Synthesis of P-decorated polysiloxane for good flame retardancy and toughness of epoxy thermosets

Polysiloxanes are modifiers that can enhance the fire safety and toughness of epoxy thermosets. However, the design of high-efficiency polysiloxane modifiers remains a formidable challenge, due to their indefinite structures as well as the complexity and high costs of the preparation process. A phosphorous-containing epoxy-functionalized polydimethylsiloxane (PDMS-DGE) has been prepared by a three-step process using octamethylcyclotetrasiloxane (D₄), 2,4,6,8-tetramethylcyclotetrasiloxane (D₄H), 1,1,3,3-tetramethyldisiloxane (TMDS), allyl glycidyl ether (AGE), and 9,10-dihydro-9-oxo-10-phosphaphenanthrene (DOPO). Incorporation of 15 wt% PDMS-DGE strongly improved the flame retardancy of epoxy thermosets. There was 38.2% increase in limiting oxygen index value (LOI), 54.4% decline in total heat release (THR), and 27.2% decrease in total smoke production (TSP) as compared to the same values for combustion of unmodified epoxy thermoset. This could be ascribed to the generation of phosphorus/silicon-containing char layers, which effectively reduced the formation of combustible gases, smoke, and heat during burning. Compared to neat EP thermoset, the flexural strength and impact strength of the 15 wt% PDMS-DGE modified epoxy thermoset was increased by 51.1% and 107.8%, respectively. This is due to the existence of epoxy groups, rigid phosphaphenanthrene structures, and flexible polydimethylsiloxane chains in PDMS-DGE. Further, the presence of PDMS-DGE provides epoxy thermosets with good moisture resistance. A facile strategy to develop polysiloxanes for epoxy thermosets with both flame retardant properties and toughness, which has vast potential for industrial applications has been proposed.