# Synthesis and Processing of Melt Spun Materials from Esterified Lignin with Lactic Acid
## 1 Introduction  
Carbon fibers are constantly being investigated considering their potential of high prospect technological application such as in the airspace or car industry, which require high performance and long-serving advanced materials for construction. Carbon fibers of exceptional mechanical properties have already been produced; however, new low-cost precursors are under investigation. At present, the need to reduce the cost of the carbon fibers for technological applications is of paramount importance and provides the opportunity to broaden their market. As a consequence, eco-friendly, renewable, and low-cost precursors of lignin have attracted major interest in order to be incorporated in the synthesis of the carbon fibers, considering both high carbon content and dense aromatic structure [1; 2; 3]. Synthesis of carbon fibers consists of several processing paths, such as chemical synthesis of the precursor polymer, fiber spinning (dry, wet, melt), stabilization in oxidative atmosphere to induce crosslinking and thermal resistance, and carbonization in inert atmosphere to induce graphitic planes in fiber structure. Regarding the chemical synthesis step, it should be mentioned that it requires careful selection and design of the reaction mechanism of the reagents in order to improve the linearity of molecular chains and reduce the polar character induced by lignin hydroxyls [4]. A potential low-cost precursor that could be used for carbon fiber production is Softwood Kraft Lignin (SKL). However, this precursor has several particularities due to the fact that it consists ofa complex three-dimensional network, while it becomes brittle upon thermal processing. Therefore, in order for lignin to be used for structural carbon-fiber production, it has to be modified so that the high-extent reactive C-O bonds in interunit lignin linkages, such as b-O-aryl ether, can be reduced [4]. On the other hand, lignin provides a low-cost and widely available alternative that could be utilized in industries, assuming that a viable modification route is established to enhance process ability and compounding to form fibers with enhanced mechanical properties and behavior overall. In general, lignin composites with epoxy resins, derivatives, and blends have already been investigated [5; 6; 7; 8], but there is no sufficient reference to their spinning ability, while [PERSON] et al. overviewed the research done on plant fiber composites [9]. Moreover, [PERSON] et al. investigated chemical modification of Sisal (a Mexican agave) with lignin and the effect of phenolic composite properties on fibers. It was concluded that phenolic functional groups enhance fiber properties. In other works considering biopolymers similar to lignin, cellulose was studied as a binder in green composites [10; 11], whereas application of plant fibers, composites, and their life cycle assessment was also reported [12; 13; 14; 15; 16; 17; 18]. Additionally, the properties of elastomeric composites coming from widely used commercial polymers are already known, as well as the effect of lignin enrichment in foams, composites, and fibers [19; 20; 21]. Furthermore, high-tech grafting of carbon nanotubes on lignin structures to manufacture fibers with advanced electrical or mechanical properties has been performed [22; 23; 24]. The motivation of the present work is the synthesis of carbon fibers using low-cost and eco-friendly precursors. Therefore, the aim in the present research is to follow a relatively simple and low-cost procedure to reach a carbon fiber production. Lignin was esterified with lactic acid (LA) and poly(lactic acid) (PLA) in order to improve its chemical affinity with high-density polyethylene (HDPE) and pave the way for the preparation of a blend that could be melt spun and then produce carbon fibers. The aforementioned organic acids were implemented in the precursor lignin-enriched fiber production via chlorination with thionyl chloride (SOCl\({}_{2}\)) and esterification with lignin.