Bio-based amine curing agents prepared from lignin by ring-opening functionalization with cyclic aza-silanes and their curing kinetics investigated for aliphatic and aromatic epoxy species

Bio-based epoxy curing agents from lignin was synthesized by ring-opening reactions of cyclic aza-silane species to obtain lignin-based curing agents with amine hydroxyl equivalent weights up to 68 g/eq. Its cure kinetics was investigated by differential scanning calorimetry in which two representative aromatic and aliphatic epoxy species were employed being diglycidyl ether bisphenol F (DGEBF) and epoxidized soybean oil (ESBO). Utilizing Borchardt and Daniels' method to obtain cure kinetic parameters, the activation energies, Ea, were determined to 101 ± 3 kJ/mol and 71 ± 7 kJ/mol and pre-exponential factors, ln(Z), to 27.6 ± 0.9 s⁻¹ and 13.3 ± 1.7 s−1 respectively for DGEBF and ESBO. The higher pre-exponential factor for DGEBF was attributed to π-π stacking interactions between the aromatic components resulting in overall faster cure kinetics of aromatic-based epoxy species. The lignin-based curing agent retained antioxidative properties which in cured DGEBF thermosets could act as an anti-corrosive agent. This enables the preparation of coatings without the need for leachable additive antioxidants.