Synthesis and characterization of bio-based benzoxazines derived from thymol

In the present study, bio-based benzoxazine resins were synthesized from bio-based phenolic compound; thymol, and three different amines; ethylamine, aniline and 1,6-diaminohexane, and paraformaldehyde by solvent-free condensation reaction. The chemical structures of bio-based benzoxazines; T-ea (thymol, ethylamine), T-a (thymol, aniline), and T-dh (thymol, 1,6-diaminohexane) were characterized by proton nuclear magnetic resonance spectroscopy, Fourier transform infrared (FTIR) spectroscopy, elemental analysis, and high-resolution mass spectrometry. The curing studies of T-ea, T-a, and T-dh bio-based benzoxazines were performed by stepwise thermal treatment at 150, 175, 200, 225, and 250 °C. The polymerization (ring-opening and crosslinking reactions) of T-ea, T-a, and T-dh bio-based benzoxazines was investigated by FTIR spectroscopy. Cure analysis was conducted using differential scanning calorimetry and the changes in thermal properties of the T-ea, T-a, and T-dh bio-based benzoxazine resins and their corresponding thermally crosslinked polybenzoxazines PT-ea, PT-a, and PT-dh were studied by thermogravimetric analyzer. The results indicated that all the thymol-based polybenzoxazines have shown enhanced thermal stability.