Experimental and Statistical Optimization of the Tensile Strength of

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Experimental and statistical optimization of the tensile strength of carbon fibers from pitches with different composition Noel Diez, Patricia Alvarez, Clara Blanco, Ricardo Santamaria, Marcos Granda, and Rosa Menendez Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04045 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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Industrial & Engineering Chemistry Research

Experimental and statistical optimization of the tensile strength of carbon fibers from pitches with different composition Noel Diez, Patricia Álvarez*, Clara Blanco, Ricardo Santamaría, Marcos Granda, Rosa Menéndez Instituto Nacional del Carbon, INCAR-CSIC, P. O.Box 73. 33080-Oviedo, Spain.

ABSTRACT: A factorial analysis was used to maximize the tensile strength of the carbon fibers obtained from pitch by means of a rational selection of variables and precursor properties. The combined influence of the fiber diameter, the soaking time at the optimum stabilization temperature and the effect of the polycondensation degree of the pitch on the tensile strength of the carbonized fibers was assessed using a factorial design. For the first time, the strong influence of the degree of polycondensation of the parent pitch was evidenced, even though less polycondensed pitches were spun at a higher yield and favored the homogeneous fixation of oxygen during stabilization. Fibers prepared from the most polycondensed pitches exhibited the best values of tensile strength. However, very highly polycondensed pitch compositions may hinder oxygen diffusion and the formation of cross-linked structures during stabilization, leading to a poorer mechanical performance that can be overcome by increasing the stabilization time. Corresponding author: Dr. Patricia Alvarez; [email protected]. Tlf:+34 985119090. Fax:+34 985297662 KEYWORDS. Stabilization, Carbon fiber, Pitch Composition, Tensile Strength, Factorial design, polymerized pitch.

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1. INTRODUCTION

Over the last years, the construction and automotive industries have been increasingly demanding new light-weight materials with enhanced mechanical properties. In response, carbon fibers (CFs) have emerged as an interesting alternative for providing useful reinforcement for composites by improving their mechanical properties and lowering their overall weight, the latter being a factor of particular importance especially in electrical vehicles1. However, the use of CFs in most of these applications is limited by their price2.While high performance carbon fibers prepared from mesophase pitches or polyacrylonitrile –with a Young´s modulus >350 GPa and a tensile strength of over 2 GPa, respectively3–have found application in highly technological sectors, their low volume of production and prohibitive cost restrict their use in the automotive and civil engineering fields4. An alternative are general purpose carbon fibers (GPCF), which are prepared from isotropic pitches or phenolic resins. These fibers have more modest tensile properties (tensile strength values of 0.5-1 GPa)5 but their costs of production are much lower. Therefore, in order to use GPCF as a suitable alternative for reinforcing and lightening advanced structural materials, it is essential to maximize their quality while maintaining affordable fabrication costs. The production of GPCF from conventional isotropic pitches requires several steps: (i) their initial filtration to remove solid particulates and/or to increase their softening points, (ii) their melt-spinning to conform the shape of the fiber, (iii) the stabilization of green fibers to avoid the melting of the pitch during processing and iv) their carbonization to obtain the final properties of the carbon fibers6,7. The conditions applied in this process will influence the mechanical characteristics of the carbon fibers. For instance, smaller diameters –a parameter that can be controlled during the spinning by modifying the temperature, pressure or winding speed- have been related to a lower density of defects and better


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tensile strength8-10. On the other hand, stabilization is a time-consuming and energy-demanding step that must be optimized to produce high quality GPCFs at low cost11. This step consists in oxidizing and cross-linking the molecules of the pitch fiber by means of controlled thermal heating under air flow12-15. The decomposition of oxidized structures during carbonization can give rise to surface defects that are directly related to the reduced tensile strength of the resulting carbon fibers16. Hence, not only the stabilization conditions but also the composition and degree of polycondensation of the parent pitch (which can be controlled by selecting an adequate precursor)17-21need to be carefully tuned11. To date, the optimization of individual parameters (e.g. stabilization or carbonization temperature) has been described for specific CF precursors, but none of the published reports address the complex relationship between the different parameters involved in the stabilization process. The present work focuses on the optimization of the preparation procedure of general purpose carbon fibers from an experimental and also a statistical point of view. By using three coal tar pitches with different characteristics as precursors9, their transformation into carbon fibers by melt spinning, stabilization and carbonization under different experimental conditions was analyzed and their properties characterized by XPS, FTIR, TG and elemental analysis. It was considered of particular importance to determine the effect of the different degree of polycondensation of the precursors on their processing. In order to rationalize the optimization of the processing while maximizing the tensile strength of the carbon fibers, the process was then addressed by developing a factorial analysis approach. This approach has already been adopted to evaluate the combined effect of different parameters on the preparation of activated carbons22-23, which cannot be assessed by traditional univariate methods. To our knowledge, no such methodology has ever been used for the optimization of the production of carbon fibers with enhanced tensile strength. This analysis can be used as a guide to optimize the preparation of carbon fibers with the best performance.


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2. MATERIALS AND METHODS 2.1. Fiber preparation Three different coal tar pitches prepared from an industrial anthracene oil were used as carbon fiber precursors. The preparation of the pitches was described in detail elsewhere24. Briefly, the anthracene oil supplied by Industrial Química del Nalón, S.A. was thermally treated in the presence of air, yielding a reaction product that was subsequently thermally treated and distilled to produce a pitch. This second step was adjusted in order to obtain three pitches (ISO1, ISO2 and ISO3) with different composition and softening point. The pitches were spun into isotropic carbon fibers by the melt-spinning procedure. As previously described9, a stainless steel laboratory-scale apparatus with capacity for 25 g of sample was used to extrude the precursors through a mono-hole spinneret of diameter of 500 µm. In a typical experiment, the precursors were heated to ~ 35 ºC above their softening point and extruded by applying a nitrogen pressure of 1 bar. The green fibers were wound on a roller able to work at different winding speeds allowing the diameter of the fiber to be controlled. The as-spun fibers were stabilized in air following a multistep time/temperature program: 160 ºC (1h), 180 ºC (1h), 200 ºC (1h), 220 ºC (1h), 240 ºC (t), using heating rates of 1 ºC min-1. The oxidized fibers were carbonized in a tube furnace under a nitrogen flow at 900 ºC for 30 min using a heating rate of 2 ºC min-1. 2.2. Characterization of materials The softening point of the pitches was determined according to the American Society for Testing and Materials (ASTM) D3104 standard. The toluene insoluble values were calculated according to the Pechiney B-16 (series PT-7/79 of STPTC) standard. The N-methyl-2-pyrrolidinone insolubles were determined following the ASTM D2318 standard for quinoline insolubles, but using N-methyl-2pyrrolidinone (NMP) instead of quinoline. Thermogravimetric experiments (TG/DTG) were performed in a TA SDT 2960 analyzer. 15 milligrams of each sample, < 0.4 mm particle size, was


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placed in a crucible which was then introduced in the thermobalance. The temperature was increased to 1000 ºC at a heating rate of 10 ºC min-1 under a nitrogen flow of 100 ml min-1. The elemental analysis (CHNS) of the green fibers was determined on a LECO-CHNS-932 microanalyzer. The oxygen content was obtained directly using a LECO-VTF-900 furnace coupled to a micro-analyzer. The analyses were performed with 1 mg of sample ground and sieved to

Experimental and Statistical Optimization of the Tensile Strength of

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