Phosphate-Based Flame Retardants

Halogen-free phosphate (phosphonate) flame retardants are relatively rare. Among the most commonly used phosphates in the production of polyester foams are dimethyl methylphosphonate (DMMP) and diethyl ethylphosphonate (DEEP).

Dimethyl Methylphosphonate (DMMP)

DMMP is a colorless or pale yellow transparent liquid. With a molecular weight of 124 and a phosphorus content of 25%, it is a halogen-free, high-phosphorus liquid flame retardant. Its properties include:

·Color index (APHA): ≤10

·Relative density (25°C): 1.16

·Viscosity: ~175 mPa·s

·Acidity: <1.0 mgKOH/g

Key features of DMMP include: 

·High phosphorus content and excellent flame retardancy

·Low required dosage

·Cost-effectiveness

·Ease of use, with good solubility in water and various organic solvents

·Low viscosity

DMMP has a decomposition temperature above 187°C, providing good thermal stability. It is effective in both flexible and rigid polyurethane foams, especially in light-colored and spray-applied foam products.

Flame Retardant Mechanism

When exposed to a flame, DMMP decomposes at high temperatures. The decomposition process produces phosphoric acid, which then loses water to form pyrophosphoric acid, metaphosphoric acid, and polyphosphoric acid. These non-volatile phosphorus compounds create a protective phosphoric acid layer on the burning surface, isolating the material from air and heat to stop combustion.

Additionally, polyphosphoric acid promotes the charring of polymer materials during combustion, reducing oxidative reactions and producing water in the process. At higher temperatures, DMMP thermally decomposes into phosphorus pentoxide, carbon dioxide, and water, diluting flammable gases and avoiding toxic gas emissions. This distinguishes DMMP from halogenated phosphate flame retardants.

Applications

·Adding 5–10% DMMP to flexible foam achieves self-extinguishing behavior when the fire source is removed.

·For rigid foam, 5% DMMP achieves a similar oxygen index (24.5) as 14% TCEP or 18% tris(2,3-dichloropropyl) phosphate. A DMMP content of 5–8% enables rigid foam to reach first-level flame-retardant performance.

Other Phosphate Compounds

Other phosphate compounds, such as trimethyl phosphate, tri(m-tolyl) phosphate, and diphenyl octyl phosphate, serve dual roles as both plasticizers and flame retardants.