High-Strength Carbon Fibers from Eco-Friendly Processing of Biomass

碳纤维是已知最强的增强纤维之一，广泛用于商业复合材料。因为碳元素不熔化，所以碳纤维需要从有机前体中获得。在可再生原料中，木质素开始发挥重要作用，作为碳纤维的潜在前体，由于其显著的碳含量。然而，木质素不规则的化学结构使得产生的碳网络效率非常低。因此，所得到的碳纤维性能仍明显低于目前使用的合成聚丙烯腈前驱体，后者涉及使用强溶剂，并导致有毒的废气，如氰化氢。本研究项目旨在开发绿色前驱体和高效的加工路线，以获得高强度碳纤维。木质素是牛皮纸造纸厂的一种廉价副产品，但这种未经处理的形式含有太多杂质，无法用于高性能产品，如碳纤维。最近，克莱姆森的研究人员发现了一种可再生的溶剂，它可以纯化卡夫木质素，并在溶剂化状态下分离出高分子量的木质素部分。克莱姆森大学的研究人员（在一个单独的项目中）也表明，非分馏、乙酰化木质素的干纺可以提高碳纤维的强度。因此，研究小组假设溶剂化木质素前驱体将实现前驱体纤维的直接干纺，然后将其转化为微结构控制的高性能碳纤维。具体的研究目标是(i)控制分馏溶剂化木质素前驱体的分子结构；（ii）对合适的溶剂化木质素馏分进行流变学评估，以测量应变硬化，从而有效地进行前驱体纤维的干纺丝；（iii）控制微结构，以获得优异的碳纤维性能。这项研究的知识意义在于，具有控制分子量和结构的木质素组分将为研究这些生物衍生流体的流变学和微观结构（流变结构）行为提供一个独特的材料平台。具体地说，将确定在拉伸流动期间导致应变硬化的长线性结构，这是纤维稳定干纺丝的理想现象。假设对木质素分馏溶剂化分子结构的控制将导致石墨碳的有效形成和所得到的碳纤维的优越性能。

Carbon fibers are among the strongest known reinforcing fibers used in a wide range of commercial composite materials. Because the element carbon does not melt, carbon fibers need to be obtained from organic precursors. Among renewable feedstocks, lignin is starting to play an important role as a potential precursor for carbon fibers, owing to its significant carbon content. However, the irregular chemical architecture of lignin makes the resulting carbon network very inefficient. Therefore, the resulting carbon-fiber properties are still significantly lower than those obtained from currently used synthetic polyacrylonitrile precursor, which involves the use of strong solvents and leads to toxic off-gases such as hydrogen cyanide. This research project is directed towards the development of a green precursor and an efficient processing route to obtain high strength carbon fibers.Lignin is readily available as an inexpensive by-product from Kraft paper mills, but this untreated form contains too many impurities to be used in high-performance products such as carbon fibers. Recently, Clemson researchers have discovered a renewable solvent that can purify Kraft lignin and isolate a high molecular weight lignin fraction in the solvated state. Clemson researchers (in a separate project) have also shown that dry-spinning of non-fractionated, acetylated lignin can lead to carbon fibers with enhanced strength. Therefore, the research team hypothesizes that the solvated lignin precursor will enable direct dry-spinning of precursor fibers, followed by their conversion to microstructure-controlled, high-performance carbon fibers. The specific research objectives are (i) control of the molecular architecture of fractionated-solvated lignin precursors (ii) rheological evaluation of suitable solvated lignin fractions to measure strain-hardening for efficient dry-spinning of precursor fibers, and (iii) microstructural control to obtain superior performance in the resulting carbon fibers. The intellectual significance of this research is that lignin fractions with controlled molecular weight and architecture will provide a unique materials platform for studying the rheological and microstructural (rheo-structural) behavior of these bio-derived fluids. Specifically, a long-linear architecture will be identified that leads to strain-hardening during extensional flow, a desirable phenomenon for stable dry-spinning of fibers. It is hypothesized that the control of molecular architecture of the fractionated-solvated lignin will lead to the efficient formation of graphitic carbon and superior properties for the resulting carbon fibers.