Halogen-Free Flame Retardant Formulation and Processing Technology for Epoxy Resin
The customer is seeking an environmentally friendly, halogen-free, and heavy-metal-free flame retardant suitable for epoxy resin with an anhydride curing system, requiring UL94-V0 compliance. The curing agent must be a high-temperature epoxy curing agent with a Tg above 125°C, requiring heat curing at 85–120°C and slow reaction at room temperature. Below is the detailed formulation as requested by the customer.
I. Flame Retardant Formulation System
1. Core Flame Retardant System: Phosphorus-Nitrogen Synergy
Flame Retardant Information Table
| Flame Retardant | Mechanism | Recommended Loading | Remarks |
|---|---|---|---|
| Aluminum hypophosphite | Condensed-phase flame retardancy, forms aluminum phosphate char layer | 10–15% | Primary flame retardant, decomposition temp. >300°C |
| Ammonium polyphosphate (APP) | Intumescent flame retardancy, synergizes with aluminum hypophosphite | 5–10% | Acid-resistant APP required |
| Melamine cyanurate (MCA) | Nitrogen source, enhances phosphorus synergy, suppresses smoke | 3–5% | Reduces dripping |
2. Auxiliary Flame Retardants and Synergists
Auxiliary Flame Retardants Information Table
| Flame Retardant | Mechanism | Recommended Loading | Remarks |
|---|---|---|---|
| Zinc borate | Promotes char formation, suppresses afterglow | 2–5% | Excessive amounts may slow curing |
| Fine aluminum hydroxide | Endothermic cooling, smoke suppression | 5–8% | Control loading (to avoid Tg reduction) |
3. Example Formulation (Total Loading: 20–30%)
Base Formulation (Relative to Total Resin Content)
| Component | Content (Relative to Resin) |
|---|---|
| Aluminum hypophosphite | 12% |
| APP | 8% |
| MCA | 4% |
| Zinc borate | 3% |
| Aluminum hydroxide | 5% |
| Total Loading | 32% (adjustable to 25–30%) |
II. Key Processing Steps
1. Mixing and Dispersion
A. Pre-Treatment:
- Dry aluminum hypophosphite, APP, and MCA at 80°C for 2 hours (prevents moisture absorption).
- Treat inorganic fillers (aluminum hydroxide, zinc borate) with a silane coupling agent (e.g., KH-550).
B. Mixing Sequence:
- Epoxy resin + Flame retardants (60°C, stir for 1h)
- Add anhydride curing agent (keep temperature <80°C)
- Vacuum degassing (-0.095 MPa, 30 min)
2. Curing Process
Step Curing (Balances flame retardant stability and high Tg):
- 85°C / 2h (slow initiation, reduces bubbles)
- 120°C / 2h (ensures complete anhydride reaction)
- 150°C / 1h (increases crosslinking density, Tg >125°C)
3. Key Notes
- Viscosity control: If viscosity is too high, add 5% reactive epoxy diluent (e.g., AGE).
- Delayed curing: Use methylhexahydrophthalic anhydride (MeHHPA) or add 0.2% 2-ethyl-4-methylimidazole (slows room-temperature reaction).
III. Performance Verification & Adjustment
1. Flame Retardancy:
- UL94 V0 test (1.6mm thickness): Ensure burning time <10 sec, no dripping.
- If fails: Increase aluminum hypophosphite (+3%) or APP (+2%).
2. Thermal Performance:
- DSC test for Tg: If Tg <125°C, reduce aluminum hydroxide (lowers Tg due to endothermic effect).
3. Mechanical Properties:
- If flexural strength drops, add 1–2% nano-silica for reinforcement.
IV. Potential Issues & Solutions
Flame Retardant Issues & Solutions Table
| Issue | Cause | Solution |
|---|---|---|
| Incomplete curing | Moisture absorption or pH interference from flame retardants | Pre-dry fillers, use acid-resistant APP |
| Poor resin flow | Excessive filler loading | Reduce aluminum hydroxide to 3% or add diluent |
| UL94 failure | Insufficient P-N synergy | Increase MCA (to 6%) or aluminum hypophosphite (to 15%) |
V. Alternative Formulation (If Needed)
Replace part of APP with DOPO derivatives (e.g., DOPO-HQ):
- 8% DOPO-HQ + 10% aluminum hypophosphite reduces total loading (~18%) while maintaining performance.
This combination balances flame retardancy, environmental safety, and high-temperature performance. Small-scale trials (500g) are recommended before full-scale production.
More info., pls contact lucy@taifeng-fr.com
Post time: Jul-25-2025
