some silicone rubber reference formulation based on halogen-free flame retardants

Here are five silicone rubber formulation designs based on halogen-free flame retardants, incorporating the flame retardants provided by the customer (aluminum hypophosphite, zinc borate, MCA, aluminum hydroxide, and ammonium polyphosphate). These designs aim to ensure flame retardancy while minimizing additive amounts to reduce the impact on the mechanical properties of the silicone rubber.

1. Phosphorus-Nitrogen Synergistic Flame Retardant System (High-Efficiency Char-Forming Type)

Target: UL94 V-0, low smoke, suitable for medium-to-high-temperature applications

Base Rubber: Methyl vinyl silicone rubber (VMQ, 100 phr)

Flame Retardants:

• Aluminum hypophosphite (AHP, phosphorus-based): 15 phr
• Provides efficient phosphorus source, promotes char formation, and suppresses gas-phase combustion.
• Melamine cyanurate (MCA, nitrogen-based): 10 phr
• Synergizes with phosphorus, releases inert gases, and dilutes oxygen.
• Zinc borate (ZnB): 5 phr
• Catalyzes char formation, suppresses smoke, and enhances char layer stability.
• Aluminum hydroxide (ATH, chemical method, 1.6–2.3 μm): 20 phr
• Endothermic decomposition, auxiliary flame retardancy, and improved dispersibility.

Additives:

• Hydroxyl silicone oil (2 phr, improves processability)
• Fumed silica (10 phr, reinforcement)
• Curing agent (Diperoxide, 0.8 phr)

Features:

• Total flame retardant loading ~50 phr, balancing flame retardancy and mechanical properties.
• Phosphorus-nitrogen synergy (AHP + MCA) reduces the required amount of individual flame retardants.

2. Intumescent Flame Retardant System (Low-Loading Type)

Target: UL94 V-1/V-0, suitable for thin products

Base Rubber: VMQ (100 phr)

Flame Retardants:

• Ammonium polyphosphate (APP, phosphorus-nitrogen-based): 12 phr
• Core of intumescent char formation, with good compatibility with silicone rubber.
• Aluminum hypophosphite (AHP): 8 phr
• Supplementary phosphorus source, reduces APP hygroscopicity.
• Zinc borate (ZnB): 5 phr
• Synergistic char catalysis and drip suppression.
• Aluminum hydroxide (ground, 3–20 μm): 15 phr
• Low-cost auxiliary flame retardant, reduces APP loading.

Additives:

• Vinyl silicone oil (3 phr, plasticization)
• Precipitated silica (15 phr, reinforcement)
• Platinum curing system (0.1% Pt)

Features:

• Total flame retardant loading ~40 phr, effective for thin products due to intumescent mechanism.
• APP requires surface treatment (e.g., silane coupling agent) to prevent migration.

3. High-Loading Aluminum Hydroxide Optimized System (Cost-Effective Type)

Target: UL94 V-0, suitable for thick products or cables

Base Rubber: VMQ (100 phr)

Flame Retardants:

• Aluminum hydroxide (ATH, chemical method, 1.6–2.3 μm): 50 phr
• Primary flame retardant, endothermic decomposition, small particle size for better dispersion.
• Aluminum hypophosphite (AHP): 5 phr
• Enhances char formation efficiency, reduces ATH loading.
• Zinc borate (ZnB): 3 phr
• Smoke suppression and anti-glowing.

Additives:

• Silane coupling agent (KH-550, 1 phr, improves ATH interface)
• Fumed silica (8 phr, reinforcement)
• Peroxide curing (DCP, 1 phr)

Features:

• Total flame retardant loading ~58 phr, but ATH dominates for cost efficiency.
• Small ATH particle size minimizes tensile strength loss.

4. Standalone Aluminum Hypophosphite (AHP) System

Application: UL94 V-1/V-2, or where nitrogen sources are undesirable (e.g., avoiding MCA foaming affecting appearance).

