Modified oxidative thermal treatment for the preparation of isotropic pitch towards cost-competitive carbon fiber

https://doi.org/10.1016/j.jiec.2017.05.039Get rights and content

Highlights

  • An O2/N2 mixed gas was used as the reaction gas, and the softening point and pitch yield variations were measured at different O2 concentrations.

  • When softening point increased from 130 to 249 °C, the pitch yield remained almost constant (only 0.6% drop).

  • Pitches prepared by the modified oxidative thermal treatment exhibited excellent spinnability without breakage for more than 10 min of spinning.

  • The obtained carbon fibers showed high tensile strengths comparable to that of a commercial isotropic-pitch-derived carbon fiber (0.83 GPa).

Abstract

A modified oxidative thermal treatment was developed to increase the softening point of a pitch precursor while minimizing the pitch yield loss. An O2/N2 mixed gas was used as the reaction gas, and the softening point and pitch yield variations were measured at different O2 concentrations. As a result, when softening point increased from 130 to 249 °C, the pitch yield remained almost constant (only 0.6% drop). Structural analysis performed via MALDI-TOF, FTIR, and 1H and 13NMR showed that condensation reaction between pitch molecules in the presence of O2-containing gas followed a different mechanism depending on the O2 concentration. In addition, the precursors were spun into pitch fibers and carbonized at 1100 °C. During the spinning, they exhibited excellent spinnability without breakage for more than 10 min of spinning. The obtained carbon fibers showed high tensile strengths comparable to that of a commercial isotropic-pitch-derived carbon fiber (0.83 GPa). The current study showed that the modified thermal treatment is useful for the preparation of cost-competitive pitch precursors suitable for carbon fiber production.

Introduction

The increasingly strict environmental policies throughout the world have forced the automotive industry to increase the fuel economy of internal combustion engine vehicles and to develop electric vehicles with longer driving range. The most feasible way to meet this demand is to reduce vehicle weight using lightweight materials. Carbon fibers as a form of carbon fiber reinforced plastic are considered one of the strongest candidates to replace conventional steel-based materials due to their outstanding mechanical properties (tensile strengths of up to 7 GPa and moduli up to 900 GPa) and low volumetric mass densities (1.75–2.00 g/cm) [1]. The automotive industry requires carbon fiber prices of less than $11/kg and tensile strengths of 1.7 GPa [2], [3]. The requirement for the mechanical properties is not challenging, but the price requirement is difficult to satisfy. It must be lower than half the current prices of carbon fibers. In recent years, many research efforts have been devoted to developing low-cost carbon fibers.

Approaches towards low-cost carbon fibers must consider the price of the raw starting materials and process costs. Polyacrylonitrile (PAN)-based carbon fibers, which currently occupy greater than 90% market share, are not feasible because of the high price of the starting material (acrylonitrile) and the low production yield. Among the other types of precursors including pitch, cellulose and lignin, pitch is considered an attractive precursor due to the low-cost starting materials, such as coal-tar (by-product of the steel industry) and petroleum residues (by-product of the petrochemical industry). There are two types of pitch precursors: mesophase and isotropic pitches. Mesophase pitch-based carbon fibers exhibit high performances but suffer from the expensive processing cost. Whereas, isotropic pitch-based carbon fibers are poor in performances but cheap in manufacturing. In this regards, researches on pitch-based carbon fibers are focusing on either decreasing the cost of mesophase pitches preparation or increasing mechanical properties of isotropic pitch-based carbon fibers. Both carbon fibers can meet the demands of the automotive industry if the problems are resolved.

Recently, researches have been focused onto improving properties of isotropic pitches and the carbon fibers based on the pitches including bromination and fractionation of molecular weight distribution [4], [5], [6], [7], [8], [9]. Also, there have been many efforts to find cheap and high performing raw materials: anthracene oil [4], [5], hyper coal [6], [7], vacuum residue (VR) [8], [9] and pyrolysis fuel oil (PFO) [10], [11]. Among the candidates, PFO has received particular research interest due to its low impurities and abundant aromatic content.

