Highly Effective P–P Synergy of a Novel DOPO-Based Flame Retardant for Epoxy Resin

A novel flame retardant (FR) DOPO-PEPA, which was synthesized via Atherton–Todd reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 1-oxo-4-hydroxymethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octane (PEPA), was used as an additive-type FR in epoxy resin (EP). The results of the limiting oxygen index (LOI), vertical burning test, and cone calorimeter test indicated that the flame retardance of FR-EP composites is dependent on the chemical structure of phosphorus-based FRs. EP/DOPO-PEPA shows pretty good mechanical properties and a relatively high degree of cross-linking. Furthermore, the synergy as DOPO-PEPA was more efficient than that of DOPO or PEPA alone to flame retardant EP. When the FR additives were 9.1%, the EP/DOPO-PEPA acquired a LOI value of 35%, UL94 V-0 rating, and the lowest peak of heat release rate (PHRR) of 595 kW/m2. Furthermore, its continuous and firm char residue layer also reinforced this kind of action.