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Meltblown Solvated Mesophase Pitch-Based Carbon Fibers: Fiber Evolution and Characteristics

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Meltblown Solvated Mesophase Pitch-Based Carbon Fibers: Fiber Evolution and Characteristics Journal of C Carbon Research Article Meltblown Solvated Mesophase Pitch-Based Carbon Fibers: Fiber Evolution and Characteristics Zhongren Yue * ID , Chang Liu and Ahmad Vakili * University of Tennessee Space Institute, 411 B.H. Goethert Parkway, Tullahoma, TN 37388, USA; [email protected] * Correspondence: [email protected] (Z.Y.); [email protected] (A.V.); Tel.: +1-931-393-7362 (Z.Y.); +1-931-393-7483 (A.V.) Received: 12 July 2017; Accepted: 7 August 2017; Published: 8 August 2017 Abstract: Potentially low-cost continuous carbon fibers are produced from solvated mesophase pitch through a patented meltblowing process. The structural evolution and properties of the fibers are characterized by various analytical methods. The meltblown fibers are continuous fibers which are collected into a fibrous web form, and the diameter of the filaments is attenuated by the flow rate of air streams. The spun fibers can be rapidly stabilized in air due to the high melting mesogens and the removable solvent. The carbonized fibers show a high carbon yield of 75 wt % (or 86 wt % if the solvents are neglected) and a mean diameter of 8–22 µm with typical fiber diameter distribution and variation. The evolution of the fiber structure depends not only on the processing temperature but also on the fiber diameter. The processed carbon fibers retain the same form as the spun fibers and have a low packing density and reasonable mechanical properties. Keywords: carbon fibers; mesophase pitch; thermal analysis; fiber conversion processes; microstructure 1. Introduction Carbon fibers (CFs) as a reinforcement in composite materials have been widely used in industry mainly due to their high specific strength and modulus. The large scale use of CFs in aircraft and aerospace is driven by maximum performance and fuel efficiency, while the cost factor and the production requirements are not critical. The use of CFs in general engineering and surface transportation is dominated by cost constraints, high production rate requirements, and generally less critical performance needs [1]. Therefore, the large-volume application of commercial CFs in the automotive industry has been hindered due to the high fiber costs and the lack of high-speed composite fabrication techniques [2]. It is believed that the use of low cost precursors, low cost spinning and thermal processes, and mass production is a critical step toward lower cost CFs and their composites for use in multiple industries [2–4]. Polyacrylonitrile (PAN) and conventional mesophase pitch (MP) are the two most important CF precursors for the manufacture of commercial CFs. Over the past two decades, many polymers, such as PE and lignin, have been evaluated as low-cost CF precursor materials. A significant amount of work has been done on engineering new precursors and processing technologies, relating fiber structure to properties, and translating the relationship into production for either reducing production cost [2,5,6] or increasing fiber properties [7–9]. Potentially low-cost CF has been produced from solvated MP pitch using a highly efficient meltblowing process [10]. Unlike conventional MP, solvated MP normally contains at least 70% by volume optical anisotropy, or MP in which at least 50 wt % is MP soluble quinoline insoluble (MSQI) pitch [11,12]. Due to its high MSQI content and adequate utilization of high melting mesogen materials, such an MP is of relatively lower cost. The added solvents make the pitch materials more spinnable at lower temperatures. When the solvent is removed, it has a high melting point, or may be unmeltable, which permits the formation of fibers that require C 2017, 3, 26; doi:10.3390/c3030026 www.mdpi.com/journal/carbon C 2017, 3, 26 2 of 14 C 2017, 3, 26 2 of 14 removed, it has a high melting point, or may be unmeltable, which permits the formation of fibers that require little or no stabilization as spun [11–13]. It is well known that the meltblowing process is littlewidely or no used stabilization to produce as spuneconomical [11–13 ].nonwoven It is well knownfiber web. that Similarly the meltblowing to the spun-bond process is process, widely usedthe tomelt-blown produce economical process directly nonwoven transforms fiber the web. spun Similarly fibers into to a the nonwoven spun-bond fabric process, in a single, the melt-blownintegrated processprocess directly [14]. Therefore, transforms the the use spun of solvated fibers into MP aan nonwovend meltblow fabric technology in a single, would integrated help to prepare process low- [14]. Therefore,cost CFs. the use of solvated MP and meltblow technology would help to prepare low-cost CFs. MostMost of of the the previous previous studiesstudies of of pitch-based pitch-based CFs CFs have have been been conducted conducted by using by using conventional conventional MP MPand and melt-spinning melt-spinning processes processes [15–22]. [15–22 Very]. Very few few studies studies focused focused on on the the newly newly