Effects of surface etching on microstructure and mechanical strength of carbon fibers

Copyright © Korean Carbon Society http://carbonlett.org Carbon fiber (CF) is recognized as an optimal reinforcement for the composites application areas which light weight and high mechanical properties are needed [1,5,11]. The use of CFs are increasing in the fields of military, aerospace, ship building, and sports goods due to these reasons. Additionally, the applications of CFs are becoming wider to the automobile industries. And it is even accelerating by the development of low cost and high performance CFs [1,2,5]. CFs can be classified by the origins of precursors. There are representatively polyacrylonitrile (PAN)-based, pitch-based and rayon-based fibers, but it is well recognized that PAN-based CFs have high mechanical properties [9]. Typical properties of PAN-based CFs are known as high tensile strength and modulus. These features are due to the ribbonlike microstructure in the fiber, and it is reported to cause the high performance of the fiber [3,8,10]. For the application of military industries, CFs are normally used as a reinforced material for structural parts. In this case, the natural properties of CFs are thinkable more important than other application areas such as sports goods and automobile industries. In order to obtain proper features of CFs, the design of manufacturing process for CFs is very crucial to determine the mechanical properties including tensile strength & modulus, elongation, and so on [3,4,6]. It is known that the production process of PAN-based CF has the basic processes such as polymerization, spinning, stretching, stabilization, carbonization and surface treatment, and that the mechanical strength of PAN fiber depends heavily on the composition of each process step [7,13]. A better PAN precursor requires relatively small diameters, high molecular orientation in the fiber axis direction, high crystallinity, and low activation energy for the cyclization reaction [28]. The spinning process includes wet spinning, melt-assisted spinning (a process in which the highly concentrated polymer solution is heated under pressure and then the solvent is removed from the spinning chamber), and dry-wet spinning (a method of spinning with a short air-gap layer so as to be molecular orientation), and the spinning process plays absolutely a key role in the molecular orientation of the precursor fibers [14,19,20]. The PAN fiber produced by the spinning process is stabilized (the process of making the stable precursor by cyclization through oxidation at a temperature of 300°C. The fiber density is round 1.30 to 1.40 g/cc) and then carbonized at 1200°C (the carbon content is increased to 90% or more. The final density of the fiber is around 1.6 to 1.8 g/cc), and finally, the CF is manufactured [12,15-18,23] In all manufacturing processes, the surface of the CF is subject to various defects for the following reasons: (1) unevenness of the polymer chain orientation due to friction between the nozzle wall and the PAN resin in the spinning process, (2) unbalance of force applied during stretching and thus surface cracking, (3) uneven reaction with oxygen during stabilization, (4) Surface cracks due to fiber volume shrinkage during carbonization process. These various surface defects of CF remain as obstacles to the development of high DOI: http://dx.doi. org/ DOI:10.5714/CL.2018. 28.100