Pigment sedimentation, sag on vertical surfaces, and shelf-life failures all trace back to the same root cause: insufficient low-shear network strength. Shanghai Semitech supplies hydrophilic and hydrophobic fumed silica (SEMISIL series, 15,000 MT/year capacity) that builds the 3D hydrogen-bonded network coating, ink and adhesive formulators rely on for anti-settling and thixotropic performance — replacing organobentonite, castor wax and polyamide-wax thixotropes in solvent-based and waterborne systems.
Anti-Settling vs Thixotropic — Two Behaviors, One Mechanism
Procurement specs often treat anti-settling agent and thixotropic agent as two SKUs. Chemically they describe two faces of the same rheology: a particulate or polymer network that holds the liquid phase rigid at rest and yields under shear.
| Behavior | State | What the formulator measures | End-user benefit |
|---|---|---|---|
| Anti-settling | Static (storage, transport) | Brookfield viscosity at 0.5 rpm; 30-day pigment-layer test | No hard-sediment after 6+ months; consistent shade at point of use |
| Thixotropic | Dynamic (spray, brush, mixing) | Thixotropic index (ηlow/ηhigh); recovery time post-shear | No sag on vertical surfaces; clean spray atomization; brush leveling |
Fumed silica delivers both because the hydrogen-bonded aggregate network reforms within seconds of shear ceasing — fast enough to stop pigment drop in cans, slow enough to allow application leveling. A single grade addresses both spec lines.
Where the Network Matters
Industrial & Protective Coatings
Sag-resistant epoxy and polyurethane topcoats on vertical steel structures, anti-corrosive primers for marine and bridges. Typical loading 1.0–2.0%.
Architectural & Decorative Paint
Pigment anti-settling in TiO₂-loaded waterborne emulsion paints; controlled drip in roller and brush application.
Automotive Refinish
Thixotropic flow control in 2K spray basecoats, metallic flake orientation, sag prevention on body panels and door frames.
Printing Inks
Solvent-based gravure and flexo inks — pigment dispersion stability and short-set rheology for high-speed web presses.
RTV Silicone Sealants
One- and two-component RTV — extrusion-quality flow under cartridge pressure, slump-free tooling after application.
Adhesives & Mastics
Epoxy, polyurethane and MS-polymer adhesives — controlled bead profile, no flow on vertical substrates.
Unsaturated Polyester & Vinyl Ester
Gel coat and laminating resin — fiber wet-out without resin drain on vertical mould surfaces; pigment anti-settling in pre-coloured gel coat.
Why Fumed Silica Outperforms Organic Thixotropes
Organic anti-settling agents — hydrogenated castor-oil wax (HCO), polyamide wax, organobentonite, organic urea derivatives — all build a network through aggregation or swelling. Each carries a process or performance limitation that fumed silica avoids.
| Property | Fumed silica (SEMISIL) | HCO castor wax | Organobentonite | Polyamide wax |
|---|---|---|---|---|
| Activation | Cold dispersion + high-shear mill | Hot dissolution 50–65 °C | Pre-gel with polar activator | Hot grind > 80 °C |
| Film transparency | Transparent at 0.5–2.0% | Slight haze | Visible tint/haze | Slight cloudiness |
| Polar/nonpolar versatility | Both (grade selection) | Polar systems only | Nonpolar preferred | Both |
| Re-dispersion after settling | Yes — gentle stir | Hard cake risk | Hard cake risk | Hard cake risk |
| Regulatory status | EINECS listed, food contact grades available | Allergen concerns (castor bean) | Contains quaternary ammonium | Clean label variants only |
Recommended SEMISIL Fumed Silica Grades
| Grade | BET (m²/g) | Surface | Best for |
|---|---|---|---|
| SEMISIL 200 | 200 ± 25 | Hydrophilic | Waterborne architectural and industrial coatings |
| SEMISIL 300 | 300 ± 30 | Hydrophilic | High-pigment-volume waterborne emulsions, inks |
| SEMISIL R-202 | 200 ± 25 | PDMS-treated hydrophobic | Solvent-based coatings, clear lacquers |
| SEMISIL R-972 | 110 ± 20 | DDS-treated hydrophobic | 2K polyurethane and epoxy coatings |
| SEMISIL R-805 | 150 ± 25 | Octylsilane hydrophobic | RTV sealants, solvent-based adhesives |
All grades are available in 10 kg multi-wall paper bags and 250 kg bulk bags. MOQ 1 MT per grade.
