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Ask Our TeamFumed Silica in Adhesives and Sealants: Thixotropy and Sag Resistance Guide
Fumed silica is the dominant rheology modifier in structural adhesives, sealants, and potting compounds. This guide covers the thixotropy mechanism, optimal dosage ranges by system type, grade selection logic, and practical formulation tips to avoid common failure modes.
How Fumed Silica Creates Thixotropy
Thixotropy is the property of a material to thin under shear stress and recover viscosity at rest. Fumed silica achieves this through a reversible physical network:
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Network Formation at Rest
Fumed silica particles carry surface silanol (–OH) groups. At rest, these form hydrogen bonds with each other and with polar groups in the base resin, creating a three-dimensional network that resists flow — giving the adhesive its no-sag character.
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Network Disruption Under Shear
When shear is applied (mixing, extrusion, spreading), the hydrogen bonds break and the silica particles disentangle, reducing viscosity dramatically. This is why fumed silica-thickened adhesives extrude easily from a gun but do not sag after application.
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Reversible Recovery
Within seconds to minutes of shear removal, the hydrogen bond network re-forms and viscosity recovers. Recovery time depends on silica loading, base resin polarity, and temperature. This is the defining feature of thixotropy versus simple viscosity increase.
Dosage Ranges by System Type
Fumed silica dosage to achieve adequate sag resistance varies significantly by base resin type, viscosity of the base, and application method. The following table gives starting-point ranges — final dosage must be validated by testing.
| System | Polarity | Recommended Grade | Typical Dosage (wt%) | Notes |
|---|---|---|---|---|
| Epoxy (liquid, low viscosity) | Polar | Aerosil R974 / R972 | 2–5% | Hydrophobic prevents moisture uptake; pre-mix with epoxy resin Part A |
| Epoxy (high viscosity paste) | Polar | Aerosil R202 | 1–3% | Lower dosage needed at high base viscosity; avoid over-thickening |
| Polyurethane (1K moisture cure) | Moderate | Aerosil R972 / R974 | 2–4% | Hydrophobic grade critical — avoids moisture reaction with NCO groups |
| Polyurethane (2K) | Moderate | Aerosil R974 | 1.5–3.5% | Add to Part A (polyol); avoids pot-life extension issues |
| Silicone (RTV-1) | Non-polar | Aerosil R202 / HDK H20 | 3–8% | PDMS-treated grades provide compatibility with silicone polymer chains |
| Silicone (RTV-2) | Non-polar | Aerosil R202 | 2–6% | Higher dosage in Part A for balanced viscosity ratio |
| MS Polymer / Hybrid | Moderate | Aerosil R972 | 2–4% | MS polymer is moisture-sensitive — hydrophobic grade essential |
| Acrylic (solvent-borne) | Moderate-polar | Aerosil 200 / R974 | 1–3% | Aerosil 200 for polar solvent systems; R974 for less polar |
| Hot Melt Adhesives | Non-polar | Aerosil R972 | 0.5–2% | Add at melt temperature; improves cohesive strength and open time stability |
Grade Selection Logic
The single most important selection criterion is system polarity and moisture sensitivity:
Use Hydrophobic (R972/R974/R202) when:
• System contains isocyanate (NCO) groups
• Base resin is non-polar (silicone, EPDM)
• Product must pass accelerated aging at high humidity
• Formulating a one-component moisture-cure product
• Long shelf life in sealed cartridges is required
Use Hydrophilic (Aerosil 200/380) when:
• System is water-based or highly polar
• Maximum thickening efficiency per gram is needed
• Two-component system mixed just before use
• Cost-in-use is the primary driver
• System tolerates some moisture uptake
BET Surface Area vs. Thickening Efficiency
Within each family, higher BET = more thickening power per gram. However, higher BET grades are also harder to disperse — they require more energy input to break aggregates and achieve uniform distribution. For most adhesive applications, 200 m²/g (hydrophilic) or 110–170 m²/g (hydrophobic) grades offer the best balance of dispersibility and performance.
Formulation Tips for Best Performance
Addition Sequence
Always add fumed silica to the liquid component, not vice versa. For 2K systems, incorporate into the component with the longer pot life (usually Part A). Pre-wetting the silica in a small portion of the resin before adding to the bulk improves dispersion quality significantly.
Mixing Protocol
- Use a high-shear dissolver (3-blade, 20–40 m/s tip speed) rather than a low-shear anchor mixer
- Mix under vacuum (≤50 mbar) where possible — removes entrained air and reduces post-mix settling
- Mix time: 15–30 minutes at high shear for full dispersion; verify by fineness of grind gauge (<20 µm agglomerate size)
- Temperature: keep below 40 °C during mixing to prevent premature cure in reactive systems
Measuring Thixotropy
The thixotropic index (TI) is the ratio of low-shear viscosity (6 rpm) to high-shear viscosity (60 rpm) measured by a Brookfield viscometer. Target TI values:
Low Sag Requirement
TI = 2–3
Typical: gap fillers, potting compounds applied horizontally
Medium Sag Resistance
TI = 3–5
Typical: structural adhesives, vertical bead application
High Sag Resistance
TI = 5–8+
Typical: overhead application, transportation adhesives, construction sealants
Troubleshooting Common Issues
| Symptom | Likely Cause | Solution |
|---|---|---|
| Product sags immediately after application | Insufficient silica loading or poor dispersion | Increase dosage by 0.5% increments; verify TI >3; check fineness of grind |
| Product too stiff to extrude from cartridge | Over-dosage or too high BET grade | Reduce loading; switch to lower BET grade (e.g., 110 m²/g from 200 m²/g) |
| Viscosity drops during shelf storage | Silica settling or network disruption by heat | Add colloidal clay (bentonite) as co-thickener; store below 25 °C |
| Rapid viscosity increase after mixing 2K components | Moisture in hydrophilic silica reacting with NCO | Switch to hydrophobic grade (R972/R974); dry ingredients before mixing |
| White streaks or undispersed lumps | Inadequate shear during incorporation | Increase mixer tip speed; pre-wet silica in portion of resin; extend mix time |
| Transparency loss in clear sealant | Silica agglomerate size >100 nm causing scattering | Extend high-shear mixing; use lower BET grade for easier dispersion |