Interlinking Lignin Molecules with Citric Acid: The Effect of Reaction Conditions on Lignin Properties

To increase the molecular weight (M_W) and glass transition temperature (T_g) of lignin, thereby improving manufacturing for applications such as polymer blends and carbon fibers, lignin is crosslinked with a renewable interlinking agent, citric acid, via esterification. M_W and T_g of a kraft (BioChoice) lignin and an alkaline-pretreated hybrid poplar lignin (HPL) were increased by up to 7× (e.g., from 20 to 146 kDa) and up to 36 °C, respectively. Moderate decreases in polarity were obtained, as observed by the solubility of the esterified lignins in toluene and ethanol. Three reaction mechanisms involving one, two, or three of the carboxylic acids in citric acid are proposed. For both biomass sources, increasing the concentration of citric acid increased the M_W (and T_g) of the lignin, whereas increasing the catalyst, sodium hypophosphite (SHP), beyond 1-2 wt%, decreases the M_W due to an unwanted side reaction between citric acid and SHP. In fact, for HPL, the highest T_g is obtained with no added catalyst, thus creating a green process that uses only renewable feed materials to build the M_W and T_g of lignin.