defects)
• Film thickness uniformity

Final takeaway

White spots are rarely a single-factor problem.
In most cases, they come from the interaction between silica properties, formulation design, and processing conditions.

Need help with grade selection?

If you are facing similar issues, you can share your system details (resin type, solids content, film thickness). Based on that, we can help evaluate the situation and suggest a more suitable silica grade and usage approach.