Surface modification of ammonium polyphosphate by kaolinite and the study on thermal decomposition behavior and flame-retardant performance

The surface of ammonium polyphosphate is modified by layered silicates (kaolinite or functionalized kaolinite). The thermogravimetric analysis data confirm that the layered silicates significantly strengthen the thermal stability of polypropylene (PP) and increase the char residue. Fire safety of PP containing different percentages of modified ammonium polyphosphate and charring–foaming agent is assessed by using limiting oxygen index (LOI) and UL-94 vertical combustion. The addition of layered silicates in polymers can effectively enhance the flame-retardant properties including the enhancements in UL-94 rating and LOI results. The flame-retardant mechanism and model are described based on the results of thermal degradation and char residue analysis.