Shanghai Semitech New Material Co., Ltd.
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TECHNICAL DATA
Silica Surface Treatment in Matting Agents: Untreated vs Wax vs Organic Coated
Surface treatment is the single biggest factor separating silica grades that perform from those that cause problems. The same particle size and OAV behave completely differently depending on whether the silica is untreated, wax-coated, or organically modified. This guide maps each treatment type to its performance characteristics and ideal applications.
Why Surface Treatment Matters
Raw precipitated or fumed silica is highly hydrophilic — its surface is covered with silanol (Si–OH) groups that attract water and interact strongly with polar solvents and binders. Surface treatment modifies these groups to control how the particle interacts with the coating matrix: how fast it disperses, how stably it suspends, and how efficiently it scatters light to reduce gloss.
Getting the treatment type wrong is one of the most common causes of silica-related formulation failures. For guidance on how treatment affects dosage selection, see our matting agent selection guide.
Treatment Type Overview
- Untreated (Hydrophilic) No surface modification. Silanol groups fully exposed. Disperses readily in polar and solventborne systems. Highest OAV of the three types — absorbs more binder. Poor suspension stability in waterborne systems; settles rapidly without adequate rheology control. Best for solventborne industrial, coil, and leather coatings where strong binder interaction is acceptable.
- Wax-Treated Silanol groups partially blocked by organic wax (polyethylene or Fischer-Tropsch wax). Reduces surface energy — improves anti-settling in waterborne and polar systems. Slightly lower OAV than untreated equivalent. Also imparts scratch and burnish resistance to the dry film. Matting efficiency is 5–15% lower than untreated at equal loading due to partial surface blocking. Most widely used treatment type across wood, coil, and waterborne applications.
- Organic-Treated Surface modified with organosilane, PDMS, or reactive coupling agents. Tailored for specific systems — UV-reactive treatments bond into the crosslinked network during cure, eliminating the oxygen inhibition and adhesion loss common with untreated silica in UV coatings. Lowest OAV, best compatibility in 100% solids UV and high-polarity waterborne systems. Premium cost but necessary for UV and specialty reactive systems.
Head-to-Head Comparison
| Property | Untreated | Wax-Treated | Organic-Treated |
|---|---|---|---|
| OAV (relative) | High | Medium | Low–Medium |
| Matting efficiency | Highest | Medium | Medium–High |
| Waterborne stability | Poor | Good | Excellent |
| UV system compatibility | Poor | Poor | Excellent |
| Anti-settling | Low | High | Medium–High |
| Scratch resistance (film) | Low | High | Medium |
| Transparency | Medium | Medium–High | High |
| Cost | Lowest | Medium | Highest |
Selection by Coating System
| Coating System | Recommended Treatment | Reason |
|---|---|---|
| Solventborne wood (PU, NC) | Wax-treated | Anti-settling + scratch resistance in clear PU finishes |
| Waterborne wood (PUD, acrylic) | Wax or organic | Stability in aqueous dispersion; no seeding or foam |
| UV 100% solids | Organic (UV-reactive) | Prevents cure inhibition and inter-coat adhesion loss |
| Coil coatings | Untreated or wax | High bake temp (230–260°C) degrades some wax treatments; untreated more stable |
| Industrial (SB epoxy, alkyd) | Untreated | Maximum matting efficiency; binder system tolerates high OAV |
| Leather coatings | Wax or organic | Flex resistance; wax improves soft-touch feel in PU leather |
| Powder coatings | Untreated | Wax causes off-gassing in bake oven; untreated preferred for electrostatic compatibility |
COIL COATING NOTE
Standard polyethylene wax treatments begin to decompose above 180°C. In coil coatings baked at 230–260°C (PMT), wax degradation products can cause surface defects and yellowing. Specify heat-stable wax grades (Fischer-Tropsch, melt point >140°C) or switch to untreated silica with a separate anti-settling additive.
Frequently Asked Questions
Can I use wax-treated silica in a UV coating system?
Generally not recommended. The wax coating is not reactive and sits at the particle surface during UV cure. This creates a weak boundary layer between the particle and the crosslinked network, reducing inter-coat adhesion in multi-layer UV systems. For UV applications, specify organic-treated grades with UV-reactive coupling. See our UV coating application guide.
Why does untreated silica settle so fast in waterborne coatings?
Untreated silica has a high surface energy and limited compatibility with the aqueous dispersion medium. Without wax or organic modification, the particles agglomerate quickly and settle under gravity. The settling rate is proportional to particle density and size squared (Stokes’ law) — untreated coarse grades settle the fastest. A wax or hydrophilic organic treatment creates steric stabilisation that slows agglomeration.
Does surface treatment affect transparency?
Yes. Organic-treated grades typically show the best transparency in clear coats because the surface modification reduces the refractive index mismatch between particle and binder. Untreated grades with high surface energy can scatter visible light more broadly, slightly increasing haze at equal loading. The difference is most visible in thin-film (<20 µm) clear coats where any haze is immediately apparent.
How do I verify the surface treatment type from a supplier TDS?
Look for: wax content (%), loss on ignition (LOI) vs. untreated baseline, or a specific statement of treatment chemistry. For organic-treated grades, request the coupling agent chemistry (silane type, PDMS content). If the TDS only states “treated” without specification, ask the supplier for the treatment decomposition temperature — this immediately distinguishes standard PE wax (<130°C) from high-stability wax (>140°C) from organosilane (>300°C).
Can surface treatment be combined with low particle size for UV systems?
Yes — and this is the standard approach for high-performance UV clearcoats. Fine particle grades (D50 3–5 µm) with UV-reactive organic treatment deliver deep matte with high transparency and no cure inhibition. The GMATT UV Series is engineered specifically for this combination. For the relationship between particle size and efficiency, see our particle size guide.
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