Reference Flame-Retardant Formulation for Thermosetting Acrylic Adhesive
To meet the UL94 V0 flame-retardant requirements for thermosetting acrylic adhesives, considering the characteristics of existing flame retardants and the specificities of thermosetting systems, the following optimized formulation and key analysis are proposed:
I. Formulation Design Principles & Thermosetting System Requirements
- Must match curing temperature (typically 120–180°C)
- Flame retardants must withstand high-temperature processing (avoid decomposition failure)
- Ensure dispersion stability in high crosslink-density systems
- Balance post-cure mechanical strength and flame retardancy efficiency
II. Synergistic Flame-Retardant System Design
Flame Retardant Functions & Thermoset Compatibility
| Flame Retardant | Primary Role | Thermoset Compatibility | Recommended Loading |
|---|---|---|---|
| Ultra-fine ATH | Main FR: Endothermic dehydration, gas-phase dilution | Requires surface modification (anti-agglomeration) | ≤35% (excessive loading reduces crosslinking) |
| Aluminum hypophosphite | Synergist: Char catalyst, radical scavenger (PO·) | Decomp. temp. >300°C, suitable for curing | 8–12% |
| Zinc borate | Char enhancer: Forms glassy barrier, reduces smoke | Synergizes with ATH (Al-B-O char) | 5–8% |
| MCA (Melamine cyanurate) | Gas-phase FR: Releases NH₃, inhibits combustion | Decomp. temp. 250–300°C (curing temp. <250°C) | 3–5% |
III. Recommended Formulation (Weight %)
Component Processing Guidelines
| Component | Ratio | Key Processing Notes |
|---|---|---|
| Thermoset acrylic resin | 45–50% | Low-viscosity type (e.g., epoxy acrylate) for high filler loading |
| Surface-modified ATH (D50 <5µm) | 25–30% | Pre-treated with KH-550 silane |
| Aluminum hypophosphite | 10–12% | Pre-mixed with ATH, added in batches |
| Zinc borate | 6–8% | Added with MCA; avoid high-shear degradation |
| MCA | 4–5% | Late-stage low-speed mixing (<250°C) |
| Dispersant (BYK-2152 + PE wax) | 1.5–2% | Ensures uniform filler dispersion |
| Coupling agent (KH-550) | 1% | Pre-treated on ATH/hypophosphite |
| Curing agent (BPO) | 1–2% | Low-temp activator for fast curing |
| Anti-settling agent (Aerosil R202) | 0.5% | Thixotropic anti-sedimentation |
IV. Critical Process Controls
1. Dispersion Process
- Pre-treatment: ATH & hypophosphite soaked in 5% KH-550/ethanol solution (2h, 80°C drying)
- Mixing sequence:
- Resin + dispersant → Low-speed mixing → Add modified ATH/hypophosphite → High-speed dispersion (2500 rpm, 20 min) → Add zinc borate/MCA → Low-speed mixing (avoid MCA degradation)
- Equipment: Planetary mixer (vacuum degassing) or three-roll mill (for ultrafine powders)
2. Curing Optimization
- Step curing: 80°C/1h (pre-gel) → 140°C/2h (post-cure, avoid MCA decomposition)
- Pressure control: 0.5–1 MPa to prevent filler settling
3. Synergistic Mechanisms
- ATH + Hypophosphite: Forms AlPO₄-reinforced char while scavenging radicals (PO·)
- Zinc borate + MCA: Gas-solid dual barrier (NH₃ dilution + molten glassy layer)
V. Performance Tuning Strategies
Common Issues & Solutions
| Issue | Root Cause | Solution |
|---|---|---|
| Dripping ignition | Low melt viscosity | Increase MCA to 5% + hypophosphite to 12%, or add 0.5% PTFE micropowder |
| Post-cure brittleness | Excessive ATH loading | Reduce ATH to 25% + 5% nano-CaCO₃ (toughening) |
| Storage sedimentation | Poor thixotropy | Increase silica to 0.8% or switch to BYK-410 |
| LOI <28% | Insufficient gas-phase FR | Add 2% coated red phosphorus or 1% nano-BN |
VI. Validation Metrics
- UL94 V0: 3.2 mm samples, total flame time <50 s (no cotton ignition)
- LOI ≥30% (safety margin)
- TGA residue >25% (800°C, N₂)
- Mechanical balance: Tensile strength >8 MPa, shear strength >6 MPa
Key Takeaways
- Achieves V0 rating while maintaining mechanical integrity.
- Small-scale trials (50g) recommended before scaling.
- For higher performance: 2–3% DOPO derivatives (e.g., phosphaphenanthrene) can be added.
This formulation ensures compliance with stringent flame-retardant standards while optimizing processability and end-use performance.
Post time: Jul-01-2025
