Polyurethane AB Adhesive Powder Flame Retardant Formulations

Polyurethane AB Adhesive Powder Flame Retardant Formulations
Based on the demand for halogen-free flame retardant formulations for polyurethane AB adhesives, combined with the characteristics and synergistic effects of flame retardants such as aluminum hypophosphite (AHP), aluminum hydroxide (ATH), zinc borate, and melamine cyanurate (MCA), the following three compounding schemes are designed. These formulations are chlorine-free and focus on optimizing flame retardant efficiency, physical performance compatibility, and process feasibility:

1. High Flame Retardancy Formulation (For electronic potting, battery encapsulation, target UL94 V-0)

Core Flame Retardant Combination:                               

• Aluminum hypophosphite (AHP): 8-12 phr (waterborne polyurethane-coated type recommended to address precipitation issues)
• Aluminum hydroxide (ATH): 20-25 phr (submicron grade, 0.2-1.0 μm, to enhance oxygen index and char compactness)
• MCA: 5-8 phr (gas-phase mechanism, synergistic with AHP in the condensed phase)
• Zinc borate: 3-5 phr (promotes ceramic char formation and inhibits smoldering)

Expected Performance:

• Oxygen index (LOI): ≥32% (pure PU ≈22%);
• UL94 rating: V-0 (1.6 mm thickness);
• Thermal conductivity: 0.45-0.55 W/m·K (contributed by ATH and zinc borate);
• Viscosity control: 25,000-30,000 cP (surface treatment required to prevent sedimentation).

Key Process:

• AHP must be pre-dispersed in the polyol component (Part A) to avoid premature reaction with isocyanate (Part B);
• ATH should be modified with a silane coupling agent (e.g., KH-550) to enhance interfacial bonding.

2. Low-Cost General Formulation (For construction sealing, furniture bonding, target UL94 V-1)

Core Flame Retardant Combination:

• Aluminum hydroxide (ATH): 30-40 phr (standard micron-grade, cost-effective, filler-type flame retardant);
• Ammonium polyphosphate (APP): 10-15 phr (combined with MCA for an intumescent system, replacing halogenated agents);
• MCA: 5-7 phr (ratio to APP 1:2~1:3, promotes foaming and oxygen isolation);
• Zinc borate: 5 phr (smoke suppression, auxiliary char formation).

Expected Performance:

• LOI: ≥28%;
• UL94 rating: V-1;
• Cost reduction: ~30% (compared to high-flame-retardancy formulation);
• Tensile strength retention: ≥80% (APP requires encapsulation to prevent hydrolysis).

Key Process:

• APP must be microencapsulated (e.g., with melamine-formaldehyde resin) to avoid moisture absorption and bubble formation;
• Add 1-2 phr hydrophobic fumed silica (e.g., Aerosil R202) for anti-settling.

3. Low-Viscosity Easy-Process Formulation (For precision electronics bonding, requiring high flowability)

Core Flame Retardant Combination:

• Aluminum hypophosphite (AHP): 5-8 phr (nanosized, D50 ≤1 μm);
• Liquid organic phosphorus flame retardant (BDP alternative): 8-10 phr (e.g., halogen-free phosphorus-based DMMP derivatives, maintaining viscosity);
• Aluminum hydroxide (ATH): 15 phr (spherical alumina composite, balancing thermal conductivity);
• MCA: 3-5 phr.

Expected Performance:

• Viscosity range: 10,000-15,000 cP (close to liquid flame retardant systems);
• Flame retardancy: UL94 V-0 (enhanced by liquid phosphorus);
• Thermal conductivity: ≥0.6 W/m·K (contributed by spherical alumina).

Key Process:

• AHP and spherical alumina must be co-mixed and dispersed under high shear (≥2000 rpm);
• Add 4-6 phr molecular sieve desiccant to Part B to prevent AHP moisture absorption.

4. Compounding Technical Points & Alternative Solutions

1. Synergistic Mechanisms:

• AHP + MCA: AHP promotes dehydration and charring, while MCA releases nitrogen gas upon heating, forming a honeycomb-like char layer.
• ATH + Zinc borate: ATH absorbs heat (1967 J/g), and zinc borate forms a borate glass layer to cover the surface.

2. Alternative Flame Retardants:

• Polyphosphazene derivatives: High-efficiency and eco-friendly, with byproduct HCl utilization;
• Epoxy silicone resin (ESR): When combined with AHP, it reduces total loading (18% for V-0) and improves mechanical properties.

3. Process Risk Control:

• Sedimentation: Anti-settling agents (e.g., polyurea-modified types) required if viscosity <10,000 cP;
• Curing inhibition: Avoid excessive alkaline flame retardants (e.g., MCA) to prevent interference with isocyanate reactions.

5. Implementation Recommendations

• Prioritize testing the high-flame-retardancy formulation: coated AHP + submicron ATH (average particle size 0.5 μm) at AHP:ATH:MCA = 10:20:5 for initial optimization.
• Key tests:
  → LOI (GB/T 2406.2) and UL94 vertical burning;
  → Bond strength after thermal cycling (-30℃~100℃, 200 hours);
  → Flame retardant precipitation after accelerated aging (60℃/7d). 

Flame Retardant Formulation Table