Preparation of carboxylated lignin-based epoxy resin with excellent mechanical properties

Lignin, a renewable biopolymer recovered from the chemical processing of woody biomass, has potential to enhance the mechanical properties of epoxy resins with the added benefit of reduced contents of non-renewable petroleum-based polymers. Lignin-based epoxy resins were prepared by employing a carboxylated lignin as a co-hardener using a “one-pot” method. The effect of lignin content and preparation method on mechanical properties, apparent morphology, and thermodynamic properties of the resulting epoxy resins were then studied. The results showed that elongation at break increased from 2.2 to 5.7%, and flexural strength increased from 23.3 to 101 MPa, with the addition of lignin up to 20 wt%. The highest increase in tensile strength was obtained for the lignin-based resin having a lignin content of 10 wt%. The incorporation of carboxylated lignin at an appropriate amount demonstrated simultaneous reinforcing and toughening effects on the epoxy resin, which can be attributed to the rigid and highly-branched structure of lignin.