Halogen-Free Flame Retardant Formulation for TPU Coating System Using DMF Solvent

Halogen-Free Flame Retardant Formulation for TPU Coating System Using DMF Solvent

For TPU coating systems using Dimethyl Formamide (DMF) as the solvent, the use of aluminum hypophosphite (AHP) and zinc borate (ZB) as flame retardants requires systematic evaluation. Below is a detailed analysis and implementation plan:

I. Feasibility Analysis of Aluminum Hypophosphite (AHP)

1. Flame Retardant Mechanism and Advantages

• Mechanism:
• Decomposes at high temperatures to generate phosphoric and metaphosphoric acids, promoting char formation in TPU (condensed-phase flame retardancy).
• Releases PO· radicals to interrupt combustion chain reactions (gas-phase flame retardancy).
• Advantages:
• Halogen-free, low smoke, low toxicity, compliant with RoHS/REACH.
• Good thermal stability (decomposition temperature ≈300°C), suitable for TPU drying processes (typically <150°C).

2. Application Challenges and Solutions

3. Recommended Formulation and Process

• Example Formulation:
• TPU/DMF base: 100 phr
• Surface-modified AHP: 20 phr
• Zinc borate (ZB): 5 phr (smoke suppression synergy)
• Dispersant (BYK-110): 1.5 phr
• Process Key Points:
• Pre-mix AHP with dispersant and partial DMF under high shear (≥3000 rpm, 30 min), then blend with TPU slurry.
• Post-coating drying: 120-150°C, extend time by 10% to ensure complete DMF evaporation.

II. Feasibility Analysis of Zinc Borate (ZB)

1. Flame Retardant Mechanism and Advantages

• Mechanism:
• Forms a B₂O₃ glass layer at high temperatures, blocking oxygen and heat (condensed-phase flame retardancy).
• Releases bound water (~13%), diluting flammable gases and cooling the system.
• Advantages:
• Strong synergistic effect with AHP or aluminum trihydroxide (ATH).
• Excellent smoke suppression, ideal for low-smoke applications.

2. Application Challenges and Solutions

3. Recommended Formulation and Process

• Example Formulation:
• TPU/DMF base: 100 phr
• Nano-sized ZB: 8 phr
• AHP: 15 phr
• Wetting agent (Tego 750W): 1 phr
• Process Key Points:
• Pre-disperse ZB in DMF via bead milling (particle size ≤2μm) before mixing with TPU slurry.
• Extend drying time (e.g., 30 min) to avoid residual moisture affecting flame retardancy.

III. Synergistic Evaluation of AHP + ZB System

1. Synergistic Flame Retardant Effects

• Gas-Phase & Condensed-Phase Synergy:
• AHP provides phosphorus for charring, while ZB stabilizes the char layer and suppresses afterglow.
• Combined LOI: 28-30%, UL94 V-0 (1.6mm) achievable.
• Smoke Suppression:
• ZB reduces smoke emission by >50% (Cone Calorimeter test).

2. Performance Balancing Recommendations

• Mechanical Property Compensation:
• Add 2-3% TPU plasticizer (e.g., polycaprolactone polyol) to maintain flexibility (elongation >300%).
• Use ultrafine powders (AHP/ZB <2μm) to minimize tensile strength loss.
• Process Stability Control:
• Maintain slurry viscosity at 2000-4000 cP (Brookfield RV, spindle 4, 20 rpm) for uniform coating.

IV. Comparison with Solvent-Based Liquid Flame Retardants

V. Recommended Implementation Steps

1. Lab-Scale Testing:

• Evaluate AHP/ZB individually and in combination (gradient loading: 10%, 15%, 20%).
• Assess dispersion stability (no sedimentation after 24h), viscosity changes, and coating uniformity.

1. Pilot-Scale Validation:

• Optimize drying conditions (time/temperature) and test flame retardancy (UL94, LOI) and mechanical properties.
• Compare costs: If AHP+ZB reduces costs by >30% vs. liquid FRs, it is economically viable.

1. Scale-Up Preparation:

• Collaborate with suppliers to develop pre-dispersed AHP/ZB masterbatches (DMF-based) for simplified production.

VI. Conclusion

With controlled dispersion processes, AHP and ZB can serve as effective flame retardants for TPU/DMF coatings, provided:

1. Surface modification + high-shear dispersion is applied to prevent particle agglomeration.
2. AHP (primary) + ZB (synergist) balances efficiency and cost.
3. For high transparency/flexibility requirements, liquid phosphorus-nitrogen FRs (e.g., Levagard 4090N) remain preferable.

Sichuan Taifeng New Flame Retardant Co., Ltd.(ISO & REACH)

Email: lucy@taifeng-fr.com