Microencapsulated ammonium polyphosphate and its application in the flame retardant polypropylene composites

In this article, a novel intumescent alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate flame retardant is prepared and filled into polypropylene as a flame retardant. The structure and properties of alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate are characterized by Fourier transform infrared, scanning electron microscopy, and thermogravimetric analysis. The results show that the Al and Si groups are attached to the surface of ammonium polyphosphate. The flame retardancy, morphology of char layers, thermal properties, and mechanical properties of the polypropylene/alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate composites are evaluated by limiting oxygen index, UL-94 test, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and mechanical properties test. The results show that alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate can improve the thermal stability and charred residues at high temperature. In addition, the combination of alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate and polyamide 6 shows more compact, firm, and continuous charred layers, resulting in better flame retardant performances. The limiting oxygen index value for polypropylene/alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate/polyamide 6 composites can reach 25.6 and obtain a UL-94 V-0 rating. Also, polypropylene/alumina–silica hydrogel double shell microencapsulated ammonium polyphosphate/polyamide 6 composites present better mechanical properties than polypropylene/ammonium polyphosphate/polyamide 6 composites at the same content of flame retardant and polyamide 6.