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Views: 0 Author: Site Editor Publish Time: 2026-08-07 Origin: Site
Ammonium polyphosphate (APP, CAS: 68333-79-9) is an inorganic salt of polyphosphoric acid and ammonia, with the general formula [NH₄PO₃]ₙ(OH)₂. As one of the most commercially significant halogen-free flame retardants, APP has become the backbone of modern intumescent fire protection systems across coatings, plastics, textiles, and construction materials.
Ammonium polyphosphate consists of chains of phosphate units linked by P–O–P bonds, with ammonium cations balancing the charge. The chain length — expressed as the degree of polymerization (n) — is the single most important structural parameter determining its performance.
Property | Typical Value |
Appearance | White, free-flowing crystalline powder |
Molecular Formula | [NH₄PO₃]ₙ(OH)₂ |
P₂O₅ Content | ≥ 58% (Phase II); ≥ 68% (high-purity grades) |
Nitrogen Content | ≥ 14% |
pH (10% aqueous suspension) | 5.5–7.5 |
Density | 1.75–1.90 g/cm³ |
Decomposition Temperature | > 275 °C |
Water Solubility | < 0.5 g/100 mL (Phase II) |
The high phosphorus and nitrogen content creates a powerful P–N synergistic effect, which is central to APP's flame-retardant mechanism.
Ammonium polyphosphate's flame retardant mechanism operates through intumescent action — a multi-stage process that converts the APP into a protective, expanded carbonaceous char layer when exposed to heat or flame.
This is APP's largest and most demanding application. Intumescent coatings protect structural steel in commercial buildings, industrial facilities, offshore platforms, and ships. When exposed to fire, the coating expands to form a thick insulating char that can maintain steel temperature below the critical failure point (typically ~550 °C) for 30, 60, 90, or even 120 minutes.
Key requirements for coating-grade APP:
● High thermal stability (> 275 °C onset decomposition)
● Low water solubility (< 0.5%)
● Consistent particle size distribution (typically D₅₀: 10–25 µm)
● Excellent dispersion in solvent-borne and water-borne binders
APP-II is widely used to flame-retard polyolefins and engineering plastics:
Polymer | Typical APP Loading | Fire Standard Met |
Polypropylene (PP) | 20–30% (IFR system) | UL 94 V-0 at 1.6 mm |
Polyethylene (PE) | 25–35% (IFR system) | UL 94 V-0 |
PVC | 5–15% (as synergist) | Enhanced LOI |
EVA | 30–40% (IFR system) | UL 94 V-0 |
Polyamide (PA 6/66) | 20–25% | UL 94 V-0 |
APP finds extensive use in:
Unsaturated polyester resins (UPR):
● Gel coats, marine applications, construction panels
Epoxy resins:
● Electronic encapsulants, aerospace composites, structural adhesives
Rigid and flexible polyurethane foams:
● Building insulation, automotive seating, furniture
APP-based back-coatings impart flame retardancy to cotton, polyester, and blended fabrics used in upholstery, curtains, protective clothing, and transportation interiors.
Wood-plastic composites (WPC), insulation foams, sealants, and fire-stop materials all benefit from APP's intumescent protection.
Rubber compounds:
● Conveyor belts, cable sheathing
Adhesives and sealants:
● Fire-rated construction joints
Paper and packaging:
● Flame-retardant treatments
Advantage | Description |
Environmentally Friendly | Halogen-free; no dioxins or furans produced during combustion |
Low Toxicity | Low acute oral and dermal toxicity; no bioaccumulation |
Low Smoke Emission | Char-forming mechanism inherently suppresses smoke |
High Efficiency | P–N synergism achieves UL 94 V-0 at moderate loadings |
Wide Polymer Compatibility | Effective in polyolefins, thermosets, elastomers, and coatings |
No Corrosive Gases | Unlike halogenated FRs, APP releases only NH₃, H₂O, and CO₂ |
Good Thermal Stability | Processing temperatures up to 280 °C (APP-II) |
Cost-Effective | Lower cost-in-use vs. many phosphorus-based alternatives |
Regulatory Compliance | Meets RoHS, REACH, WEEE requirements globally |
APP-I has a low degree of polymerization (n < 100) and higher water solubility, making it more suitable for fertilizers and liquid formulations. APP-II has a high degree of polymerization (n > 1000), low water solubility, and superior thermal stability — it is the standard grade for flame retardant applications in plastics, coatings, and construction materials.
No. Ammonium polyphosphate is considered a low-toxicity, halogen-free flame retardant. It does not release dioxins, furans, or corrosive gases during combustion. APP holds food-contact approvals in certain applications (as a food additive, E452), though this article focuses exclusively on its industrial flame retardant uses. Standard industrial hygiene practices (avoiding dust inhalation, wearing PPE) are recommended during handling.
The classic empirically verified ratio is APP : Pentaerythritol : Melamine = 3:1:1 by weight. However, modern formulations often adjust this ratio based on specific binder chemistry, substrate, and fire rating requirements. Titanium dioxide (TiO₂) is commonly added at ~5–10% as a synergist and pigment.
Standard APP-II has some moisture sensitivity over extended outdoor exposure. For long-term outdoor applications, silane-coated APP-II or MF-encapsulated APP-II is strongly recommended to prevent leaching and maintain flame-retardant performance.
Phase II ammonium polyphosphate begins thermal decomposition at approximately 275 °C (5% weight loss by TGA). It is suitable for most polyolefin processing (180–250 °C) but may not be appropriate for high-temperature engineering thermoplastics processed above 280 °C without special stabilization.