One of the most important characteristics of spinnable pitches is a softening point. Standard procedures to make carbon fibers are pitch synthesis, spinning, stabilization, and carbonization. Among the procedures, the pitch synthesis and stabilization are the most time and energy consuming and interdependent steps. As-spun fibers from the pitches with softening points lower than 250 °C are not suitable for stabilization, a process to convert as-spun pitch fibers into thermoset fibers. Therefore it is essential to keep softening points higher than 250 °C if the pitches used for fiber purpose. However, pitches generally have low softening points between 100 and 200 °C once synthesized. Therefore, an additional process to increase the softening point should be performed. In most cases, the softening point can be adjusted by removing low molecular weight components [6], [12] or solvent extraction [13]. However, these methods result in substantial pitch yield loss, thereby increasing the production cost. Therefore, a new method for increasing the softening point without yield loss must be developed for the preparation of low-cost carbon fiber.

To adjust softening points of pitches, air-blowing has been widely investigated by researchers. Previous studies demonstrated that air-blowing is a simple and effective way to increase the softening point [14], [15], [16], [17], [18], [19], [20]. Air produces oxidative thermal condensation between the molecular components at elevated temperatures, increasing the average molecular weight and softening point [21], [22]. However, air consists of not only 21% O2 but also inert gases, including 78% N2. These inert gases facilitate volatilization of light components via the blowing effect and reduce the pitch yield. We hypothesize that using gas with a higher O2 concentration than air would be a more effective way to increase the softening point because a higher O2 concentration would enable the condensation reaction to progress at a lower gas flow rate, suppressing the volatilization of light components that can minimize the pitch yield loss. Many studies were performed on pitch air-blowing, but few studies proposed its feasibility for carbon fiber applications, especially with respect to the mechanical properties of carbon fibers derived from air-blown pitches and their spinnability.

Herein, we investigated oxidative thermal treatment using O2/N2 mixed gas with varying O2 concentrations as an effective way to increase the softening point of pitch without loss of pitch yield. PFO was used as the starting material. The effects on the softening point and yield were examined at varying O2 concentrations. For comparison, the effects of vacuum distillation and the gas flow rate were also examined. The reaction between O2 and hydrocarbons is exothermic and highly dependent on the concentration [23]; thus, the O2 concentration was limited to 50% for safety. To ensure the possibility of the pitches via the treatment for carbon fiber production, the pitches were melt-spun to analyze their spinnability, and the mechanical properties of the resulting carbon fibers were measured and compared with the control.

Section snippets

Preparation of pitch precursor

For the petroleum raw material, PFO (Hanwha-Total Co., South Korea) were used to synthesize pitch precursor. The pitch precursor was prepared via oxidative thermal treatment. In a typical process, 700 g of PFO was added to a 1 L reactor (Autoclave Korea, South Korea) and heat treated at 360 °C for 5 h under vigorous agitation. To create an oxidative atmosphere, O2-containing reaction gas was blown into the reactor at a flow rate of 0.5 L/min throughout the entire reaction. The O2-containing reaction

Characterization of the starting material

The compositional properties of PFO are presented in Table 1. The PFO was composed of greater than 92% carbon, without any other heteroatoms, such as nitrogen and sulfur. The absence of such impurities is desirable in the starting material for a carbon fiber precursor because these elements hinder graphitic structure development of pitch-based carbon fibers. In particular, sulfur can affect the mechanical properties of the carbon fiber. During heat treatment, sulfur-bearing gases, such as CS2

Conclusion

We applied a modified oxidative thermal treatment to prepare spinnable isotropic pitches using O2/N2 mixed gas with varying O2 concentrations. The treatment was highly effective at increasing the softening point while maintaining high pitch yield by facilitating oxidative thermal condensation and suppressing the volatilization of light components. Compared to a commercial pitch, the experimental pitches prepared by the treatment showed excellent spinnability. In conclusion, the current research

Acknowledgment

This Research was performed as a part of project No KK1701-G00 (Research on petroleum based carbon fiber and its composite) and No SKO1706M01 (Research on petroleum impregnating pitch) and was supported by the Korea Research Institute of Chemical Technology (KRICT).

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