Towards superb toughness, strength, and flame retardancy in epoxy resins via molecular interface engineering

Abstract

Epoxy resins (EP), as prototypical highly crosslinked thermosets, face challenges in balancing strength with toughness, while simultaneously incorporating multi-functionalities, such as flame retardancy, thermal properties, transparency, and ultraviolet (UV) shielding capabilities. To overcome these limitations, this study designs and synthesizes two phosphaphenanthrene-modified poly-Schiff base “all-in-one” modifiers with tailored flexible segments, yielding two binary systems named EP-PPSR and EP-PPSi. These modifiers, through subtle structural variations, effectively regulate interfacial interactions and stress responsiveness in the EP matrix, enabling a unique balance between toughness and strength. Notably, EP-PPSi, featuring a highly flexible chain, exhibits superior tensile and flexural properties, with tensile and flexural strengths increasing by up to 34.7 % and 25.4 %, respectively, compared to neat EP. However, EP-PPSR, with a larger toughening interface, shows higher impact strength and fracture toughness, achieving an impact strength of 35.4 kJ m⁻², nearly 2.9 times to that of neat EP. Both systems demonstrate excellent flame retardancy, self-extinguishing properties, and ignition resistance, with limiting oxygen index (LOI) values reaching up to 35.5 % and 37.5 %, respectively. At 4 wt% addition, both systems achieve a UL-94 vertical burning V-0 rating. EP-PPSi, leveraging a P–Si synergistic effect, outperforms EP-PPSR in suppressing heat release, smoke generation, and flame propagation under intense heat. Additionally, both materials boast remarkable transparency, UV shielding capacity, dielectric and thermomechanical properties, as well as a high glass transition temperature. This work presents a novel strategy for crafting multifunctional, high-performance flame-retardant thermosets, broadening their application horizons in demanding fields.