Jie Luo

Date of Award

5-2010

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

Joseph M. Genco

Second Committee Member

David J. Neivandt

Third Committee Member

Barbara J.W. Cole

Abstract

This study was undertaken in support of the Biorefinery concept applicable to hardwood Kraft mills. The “near neutral hemicellulose extraction process” uses sodium carbonate and sodium sulphide (green liquor) to extract acetyl groups and carbohydrates from hardwood chips for conversion into acetic acid and five and six carbon sugars respectively. Lignin, although extracted in the process, is underutilized and is potentially a valuable by-product. The objective of this study was to investigate methods of recovering lignin as a possible precursor material suitable for production of carbon fiber. Recovered hardwood lignin would serve as an alternative raw material to supplement commercial precursors such as polyacrylonitrile (PAN) and pitch from petroleum and coal. Previous lignin studies are reviewed that use a variety of lignin types and lignin-polymer blends as precursors for carbon fiber. Few however explore using pre-extracted lignin from a Biorefinery as a carbon fiber precursor.The most suitable method developed in the present study for recovery of pre-extracted lignin involved a four step procedure. Crude lignin is iv precipitated from the wood extract by adding sulfuric acid to lower the pH value to 1.0. The crude lignin extract is then upgraded by using hydrolysis to cleave lignin carbohydrate bonds and to remove carbohydrates that contaminate the lignin. The precipitated lignin solids were separated by filtration, washed with water and then dried in an oven. This procedure was found superior to a technique predicated upon using ethanol to precipitate and remove carbohydrate contamination. Lignin recovered using the hydrolysis method developed here 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 readily spun into lignin fibers, but the spun fibers proved to be extremely brittle. Of all of the samples evaluated, Alcell lignin fiber was found to be the least brittle. This decrease in brittleness in the Alcell lignin fiber was attributed to a low glass transition temperature (Tg) measured as (108.6°C) and found to be the lowest of any of the samples evaluated. The spun lignin fiber was stabilized thermally in air at 200°C and converted into carbon fiber in an argon atmosphere at 1,000°C. Micrographs obtained using scanning electron microscopy (SEM) illustrated numerous 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 a heating rate that was too high in the carbonization process. To utilize pre-extracted lignin as a raw material for producing carbon fibers the brittleness problem must be overcome. Methods for solving this problem are discussed, including hydrogenation and acetylation of the lignin to reduce cross-linkages and lower its glass transition temperature (Tg).

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