How to reduce the smoke density for epoxy adhesive with flame retardant AHP and MCA ?

The addition of aluminum hypophosphite and MCA to epoxy adhesive results in high smoke emission. Using zinc borate to reduce smoke density and emission is feasible, but the existing formulation needs to be optimized for the ratio.

1. Smoke Suppression Mechanism of Zinc Borate

Zinc borate is an efficient smoke suppressant and flame-retardant synergist. Its mechanisms include:

• Char Formation Promotion: Forms a dense char layer during combustion, isolating oxygen and heat, and reducing flammable gas release.
• Smoke Inhibition: Catalyzes cross-linking reactions to reduce smoke particle generation, lowering smoke density (especially effective for polymers like epoxy).
• Synergistic Effect: Enhances flame retardancy when combined with phosphorus-based (e.g., aluminum hypophosphite) and nitrogen-based (e.g., MCA) flame retardants.

2. Alternative or Supplementary Smoke Suppressants

For further optimization of smoke suppression, consider the following synergistic solutions:

• Molybdenum Compounds (e.g., zinc molybdate, molybdenum trioxide): More effective than zinc borate but costlier; recommended to blend with zinc borate (e.g., zinc borate : zinc molybdate = 2:1).
• Aluminum/Magnesium Hydroxide: Requires high loading (20-40 phr), which may affect epoxy’s mechanical properties—adjust cautiously.

3. Recommended Formulation Adjustments

Assuming the original formulation is aluminum hypophosphite + MCA, here are optimization directions (based on 100 parts epoxy resin):

Option 1: Direct Addition of Zinc Borate

• Aluminum hypophosphite: Reduce from 20-30 phr to 15-25 phr
• MCA: Reduce from 10-15 phr to 8-12 phr
• Zinc borate: Add 5-15 phr (start testing at 10 phr)
• Total flame retardant content: Keep at 30-40 phr (avoid excessive amounts affecting adhesive performance).

Option 2: Zinc Borate + Zinc Molybdate Synergy

• Aluminum hypophosphite: 15-20 phr
• MCA: 5-10 phr
• Zinc borate: 8-12 phr
• Zinc molybdate: 4-6 phr
• Total flame retardant content: 30-35 phr.

4. Key Validation Metrics

• Flame Retardancy: UL-94 vertical burning, LOI tests (target: V-0 or LOI >30%).
• Smoke Density: Use a smoke density tester (e.g., NBS smoke chamber) to compare reduction in Smoke Density Rating (SDR).
• Mechanical Properties: Ensure tensile strength and adhesion strength meet requirements after curing.
• Processability: Confirm uniform dispersion of flame retardants without affecting viscosity or curing time.

5. Considerations

• Particle Size Control: Opt for nano-sized zinc borate (e.g., particle size <1 μm) to improve dispersion.
• Surface Modification: Treat zinc borate with a silane coupling agent to enhance compatibility with epoxy resin.
• Regulatory Compliance: Ensure selected flame retardants meet RoHS, REACH, and other regulations.

6. Example Formulation (Reference)

7. Summary

• Zinc borate is an effective choice for reducing smoke emission. Recommend adding 10-15 phr while moderately reducing aluminum hypophosphite/MCA content.
• For further smoke suppression, blend with molybdenum compounds (e.g., 4-6 phr).
• Experimental validation is necessary to balance flame retardancy, smoke suppression, and mechanical properties.

Let me know if you’d like any refinements! Lucy@taifeng-fr.com