1,000,000 cSt dimethyl silicone oil (high-molecular-weight PDMS with a molecular weight of one million) can be incorporated into EVA. EVA footwear foaming system is a classic application scenario for this material. However, direct feeding of neat paste-like silicone oil is not recommended; silicone masterbatch with EVA carrier is preferred.
I. Functions (for EVA foaming / sheet materials)
Internal lubrication & improved mold release: reduces friction between melt and mold, mitigates mold sticking and reduces external spray-type release agents.
Cell optimization: lowers melt surface tension to achieve more uniform cells and minimize pinholes and bubble defects.
Enhanced wear resistance and reduced coefficient of friction: imparts a slippery surface to finished products and improves DIN abrasion resistance.
Superior thermal stability over low-viscosity silicone oils: EVA processing temperature range is 140~180°C. The 1,000,000 cSt silicone oil hardly undergoes thermal decomposition to form black streaks. Compared with low-molecular-weight silicone oils of 10,000 and 100,000 cSt, it carries a much lower risk of migration and bloom.
Mechanism: EVA is a non-polar polyolefin, and dimethyl silicone oil is also non-polar, so certain compatibility exists between them. The long chains of high-molecular-weight silicone oil are not prone to massive migration toward the surface.
II. Key Limitations (Highlights)
✅ Advantages: low blooming, high temperature resistance, good wear resistance and mold release performance
❌ Disadvantages: neat 1,000,000 cSt silicone oil is a semi-solid paste with extremely poor flowability
Direct feeding of neat silicone oil into internal mixers / extruders results in poor dispersion; it readily agglomerates and forms fisheye particles.
Paste materials are difficult for automatic metering and feeding.
Excessive dosage causes screw slippage and extrusion fluctuation. Any silicone content will reduce the surface adhesion performance of EVA (for gluing and painting) to a certain extent, and the adverse effect becomes more pronounced at higher loadings.
III. Two Feeding Strategies
Strategy 1: Recommended → EVA-carrier silicone masterbatch (40~50% silicone oil content)
The million-viscosity silicone oil is pre-dispersed in EVA carrier to form pelletized masterbatch, which can be dry-blended and fed directly with EVA pellets.
Dosage: 3–5 phr masterbatch (equivalent to 1.5–2.5 phr neat silicone oil).
Application: internally mixed foamed shoe soles and EVA sheets; this is the mainstream industrial practice with stable dispersion.
Strategy 2: Direct addition of neat 1,000,000 cSt silicone oil (internal mixer only)
Only suitable for high-shear compounding in internal mixers. Direct feeding of paste silicone oil in single-screw extrusion is not recommended as satisfactory dispersion is hard to achieve.
Neat silicone oil loading: ≤2%.
Feeding timing: Pre-mix and heat EVA and fillers first, then add silicone oil, and extend compounding time to ensure sufficient dispersion.
IV. Formulation & Process Considerations
Crosslinking system: The million-viscosity silicone oil does not interfere with DCP crosslinking, which is an advantage over low-molecular-weight silicone oils.
Adhesion requirement: If subsequent bonding or surface painting is required for finished parts, minimize silicone oil dosage. Lab adhesion tests are mandatory.
Avoid over-dosing: neat silicone oil above 2% will lead to coarse cells, reduced mechanical strength and significant deterioration of adhesion.
Comparison for Material Selection
| Silicone Oil Type | Advantages | Disadvantages | Suitable for EVA? |
|---|---|---|---|
| 1,000,000 cSt high-viscosity silicone oil | Low blooming, heat resistance, good wear & mold release | Paste form, difficult to disperse | Recommended for use as masterbatch |
| 10,000～100,000 cSt low-viscosity silicone oil | Good flowability, easy feeding | Prone to migration & bloom; thermal decomposition generates black streaks | Not recommended for foamed shoe materials |
V. Lab Trial Recommendations
Conduct lab trials first: keep formulation fixed, test neat silicone oil at 1% and 2% (or masterbatch at 3% and 5%) in gradient. Test items: dispersion appearance, cell structure, mold release performance, abrasion resistance and bonding strength.
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