Recommended Formulation:

• Base Rubber: VMQ (100 phr)
• Aluminum hypophosphite (AHP): 20–30 phr
• High phosphorus content (40%); 20 phr provides ~8% phosphorus for basic flame retardancy.
• For UL94 V-0, increase to 30 phr (may impair mechanical properties).
• Reinforcing Filler: Silica (10–15 phr, maintains strength)
• Additives: Hydroxyl silicone oil (2 phr, processability) + curing agent (Diperoxide or platinum system).

Features:

• Relies on condensed-phase flame retardancy (char formation), significantly improves LOI but has limited smoke suppression.
• High loading (>25 phr) may stiffen the material; recommend adding 3–5 phr ZnB to improve char quality.

5. Aluminum Hypophosphite (AHP) + MCA Blend

Application: UL94 V-0, low loading with gas-phase flame retardant synergy.

Recommended Formulation:

• Base Rubber: VMQ (100 phr)
• Aluminum hypophosphite (AHP): 12–15 phr
• Phosphorus source for char formation.
• MCA: 8–10 phr
• Nitrogen source for P-N synergy, releases inert gases (e.g., NH₃) to suppress flame propagation.
• Reinforcing Filler: Silica (10 phr)
• Additives: Silane coupling agent (1 phr, dispersion aid) + curing agent.

Features:

• Total flame retardant loading ~20–25 phr, significantly lower than standalone AHP.
• MCA reduces AHP requirement but may slightly affect transparency (use nano-MCA if clarity is needed).

Flame Retardant Formulation Summary

Experimental Recommendations

1. Priority Testing: AHP + MCA (15+10 phr). If V-0 is achieved, gradually reduce AHP (e.g., 12+10 phr).
2. Standalone AHP Test: Start at 20 phr, increment by 5 phr to evaluate LOI and UL94, monitoring mechanical properties.
3. Smoke Suppression: Add 3–5 phr ZnB to any formulation without compromising flame retardancy.
4. Cost Optimization: Incorporate 10–15 phr ATH to reduce cost, though total filler loading increases.

Recommended Mixing Process

(For two-part addition-cure silicone rubber)

1. Base Rubber Pre-Treatment:

• Load silicone rubber (e.g., 107 gum, vinyl silicone oil) into a planetary mixer, degas under vacuum if needed.

1. Flame Retardant Addition:

• Powdered flame retardants (e.g., ATH, MH):
• Add in batches, pre-mix with base rubber (low-speed mixing, 10–15 min) to avoid agglomeration.
• Dry at 80–120°C if hygroscopic.
• Liquid flame retardants (e.g., phosphates):
• Blend directly with silicone oil, crosslinker, etc., under high shear (20–30 min).

1. Other Additives:

• Sequentially add fillers (e.g., silica), crosslinker (hydrosilane), catalyst (platinum), and inhibitors.

1. Homogenization:

• Further refine dispersion using a three-roll mill or high-shear emulsifier (critical for nano-additives like CNTs).

1. Degassing & Filtration:

• Vacuum degas (-0.095 MPa, 30 min), filter for high-purity requirements.

Key Considerations

• Flame Retardant Selection:
• Halogen-free retardants (e.g., ATH) require fine particle size (1–5 μm); excessive loading harms mechanical properties.
• Silicone-based retardants (e.g., phenyl silicone resins) offer better compatibility but at higher cost.
• Process Control:
• Temperature ≤ 60°C (prevents platinum catalyst poisoning or premature curing).
• Humidity ≤ 50% RH (avoids reactions between hydroxyl silicone oil and flame retardants).

Conclusion

• Mass Production: Pre-mix flame retardants with base rubber for efficiency.
• High-Stability Requirements: Blend during compounding to minimize storage risks.
• Nano-Flame Retardant Systems: Mandatory high-shear dispersion to prevent agglomeration.

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