A novel nitrogen, phosphorus, and boron ionic pair compound toward fire safety and mechanical enhancement effect for epoxy resin

In this work, a novel nitrogen, phosphorus and boron ionic pair compound (DTPA[AZB]), composed of a protonated flame retardant (DTPA) 6,6'-(1,4-phenylenebis((pyrazin-2-ylamino)methylene))bis(dibenzo[c,e][1,2]oxaphosphinine 6-oxide) and a counter anion alizarin borate (AZB), has been prepared and fully characterized, AZB was synthesized by the reaction of alizarin with boric acid. DTPA was produced in two steps. First, terephthalaldehyde was condensed with aminopyrazine to form the corresponding imine. This was treated with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to generate DTPA. Blending with DTPA greatly reduced the flammability of epoxy resin. When the amount of DTPA added was 4%, a modified epoxy resin passed the V-0 rating and the limiting oxygen index (LOI) reached 32.5%. With the introduction of 3% AZB into the EP/DTPA material, the LOI reached 33.5%. Simultaneously, compared with that of neat EP, the peak heat release rate and smoke production rate for EP/DTPA-4 was decreased by 24.1% and 40.7%, respectively, and the peak heat release rate and smoke production rate for EP/DTPA[AZB]-3 was decreased by 32.9% and 43.4%, respectively. The results indicate that AZB and DTPA show good cooperative flame retardant effects. The flame retardancy of the modified epoxy is improved with greater heat release suppression combustion of the resin. A mode of flame retardant action has been proposed based on analysis results from Py-GC/MS for DTPA, and SEM, IR and Raman for the residual carbon from cone calorimeter and UL-94 tests, respectively. Importantly, the tensile strength, fexural strength, and fexural modulus of the EP/DTPA[AZB] increased compared with the same properties of neat EP.