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

碳纤维是已知的最坚固的增强纤维之一，用于各种商业复合材料。由于碳元素不会熔化，碳纤维需要从有机前驱体中获得。在可再生原料中，木质素因其碳含量高而开始作为碳纤维的潜在前体发挥重要作用。然而，木质素的不规则化学结构使得由此产生的碳网络非常低效。因此，得到的碳纤维性能仍然显著低于目前使用的合成聚丙烯腈前驱体，这涉及到使用强溶剂并导致有毒的尾气，如氰化氢。这项研究项目旨在开发一种绿色前驱体和一种有效的加工方法来获得高强度碳纤维。木质素是一种廉价的副产品，从硫酸盐纸厂很容易获得，但这种未经处理的形式含有太多杂质，不能用于高性能产品，如碳纤维。最近，克莱姆森研究人员发现了一种可再生的溶剂，可以提纯硫酸盐木质素，并在溶剂化状态下分离出高相对分子质量的木质素组分。克莱姆森的研究人员(在另一项研究中)还表明，对非分级的乙酰化木质素进行干法纺丝可以获得强度更高的碳纤维。因此，研究小组假设，溶剂化的木质素前体将使原丝纤维能够直接干法纺丝，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)控制分级溶剂化木素前体的分子结构；(Ii)对合适的溶剂化木素组分进行流变学评估，以测量用于原丝纤维高效干纺的应变硬化；以及(Iii)微观结构控制，以获得所得碳纤维的优异性能。这项研究的智力意义在于，具有可控分子量和结构的木质素馏分将为研究这些生物衍生流体的流变性和微观结构(流变性)行为提供独特的材料平台。具体地说，将确定在拉伸流动过程中导致应变硬化的长线性结构，这是纤维稳定干纺的理想现象。通过对木素分子构型的控制，可以有效地生成石墨炭，从而获得性能优异的碳纤维。

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.