Formulation Conversion for Halogen-Free Flame Retardant PVC Leather

Formulation Conversion  for Halogen-Free Flame Retardant PVC Leather

Introduction

The client produces flame-retardant PVC leather and previously used antimony trioxide (Sb₂O₃). They now aim to eliminate Sb₂O₃ and switch to halogen-free flame retardants. The current formulation includes PVC, DOP, EPOXY, BZ-500, ST, HICOAT-410, and antimony. Transitioning from an antimony-based PVC leather formulation to a halogen-free flame-retardant system represents a significant technological upgrade. This shift not only complies with increasingly stringent environmental regulations (e.g., RoHS, REACH) but also enhances the product’s “green” image and market competitiveness.

Key Challenges

1. Loss of Synergistic Effect:
   • Sb₂O₃ is not a strong flame retardant on its own but exhibits excellent synergistic flame-retardant effects with chlorine in PVC, significantly improving efficiency. Removing antimony requires finding an alternative halogen-free system that replicates this synergy.
2. Flame Retardancy Efficiency:
   • Halogen-free flame retardants often require higher loadings to achieve equivalent flame-retardant ratings (e.g., UL94 V-0), which may impact mechanical properties (softness, tensile strength, elongation), processing performance, and cost.
3. PVC Leather Characteristics:
   • PVC leather demands excellent softness, hand feel, surface finish (embossing, gloss), weather resistance, migration resistance, and low-temperature flexibility. The new formulation must maintain or closely match these properties.
4. Processing Performance:
   • High loadings of halogen-free fillers (e.g., ATH) may affect melt flow and processing stability.
5. Cost Considerations:
   • Some high-efficiency halogen-free flame retardants are expensive, necessitating a balance between performance and cost.

Selection Strategy for Halogen-Free Flame Retardant Systems (for PVC Artificial Leather)

1. Primary Flame Retardants – Metal Hydroxides

• Aluminum Trihydroxide (ATH):
  • Most common, cost-effective.
  • Mechanism: Endothermic decomposition (~200°C), releasing water vapor to dilute flammable gases and oxygen while forming a protective surface layer.
  • Drawbacks: Low efficiency, high loading required (40–70 phr), significantly reduces softness, elongation, and processability; decomposition temperature is low.
• Magnesium Hydroxide (MDH):
  • Higher decomposition temperature (~340°C), better suited for PVC processing (160–200°C).
  • Drawbacks: Similar high loadings (40–70 phr) needed; slightly higher cost than ATH; may have higher moisture absorption.

Strategy:

• Prefer MDH or an ATH/MDH blend (e.g., 70/30) to balance cost, processing temperature adaptability, and flame retardancy.
• Surface-treated (e.g., silane-coupled) ATH/MDH improves compatibility with PVC, mitigates property degradation, and enhances flame retardancy.

2. Flame Retardant Synergists

To reduce primary flame retardant loadings and improve efficiency, synergists are essential:

• Phosphorus-Nitrogen Flame Retardants: Ideal for halogen-free PVC systems.
  • Ammonium Polyphosphate (APP): Promotes charring, forming an intumescent insulating layer.
    • Note: Use high-temperature-resistant grades (e.g., Phase II, >280°C) to avoid decomposition during processing. Some APPs may affect transparency and water resistance.
  • Aluminum Diethylphosphinate (ADP): Highly efficient, low loading (5–20 phr), minimal impact on properties, good thermal stability.
    • Drawback: Higher cost.
  • Phosphate Esters (e.g., RDP, BDP, TCPP): Function as plasticizing flame retardants.
    • Pros: Dual role (plasticizer + flame retardant).
    • Cons: Small molecules (e.g., TCPP) may migrate/volatilize; RDP/BDP have lower plasticizing efficiency than DOP and may reduce low-temperature flexibility.
• Zinc Borate (ZB):
  • Low-cost, multifunctional (flame retardant, smoke suppressant, char promoter, anti-dripping). Synergizes well with ATH/MDH and phosphorus-nitrogen systems. Typical loading: 3–10 phr.
• Zinc Stannate/Hydroxy Stannate:
  • Excellent smoke suppressants and flame retardant synergists, particularly for chlorine-containing polymers (e.g., PVC). Can partially replace antimony’s synergistic role. Typical loading: 2–8 phr.
• Molybdenum Compounds (e.g., MoO₃, Ammonium Molybdate):
  • Strong smoke suppressants with flame retardant synergy. Typical loading: 2–5 phr.
• Nano Fillers (e.g., Nanoclay):
  • Low loadings (3–8 phr) improve flame retardancy (char formation, reduced heat release rate) and mechanical properties. Dispersion is critical.

