Synthesis and Flame-Retardant Performance of Epoxy Resins Containing Aromatic Imine from Different Bisphenols Based on the Duff Reaction

A series of epoxy resins containing aromatic imine are synthesized via the Duff reaction by using various bisphenols, which show significantly reduced heat release capacity (HRC) and total heat release (THR) after curing. Compared to diglycidyl ether bisphenol A/4,4-diaminodiphenyl methane (DGEBA/DDM), the HRC and THR of phenolphthalein-based epoxy resin containing aromatic imine/DDM (BPP-E/DDM) are reduced by 87% (464 J/g K vs 58 J/g K) and 62% (30.2 kJ/g vs 11.4 kJ/g), respectively. Furthermore, the BPP-E/DDM exhibits a UL-94 V-0 rating (1.6 mm) and a limiting oxygen index of 48% (3.2 mm). Compared to DGEBA/DDM in the cone calorimeter test, the peak heat release rate, THR, peak smoke produce rate, and total smoke production of BPP-E/DDM were reduced by 91% (1006 kW/m2 vs 87 kW/m2), 51% (89 MJ/m2 vs 44 MJ/m2), 89% (0.36 m2/s vs 0.09 m2/s), and 79% (38 m2 vs 8 m2), respectively. The influences of aromatic imine and the chemical structures between two benzene rings in the bisphenols on HRC and THR of the resultant epoxy thermosets are investigated. The mechanism studies show that the presence of aromatic imine in the epoxy resins completely changes the decomposition process to generate the products containing aromatic N-heterocycle, which contributes greatly to the improved char-forming ability and flame retardancy. Meanwhile the thermosets bearing carbonyl, 3-phthalidylene, and sulfonyl between two benzene rings in the bisphenols show better char-forming ability and flame retardancy among the resultant epoxy resins containing aromatic imine, due to more polycyclic aromatic hydrocarbons produced in the condensed phase.