Novel reactive benzoxazine resins using renewables: Leveraging inherent chemistries to produce novel thermoset materials

Abstract

Novel thermoset tannin-based oxazolidine materials have been prepared in a one pot reaction combining condensed tannins with N,N-alkyldiamines and various aldehydes. Reaction stoichiometry was important in O,N-oxazole hetereocycle formation with N-methylene coupling and ammonium ion formation also observed. For this tannin-diamine system, a tannin :1,6-hexanediamine:formaldehyde mole ratio of 1:1:2 was favoured for oxazole product formation, cure and material properties. Thermal analysis (DSC, DMTA) established that these materials exhibit benzoxazole crosslinking reactivity, with higher alkyldiamine contents found to contribute to greater cure onset temperatures that ranged between 130 and 180 °C. Consolidated, crosslinked tannin-diamine materials exhibit consistent mechanical properties at elevated temperatures (>200 °C). Tannin-diamine materials could be combined with wood fibre , either as a powder or aqueous dispersion, to give composite materials (30–40 % fibre fractions, o.d.) with similar high temperature mechanical properties. Substitutions of glyoxal or furfural for formaldehyde produced comparable tannin-diamine products exhibiting similar material performance when hot-pressed. These tannin-based benzoxazoles have potential for utilisation as a matrix or binder with natural fibre reinforcements and composite materials. • Tannin-based oxazolidine materials have been synthesised via a one pot reaction. • Condensed tannins react with N,N-alkyldiamines and aldehydes to form O,N-oxazoles. • O,N-oxazoles are formed via N-methylene coupling and ammonium ion formation. • Tannin-based benzoxazoles exhibit similar crosslinking as synthetic oxazole systems. • Tannin-diamines can be consolidated either pure or with fibre to form composites.