Abstract
Phenolic-based thermosets are generally obtained through the cross-linking of formo-phenolic precursors (called novolac) by condensation reaction with formaldehyde, thus creating highly stable methylene bridges. Herein, reprocessable and depolymerizable covalent adaptable networks containing dynamic phenol-carbamate cross-links were prepared via a two-step process from (biobased) novolac resins and partially biobased 1,5-pentamethylene diisocyanate derivatives, using dibutyltin dilaurate (DBTDL) as catalyst. These CANs retained the mechanical strength (flexural strength up to 94 MPa) and high glass transition temperatures (67 °C < T α <175 °C) of conventional formo-phenolic thermosets while acquiring fast stress relaxation properties at high temperature (T ≥ 160 °C) thanks to the introduction of exchangeable urethane groups. Characterizations by FTIR, stress relaxation experiments and tensile tests demonstrated that networks obtained from commercially available novolac and isocyanate trimer (STABiO-NR1) could be reprocessed up to two times while maintaining mechanical properties superior to those of permanently cross-linked materials prepared from novolac and formaldehyde. Additionally, novolac obtained by polycondensation of phenol and formaldehyde was replaced by biobased cashew nutshell liquid (CNSL) yielding reprocessable materials containing up to 87 wt.% of renewable atoms. These resins were successfully employed as organic matrices for the preparation of glass fiber composites. Composites prepared from CNSL and polyisocyanurate cross-linker achieved 87 % of the reference's ultimate flexural strength with a toughness more than four times higher than that of the non-recyclable formo-phenolic reference. Furthermore, neat and reusable glass fibers could be recovered by rapid depolymerisation (alcoholysis) of these phenol-carbamate-based composites. These results highlight this strategy as a promising and sustainable alternative in the field of non-recyclable formo-phenolic thermosets.