Synthesis and Processing of Melt Spun Materials from Esteriﬁed Lignin with Lactic Acid

: In this study, the carbon ﬁber manufacturing process is investigated, using high-density polyethylene (HDPE) and esteriﬁed lignin either with lactic acid (LA) or with poly(lactic acid) (PLA) as precursors. More speciﬁcally, lignin was modiﬁed using either LA or PLA in order to increase its chemical a ﬃ nity with HDPE. The modiﬁed compounds were continuously melt spun to ﬁbrous materials by blending with HDPE in order to fabricate a carbon ﬁber precursor. The obtained products were characterized with respect to their morphology, as well as their structure and chemical composition. Moreover, an assessment of both physical and structural transformations after modiﬁcation of lignin with LA and PLA was performed in order to evaluate the spinning ability of the composite ﬁbers, as well as the thermal processing to carbon ﬁbers. This bottom–up approach seems to be able to provide a viable route considering large scale production in order to transform lignin in value-added product. Tensile tests revealed that the chemical lignin modiﬁcation allowed an enhancement in its spinning ability due to its compatibility improvement with the commercial low-cost and thermoplastic HDPE polymer. Finally, stabilization and carbonization thermal processing was performed in order to obtain carbon ﬁbers. a peak of medium intensity at 2900 cm − 1 , which is attributed to C–H stretches. The peaks at 2900 cm − 1 result from various C–H bonds due to the aromatic methoxy groups and the methyl / methylene side groups. The characteristic absorption bands for aromatic rings of lignin are at 1600, 1500, and 1410 cm − 1 . The peak at 1600 cm − 1 also implies the presence of a conjugated structure. There is also a shoulder at 1750 cm − 1 , corresponding to lignin’s carbonyl groups [43–45]. The FTIR most important peaks are tabulated in Table 7.


