Characteristics and Properties of Phosphorous-Based Flame Retardants

Phosphorus-based flame retardants have been used as flame retardants for a long time due to their characteristics and performance. The phosphorus-based flame retardants not only overcome the shortcomings of halogen-based flame retardants, such as large smoke generation during combustion and the release of toxic and corrosive gases, but also mitigate the inorganic flame retardants' shortcomings of requiring a high addition amount and seriously affecting the materials' physical and mechanical properties. They have been widely used because of their excellent flame retardancy, low smoke, low toxicity, and no corrosive gas generation characteristics.

According to the different flame-retardant effects of phosphorus compounds in different reaction zones, the flame-retardant mechanism can be divided into condensed phase flame retardant mechanism and gas phase flame retardant mechanism. During combustion, phosphorus-based flame retardants decompose into oxyacid of phosphorus, such as phosphoric acid, hypophosphorous acid, and polyphosphoric acid, which have strong dehydration properties and can cause the material to dehydrate and carbonize into a film, reducing the amount of combustible products. At the same time, the oxyacid of phosphorus often are viscous, semi-solid substances that can form a glassy melt on the surface of the material, covering the char layer, thereby reducing the permeability of the char layer, and protects it from further oxidation, and inhibits the spread of combustion.

Phosphorus-based flame retardants are free radical scavengers. During combustion, PO· free radicals are formed, which can combine with H· free radicals in the flame, therefore the phosphorus-based flame retardants can play a flame-retardant role in the gas phase. In addition, the water produced by phosphorus-based flame retardants during the flame-retardant process can not only lower the temperature of the condensed phase but also dilute the concentration of combustibles in the gas phase, thus achieving a good flame-retardant effect.

Inorganic phosphorus-based flame retardants mainly include red phosphorus (microencapsulated red phosphorus), ammonium polyphosphate and phosphates (such as diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, etc.). Phosphorus-containing inorganic flame retardants are widely used because of their good thermal stability, non-volatility, non-corrosive gas generation, long-lasting effect and low toxicity characteristics.

Organic phosphorus-based flame retardants mainly include triphenyl phosphate, dimethylbenzene phosphate ester, butylbenzene phosphate ester and so on. The characteristic of phosphate esters is that they have both flame-retardant and plasticizing functions.