Add a small amount of polydimethylsiloxane to varnish. It mainly delivers excellent leveling, slip, defoaming, anti-blocking and hydrophobic effects. However, excessive dosage will cause gloss loss and compromise recoatability and adhesion. Viscosity and addition amount must be strictly controlled.
I. Functions & Effects (Excellent Performance with Proper Dosage)
Leveling & Crater Prevention
It reduces surface tension and enables rapid wetting, minimizing pinholes, craters, fish eyes and orange peel defects for a flatter, brighter film.
Slip, Scuff Resistance & Anti-blocking
An ultra-thin silicone film forms on the surface to provide smooth touch and superior scuff resistance. It effectively prevents page sticking and blocking during printing and stacking.
Defoaming & Foam Suppression
A tiny dosage can eliminate and inhibit foam, avoiding pinholes during application. It is especially suitable for heavy coating and high-speed coating processes.
Hydrophobicity, Water Resistance & Stain Resistance
The silicone film is water-repellent, improving water resistance, moisture resistance and salt spray resistance, and making the surface easy to clean.
Gloss Enhancement & Weather Resistance
A small addition increases surface gloss, resists aging and yellowing, and extends the service life of the coating.
II. Risks & Adverse Effects (Problems Caused by Overdosage)
- Gloss Loss & Hazing: Dosage over 0.5% will lead to sharp gloss drop, fogging and whitening.
- Recoating Failure: Silicone migrates to the film surface, drastically reducing interlayer adhesion. Peeling and craters often occur during rework and topcoating.
- Cross-contamination of Craters: It may contaminate other batches or substrates, and silicone contamination is difficult to remove afterwards.
- Poor Compatibility: Conventional polydimethylsiloxane tends to separate and precipitate in acrylic and polyurethane varnishes. Modified silicone is recommended instead.
III. Practical Formulas & Ratios (Key Parameters)
1. Product Selection (Preferred Options)
- Solvent-based varnish: Polydimethylsiloxane with viscosity of 50–500 mPa·s (25℃) (e.g. 100cs, 350cs).
- Water-based varnish: Polyether-modified silicone (e.g. DC-51, BYK-346) to avoid phase separation.
2. Dosage Control (Strict Compliance Required)
- Solvent-based system: 0.05%–0.3% by weight. 0.1%–0.2% for leveling and anti-blocking; 0.05%–0.1% for defoaming.
- Water-based system: 0.2%–0.5%. Modified silicone features better compatibility.
- Maximum limit: ≤0.5%. Exceeding this value will definitely cause gloss loss and poor recoatability.
3. Addition Procedure
- Dilute silicone oil with xylene, ethanol or water to prepare a 5%–10% stock solution.
- Add the solution slowly under low-speed stirring (300–500 rpm) to avoid air entrapment.
- Continue stirring for 5–10 minutes after addition, then let it stand for defoaming before use.
IV. Application Reference
- Water-based overprint varnish for printing: Add 0.2%–0.4% modified silicone for anti-blocking, slip improvement and leveling.
- Solvent-based varnish for woodwork & furniture: Add 0.1%–0.2% 350cs silicone oil for scuff resistance and crater prevention.
- Automotive polishing varnish: Add 0.5%–1.0% 100–500cs silicone oil to boost gloss, enhance hydrophobicity and protect the coating film.
- Topcoat varnish for leather: Add 0.3%–0.5% modified silicone to achieve smooth feel, water resistance and dry/wet rub resistance.
V. Key Notes to Avoid Problems
- Use the minimum effective dosage: Always start with a low dosage. Conduct pre-tests first (add 0.1 g silicone oil to 100 g varnish) to inspect gloss, recoatability and adhesion.
- Prioritize modified silicone: Choose polyether-modified silicone for water-based and high-polarity systems for better compatibility and less precipitation.
- Avoid incompatible combinations: Do not mix with fluorinated leveling agents, as antagonistic reaction and craters may occur.
- Remove silicone before recoating: For rework or topcoating, thoroughly clean the surface with isopropanol or silicone remover, and test adhesion in advance.
Links