Why red phosphorus can be flame retardant?

Why red phosphorus can be flame retardant?

Red phosphorus is flammable, but in the resin, the action process of red phosphorus and other phosphorus-containing additives is not simple oxidation. The phosphorus-containing additives mainly work in the condensed phase. The flame retardant mechanism is:

I. When the resin burns it combines with the oxygen in the air to produce carbon dioxide, water and soot (carbon).

CmHm O₂ → CO₂ H2O C

II. Red phosphorus combines with the oxygen and water in the air to produce condensed phosphoric acid (a polymer of phosphoric acid).

PO₂ H₂O → (HPOx)n condensed phosphoric acid

III. The soot (carbon) produced in step I mixes with the condensed phosphoric acid produced in step II to form a thin film on the surface of the plastic.

This film, known as “char,” keeps the oxygen out, so that the flame goes out.

The flame retardant mechanism of each phosphorus-based flame retardant is determined by the structure of the flame retardant and the combustion product.

Flame Retardant Mechanism of Red Phosphorus

The flame retardant mechanism of red phosphorus is generally considered to be carried out in the condensed phase. When the polymer added with red phosphorus flame retardant is burned, the red phosphorus is first oxidized to generate phosphoric acid, and then the phosphoric acid is further dehydrated to generate metaphosphoric acid and polymetaphosphoric acid.

Therefore, a non-flammable liquid protective film is formed on the surface of the polymer; at the same time, due to the strong dehydration of metaphosphoric acid and polymetaphosphoric acid, the surface of the polymer material is dehydrated and carbonized to form a dense carbonized layer.

On the one hand, the liquid film and the carbonized layer can prevent heat and oxygen, and at the same time effectively prevent the diffusion of flammable gas generated by the high temperature decomposition of the polymer, thereby inhibiting the combustion of the material and achieving the purpose of flame retardant.

Compared with other additive phosphorus flame retardants, red phosphorus has a high phosphorus content. Therefore, in order to achieve the same flame retardant grade, the amount of red phosphorus added is less than that of other flame retardants, and the polymer flame retardant with it has better physical and mechanical properties.