Structural modification of ammonium polyphosphate by DOPO to achieve high water resistance and hydrophobicity

As one of the important flame retardants, ammonium polyphosphate (APP) usually needs surface modification or microencapsulation to lower its solubility in water and to improve its compatibility with polymers before being applied to the flame retardant system. Herein, we reported a new way to obtain highly water-resistant hydrophobic APP by introduction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) into its molecular structure. The contact angle of obtained DOPO-modified APP is 105°, and its solubility in water at 25 and 80 °C was 0.08 and 0.2 g·100 mL− 1 H2O, respectively, lower than that of the commercial microencapsulated APP (0.2 and 0.4 g·100 mL− 1 H2O). The hydrophobic mechanism of DOPO-modified APP was discussed in detail. For PP loaded with 30% of DOPO-modified APP or commercial APP, the former could pass V0 rating (1.6 mm) at the vertical burning test (UL-94), while the latter failed. The LOI value of the former increased to 30.1% and that of the latter was only 24.2%. Moreover, cone calorimeter (CC) testing results also indicated that the former exhibited better flame retardant performance.