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 of

Synthesis of Fibers
In a 1000 mL three-neck round bottom flask, 100 mL of LA was dissolved in 230 mL of CHCl 3 , under vigorous stirring at room temperature for 1 h. Then, 100 mL of SOCl 2 was added to the mixture, dripping slowly and in small amounts in order to perform the chlorination reaction. As the SOCl 2 was Appl. Sci. 2019, 9,5361 3 of 17 added to the flask, vapors of SO 2 were released from the chemical system according to the reaction in Figure 1 [25]. According to this reaction, a nucleophilic substitution pathway occurs, in which the carboxylic acid is first converted into a chlorosulfite intermediate. Then, the chlorosulfite reacts with a nucleophilic chloride ion in order lactic acid chloride (LCl) to be synthesized.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 16 which the carboxylic acid is first converted into a chlorosulfite intermediate. Then, the chlorosulfite reacts with a nucleophilic chloride ion in order lactic acid chloride (LCl) to be synthesized. The released vapors indicate that the chlorination reaction has progressed. After partial evaporation of the solvent (CHCl3), the solution was added to 250 mL of pyridine. Then, 100 g of washed SKL was added to the mixture, and the blend was left under vigorous stirring at room temperature for 1 h. The temperature was then elevated at 120 °C, and a reflux condenser was adapted onto the flask. The esterification reaction duration was 5 h at the aforementioned conditions; the mechanism of the corresponding esterification reaction (alcoholysis) between the hydroxyl groups of SKL and lactic chloride is depicted in Figure 2 [26]. According to the esterification process, an addition of oxygen (nucleophile) to the carbonyl group occurs, yielding a tetrahedral intermediate. Then, an electron pair from oxygen displaces the chloride (leaving group), generating a new carbonyl compound as a product. After the completion of the reaction, the solvent (pyridine) was evaporated. The solid product (SKL esterified with lactic chloride, SKL-LA) was oven-dried overnight at 70 °C. In a 1000 mL glass beaker, 45 g of poly(lactic acid) (PLA) was dissolved in 600 mL of chloroform under vigorous mechanical stirring. The blend was left under stirring overnight in order to ensure complete dissolution. Afterwards, 100 mL of SOCl2 was added dripping in the solution under mechanical stirring. The SOCl2 reacted with the hydroxyl groups of PLA and produced poly(lactic acid) chloride (PLCl) according to the reaction in Figure 3 [25]. During the SOCl2 addition, there was a release of SO2 vapors that indicated the reaction progress. After completion of the reaction, the obtained product (PLCl) was dried and pulverized. Then, 25 g of the synthesized PLCl was dissolved in 270 mL of pyridine in a 500 mL three-neck round bottom spherical flask and was left under vigorous stirring overnight at room temperature. In the next step, 40 g of SKL was added, and the mixture was left to react at 120 °C for 5 h under vigorous stirring. After that, the solvent was The released vapors indicate that the chlorination reaction has progressed. After partial evaporation of the solvent (CHCl 3 ), the solution was added to 250 mL of pyridine. Then, 100 g of washed SKL was added to the mixture, and the blend was left under vigorous stirring at room temperature for 1 h. The temperature was then elevated at 120 • C, and a reflux condenser was adapted onto the flask. The esterification reaction duration was 5 h at the aforementioned conditions; the mechanism of the corresponding esterification reaction (alcoholysis) between the hydroxyl groups of SKL and lactic chloride is depicted in Figure 2 [26]. According to the esterification process, an addition of oxygen (nucleophile) to the carbonyl group occurs, yielding a tetrahedral intermediate. Then, an electron pair from oxygen displaces the chloride (leaving group), generating a new carbonyl compound as a product. After the completion of the reaction, the solvent (pyridine) was evaporated. The solid product (SKL esterified with lactic chloride, SKL-LA) was oven-dried overnight at 70 • C.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 16 which the carboxylic acid is first converted into a chlorosulfite intermediate. Then, the chlorosulfite reacts with a nucleophilic chloride ion in order lactic acid chloride (LCl) to be synthesized. The released vapors indicate that the chlorination reaction has progressed. After partial evaporation of the solvent (CHCl3), the solution was added to 250 mL of pyridine. Then, 100 g of washed SKL was added to the mixture, and the blend was left under vigorous stirring at room temperature for 1 h. The temperature was then elevated at 120 °C, and a reflux condenser was adapted onto the flask. The esterification reaction duration was 5 h at the aforementioned conditions; the mechanism of the corresponding esterification reaction (alcoholysis) between the hydroxyl groups of SKL and lactic chloride is depicted in Figure 2 [26]. According to the esterification process, an addition of oxygen (nucleophile) to the carbonyl group occurs, yielding a tetrahedral intermediate. Then, an electron pair from oxygen displaces the chloride (leaving group), generating a new carbonyl compound as a product. After the completion of the reaction, the solvent (pyridine) was evaporated. The solid product (SKL esterified with lactic chloride, SKL-LA) was oven-dried overnight at 70 °C. In a 1000 mL glass beaker, 45 g of poly(lactic acid) (PLA) was dissolved in 600 mL of chloroform under vigorous mechanical stirring. The blend was left under stirring overnight in order to ensure complete dissolution. Afterwards, 100 mL of SOCl2 was added dripping in the solution under mechanical stirring. The SOCl2 reacted with the hydroxyl groups of PLA and produced poly(lactic acid) chloride (PLCl) according to the reaction in Figure 3 [25]. During the SOCl2 addition, there was a release of SO2 vapors that indicated the reaction progress. After completion of the reaction, the obtained product (PLCl) was dried and pulverized. Then, 25 g of the synthesized PLCl was dissolved in 270 mL of pyridine in a 500 mL three-neck round bottom spherical flask and was left under vigorous stirring overnight at room temperature. In the next step, 40 g of SKL was added, and the mixture was left to react at 120 °C for 5 h under vigorous stirring. After that, the solvent was In a 1000 mL glass beaker, 45 g of poly(lactic acid) (PLA) was dissolved in 600 mL of chloroform under vigorous mechanical stirring. The blend was left under stirring overnight in order to ensure complete dissolution. Afterwards, 100 mL of SOCl 2 was added dripping in the solution under mechanical stirring. The SOCl 2 reacted with the hydroxyl groups of PLA and produced poly(lactic acid) chloride (PLCl) according to the reaction in Figure 3 [25]. During the SOCl 2 addition, there was a release of SO 2 vapors that indicated the reaction progress. After completion of the reaction, the obtained product (PLCl) was dried and pulverized. Then, 25 g of the synthesized PLCl was dissolved in 270 mL of pyridine in a 500 mL three-neck round bottom spherical flask and was left under vigorous stirring overnight at room temperature. In the next step, 40 g of SKL was added, and the mixture was left to react at 120 • C for 5 h under vigorous stirring. After that, the solvent was evaporated, and the solid collected product of SKL-PLA (SKL esterified with PLA) was dried at 80 • C. The chemical reaction of the esterification process (alcoholysis) is illustrated in Figure 4 [26].
Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 16 evaporated, and the solid collected product of SKL-PLA (SKL esterified with PLA) was dried at 80 °C. The chemical reaction of the esterification process (alcoholysis) is illustrated in Figure 4 [26].  The abovementioned materials were mixed with HDPE, and the resulting blends were loaded in an extruder line, optimizing spinning parameters of extruder temperature, roller distance from the extruder, and extrusion speed for each precursor blend. The parameters and conditions for the optimization process for the HDPE spinning for the production of HDPE/SKL-LA (80:20) and HDPE/SKL-PLA (80:20) fibers are tabulated in Tables A1 and A2. The best results concerning the spinning ability of the blends were obtained for the conditions that are presented in Table 1. The blend mass ratio of the HDPE/esterified lignin was 80/20 by weight (HDPE/SKL-LA (80:20), HDPE/SKL-PLA (80:20)). The extruded precursor HDPE/SKL-PLA fibers were fixed in bundles and were thermally treated following four different thermal stages. Firstly, they were subjected to a 1 h long prestabilization step at 150 °C, followed by a second stage at 220 °C for 3 h and finally at 300 °C for 10 min to maximize crosslinking. This thermal procedure enhanced crosslinking through oxidation and prepared the structure to withstand carbonization. Carbonization was performed at 800 °C, under nitrogen atmosphere with a residence time of 5 min. The thermal treatment effect is presented in Table 2.    The abovementioned materials were mixed with HDPE, and the resulting blends were loaded in an extruder line, optimizing spinning parameters of extruder temperature, roller distance from the extruder, and extrusion speed for each precursor blend. The parameters and conditions for the optimization process for the HDPE spinning for the production of HDPE/SKL-LA (80:20) and HDPE/SKL-PLA (80:20) fibers are tabulated in Tables A1 and A2. The best results concerning the spinning ability of the blends were obtained for the conditions that are presented in Table 1. The blend mass ratio of the HDPE/esterified lignin was 80/20 by weight (HDPE/SKL-LA (80:20), HDPE/SKL-PLA (80:20)). The extruded precursor HDPE/SKL-PLA fibers were fixed in bundles and were thermally treated following four different thermal stages. Firstly, they were subjected to a 1 h long prestabilization step at 150 °C, followed by a second stage at 220 °C for 3 h and finally at 300 °C for 10 min to maximize crosslinking. This thermal procedure enhanced crosslinking through oxidation and prepared the structure to withstand carbonization. Carbonization was performed at 800 °C, under nitrogen atmosphere with a residence time of 5 min. The thermal treatment effect is presented in Table 2.  The abovementioned materials were mixed with HDPE, and the resulting blends were loaded in an extruder line, optimizing spinning parameters of extruder temperature, roller distance from the extruder, and extrusion speed for each precursor blend. The parameters and conditions for the optimization process for the HDPE spinning for the production of HDPE/SKL-LA (80:20) and HDPE/SKL-PLA (80:20) fibers are tabulated in Tables A1 and A2. The best results concerning the spinning ability of the blends were obtained for the conditions that are presented in Table 1. The blend mass ratio of the HDPE/esterified lignin was 80/20 by weight (HDPE/SKL-LA (80:20), HDPE/SKL-PLA (80:20)). The extruded precursor HDPE/SKL-PLA fibers were fixed in bundles and were thermally treated following four different thermal stages. Firstly, they were subjected to a 1 h long pre-stabilization step at 150 • C, followed by a second stage at 220 • C for 3 h and finally at 300 • C for 10 min to maximize crosslinking. This thermal procedure enhanced crosslinking through oxidation and prepared the structure to withstand carbonization. Carbonization was performed at 800 • C, under nitrogen atmosphere with a residence time of 5 min. The thermal treatment effect is presented in Table 2. The carbonization of the fibers was conducted via a high-temperature furnace (Protherm-PTF Series, Single-Zone tube furnace, Maximum Temperature: 1200 • C) under nitrogen atmosphere at 800 • C for 5 min.
The melt spinning experiments were carried out on a Dynisco extruder. The Dynisco LME (Laboratory Mixing Extruder) is a laboratory tool developed to produce fibers continuously derived from thermoplastic polymers. The specimen is conveyed in a cooled hopper right after exiting the hot surface of the cylindrical rotor, against the inclined surface of the stationary scroll, and moves toward the outlet die.

