Technical Guide Film Additives 2026
Anti-Blocking Silica
Selecting the Right Grade for Plastic Film Applications
How to eliminate film blocking in BOPP, LLDPE, CPP, and PET production. Built for film compound formulators, masterbatch technologists, and packaging film procurement engineers.
Contents
What Is Film Blocking?
Film blocking is unwanted adhesion of two polymer film surfaces under pressure — caused by van der Waals forces and polymer chain entanglement at the interface. It tears film, jams slitters and bag-making equipment, and causes print registration defects. Blocking worsens with higher temperature, winding tension, surface smoothness, and thinner gauge.
How Silica Anti-Blocking Agents Work
Synthetic amorphous silica particles (d50 3–5 µm) migrate toward the film surface during extrusion. Protruding particles create micro-rough topography that physically separates wound layers, reducing real contact area to near-zero. Unlike organic slip agents (erucamide, oleamide), silica anti-blocking is permanent, temperature-stable, and print/lamination safe.
Particle Size vs Film Gauge
The rule: target silica d50 of 0.2–0.4x film gauge. Below 0.2x, particles are buried and ineffective. Above 0.4x, particles protrude excessively causing haze and pullout.
| Film Gauge | Target d50 | Recommended Grade |
|---|---|---|
| 10–15 µm (ultra-thin BOPP, BOPET skin) | 2–5 µm | SA-23 |
| 15–25 µm (standard BOPP, thin CPP) | 3–6 µm | SA-23 |
| 25–40 µm (thick CPP, LLDPE stretch) | 4–8 µm | SA-25 |
| 40–80 µm (heavy CPP, LDPE agricultural) | 5–12 µm | SA-25 |
| 80–200 µm (blown sacks, heavy PE) | 5–15 µm | SA-25 at higher loading |
SA-23 (d50 3.0–4.0 µm) is specified for clarity-grade BOPP/BOPET where haze is a release criterion. SA-25 (d50 4.0–5.0 µm) is acceptable where optical clarity is not primary.
COF Measurement and Targets
COF is measured per ASTM D1894 or ISO 8295. Static COF governs film feeding; dynamic COF governs winding and slitting speed.
| COF Type | Problem Range | Acceptable | Good |
|---|---|---|---|
| Static | > 0.6 | 0.3–0.5 | < 0.3 |
| Dynamic | > 0.5 | 0.2–0.4 | < 0.3 |
SEMITECH SA-23 at 0.10–0.20 wt% achieves dynamic COF < 0.3 in 20 µm BOPP film under standard compounding conditions.
Masterbatch Compounding Process
- Carrier resin: LLDPE for PE lines; PP homo/random for BOPP/CPP. Carrier MFI should exceed film resin MFI.
- Active content: 5–10 wt% standard; up to 20 wt% for heavy-gauge PE.
- Twin-screw extrusion: 180–230°C (PE), 200–240°C (PP). No pre-drying or pre-grinding needed for SA-23/SA-25.
- Let-down: 1–5% masterbatch to achieve 0.05–0.30 wt% finished-film loading.
Common errors: adding silica to sealant layer (raises seal initiation temp), overloading beyond 0.30 wt% without validating haze, carrier with mismatched density/MFI.
Grade Selection Summary
| Criterion | SA-23 | SA-25 |
|---|---|---|
| d50 | 3.0–4.0 µm | 4.0–5.0 µm |
| Film gauge | < 25 µm | ≥ 25 µm |
| Optical clarity | Higher | Moderate |
| BOPP clarity / thin OPP | ✓ Primary | Not recommended |
| Heavy-duty PE blown sacks | Under-performs | ✓ Primary |
| Evonik equivalent | SYLOID® FP244 | SYLOID® FP246 |
| Cost advantage vs Evonik | 30–45% lower landed | 30–45% lower landed |
Silica vs Organic Slip Agents
| Property | Silica (SA-23/SA-25) | Erucamide / Oleamide |
|---|---|---|
| Mechanism | Physical micro-spacer (permanent) | Surface bloom (migration) |
| Durability | Permanent | Depletes over time |
| Temperature stability | Stable to 300°C+ | Blooms faster at elevated temperature |
| Print/lamination safety | No contamination | Risk of bloom on ink/adhesive |
| Food contact | E551, FDA 21 CFR 172.480 | Subject to migration limits |
Many formulations combine silica (long-term anti-blocking) with a low-level slip agent (short-term COF reduction) for best total performance.
Troubleshooting
| Symptom | Likely Cause | Corrective Action |
|---|---|---|
| Film blocking despite silica addition | Loading too low, or d50 too small for gauge | Increase loading or switch to SA-25 |
| Haze increase after silica addition | d50 too large, or loading too high | Switch to SA-23 or reduce loading |
| Inconsistent COF across roll width | Uneven silica dispersion | Improve masterbatch mixing, check carrier MFI compatibility |
| Heat-seal strength reduction | Silica added to sealant layer | Move silica to skin layer; reduce loading |
| Blocking worsens after summer storage | Thermal blocking — loading insufficient | Increase loading; consider combination with slip agent |
| Surface specks / particle pullout | Agglomerates or d50 too large | Check masterbatch dispersion; confirm d50 by laser diffraction |
FAQ
+What test method is used to measure film blocking force?
ASTM D3354 — two film samples pressed together at controlled conditions, then peeled apart. COF is measured separately per ASTM D1894 or ISO 8295. Run both tests at conditions representative of production.
+Can SEMITECH SA-23/SA-25 be used in recycled or regrind PE compounds?
Yes, with the same compounding process. Validate COF on the regrind compound rather than assuming target values from virgin material specifications.
+What documentation does SEMITECH provide per batch?
Full CoA covering d50 (laser diffraction), pH, loss on drying, loss on ignition, oil absorption, and SiO₂ content. REACH compliance and food-contact declarations available on request.