Effect of Aliphatic Chain Length of Bio-based Amines on The Molecular Structure, Curing Mechanism, and Rheological Properties of Cardanol-based Polybenzoxazine

The use of agricultural products and their waste as raw materials for the synthesis of polybenzoxazine is a highly strategic initiative to address environmental sustainability issues and the limited availability of petroleum resources. The implementation of this initiative includes the utilization of cardanol as a natural source of phenol from cashew nut shell oil and primary amines from agricultural products as reactants for benzoxazine monomers synthesized through the Mannich condensation reaction. Two types of benzoxazine monomers synthesized with bio-based amines containing different aliphatic chain lengths were investigated in this study. FTIR and Raman spectroscopy measurements show that the aliphatic chain length of bio-based amines affects the formation of hydrogen bonds among cardanol- based benzoxazine monomers. Additionally, the thermal and rheological properties of these benzoxazine monomers, as measured by DSC and rheometer, are significantly influenced by the length of the aliphatic chain of the primary amine used in the benzoxazine monomer formation reaction. A key finding of the cardanol-based benzoxazine reaction highlighted in this study is that the longer the aliphatic chain of the bio-based primary amine, the fewer hydrogen bonds are formed, the lower the enthalpy of polymer formation, and the lower the viscosity of the polymer.