Hydrophobicity, transparency, mechanically excellent and fire safety multifunctional epoxy resin based on a novel DOPO derivative with cyano group

A novel derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) bearing cyano groups, referred to as PDPO, was successfully synthesized and applied as an additive for epoxy resin (E-44, with epoxide equivalent weights ranging from 201 to 240 g/epoxide), along with the curing agent 4,4′-diaminodiphenylmethane (DDM). Comprehensive characterization techniques, including Fourier transform infrared (FTIR) spectroscopy, ¹H and ³¹P nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis (EA), were employed to confirm the chemical structure of PDPO. Thermogravimetric analysis (TGA) revealed remarkable thermal stability for PDPO, with a substantial char residue of 43.1 wt% at 800 °C. Introducing 5 wt% PDPO into the EP formulation resulted in a notable enhancement of fire retardancy, as evident from the increased Limiting Oxygen Index (LOI) value of 32.2 % and the attainment of UL-94 V-0 rating for the EP/PDPO-5. The efficacy of fire retardancy was further underscored by cone calorimeter (CC) testing, which showcased a reduction in heat release rate and smoke production. An in-depth analysis of pyrolysis behavior and char formation demonstrated that PDPO exhibited multifaceted roles in both the gas and condensed phases, contributing to the fire retardant properties of the material. Additionally, the incorporation of PDPO at a concentration of 1 wt% led to notable enhancements in the tensile and flexural strength of EP/PDPO-1 by 15.2 % and 10.4 %, respectively, compared to neat EP. Remarkably, the transparency of EP/PDPO-1 was preserved at 95.6 %, while the water contact angle was elevated to 115.5°, indicating improved hydrophobicity. These results collectively highlight the potential of PDPO as a fire-retardant additive, demonstrating its ability to simultaneously stay transparency and enhance mechanical properties and hydrophobic characteristics in the EP/PDPO systems.