Synthesis of bio-based flame-retardant epoxy co-curing agent and application in wood surface coating

A phosphaphenanthrene-containing epoxy co-curing agent (PDD) was successfully synthesized by a one-pot nucleophilic addition reaction from protocatechualdehyde (PH), 4,4′-diaminodiphenylether (DDE) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The chemical structure of the PDD was characterized with FTIR, NMR and HRMS techniques. The physical properties of the coatings containing PDD were comparable to the EP-0 (control group phosphorus content with of 0). Various loadings of PDD and 4,4′-diaminodiphenylmethane (DDM) were incorporated into commercial epoxy resin to fabricate epoxy coating on wood surface to achieve flame retardancy. The thermal stability and fire-retardant properties of epoxy coatings were evaluated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), differential scanning calorimetry (DSC) and cone calorimetry (CCT). With 1.09 wt% phosphorus content, the epoxy coating exhibited excellent flame retardancy, LOI value reached 32.6%, total heat release (THR) and total smoke release (TSP) of the EP-1.09 coated wood samples were declined by 39.8% and 61.4%, respectively. Moreover, PDD played an effective flame-retardant action both in the gas and condensed phase, not only promoting formation of continuous and dense char layer but also releasing phosphorous radicals and nonflammable gases. Hence, the bio-based co-curing agent endowed flame-retardant performance to epoxy and wood for developing functional thermosetting coating.