Lignin recovered from the near-neutral hemicellulose extraction process as a precursor for carbon fiber

Lignin recovered from the near-neutral hemicellulose extraction process was investigated as a precursor suitable for production of carbon fiber. Crude lignin was precipitated from the wood extract by adding sulfuric acid to lower the pH value to 1.0. The crude lignin extract was then upgraded by using hydrolysis to cleave lignin-carbohydrate bonds and to remove carbohydrates that contaminate the lignin. The precipitated solids were separated by filtration, washed with water, and then dried. Lignin recovered using the hydrolysis method was found to be high in carbon, high in total lignin, low in inorganic contamination, and low in insoluble material, but high in volatile material. The recovered lignin could be thermally spun into lignin fibers, but the spun fibers proved to be brittle, which was thought to be due to its low-molecular mass and the glassy nature of lignin. Micrographs obtained using scanning electron microscopy (SEM) illustrated imperfections on the surface and in the interior microstructure of the carbon fiber when compared to micrographs taken of commercial carbon fiber manufactured using PAN and pitch. These imperfections were thought to be related to the high volatile material content in the samples and the slow rate of heating during the carbonization process. Baker (2011) suggests that increasing the rate of heating during carbonization can reduce the degree of brittleness and improve mechanical properties.