Flame-Retardant and Transparent Poly(methyl methacrylate) Composites Based on Phosphorus–Nitrogen Flame Retardants

In this article, dimethyl methylphosphonate (DMMP) and methacrylamide (MAM) were used as flame retardants to develop an efficient synergistic flame-retardant system of phosphorus and nitrogen, and P(MMA-co-MAM)/DMMP products were prepared by free-radical polymerization, which had an excellent flame-retardant effect and high transparency. When only 14 wt % DMMP/MAM is added, the limiting oxygen index (LOI) value of poly(methyl methacrylate) (PMMA) can reach 23.1%, and the vertical combustion grade can attain V-0. The flame-retardant pathways of the DMMP/MAM flame retardant mainly include terminating a free-radical chain reaction and gas phase dilution, which indicates that the gas-phase flame retardant plays an important role in the PMMA flame retardant. Meanwhile, the optical properties of P(MMA-co-MAM)/DMMP products are almost the same as those of pure PMMA. In addition, when 14 wt % DMMP/MAM is added, the mechanical properties of P(MMA-co-MAM)/DMMP products are only slightly changed, and the Vicat softening temperature is 65 °C higher than the long-term service temperature of commercial PMMA, which can fulfill the needs of most markets. This research provides a valuable method for preparing transparent flame-retardant PMMA products.