Role of Silicone Additives in Waterborne Systems
The shift from solventborne to waterborne coatings creates a persistent surface-defect problem. Solventborne systems carry surface tensions of 22–25 mN/m, close to most substrate surfaces. Waterborne systems sit at 45–55 mN/m — far above the critical surface energy of many substrates and well above the surface energy of any contaminants present. The result is cratering, fish-eye, and pinholes as the high-surface-tension film dewets from low-energy areas during application and drying.
Polyether-modified silicone additives solve this by migrating to the air–film interface and reducing the coating surface tension to 22–28 mN/m at loadings of 0.1–0.5%. At these concentrations the silicone forms a monomolecular surface layer that wets the substrate, allows air bubbles to escape, and promotes flow into surface irregularities before the film locks up. Two functional types exist: wetting agents (EO-rich polyether silicones, which reduce dynamic surface tension fastest) and leveling agents (EO/PO block polysiloxanes, which provide slower, more sustained flow and improve final gloss).
Selection Table
| Function | Silicone Type | Loading | Compatible Resins |
|---|---|---|---|
| Substrate wetting | Polyether silicone (EO-rich) | 0.05–0.2% | Acrylic, PU, alkyd |
| Leveling / anti-cratering | Polyether-polysiloxane block | 0.1–0.3% | All waterborne |
| Slip / scratch resistance | PDMS emulsion | 0.2–0.5% | Topcoats |
| Defoaming | Polydimethylsiloxane emulsion | 0.05–0.15% | All systems |
Compatibility and Overdose Risk
Silicone additives in waterborne coatings are potent surface-active compounds; overdose creates more problems than it solves.
- Overdose (>0.5%) causes silicone crawling (retraction of the film from edges and fasteners), recoat adhesion loss, and inter-coat delamination in multi-layer systems.
- Test compatibility by drawdown on representative substrate before full batch — some polyether silicones are incompatible with associative thickeners (HEUR type) and break the thickener network, causing sag.
- EO/PO ratio governs the foam–wetting trade-off: high EO gives maximum substrate wetting but generates foam; high PO suppresses foam but reduces dynamic wetting efficiency. Most commercial wetting agents are optimized blends.
- Never add neat PDMS fluid directly to the aqueous phase — it will not disperse and will produce persistent macro-defects. Always use the emulsified grade or pre-dilute in a water-miscible co-solvent.
Dosage and Addition Sequence
Addition order matters in waterborne coatings manufacturing:
- Add wetting agents to the grind phase (along with pigments and dispersants) — early addition ensures the wetting agent is present during pigment incorporation and maximizes pigment surface coverage.
- Add leveling agents to the let-down phase after pigment grinding is complete — adding leveling agents during grind causes excessive foam and can disrupt the dispersion.
- Split defoamer addition: add 50% to the grind phase to control foam during high-shear dispersing, and hold 50% for the let-down phase to knock back foam introduced during thinning and mixing.
- Start at the low end of the recommended range and adjust up in 0.05% increments, evaluating each drawdown for defect elimination versus overdose symptoms.
Related Reading
Polyether Silicone Oil · Silicone Emulsion · Silicone Oil for Defoamers