Bio-based, main-chain type polybenzoxazine precursor derived from sustainable furfurylamine and salicylaldehyde: Synthesis, characterization and properties

Abstract

Main-chain type polybenzoxazine precursors (MCPPs) are a current focus in the polybenzoxazine field for their improved char yield, thermal stability and film-forming ability compared to monomeric benzoxazines resins. To improve sustainability, there is an urgent need to utilize renewable feedstocks instead of fossil-based chemicals in the preparation of MCPPs. Here, renewable furfurylamine and salicylaldehyde were used as staring materials to synthesize high biomass content MCPPs through a three-steps polymerization route. The number-average molecular weight of a bio-based MCPP is 6100 g/mol, and it has a polydispersity index of 1.7. The furan-containing MCPP shows two polymerization stages during heating; the main exothermic stage at approximately 250 °C is attributed to the ring-opening polymerization of benzoxazine units, and the secondary stage at higher temperature is due to a crosslinking reaction between furan rings, forming furfurylamine Mannich bridges, and an oxidization-induced coupling reaction. The average activation energy of an MCPP is approximately 173.8 kJ/mol. The 240 °C-cured film is transparent with a glass transition temperature of over 300 °C. The crosslinked film shows a high thermal stability and a char yield of greater than 66% in a nitrogen atmosphere. Scanning electron microscopy (SEM) images indicated that a defect-free film can be made by controlling the curing parameters.