Spectral and thermal studies on the synthesis and oligomerization of novel cardanol-based benzoxazines

Abstract

A new class of polymers that is gaining attention of industry and academia are the polybenzoxazines, which can be obtained after the thermal polymerization of their monomers, benzoxazines. Due to the great versatility of the synthesis of these monomers, which uses a phenolic compound and primary amine, it is possible to use renewable sources such as cardanol and the manipulation of the monomer and polymer properties. However, an unexplored area is the understanding of the type of starting material used with the synthesis and polymerization of benzoxazines. Therefore, the aim of this study was to investigate how different primary amines structures influence on the synthesis and polymerization of novel cardanol-based benzoxazines. The monomers were characterized and compared using NMR (1H and 13C), FT-IR, DSC and TGA. It was demonstrated how primary amines influence on the stability of the oxazine ring, which relates to the onset polymerization temperature (Te). Aniline-type benzoxazines provided higher Te than aliphatic ones. Knowing that elevate Te, around 230 to 295 oC, could degrade the long alkyl chain of cardanol, the use of catalysts was also evaluated, showing the innocuous MgCl2 as very promising catalyst. The polymerization of these monomers with MgCl2 (1%) were studied using FT-IR, DSC and GPC, showing that according to the polymerization behaviour of each benzoxazine, this approach can be applied successfully for a more effective synthesis of cardanol-based polybenzoxazines.