Dosage & Processing — Practical Formulation Guidance
Typical loading
- Architectural emulsion paint: 0.3–0.8% on total formulation weight
- Industrial solvent-based coatings: 0.8–1.5%
- RTV sealants and adhesives: 5–12% (acts as both reinforcing filler and thixotrope)
- Unsaturated polyester gel coat: 1.5–2.5%
- Printing inks: 0.5–1.5%
Dispersion requirements
- Peripheral disc speed 10–15 m/s minimum to break primary aggregates — without it, thickening efficiency drops by 40–60% and visible seeds appear in clear systems
- Add silica to the resin / vehicle before pigment grind — silica wets best in low-viscosity polar fluid
- For waterborne systems, pre-wet hydrophobic grades in a co-solvent (glycol ether) before water-phase addition
- Avoid prolonged high-shear post-grind — over-shearing breaks the aggregate skeleton and reduces low-shear viscosity
Order of addition
- Charge resin and solvent to mixer
- Add SEMISIL fumed silica slowly under increasing shear (avoid dusting)
- Disperse 15–20 min at 10–15 m/s peripheral speed
- Add pigment, extenders, additives
- Let down to final viscosity
FAQ — Anti-Settling & Thixotropic Formulation
Q1: How does fumed silica prevent pigment hard-settling during 6+ months of storage?
A 0.5–1.5% loading of hydrophilic fumed silica creates a continuous 3D hydrogen-bonded network through the liquid phase that physically suspends pigment particles. The yield stress of this network (typically 1–10 Pa) exceeds the gravitational stress on suspended TiO₂, iron oxide and organic pigments, preventing them from migrating downward. Even if a soft sediment forms, it can be re-incorporated by gentle stirring — there is no hard-cake compaction.
Q2: What is the difference between hydrophilic and hydrophobic fumed silica for anti-settling?
Hydrophilic grades (SEMISIL 200/300) work best in polar systems — waterborne paints, alcohol-based inks, polar-solvent coatings — because the silanol surface forms strong hydrogen bonds in the polar medium. Hydrophobic grades (SEMISIL R-202/R-972/R-805) are PDMS- or alkyl-treated to repel water, making them ideal for solvent-based systems where water sensitivity would otherwise damage the network and the final film.
Q3: Why is fumed silica preferred over organobentonite for clear topcoats?
Fumed silica is optically transparent at typical loadings (0.5–2.0%) because its primary aggregates are below 200 nm and refractive index closely matches polymer binders. Organobentonite particles are larger and carry surface pigmentation that produces a visible tint and haze. For automotive clearcoats, wood lacquers and high-end industrial finishes, fumed silica preserves film clarity while delivering equivalent sag resistance.
Q4: Can fumed silica replace polyamide wax thixotropes one-to-one?
Loading is not 1:1 but the result is equivalent. Polyamide wax is typically used at 0.8–2.0% and requires hot grinding above 80 °C. SEMISIL 200 achieves the same thixotropic index at 0.5–1.5%, disperses cold in the standard pigment-grind step, and avoids the process risk of waxy phase separation when re-heated. Formulators replacing polyamide wax should start at 70% of the original wax loading and adjust by Brookfield reading.
Q5: What grade of fumed silica works best for waterborne architectural paint?
SEMISIL 200 is the default choice for waterborne emulsion paints. Its hydrophilic silanol surface disperses readily in water without pre-wetting, and at 0.3–0.8% loading it controls TiO₂ anti-settling and brush-drag rheology without affecting open time or recoat properties. For higher pigment volume concentration (PVC > 60%) formulations, SEMISIL 300 provides stronger network structure.
Q6: How much shear is needed to fully disperse fumed silica?
A high-speed disperser with 10–15 m/s peripheral disc speed for 15–20 minutes is the minimum. Below 8 m/s, primary silica aggregates do not break apart and thickening efficiency drops sharply. Above 20 m/s and beyond 30 minutes, over-shearing fractures the aggregate skeleton itself and the network can no longer rebuild — both yield stress and thixotropic recovery drop.