Design and development of disulfide and thioether-linked bio-based bisbenzoxazines for low-curing, thermally stable and corrosion-resistant coating applications

Polybenzoxazines derived from green precursors face significant challenges due to the high temperatures required for ring-opening polymerization (ROP), limiting industrial scalability. To address this, the present study introduces two novel series of bisbenzoxazine monomers containing disulfide (-S-S-) and thioether (-C-S-C-) linkages, synthesized from bio-based phenolic precursors (guaiacol, cardanol, eugenol, thymol) were paired with dihydrazides (thiodipropionate and dithiodipropionate). The structural confirmation of these monomers was confirmed by ATR-FTIR, ¹H NMR and ¹³C NMR spectral techniques. Thermal properties were studied using DSC and TGA analyses. The synergistic combination of -S-S-, C-S-C and hydrazide groups have enabled the development of novel benzoxazine systems with lower curing behavior and higher thermal stability. Also, from curing kinetics the ROP temperature of benzoxazine monomers were reduced to 142 °C and subsequently confirmed through ATR-FTIR. Further, TGA results revealed that eugenol containing polybenzoxazines possess higher thermal stability among the series. Additionally, the corrosion resistant performance of polybenzoxazines was carried out using electrochemical methods on mild steel (MS) substrates. Results indicated excellent anti-corrosion properties with better corrosion inhibition efficiency of 99 %. All the results were compared with conventional phenol-based monomers containing same functionalities. This work highlights the potential utility of sustainable feedstocks for benzoxazine with lower energy demands and high-performance coating applications.