A phosphaphenanthrene-containing vanillin derivative as co-curing agent for flame-retardant and antibacterial epoxy thermoset

Abstract

A phosphaphenanthrene-containing vanillin-derived compound (VDG) was synthesized by a one-pot nucleophilic addition reaction from vanillin, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and 3,5‐diamino‐1,2,4‐triazole. The molecular structure of VDG was characterized by Fourier transform infrared (FTIR), and 1H, 13C, and 31P nuclear magnetic resonance (NMR) spectrometer. Various loadings of VDG were incorporated into diglycidyl ether of bisphenol A (DGEBA) as a co-curing agent together with 4, 4′-diaminodiphenylmethane (DDM) to produce an epoxy thermoset. The thermomechanical, thermal and fire-retardant properties of EP thermosets were evaluated by dynamic mechanical analyzer (DMA), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), limiting oxygen index (LOI), UL-94, and cone calorimetry. The results showed that a linear decrease in glass transition temperature with increasing loading of VDG owing to the declined cross-linking density. The cured DGEBA/DDM system with 2.0 wt% of VDG exhibited super anti-flammability in terms of a high LOI value of 37.0% and UL-94 V-0 rating, whereas the cured DGEBA/DDM and DGEBA/DDM with 2.0 wt% of DOPO systems showed no rating in UL-94 test. In addition, cone calorimetry test showed that the cured DGEBA/DDM system with 2.0 wt% of VDG exhibited a reduction of up to 47.5% and 34% for peak heat release rate and total heat release, respectively, compared to the cured DGEBA/DDM system. The enhanced flame-retardant property was attributed to the formation of the compact and continual structure of char layer residue with good thermo-oxidative stability. In addition to good anti-flammability, the cured DGEBA/DDM/VDG thermoset showed antibacterial property against E. coli, with an inhibition zone diameter of 10 mm. Thus, this bio-based co-curing agent could impart flame-retardant and antibacterial performance to epoxy for developing high performance functional thermosetting materials.