High strength, low flammability, and smoke suppression for epoxy thermoset enabled by a low-loading phosphorus-nitrogen-silicon compound

In order to develop high-performance epoxy resins (EP), a novel phosphorus/nitrogen-containing phenylsiloxane macromolecule (DP-PPD) has been synthesized as a multi-functional additive. The DP-PPD macromolecules can simultaneously enhance the mechanical strength, flame retardancy, and smoke suppression performance for EP at low loading. Compared with unmodified EP, only by introducing 2 wt% DP-PPD, the impact strength, flexural strength and flexural modulus are greatly increased by 100%, 12.9% and 38.7%, respectively, due to flexible phenylsiloxane units and covalent interaction with EP matrix. Meanwhile, the resultant EP containing barely 3 wt% DP-PPD reaches V-0 rating in UL-94 test and shows a high limiting oxygen index of 30.1%. The cone calorimeter test results reveal that the peak of heat release rate, total heat release, and total smoke release of 9%DP-PPD/EP are decreased by 31.4%, 35.4%, and 20.0%, respectively, compared to those of unmodified EP. The mechanism analysis confirms that abundant aromatic rings, siloxane, and DOPO units in DP-PPD can synergistically promote the formation of P–Si-containing hybrid char layer for EP resins during combustion, thus leading to satisfied flame retardancy and smoke suppression. Meanwhile, the reduction of volatilized organic products and the emergence of phosphorous fragments indicates the flame inhibition effect of DP-PPD. Moreover, DP-PPD has little negative effect on the heat resistance of EP. This efficient single-component multi-functional additive provides a new strategy for the construction of high-performance macromolecular materials.