P/N Flame Retardant Based on a Pyrimidine Ring for Improving the Flame Retardancy, Mechanical Properties, and Smoke Suppression of Epoxy Resin

A P/N flame-retardant TBD based on a pyrimidine ring was devised and developed by reacting 2,4,6-triaminopyrimidine, benzaldehyde, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to create a less toxic, low-smoke, and mechanically improved the flame-retardant epoxy resin (EP). The EP modified using TBD exhibited good transparency, which was attributed to the outstanding compatibility of TBD with the EP. With a phosphorus content of only 0.65%, EP/TBD10 passed the V-0 rating of the UL-94 test, and the limited oxygen index increased from 27.1% (pure EP) to 38.1%. In the cone calorimetry test (CCT), the peak heat release rate and the total heat release decreased by 34.9 and 45.7%, respectively, compared to pure EP. In addition, the modified cured EP exhibited good mechanical properties owing to the action of rigid groups and the formation of hydrogen bonds, and the tensile strength and modulus as well as the flexural strength and modulus of the samples were improved. In the smoke density test, EP/TBD20 received the ratings of HL1 and HL2 in Ds(4) and VOF4, respectively, according to EN45545-2. All results indicate that TBD is an efficient flame retardant that can enhance the flame retardancy and mechanical properties of the curing EP and reduce its smoke density, making it suitable for use in the rail transportation industry.