Characterization
The fibers composition and their morphology were determined with a Hitachi Electron Microscope TM3030 coupled with an Ultra-High Resolution Scanning Electron Microscope (UHR-SEM) using NOVA NANOSEM 230 (FEI Company, Hillsboro, United States, 2009) and with an Energy Dispersive X-ray Spectrophotometer (EDS) (QUANTAX 70, BRUKER, Billerica, Massachusettes, United States, 2016). The Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (FTIR) instrument Cary 630 spectrometer (Agilent, Santa Clara, California, United States, 2016) was used to analyze the FTIR spectra of the fibers. This instrument has a resolution of 4 cm −1 and an operating wavelength range of 4000-400 cm −1 .
The Thermogravimetric/Differential Scanning Calorimetry (TGA/DSC) instrument STA 449 F5 Jupiter (NETZSCH-Gerätebau GmbH, Selb, Germany, 2017) was used for the thermal analysis of the fibers. The system consists of a SiC furnace with an operation temperature varying between 25 and 1550 • C. The maximum heating rate is 50 K/min. The gas used was nitrogen at a volumetric rate of 50 mL/min and at a heating rate of 20 • C/min. A Renishaw inVia spectrometer was used to perform micro-Raman measurements coupled with a green diode laser with emission at 532 nm (Renishaw plc, Gloucestershire, United Kingdom, 2013). The backscattering configuration was used. A beam spot size of 1 micrometer for a 1 mW beam at 1% intensity was used. The MATLAB 8.0 and Statistics Toolbox 8.1 (The MathWorks, Inc., Natick, Massachusetts, United States, 2012) was used for fitting the Raman signal to calculate the integrals of D and G peaks and quantify ID/IG ratio.
The structure of the fibers was observed and collected by the instrument SkyScan 1272, a 3D X-ray scan system (BRUKER, Billerica, Massachusettes, United States, 2017). The system disposes an X-ray source which operates at 20-100 kV and 10 W (<5 µm spot size @ 4W), an X-ray detector with a maximum resolution of 11 Mp (4032 × 2688 pixels), and a CCD fiber of 14 bits, cooled and optically joined to a scintillator. During the observation, the specimens were fixed on a rotational stage, the distance between the rotation axis and the radiation source is set from 6 to 12 mm, and the pixel resolution is set to 2016 × 1344 in order to obtain reliable images. Several stacked images were used for rendering, calculations, and image analysis for each sample.
The tensile strength at the breaking point of the fiber was determined according to the scale of the dynamometer of the MPM-10 tensile test machine, based on the standards ISO 11566:1996 and ASTM D 3379-89.

Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy
Taking into account the SEM images of the fibers HDPE/SKL-LA, HDPE/SKL-PLA (precursor) and HDPE/SKL-PLA (carbonized) (Figures 5-7, respectively), it can be observed that the diameter of the fibrous materials ranges from 158 to 170 µm (Table 3). It should be stated that the SEM characterization of the fibers in respect of their diameter (Table 3) includes the measurement of 10 fibers of each sample. Moreover, two measurements were conducted in each fiber.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 16 Taking into account the SEM images of the fibers HDPE/SKL-LA, HDPE/SKL-PLA (precursor) and HDPE/SKL-PLA (carbonized) (Figures 5-7, respectively), it can be observed that the diameter of the fibrous materials ranges from 158 to 170 μm (Table 3). It should be stated that the SEM characterization of the fibers in respect of their diameter (Table 3) includes the measurement of 10 fibers of each sample. Moreover, two measurements were conducted in each fiber.    Although the typical diameter of a PAN-based precursor fiber is only 15 μm, materials with a bigger diameter than that were produced because of the low elasticity of the HDPE and the use of a Taking into account the SEM images of the fibers HDPE/SKL-LA, HDPE/SKL-PLA (precursor) and HDPE/SKL-PLA (carbonized) (Figures 5-7, respectively), it can be observed that the diameter of the fibrous materials ranges from 158 to 170 μm (Table 3). It should be stated that the SEM characterization of the fibers in respect of their diameter (Table 3) includes the measurement of 10 fibers of each sample. Moreover, two measurements were conducted in each fiber.    Although the typical diameter of a PAN-based precursor fiber is only 15 μm, materials with a bigger diameter than that were produced because of the low elasticity of the HDPE and the use of a Taking into account the SEM images of the fibers HDPE/SKL-LA, HDPE/SKL-PLA (precursor) and HDPE/SKL-PLA (carbonized) (Figures 5-7, respectively), it can be observed that the diameter of the fibrous materials ranges from 158 to 170 μm (Table 3). It should be stated that the SEM characterization of the fibers in respect of their diameter (Table 3) includes the measurement of 10 fibers of each sample. Moreover, two measurements were conducted in each fiber.    Although the typical diameter of a PAN-based precursor fiber is only 15 μm, materials with a bigger diameter than that were produced because of the low elasticity of the HDPE and the use of a  Although the typical diameter of a PAN-based precursor fiber is only 15 µm, materials with a bigger diameter than that were produced because of the low elasticity of the HDPE and the use of a 1.8 mm spinneret. Additionally, the addition of lignin reduces the tension capacity of the material [27]. The thermal behavior of HDPE/SKL-LA fibers was not proper to withstand carbonization, due to the formation of agglomerates and heterogeneity on the extruded fiber. On the other hand, HDPE/SKL-PLA fibers were carbonized due to the fact that SKL-PLA presents enhanced mixing ability with HDPE blends. Furthermore, it should be mentioned that HDPE/SKL-PLA carbon fiber shrunk in length about 6% compared to the precursor, owing to densification and the increase of sturdiness after carbonization treatment [28]. Due to the same fact, the diameter of the fiber is increased. Furthermore, combined SEM-EDS analysis was conducted on the produced samples in order to provide information of the chemical composition, with the percentage of each chemical element in the fibers, as well as to support the chlorination mechanism of LA and PLA reaction and to confirm the successful synthesis of modified lignin by the corroboration of the absence of Cl element in the observed specimens. Table 4 presents the tabulated values of wt.% elemental composition of the produced HDPE/SKL-PLA fibers prior to and after carbonization process. Regarding the obtained results, it should be remarked that the percentage of chlorine in the samples was practically zero for both the HDPE/SKL-PLA precursor and carbon fiber specimens.

