Investigation on the role of the alkyl side chain of cardanol on benzoxazine polymerization and polymer properties

Abstract

Cardanol is a bio-phenol used in the field of polybenzoxazine (polyBZ) due to its abundance, low cost, effectiveness, and ease of reaction. Its impact on the polymerization kinetic of benzoxazine as well as on the thermal stability has been hereby investigated. To this aim, three di-functional benzoxazine monomers (BZ) are synthesized by bridging with ethylene diamine either two phenols (di-phenol), one phenol and one cardanol (asymmetric card-phenol), or two cardanol moieties (di-cardanol). FT-IR measurements show that the presence of the cardanol aliphatic chain modifies the formation of hydrogen bonding network. These differences are highlighted in the chemorheology of polymerization, where the di-phenol BZ monomer shows an early vitrification explained by the formation of a high degree of H-bonding interactions during curing. An important feature of benzoxazines reactions, highlighted in this study, is that vitrification does not stop the polymerization process. Cardanol long alkyl side chain can cause a plasticizing effect and induces a lower degree of polymerization. This is observed through the decrease of the glass transition temperature (Tg) and the apparition of a β relaxation for cardanol-based polymers. Interestingly, the thermal stability is significantly improved for cardanol-based polyBZ, which can be the result of H-bonds reorganization in the system due to the cardanol aliphatic chains.