Use of Polycarboxylic Acid Derived from Partially Depolymerized Lignin As a Curing Agent for Epoxy Application

In this study, Kraft lignin was partially depolymerized through base catalyzed depolymerization (BCD) in supercritical methanol to increase its solubility in organic solvents. The resulting partially depolymerized lignin (PDL) was then converted to lignin-based polycarboxylic acid (LPCA) by reacting with succinic anhydride. The hydroxyl value of PDL and acid value of LPCA were determined using 31P NMR. LPCA was used as a curing agent to cure a commercial epoxy (DER353). Because LPCA was a soft solid, glycerol tris(succinate monoester) (GTA) which was liquid at room temperature was also synthesized and used as cocuring agent and a diluent for LPCA. Dynamic mechanical properties and thermal stability of the cured epoxy resins were examined using dynamic mechanical analysis and thermogravimetric analysis, respectively. Results showed that curing of DER353 with the LPCA resulted in an epoxy resin with a moderate glass transition temperature (Tg) and a storage modulus comparable to the resin cured with the commercial hexahydrophthalic anhydride (HHPA). However, thermal and thermal mechanical properties of the cured resins could be greatly regulated by using GTA or HHPA as a cocuring agent.