Thermogravimetric Analysis and Differential Scanning Calorimetry
The TGA and DSC diagrams of lignin compounds are illustrated in Figure 8. The gas used was nitrogen and the temperature ramp was 10 • C/min. Taking into consideration the TGA and DSC diagrams of the compounds SKL-LA ( Figure 8b) and SKL-PLA (Figure 8c), it may be remarked that both compounds present an initial peak of thermal degradation at 280 • C, whereas their thermal degradation end temperature is located at 800 • C leaving a residue of approximately 22% and 35% mass, respectively ( Table 5). The residual mass is attributed to the autocatalytic stabilization effect of lignin derived by high elemental composition in oxygen [29][30][31][32]. The PLCl is degraded almost quantitatively at 400 • C (Figure 8a). The DSC curve of PLCl presents two characteristic peaks, an endothermic at 340 • C and an exothermic at 480 • C, which correspond to its thermal degradation peak and crosslinking, respectively [33] (Figure 8a). As far as SKL-LA is concerned, the DSC graph presents a wide exothermic curve, which implies a steady rate and slow evolution of gases as crosslinking byproducts of the material, considering the mass loss as provided by TGA (Figure 8b). Finally the DSC graph for the SKL-PLA sample demonstrates a more intense exothermic peak ascribed to oxygen-derived autocatalytic crosslinking, generated at multiple starting points due to its size, but there is only a small endothermic peak at 700 • C, relating to its degradation and state transition from crystal-like to amorphous [34,35] (Figure 8c). Concerning the TGA/DSC graphs of the HDPE/SKL-PLA carbon fiber, it can be easily observed that the carbon residue after thermal treatment is about 85%, meaning that the whole procedure successfully produced a carbon fiber with desired thermal behavior, resistance, and adequate carbon yield. The wide exothermic DSC curve is ascribed to intramolecular bond forming reactions as well as to polycondensation and denitrification reactions (Figure 8d) [36].

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy
As evidenced in the aforementioned SEM/EDS micrographs as well as in the FTIR spectrogram in Figure 9, the chlorination reaction of PLA to PLCl was successful. The major differences in FTIR spectra after chlorination of PLA and LA are pointed out in the peaks at 1558 and 1640 cm −1 [37] that are attributed to asymmetric CO2 − stretching vibration, which is mainly reacted with SOCl2 [38][39][40][41]. Further peaks are similar between PLA and PLCl, slightly more intense in case of PLCl. This is due to the -Cl group electronegativity and the use of SOCl2 in the chlorination reaction [25,42]. The FTIR analysis results are presented in Table 6.

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy
As evidenced in the aforementioned SEM/EDS micrographs as well as in the FTIR spectrogram in Figure 9, the chlorination reaction of PLA to PLCl was successful. The major differences in FTIR spectra after chlorination of PLA and LA are pointed out in the peaks at 1558 and 1640 cm −1 [37] that are attributed to asymmetric CO 2 − stretching vibration, which is mainly reacted with SOCl 2 [38][39][40][41].
Further peaks are similar between PLA and PLCl, slightly more intense in case of PLCl. This is due to the -Cl group electronegativity and the use of SOCl 2 in the chlorination reaction [25,42]. The FTIR analysis results are presented in Table 6. The FTIR spectra of SKL-LA and SKL-PLA in Figure 10 demonstrate similar peaks. The small band at about 3500 cm −1 is attributed to the hydroxyl group of phenolic and aliphatic structures of lignin. This peak is more intense for SKL-LA due to the fact that the LA monomers which are esterified to the lignin hydroxyl groups have one hydroxyl group at a single end. The PLCl chains esterified to lignin (SKL-PLA) own only one hydroxyl group at the end of each chain. Therefore, the only signal that SKL-PLA presents at 3500 cm −1 is ascribed almost completely on the lignin hydroxyls, because the contribution from the PLCl hydroxyls to this signal is negligible. Moreover, SKL-LA demonstrates a peak of medium intensity at 2900 cm −1 , which is attributed to C-H stretches. The peaks at 2900 cm −1 result from various C-H bonds due to the aromatic methoxy groups and the methyl/methylene side groups. The characteristic absorption bands for aromatic rings of lignin are at 1600, 1500, and 1410 cm −1 . The peak at 1600 cm −1 also implies the presence of a conjugated structure. There is also a shoulder at 1750 cm −1 , corresponding to lignin's carbonyl groups [43][44][45]. The FTIR most important peaks are tabulated in Table 7.

