Synthesis, Characterization, and Flame-Retardant Performance of Dicyandiamide–Formaldehyde Modified Polyester Composites

Abstract

The present study investigates the synthesis of dicyandiamide–formaldehyde (DCF) resin and its application as a nitrogen-containing flame-retardant additive for polyester composites. DCF resin was synthesized through condensation polymerization and incorporated into polyester at concentrations of 5–15 wt.% to prepare flame-retardant polyester/DCF blends. The prepared composites were characterized by Limiting Oxygen Index (LOI), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and UL-94 vertical burning tests. Results demonstrated a significant improvement in flame-retardant performance with increasing DCF content. LOI values increased from 20.8% for pure polyester to 28.3% for the composite containing 15 wt.% DCF. Thermal stability improved substantially, with degradation onset temperature increasing from 312°C to 356°C and char residue increasing from 3.1% to 17.6%. DSC analysis revealed enhanced glass transition temperatures due to increased crosslink density within the polymer matrix. SEM observations confirmed the formation of compact carbonaceous char layers that effectively protected the substrate during combustion. UL-94 testing demonstrated a progressive improvement in flame-retardant behavior, and the formulation containing 15 wt.% DCF achieved a V-0 rating. The results indicate that DCF functions through both gas-phase and condensed-phase flame-retardant mechanisms, making it an effective nitrogen-based flame retardant for advanced polyester composites.