Design and synthesis of polyetherimides as a flame-retarded thermolatent hardener for high-performance epoxy thermosets

To simultaneously improve the storage stability of epoxy/imidazole systems and flame retardancy without sacrificing the thermal, mechanical and dielectric properties of thermosets, a novel multifunctional polyetherimide containing benzimidazole and phosphorus (mPI) was synthesized and used as a hardener for diglycidyl ether of bisphenol A (DGEBA). In addition to retaining the effective curing behavior of imidazole, mPI, a macromolecular hardener and flame retardant, could endow the mixture of DGEBA and mPI (EP/mPI) with the long latent period and thermosets with the high performance. The results showed that mPI was unreactive at ambient temperature and underwent the rapid curing reaction at 170 °C that is a little higher than its glass transition temperature (Tg) of 164.7 °C, and EP/mPI showed the long thermal latent period of 70 days or more. Meanwhile, the cured EP/mPI attained the UL-94 V-0 rating with the LOI value of 31.2%. Besides, EP/mPI thermoset showed a 42.9 °C, 47.6 °C, 18.2 °C, 135.4% and 60.6% increase in the Tg, heat deflection temperature, temperature at 5% mass loss, storage modulus and Young's modulus, respectively, as well as a 3.6% decrease in the dielectric constant at 100 MHz, in comparison with those of the traditional cured DGEBA/benzimidazole (EP/BIM). This work provides a facile strategy to prepare one-pot epoxy/imidazole formulations and flame-retarded epoxy thermosets with high performance by using the well-designed multifunctional polyetherimides as a hardener and flame retardant, which has good application prospects in electronics.