Submitted to: Carbon
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/24/2005
Publication Date: 5/12/2006
Citation: Braun, J.L., Holtman, K.M., Kadla, J.F. 2005. Lignin-based carbon fibers: oxidative themostabilization of kraft lignin. Carbon. 43 (2005): 385-394. Interpretive Summary: Lignin is a major constituent of agricultural residues and is essentially a waste product in the isolation of cellulose. There are few, if any, avenues currently existing to produce value-added products from lignin and it is usually burnt to obtain energy. Lignin may be a potentially cheap raw material for carbon fibers, however its glass transition temperature is below the temperature required for carbonization. In this study we show that by slow heating we can thermostabilize the lignin (i.e. retain its glassy state) enabling us to maintain fiber form during the subsequent carbonization process. This research may be particularly useful for utilization of residual lignin from enzymatic hydrolysis in biofuel applications.
Technical Abstract: The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 0C at various heating rates in the presence of air. The glass transition temperature (Tg) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 0C/min or lower was required in order to maintain Tg > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200–250 0C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO2 and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures (Tg) as opposed to faster heating rates.