Fully Bio-based Polybenzoxazines Derived from Thymol: Thermal Stability, Hydrophobicity and Corrosion Resistant Properties

In the present work, three types of bio-benzoxazines were synthesized from thymol using three structurally different amines viz. octadecylamine, dodecylamine and furfurylamine through Mannich condensation. The molecular structure of bio-based benzoxazines namely, TH-dda (thymol, dodecylamine), TH-oda (thymol, octadecylamine) and TH-ffa (thymol, furfurylamine) were characterized by Fourier transform infrared (FTIR) and 1 H-NMR spectroscopic techniques. The curing behaviour of synthesized benzoxazines was studied using differential scanning calorimetry (DSC). The occurrence of ring-opening and cross-linking polymerization of TH-dda, TH-oda and TH-ffa bio-based benzoxazines were studied by FTIR spectroscopy. The thermal stability of thermally cross-linked polybenzoxazines, poly(TH-dda), poly(TH-oda) and poly(TH-ffa) were studied by thermogravimetric analysis (TGA). The results obtained from TGA indicate that among the polybenzoxazines studied, poly(TH-oda) possesses the higher thermal stability than that of other polybenzoxazine samples viz., [poly(TH-dda) and poly(TH-ffa)]. The value of limiting oxygen index (LOI) for polybenzoxazine samples was calculated using char yield obtained from TGA indicate that these benzoxazines exhibit good flame retardant characteristics. Data from water contact angle studies infer that these samples possess good hydrophobic properties in the range of 134–139°. Results from corrosion studies infer that mild steel specimens coated with these benzoxazines were found to be more stable towards corrosion.