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Modification into Spinning Material Appropriate for Carbon Fiber Employing Hydrogen

In order to modify the impregnated pitch procured from C-Chem into spinning material suitable for the production of carbon fiber, after hydrogenation, the pitch is first subjected to thermal polymerization*1, in which lightweight substances are blown off by means of high-precision distillation, and are then removed of minute impurities by means of filtration (Fig. 2).

Fig. 2 Raw Material Modification Process

The hydrogen plays two roles: to remove the sulfur and nitrogen contained in coal tar, and to change the molecular structure of the impregnated pitch so as to convert it into 6-membered ring carbon*2 in which graphite crystals can easily grow. The 6-membered ring carbon is converted to graphite crystals by means of heating, and as the crystals grow, their strength and modulus increase. Impregnated pitch in its original form cannot be sufficiently converted to graphite crystals, and even when subjected to heat treatment, a fully developed graphite structure does not result (isotropic pitch). However, a regularly-aligned molecular (liquid crystal) structure can be produced, even in a liquid state, by adding hydrogen to adjust the molecular structure of the impregnated pitch (mesophase pitch). Mesophase pitch that is liquid-crystallized in a well balanced manner has a softening point (the temperature at which the substance starts to melt) below the thermal decomposition temperature and, therefore, can be used as a material for producing high-quality carbon fiber (Photo 1). (Isotropic pitch can be used for producing general-purpose low modulus carbon fiber.)

Photo 1 Change of Carbon Fiber Structures due to Difference of Material Pitch

*1 Polymerization: Forming of compounds larger in size than the original substance through the chemical bonding of two or more molecules composed of one or more kinds of substances

*2 6-membered ring: Chemical material having 6 atoms bonded in a ring state

Control of Crystals in the Spinning Process to Further Improve Strength and Modulus by Heat Treatment

Pitch modified as the spinning material becomes carbon fiber via the production process shown in Fig. 3. In the spinning process by which pitch is converted to fi bers having a diameter of about 10 μm (one-tenth of a hair), the pitch crystals are aligned longitudinally by means of die (nozzle) configuration and the stirring method; at the same time, physical properties such as modulus and strength are optimized by controlling the method used to align and laminate the layers in which the crystals are arranged or by controlling the sectional structure (Fig. 4). Only three companies in the world can produce high-strength and high modulus carbon fiber employing mesophase pitch as the spinning material. Among them, only NGF can produce pitch-based carbon fiber having a crystal orientation controlled so as not to allow cross-sectional cracking of the fiber, a quality defect (Photo 2).

Photo 2 Carbon Fiber with Cross-sectional Cracking

Yarn produced by the spinning of as many as 12,000 ultrafine, non-processed pitch-based carbon fibers has a low softening point (300°C), which causes melting when the fibers are subjected to high-temperature heat treatment. To solve this problem, oxygen and other elements are added in advance to eliminate hydrogen and other elemental impurities, while at the same time the molecular bonding capability is improved by precisely controlling the chemical reaction used to raise the softening point (infusibilization treatment by means of gaseous phase oxidation). The resulting infusibilized fiber is heated and baked in an oxygen-free state to remove impurities and elements other than carbon in order to raise the carbon density (carbonization). Following this, the carbon crystals are regularly rearranged by further raising the heat-treatment temperature to improve the modulus and strength (graphitization; the same crystal structure found in pencil lead). Because the carbon fi ber thus manufactured is most commonly applied in concert with resin, the fiber is subjected to surface treatment to improve its bonding capacity with resin and its workability during secondary processing.

Fig. 3 Production Process for Pitch-based Carbon Fiber

Fig. 4 Crystal Orientation of Mesophase Pitch in Spinning Process

Commonly, pitch-based carbon fiber material is produced by bundling 6,000~12,000 carbon fibers with a diameter of 10 μm (Photo 3). To meet the need for lighter weight, NGF has initiated the industrial production of carbon fiber material manufactured by bundling 400 fibers with the world’s finest diameter of 7 μm and modulus similar to that of 10 μm-diameter carbon fibers.

Photo 3 Carbon Fiber Production Line

New Market Development Capitalizing on the Superior Physical Properties peculiar to Pitch-based Carbon Fibers

In order to meet various applications, NGF supplies a variety of pitch-based carbon fibers of different moduli, such as yarn, fabric, and prepreg manufactured by impregnating thermosetting resins (Photo 4). Extensive application development of carbon fiber, such as the CFRP composite material developed by Nippon Steel Composites Company (established in 1988 and integrated into Nippon Steel Materials in 2010), is one of the strengths of the new market development program being promoted by NGF.

Photo 4 High-performance Pitch-based Carbon Fiber Products

Currently, carbon fiber with a low modulus (50~150 GPa class) is increasingly being applied in golf club shafts and fishing rods. On the other hand, high-modulus carbon fiber (600 GPa or more), which is difficult to produce for PAN-based carbon fiber, is used in various rolls for printing and film production, in the arms of liquid crystal and semiconductor transfer robots and in construction reinforcing members by fully capitalizing on such performances as zero thermal deformation, lightness of weight and high strength. (Refer to Photo 5) More recently, high modulus material is applied in racing bicycle frames requiring both lightness of weight and high rigidity.

Photo 5 Examples of Conventional Applications

The fields with high expectations for expanded application growth include the shafts of machine tool motors, and robot arms and beams. In particular, the long beams of large-capacity machine tools are heavy and suffer reduced fabrication accuracy due to vibration. Thus, fabrication accuracy improves with the adoption of lightweight carbon fiber-reinforced plastic with high vibration damping capacity (Photo 6).

Fig. 5 Thermal Conductivity of Pitch-based Carbon Fiber

Fig. 6 Coefficient of Thermal Expansion of Various Materials

High modulus carbon fiber has high thermal conductivity (ease of thermal conduction), and its coefficient of thermal expansion can be minimized to zero by composite use with other materials (Figs. 5 and 6). Due to these characteristic performances, pitch-based carbon fiber has recently been adopted for the heat radiation members of electronic equipment, solar panel members, and the antenna members of artificial satellites operating in space under temperature fluctuations as great as 200°C or more, depending on the extent of exposure to sun light (Photo 7).

Photo 6 Automobile Panel Fabrication Equipment

Pitch-based carbon fiber offers characteristic performances such as high elasticity (900 GPa) and high thermal conductivity (900 W) not found in any other materials in practical use. NGF is promoting the market development of these fibers in two directions.

One is to replace metallic materials with carbon fiber in industrial applications. With energy savings being called for in diverse industrial fields, lightweight pitch-based carbon fiber with high rigidity can contribute to the reduction of weight in production equipment and devices.

Another direction of market development is application development in the consumer product field, which capitalizes on such performances of pitch-based carbon fiber as high thermal conductivity and zero C.T.E. (coefficient of thermal expansion). For example, with the growing need for higher functionality and higher density in electronic equipment devices, carbon fiber is seen to have a high heat-radiation capacity, which is required of high thermal-conducting electronic materials capable of improving device reliability.

On the strength of reinforced supply capacity realized by the startup of new production lines in the fall of 2010, NGF is further promoting new market development for high-performance pitch-based carbon fiber.