A reference flame retardant formulation for SK Polyester ES500 (UL94 V0 rating).
I. Formulation Design Approach
- Substrate Compatibility
- SK Polyester ES500: A thermoplastic polyester with a typical processing temperature of 220–260°C. The flame retardant must withstand this temperature range.
- Key Requirements: Balance flame retardancy (V0), mechanical properties (tensile/impact strength), and processing fluidity.
- Synergistic Flame Retardant System
- Ultrafine Aluminum Hydroxide (ATH): Primary flame retardant, endothermic dehydration. Loading must balance flame retardancy and mechanical properties.
- Aluminum Hypophosphite: Char-forming synergist, works with ATH to create a phosphorus-aluminum synergistic effect, improving char quality.
- Zinc Borate: Char enhancer, suppresses smoke, and forms a dense barrier with ATH.
- MCA (Melamine Cyanurate): Gas-phase flame retardant, dilutes oxygen and inhibits melt dripping.
II. Recommended Formulation (Weight Percentage)
| Component | Ratio | Processing Notes |
|---|---|---|
| SK Polyester ES500 | 45–50% | Base resin; select high-fluidity grade to compensate for filler viscosity. |
| Ultrafine ATH | 25–30% | Surface-modified with silane coupling agent (KH-550), D50 < 3 μm. |
| Aluminum Hypophosphite | 10–12% | Heat-resistant (>300°C), pre-mixed with ATH and added in stages. |
| Zinc Borate | 6–8% | Added with MCA to avoid high-shear structural damage. |
| MCA | 4–5% | Process temperature < 250°C, low-speed dispersion. |
| Dispersant | 2–3% | Polyester-compatible dispersant (e.g., BYK-161) + polyethylene wax composite. |
| Coupling Agent (KH-550) | 1% | Pre-treats ATH and aluminum hypophosphite; ethanol immersion followed by drying. |
| Anti-Dripping Agent | 0.5–1% | PTFE micropowder to suppress melt ignition. |
| Processing Aid | 0.5% | Zinc stearate (lubrication and anti-sticking). |
III. Key Process Controls
- Dispersion Optimization
- Pre-treatment: Soak ATH and aluminum hypophosphite in 1% KH-550 ethanol solution for 2 hours, then dry at 80°C.
- Mixing Sequence:
- Base resin + dispersant + coupling agent → Low-speed mixing (500 rpm, 5 min).
- Add modified ATH/aluminum hypophosphite → High-speed shear (2500 rpm, 20 min).
- Add zinc borate/MCA/PTFE → Low-speed mixing (800 rpm, 10 min).
- Equipment: Twin-screw extruder (temperature zones: feed zone 200°C, melting zone 230°C, die 220°C).
- Processing Temperature Control
- Ensure melt temperature < 250°C to prevent MCA decomposition (MCA decomposes at 250–300°C).
- Water-cool pellets after extrusion to prevent flame retardant migration.
IV. Flame Retardant Synergistic Mechanism
- ATH + Aluminum Hypophosphite
- ATH absorbs heat and releases water vapor, diluting flammable gases.
- Aluminum hypophosphite catalyzes dense char formation (AlPO₄), blocking heat transfer.
- Zinc Borate + MCA
- Zinc borate forms a glassy barrier over char cracks.
- MCA decomposes to release NH₃, diluting oxygen and inhibiting free radical reactions.
- PTFE Anti-Dripping
- PTFE micropowder forms a fibrous network, reducing melt-drip ignition risk.
V. Performance Tuning & Troubleshooting
| Common Issue | Solution |
|---|---|
| Flame retardancy below V0 (V1/V2) | Increase aluminum hypophosphite to 12% + MCA to 5%, or add 2% encapsulated red phosphorus (synergistic with aluminum hypophosphite). |
| Reduced mechanical properties | Reduce ATH to 25%, add 5% glass fiber (reinforcement) or 3% maleic anhydride-grafted POE (toughening). |
| Poor processing fluidity | Increase dispersant to 3%, or add 0.5% low-MW polyethylene wax (lubrication). |
| Surface blooming | Optimize coupling agent dosage or switch to titanate coupling agent (NDZ-201) for better interfacial bonding. |
VI. Validation Metrics
- UL94 V0 Test:
- 1.6 mm and 3.2 mm specimens, total burn time < 50 sec after two ignitions, no dripping ignition.
- LOI: Target ≥30% (actual ≥28%).
- Mechanical Properties:
- Tensile strength > 40 MPa, impact strength > 5 kJ/m² (ASTM standard).
- Thermal Stability (TGA):
- Char residue at 800°C > 20%, initial decomposition temperature > 300°C.
VII. Example Reference Formulation
| Component | Content (%) |
|---|---|
| SK Polyester ES500 | 48% |
| Ultrafine ATH (modified) | 28% |
| Aluminum Hypophosphite | 11% |
| Zinc Borate | 7% |
| MCA | 4% |
| BYK-161 Dispersant | 2.5% |
| KH-550 Coupling Agent | 1% |
| PTFE Anti-Dripping Agent | 0.8% |
| Zinc Stearate | 0.5% |
This formulation and process design effectively achieve UL94 V0 flame retardancy for SK Polyester ES500 while balancing processability and mechanical properties. Small-scale trials are recommended to verify dispersion before fine-tuning ratios (e.g., balancing aluminum hypophosphite and MCA). For further flame retardancy enhancement, consider adding 2% boron nitride nanosheets (BNNS) as a dual-functional thermal conductive/flame retardant filler.
More info., pls contact lucy@taifeng-fr.com
Post time: Jul-01-2025
