Organophosphorus heteroaromatic compound towards mechanically reinforced and low-flammability epoxy resin

In order to solve the contradiction between the flammability and mechanical properties of epoxy resins, a novel organophosphorus heteroaromatic compound (DMBT) was designed and synthesized derived from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 2-mercaptobenzothiazole (MBT). In the existence of DMBT, the properties of the resultant epoxy thermosets (EP) were improved obviously in the aspect of flame retardancy and mechanical properties. Benefitting from the design of strong intermolecular action between DMBT and EP, the tensile strength, flexural strength and modulus of EP/10 wt% DMBT largely increased to 82.2 +/- 4.0 MPa, 122.2 +/- 1.7 MPa and 3426 81 MPa from 70.2 +/- 5.9, 94.0 +/- 2.0 and 2654 +/- 139 MPa of EP, respectively. Meanwhile, under the action of DMBT, EP/10 wt% DMBT passed UL-94 V-1 rating and possessed a LOI value of 33.5%. The results of calorimeter tests confirmed that the incorporation of DMBT could effectively reduce the heat release rates (HRR) and total heat release (THR) of materials during combustion. Furthermore, the analysis of the char residue after burning and the volatile products in pyrolysis revealed that the flame-retardant mechanism of DMBT was attributed to improving the charring ability and releasing phosphorus-containing free radicals. This work provides a new strategy to prepare the mechanically-strong and flame-retardant epoxy resins.