Can Phosphorus-Nitrogen Flame Retardants Achieve V0 Rating in Silicone Rubber?
When customers inquire about using only aluminum hypophosphite (AHP) or AHP + MCA combinations for halogen-free flame retardancy in silicone rubber to achieve V0 rating, the answer is yes—but dosage adjustments are required based on the flame retardancy requirements. Below are specific recommendations for different scenarios:
1. Using Aluminum Hypophosphite (AHP) Alone
Applicable scenarios: For UL94 V-1/V-2 requirements or applications sensitive to nitrogen sources (e.g., avoiding foaming effects from MCA that may affect appearance).
Recommended formulation:
- Base rubber: Methyl vinyl silicone rubber (VMQ, 100 phr)
- Aluminum hypophosphite (AHP): 20–30 phr
- High phosphorus content (40%); 20 phr provides ~8% phosphorus content for basic flame retardancy.
- For UL94 V-0, increase to 30 phr (may affect mechanical properties).
- Reinforcing filler: Fumed silica (10–15 phr, maintains strength)
- Additives: Hydroxyl silicone oil (2 phr, improves processing) + curing agent (peroxide or platinum system)
Characteristics:
- AHP alone relies on condensed-phase flame retardancy (char formation), significantly improving the oxygen index (LOI) of silicone rubber but with limited smoke suppression.
- High dosage (>25 phr) may increase material hardness; adding 3–5 phr zinc borate can improve char layer quality.
2. AHP + MCA Combination
Applicable scenarios: UL94 V-0 requirements, aiming for low additive dosage with gas-phase flame retardant synergy.
Recommended formulation:
- Base rubber: VMQ (100 phr)
- Aluminum hypophosphite (AHP): 12–15 phr
- Provides phosphorus source, promotes char formation.
- MCA: 8–10 phr
- Nitrogen source synergizes with AHP (P-N effect), releasing inert gases (e.g., NH₃) to suppress flame spread.
- Reinforcing filler: Fumed silica (10 phr)
- Additives: Silane coupling agent (1 phr, improves dispersion) + curing agent
Characteristics:
- Total flame retardant dosage: ~20–25 phr, significantly lower than AHP alone.
- MCA reduces AHP dosage but may slightly affect transparency (nano-MCA recommended if transparency is required).
3. Key Parameter Comparison
| Formulation | Expected Flame Retardancy | Total Dosage (phr) | Pros & Cons |
|---|---|---|---|
| AHP alone (20 phr) | UL94 V-1 | 20 | Simple, low cost; V-0 requires ≥30 phr, with performance degradation. |
| AHP alone (30 phr) | UL94 V-0 | 30 | High flame retardancy but increased hardness and reduced elongation. |
| AHP 15 + MCA 10 | UL94 V-0 | 25 | Synergistic effect, balanced performance—recommended for initial trials. |
4. Experimental Recommendations
- Priority test for AHP + MCA (15+10 phr): If V-0 is achieved, gradually reduce AHP (e.g., 12+10).
- AHP alone verification: Start at 20 phr, increase by 5 phr per test to evaluate LOI and UL94, monitoring mechanical property changes.
- Smoke suppression needs: Add 3–5 phr zinc borate to the above formulations to reduce smoke without compromising flame retardancy.
5. Some Coated Ammonium Polyphosphate
We have some customers successfully using TF-201G for silicon rubber .
For further optimization, consider incorporating small amounts of aluminum hydroxide (10–15 phr) to reduce overall costs, though this increases total filler content.
More inof., pls contact lucy@taifeng-fr.com
Post time: Jul-25-2025