.3. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy
As evidenced in the aforementioned SEM/EDS micrographs as well as in the FTIR spectrogram n Figure 9, the chlorination reaction of PLA to PLCl was successful. The major differences in FTIR pectra after chlorination of PLA and LA are pointed out in the peaks at 1558 and 1640 cm −1 [37] that re attributed to asymmetric CO2 − stretching vibration, which is mainly reacted with SOCl2 [38][39][40][41]. urther peaks are similar between PLA and PLCl, slightly more intense in case of PLCl. This is due o the -Cl group electronegativity and the use of SOCl2 in the chlorination reaction [25,42]. The FTIR nalysis results are presented in Table 6. The FTIR spectra of SKL-LA and SKL-PLA in Figure 10 demonstrate similar peaks. The small band at about 3500 cm −1 is attributed to the hydroxyl group of phenolic and aliphatic structures of lignin. This peak is more intense for SKL-LA due to the fact that the LA monomers which are esterified to the lignin hydroxyl groups have one hydroxyl group at a single end. The PLCl chains esterified to lignin (SKL-PLA) own only one hydroxyl group at the end of each chain. Therefore, the only signal that SKL-PLA presents at 3500 cm −1 is ascribed almost completely on the lignin hydroxyls, because the contribution from the PLCl hydroxyls to this signal is negligible. Moreover, SKL-LA demonstrates a peak of medium intensity at 2900 cm −1 , which is attributed to C-H stretches. The peaks at 2900 cm −1 result from various C-H bonds due to the aromatic methoxy groups and the methyl/methylene side groups. The characteristic absorption bands for aromatic rings of lignin are at 1600, 1500, and 1410 cm −1 . The peak at 1600 cm −1 also implies the presence of a conjugated structure. There is also a shoulder at 1750 cm −1 , corresponding to lignin's carbonyl groups [43][44][45]. The FTIR most important peaks are tabulated in Table 7.

Thermal Processing (Stabilization-Carbonization)
Taking into consideration Table 5, it may be remarked that the HDPE/SKL-PLA fibers present an enhanced thermal behavior compared to those of HDPE/SKL-LA, because they present a higher onset, peak, and end temperature of thermal degradation and considering the fact that HDPE has the same thermal behavior in both samples. Moreover, taking into consideration the two fibers, the HDPE/SKL-PLA precursor is expected to exhibit an extended linear-like polymeric chain and bigger molecular weight, in comparison with the much smaller HDPE/SKL-LA fibers, which were not successfully carbonized, due to the aforementioned reasons. The yield of the carbon fiber after carbonization of the HDPE/SKL-PLA fibers was 85%. Based on the higher end temperature and lower weight loss as given in Table 5, HDPE/SKL-PLA is more thermally stable compared to HDPE/SKL-LA. According to the literature, although the carbonization of PAN precursor fibers is carried out between 1000 and 1800 • C, in our study, the fibrous materials were carbonized at 800 • C for 5 min (in a horizontal furnace used for CVD), due to the different texture and thermal behavior of the materials.

Tensile Testing of Precursor Fibers
A tensile test was performed on the precursor fibers. Regarding the obtained results, the measured average tensile strength at the breaking point varied between the fibers. The fibers which presented the optimum properties were HDPE/SKL-PLA fibers, at 32.33 MPa demonstrating an average diameter of 159.5 µm, while the HDPE/SKL-LA demonstrated 25.12 MPa of tensile strength. The pure HDPE fibers showed a tensile strength of 19.87 MPa. All tests were conducted three times, and then the average value was calculated (3 fibers of each different type were used, 9 fibers in total, 27 measurements in total). The crosshead speed used for testing was 0.2 mm/min. According to the literature [46,47], as far as the orientation is concerned, it was not substantially modified after the tensile test, which was a promising indication for potential thermal treatment (stabilization, carbonization), because during these processes, the fibers tend to shrink and become thicker. Furthermore, these values are related and comply with the tensile properties of natural fiber composites [48], while the difference between the precursor fiber in tensile strength is ascribed to their slightly different microstructure, which is affected by extrusion speed, temperature, and residence time of the materials in the extrusion line and the size of the spinneret [49]. Further, the tensile deformation and damage of fibers are strongly affected by both defects and orientation [50], while the chemical bonding has a severe impact on stiffness and tensile strength of fiber network [51].

