Hexaphenoxycyclotriphosphazene (HPCTP)

Hexaphenoxycyclotriphosphazene (HPCTP)

CAS：1184-10-7

1. Chemical Structure and Fundamental Characteristics

HPCTP is the fully substituted, aryloxy derivative of the inorganic cyclotriphosphazene ring. Its core is a planar, six-membered heterocyclic ring consisting of three phosphorus (P) and three nitrogen (N) atoms arranged alternately. Each phosphorus atom is doubly bonded to an oxygen atom and singly bonded to two phenoxy (-OC₆H₅) groups, replacing all the original chlorine atoms found in its precursor.

Molecular Formula: C₃₆H₃₀N₃O₆P₃ or more representatively, [NP(OPh)₂]₃, where Ph = C₆H₅.

Molecular Weight: 693.58 g/mol.

Appearance: A white to off-white crystalline powder.

Melting Point: Typically in the range of 110–115 °C.

Solubility: Highly soluble in common organic solvents such as tetrahydrofuran (THF), toluene, chloroform, dichloromethane, and dimethylformamide (DMF). It is insoluble in water and lower alcohols (e.g., methanol, ethanol), due to its bulky, hydrophobic organic substituents.

Thermal Stability: Exhibits excellent thermal stability, with decomposition temperatures typically above 300 °C under inert atmospheres.

2. Synthesis

HPCTP is synthesized via a complete nucleophilic substitution reaction.

Precursor: Hexachlorocyclotriphosphazene(HCCP,N₃P₃Cl₆), the fundamental trimeric phosphazene.

Reagents: An excess of sodium phenoxide (NaOPh) or phenol (PhOH) in the presence of a base (e.g., K₂CO₃) to scavenge the generated acid.

Reaction: N₃P₃Cl₆ + 6 NaOPh → N₃P₃(OPh)₆ + 6 NaCl

Process: The reaction is usually carried out in a dry, aprotic solvent (e.g., THF or diglyme) under an inert atmosphere to prevent hydrolysis of the sensitive P-Cl bonds in the intermediate stages. The product is purified by recrystallization from a suitable solvent like toluene or a toluene/ethanol mixture.

3. Key Properties and Mechanism of Action

Flame Retardancy: HPCTP is a highly effective halogen-free flame retardant. Its action is based on a synergistic phosphorus-nitrogen (P-N) mechanism:

1. Condensed Phase Activity: Upon heating, the P-N ring structure promotes the formation of a thick, thermally stable, and intumescent char layer on the polymer surface. This char acts as a physical barrier, insulating the underlying material, limiting the release of flammable volatiles, and blocking oxygen access.

2. Gas Phase Activity: During decomposition, it can release non-combustible gases (e.g., NH₃, N₂) which dilute flammable gases and radicals in the flame zone.

Plasticizing Effect: It often acts as a secondary plasticizer, improving the processability and flexibility of some polymer systems without significant migration issues common with small-molecule plasticizers.

Compatibility: Its aromatic phenoxy groups provide good compatibility with a wide range of engineering thermoplastics and resins.

4. Primary Applications

HPCTP is predominantly used as a high-performance additive in polymer science and industry:

Flame Retardant for Engineering Plastics: Extensively used in epoxy resins (for printed circuit boards and encapsulants), polycarbonates (and PC/ABS blends), polyurethanes, and unsaturated polyesters. It is prized for its good transparency retention and minimal impact on the inherent mechanical properties of the base polymer.

Synergist: Often used in combination with other flame retardants like metal phosphinates, melamine polyphosphate, or alumina trihydrate to enhance efficiency and reduce overall loading.

Polymer Modifier: Imparts improved thermal stability and can modify rheological properties.

Chemical Intermediate: Serves as a model compound or starting material for further derivatization in research to create novel cyclophosphazene-based materials for advanced applications (e.g., liquid crystals, dendrimers, polymeric networks).

5. Safety and Handling

Toxicity: Considered to have low to moderate acute toxicity. However, like many fine organic powders, it can cause mechanical irritation to the eyes, skin, and respiratory tract.

Handling: Standard laboratory precautions should be observed. Use with adequate ventilation (fume hood), and wear appropriate personal protective equipment (PPE) including safety glasses, gloves, and a dust mask.

Stability:It is hydrolytically stable under normal conditions, unlike its chlorinated precursor. No special stabilizers are required for storage.

Regulatory Status: Its halogen-free nature makes it an attractive alternative to brominated flame retardants, which face increasing regulatory scrutiny (e.g., RoHS, REACH).

Summary

Hexaphenoxycyclotriphosphazene (HPCTP) is a versatile, halogen-free cyclic phosphazene characterized by a robust P₃N₃ ring core surrounded by six phenoxy groups. Its primary industrial significance lies in its role as a highly effective flame-retardant additive and synergist for high-performance engineering thermoplastics and thermosets,where it provides excellent fire resistance through a condensed-phase char-forming mechanism while maintaining key physical properties of the host polymer.

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