Dissolution and Dispersion Process of Solid Flame Retardants in Polyurethane AB Adhesive System

Dissolution and Dispersion Process of Solid Flame Retardants in Polyurethane AB Adhesive System

For the dissolution/dispersion of solid flame retardants such as aluminum hypophosphite (AHP), aluminum hydroxide (ATH), zinc borate, and melamine cyanurate (MCA) in a polyurethane AB adhesive system, the key steps involve pretreatment, stepwise dispersion, and strict moisture control. Below is the detailed process (for high flame-retardant formulations; other formulations can be adjusted accordingly).

I. Core Principles

1. “Dissolution” is essentially dispersion: Solid flame retardants must be uniformly dispersed in the polyol (A-component) to form a stable suspension.
2. Pretreatment of flame retardants: Address issues of moisture absorption, agglomeration, and reactivity with isocyanates.
3. Stepwise addition: Add materials in order of density and particle size to avoid localized high concentrations.
4. Strict moisture control: Water consumes the isocyanate (-NCO) in the B-component, leading to poor curing.

II. Detailed Operating Procedure (Based on 100 parts polyol in A-component)

Step 1: Flame Retardant Pretreatment (24 hours in advance)

• Aluminum Hypophosphite (AHP, 10 parts):
  • Surface coating with silane coupling agent (KH-550) or titanate coupling agent (NDZ-201):
    • Mix 0.5 parts coupling agent + 2 parts anhydrous ethanol, stir for 10 min for hydrolysis.
    • Add AHP powder and stir at high speed (1000 rpm) for 20 min.
    • Dry in an oven at 80°C for 2 hours, then store sealed.
• Aluminum Hydroxide (ATH, 25 parts):
  • Use submicron-sized, silane-modified ATH (e.g., Wandu WD-WF-20). If unmodified, treat similarly to AHP.
• MCA (6 parts) & Zinc Borate (4 parts):
  • Dry at 60°C for 4 hours to remove moisture, then sieve through a 300-mesh screen.

Step 2: A-Component (Polyol Side) Dispersion Process

1. Base Mixing:
   • Add 100 parts polyol (e.g., polyether polyol PPG) to a dry container.
   • Add 0.3 parts polyether-modified polysiloxane leveling agent (e.g., BYK-333).
2. Low-Speed Pre-Dispersion:
   • Add flame retardants in order: ATH (25 parts) → AHP (10 parts) → zinc borate (4 parts) → MCA (6 parts).
   • Stir at 300-500 rpm for 10 min until no dry powder remains.
3. High-Shear Dispersion:
   • Switch to a high-speed disperser (≥1500 rpm) for 30 min.
   • Control temperature ≤50°C (to prevent polyol oxidation).
4. Grinding & Refinement (Critical!):
   • Pass through a three-roll mill or basket sand mill 2-3 times until fineness ≤30μm (tested via Hegman gauge).
5. Viscosity Adjustment & Defoaming:
   • Add 0.5 parts hydrophobic fumed silica (Aerosil R202) to prevent settling.
   • Add 0.2 parts silicone defoamer (e.g., Tego Airex 900).
   • Stir at 200 rpm for 15 min for degassing.

Step 3: B-Component (Isocyanate Side) Treatment

• Add 4-6 parts molecular sieve (e.g., Zeochem 3A) to the B-component (e.g., MDI prepolymer) for moisture absorption.
• If using liquid phosphorus flame retardants (low-viscosity option), directly mix into the B-component and stir for 10 min.

Step 4: AB Component Mixing & Curing

• Mixing ratio: Follow the original AB adhesive design (e.g., A:B = 100:50).
• Mixing process:
  • Use a dual-component planetary mixer or static mixing tube.
  • Mix for 2-3 min until uniform (no stringing).
• Curing conditions:
  • Room temperature curing: 24 hours (extended by 30% due to flame retardant heat absorption).
  • Accelerated curing: 60°C/2 hours (validate for bubble-free results).

III. Key Process Control Points

IV. Alternative Dispersion Methods (Without Grinding Equipment)

1. Ball milling pretreatment:
   • Mix flame retardants and polyol at 1:1 ratio, ball mill for 4 hours (zirconia balls, 2mm size).
2. Masterbatch method:
   • Prepare a 50% flame retardant masterbatch (polyol as carrier), then dilute before use.
3. Ultrasonic dispersion:
   • Apply ultrasonication (20kHz, 500W, 10 min) to premixed slurry (suitable for small batches).

V. Implementation Recommendations

1. Small-scale testing first: Test with 100g of A-component, focusing on viscosity stability (24h change <10%) and curing speed.
2. Flame retardant addition sequence rule:
   • “Heavy first, light later; fine first, coarse later” → ATH (heavy) → AHP (fine) → zinc borate (medium) → MCA (light/coarse).
3. Emergency troubleshooting:
   • Sudden viscosity increase: Add 0.5% propylene glycol methyl ether acetate (PMA) to dilute.
   • Poor curing: Add 5% modified MDI (e.g., Wanhua PM-200) to the B-component.