Synthesis, characterization of phosphorus containing diamide-diimide-tetraamines based on L-tryptophan amino acid and their effect on flame retardancy of epoxy resins

The curing behavior of diglycidyl ether of bisphenol-A (DGEBA) with different phosphorus containing diamide-diimide-tetraamines (DADITAs) was studied by DSC. Eight DADITAs of varying structures were synthesized by reacting 1 mole of pyromellitic anhydride (PMDA)/3,3'-benzophenone tetracarboxylic dianhydride (BTDA)/1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA)/4,4'-oxydiphthalic anhydride (ODPA) with 2 mole of L-tryptophan (T) in a mixture of acetic acid and pyridine (3:2 V/V) followed by activaton with thionyl chloride and then condensation with excess of phosphorus containing triamines tris(3-aminophenyl) phosphine (TAP) and tris(3-aminophenyl) phosphine oxide (TAPO). DADITAs obtained by reacting PMDA/BTDA/NTDA/ODPA with L-tryptophan followed by condensation with TAP/TAPO were designated as PTAP, PTAPO, BTAP, BTAPO, NTAP, NTAPO, OTAP and OTAPO respectively. The structural characterization of synthesized DADITAs was done by FTIR, H-1-NMR, C-13-NMR, P-31-NMR spectroscopic techniques and elemental analysis. Thermal stability of the isothermally cured epoxy was investigated using dynamic thermogravimetry analysis. The glass transition temperature (T (g)) was highest in DGEBA cured using PTAP. All epoxy thermosets exhibited excellent flame retardancy, moderate changes in T (g) and thermal stability. Due to presence of phosphorus in curing agents, all epoxy resin systems met the UL-94 V-0 classification and the limiting oxygen index (LOI) reached up to 38.5, probably because of the nitrogen-phosphorus synergistic effect.