Basic Information

Ammonium Polyphosphate (APP) is an inorganic compound widely used as an eco-friendly, halogen-free flame retardant. Its chemical formula is typically represented as ((NH_4PO_3)_n), where (n) denotes the degree of polymerization. APP is classified into two main types:

  • Type I (Short-Chain APP): Lower polymerization degree, primarily used in fire-resistant coatings and liquid formulations.
  • Type II (Long-Chain APP): Higher thermal stability and water resistance, ideal for plastics and polymers.

APP is favored for its low toxicity, low smoke emission, and compliance with global environmental regulations (e.g., REACH, RoHS).

Ammonium Polyphosphate

Physical Properties

  • Appearance: White powder or granules.
  • Density: ~1.9 g/cm³.
  • Solubility: Low water solubility (Type II APP is further modified for enhanced moisture resistance).
  • Thermal Stability: Decomposes at 250–300°C, releasing ammonia and forming polyphosphoric acid—a key step in its flame-retardant mechanism.
  • Particle Size: Adjustable from 1–50 μm to suit different applications.

Chemical Properties

Flame Retardant Mechanism

APP acts through intumescent flame retardancy:

  • Acid Source: Upon heating, APP decomposes to generate phosphoric acid.
  • Carbonization: The acid catalyzes dehydration of carbon-rich materials (e.g., polymers), forming a protective char layer.
  • Gas Phase Action: Releases non-flammable gases (e.g., NH₃) to dilute oxygen and suppress combustion.
  • pH Stability: Neutral to slightly acidic (pH 5.5–7.0), minimizing corrosion risks.
  • Compatibility: Works synergistically with char-forming agents (e.g., pentaerythritol) and nitrogen sources (e.g., melamine).

Application Highlights

APP’s versatility makes it critical in industries requiring high fire safety standards:

Plastics & Polymers:

  • Used in polypropylene (PP), polyethylene (PE), and ethylene-vinyl acetate (EVA) for cables, automotive parts, and electronics.
  • Typical dosage: 20–30% in PP/PE composites to achieve UL94 V-0 rating.

Fire-Resistant Coatings:

  • Key component in intumescent coatings for steel structures and wood. Expands up to 100x under heat, forming an insulating char layer.

Textiles:

  • Impregnated into fabrics for curtains, upholstery, and industrial protective gear.

Electronics:

  • Protects battery casings and circuit boards in EVs and energy storage systems.

Case Study: A European automotive supplier achieved a 40% reduction in smoke density by replacing halogenated flame retardants with APP in EV battery housings.

intumescent coating

Conclusion

Ammonium Polyphosphate (APP) stands out as a high-performance, sustainable flame retardant solution. Its unique combination of thermal stability, low environmental impact, and broad applicability positions it as a cornerstone of modern fire safety technology. As industries increasingly prioritize green chemistry and stringent safety standards, APP continues to drive innovation in materials science.

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