Silicone as a Processing Aid in Plastic Masterbatch
Silicone-based processing aids are incorporated into plastic masterbatch concentrates to deliver mold release, surface slip, and scratch resistance to finished parts — without the bloom, extraction, or migration issues associated with conventional fatty acid amide or wax-based slip agents. The masterbatch approach allows compounders to pre-disperse ultra-high molecular weight (ultra-HMWS) silicone into a carrier resin at 20–50% loading, which end-users let down to 0.5–3% in the final part.
The three silicone material types used in plastic masterbatch are distinct in function and should not be conflated:
- Ultra-high MW PDMS (>500,000 cSt or solid rubber form): primary processing aid and slip/scratch modifier
- Fumed silica (ZEMSIL FS series): anti-block and reinforcing filler in film applications
- Silane coupling agents (CS series): surface treatment of mineral fillers to improve polymer-filler compatibility
Ultra-High MW Silicone: Mold Release and Scratch Resistance
Conventional PDMS fluids (≤100,000 cSt) migrate to the surface and bleed out under processing temperatures, causing contamination of mold tooling and print/paint adhesion problems. Ultra-HMWS silicone — supplied as a gum or in a carrier resin pellet — does not migrate freely. Instead, it forms a concentrated surface layer during molding through shear-induced alignment, providing:
- Mold release: reduction in ejection force by 30–60% in complex geometries (ribs, deep draws). Particularly valuable in glass-filled PA66 and PBT, where abrasive filler causes mold wear.
- Scratch resistance: surface CoF reduction in PP/PE automotive interior trim and appliance housings. Evaluated by Erichsen scratch tester (ISO 1518) or Mar resistance cross-hatch method.
- Surface slip: packaging films and liners benefit from reduced CoF (ASTM D1894) for high-speed form-fill-seal operation.
Effective loading in the final part: 0.3–1.5% for PP/PE; 0.5–2.0% for PA, ABS, PC/ABS; 1.0–3.0% for high-filler compounds. Excessive loading above 3% degrades weld-line strength and impairs paint adhesion by creating a silicone-rich skin.
| Polymer | Application | Recommended Silicone Form | Treat Level (finished part) |
|---|---|---|---|
| PP homopolymer | Automotive trim, film | Ultra-HMWS pellet masterbatch | 0.5–1.5% |
| HDPE film | Packaging slip film | Ultra-HMWS + FS-200 anti-block | 0.3–1.0% + 0.5–1.0% |
| PA66 GF30 | Engineering molding | Ultra-HMWS gum carrier | 1.0–2.0% |
| ABS | Appliance housing | Ultra-HMWS in ABS carrier | 0.5–1.5% |
| TPU | Film, hose | Ultra-HMWS PDMS | 1.0–3.0% |
Fumed Silica as Anti-Block in Film Applications
Polyethylene and polypropylene blown films used in packaging require anti-blocking agents to prevent adjacent film layers from adhering during storage and unwinding. Conventional anti-block agents (diatomaceous earth, talc, synthetic silica) are inorganic particles that create surface micro-roughness. ZEMSIL FS-150 and FS-200 fumed silica — with primary particle size 12–16 nm and BET surface area 150–200 m²/g — function as anti-block agents with several advantages over coarser minerals:
- Lower treat level (0.1–0.3% vs. 0.3–0.8% for diatomite) due to higher surface area and finer dispersion
- Minimal haze increase — critical for transparent food packaging films
- Secondary benefit: slight rheology reinforcement in the melt, improving bubble stability in blown film
Dispersion is critical: fumed silica must be pre-compounded into a masterbatch (typically in LDPE or LLDPE carrier at 10–20% loading) using a twin-screw extruder. Direct feeding into the blown film extruder leads to agglomeration, gels, and optical defects.
Silane as Polymer-Filler Compatibilizer
Mineral-filled PP compounds (talc, calcium carbonate, glass fiber) require coupling agents to improve stress transfer between polar mineral surfaces and the non-polar PP matrix. ZEMSIL CS-151 (vinyltriethoxysilane) and CS-171 (vinyltrimethoxysilane) are used for glass fiber sizing and CaCO₃ surface treatment. The mechanism:
- Silane methoxy/ethoxy groups hydrolyze and condense onto mineral surface hydroxyl groups → covalent Si–O–mineral bond
- Organic functional group (vinyl, methacrylate, amino) interpenetrates or reacts with polymer matrix
- Result: improved tensile strength (+15–30%), impact strength, and reduced water uptake in filled PP/PA compounds
For reactive extrusion (REX) with maleic anhydride-grafted PP (MAH-PP), aminosilane CS-550 (KH-550) is used: the primary amine reacts with anhydride groups to form imide linkages, creating strong covalent filler-matrix coupling. This is the standard approach in long glass fiber (LGF) PP and short glass fiber (SGF) PA engineering compounds.
Handling and Masterbatch Production
Key process parameters for silicone masterbatch manufacture:
- Ultra-HMWS silicone: requires high-shear twin-screw extrusion with L/D ≥ 40 and kneading block configuration to break down the gum structure. Processing temperature 180–220 °C (matched to carrier resin). Screws must be cleaned with purge compound after every run — residual silicone causes contamination in the next product.
- Fumed silica masterbatch: use co-rotating twin-screw with high-vacuum vent to remove entrained air. Feed fumed silica through a loss-in-weight feeder; starve-feed to avoid bridging. Specific energy input 0.15–0.25 kWh/kg for good dispersion.
- Pelletizing: underwater pelletizing preferred for crisp cut; strand pelletizing acceptable but requires silicone die lip for anti-stick.
Related Reading
Fumed Silica — FS Series · Silane Coupling Agents — CS-151 / CS-171 · Applications: Reinforcement · Applications: Mold Release