Eco-friendly synthesis of cardanol-based AB monomer for formaldehyde-free phenolic thermosets

Phenolic thermosets are mainly synthesized from two very toxic and petroleum-based molecules, phenol and formaldehyde. This study aims at replacing these two species with a unique bio-based phenolic molecule bearing an aldehyde function. For this purpose, cardanol, a phenolic component extracted from cashew nut shells, has been chemically modified to give 8-(3-hydroxyphenyl)octanal. Two synthetic routes were investigated, each one consisting in three steps and sharing the same key intermediate. The first route consists in the epoxidation of cardanol, followed by hydrolysis to form diols, and finally an oxidative cleavage of these diols using NaIO4. A second route has been considered consisting in an epoxidation, followed by the formation of beta-hydroxy hydroperoxides with H2O2, and finally a thermal cleavage. The green potential of these two methods was compared and discussed with other routes described in the literature. The resulting aldehyde was subsequently used as an AB monomer to obtain phenolic network by direct step growth homopolymerization, enabling the substitution of both phenol and formaldehyde at the same time. The resin formed thereby shows a low Tg due to the C8 alkyl chain of 8-(3-hydroxyphenyl)octanal as well as a good thermal stability. Oxidative cleavage of cardanol to a single-component AB monomer allows the preparation of bio-based formaldehyde-free phenolic resins.