Synthesis of a Novel Derivative Containing Phosphaphenanthrene and Phenyl Propargyl Ether Groups Toward Enhanced Flame Retardance and Mechanical Properties of Epoxy Resin

To achieve effective flame retardancy and improved toughness in epoxy resin (EP) without compromising strength and modulus, a new derivative containing phosphaphenanthrene and phenyl propyl ether groups (HPMD) has been developed and used as an additive. The structure of HPMD has been fully characterized, and the properties of HPMD and modified EP have been thoroughly studied. The phenyl propyl ether group in HPMD can undergo Claisen type rearrangement to form benzopyran during heat treatment and subsequently polymerize by curing via the C-C bond in the benzopyran. The unique rigid and flexible structures of HPMD allow it to disperse in the EP matrix through melting. Dynamic mechanical analysis suggests that HPMD has a high degree of interpenetration in the EP matrix, resulting in a single-phase structure for the epoxy resins containing HPMD. Notably, incorporating only 4 wt% HPMD content into the polymer matrix yields a flame-retarded, toughened, and strengthened EP. EP containing 4 wt% HPMD (EP4; phosphorus content of only 0.33%) displays a limiting oxygen index value of 31.1% and exhibits a UL94 V-0 rating. Compared with that of EP, the combustion of EP4 exhibits a decrease in the peak of heat release rate, maximum average heat rate emission, average heat release rate, and total heat release, along with an increase in the char yield, further indicating that HPMD effectively retards the combustion of EP. Moreover, EP4 displays improvements in tensile strength, elongation at break, flexural strength, flexural modulus, and impact strength of 18.4%, 9.7%, 14.9%, 19.1%, and 26.7%, respectively, compared with unmodified EP. Analysis of HPMD's flame-retardant action suggests that it exhibits flame-retardant activity simultaneously in the gas and condensed phases. Significant progress has been demonstrated toward the development of a meltable epoxy resin additive that combines flame-retardant, toughening, and strengthening functions while avoiding phase separation from the epoxy resin.