P- and Si-modified shellac for flame-retardant epoxy-based coatings

This is an attempt to develop thermally stable shellac–epoxy coating blends with flame-retardant properties. The monophosphazene and silicon precursors were synthesized with PCl5 and dichlorodimethylsilane (DCMS) as phosphorus and silicon sources, respectively. The synthesized phosphazene and silicon precursor moieties were then reacted with shellac in various concentrations to prepare the modified shellac. The shellac/modified shellac and epoxy coatings were then mixed to form the blend formulations and coated onto the mild steel panels. The structure confirmation of both the precursors was achieved using physicochemical analysis, FTIR and NMR, while the modified shellac was characterized using FTIR. Furthermore, the thermal, mechanical and flame-retardant properties of the coating blends were analyzed. The Tg values and the degradation curves showed that the coating blends with Si and P modification were more thermally stable than the unmodified shellac blends. The maximum LOI obtained was 27 for the highest concentrations of P and Si in the coating, similarly, the calculations based on the char yields of LOI also showed the similar trend of increasing values although the values were lower than the practical LOI values. The barrier properties were predicted depending on the results of water absorption and gel content values which showed that the crosslinking density was increasedwith increasing concentrations of shellac in the coating.