1. What a silica sol is
Colloidal silica is a slightly bluish, transparent dispersion of spherical SiO₂ particles, typically 6–50 nm across. For composite and functional coatings, 10–20 nm is most common, because smaller particles penetrate better and give stronger adhesion. Inside each particle, Si–O–Si bonds form a three-dimensional network. At the surface, a tightly bound layer of silanol (Si–OH) groups and structural water remains. It survives heating and reduced pressure and is only fully removed at around 700 °C.
The surface carries a large number of negatively charged hydroxyl groups. Electrostatic repulsion keeps the particles apart. When the particles collide, or when the electrolyte content is too high, they aggregate into a gel. When the sol dries, the particles join through siloxane bonds into a clear film that no longer dissolves in water. This is why silica-based coatings cure even in damp places such as tunnels, without blistering or peeling.
A small amount of Na⁺ sits at the ends of the surface silanol groups as a stabiliser. Ammonium (NH₄⁺) can also be used. Our acidic grades CH83-A30 and CH83-A1030 contain almost no sodium (Na₂O ≤ 0.05%).
2. pH: the first thing to check
| pH | Behaviour |
|---|---|
| 2–3 | Metastable zone |
| 5–6 | Least stable: gels within hours |
| 8–10 | Stable range for alkaline sols |
| > 10.5 | Silica begins to dissolve and forms alkali silicate |
Viscosity also depends on pH. As pH rises, the particles carry a thicker solvation layer and the viscosity increases. Acidic sols are less charged and less viscous. For coatings, use a sol at pH 8.5–9.5 and keep the finished paint at pH 8.5–9.5 as well.
3. Salts and electrolytes
Salts of multivalent metal ions gel colloidal silica. The released ions neutralise the surface charge that keeps the particles apart, and the particles aggregate. How fast this happens depends on the type and concentration of electrolyte and on temperature:
- Monovalent salts act slowly.
- Multivalent salts act quickly above pH 9.
- Below pH 9, the effect of salts such as alum and magnesium sulfate is smaller.
This is not always a problem. The same effect can be used on purpose to speed up setting, for example in cement systems.
4. Organic solvents
Colloidal silica mixes with ethanol, acetates and similar solvents up to a limit. Beyond it, the mixture solidifies. Compatibility also changes with pH, which matters in water-based topcoats and paper finishes.
5. Surfactants and additives
Colloidal silica is generally compatible with anionic and non-ionic surfactants. Surfactants, dispersants and levelling agents that contain electrolyte impurities easily cause gelation and should not be used. When an additive kills a batch, check its salt content first.
6. Emulsions and water-soluble resins
Resins whose pH and charge match the sol, and which contain few carboxyl groups, generally mix without trouble. With emulsions, make sure the emulsifier system does not trigger gelation.
Gelation is in essence a drying process. As water is lost and the concentration rises, Brownian motion brings particles into contact more often, and the surface silanols condense into a silica gel. The more completely the gel is dehydrated, the denser and stronger it becomes. This step cannot be reversed: the gel does not redissolve even in boiling water.
7. Viscosity
Viscosity rises with SiO₂ content. It is driven mainly by the manufacturing process, the stabiliser and the pH. Sols used as binders and in base coats need some viscosity to bond well; our lab works to about 3–6 mPa·s for that use.
8. Particle size and storage stability
Smaller particles mean more particles per unit volume, a larger specific surface area, and stronger bonding and adsorption. In elastic coatings, small high-surface-area particles raise the film modulus, reduce cracking and improve water and corrosion resistance. Our recommendation for these coatings is 10–20 nm, with a specific surface area of about 200 m²/g.
| SiO₂ particle size | 5–10 nm | 10–15 nm | 25 nm | 30 nm |
|---|---|---|---|---|
| After 6 months | No change | No change | Caking | Caking |
Checklist before you blame the silica
- Is the paint pH still between 8.5 and 9.5?
- Has any additive brought in multivalent salts or electrolyte impurities?
- Is a solvent addition beyond the miscibility limit?
- Is the emulsifier system of the resin compatible with the sol?
- Has the container lost water, or has the product been frozen?
- Is the particle size right for a 6-month shelf life? Use 10–15 nm or smaller.
Still stuck? Send us your formulation. Our coatings research institute will look at it.