developed, developed, highly highly efficientefficient meltblowing meltblowing process process of of lower-cost lower-cost solvatedsolvated MP. ItIt was reported that that short short CF CF was was prepared prepared fromfrom naphthalene naphthalene MP MP through through the the meltblowing meltblowing processprocess [[23].23]. However However the the short short fibers fibers will will reduce reduce theirtheir reinforcement reinforcement in in the the composites composites and and limit limit their their application.application. InIn the the present present study, study, low-cost, low-cost, continuous continuous CFs wereCFs were prepared prepared from solvatedfrom solvated MP by MP using by a using patented a meltblownpatented fibermeltblown spinning fiber technique spinning [24 technique] and relatively [24] and fast relatively thermal stabilization fast thermal in stabilization air and carbonization in air and in carbonization in N2. The structure, evolution and properties of the fibers were then characterized N2. The structure, evolution and properties of the fibers were then characterized with various analyses and comparedwith various with commerciallyanalyses and availablecompared CFs. with The commercia results helpedlly available us to optimize CFs. The the CFresults preparation helped process,us to optimize the CF preparation process, to better understand the characteristics of the prepared CFs, to better understand the characteristics of the prepared CFs, and improve their uses in composite materials. and improve their uses in composite materials. 2. Results and Discussion 2. Results and Discussion 2.1. Characteristics of the Spun Fibers 2.1. Characteristics of the Spun Fibers The solvated MP fibers (Figure1) were continuously produced in the meltblown fiber spinning apparatus.The solvated The meltblown MP fibers fibers (Figure were 1) collected were continuously as shown inproduced Figure1 ina, andthe meltblown are continuous fiber longspinning fiber websapparatus. with a width The meltblown and a thickness fibers were of approximately collected as shown 10 cm in and Figure 1 cm, 1a, respectively. and are continuous In the web long of fiber fibers, webs with a width and a thickness of approximately 10 cm and 1 cm, respectively. In the web of individual filaments are continuous fibers and are loosely assembled together, their orientation is fibers, individual filaments are continuous fibers and are loosely assembled together, their controlled if needed, here only slightly toward the length of the web, as shown in Figure1b. The rate orientation is controlled if needed, here only slightly toward the length of the web, as shown in Figure of spinning, compared to the rate of collection of the fibers in a nonvowen web format, determines 1b. The rate of spinning, compared to the rate of collection of the fibers in a nonvowen web format, the orientation of the fibers in the web. The spun fibers are very brittle and need to be very carefully determines the orientation of the fibers in the web. The spun fibers are very brittle and need to be handled when transported. very carefully handled when transported. Figure 1. Typical continuous pitch fiber web (a) collected from the meltblown fiber spinning Figure 1. Typical continuous pitch fiber web (a) collected from the meltblown fiber spinning apparatus, apparatus, and (b) showing the orientation of the individual long fibers. and (b) showing the orientation of the individual long fibers. Due to the use of a solvated MP precursor, the spun fibers contained solvent. A thermogravimetricDue to the use of analysis a solvated (TGA) MP precursor,running in the N2spun was fibersperformed contained to compare solvent. the A spun thermogravimetric fibers (spun analysiswith a (TGA)hot air flow running rate of in 40 N 2andwas 100 performed L per minute to compare(Lpm), respectively) the spun fibers with (spuntheir precursor with a hot (solvated air flow rateMP) of and 40 andthe conventional 100 L per minute isotropic (Lpm), and MP respectively) materials (both with solvent their precursor free). The (solvatedresults of the MP) TGA and are the conventionalshown in Figure isotropic 2. Without and MP solvents, materials both (bothisotropic solvent and MP free). materials The results showed of a therelatively TGA high are shown weight- in Figureloss starting2. Without point solvents, (335 °C). In both contrast, isotropic the solvated and MP MP materials and its showedfibers showed a relatively a relatively high low weight-loss weight- startingloss starting point(335 point◦C). (175 In °C) contrast, due to the the solvated solvent remo MP andval. itsBased fibers on showed the TGA a relativelyresults, the low solvated weight-loss MP startingand its point fiber contained (175 ◦C) due 12–16 to wt the % solvent solvent. removal. The solvent Based content on the of the TGA spun results, fibers the was solvated less than MP 1% andof itsits fiber precursor, contained indicating 12–16 wt that % solvent.only a small The solvent amount content of solvents of the was spun removed fibers was during less the than spinning 1% of its precursor, indicating that only a small amount of solvents was removed during the spinning process.
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