Micro-CT Analysis
The micro-CT analysis results are depicted in Table 8, and the volume rendering images of carbon fibers are presented in Figure 11. Concerning the X-ray tomography obtained results, micro-CT studies on inner geometry and morphology of carbon fiber tapes [52] demonstrated a similar morphology as the one obtained in the present work. Additionally, tomography performed on low-density carbon fiber materials and composites exhibits similar physical properties with home-grown carbon fibers [53,54]. Furthermore, investigation of carbon fiber failure and breakage via micro-CT modeling showed similar values [55][56][57] and supports effective carbonization in the present study. The surface density is the mass per unit area (mm 2 ). Linear density is the density that the material demonstrates in relation to its length [58]. In the presented work, it is the amount of mass per millimeter. The degree of anisotropy (DA) is an index that shows the alignment of the materials (fibers) into the total scanned volume [59], and in our study, it is a measure of the orientation of the fibers within the rectangles of the Figure 8.Fractal dimension is a measure showing the change in the detail of a specific object (fibers in this case) as the measuring scale changes [60]. The micro-CT is useful for the scope of the current research as it gives information about the porosity and the morphology of the final material. The innovation of the presented technique applied to the samples produced in this study is the demonstration of a porosity below 2%. This fact underlines the durability and the sturdiness of the synthesized materials. fibers within the rectangles of the Figure 8.Fractal dimension is a measure showing the change in the detail of a specific object (fibers in this case) as the measuring scale changes [60]. The micro-CT is useful for the scope of the current research as it gives information about the porosity and the morphology of the final material. The innovation of the presented technique applied to the samples produced in this study is the demonstration of a porosity below 2%. This fact underlines the durability and the sturdiness of the synthesized materials.

Raman Spectroscopy
Raman spectroscopy measurements provided insight into the carbonization procedure ( Figure  12). More specifically, taking into consideration that the presence of G-bands around 1650 cm −1 is a straightforward indication of graphite planes formation, it should be mentioned that the ID/IG ratio of HDPE/SKL-PLA carbon fibers was 0.847, denoting a lower intensity of D-band at 1300 cm −1 compared to G-band, denoting that the sp 3 bonds are more than the sp 2 bonds.
Raman investigation of the produced carbon fibers was performed in order to characterize the structural transformation of the precursor fiber to carbon fiber; the formation of a graphite-like sp 2 plane is expected to be formed as a result of carbonization. Lignin-based carbon fibers presented a promising response to electromagnetic radiation in Raman characterization, with all D, G, and 2D

Raman Spectroscopy
Raman spectroscopy measurements provided insight into the carbonization procedure ( Figure 12). More specifically, taking into consideration that the presence of G-bands around 1650 cm −1 is a straightforward indication of graphite planes formation, it should be mentioned that the I D /I G ratio of HDPE/SKL-PLA carbon fibers was 0.847, denoting a lower intensity of D-band at 1300 cm −1 compared to G-band, denoting that the sp 3 bonds are more than the sp 2 bonds.
Raman investigation of the produced carbon fibers was performed in order to characterize the structural transformation of the precursor fiber to carbon fiber; the formation of a graphite-like sp 2 plane is expected to be formed as a result of carbonization. Lignin-based carbon fibers presented a promising response to electromagnetic radiation in Raman characterization, with all D, G, and 2D peaks to appear. A trend of forming a sharper and of higher intensity G to D peak was recorded in Raman spectra, which seems to provoke a proper structure reorientation [61].
Looking at the envelope of fit reported in Figure 12, it can be clearly stated that D and G regions encompass four different Gaussian components. Two of them were used to fit the D peak and two of them were used for the G peak. The D peak is described by two components centered at 1330 and 1360 cm −1 which present low-intensity variation changing from LA to PLA. The G peak demonstrates a more stable component around 1590-1600 cm −1 [62]. To obtain quantitative information by Raman spectroscopy, the ratio of the intensity of D to G peak (I D /I G ) is measured, and I D /I G obtained a value of 0.847, indicating the strong presence of graphitic-like planes present in carbon fiber structure [63][64][65].

