Formaldehyde-Free Synthesis of Fully Bio-Based Multifunctional Bisbenzoxazine Resins from Natural Renewable Starting Materials

Although bio-based benzoxazines (BZs) have been explored widely as sustainable thermosetting resins, few high-performance BZs have been prepared completely from natural renewable resources. In this study we synthesized a fully bio-based multifunctional bisbenzoxazine (AP-fa-BZ) in high yield and purity from apigenin (AP), furfurylamine (fa), and benzaldehyde by using both solvent and solventless approaches. Fourier transform infrared (FTIR) spectroscopy, high-resolution mass spectrometry, and one- and two-dimensional nuclear magnetic resonance spectroscopy confirmed the chemical structure of AP-fa-BZ. We then used dynamic mechanical analysis, differential scanning calorimetry (DSC), thermogravimetric analysis, and in situ FTIR spectroscopy to examine the thermal characteristics of AP-fa-BZ before and after its ring-opening polymerization (ROP). DSC revealed that the temperature required for the formation of poly(AP-fa-BZ) through ROP (236.3 °C) was significantly lower than that of a typical 4-phenyl-3,4-dihydro-2H-1,3-benzoxazine (Pa-type) monomer due to the positive catalytic effect of the phenolic OH groups in the AP structure. After thermal polymerization at 250 °C, the resulting poly(AP-fa-BZ) possessed a high thermal decomposition temperature (Td10 = 395 °C), a high char yield (52 wt %), and a high glass transition temperature (Tg = 283 °C). Contact angle measurements revealed the tunable surface properties of AP-fa-BZ. Finally, the AP-fa-BZ resin functioned as an antibacterial agent against both Staphylococcus aureus and Escherichia coli.