Polypropylene (PP) offers good physical and mechanical properties, leading to widespread use for applications in transport, electronics and electrical equipment, architectural materials, furniture and interior decorations. The inorganic PIN flame retardant ammonium polyphosphate (APP) offers advantages of low toxicity and low smoke, but poses application challenges including polymer compatibility, migration and moisture resistance.
To improve performance of ammonium polyphosphate PIN flame retardant, microencapsulation is increasingly proposed, where APP particles are enclosed in thin polymer layers (e.g. water resistant polymers), enabling improved dosing, polymer integration and stability. The authors take this further, by microencapsulation in alumina-silica hydrogel, so that the enclosing material offers both the microencapsulation functionality, but also flame retardant synergistic effect through presence of silicon and aluminium hydroxide. The synthesised alumina-silica hydrogel double shell microencapsulated ammonium polyphosphate (MDAPP) was tested for fire performance in a polypropylene – polyamide 6 composite, showing improved mechanical properties compared to the composite plus standard APP and achieving UL94 V0 fire rating.“Microencapsulated ammonium polyphosphate and its application in the flame retardant polypropylene composites”, M. Chen et al., Journal of Fire Sciences 2015 http://jfs.sagepub.com/content/early/2015/08/05/0734904115598163.abstract