Microfluidic fabrication of polysiloxane/dimethyl methylphosphonate flame-retardant microcapsule and its application in silicone foams

A novel and versatile route for fabricating flame-retardant microcapsules via microfluidics technology is reported. The flame-retardant microcapsules were prepared with a dimethyl methylphosphonate (DMMP) core and an ultraviolet-curable (UV-curable) polysiloxane shell. Furthermore, a UV-curable polysiloxane was synthesized. The synthesis mechanism of UV-curable polysiloxane and the curing mechanism of flame-retardant microcapsules were analyzed. To verify that DMMP was encapsulated in the microcapsules, X-ray fluorescence was used before and after microencapsulation. The resulting microcapsules were well monodispersed and exhibited a good spherical shape with a smooth surface. In addition, the size of the microcapsules decreased dramatically with an increasing flow-rate ratio of the middle-/inner-phase or outer-phase flow rate. The thermal stability of the microcapsules was worse than shell materials but superior to DMMP. Silicone foams (SiFs) with microcapsules prepared using a dehydrogenation method achieved a relatively higher limiting oxygen-index value than the pure SiF, which indicated that the microcapsules could enhance the flame retardation of SiFs effectively. Because of the polysiloxane shell, the microcapsules had good compatibility with SiFs, and the influence of microcapsules on the mechanical properties of SiFs was unremarkable.