Synthesis of epoxy resins using phosphorus-based precursors for flame-retardant coating

Abstract

Materials imparting flame retardancy play a pivotal role in the development of advanced materials due to their ability to reduce the annihilating effect of fire on man and environment. Realizing the commercial significance of epoxy resins, an attempt has been made to enhance their flame retardancy even under demanding conditions. Among the various flame-retardant materials, phosphorus-containing compounds are gaining interest both in academic and industrial areas due to their simple mechanism of action which involves char formation, a barrier between the substrate and the fire which protects the substrate from being burnt. In the present research work, unconventional starting material—diethanolamine—was used to synthesize phosphorus-based flame-retardant precursors. Intermediates formed in first step were reacted with epichlorohydrin to form epoxy resins. Structure of the compounds was confirmed by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, hydroxyl, and epoxy values. Resins were cured with commercial polyamide, and properties were compared with commercial epoxy resin. Coating films were characterized by various physical, mechanical, and thermal properties like impact resistance, pencil hardness, gloss, solvent resistance, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The TGA and DSC results showed that the char yield value has increased from 3.36 to 8.53%, and the glass transition temperature (Tg) has increased from 72.86 to 82.05°C while the initial degradation temperature decreased. Coatings showed excellent flame retardancy with maximum Limiting Oxygen Index value of 35 and a self-extinguishing behavior as confirmed by the UL-94 test.