Preparation of ammonium polyphosphate coated with aluminium hydroxide and its application in polypropylene as flame retardant

A new surface-modification method was adopted to decrease water solubility and enhance flame retradancy of ammonium polyphosphate (APP) by means of coating with aluminium hydroxide (ATH). The data of X-Ray Fluorescence Spectrometer (XRF) and scanning electron microscope (SEM) demonstrated that ATH was deposited on the surface of APP particles, and lower water resistance and rougher surface were verified. Furthermore, the modified APP was applied with dipentaerythritol (DPER) to prepare flame retarded polypropylene (FR-PP). The combustion behaviors of FR-PP were studied by limiting oxygen index (LOI), UL 94 vertical burning and cone calorimeter tests. Results show that loading of the coated APP has improved flame retardancy of FR-PP composites remarkably. Compared with unmodified APP, PP/DPER/coated APP can pass V-0 in UL 94 test, LOI value increases from 26.6% to 31.8%, and the peak heat release rate (PHRR) decreases from 475 kW/m² to 308 kW/m². Finally, study on char residues of FR-PP composites indicates that APP coated with ATH makes char layer more complete and more compact, which contributes to the improvement of flame retardancy.