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

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 degrees C. Consolidated, crosslinked tannin-diamine materials exhibit consistent mechanical properties at elevated temperatures (>200 degrees 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.

Automated summary

Novel thermoset tannin-based oxazolidine materials were synthesized by a one pot reaction, which can be combined with wood fibers to form composite materials with high temperature mechanical properties.