Outcome Analysis
In the present research, a relatively low-cost procedure was followed to reach a carbon fiber production, including a comprehensive stage of organic synthesis, three stabilization stages, and one carbonization stage, with the precursor fiber containing 20 wt.% content of lignin. Moreover, the aim of this study is the industrial upscalability along with simplicity (high profit, high spinning speed and production rate, low cost, one pot reactions), a fact which seems feasible according to this study. According to the literature, despite the fact that chemical modifications of lignin have been investigated [24], there are no references of successful melt-spun lignin conjugation with high molecular weight commercial spinnable polymers, using esterification after a chlorination reaction, so as to enhance the intermolecular bonds between the compounds. The successful chemical synthesis, which was conducted in this study without the use of plasticization [66] and confirmed by characterization, provided thermal process ability to the precursor fiber. Anthracene-oil-derived pitches as precursors have led to green carbon fibers [67], but the diameter of the fibers varied depending on the winding speed, and overall, that method was more expensive than using lignin in the blends. Additionally, carbon fibers from low molecular weight polymers by solvent extraction [68] and from polyacrylonitrile combined with CNTs [69] showed good strength but lacked the lowcost synthesis. While cellulose has been used in the fabrication of carbon fibers [70], the potential of lignin has been acknowledged already [2], but its handling and treatment remained an issue. Finally, lignin, compared to cellulose, is more suitable as a precursor for carbon fiber due to its aromatic groups, which strengthen the material structure after carbonization (higher carbon yield) [71].

Conclusions
In the present study, lactic acid and poly(lactic acid) were modified by chlorination, in order to produce the corresponding organic chlorides. Then, these compounds were esterified with SKL. Subsequently, the obtained product was compounded and spun with HDPE thermoplastic polymer to form composite precursors suitable for carbon fiber production. Stabilization and carbonization were conducted, and the obtained HDPE/SKL-PLA fibers demonstrated lower ID/IG, by Raman characterization for carbonization at 800 °C, which was adopted as optimal thermal processing. The relative low ID/IG ratio, compared to carbon fibers reported in literature, indicates that graphitic sp 2 planes are substantially formed and oriented, and thus, it can be feasible to enhance the electrical

Outcome Analysis
In the present research, a relatively low-cost procedure was followed to reach a carbon fiber production, including a comprehensive stage of organic synthesis, three stabilization stages, and one carbonization stage, with the precursor fiber containing 20 wt.% content of lignin. Moreover, the aim of this study is the industrial upscalability along with simplicity (high profit, high spinning speed and production rate, low cost, one pot reactions), a fact which seems feasible according to this study. According to the literature, despite the fact that chemical modifications of lignin have been investigated [24], there are no references of successful melt-spun lignin conjugation with high molecular weight commercial spinnable polymers, using esterification after a chlorination reaction, so as to enhance the intermolecular bonds between the compounds. The successful chemical synthesis, which was conducted in this study without the use of plasticization [66] and confirmed by characterization, provided thermal process ability to the precursor fiber. Anthracene-oil-derived pitches as precursors have led to green carbon fibers [67], but the diameter of the fibers varied depending on the winding speed, and overall, that method was more expensive than using lignin in the blends. Additionally, carbon fibers from low molecular weight polymers by solvent extraction [68] and from polyacrylonitrile combined with CNTs [69] showed good strength but lacked the low-cost synthesis. While cellulose has been used in the fabrication of carbon fibers [70], the potential of lignin has been acknowledged already [2], but its handling and treatment remained an issue. Finally, lignin, compared to cellulose, is more suitable as a precursor for carbon fiber due to its aromatic groups, which strengthen the material structure after carbonization (higher carbon yield) [71].

Conclusions
In the present study, lactic acid and poly(lactic acid) were modified by chlorination, in order to produce the corresponding organic chlorides. Then, these compounds were esterified with SKL. Subsequently, the obtained product was compounded and spun with HDPE thermoplastic polymer to form composite precursors suitable for carbon fiber production. Stabilization and carbonization were conducted, and the obtained HDPE/SKL-PLA fibers demonstrated lower I D /I G , by Raman characterization for carbonization at 800 • C, which was adopted as optimal thermal processing. The relative low I D /I G ratio, compared to carbon fibers reported in literature, indicates that graphitic sp 2 planes are substantially formed and oriented, and thus, it can be feasible to enhance the electrical properties of composites. The results from this work can constitute a solid basis for scaling up similar procedures.
Author Contributions: P.G., I.A.K., and C.A.C. conceived and designed the experiments. P.G. conducted the experiments. P.G., G.K., and I.A.K. performed the characterization and evaluation of the data. P.G., I.A.K., and C.A.C. discussed the data and wrote the paper.

Conflicts of Interest:
The authors declare no conflict of interest.