Lignin has great potential to provide revenue for lignocellulosic biofuels production and is considered an interesting feedstock for the manufacture of high value products. A product of particular interest is lignin-based low-cost carbon fiber (L-LCCF), where the low cost of the lignin precursor becomes an attractive option in considering the possible use of commodity grade carbon fiber in, for example, reducing the weight of vehicles to reduce fuel consumption.
Previous work on L-LCCF used lignins, which were modified to become fully thermoplastic materials, then melt-spun to give lignin fiber, converted to a thermoset fiber and then carbonized. The main technical barrier to manufacture L-LCCF is the lignin glass transition temperature (Tg), which has to be low enough for melt-spinning to proceed successfully, but high enough to ensure that conversion of the thermoplastic lignin fiber to a thermoset fiber proceeds at an acceptable rate (<3 hours cf. 3-21 days reported). The thermal window of opportunity is thus narrow and much preparation of the lignin is required to satisfy this technical barrier, which has yet to be sufficiently overcome. However, lignin has advantages over other precursors: it is very inexpensive, a renewable product, and is already substantially oxidized so that it can be oxidatively thermostabilized at much higher rates than either MPP or PAN. This could allow for substantial cost reduction in the manufacture of carbon fiber.
In previous communications Baker described the properties of two lignins, one of which was a hardwood kraft lignin product isolated from black liquor, and the other was an organic purified version of the first (OP86; i.e. Tg~86°C). It was found that while the Kraft lignin was not readily melt-spinable, the organic purified lignin was, and continuous lignin tows could be spun to diameters as low as 10μm and with high speed. In continuing work, the use of a thermal pretreatment to adjust the Tg and Tsoftening of a lignin starting material would be more effectively employed using a lignin with a more narrow molecular weight distribution. The thermal pretreatment of OP86 progressed and gave a selection of lignins with varying Tg and Tmf properties, which were evaluated for their melt-spinning properties. The lignin fiber tows were studied extensively for their oxidative thermostabilization and carbonization properties. Ultimately, the best carbon fibers produced in this work had a tensile strength of 185ksi (cf. OP86 lignin at 75ksi) with a carbon yield of 55% (cf. 33% for the OP86 lignin). The time required for oxidative thermostabilization could be reduced to 13 minutes and for the overall process, less than one hour – many times faster than any other carbon fiber process.
In evaluating this work, it was determined that the lignins used for CF manufacture should be of optimal quality in terms of purity, thermal, and molecular weight properties. An organosolv process was therefore applied to several biomasses (hardwood, softwood, switchgrass …) and optimized to produce the most optimal lignins. Several were melt spun and converted to CF. The data obtained suggested that scale-up of the organosolv process would lead to lignins that were directly suitable for use in CF manufacture and that the CFs produced would have superior strengths. Further to this, technical lignins were refined and also converted to CF with increased strengths compared to those produced before.
Dr. Baker consults and performs research on the preparation of specialty lignins from various biomasses by the use of separation/ fractionation processes and the refining of commercial lignin streams. The enhanced lignins are directed towards utilization in high value products and carbon materials.
His main research thrust concerns the manufacture of low-cost carbon fiber from refined lignins. Recent work has also resulted in the manufacture of novel carbon nanofiber materials from lignin with desirable morphologies, low cost, and short processing times. This represents a significant milestone in carbon nanofiber manufacture that could allow for applications in energy storage, electrical devices and in lower-cost filtration and separation applications.
His long-term goal is to develop novel and effective strategies for the preparation and utilization of lignin and cellulose polymers to produce high-value materials.
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