Abstract: Ammonium polyphosphate（APP）with high polymerization degree was prepared using phosphoric acid and urea as raw materials.Preparation process was optimized by single factor experiments and the effects of mix ratio of raw materials,heating rate,pre-polymerization temperature,curing temperature,and curing time on quality of APP were investigated.Average polymerization degree of the products was measured by means of nuclear magnetic resonance（NMR） and crystal structure of the products was characterized by means of X-ray diffraction (XRD) and infrared (IR).Solubility and total phosphorus content of APP were also measured.Results showed that the optimal conditions were as follows：n(H₃PO₄)∶n[CO(NH₂)₂]=1∶1.9,heating rate of pre-polymerization stage was at 2～3 ℃/min,pre-polymerization temperature was 130 ℃,curing temperature was 230 ℃,and curing time was 90 min.Under the optimal conditions,average polymerization degree of prepared APP was 114,solubility of APP was 0.492 g,and total phosphorus content of APP was 31.75%(mass fraction).XRD result identified the prepared product was APP-I.

Key words: flame retardants, ammonium polyphosphate, phosphoric acid, urea