Synthesis of novel phosphorus‐containing epoxy hardeners and thermal stability and flame‐retardant properties of cured products

Two novel flame‐retardant curing agents for epoxy resins, 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO)‐containing 4‐[(phenylamino)methyl]phenol (P‐Ph) and DOPO‐containing Mannich‐type bases (P‐DDS‐Ph), were synthesized by the condensation of 4‐hydroxybenzaldehyde with 4‐aminophenol and DDS, respectively, followed by the addition of DOPO to the resulting imine linkage. Chemical structures of these materials were characterized with FTIR, 1 H‐NMR spectra, 31 P‐NMR spectra, and elemental analysis. The thermal properties and flame retardancy of o ‐cresol novolac epoxy resin (CNE) cured with different contents of the phosphorus‐containing compounds were investigated by nonisothermal differential scanning calorimetry, thermogravimetric analysis, and limiting oxygen index (LOI). The obtained results showed that more char was formed while containing lower contents of the phosphorus‐containing compounds in the P‐Ph/CNE and P‐DDS‐Ph/CNE indicating their excellent flame retardancy. Moreover, the P‐DDS‐Ph/CNE exhibited higher T g (224°C) and better thermal stability ( T 10% , 330°C) than that of P‐Ph/CNE. Therefore, the developed P‐DDS‐Ph/CNE may be potentially used as environmentally preferable products in electronic fields.