Encapsulating montmorillonite and ammonium polyphosphate in one polyurea microcapsule to improve flame-retardant and mechanical properties of polypropylene composites

Ammonium polyphosphate (APP) and montmorillonite (MMT) have been widely used in the flame-retardant of polypropylene (PP), but the low synergistic flame-retardant efficiency and their poor compatibility with PP matrix need to be greatly improved. In this work, APP and MMT were encapsulated in the one microcapsule (PU@A-M) at the determined optimal ratio through the “bridging” reactions of diethylenetriamine (DETA) with APP and MMT. Compared with the PP/A + M composites with the physical mixture of APP and MMT, the limiting oxygen index, peak of heat release rate, total heat release, and total smoke production of PP/PU@A-M were decreased by 5.7%, 48.8%, 3.1%, and 20%, respectively. The well-dispersed PU@A-M with charring-forming agent (CFA) generated continuous and compact char layers with C  O  P, C  O  Si, and Si  O  P crosslinking structure. Furthermore, the tensile strength and elongation at break of PP/PU@A-M were enhanced by 4.8% and 36.9%, respectively, as compared with PP/A + M because of good dispersibility and compatibility of PU@A-M in PP matrix.