Synergistic effect of organo-montmorillonite and DOPO-based oligomer on improving the flame retardancy of epoxy thermoset

The organo-montmorillonite (MT), combined with a DOPO-based oligomer (PDAP), was used to improve the flame retardancy of epoxy thermoset. The thermal stabilities and flame-retardant properties of thermosets were investigated by thermogravimetric analysis, limiting oxygen index (LOI) and UL-94 tests. The synergistic effect of MT and PDAP was studied by Py-GC/MS, Fourier transform infrared spectroscopy, elemental analysis, laser Raman spectroscopy and scanning electron microscope. Results revealed that 0.5 mass% MT combined with 4 mass% PDAP showed obvious synergistic effect on enhancing the flame retardancy of thermoset. The corresponding thermoset achieved an LOI value of 35.5% and V-0 rating in UL-94 test, which was attributed to the intense blowing-out effect during combustion. The synergistic mechanism was probably ascribed to the formation of silicoaluminophosphate (SAPO) originating from the reaction between MT and PDAP. The SAPO serving as a solid acidic catalyst, coupled with the acid sites from the decomposition of organomodifier in MT, could promote the charring process. With the further increase in MT content, the charring process was strongly promoted and more phosphorus element was retained in the condensed phase, which inevitably resulted in the remarkable decrease of the amount of pyrolytic gases containing phosphorus-based radicals and nonflammable gases. These factors were responsible for the diminished blowing-out effect during combustion, which was adverse to the further improvement of flame retardancy.