3. Smoke Suppressants

PVC produces heavy smoke during combustion. Halogen-free formulations often require smoke suppression. Zinc borate, zinc stannate, and molybdenum compounds are excellent choices.

Proposed Halogen-Free Flame Retardant Formulation (Based on Client’s Original Formulation)

Target: Achieve UL94 V-0 (1.6 mm or thicker) while maintaining softness, processability, and key properties.

Assumptions:

• Original formulation:
  • DOP: 50–70 phr (plasticizer).
  • ST: Likely stearic acid (lubricant).
  • HICOAT-410: Ca/Zn stabilizer.
  • BZ-500: Likely a lubricant/processing aid (to confirm).
  • EPOXY: Epoxidized soybean oil (co-stabilizer/plasticizer).
  • Antimony: Sb₂O₃ (to be removed).

1. Recommended Formulation Framework (per 100 phr PVC resin)

2. Example Formulation (Requires Optimization)

Critical Implementation Steps

1. Confirm Raw Material Details:
   • Clarify the chemical identities of BZ-500 and ST (consult supplier datasheets).
   • Verify exact loadings of DOP, EPOXY, and HICOAT-410.
   • Define client requirements: Target flame retardancy (e.g., UL94 thickness), softness (hardness), application (automotive, furniture, bags?), special needs (cold resistance, UV stability, abrasion resistance?), cost limits.
2. Select Specific Flame Retardant Grades:
   • Request halogen-free flame retardant samples tailored for PVC leather from suppliers.
   • Prioritize surface-treated ATH/MDH for better dispersion.
   • For APP, use high-temperature-resistant grades.
   • For phosphate esters, prefer RDP/BDP over TCPP for lower migration.
3. Lab-Scale Testing & Optimization:
   • Prepare small batches with varying loadings (e.g., adjust MDH/APP/ZB/ZS ratios).
   • Mixing: Use high-speed mixers (e.g., Henschel) for uniform dispersion. Add liquids (plasticizers, stabilizers) first, then powders.
   • Processing Trials: Test on production equipment (e.g., Banbury mixer + calendering). Monitor plastification time, melt viscosity, torque, surface quality.
   • Performance Testing:
     • Flame retardancy: UL94, LOI.
     • Mechanical properties: Hardness (Shore A), tensile strength, elongation.
     • Softness/hand feel: Subjective + hardness tests.
     • Low-temperature flexibility: Cold bend test.
     • Thermal stability: Congo red test.
     • Appearance: Color, gloss, embossing.
     • (Optional) Smoke density: NBS smoke chamber.
4. Troubleshooting & Balancing:

5. Pilot & Production: After lab optimization, conduct pilot trials to verify stability, consistency, and cost. Scale up only after validation.

Conclusion

Transitioning from antimony-based to halogen-free flame-retardant PVC leather is feasible but requires systematic development. The core approach combines metal hydroxides (preferably surface-treated MDH), phosphorus-nitrogen synergists (APP or ADP), and multifunctional smoke suppressants (zinc borate, zinc stannate). Simultaneously, optimizing plasticizers, stabilizers, lubricants, and processing aids is critical.

Keys to Success:

1. Define clear targets and constraints (flame retardancy, properties, cost).
2. Select proven halogen-free flame retardants (surface-treated fillers, high-temp APP).
3. Conduct rigorous lab testing (flame retardancy, properties, processing).
4. Ensure uniform mixing and process compatibility.

   More info., you can contact lucy@taifeng-fr.com