The phosphorus content of ammonium polyphosphate is as high as 30% to 32%, and the nitrogen content is 14% to 16%. The most prominent feature of this type of flame retardant is that the amount of smoke generated during combustion is extremely low, and no hydrogen halide is produced. Ammonium polyphosphate can replace ammonium phosphate due to its good thermal stability.

Ammonium polyphosphate is white crystal or amorphous fine powder. The water solubility and hygroscopicity of APP decreased with increasing polymer. Domestically, according to the degree of polymerization n, it can be divided into two types: water-soluble (n=10-20, relative molecular mass 1000-2000) and water-insoluble (n>20, relative molecular mass greater than 2000). n can be greater than 1000. In foreign countries, n<100 is called crystal phase I ammonium polyphosphate (APPI), and the branched APP with n>1000 is called crystal phase II ammonium polyphosphate (APPII). Short-chain APP with n<100 is more sensitive to water (hydrolyzability) than ultra-long-chain (M>1000) APP, which has higher thermal stability and hydrolysis resistance. Long-chain APP begins to decompose into phosphoric acid and ammonia at above 300 °C, while short-chain APP begins to decompose above 150 °C.

The commonly used crystalline APP is a water-insoluble long-chain ammonium polyphosphate. APP has a large phosphorus content and a high nitrogen content, and the phosphorus-nitrogen system produces a synergistic effect and has good flame retardancy. Low relative density, good dispersibility, good chemical stability, smoke elimination and low toxicity.