Silicone Additives in Printing Inks and Overprint Varnishes
Silicone oils — specifically low-viscosity polydimethylsiloxane (PDMS) and polyether-modified silicone surfactants — are widely used as functional additives in printing inks (offset, flexo, gravure, UV-cure) and overprint varnishes (OPV). Their primary roles are slip enhancement, anti-blocking, surface leveling, and controlled wettability modification. Because silicones migrate to the air-liquid interface during film formation, very low treat rates (0.05–1.0%) produce disproportionately large surface effects.
The challenge for ink and OPV formulators is that silicones simultaneously improve some properties and degrade others. Managing these trade-offs — slip vs. re-coatability, gloss vs. blocking resistance, wettability vs. overprint adhesion — is the core formulation task.
Functional Roles and Mechanism
Slip and scratch resistance. PDMS-based slip additives reduce the kinetic coefficient of friction (CoF) of a cured ink or varnish film from >0.5 to below 0.2. The silicone fluid migrates to the surface during drying/curing, forming a low-surface-energy boundary layer. This is critical for packaging substrates that pass through converting equipment (slitters, folders, case erectors) at high speed.
Anti-blocking. Blocking occurs when stacked or wound printed sheets bond under pressure and heat. Surface-oriented silicone reduces the contact area and adhesion energy between layers. For heat-set web offset and solvent-based gravure inks, PDMS at 0.1–0.3% is standard practice.
Leveling and crater prevention. Polyether-modified silicones (ZEMSIL PE series) reduce dynamic surface tension faster than conventional surfactants. In UV-cure inkjet and OPV systems, this prevents crater formation from substrate contamination and improves dot gain uniformity.
Wettability control. Linear PDMS reduces surface tension of the ink vehicle, improving wetting on low-energy substrates such as PP films and PE-coated board. However, excessive PDMS loading lowers surface tension below the critical wetting threshold of subsequent coatings or adhesives — a primary cause of delamination in laminated packaging.
Product Selection and Trade-Offs
| Property | Low-viscosity PDMS (e.g., DM-50) | Polyether-modified silicone (PE series) | Reactive silicone acrylate |
|---|---|---|---|
| Slip (CoF reduction) | Excellent | Moderate | Good |
| Leveling | Moderate | Excellent | Good |
| Gloss contribution | Neutral/slight reduction | Slight reduction | Neutral |
| Re-coatability / overprint adhesion | Risk if overdosed | Good (water-dilutable grades) | Excellent (bonds into network) |
| UV-cure compatibility | Inhibition risk (high loading) | Generally compatible | Covalently bonded — no migration |
| Recommended loading | 0.05–0.3% | 0.1–0.5% | 0.5–2.0% |
Gloss trade-off. High-gloss OPVs are adversely affected by surface-migrating PDMS at loadings above 0.2%. The silicone film at the surface scatters light. Where 85° gloss targets exceed 85 GU, reactive silicone acrylates (which bond into the UV network and do not migrate) are preferred over free PDMS.
Re-coatability and overprint adhesion. The most common complaint with silicone slip additives is failure of subsequent overprint varnish or adhesive layers. Surface PDMS creates a low-energy barrier that reduces adhesion of the next applied layer. Mitigation strategies include: using polyether-modified silicone (more hydrophilic, better re-wettability), using reactive silicone acrylates, or limiting PDMS loading to ≤0.1% and corona-treating the substrate before lamination.
Formulation Guidelines by Ink Type
Offset lithography (heatset and coldset). Paste inks with high viscosity (40–100 Pa·s) require silicone compatibility testing with the hydrocarbon vehicle. PDMS at 5–20 cSt is preferred; high-viscosity grades do not disperse well in paste ink. Loading: 0.05–0.15%.
UV flexo and UV offset. PDMS can retard free-radical cure by diluting photoinitiator and by oxygen barrier disruption at the surface. Acrylate-functional silicones (ZEMSIL SR-AC reactive silicone) eliminate this risk by copolymerizing during cure. Non-reactive loading must not exceed 0.3% in photoinitiator-lean systems.
Solvent-based gravure and flexo. PDMS is highly soluble in aromatic and ester solvents. Loading 0.1–0.3% DM-50 or DM-100 provides full slip benefit. Polyether silicones can cause foam in high-shear gravure cells; use foam-tested grades or substitute PDMS.
Water-based flexo. PDMS is incompatible with water-based systems without emulsification. Use ZEMSIL SE-DM silicone emulsion or polyether-modified silicone (PE series) that is water-soluble. Loading: 0.2–0.5% on active silicone basis.
Quality Control and Testing
Key test methods for silicone performance in inks:
- CoF (ASTM D1894): measure static and kinetic CoF on printed/cured drawdown; target kinetic CoF <0.25 for packaging
- Blocking resistance (ASTM D2793 / DIN 16901 variant): stack printed sheets under 100–500 g/cm² at 50 °C/24 h; assess peel and surface transfer
- Overprint adhesion (cross-hatch, ISO 2409): apply second OPV layer over slip-additivated substrate; cross-hatch tape test after 24 h cure
- 60° gloss (ASTM D523): benchmark against silicone-free control; accept <3 GU reduction for high-gloss OPV
- Surface tension (Wilhelmy plate or pendant drop): monitor that ink surface tension stays above 30 mN/m to preserve lamination bond strength
Related Reading
Silicone Oil — ZEMSIL DM Series · Polyether Silicone — PE Series · Applications: Release Liner Coating · Applications: Waterborne Coating Additive