Synergistic flame-retardant behavior and mechanism of tris(3-nitrophenyl) phosphine and DOPO in epoxy resins

Abstract

A novel flame-retarded epoxy resins system is prepared by copolymerizing diglycidyl ether of bisphenol A (EP) with tris(3-nitrophenyl) phosphine (NPPh3), 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and 4,4-diaminodiphenylmethane (DDM). The thermogravimetric curves suggest that there is an obvious synergistic effect between NPPh3 and DOPO. Flame-retardant properties of the cured products are evaluated using limited oxygen index (LOI) and vertical burning tests (UL-94). The results indicate that the flame retardancy of NPPh3/DOPO/EP thermosets is enhanced. 2%NPPh3/4%DOPO/EP achieves a LOI value of 33.8% and V-0 rating in UL-94 test. The thermal stability of the EP composites is detected by thermogravimetric analysis (TG) and differential scanning calorimetry. The results demonstrate that the thermal stability of NPPh3/DOPO cured epoxy resins displays an improvement in the high-temperature region and the glass transition temperature decreases slightly compared with pure EP. The pyrolytic gases are characterized using thermogravimetric analysis/infrared spectrometry (TG-FTIR) in an air atmosphere. The gaseous species produced by the flame-retarded EP composites are the same as those from EP. Additionally, the morphology and the structure of char residues are studied by scanning electron microscopy and Fourier transform infrared spectra (FTIR). The morphology of the residual char for flame-retarded EP composites shows a compact, smooth, and tight structure. These outstanding integrated properties will make EP composites attractive for practical applications.