A phosphorus-containing indole derivatives endowing epoxy resin with excellent flame retardance and mechanical properties

The phosphorus-containing indole bio-based flame retardant of IPD was successfully synthesized by a one-pot method using tryptamine (PTM), indole-3-carboxaldehyde (3-IAID) and 9,10-dihydro-9-oxo-10-phosphophenanthrene-10-oxide (DOPO) as the reactive monomers and its effects on the curing behaviour, thermal stability and mechanical properties of the epoxy resins (EPs) were researched. With the addition of 5 wt% IPD (only 0.31 wt% phosphorus), the apparent activation energy (Ea) of the EP/IPD was reduced by 8.8 %, the limiting oxygen index (LOI) reached 33.7 % and achieved UL-94 V-0 rating. Cone calorimetry test (CCT) data indicated that the peak heat release rate (PHRR) and total heat release (THR) of EP/IPD-5 were decreased approximately 33.0 % and 25.9 %, respectively, and the flame growth index (FIGRA) was reduced by approximately 40.8 %, compared with EP. The biphasic action of IPD in the combustion process provides the system with good fire retardancy and charring properties. In addition, owing to the hydrogen bonding and it-it conjugation effects formed between the IPD and epoxy groups, the flexural strength was enhanced to 16.5 %. This work provides a simple strategy for expanding the application areas of EP.